Sockets API Extensions for Stream Control Transmission Protocol (SCTP)
draft-ietf-tsvwg-sctpsocket-19.txt

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Abstract

This document describes a mapping of the Stream Control Transmission Protocol SCTP into a sockets API. The benefits of this mapping include compatibility for TCP applications, access to new SCTP features and a consolidated error and event notification scheme.

Table of Contents

1.  Introduction
2.  Conventions
3.  Data Types
4.  One-to-Many Style Interface
    4.1.  Basic Operation
        4.1.1.  socket()
        4.1.2.  bind()
        4.1.3.  listen()
        4.1.4.  sendmsg() and recvmsg()
        4.1.5.  close()
        4.1.6.  connect()
    4.2.  Implicit Association Setup
    4.3.  Non-blocking mode
    4.4.  Special considerations
5.  One-to-One Style Interface
    5.1.  Basic Operation
        5.1.1.  socket()
        5.1.2.  bind()
        5.1.3.  listen()
        5.1.4.  accept()
        5.1.5.  connect()
        5.1.6.  close()
        5.1.7.  shutdown()
        5.1.8.  sendmsg() and recvmsg()
        5.1.9.  getpeername()
6.  Data Structures
    6.1.  The msghdr and cmsghdr Structures
    6.2.  SCTP msg_control Structures
        6.2.1.  SCTP Initiation Structure (SCTP_INIT)
        6.2.2.  SCTP Header Information Structure (SCTP_SNDRCV)
        6.2.3.  Extended SCTP Header Information Structure (SCTP_EXTRCV)
    6.3.  SCTP Events and Notifications
        6.3.1.  SCTP Notification Structure
        6.3.2.  SCTP_ASSOC_CHANGE
        6.3.3.  SCTP_PEER_ADDR_CHANGE
        6.3.4.  SCTP_REMOTE_ERROR
        6.3.5.  SCTP_SEND_FAILED
        6.3.6.  SCTP_SHUTDOWN_EVENT
        6.3.7.  SCTP_ADAPTATION_INDICATION
        6.3.8.  SCTP_PARTIAL_DELIVERY_EVENT
        6.3.9.  SCTP_AUTHENTICATION_EVENT
        6.3.10.  SCTP_SENDER_DRY_EVENT
    6.4.  Ancillary Data Considerations and Semantics
        6.4.1.  Multiple Items and Ordering
        6.4.2.  Accessing and Manipulating Ancillary Data
        6.4.3.  Control Message Buffer Sizing
7.  Common Operations for Both Styles
    7.1.  send(), recv(), sendto(), and recvfrom()
    7.2.  setsockopt() and getsockopt()
    7.3.  read() and write()
    7.4.  getsockname()
8.  Socket Options
    8.1.  Read / Write Options
        8.1.1.  Retransmission Timeout Parameters (SCTP_RTOINFO)
        8.1.2.  Association Parameters (SCTP_ASSOCINFO)
        8.1.3.  Initialization Parameters (SCTP_INITMSG)
        8.1.4.  SO_LINGER
        8.1.5.  SCTP_NODELAY
        8.1.6.  SO_RCVBUF
        8.1.7.  SO_SNDBUF
        8.1.8.  Automatic Close of Associations (SCTP_AUTOCLOSE)
        8.1.9.  Set Primary Address (SCTP_PRIMARY_ADDR)
        8.1.10.  Set Adaptation Layer Indicator (SCTP_ADAPTATION_LAYER)
        8.1.11.  Enable/Disable Message Fragmentation (SCTP_DISABLE_FRAGMENTS)
        8.1.12.  Peer Address Parameters (SCTP_PEER_ADDR_PARAMS)
        8.1.13.  Set Default Send Parameters (SCTP_DEFAULT_SEND_PARAM)
        8.1.14.  Set Notification and Ancillary Events (SCTP_EVENTS)
        8.1.15.  Set/Clear IPv4 Mapped Addresses (SCTP_I_WANT_MAPPED_V4_ADDR)
        8.1.16.  Get or Set the Maximum Fragmentation Size (SCTP_MAXSEG)
        8.1.17.  Get or Set the List of Supported HMAC Identifiers (SCTP_HMAC_IDENT)
        8.1.18.  Get or Set the Active Shared Key (SCTP_AUTH_ACTIVE_KEY)
        8.1.19.  Get or Set Delayed SACK Timer (SCTP_DELAYED_SACK)
        8.1.20.  Get or Set Fragmented Interleave (SCTP_FRAGMENT_INTERLEAVE)
        8.1.21.  Set or Get the SCTP Partial Delivery Point (SCTP_PARTIAL_DELIVERY_POINT)
        8.1.22.  Set or Get the Use of Extended Receive Info (SCTP_USE_EXT_RCVINFO)
        8.1.23.  Set or Get the Auto ASCONF Flag (SCTP_AUTO_ASCONF)
        8.1.24.  Set or Get the Maximum Burst (SCTP_MAX_BURST)
        8.1.25.  Set or Get the Default Context (SCTP_CONTEXT)
        8.1.26.  Enable or Disable Explicit EOR Marking (SCTP_EXPLICIT_EOR)
        8.1.27.  Enable SCTP Port Reusage (SCTP_REUSE_PORT)
    8.2.  Read-Only Options
        8.2.1.  Association Status (SCTP_STATUS)
        8.2.2.  Peer Address Information (SCTP_GET_PEER_ADDR_INFO)
        8.2.3.  Get the List of Chunks the Peer Requires to be Authenticated (SCTP_PEER_AUTH_CHUNKS)
        8.2.4.  Get the List of Chunks the Local Endpoint Requires to be Authenticated (SCTP_LOCAL_AUTH_CHUNKS)
        8.2.5.  Get the Current Number of Associations (SCTP_GET_ASSOC_NUMBER)
        8.2.6.  Get the Current Identifiers of Associations (SCTP_GET_ASSOC_ID_LIST)
    8.3.  Write-Only Options
        8.3.1.  Set Peer Primary Address (SCTP_SET_PEER_PRIMARY_ADDR)
        8.3.2.  Add a Chunk That Must Be Authenticated (SCTP_AUTH_CHUNK)
        8.3.3.  Set a Shared Key (SCTP_AUTH_KEY)
        8.3.4.  Deactivate a Shared Key (SCTP_AUTH_DEACTIVATE_KEY)
        8.3.5.  Delete a Shared Key (SCTP_AUTH_DELETE_KEY)
    8.4.  Ancillary Data and Notification Interest Options
9.  New Functions
    9.1.  sctp_bindx()
    9.2.  sctp_peeloff()
    9.3.  sctp_getpaddrs()
    9.4.  sctp_freepaddrs()
    9.5.  sctp_getladdrs()
    9.6.  sctp_freeladdrs()
    9.7.  sctp_sendmsg()
    9.8.  sctp_recvmsg()
    9.9.  sctp_connectx()
    9.10.  sctp_send()
    9.11.  sctp_sendx()
    9.12.  sctp_getaddrlen()
10.  IANA Considerations
11.  Security Considerations
12.  Acknowledgments
13.  Normative References
Appendix A.  One-to-One Style Code Example
Appendix B.  One-to-Many Style Code Example
§  Authors' Addresses

1.  Introduction

The sockets API has provided a standard mapping of the Internet Protocol suite to many operating systems. Both TCP [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.) and UDP [RFC0768] (Postel, J., “User Datagram Protocol,” August 1980.) have benefited from this standard representation and access method across many diverse platforms. SCTP is a new protocol that provides many of the characteristics of TCP but also incorporates semantics more akin to UDP. This document defines a method to map the existing sockets API for use with SCTP, providing both a base for access to new features and compatibility so that most existing TCP applications can be migrated to SCTP with few (if any) changes.

There are three basic design objectives:

1. Maintain consistency with existing sockets APIs: We define a sockets mapping for SCTP that is consistent with other sockets API protocol mappings (for instance, UDP, TCP, IPv4, and IPv6).
2. Support a one-to-many style interface This set of semantics is similar to that defined for connection-less protocols, such as UDP. A one-to-many style SCTP socket should be able to control multiple SCTP associations. This is similar to an UDP socket, which can communicate with many peer end points. Each of these associations is assigned an association ID so that an applications can use the ID to differentiate them. Note that SCTP is connection-oriented in nature, and it does not support broadcast or multicast communications, as UDP does.
3. Support a one-to-one style interface This interface supports a similar semantics as sockets for connection-oriented protocols, such as TCP. A one-to-one style SCTP socket should only control one SCTP association. One purpose of defining this interface is to allow existing applications built on other connection-oriented protocols be ported to use SCTP with very little effort. And developers familiar with those semantics can easily adapt to SCTP. Another purpose is to make sure that existing mechanisms in most OSes to deal with socket, such as select(), should continue to work with this style of socket. Extensions are added to this mapping to provide mechanisms to exploit new features of SCTP.

Goals 2 and 3 are not compatible, so in this document we define two modes of mapping, namely the one-to-many style mapping and the one-to-one style mapping. These two modes share some common data structures and operations, but will require the use of two different application programming styles. Note that all new SCTP features can be used with both styles of socket. The decision on which one to use depends mainly on the nature of applications.

A mechanism is defined to extract a one-to-many style SCTP association into a one-to-one style socket.

Some of the SCTP mechanisms cannot be adequately mapped to existing socket interface. In some cases, it is more desirable to have new interface instead of using existing socket calls. Section 9 (New Functions) of this document describes those new interface.

2.  Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Data Types

Whenever possible, data types from Draft 6.6 (March 1997) of POSIX 1003.1g are used: uintN_t means an unsigned integer of exactly N bits (e.g., uint16_t). We also assume the argument data types from 1003.1g when possible (e.g., the final argument to setsockopt() is a size_t value). Whenever buffer sizes are specified, the POSIX 1003.1 size_t data type is used.

4.  One-to-Many Style Interface

The one-to-many style interface has the following characteristics:

• Outbound association setup is implicit.
• Messages are delivered in complete messages (with one notable exception).
• There is a 1 to MANY relationship between socket and association.

4.1.  Basic Operation

A typical server in this style uses the following socket calls in sequence to prepare an endpoint for servicing requests:

• socket()
• bind()
• listen()
• recvmsg()
• sendmsg()
• close()

A typical client uses the following calls in sequence to setup an association with a server to request services:

• socket()
• sendmsg()
• recvmsg()
• close()

In this style, by default, all the associations connected to the endpoint are represented with a single socket. Each associations is assigned an association ID (type is sctp_assoc_t) so that an application can use it to differentiate between them. In some implementations, the peer endpoints addresses can also be used for this purpose. But this is not required for performance reasons. If an implementation does not support using addresses to differentiate between different associations, the sendto() call can only be used to setup an association implicitly. It cannot be used to send data to an established association as the association ID cannot be specified.

Once as association ID is assigned to an SCTP association, that ID will not be reused until the application explicitly terminates the association. The resources belonged to that association will not be freed until that happens. This is similar to the close() operation on a normal socket. The only exception is when the SCTP_AUTOCLOSE option (section 7.1.8) is set. In this case, after the association is terminated gracefully automatically, the association ID assigned to it can be reused. All applications using this option should be aware of this to avoid the possible problem of sending data to an incorrect peer end point.

If the server or client wishes to branch an existing association off to a separate socket, it is required to call sctp_peeloff() and in the parameter specifies the association identification. The sctp_peeloff() call will return a new socket which can then be used with recv() and send() functions for message passing. See Section 9.2 (sctp_peeloff()) for more on branched-off associations.

Once an association is branched off to a separate socket, it becomes completely separated from the original socket. All subsequent control and data operations to that association must be done through the new socket. For example, the close operation on the original socket will not terminate any associations that have been branched off to a different socket.

We will discuss the one-to-many style socket calls in more details in the following subsections.

4.1.1.  socket()

Applications use socket() to create a socket descriptor to represent an SCTP endpoint.

The function prototype is

int socket(int domain,
           int type,
           int protocol);

and one uses PF_INET or PF_INET6 as the domain, SOCK_SEQPACKET as the type and IPPROTO_SCTP as the protocol.

Here, SOCK_SEQPACKET indicates the creation of a one-to-many style socket.

Using the PF_INET domain indicates the creation of an endpoint which can use only IPv4 addresses, while PF_INET6 creates an endpoint which can use both IPv6 and IPv4 address.

4.1.2.  bind()

Applications use bind() to specify which local address the SCTP endpoint should associate itself with.

An SCTP endpoint can be associated with multiple addresses. To do this, sctp_bindx() is introduced in Section 9.1 (sctp_bindx()) to help applications do the job of associating multiple addresses.

These addresses associated with a socket are the eligible transport addresses for the endpoint to send and receive data. The endpoint will also present these addresses to its peers during the association initialization process, see [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).

After calling bind(), if the endpoint wishes to accept new associations on the socket, it must call listen() (see Section 4.1.3 (listen())).

The function prototype of bind() is

int bind(int sd,
         struct sockaddr *addr,
         socklen_t addrlen);

and the arguments are

sd:

The socket descriptor returned by socket().

addr:

The address structure (struct sockaddr_in or struct sockaddr_in6, see [RFC3493] (Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. Stevens, “Basic Socket Interface Extensions for IPv6,” February 2003.)).

addrlen:

The size of the address structure.

If sd is an IPv4 socket, the address passed must be an IPv4 address. If the sd is an IPv6 socket, the address passed can either be an IPv4 or an IPv6 address.

Applications cannot call bind() multiple times to associate multiple addresses to an endpoint. After the first call to bind(), all subsequent calls will return an error.

If addr is specified as a wildcard (INADDR_ANY for an IPv4 address, or as IN6ADDR_ANY_INIT or in6addr_any for an IPv6 address), the operating system will associate the endpoint with an optimal address set of the available interfaces.

If a bind() is not called prior to a sendmsg() call that initiates a new association, the system picks an ephemeral port and will choose an address set equivalent to binding with a wildcard address. One of those addresses will be the primary address for the association. This automatically enables the multi-homing capability of SCTP.

4.1.3.  listen()

By default, new associations are not accepted for one-to-many style sockets. An application uses listen() to mark a socket as being able to accept new associations.

The function prototype is

int listen(int sd,
           int backlog);

and the arguments are

sd:

The socket descriptor of the endpoint.

backlog:

If backlog is non-zero, enable listening else disable listening.

Note that one-to-many style socket consumers do not need to call accept to retrieve new associations. Calling accept() on a one-to-many style socket should return EOPNOTSUPP. Rather, new associations are accepted automatically, and notifications of the new associations are delivered via recvmsg() with the SCTP_ASSOC_CHANGE event (if these notifications are enabled). Clients will typically not call listen(), so that they can be assured that the only associations on the socket will be ones they actively initiated. Server or peer-to-peer sockets, on the other hand, will always accept new associations, so a well-written application using server one-to-many style sockets must be prepared to handle new associations from unwanted peers.

Also note that the SCTP_ASSOC_CHANGE event provides the association ID for a new association, so if applications wish to use the association ID as input to other socket calls, they should ensure that the SCTP_ASSOC_CHANGE event is enabled.

4.1.4.  sendmsg() and recvmsg()

An application uses sendmsg() and recvmsg() call to transmit data to and receive data from its peer.

The function prototypes are

ssize_t sendmsg(int sd,
                const struct msghdr *message,
                int flags);

and

ssize_t recvmsg(int sd,
                struct msghdr *message,
                int flags);

using the arguments:

sd:

The socket descriptor of the endpoint.

message:

Pointer to the msghdr structure which contains a single user message and possibly some ancillary data. See Section 6 (Data Structures) for complete description of the data structures.

flags:

No new flags are defined for SCTP at this level. See Section 6 (Data Structures) for SCTP-specific flags used in the msghdr structure.

As we will see in Section 6 (Data Structures), along with the user data, the ancillary data field is used to carry the sctp_sndrcvinfo and/or the sctp_initmsg structures to perform various SCTP functions including specifying options for sending each user message. Those options, depending on whether sending or receiving, include stream number, stream sequence number, various flags, context and payload protocol Id, etc.

When sending user data with sendmsg(), the msg_name field in msghdr structure will be filled with one of the transport addresses of the intended receiver. If there is no association existing between the sender and the intended receiver, the sender's SCTP stack will set up a new association and then send the user data (see Section 4.2 (Implicit Association Setup) for more on implicit association setup). If an SCTP_INIT cmsg structure is used with NULL data, an association will be established using the parameters from the struct sctp_initmsg structure. If no SCTP_INIT cmsg structure is used in combination with NULL data, an association is established using the default parameters. If NULL data is used, no association exists and the SCTP_ABORT or SCTP_EOF -1 MUST be returned and an errno SHOULD be set to something like EDONOTBESTUPID. Sending a message using sendmsg() is atomic unless explicit EOR marking is enabled on the socket specified by sd.

If a peer sends a SHUTDOWN, a SCTP_SHUTDOWN_EVENT notification will be delivered if that notification has been enabled, and no more data can be sent to that association. Any attempt to send more data will cause sendmsg() to return with an ESHUTDOWN error. Note that the socket is still open for reading at this point so it is possible to retrieve notifications.

When receiving a user message with recvmsg(), the msg_name field in msghdr structure will be populated with the source transport address of the user data. The caller of recvmsg() can use this address information to determine to which association the received user message belongs. Note that if SCTP_ASSOC_CHANGE events are disabled, applications must use the peer transport address provided in the msg_name field by recvmsg() to perform correlation to an association, since they will not have the association ID.

If all data in a single message has been delivered, MSG_EOR will be set in the msg_flags field of the msghdr structure (see section Section 6.1 (The msghdr and cmsghdr Structures)).

If the application does not provide enough buffer space to completely receive a data message, MSG_EOR will not be set in msg_flags. Successive reads will consume more of the same message until the entire message has been delivered, and MSG_EOR will be set.

If the SCTP stack is running low on buffers, it may partially deliver a message. In this case, MSG_EOR will not be set, and more calls to recvmsg() will be necessary to completely consume the message. Only one message at a time can be partially delivered in any stream. The socket option SCTP_FRAGMENT_INTERLEAVE controls various aspects of what interlacing of messages occurs for both the one-to-one and the one-to-many model sockets. Please consult Section 8.1.20 (Get or Set Fragmented Interleave (SCTP_FRAGMENT_INTERLEAVE)) for further details on message delivery options.

Note, if the socket is a branched-off socket that only represents one association (see Section 4.1 (Basic Operation)), the msg_name field can be used to override the primary address when sending data.

4.1.5.  close()

Applications use close() to perform graceful shutdown (as described in Section 10.1 of [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.)) on ALL the associations currently represented by a one-to-many style socket.

The function prototype is

int close(int sd);

and the argument is

sd:

The socket descriptor of the associations to be closed.

To gracefully shutdown a specific association represented by the one-to-many style socket, an application should use the sendmsg() call, and including the SCTP_EOF flag. A user may optionally terminate an association non-gracefully by sending with the SCTP_ABORT flag and possibly passing a user specified abort code in the data field. Both flags SCTP_EOF and SCTP_ABORT are passed with ancillary data (see Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV))) in the sendmsg call.

If sd in the close() call is a branched-off socket representing only one association, the shutdown is performed on that association only.

4.1.6.  connect()

An application may use the connect() call in the one-to-many style to initiate an association without sending data.

The function prototype is

int connect(int sd,
            const struct sockaddr *nam,
            socklen_t len);

and the arguments are

sd:

The socket descriptor to have a new association added to.

nam:

The address structure (either struct sockaddr_in or struct sockaddr_in6 defined in [RFC3493] (Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. Stevens, “Basic Socket Interface Extensions for IPv6,” February 2003.)).

len:

The size of the address.

Multiple connect() calls can be made on the same socket to create multiple associations. This is different from the semantics of connect() on a UDP socket.

4.2.  Implicit Association Setup

Implicit association setup applies only to one-to-many style sockets. For one-to-one style sockets implicit association setup MUST NOT be used.

Once the bind() call is complete on a one-to-many style socket, the application can begin sending and receiving data using the sendmsg()/recvmsg() or sendto()/recvfrom() calls, without going through any explicit association setup procedures (i.e., no connect() calls required).

Whenever sendmsg() or sendto() is called and the SCTP stack at the sender finds that there is no association existing between the sender and the intended receiver (identified by the address passed either in the msg_name field of msghdr structure in the sendmsg() call or the dest_addr field in the sendto() call), the SCTP stack will automatically setup an association to the intended receiver.

Upon the successful association setup a SCTP_COMM_UP notification will be dispatched to the socket at both the sender and receiver side. This notification can be read by the recvmsg() system call (see Section 4.1.3 (listen())).

Note, if the SCTP stack at the sender side supports bundling, the first user message may be bundled with the COOKIE ECHO message [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).

When the SCTP stack sets up a new association implicitly, it first consults the sctp_initmsg structure, which is passed along within the ancillary data in the sendmsg() call (see Section 6.2.1 (SCTP Initiation Structure (SCTP_INIT)) for details of the data structures), for any special options to be used on the new association.

If this information is not present in the sendmsg() call, or if the implicit association setup is triggered by a sendto() call, the default association initialization parameters will be used. These default association parameters may be set with respective setsockopt() calls or be left to the system defaults.

Implicit association setup cannot be initiated by send()/recv() calls.

4.3.  Non-blocking mode

Some SCTP users might want to avoid blocking when they call socket interface function.

Once all bind() calls are complete on a one-to-many style socket, the application must set the non-blocking option by a fcntl() (such as O_NONBLOCK). After which the sendmsg() function returns immediately, and the success or failure of the data message (and possible SCTP_INITMSG parameters) will be signaled by the SCTP_ASSOC_CHANGE event with SCTP_COMM_UP or CANT_START_ASSOC. If user data could not be sent (due to a CANT_START_ASSOC), the sender will also receive a SCTP_SEND_FAILED event. Those event(s) can be received by the user calling of recvmsg(). A server (having called listen()) is also notified of an association up event by the reception of a SCTP_ASSOC_CHANGE with SCTP_COMM_UP via the calling of recvmsg() and possibly the reception of the first data message.

In order to shutdown the association gracefully, the user must call sendmsg() with no data and with the SCTP_EOF flag set. The function returns immediately, and completion of the graceful shutdown is indicated by an SCTP_ASSOC_CHANGE notification of type SHUTDOWN_COMPLETE (see Section 6.3.2 (SCTP_ASSOC_CHANGE)). Note that this can also be done using the sctp_send() call described in Section 9.10 (sctp_send()).

An application is recommended to use caution when using select() (or poll()) for writing on a one-to-many style socket. The reason being that interpretation of select on write is implementation specific. Generally a positive return on a select on write would only indicate that one of the associations represented by the one-to-many socket is writable. An application that writes after the select return may still block since the association that was writeable is not the destination association of the write call. Likewise select (or poll()) for reading from a one-to-many socket will only return an indication that one of the associations represented by the socket has data to be read.

An application that wishes to know that a particular association is ready for reading or writing should either use the one-to-one style or use the sctp_peeloff() (see Section 9.2 (sctp_peeloff())) function to separate the association of interest from the one-to-many socket.

4.4.  Special considerations

The fact that a one-to-many style socket can provide access to many SCTP associations through a single socket descriptor has important implications for both application programmers and system programmers implementing this API. A key issue is how buffer space inside the sockets layer is managed. Because this implementation detail directly affects how application programmers must write their code to ensure correct operation and portability, this section provides some guidance to both implementers and application programmers.

An important feature that SCTP shares with TCP is flow control: specifically, a sender may not send data faster than the receiver can consume it.

For TCP, flow control is typically provided for in the sockets API as follows. If the reader stops reading, the sender queues messages in the socket layer until it uses all of its socket buffer space allocation creating a "stalled connection". Further attempts to write to the socket will block or return the error EAGAIN or EWOULDBLOCK for a non-blocking socket. At some point, either the connection is closed, or the receiver begins to read again freeing space in the output queue.

For one-to-one style SCTP sockets (this includes sockets descriptors that were separated from a one-to-many style socket with sctp_peeloff()) the behavior is identical. For one-to-many style SCTP sockets, the fact that we have multiple associations on a single socket makes the situation more complicated. If the implementation uses a single buffer space allocation shared by all associations, a single stalled association can prevent the further sending of data on all associations active on a particular one-to-many style socket.

For a blocking socket, it should be clear that a single stalled association can block the entire socket. For this reason, application programmers may want to use non-blocking one-to-many style sockets. The application should at least be able to send messages to the non-stalled associations.

But a non-blocking socket is not sufficient if the API implementer has chosen a single shared buffer allocation for the socket. A single stalled association would eventually cause the shared allocation to fill, and it would become impossible to send even to non-stalled associations.

The API implementer can solve this problem by providing each association with its own allocation of outbound buffer space. Each association should conceptually have as much buffer space as it would have if it had its own socket. As a bonus, this simplifies the implementation of sctp_peeloff().

To ensure that a given stalled association will not prevent other non-stalled associations from being writable, application programmers should either:

• demand that the underlying implementation dedicates independent buffer space allotments to each association (as suggested above), or
• verify that their application layer protocol does not permit large amounts of unread data at the receiver (this is true of some request-response protocols, for example), or
• use one-to-one style sockets for association which may potentially stall (either from the beginning, or by using sctp_peeloff before sending large amounts of data that may cause a stalled condition).

5.  One-to-One Style Interface

The goal of this style is to follow as closely as possible the current practice of using the sockets interface for a connection oriented protocol, such as TCP. This style enables existing applications using connection oriented protocols to be ported to SCTP with very little effort.

Note that some new SCTP features and some new SCTP socket options can only be utilized through the use of sendmsg() and recvmsg() calls, see Section 5.1.8 (sendmsg() and recvmsg()). Also note that some socket interfaces may not be able to provide data on the third leg of the association set up with this interface style.

5.1.  Basic Operation

A typical server in one-to-one style uses the following system call sequence to prepare an SCTP endpoint for servicing requests:

• socket()
• bind()
• listen()
• accept()

The accept() call blocks until a new association is set up. It returns with a new socket descriptor. The server then uses the new socket descriptor to communicate with the client, using recv() and send() calls to get requests and send back responses.

Then it calls

• close()

to terminate the association.

A typical client uses the following system call sequence to setup an association with a server to request services:

• socket()
• connect()

After returning from connect(), the client uses send() and recv() calls to send out requests and receive responses from the server.

The client calls

• close()

to terminate this association when done.

5.1.1.  socket()

Applications calls socket() to create a socket descriptor to represent an SCTP endpoint.

The function prototype is

int socket(int domain,
           int type,
           int protocol);

and one uses PF_INET or PF_INET6 as the domain, SOCK_STREAM as the type and IPPROTO_SCTP as the protocol.

Here, SOCK_STREAM indicates the creation of a one-to-one style socket.

Using the PF_INET domain indicates the creation of an endpoint which can use only IPv4 addresses, while PF_INET6 creates an endpoint which can use both IPv6 and IPv4 address.

5.1.2.  bind()

Applications use bind() to pass an address to be associated with an SCTP endpoint to the system. bind() allows only either a single address or a IPv4 or IPv6 wildcard address to be bound. An SCTP endpoint can be associated with multiple addresses. To do this, sctp_bindx() is introduced in Section 9.1 (sctp_bindx()) to help applications do the job of associating multiple addresses.

These addresses associated with a socket are the eligible transport addresses for the endpoint to send and receive data. The endpoint will also present these addresses to its peers during the association initialization process, see [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).

The function prototype of bind() is

int bind(int sd,
         struct sockaddr *addr,
         socklen_t addrlen);

and the arguments are

sd:

The socket descriptor returned by socket().

addr:

The address structure (struct sockaddr_in or struct sockaddr_in6, see [RFC3493] (Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. Stevens, “Basic Socket Interface Extensions for IPv6,” February 2003.)).

addrlen:

The size of the address structure.

If sd is an IPv4 socket, the address passed must be an IPv4 address. Otherwise, i.e., the sd is an IPv6 socket, the address passed can either be an IPv4 or an IPv6 address.

Applications cannot call bind() multiple times to associate multiple addresses to the endpoint. After the first call to bind(), all subsequent calls will return an error.

If addr is specified as a wildcard (INADDR_ANY for an IPv4 address, or as IN6ADDR_ANY_INIT or in6addr_any for an IPv6 address), the operating system will associate the endpoint with an optimal address set of the available interfaces.

If a bind() is not called prior to the connect() call, the system picks an ephemeral port and will choose an address set equivalent to binding with a wildcard address. One of those addresses will be the primary address for the association. This automatically enables the multi-homing capability of SCTP.

The completion of this bind() process does not ready the SCTP endpoint to accept inbound SCTP association requests. Until a listen() system call, described below, is performed on the socket, the SCTP endpoint will promptly reject an inbound SCTP INIT request with an SCTP ABORT.

5.1.3.  listen()

Applications use listen() to ready the SCTP endpoint for accepting inbound associations.

The function prototype is

int listen(int sd,
           int backlog);

and the arguments are

sd:

the socket descriptor of the SCTP endpoint.

backlog:

this specifies the max number of outstanding associations allowed in the socket's accept queue. These are the associations that have finished the four-way initiation handshake (see Section 5 of [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.)) and are in the ESTABLISHED state. Note, a backlog of '0' indicates that the caller no longer wishes to receive new associations.

5.1.4.  accept()

Applications use accept() call to remove an established SCTP association from the accept queue of the endpoint. A new socket descriptor will be returned from accept() to represent the newly formed association.

The function prototype is

int accept(int sd,
           struct sockaddr *addr,
           socklen_t *addrlen);

and the arguments are

sd:

The listening socket descriptor.

addr:

On return, will contain the primary address of the peer endpoint.

addrlen:

On return, will contain the size of addr.

The functions returns the socket descriptor for the newly formed association.

5.1.5.  connect()

Applications use connect() to initiate an association to a peer.

The function prototype is

int connect(int sd,
            const struct sockaddr *addr,
            socklen_t addrlen);

and the arguments are

sd:

The socket descriptor of the endpoint.

addr:

The peer's address.

addrlen:

The size of the address.

This operation corresponds to the ASSOCIATE primitive described in section 10.1 of [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).

By default, the new association created has only one outbound stream. The SCTP_INITMSG option described in Section 8.1.3 (Initialization Parameters (SCTP_INITMSG)) should be used before connecting to change the number of outbound streams.

If a bind() is not called prior to the connect() call, the system picks an ephemeral port and will choose an address set equivalent to binding with INADDR_ANY and IN6ADDR_ANY for IPv4 and IPv6 socket respectively. One of those addresses will be the primary address for the association. This automatically enables the multi-homing capability of SCTP.

Note that SCTP allows data exchange, similar to T/TCP [RFC1644] (Braden, B., “T/TCP -- TCP Extensions for Transactions Functional Specification,” July 1994.), during the association set up phase. If an application wants to do this, it cannot use connect() call. Instead, it should use sendto() or sendmsg() to initiate an association. If it uses sendto() and it wants to change initialization behavior, it needs to use the SCTP_INITMSG socket option before calling sendto(). Or it can use SCTP_INIT type sendmsg() to initiate an association without doing the setsockopt(). Note that some sockets implementations may not support the sending of data to initiate an association with the one-to-one style (implementations that do not support T/TCP normally have this restriction).

SCTP does not support half close semantics. This means that unlike T/TCP, MSG_EOF should not be set in the flags parameter when calling sendto() or sendmsg() when the call is used to initiate a connection. MSG_EOF is not an acceptable flag with SCTP socket.

5.1.6.  close()

Applications use close() to gracefully close down an association.

The function prototype is

int close(int sd);

and the argument is

sd:

The socket descriptor of the associations to be closed.

After an application calls close() on a socket descriptor, no further socket operations will succeed on that descriptor.

5.1.7.  shutdown()

SCTP differs from TCP in that it does not have half closed semantics. Hence the shutdown() call for SCTP is an approximation of the TCP shutdown() call, and solves some different problems. Full TCP-compatibility is not provided, so developers porting TCP applications to SCTP may need to recode sections that use shutdown(). (Note that it is possible to achieve the same results as half close in SCTP using SCTP streams.)

The function prototype is

int shutdown(int sd,
             int how);

and the arguments are

sd:

The socket descriptor of the association to be closed.

how:

Specifies the type of shutdown. The values are as follows:

SHUT_RD:

Disables further receive operations. No SCTP protocol action is taken.

SHUT_WR:

Disables further send operations, and initiates the SCTP shutdown sequence.

SHUT_RDWR:

Disables further send and receive operations and initiates the SCTP shutdown sequence.

The major difference between SCTP and TCP shutdown() is that SCTP SHUT_WR initiates immediate and full protocol shutdown, whereas TCP SHUT_WR causes TCP to go into the half closed state. SHUT_RD behaves the same for SCTP as TCP. The purpose of SCTP SHUT_WR is to close the SCTP association while still leaving the socket descriptor open, so that the caller can receive back any data SCTP was unable to deliver (see Section 6.3.5 (SCTP_SEND_FAILED) for more information).

To perform the ABORT operation described in [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) section 10.1, an application can use the socket option SO_LINGER. It is described in Section 8.1.4 (SO_LINGER).

5.1.8.  sendmsg() and recvmsg()

With a one-to-one style socket, the application can also use sendmsg() and recvmsg() to transmit data to and receive data from its peer. The semantics is similar to those used in the one-to-many style (section Section 4.1.3 (listen())), with the following differences:

1. When sending, the msg_name field in the msghdr is not used to specify the intended receiver, rather it is used to indicate a preferred peer address if the sender wishes to discourage the stack from sending the message to the primary address of the receiver. If the socket is connected and the transport address given is not part of the current association, the data will not be sent and a SCTP_SEND_FAILED event will be delivered to the application if send failure events are enabled.
2. Using sendmsg() on a non-connected one-to-one style socket for implicit connection setup may or may not work depending on the SCTP implementation.

5.1.9.  getpeername()

Applications use getpeername() to retrieve the primary socket address of the peer. This call is for TCP compatibility, and is not multi-homed. It does not work with one-to-many style sockets. See Section 9.3 (sctp_getpaddrs()) for a multi-homed/one-to-many style version of the call.

The function prototype is

int getpeername(int sd,
                struct sockaddr *address,
                socklen_t *len);

and the arguments are:

sd:

The socket descriptor to be queried.

address:

On return, the peer primary address is stored in this buffer. If the socket is an IPv4 socket, the address will be IPv4. If the socket is an IPv6 socket, the address will be either an IPv6 or IPv4 address.

len:

The caller should set the length of address here. On return, this is set to the length of the returned address.

If the actual length of the address is greater than the length of the supplied sockaddr structure, the stored address will be truncated.

6.  Data Structures

We discuss in this section important data structures which are specific to SCTP and are used with sendmsg() and recvmsg() calls to control SCTP endpoint operations and to access ancillary information and notifications.

6.1.  The msghdr and cmsghdr Structures

The msghdr structure used in the sendmsg() and recvmsg() calls, as well as the ancillary data carried in the structure, is the key for the application to set and get various control information from the SCTP endpoint.

The msghdr and the related cmsghdr structures are defined and discussed in details in [RFC3542] (Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, “Advanced Sockets Application Program Interface (API) for IPv6,” May 2003.). Here we will cite their definitions from [RFC3542] (Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, “Advanced Sockets Application Program Interface (API) for IPv6,” May 2003.).

The msghdr structure:

struct msghdr {
  void *msg_name;           /* ptr to socket address structure */
  socklen_t msg_namelen;    /* size of socket address structure */
  struct iovec *msg_iov;    /* scatter/gather array */
  size_t msg_iovlen;        /* # elements in msg_iov */
  void *msg_control;        /* ancillary data */
  socklen_t msg_controllen; /* ancillary data buffer length */
  int msg_flags;            /* flags on received message */
};

and the cmsghdr structure:

struct cmsghdr {
  socklen_t cmsg_len; /* #bytes, including this header */
  int cmsg_level;     /* originating protocol */
  int cmsg_type;      /* protocol-specific type */
                      /* followed by unsigned char cmsg_data[]; */
  };

In the msghdr structure, the usage of msg_name has been discussed in previous sections (see Section 4.1.3 (listen()) and Section 5.1.8 (sendmsg() and recvmsg())).

The scatter/gather buffers, or I/O vectors (pointed to by the msg_iov field) are treated as a single SCTP data chunk, rather than multiple chunks, for both sendmsg() and recvmsg().

The msg_flags are not used when sending a message with sendmsg().

If a notification has arrived, recvmsg() will return the notification with the MSG_NOTIFICATION flag set in msg_flags. If the MSG_NOTIFICATION flag is not set, recvmsg() will return data. See Section 6.3 (SCTP Events and Notifications) for more information about notifications.

If all portions of a data frame or notification have been read, recvmsg() will return with MSG_EOR set in msg_flags.

6.2.  SCTP msg_control Structures

A key element of all SCTP-specific socket extensions is the use of ancillary data to specify and access SCTP-specific data via the struct msghdr's msg_control member used in sendmsg() and recvmsg(). Fine-grained control over initialization and sending parameters are handled with ancillary data.

Each ancillary data item is proceeded by a struct cmsghdr (see Section 6.1 (The msghdr and cmsghdr Structures)), which defines the function and purpose of the data contained in in the cmsg_data[] member.

There are two kinds of ancillary data used by SCTP: initialization data, and, header information (SNDRCV). Initialization data (one-to-many style only) sets protocol parameters for new associations. Section 6.2.1 (SCTP Initiation Structure (SCTP_INIT)) provides more details. Header information can set or report parameters on individual messages in a stream. See Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)) for how to use SNDRCV ancillary data.

By default on a one-to-one style socket, SCTP will pass no ancillary data; on a one-to-many style socket, SCTP will only pass SCTP_SNDRCV and SCTP_ASSOC_CHANGE information. Specific ancillary data items can be enabled with socket options defined for SCTP; see Section 8.4 (Ancillary Data and Notification Interest Options).

Note that all ancillary types are fixed length; see Section 6.4 (Ancillary Data Considerations and Semantics) for further discussion on this. These data structures use struct sockaddr_storage (defined in [RFC3493] (Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. Stevens, “Basic Socket Interface Extensions for IPv6,” February 2003.)) as a portable, fixed length address format.

Other protocols may also provide ancillary data to the socket layer consumer. These ancillary data items from other protocols may intermingle with SCTP data. For example, the IPv6 socket API definitions ([RFC3542] (Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, “Advanced Sockets Application Program Interface (API) for IPv6,” May 2003.) and [RFC3493] (Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. Stevens, “Basic Socket Interface Extensions for IPv6,” February 2003.)) define a number of ancillary data items. If a socket API consumer enables delivery of both SCTP and IPv6 ancillary data, they both may appear in the same msg_control buffer in any order. An application may thus need to handle other types of ancillary data besides that passed by SCTP.

The sockets application must provide a buffer large enough to accommodate all ancillary data provided via recvmsg(). If the buffer is not large enough, the ancillary data will be truncated and the msghdr's msg_flags will include MSG_CTRUNC.

6.2.1.  SCTP Initiation Structure (SCTP_INIT)

This cmsghdr structure provides information for initializing new SCTP associations with sendmsg(). The SCTP_INITMSG socket option uses this same data structure. This structure is not used for recvmsg().

Here is the definition of the sctp_initmsg structure:

struct sctp_initmsg {
  uint16_t sinit_num_ostreams;
  uint16_t sinit_max_instreams;
  uint16_t sinit_max_attempts;
  uint16_t sinit_max_init_timeo;
};

sinit_num_ostreams:

This is an integer number representing the number of streams that the application wishes to be able to send to. This number is confirmed in the SCTP_COMM_UP notification and must be verified since it is a negotiated number with the remote endpoint. The default value of 0 indicates to use the endpoint default value.

sinit_max_instreams:

This value represents the maximum number of inbound streams the application is prepared to support. This value is bounded by the actual implementation. In other words the user MAY be able to support more streams than the Operating System. In such a case, the Operating System limit overrides the value requested by the user. The default value of 0 indicates to use the endpoints default value.

sinit_max_attempts:

This integer specifies how many attempts the SCTP endpoint should make at resending the INIT. This value overrides the system SCTP 'Max.Init.Retransmits' value. The default value of 0 indicates to use the endpoints default value. This is normally set to the system's default 'Max.Init.Retransmit' value.

sinit_max_init_timeo:

This value represents the largest Time-Out or RTO value (in milliseconds) to use in attempting an INIT. Normally the 'RTO.Max' is used to limit the doubling of the RTO upon timeout. For the INIT message this value MAY override 'RTO.Max'. This value MUST NOT influence 'RTO.Max' during data transmission and is only used to bound the initial setup time. A default value of 0 indicates to use the endpoints default value. This is normally set to the system's 'RTO.Max' value (60 seconds).

6.2.2.  SCTP Header Information Structure (SCTP_SNDRCV)

This cmsghdr structure specifies SCTP options for sendmsg() and describes SCTP header information about a received message through recvmsg().

Here is the definition of sctp_sndrcvinfo:

struct sctp_sndrcvinfo {
  uint16_t sinfo_stream;
  uint16_t sinfo_ssn;
  uint16_t sinfo_flags;
  uint16_t sinfo_pr_policy;
  uint32_t sinfo_ppid;
  uint32_t sinfo_context;
  uint32_t sinfo_pr_value;
  uint32_t sinfo_tsn;
  uint32_t sinfo_cumtsn;
  sctp_assoc_t sinfo_assoc_id;
};

sinfo_stream:

For recvmsg() the SCTP stack places the message's stream number in this value. For sendmsg() this value holds the stream number that the application wishes to send this message to. If a sender specifies an invalid stream number an error indication is returned and the call fails.

sinfo_ssn:

For recvmsg() this value contains the stream sequence number that the remote endpoint placed in the DATA chunk. For fragmented messages this is the same number for all deliveries of the message (if more than one recvmsg() is needed to read the message). The sendmsg() call will ignore this parameter.

sinfo_ppid:

This value in sendmsg() is an unsigned integer that is passed to the remote end in each user message. In recvmsg() this value is the same information that was passed by the upper layer in the peer application. Please note that the SCTP stack performs no byte order modification of this field. For example, if the DATA chunk has to contain a given value in network byte order, the SCTP user has to perform the htonl() computation.

sinfo_context:

This value is an opaque 32 bit context datum that is used in the sendmsg() function. This value is passed back to the upper layer if a error occurs on the send of a message and is retrieved with each undelivered message (Note: if a endpoint has done multiple sends, all of which fail, multiple different sinfo_context values will be returned. One with each user data message).

sinfo_flags:

This field may contain any of the following flags and is composed of a bitwise OR of these values.

recvmsg() flags:

SCTP_UNORDERED:

This flag is present when the message was sent non-ordered.

sendmsg() flags:

SCTP_UNORDERED:

This flag requests the un-ordered delivery of the message. If this flag is clear the datagram is considered an ordered send.

SCTP_ADDR_OVER:

This flag, in the one-to-many style, requests the SCTP stack to override the primary destination address with the address found with the sendto/sendmsg call.

SCTP_ABORT:

Setting this flag causes the specified association to abort by sending an ABORT message to the peer (one-to-many style only). The ABORT chunk will contain an error cause 'User Initiated Abort' with cause code 12. The cause specific information of this error cause is provided in msg_iov.

SCTP_EOF

Setting this flag invokes the SCTP graceful shutdown procedures on the specified association. Graceful shutdown assures that all data queued by both endpoints is successfully transmitted before closing the association (one-to-many style only).

SCTP_SENDALL:

This flag, if set, will cause a one-to-many model socket to send the message to all associations that are currently established on this socket. For the one-to-one socket, this flag has no effect.

sinfo_pr_policy:

For the sending side, this specifies which PR-SCTP policy is used. Using SCTP_PR_SCTP_NONE results in a reliable transmission. When SCTP_PR_SCTP_TTL is used, the PR-SCTP policy "timed reliability" defined in [RFC3758] (Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad, “Stream Control Transmission Protocol (SCTP) Partial Reliability Extension,” May 2004.) is used. In this case, the lifetime is provided in sinfo_pr_value.

sinfo_pr_value:

The meaning of this field depends of the PR-SCTP policy specified by the sinfo_pr_policy field. It is ignored when SCTP_PR_SCTP_NONE is specified. In case of SCTP_PR_SCTP_TTL the lifetime is specified.

sinfo_tsn:

For the receiving side, this field holds a TSN that was assigned to one of the SCTP Data Chunks.

sinfo_cumtsn:

This field will hold the current cumulative TSN as known by the underlying SCTP layer. Note this field is ignored when sending and only valid for a receive operation when sinfo_flags are set to SCTP_UNORDERED.

sinfo_assoc_id:

The association handle field, sinfo_assoc_id, holds the identifier for the association announced in the SCTP_COMM_UP notification. All notifications for a given association have the same identifier. Ignored for one-to-one style sockets.

A sctp_sndrcvinfo item always corresponds to the data in msg_iov.

6.2.3.  Extended SCTP Header Information Structure (SCTP_EXTRCV)

This cmsghdr structure specifies SCTP options for SCTP header information about a received message via recvmsg(). Note that this structure is an extended version of SCTP_SNDRCV (see Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV))) and will only be received if the user has set the socket option SCTP_USE_EXT_RCVINFO to true in addition to any event subscription needed to receive ancillary data. Note that next message data is not valid unless the current message is completely read, i.e. the MSG_EOR is set, in other words if you have more data to read from the current message then no next message information will be available.

Here is the definition of sctp_extrcvinfo structure:

struct sctp_extrcvinfo {
  struct sctp_sndrcvinfo serinfo_sinfo;
  uint16_t serinfo_next_flags;
  uint16_t serinfo_next_stream;
  uint32_t serinfo_next_aid;
  uint32_t serinfo_next_length;
  uint32_t serinfo_next_ppid;
};

serinfo_sinfo:

Please see Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)) for the details for this structure.

serinfo_next_flags:

This bitmask will hold one or more of the following values:

SCTP_NEXT_MSG_AVAIL:

This bit, when set to 1, indicates that next message information is available i.e.: next_stream, next_asocid, next_length and next_ppid fields all have valid values. If this bit is set to 0, then these fields are not valid and should be ignored.

SCTP_NEXT_MSG_ISCOMPLETE:

This bit, when set, indicates that the next message is completely in the receive buffer. The next_length field thus contains the entire message size. If this flag is set to 0, then the next_length field only contains part of the message size since the message is still being received (it is being partially delivered).

SCTP_NEXT_MSG_IS_UNORDERED:

This bit, when set, indicates that the next message to be received was sent by the peer as unordered. If this bit is not set (i.e the bit is 0) the next message to be read is an ordered message in the stream specified.

SCTP_NEXT_MSG_IS_NOTIFICATION:

This bit, when set, indicates that the next message to be received is not a message from the peer, but instead is a MSG_NOTIFICATION from the local SCTP stack.

serinfo_next_stream

This value, when valid (see sreinfo_next_flags), contains the next stream number that will be received on a subsequent call to one of the receive message functions.

serinfo_next_aid:

This value, when valid (see next_flags), contains the next association identification that will be received on a subsequent call to one of the receive message functions.

sreinfo_next_length:

This value, when valid (see sreinfo_next_flags), contains the length of the next message that will be received on a subsequent call to one of the receive message functions. Note that this length may be a partial length depending on the settings of next_flags.

sreinfo_next_ppid:

This value, when valid (see sreinfo_next_flags), contains the ppid of the next message that will be received on a subsequent call to one of the receive message functions.

6.3.  SCTP Events and Notifications

An SCTP application may need to understand and process events and errors that happen on the SCTP stack. These events include network status changes, association startups, remote operational errors and undeliverable messages. All of these can be essential for the application.

When an SCTP application layer does a recvmsg() the message read is normally a data message from a peer endpoint. If the application wishes to have the SCTP stack deliver notifications of non-data events, it sets the appropriate socket option for the notifications it wants. See Section 8.4 (Ancillary Data and Notification Interest Options) for these socket options. When a notification arrives, recvmsg() returns the notification in the application-supplied data buffer via msg_iov, and sets MSG_NOTIFICATION in msg_flags.

This section details the notification structures. Every notification structure carries some common fields which provides general information.

A recvmsg() call will return only one notification at a time. Just as when reading normal data, it may return part of a notification if the msg_iov buffer is not large enough. If a single read is not sufficient, msg_flags will have MSG_EOR clear. The user MUST finish reading the notification before subsequent data can arrive.

6.3.1.  SCTP Notification Structure

The notification structure is defined as the union of all notification types.

union sctp_notification {
  struct {
    uint16_t sn_type; /* Notification type. */
    uint16_t sn_flags;
    uint32_t sn_length;
  } sn_header;
  struct sctp_assoc_change sn_assoc_change;
  struct sctp_paddr_change sn_paddr_change;
  struct sctp_remote_error sn_remote_error;
  struct sctp_send_failed sn_send_failed;
  struct sctp_shutdown_event sn_shutdown_event;
  struct sctp_adaptation_event sn_adaptation_event;
  struct sctp_pdapi_event sn_pdapi_event;
  struct sctp_authkey_event sn_auth_event;
  struct sctp_sender_dry_event sn_sender_dry_event;
};

sn_type:

The following list describes the SCTP notification and event types for the field sn_type.

SCTP_ASSOC_CHANGE:

This tag indicates that an association has either been opened or closed. Refer to Section 6.3.2 (SCTP_ASSOC_CHANGE) for details.

SCTP_PEER_ADDR_CHANGE:

This tag indicates that an address that is part of an existing association has experienced a change of state (e.g. a failure or return to service of the reachability of a endpoint via a specific transport address). Please see Section 6.3.3 (SCTP_PEER_ADDR_CHANGE) for data structure details.

SCTP_REMOTE_ERROR:

The attached error message is an Operational Error received from the remote peer. It includes the complete TLV sent by the remote endpoint. See Section 6.3.4 (SCTP_REMOTE_ERROR) for the detailed format.

SCTP_SEND_FAILED:

The attached datagram could not be sent to the remote endpoint. This structure includes the original SCTP_SNDRCVINFO that was used in sending this message i.e. this structure uses the sctp_sndrecvinfo per Section 6.3.5 (SCTP_SEND_FAILED).

SCTP_SHUTDOWN_EVENT:

The peer has sent a SHUTDOWN. No further data should be sent on this socket.

SCTP_ADAPTATION_INDICATION:

This notification holds the peers indicated adaptation layer. Please see Section 6.3.7 (SCTP_ADAPTATION_INDICATION).

SCTP_PARTIAL_DELIVERY_EVENT:

This notification is used to tell a receiver that the partial delivery has been aborted. This may indicate the association is about to be aborted. Please see Section 6.3.8 (SCTP_PARTIAL_DELIVERY_EVENT)

SCTP_AUTHENTICATION_EVENT:

This notification is used to tell a receiver that either an error occurred on authentication, or a new key was made active. Section 6.3.9 (SCTP_AUTHENTICATION_EVENT)

SCTP_SENDER_DRY_EVENT:

This notification is used to inform the application that the sender has no user data queued anymore, neither for transmission or retransmission. Section 6.3.10 (SCTP_SENDER_DRY_EVENT)

All standard values for sn_type are greater than 2^15. Values from 2^15 and down are reserved.

sn_flags:

These are notification-specific flags.

sn_length:

This is the length of the whole sctp_notification structure including the sn_type, sn_flags, and sn_length fields.

6.3.2.  SCTP_ASSOC_CHANGE

Communication notifications inform the ULP that an SCTP association has either begun or ended. The identifier for a new association is provided by this notification. The notification information has the following format:

struct sctp_assoc_change {
  uint16_t sac_type;
  uint16_t sac_flags;
  uint32_t sac_length;
  uint16_t sac_state;
  uint16_t sac_error;
  uint16_t sac_outbound_streams;
  uint16_t sac_inbound_streams;
  sctp_assoc_t sac_assoc_id;
  uint8_t  sac_info[];
};

sac_type:

It should be SCTP_ASSOC_CHANGE.

sac_flags:

Currently unused.

sac_length:

This field is the total length of the notification data, including the notification header.

sac_state:

This field holds one of a number of values that communicate the event that happened to the association. They include:

SCTP_COMM_UP:

A new association is now ready and data may be exchanged with this peer. When an association has been established successfully, this notification should be the first one.

SCTP_COMM_LOST:

The association has failed. The association is now in the closed state. If SEND FAILED notifications are turned on, a SCTP_COMM_LOST is followed by a series of SCTP_SEND_FAILED events, one for each outstanding message.

SCTP_RESTART:

SCTP has detected that the peer has restarted.

SCTP_SHUTDOWN_COMP:

The association has gracefully closed.

SCTP_CANT_STR_ASSOC -

The association failed to setup. If non blocking mode is set and data was sent (in the udp mode), a SCTP_CANT_STR_ASSOC is followed by a series of SCTP_SEND_FAILED events, one for each outstanding message.

sac_error:

If the state was reached due to a error condition (e.g. SCTP_COMM_LOST) any relevant error information is available in this field. This corresponds to the protocol error codes defined in [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).

sac_outbound_streams:

sac_inbound_streams:

The maximum number of streams allowed in each direction are available in sac_outbound_streams and sac_inbound streams.

sac_assoc_id:

The association id field, holds the identifier for the association. All notifications for a given association have the same association identifier. For one-to-one style socket, this field is ignored.

sac_info:

If the sac_state is SCTP_COMM_LOST and an ABORT chunk was received for this association, sac_info[] contains the complete ABORT chunk as defined in the SCTP specification [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) section 3.3.7. If the sac_state is SCTP_COMM_UP or SCTP_RESTART, sac_info may contain an array of features that the current association supports. Features may include

SCTP_PR:

The both endpoints support the protocol extension described in [RFC3758] (Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad, “Stream Control Transmission Protocol (SCTP) Partial Reliability Extension,” May 2004.).

SCTP_AUTH:

The both endpoints support the protocol extension described in [RFC4895] (Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, “Authenticated Chunks for the Stream Control Transmission Protocol (SCTP),” August 2007.).

SCTP_ASCONF:

The both endpoints support the protocol extension described in [RFC5061] (Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, “Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration,” September 2007.).

SCTP_MULTIBUF:

For an one-to-many style socket, the local endpoints uses separate send and/or receive buffers for each SCTP association.

6.3.3.  SCTP_PEER_ADDR_CHANGE

When a destination address on a multi-homed peer encounters a change an interface details event is sent. The information has the following structure:

struct sctp_paddr_change {
  uint16_t spc_type;
  uint16_t spc_flags;
  uint32_t spc_length;
  struct sockaddr_storage spc_aaddr;
  uint32_t spc_state;
  uint32_t spc_error;
  sctp_assoc_t spc_assoc_id;
}

spc_type:

It should be SCTP_PEER_ADDR_CHANGE.

spc_flags:

Currently unused.

spc_length:

This field is the total length of the notification data, including the notification header.

spc_aaddr:

The affected address field, holds the remote peer's address that is encountering the change of state.

spc_state:

This field holds one of a number of values that communicate the event that happened to the address. They include:

SCTP_ADDR_AVAILABLE:

This address is now reachable.

SCTP_ADDR_UNREACHABLE:

The address specified can no longer be reached. Any data sent to this address is rerouted to an alternate until this address becomes reachable.

SCTP_ADDR_REMOVED:

The address is no longer part of the association.

SCTP_ADDR_ADDED:

The address is now part of the association.

SCTP_ADDR_MADE_PRIM:

This address has now been made to be the primary destination address.

SCTP_ADDR_CONFIRMED:

This address has now been confirmed as a valid address.

spc_error:

If the state was reached due to any error condition (e.g. SCTP_ADDR_UNREACHABLE) any relevant error information is available in this field.

spc_assoc_id:

The association id field, holds the identifier for the association. All notifications for a given association have the same association identifier. For one-to-one style socket, this field is ignored.

6.3.4.  SCTP_REMOTE_ERROR

A remote peer may send an Operational Error message to its peer. This message indicates a variety of error conditions on an association. The entire ERROR chunk as it appears on the wire is included in a SCTP_REMOTE_ERROR event. Please refer to the SCTP specification [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) and any extensions for a list of possible error formats. SCTP error notifications have the format:

struct sctp_remote_error {
  uint16_t sre_type;
  uint16_t sre_flags;
  uint32_t sre_length;
  uint16_t sre_error;
  sctp_assoc_t sre_assoc_id;
  uint8_t sre_data[];
};

sre_type:

It should be SCTP_REMOTE_ERROR.

sre_flags:

Currently unused.

sre_length:

This field is the total length of the notification data, including the notification header and the contents of sre_data.

sre_error:

This value represents one of the Operational Error causes defined in the SCTP specification, in network byte order.

sre_assoc_id:

The association id field, holds the identifier for the association. All notifications for a given association have the same association identifier. For one-to-one style socket, this field is ignored.

sre_data:

This contains the ERROR chunk as defined in the SCTP specification [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) section 3.3.10.

6.3.5.  SCTP_SEND_FAILED

If SCTP cannot deliver a message it may return the message as a notification.

struct sctp_send_failed {
  uint16_t ssf_type;
  uint16_t ssf_flags;
  uint32_t ssf_length;
  uint32_t ssf_error;
  struct sctp_sndrcvinfo ssf_info;
  sctp_assoc_t ssf_assoc_id;
  uint8_t ssf_data[];
};

ssf_type:

It should be SCTP_SEND_FAILED.

ssf_flags:

The flag value will take one of the following values:

SCTP_DATA_UNSENT:

Indicates that the data was never put on the wire.

SCTP_DATA_SENT:

Indicates that the data was put on the wire. Note that this does not necessarily mean that the data was (or was not) successfully delivered.

ssf_length:

This field is the total length of the notification data, including the notification header and the payload in ssf_data.

ssf_error:

This value represents the reason why the send failed, and if set, will be a SCTP protocol error code as defined in [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) section 3.3.10.

ssf_info:

The send information associated with the undelivered message. The sinfo_flags field will also contain an indication if the beginning of the message and/or end of the message is present. In cases where no data has been sent on the wire, this field will have or'ed in the value SCTP_DATA_NOT_FRAG, which is a composition of both a "BEGIN" and "END" fragmentation bit. In cases where part of the data has been sent this field will hold an indication that just the part not sent is present SCTP_DATA_LAST_FRAG which corresponds to the "END" bit. Note that the message itself may be more than one chunk. If the sinfo_flags field holds neither of these two values then a piece that has been fragmented and sent but not acknowledged is present. This piece is from an unspecified position in the message and the application can make no assumptions about the data itself. Applications wanting to examine a recovered message should look for the SCTP_DATA_NOT_FRAG. Without this flag the application should assume part of the message arrived and take appropriate steps to audit and recover any lost or missing data. Note also that the partial reliablity information that was sent down to the SCTP stack is NOT present in the sinfo_flags field since it occupies the same bit positions as the SCTP_DATA_NOT_FRAG and the SCTP_DATA_LAST_FRAG bits.

ssf_assoc_id:

The association id field, sf_assoc_id, holds the identifier for the association. All notifications for a given association have the same association identifier. For one-to-one style socket, this field is ignored.

ssf_data:

The undelivered message or part of the undelivered message will be present in the sf_data field. Note that the sinfo_flags field as noted above should be used to determine if a complete message is present or just a piece of the message. Note that only user data is present in this field, any chunk headers or SCTP common headers must be removed by the SCTP stack.

6.3.6.  SCTP_SHUTDOWN_EVENT

When a peer sends a SHUTDOWN, SCTP delivers this notification to inform the application that it should cease sending data.

    struct sctp_shutdown_event {
        uint16_t sse_type;
        uint16_t sse_flags;
        uint32_t sse_length;
        sctp_assoc_t sse_assoc_id;
    };

sse_type

It should be SCTP_SHUTDOWN_EVENT.

sse_flags:

Currently unused.

sse_length:

This field is the total length of the notification data, including the notification header. It will generally be sizeof (struct sctp_shutdown_event).

sse_flags:

Currently unused.

sse_assoc_id:

The association id field, holds the identifier for the association. All notifications for a given association have the same association identifier. For one-to-one style socket, this field is ignored.

6.3.7.  SCTP_ADAPTATION_INDICATION

When a peer sends a Adaptation Layer Indication parameter , SCTP delivers this notification to inform the application that of the peers requested adaptation layer.

struct sctp_adaptation_event {
  uint16_t sai_type;
  uint16_t sai_flags;
  uint32_t sai_length;
  uint32_t sai_adaptation_ind;
  sctp_assoc_t sai_assoc_id;
};

sai_type:

It should be SCTP_ADAPTATION_INDICATION.

sai_flags:

Currently unused.

sai_length:

This field is the total length of the notification data, including the notification header. It will generally be sizeof (struct sctp_adaptation_event).

sai_adaptation_ind:

This field holds the bit array sent by the peer in the adaptation layer indication parameter. The bits are in network byte order.

sai_assoc_id:

The association id field, holds the identifier for the association. All notifications for a given association have the same association identifier. For one-to-one style socket, this field is ignored.

6.3.8.  SCTP_PARTIAL_DELIVERY_EVENT

When a receiver is engaged in a partial delivery of a message this notification will be used to indicate various events.

struct sctp_pdapi_event {
  uint16_t pdapi_type;
  uint16_t pdapi_flags;
  uint32_t pdapi_length;
  uint32_t pdapi_indication;
  uint32_t pdapi_stream;
  uint32_t pdapi_seq;
  sctp_assoc_t pdapi_assoc_id;
};

pdapi_type:

It should be SCTP_PARTIAL_DELIVERY_EVENT.

pdapi_flags:

Currently unused.

pdapi_length:

This field is the total length of the notification data, including the notification header. It will generally be sizeof(struct sctp_pdapi_event).

pdapi_indication:

This field holds the indication being sent to the application possible values include:

SCTP_PARTIAL_DELIVERY_ABORTED:

This notification indicates that the partial delivery of a user message has been aborted.

pdapi_stream:

This field holds the stream on which the partial delivery event happened.

pdapi_seq:

This field holds the stream sequence number which was being partially delivered.

pdapi_assoc_id:

The association id field, holds the identifier for the association. All notifications for a given association have the same association identifier. For one-to-one style socket, this field is ignored.

6.3.9.  SCTP_AUTHENTICATION_EVENT

When a receiver is using authentication this message will provide notifications regarding new keys being made active as well as errors.

struct sctp_authkey_event {
  uint16_t auth_type;
  uint16_t auth_flags;
  uint32_t auth_length;
  uint16_t auth_keynumber;
  uint16_t auth_altkeynumber;
  uint32_t auth_indication;
  sctp_assoc_t auth_assoc_id;
};

auth_type:

It should be SCTP_AUTHENTICATION_EVENT.

auth_flags:

Currently unused.

auth_length:

This field is the total length of the notification data, including the notification header. It will generally be sizeof (struct sctp_authkey_event).

auth_keynumber:

This field holds the keynumber set by the user for the effected key. If more than one key is involved, this will contain one of the keys involved in the notification.

auth_altkeynumber:

This field holds an alternate keynumber which is used by some notifications.

auth_indication:

This field hold the error or indication being reported. The following values are currently defined:

SCTP_AUTH_NEWKEY:

This report indicates that a new key has been made active (used for the first time by the peer) and is now the active key. The auth_keynumber field holds the user specified key number.

SCTP_AUTH_NO_AUTH:

This report indicates that the peer does not support the SCTP-AUTH.

SCTP_AUTH_FREE_KEY:

This report indicates that the SCTP implementation will not use the key identifier specified in auth_keynumber anymore.

auth_assoc_id:

The association id field, holds the identifier for the association. All notifications for a given association have the same association identifier.

6.3.10.  SCTP_SENDER_DRY_EVENT

When the SCTP implementation has no user data anymore to send or retransmit this notification is given to the user. If the user subscribes to this event and the SCTP has at this point of time no user data to send or retransmit this notification is also given to the user.

struct sctp_sender_dry_event {
  uint16_t sender_dry_type;
  uint16_t sender_dry_flags;
  uint32_t sender_dry_length;
  sctp_assoc_t sender_dry_assoc_id;
};

sender_dry_type:

It should be SCTP_SENDER_DRY_EVENT.

sender_dry_flags:

Currently unused.

sender_dry_length:

This field is the total length of the notification data, including the notification header. It will generally be sizeof (struct sctp_sender_dry_event).

6.4.  Ancillary Data Considerations and Semantics

Programming with ancillary socket data contains some subtleties and pitfalls, which are discussed below.

6.4.1.  Multiple Items and Ordering

Multiple ancillary data items may be included in any call to sendmsg() or recvmsg(); these may include multiple SCTP or non-SCTP items, or both.

The ordering of ancillary data items (either by SCTP or another protocol) is not significant and is implementation-dependent, so applications must not depend on any ordering.

SCTP_SNDRCV items must always correspond to the data in the msghdr's msg_iov member. There can be only a single SCTP_SNDRCV info for each sendmsg() or recvmsg() call.

6.4.2.  Accessing and Manipulating Ancillary Data

Applications can infer the presence of data or ancillary data by examining the msg_iovlen and msg_controllen msghdr members, respectively.

Implementations may have different padding requirements for ancillary data, so portable applications should make use of the macros CMSG_FIRSTHDR, CMSG_NXTHDR, CMSG_DATA, CMSG_SPACE, and CMSG_LEN. See [RFC3542] (Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, “Advanced Sockets Application Program Interface (API) for IPv6,” May 2003.) and your SCTP implementation's documentation for more information. Following is an example, from [RFC3542] (Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, “Advanced Sockets Application Program Interface (API) for IPv6,” May 2003.), demonstrating the use of these macros to access ancillary data:

struct msghdr msg;
struct cmsghdr *cmsgptr;

/* fill in msg */

/* call recvmsg() */

for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
     cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
  if (cmsgptr->cmsg_level == ... && cmsgptr->cmsg_type == ... ) {
    u_char  *ptr;

    ptr = CMSG_DATA(cmsgptr);
    /* process data pointed to by ptr */
  }
}

6.4.3.  Control Message Buffer Sizing

The information conveyed via SCTP_SNDRCV events will often be fundamental to the correct and sane operation of the sockets application. This is particularly true of the one-to-many semantics, but also of the one-ton-one semantics. For example, if an application needs to send and receive data on different SCTP streams, SCTP_SNDRCV events are indispensable.

Given that some ancillary data is critical, and that multiple ancillary data items may appear in any order, applications should be carefully written to always provide a large enough buffer to contain all possible ancillary data that can be presented by recvmsg(). If the buffer is too small, and crucial data is truncated, it may pose a fatal error condition.

Thus it is essential that applications be able to deterministically calculate the maximum required buffer size to pass to recvmsg(). One constraint imposed on this specification that makes this possible is that all ancillary data definitions are of a fixed length. One way to calculate the maximum required buffer size might be to take the sum the sizes of all enabled ancillary data item structures, as calculated by CMSG_SPACE. For example, if we enabled SCTP_SNDRCV_INFO and IPV6_RECVPKTINFO [RFC3542] (Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, “Advanced Sockets Application Program Interface (API) for IPv6,” May 2003.), we would calculate and allocate the buffer size as follows:

size_t total;
void *buf;

total = CMSG_SPACE(sizeof (struct sctp_sndrcvinfo)) +
        CMSG_SPACE(sizeof (struct in6_pktinfo));

buf = malloc(total);

We could then use this buffer for msg_control on each call to recvmsg() and be assured that we would not lose any ancillary data to truncation.

7.  Common Operations for Both Styles

7.1.  send(), recv(), sendto(), and recvfrom()

Applications can use send() and sendto() to transmit data to the peer of an SCTP endpoint. recv() and recvfrom() can be used to receive data from the peer.

The function prototypes are

ssize_t send(int sd,
             const void *msg,
             size_t len,
             int flags);

ssize_t sendto(int sd,
               const void *msg,
               size_t len,
               int flags,
               const struct sockaddr *to,
               socklen_t tolen);

ssize_t recv(int sd,
             void *buf,
             size_t len,
             int flags);

ssize_t recvfrom(int sd,
                 void *buf,
                 size_t len,
                 int flags,
                 struct sockaddr *from,
                 socklen_t *fromlen);

and the arguments are

sd:

The socket descriptor of an SCTP endpoint.

msg:

The message to be sent.

len:

the size of the message or the size of buffer.

to:

one of the peer addresses of the association to be used to send the message.

tolen:

The size of the address.

buf:

The buffer to store a received message.

from:

The buffer to store the peer address used to send the received message.

fromlen:

The size of the from address.

flags:

(described below).

These calls give access to only basic SCTP protocol features. If either peer in the association uses multiple streams, or sends unordered data these calls will usually be inadequate, and may deliver the data in unpredictable ways.

SCTP has the concept of multiple streams in one association. The above calls do not allow the caller to specify on which stream a message should be sent. The system uses stream 0 as the default stream for send() and sendto(). recv() and recvfrom() return data from any stream, but the caller can not distinguish the different streams. This may result in data seeming to arrive out of order. Similarly, if a data chunk is sent unordered, recv() and recvfrom() provide no indication.

SCTP is message based. The msg buffer above in send() and sendto() is considered to be a single message. This means that if the caller wants to send a message which is composed by several buffers, the caller needs to combine them before calling send() or sendto(). Alternately, the caller can use sendmsg() to do that without combining them. Sending a message using send() or sendto() is atomic unless explicit EOR marking is enabled on the socket specified by sd. Using sendto() on a non-connected one-to-one style socket for implicit connection setup may or may not work depending on the SCTP implementation. recv() and recvfrom() cannot distinguish message boundaries.

In receiving, if the buffer supplied is not large enough to hold a complete message, the receive call acts like a stream socket and returns as much data as will fit in the buffer.

Note, the send() and recv() calls may not be used for a one-to-many style socket.

Note, if an application calls a send function with no user data and no ancillary data the SCTP implementation should reject the request with an appropriate error message. An implementation is NOT allowed to send a Data chunk with no user data [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).

7.2.  setsockopt() and getsockopt()

Applications use setsockopt() and getsockopt() to set or retrieve socket options. Socket options are used to change the default behavior of sockets calls. They are described in Section 8 (Socket Options).

The function prototypes are

int getsockopt(int sd,
               int level,
               int optname,
               void *optval,
               socklen_t *optlen);

and

int setsockopt(int sd,
               int level,
               int optname,
               const void *optval,
               socklen_t optlen);

and the arguments are

sd:

The socket descriptor.

level:

Set to IPPROTO_SCTP for all SCTP options.

optname -

The option name.

optval:

The buffer to store the value of the option.

optlen:

The size of the buffer (or the length of the option returned).

All socket options set on a 1-to-1 listening sockets also apply all accepted sockets. All socket options set on a 1-to-many socket using the assoc_id 0 applies for all future associations on the socket.

7.3.  read() and write()

Applications can use read() and write() to send and receive data to and from peer. They have the same semantics as send() and recv() except that the flags parameter cannot be used.

Note, these calls, when used in the one-to-many style, may only be used with branched off socket descriptors (see Section 9.2 (sctp_peeloff())).

7.4.  getsockname()

Applications use getsockname() to retrieve the locally-bound socket address of the specified socket. This is especially useful if the caller let SCTP chose a local port. This call is for where the endpoint is not multi-homed. It does not work well with multi-homed sockets. See Section 9.5 (sctp_getladdrs()) for a multi-homed version of the call.

The function prototype is

int getsockname(int sd,
                struct sockaddr *address,
                socklen_t *len);

and the arguments are

sd:

The socket descriptor to be queried.

address:

On return, one locally bound address (chosen by the SCTP stack) is stored in this buffer. If the socket is an IPv4 socket, the address will be IPv4. If the socket is an IPv6 socket, the address will be either an IPv6 or IPv4 address.

len:

The caller should set the length of address here. On return, this is set to the length of the returned address.

If the actual length of the address is greater than the length of the supplied sockaddr structure, the stored address will be truncated.

If the socket has not been bound to a local name, the value stored in the object pointed to by address is unspecified.

8.  Socket Options

The following sub-section describes various SCTP level socket options that are common to both styles. SCTP associations can be multi-homed. Therefore, certain option parameters include a sockaddr_storage structure to select which peer address the option should be applied to.

For the one-to-many style sockets, an sctp_assoc_t structure (association ID) is used to identify the association instance that the operation affects. So it must be set when using this style.

For the one-to-one style sockets and branched off one-to-many style sockets (see Section 9.2 (sctp_peeloff())) this association ID parameter is ignored.

Note that socket or IP level options are set or retrieved per socket. This means that for one-to-many style sockets, those options will be applied to all associations belonging to the socket. And for one-to-one style, those options will be applied to all peer addresses of the association controlled by the socket. Applications should be very careful in setting those options.

For some IP stacks getsockopt() is read-only; so a new interface will be needed when information must be passed both in to and out of the SCTP stack. The syntax for sctp_opt_info() is

int sctp_opt_info(int sd,
                  sctp_assoc_t id,
                  int opt,
                  void *arg,
                  socklen_t *size);

The sctp_opt_info() call is a replacement for getsockopt() only and will not set any options associated with the specified socket. A setsockopt() must be used to set any writeable option.

For one-to-many style sockets, id specifies the association to query. For one-to-one style sockets, id is ignored.

opt specifies which SCTP socket option to get. It can get any socket option currently supported that requests information (either read/write options or read only) such as:

SCTP_RTOINFO

SCTP_ASSOCINFO

SCTP_DEFAULT_SEND_PARAM

SCTP_GET_PEER_ADDR_INFO

SCTP_PRIMARY_ADDR

SCTP_PEER_ADDR_PARAMS

SCTP_STATUS

SCTP_CONTEXT

SCTP_AUTH_ACTIVE_KEY

SCTP_PEER_AUTH_CHUNKS

SCTP_LOCAL_AUTH_CHUNKS

arg is an option-specific structure buffer provided by the caller. See Section 9.5 (sctp_getladdrs()) subsections for more information on these options and option-specific structures.

sctp_opt_info() returns 0 on success, or on failure returns -1 and sets errno to the appropriate error code.

All options that support specific settings on an association by filling in either an association id variable or a sockaddr_storage SHOULD also support setting of the same value for the entire endpoint (i.e. future associations). To accomplish this the following logic is used when setting one of these options:

• If an address is specified via a sockaddr_storage that is included in the structure, the address is used to lookup the association and the settings are applied to the specific address (if appropriate) or to the entire association.
• If an association identification is filled in but not a sockaddr_storage (if present), the association is found using the association identification and the settings should be applied to the entire association (since a specific address is not specified). Note this also applies to options that hold an association identification in their structure but do not have a sockaddr_storage field.
• If neither the sockaddr_storage or association identification is set, i.e. the sockaddr_storage is set to all 0's (INADDR_ANY) and the association identification is 0, the settings are a default and to be applied to the endpoint (all future associations).

8.1.  Read / Write Options

8.1.1.  Retransmission Timeout Parameters (SCTP_RTOINFO)

The protocol parameters used to initialize and bound retransmission timeout (RTO) are tunable. See [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) for more information on how these parameters are used in RTO calculation.

The following structure is used to access and modify these parameters:

struct sctp_rtoinfo {
  sctp_assoc_t srto_assoc_id;
  uint32_t srto_initial;
  uint32_t srto_max;
  uint32_t srto_min;
};

srto_initial:

This contains the initial RTO value.

srto_max and srto_min:

These contain the maximum and minimum bounds for all RTOs.

srto_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets the following applies: This is filled in the application, and identifies the association for this query. If this parameter is '0', then the change affects the entire endpoint.

All times are given in milliseconds. A value of 0, when modifying the parameters, indicates that the current value should not be changed.

To access or modify these parameters, the application should call getsockopt or setsockopt() respectively with the option name SCTP_RTOINFO.

8.1.2.  Association Parameters (SCTP_ASSOCINFO)

This option is used to both examine and set various association and endpoint parameters. See [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) for more information on how this parameter is used.

The following structure is used to access and modify this parameters:

struct sctp_assocparams {
  sctp_assoc_t sasoc_assoc_id;
  uint16_t sasoc_asocmaxrxt;
  uint16_t sasoc_number_peer_destinations;
  uint32_t sasoc_peer_rwnd;
  uint32_t sasoc_local_rwnd;
  uint32_t sasoc_cookie_life;
};

sasoc_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets the following applies: This information may be examined for either the endpoint or a specific association. To examine a endpoints default parameters the association id (sasoc_assoc_id) should must be set to the value '0'.

sasoc_asocmaxrxt:

This contains the maximum retransmission attempts to make for the association.

sasoc_number_peer_destinations:

This is the number of destination addresses that the peer has.

sasoc_peer_rwnd:

This holds the current value of the peers rwnd (reported in the last SACK) minus any outstanding data (i.e. data inflight).

sasoc_local_rwnd:

This holds the last reported rwnd that was sent to the peer.

sasoc_cookie_life:

This is the associations cookie life value used when issuing cookies.

The values of the sasoc_peer_rwnd is meaningless when examining endpoint information.

All time values are given in milliseconds. A value of 0, when modifying the parameters, indicates that the current value should not be changed.

The values of the sasoc_asocmaxrxt and sasoc_cookie_life may be set on either an endpoint or association basis. The rwnd and destination counts (sasoc_number_peer_destinations, sasoc_peer_rwnd, sasoc_local_rwnd) are NOT settable and any value placed in these is ignored.

To access or modify these parameters, the application should call getsockopt or setsockopt() respectively with the option name SCTP_ASSOCINFO.

The maximum number of retransmissions before an address is considered unreachable is also tunable, but is address-specific, so it is covered in a separate option. If an application attempts to set the value of the association maximum retransmission parameter to more than the sum of all maximum retransmission parameters, setsockopt() shall return an error. The reason for this, from [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) section 8.2:

Note: When configuring the SCTP endpoint, the user should avoid having the value of 'Association.Max.Retrans' larger than the summation of the 'Path.Max.Retrans' of all the destination addresses for the remote endpoint. Otherwise, all the destination addresses may become inactive while the endpoint still considers the peer endpoint reachable.

8.1.3.  Initialization Parameters (SCTP_INITMSG)

Applications can specify protocol parameters for the default association initialization. The structure used to access and modify these parameters is defined in Section 6.2.1 (SCTP Initiation Structure (SCTP_INIT))). The option name argument to setsockopt() and getsockopt() is SCTP_INITMSG.

Setting initialization parameters is effective only on an unconnected socket (for one-to-many style sockets only future associations are effected by the change). With one-to-one style sockets, this option is inherited by sockets derived from a listener socket.

8.1.4.  SO_LINGER

An application can use this option to perform the SCTP ABORT primitive. This option affects all associations related to the socket.

The linger option structure is:

struct linger {
  int l_onoff;  /* option on/off */
  int l_linger; /* linger time   */
};

To enable the option, set l_onoff to 1. If the l_linger value is set to 0, calling close() is the same as the ABORT primitive. If the value is set to a negative value, the setsockopt() call will return an error. If the value is set to a positive value linger_time, the close() can be blocked for at most linger_time ms. If the graceful shutdown phase does not finish during this period, close() will return but the graceful shutdown phase continues in the system.

Note, this is a socket level option NOT an SCTP level option. So when setting SO_LINGER you must specify a level of SOL_SOCKET in the setsockopt() call.

8.1.5.  SCTP_NODELAY

Turn on/off any Nagle-like algorithm. This means that packets are generally sent as soon as possible and no unnecessary delays are introduced, at the cost of more packets in the network. Expects an integer boolean flag.

8.1.6.  SO_RCVBUF

Sets receive buffer size in octets. For SCTP one-to-one style sockets, this controls the receiver window size. For one-to-many style sockets the meaning is implementation dependent. It might control the receive buffer for each association bound to the socket descriptor or it might control the receive buffer for the whole socket. The call expects an integer.

8.1.7.  SO_SNDBUF

Sets send buffer size. For SCTP one-to-one style sockets, this controls the amount of data SCTP may have waiting in internal buffers to be sent. This option therefore bounds the maximum size of data that can be sent in a single send call. For one-to-many style sockets, the effect is the same, except that it applies to one or all associations (see Section 4.4 (Special considerations)) bound to the socket descriptor used in the setsockopt() or getsockopt() call. The option applies to each association's window size separately. The call expects an integer.

8.1.8.  Automatic Close of Associations (SCTP_AUTOCLOSE)

This socket option is applicable to the one-to-many style socket only. When set it will cause associations that are idle for more than the specified number of seconds to automatically close using the graceful shutdown procedure. An association being idle is defined as an association that has NOT sent or received user data. The special value of '0' indicates that no automatic close of any associations should be performed, this is the default value. The option expects an integer defining the number of seconds of idle time before an association is closed.

An application using this option should enable receiving the association change notification. This is the only mechanism an application is informed about the closing of an association. After an association is closed, the association ID assigned to it can be reused. An application should be aware of this to avoid the possible problem of sending data to an incorrect peer end point.

8.1.9.  Set Primary Address (SCTP_PRIMARY_ADDR)

Requests that the local SCTP stack use the enclosed peer address as the association primary. The enclosed address must be one of the association peer's addresses.

The following structure is used to make a set peer primary request:

struct sctp_setprim {
  sctp_assoc_t ssp_assoc_id;
  struct sockaddr_storage ssp_addr;
};

ssp_addr:

The address to set as primary.

ssp_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets it identifies the association for this request.

8.1.10.  Set Adaptation Layer Indicator (SCTP_ADAPTATION_LAYER)

Requests that the local endpoint set the specified Adaptation Layer Indication parameter for all future INIT and INIT-ACK exchanges.

The following structure is used to access and modify this parameter:

struct sctp_setadaptation {
  uint32_t   ssb_adaptation_ind;
};

ssb_adaptation_ind:

The adaptation layer indicator that will be included in any outgoing Adaptation Layer Indication parameter.

8.1.11.  Enable/Disable Message Fragmentation (SCTP_DISABLE_FRAGMENTS)

This option is a on/off flag and is passed an integer where a non-zero is on and a zero is off. If enabled no SCTP message fragmentation will be performed. Instead if a message being sent exceeds the current PMTU size, the message will NOT be sent and instead a error will be indicated to the user.

8.1.12.  Peer Address Parameters (SCTP_PEER_ADDR_PARAMS)

Applications can enable or disable heartbeats for any peer address of an association, modify an address's heartbeat interval, force a heartbeat to be sent immediately, and adjust the address's maximum number of retransmissions sent before an address is considered unreachable.

The following structure is used to access and modify an address's parameters:

struct sctp_paddrparams {
  sctp_assoc_t spp_assoc_id;
  struct sockaddr_storage spp_address;
  uint32_t ssp_hbinterval;
  uint16_t ssp_pathmaxrxt;
  uint32_t ssp_pathmtu;
  uint32_t ssp_flags;
  uint32_t ssp_ipv6_flowlabel;
  uint8_t ssp_ipv4_tos;
};

spp_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets it identifies the association for this query.

spp_address:

This specifies which address is of interest.

ssp_hbinterval:

This contains the value of the heartbeat interval, in milliseconds. Note that unless the spp_flag is set to SPP_HB_ENABLE the value of this field is ignored. Note also that a value of zero indicates the current setting should be left unchanged. To set an actual value of zero the use of the flag SPP_HB_TIME_IS_ZERO should be used.

ssp_pathmaxrxt:

This contains the maximum number of retransmissions before this address shall be considered unreachable. Note that a value of zero indicates the current setting should be left unchanged.

ssp_pathmtu:

When Path MTU discovery is disabled the value specified here will be the "fixed" path MTU (i.e. the value of the spp_flags field must include the flag SPP_PMTUD_DISABLE). Note that if the spp_address field is empty then all destinations for this association will have this fixed path MTU set upon them. If an address is specified, then only that address will be effected. Note also that this option cannot be set on the endpoint, but must be set on each individual association. Also, when disabling PMTU discovery, the implementation may disallow this behavior if the "fixed" path MTU is below the constant value SCTP_SMALLEST_PMTU.

spp_ipv6_flowlabel:

This field is used in conjunction with the SPP_IPV6_FLOWLABEL flag.

spp_ipv4_tos:

This field is used in conjunction with the SPP_IPV4_TOS flag.

spp_flags:

These flags are used to control various features on an association. The flag field is a bit mask which may contain zero or more of the following options:

SPP_HB_ENABLE:

Enable heartbeats on the specified address. Note that if the address field is empty all addresses for the association have heartbeats enabled upon them.

SPP_HB_DISABLE:

Disable heartbeats on the specified address. Note that if the address field is empty all addresses for the association will have their heartbeats disabled. Note also that SPP_HB_ENABLE and SPP_HB_DISABLE are mutually exclusive, only one of these two should be specified. Enabling both fields will have undetermined results.

SPP_HB_DEMAND:

Request a user initiated heartbeat to be made immediately.

SPP_HB_TIME_IS_ZERO:

Specify's that the time for heartbeat delay is to be set to the value of 0 milliseconds.

SPP_PMTUD_ENABLE:

This field will enable PMTU discovery upon the specified address. Note that if the address field is empty then all addresses on the association are effected.

SPP_PMTUD_DISABLE:

This field will disable PMTU discovery upon the specified address. Note that if the address field is empty then all addresses on the association are effected. Note also that SPP_PMTUD_ENABLE and SPP_PMTUD_DISABLE are mutually exclusive. Enabling both will have undetermined results.

SPP_IPV6_FLOWLABEL:

Setting this flag enables setting of the IPV6 flowlabel value associated with either the association or the specific address. If the address field is filled in, then the specific destination address has this value set upon it. If the association is specified, but not the address, then the flowlabel value is set for any future destination addresses that may be added. The value is obtained in the spp_ipv6_flowlabel field.
Upon retrieval, this flag will be set to indicate that the spp_ipv6_flowlabel field has a valid value returned. If a specific destination addresses is set (in the spp_address field) when called then the value returned is that of the address. If just an association is specified (and no address) then the association default flowlabel is returned. If neither an association nor an destination is specified, then the sockets default flowlabel is returned. For non IPv6 sockets, then this flag will be left cleared.

SPP_IPV4_TOS:

Setting this flag enables setting of the IPV4 tos value associated with either the association or specific address. If the address field is filled in, then the specific destination address has this value set upon it. If the association is specified, but not the address, then the tos value is set for any future destination addresses that may be added. The value is obtained in the spp_ipv4_tos field.
Upon retrieval, this flag will be set to indicate that the spp_ipv4_tos field has a valid value returned. If a specific destination addresses is set when called (in the spp_address field) then that specific destination addresses tos value is returned. If just an association is specified then the association default tos is returned. If neither an association nor an destination is specified, then the sockets default tos is returned. For non IPv4 sockets, then this flag will be left cleared.

To read or modify these parameters, the application should call sctp_opt_info() with the SCTP_PEER_ADDR_PARAMS option.

8.1.13.  Set Default Send Parameters (SCTP_DEFAULT_SEND_PARAM)

Applications that wish to use the sendto() system call may wish to specify a default set of parameters that would normally be supplied through the inclusion of ancillary data. This socket option allows such an application to set the default sctp_sndrcvinfo structure. The application that wishes to use this socket option simply passes in to this call the sctp_sndrcvinfo structure defined in Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)). The input parameters accepted by this call include sinfo_stream, sinfo_flags, sinfo_ppid, sinfo_context, sinfo_pr_policy and sinfo_pr_value. The sinfo_flags is composed of a bitwise OR of SCTP_UNORDERED, SCTP_EOF, and SCTP_SENDALL. The sinfo_assoc_id field specifies the association to apply the parameters to in a one-to-many style sockets. It is ignored on the one-to-one style. Note that setting the sinfo_assoc_id field to zero indicates that the users wishes to set the endpoint default send parameters for all future associations.

8.1.14.  Set Notification and Ancillary Events (SCTP_EVENTS)

This socket option is used to specify various notifications and ancillary data the user wishes to receive. Please see Section 8.4 (Ancillary Data and Notification Interest Options) for a full description of this option and its usage.

8.1.15.  Set/Clear IPv4 Mapped Addresses (SCTP_I_WANT_MAPPED_V4_ADDR)

This socket option is a boolean flag which turns on or off mapped V4 addresses. If this option is turned on and the socket is type PF_INET6, then IPv4 addresses will be mapped to V6 representation. If this option is turned off, then no mapping will be done of V4 addresses and a user will receive both PF_INET6 and PF_INET type addresses on the socket.

By default this option is turned off and expects an integer to be passed where non-zero turns on the option and zero turns off the option.

8.1.16.  Get or Set the Maximum Fragmentation Size (SCTP_MAXSEG)

This option will get or set the maximum size to put in any outgoing SCTP DATA chunk. If a message is larger than this size it will be fragmented by SCTP into the specified size. Note that the underlying SCTP implementation may fragment into smaller sized chunks when the PMTU of the underlying association is smaller than the value set by the user. The default value for this option is '0' which indicates the user is NOT limiting fragmentation and only the PMTU will effect SCTP's choice of DATA chunk size. Note also that values set larger than the maximum size of an IP datagram will effectively let SCTP control fragmentation (i.e. the same as setting this option to 0).

The following structure is used to access and modify this parameter:

struct sctp_assoc_value {
  sctp_assoc_t assoc_id;
  uint32_t assoc_value;
};

assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets this parameter indicates which association the user is performing an action upon. Note that if this field's value is zero then the endpoints default value is changed (effecting future associations only).

assoc_value:

This parameter specifies the maximum size in bytes.

8.1.17.  Get or Set the List of Supported HMAC Identifiers (SCTP_HMAC_IDENT)

This option gets or sets the list of HMAC algorithms that the local endpoint requires the peer to use.

The following structure is used to get or set these identifiers:

struct sctp_hmacalgo {
  uint32_t shmac_number_of_idents;
  uint16_t shmac_idents[];
};

shmac_number_of_idents:

This field gives the number of elements present in the array shmac_idents.

shmac_idents:

This parameter contains an array of HMAC Identifiers that the local endpoint is requesting the peer to use, in priority order. The following identifiers are valid:

• SCTP_AUTH_HMAC_ID_SHA1
• SCTP_AUTH_HMAC_ID_SHA256

Note that the list supplied must include SCTP_AUTH_HMAC_ID_SHA1 and may include any of the other values in its preferred order (lowest list position has the most preference in algorithm selection). Note also that the lack of SCTP_AUTH_HMAC_ID_SHA1, or the inclusion of an unknown HMAC identifier (including optional identifiers unknown to the implementation) will cause the set option to fail and return an error.

8.1.18.  Get or Set the Active Shared Key (SCTP_AUTH_ACTIVE_KEY)

This option will get or set the active shared key to be used to build the association shared key.

The following structure is used to access and modify these parameters:

struct sctp_authkeyid {
  sctp_assoc_t scact_assoc_id;
  uint16_t scact_keynumber;
};

scact_assoc_id:

This parameter, if non-zero, indicates what association that the shared key identifier is being set active upon. Note that if this element contains zero, then the activation applies to the endpoint and all future associations will use the specified shared key identifier. For one-to-one sockets, this parameter is ignored. Note, however, that this option will set the active key on the association if the socket is connected, otherwise this will set the default active key for the endpoint.

scact_keynumber:

This parameter is the shared key identifier which the application is requesting to become the active shared key to be used for sending authenticated chunks. The key identifier MUST correspond to an existing shared key. Note that shared key identifier '0' defaults to a null key.

When used with setsockopt() the SCTP implementation MUST use the indicated shared key identifier for all messages being given to an SCTP implementation via a send call after the setsockopt() call until changed again. Therefore the SCTP implementation MUST NOT bundle user messages which should be authenticated using different shared key identifiers.

Initially the key with key identifier 0 is the active key.

8.1.19.  Get or Set Delayed SACK Timer (SCTP_DELAYED_SACK)

This option will effect the way delayed acks are performed. This option allows you to get or set the delayed ack time, in milliseconds. It also allows changing the delayed ack frequency. Changing the frequency to 1 disables the delayed sack algorithm. If the sack_assoc_id is 0, then this sets or gets the endpoints default values. If the sack_assoc_id field is non-zero, then the set or get effects the specified association for the one to many model (the assoc_id field is ignored by the one to one model). Note that if sack_delay or sack_freq are 0 when setting this option, then the current values will remain unchanged.

The following structure is used to access and modify these parameters:

struct sctp_sack_info {
  sctp_assoc_t sack_assoc_id;
  uint32_t sack_delay;
  uint32_t sack_freq;
};

sack_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets this parameter indicates which association the user is performing an action upon. Note that if this field's value is zero then the endpoints default value is changed (effecting future associations only).

sack_delay:

This parameter contains the number of milliseconds that the user is requesting the delayed ACK timer be set to. Note that this value is defined in the standard to be between 200 and 500 milliseconds.

sack_freq:

This parameter contains the number of packets that must be received before a sack is sent without waiting for the delay timer to expire. The default value for this is 2, setting this value to 1 will disable the delayed sack algorithm.

8.1.20.  Get or Set Fragmented Interleave (SCTP_FRAGMENT_INTERLEAVE)

Fragmented interleave controls how the presentation of messages occurs for the message receiver. There are three levels of fragment interleave defined. Two of the levels effect the one-to-one model, while the one-to-many model is effected by all three levels.

This option takes an integer value. It can be set to a value of 0, 1 or 2. Attempting to set this level to other values will return an error.

Setting the three levels provides the following receiver interactions:

level 0:

Prevents the interleaving of any messages. This means that when a partial delivery begins, no other messages will be received except the message being partially delivered. If another message arrives on a different stream (or association) that could be delivered, it will be blocked waiting for the user to read all of the partially delivered message.

level 1:

Allows interleaving of messages that are from different associations. For the one-to-one model, level 0 and level 1 thus have the same meaning since a one-to-one socket always receives messages from the same association. Note that setting the one-to-many model to this level may cause multiple partial delivers from different associations but for any given association, only one message will be delivered until all parts of a message have been delivered. This means that one large message, being read with an association identification of "X", will block other messages from association "X" from being delivered.

level 2:

Allows complete interleaving of messages. This level requires that the sender carefully observe not only the peer association identification (or address) but also must pay careful attention to the stream number. With this option enabled a partially delivered message may begin being delivered for association "X" stream "Y" and the next subsequent receive may return a message from association "X" stream "Z". Note that no other messages would be delivered for association "X" stream "Y" until all of stream "Y"'s partially delivered message was read. Note that this option also effects the one-to-one model. Also note that for the one-to-many model not only may another streams message from the same association be delivered from the next receive, some other associations message may be delivered upon the next receive.

An implementation should default the one-to-many model to level 1. The reason for this is that otherwise it is possible that a peer could begin sending a partial message and thus block all other peers from sending data. However a setting of level 2 requires the application to not only be aware of the association (via the association id or peers address) but also the stream number. The stream number is NOT present unless the user has subscribed to the sctp_data_io_events (see Section 8.4 (Ancillary Data and Notification Interest Options)). This is also why we recommend that the one-to-one model be defaulted to level 0 (level 1 for the one-to-one model has no effect). Note that an implementation should return an error if a application attempts to set the level to 2 and has NOT subscribed to the sctp_data_io_events.

8.1.21.  Set or Get the SCTP Partial Delivery Point (SCTP_PARTIAL_DELIVERY_POINT)

This option will set or get the SCTP partial delivery point. This point is the size of a message where the partial delivery API will be invoked to help free up rwnd space for the peer. Setting this to a lower value will cause partial deliveries to happen more often. The calls argument is an integer that sets or gets the partial delivery point. Note also that the call will fail if the user attempts to set this value larger than the socket receive buffer size.

Note that any single message having a length smaller than or equal to the SCTP partial delivery point will be delivered in one single read call as long as the user provided buffer is large enough to hold the message.

8.1.22.  Set or Get the Use of Extended Receive Info (SCTP_USE_EXT_RCVINFO)

This option will enable or disable the use of the extended version of the sctp_sndrcvinfo structure. If this option is disabled, then the normal sctp_sndrcvinfo structure is returned in all receive message calls. If this option is enabled then the sctp_extrcvinfo structure is returned in all receive message calls.

Note that the sctp_extrcvinfo structure is never used in any send call.

8.1.23.  Set or Get the Auto ASCONF Flag (SCTP_AUTO_ASCONF)

This option will enable or disable the use of the automatic generation of ASCONF chunks to add and delete addresses to an existing association. Note that this option has two caveats namely: a) it only effects sockets that are bound to all addresses on the machine, and b) the system administrator may have an overriding control that turns the ASCONF feature off no matter what setting the socket option may have.

8.1.24.  Set or Get the Maximum Burst (SCTP_MAX_BURST)

This option will allow a user to change the maximum burst of packets that can be emitted by this association. Note that the default value is 4, and some implementations may restrict this setting so that it can only be lowered.

To set or get this option the user fills in the following structure:

struct sctp_assoc_value {
  sctp_assoc_t assoc_id;
  uint32_t assoc_value;
};

assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets this parameter indicates which association the user is performing an action upon. Note that if this field's value is zero then the endpoints default value is changed (effecting future associations only).

assoc_value:

This parameter contains the maximum burst.

8.1.25.  Set or Get the Default Context (SCTP_CONTEXT)

The context field in the sctp_sndrcvinfo structure is normally only used when a failed message is retrieved holding the value that was sent down on the actual send call. This option allows the setting of a default context on an association basis that will be received on reading messages from the peer. This is especially helpful in the one-2-many model for an application to keep some reference to an internal state machine that is processing messages on the association. Note that the setting of this value only effects received messages from the peer and does not effect the value that is saved with outbound messages.

To set or get this option the user fills in the following structure:

struct sctp_assoc_value {
  sctp_assoc_t assoc_id;
  uint32_t assoc_value;
};

assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets this parameter indicates which association the user is performing an action upon. Note that if this field's value is zero then the endpoints default value is changed (effecting future associations only).

assoc_value:

This parameter contains the context.

8.1.26.  Enable or Disable Explicit EOR Marking (SCTP_EXPLICIT_EOR)

This boolean flag is used to enable or disable explicit end of record (EOR) marking. When this option is enabled, a user may make multiple send system calls to send a record and must indicate that they are finished sending a particular record by including on the send the SCTP_EOR flag. If this boolean flag is disabled then each individual send system call is considered to have a SCTP_EOR indicator set on it implicitly without the user having to explicitly add this flag.

8.1.27.  Enable SCTP Port Reusage (SCTP_REUSE_PORT)

This option only supports one-to-one style SCTP sockets. If used on a one-to-many style SCTP socket an error is indicated.

The setsockopt() call MUST NOT be used after calling bind() or sctp_bindx() for a one-to-one style SCTP socket. If using bind() or sctp_bindx() on a socket with the SCTP_REUSE_PORT option, all other SCTP sockets bound to the same port MUST have set the SCTP_REUSE_PORT. Calling bind() or sctp_bindx() for a socket without having set the SCTP_REUSE_PORT option will fail if there are other sockets bound to the same port. At most one socket being bound to the same port may be listening.

It should be noted that the behaviour of the socket level socket option to reuse ports and/or addresses for SCTP sockets is unspecified.

8.2.  Read-Only Options

The options defined in this subsection are read-only. Using this option in a setsockopt() call will result in a error indicating EOPNOTSUPP.

8.2.1.  Association Status (SCTP_STATUS)

Applications can retrieve current status information about an association, including association state, peer receiver window size, number of unacked data chunks, and number of data chunks pending receipt. This information is read-only.

The following structure is used to access this information:

struct sctp_status {
  sctp_assoc_t sstat_assoc_id;
  int32_t sstat_state;
  uint32_t sstat_rwnd;
  uint16_t sstat_unackdata;
  uint16_t sstat_penddata;
  uint16_t sstat_instrms;
  uint16_t sstat_outstrms;
  uint32_t sstat_fragmentation_point;
  struct sctp_paddrinfo sstat_primary;
};

sstat_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets it holds the identifier for the association. All notifications for a given association have the same association identifier.

sstat_state:

This contains the association's current state one of the following values:

• SCTP_CLOSED
• SCTP_BOUND
• SCTP_LISTEN
• SCTP_COOKIE_WAIT
• SCTP_COOKIE_ECHOED
• SCTP_ESTABLISHED
• SCTP_SHUTDOWN_PENDING
• SCTP_SHUTDOWN_SENT
• SCTP_SHUTDOWN_RECEIVED
• SCTP_SHUTDOWN_ACK_SENT

sstat_rwnd:

This contains the association peer's current receiver window size.

sstat_unackdata:

This is the number of unacked data chunks.

sstat_penddata:

This is the number of data chunks pending receipt.

sstat_primary:

This is information on the current primary peer address.

sstat_instrms:

The number of streams that the peer will be using inbound.

sstat_outstrms:

The number of streams that the endpoint is allowed to use outbound.

sstat_fragmentation_point -

The size at which SCTP fragmentation will occur.

To access these status values, the application calls getsockopt() with the option name SCTP_STATUS.

8.2.2.  Peer Address Information (SCTP_GET_PEER_ADDR_INFO)

Applications can retrieve information about a specific peer address of an association, including its reachability state, congestion window, and retransmission timer values. This information is read-only.

The following structure is used to access this information:

struct sctp_paddrinfo {
  sctp_assoc_t spinfo_assoc_id;
  struct sockaddr_storage spinfo_address;
  int32_t spinfo_state;
  uint32_t spinfo_cwnd;
  uint32_t spinfo_srtt;
  uint32_t spinfo_rto;
  uint32_t spinfo_mtu;
};

spinfo_assoc_id:

This is parameter is ignored for one-to-one style sockets. For one-to-many style sockets the following applies: This is field may be filled in by the application, if so, this field will have priority in looking up the association over the address specified in spinfo_address. Note that if the address does not belong to the association specified then this call will fail. If the application does NOT fill in the spinfo_assoc_id, then the address will be used to lookup the association and on return this field will have the valid association id. In other words, this call can be used to translate a address into an association id.

spinfo_address:

This is filled in the application, and contains the peer address of interest.

spinfo_state:

This contains the peer addresses's state (either SCTP_ACTIVE or SCTP_INACTIVE and possibly the modifier SCTP_UNCONFIRMED).

spinfo_cwnd:

This contains the peer addresses's current congestion window.

spinfo_srtt:

This contains the peer addresses's current smoothed round-trip time calculation in milliseconds.

spinfo_rto:

This contains the peer addresses's current retransmission timeout value in milliseconds.

spinfo_mtu:

The current P-MTU of this address.

8.2.3.  Get the List of Chunks the Peer Requires to be Authenticated (SCTP_PEER_AUTH_CHUNKS)

This option gets a list of chunks for a specified association that the peer requires to be received authenticated only.

The following structure is used to access these parameters:

struct sctp_authchunks {
  sctp_assoc_t gauth_assoc_id;
  guint32_t gauth_number_of_chunks
  uint8_t gauth_chunks[];
};

gauth_assoc_id:

This parameter, indicates which association the user is requesting the list of peer authenticated chunks. For one-to-one sockets, this parameter is ignored.

gauth_number_of_chunks:

This parameter gives the number of elements in the array gauth_chunks.

gauth_chunks:

This parameter contains an array of chunks that the peer is requesting to be authenticated.

8.2.4.  Get the List of Chunks the Local Endpoint Requires to be Authenticated (SCTP_LOCAL_AUTH_CHUNKS)

This option gets a list of chunks for a specified association that the local endpoint requires to be received authenticated only.

The following structure is used to access these parameters:

struct sctp_authchunks {
  sctp_assoc_t gauth_assoc_id;
  uint32_t gauth_number_of_chunks;
  uint8_t gauth_chunks[];
};

gauth_assoc_id:

This parameter, indicates which association the user is requesting the list of local authenticated chunks. For one-to-one sockets, this parameter is ignored.

gauth_number_of_chunks:

This parameter gives the number of elements in the array gauth_chunks.

gauth_chunks:

This parameter contains an array of chunks that the local endpoint is requesting to be authenticated.

8.2.5.  Get the Current Number of Associations (SCTP_GET_ASSOC_NUMBER)

This option gets the current number of associations that are attached to a one-to-many style socket. The option value is an uint32_t.

8.2.6.  Get the Current Identifiers of Associations (SCTP_GET_ASSOC_ID_LIST)

This option gets the current list of SCTP association identifiers of the SCTP associations handled by a one-to-many style socket.

The option value has the structure

struct sctp_assoc_ids {
  uint32_t gaids_number_of_ids;
  sctp_assoc_t gaids_assoc_id[];
};

The caller MUST provide a large enough buffer to hold all association identifiers. If the buffer is too small, an error MUST be returned. The user can use the SCTP_GET_ASSOC_NUMBER socket option to get an idea how large the buffer has to be. gaids_number_of_ids gives the number of elements in the array gaids_assoc_id.

8.3.  Write-Only Options

The options defined in this subsection are write-only. Using this option in a getsockopt() or sctp_opt_info() call will result in a error indicating EOPNOTSUPP.

8.3.1.  Set Peer Primary Address (SCTP_SET_PEER_PRIMARY_ADDR)

Requests that the peer mark the enclosed address as the association primary. The enclosed address must be one of the association's locally bound addresses.

The following structure is used to make a set peer primary request:

struct sctp_setpeerprim {
  sctp_assoc_t sspp_assoc_id;
  struct sockaddr_storage sspp_addr;
};

sspp_addr:

The address to set as primary.

sspp_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets it identifies the association for this request.

8.3.2.  Add a Chunk That Must Be Authenticated (SCTP_AUTH_CHUNK)

This set option adds a chunk type that the user is requesting to be received only in an authenticated way. Changes to the list of chunks will only effect future associations on the socket.

The following structure is used to add a chunk:

struct sctp_authchunk {
  uint8_t sauth_chunk;
};

sauth_chunk:

This parameter contains a chunk type that the user is requesting to be authenticated.

The chunk types for INIT, INIT-ACK, SHUTDOWN-COMPLETE, and AUTH chunks MUST NOT be used. If they are used, an error MUST be returned. The usage of this option enables SCTP-AUTH in cases where it is not required by other means (for example the use of dynamic address reconfiguration).

8.3.3.  Set a Shared Key (SCTP_AUTH_KEY)

This option will set a shared secret key which is used to build an association shared key.

The following structure is used to access and modify these parameters:

struct sctp_authkey {
  sctp_assoc_t sca_assoc_id;
  uint16_t sca_keynumber;
  uint16_t sca_keylength;
  uint8_t sca_key[];
};

sca_assoc_id:

This parameter, if non-zero, indicates what association that the shared key is being set upon. Note that if this element contains zero, then the shared key is set upon the endpoint and all future associations will use this key (if not changed by subsequent calls to SCTP_AUTH_KEY). For one-to-one sockets, this parameter is ignored. Note, however, that this option will set a key on the association if the socket is connected, otherwise this will set a key on the endpoint.

sca_keynumber:

This parameter is the shared key identifier by which the application will refer to this key. If a key of the specified index already exists, then this new key will replace the old existing key. Note that shared key identifier '0' defaults to a null key.

sca_keylength:

This parameter is the length of the array sca_key.

sca_key:

This parameter contains an array of bytes that is to be used by the endpoint (or association) as the shared secret key. Note, if the length of this field is zero, a null key is set.

8.3.4.  Deactivate a Shared Key (SCTP_AUTH_DEACTIVATE_KEY)

This set option indicates that the application will not send user messages anymore requiring the usage of the indicated key identifier.

struct sctp_authkeyid {
  sctp_assoc_t scact_assoc_id;
  uint16_t scact_keynumber;
};

scact_assoc_id:

This parameter, if non-zero, indicates what association that the shared key identifier is being deactivated. Note that if this element contains zero, then the shared key is deactivated for the endpoint and all associations will no longer use the specified shared key identifier (unless otherwise set on the association using SCTP_AUTH_KEY). For one-to-one sockets, this parameter is ignored. Note, however, that this option will deactivate the key from the association if the socket is connected, otherwise this will deactivate the key from the endpoint.

scact_keynumber:

This parameter is the shared key identifier which the application is requesting to be deactivated. The key identifier MUST correspond to an existing shared key. Note if this parameter is zero, use of the null key identifier '0' is deactivated on the endpoint and/or association.

The currently active key can not be deactivated.

8.3.5.  Delete a Shared Key (SCTP_AUTH_DELETE_KEY)

This set option will delete a shared secret key in the SCTP implementation.

struct sctp_authkeyid {
  sctp_assoc_t scact_assoc_id;
  uint16_t scact_keynumber;
};

scact_assoc_id:

This parameter, if non-zero, indicates what association that the shared key identifier is being deleted from. Note that if this element contains zero, then the shared key is deleted from the endpoint and all associations will no longer use the specified shared key identifier (unless otherwise set on the association using SCTP_AUTH_KEY). For one-to-one sockets, this parameter is ignored. Note, however, that this option will delete the key from the association if the socket is connected, otherwise this will delete the key from the endpoint.

scact_keynumber:

This parameter is the shared key identifier which the application is requesting to be deleted. The key identifier MUST correspond to an existing shared key and MUST NOT be in use for any packet being sent by the SCTP implementation. This means in particular, that it MUST be deactivated first. Note if this parameter is zero, use of the null key identifier '0' is deleted from the endpoint and/or association.

Only deactivated keys which are no longer used by the kernel can be deleted.

8.4.  Ancillary Data and Notification Interest Options

Applications can receive per-message ancillary information and notifications of certain SCTP events with recvmsg().

The following optional information is available to the application:

SCTP_SNDRCV (sctp_data_io_event):

Per-message information (i.e. stream number, TSN, SSN, etc. described in Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)))

SCTP_ASSOC_CHANGE (sctp_association_event):

described in Section 6.3.2 (SCTP_ASSOC_CHANGE)

SCTP_PEER_ADDR_CHANGE (sctp_address_event):

described in Section 6.3.3 (SCTP_PEER_ADDR_CHANGE)

SCTP_SEND_FAILED (sctp_send_failure_event):

described in Section 6.3.5 (SCTP_SEND_FAILED)

SCTP_REMOTE_ERROR (sctp_peer_error_event):

described in Section 6.3.4 (SCTP_REMOTE_ERROR)

SCTP_SHUTDOWN_EVENT (sctp_shtudown_event):

described in Section 6.3.6 (SCTP_SHUTDOWN_EVENT)

SCTP_PARTIAL_DELIVERY_EVENT (sctp_partial_delivery_event):

described in Section 6.3.8 (SCTP_PARTIAL_DELIVERY_EVENT)

SCTP_ADAPTATION_INDICATION (sctp_adaptation_layer_event):

described in Section 6.3.7 (SCTP_ADAPTATION_INDICATION)

SCTP_AUTHENTICATION_EVENT (sctp_authentication_event):

described in Section 6.3.9 (SCTP_AUTHENTICATION_EVENT))

SCTP_SENDER_DRY_EVENT (sctp_sender_dry_event):

described in Section 6.3.10 (SCTP_SENDER_DRY_EVENT)

To receive any ancillary data or notifications, first the application registers its interest by calling the SCTP_EVENTS setsockopt() with the following structure:

struct sctp_event_subscribe{
  uint8_t sctp_data_io_event;
  uint8_t sctp_association_event;
  uint8_t sctp_address_event;
  uint8_t sctp_send_failure_event;
  uint8_t sctp_peer_error_event;
  uint8_t sctp_shutdown_event;
  uint8_t sctp_partial_delivery_event;
  uint8_t sctp_adaptation_layer_event;
  uint8_t sctp_authentication_event;
  uint8_t sctp_sender_dry_event;
};

sctp_data_io_event:

Setting this flag to 1 will cause the reception of SCTP_SNDRCV information on a per message basis. The application will need to use the recvmsg() interface so that it can receive the event information contained in the msg_control field. Setting the flag to 0 will disable reception of the message control information.

sctp_association_event:

Setting this flag to 1 will enable the reception of association event notifications. Setting the flag to 0 will disable association event notifications.

sctp_address_event:

Setting this flag to 1 will enable the reception of address event notifications. Setting the flag to 0 will disable address event notifications.

sctp_send_failure_event:

Setting this flag to 1 will enable the reception of send failure event notifications. Setting the flag to 0 will disable send failure event notifications.

sctp_peer_error_event:

Setting this flag to 1 will enable the reception of peer error event notifications. Setting the flag to 0 will disable peer error event notifications.

sctp_shutdown_event:

Setting this flag to 1 will enable the reception of shutdown event notifications. Setting the flag to 0 will disable shutdown event notifications.

sctp_partial_delivery_event:

Setting this flag to 1 will enable the reception of partial delivery notifications. Setting the flag to 0 will disable partial delivery event notifications.

sctp_adaptation_layer_event:

Setting this flag to 1 will enable the reception of adaptation layer notifications. Setting the flag to 0 will disable adaptation layer event notifications.

sctp_authentication_event:

Setting this flag to 1 will enable the reception of authentication layer notifications. Setting the flag to 0 will disable authentication layer event notifications.

sctp_sender_dry_event:

Setting this flag to 1 will enable the reception of sender dry notifications. Setting the flag to 0 will disable sender dry event notifications.

An example where an application would like to receive data io events and association events but no others would be as follows:

{
  struct sctp_event_subscribe event;

  memset(&event,0,sizeof(event));

  event.sctp_data_io_event = 1;
  event.sctp_association_event = 1;

  setsockopt(fd, IPPROTO_SCTP, SCTP_EVENTS, &event, sizeof(event));
}

Note that for one-to-many style SCTP sockets, the caller of recvmsg() receives ancillary data and notifications for ALL associations bound to the file descriptor. For one-to-one style SCTP sockets, the caller receives ancillary data and notifications for only the single association bound to the file descriptor.

By default both the one-to-one style and one-to-many style socket has all options off.

9.  New Functions

Depending on the system, the following interface can be implemented as a system call or library function.

9.1.  sctp_bindx()

This function allows the user to bind a specific subset of addresses or, if the SCTP extension described in [RFC5061] (Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, “Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration,” September 2007.) is supported, add or delete specific addresses.

The function prototype is

int sctp_bindx(int sd,
               struct sockaddr *addrs,
               int addrcnt,
               int flags);

If sd is an IPv4 socket, the addresses passed must be IPv4 addresses. If the sd is an IPv6 socket, the addresses passed can either be IPv4 or IPv6 addresses.

A single address may be specified as INADDR_ANY or IN6ADDR_ANY, see Section 4.1.2 (bind()) for this usage.

addrs is a pointer to an array of one or more socket addresses. Each address is contained in its appropriate structure. For an IPv6 socket, an array of sockaddr_in6 would be returned. For a IPv4 socket, an array of sockaddr_in would be returned. The caller specifies the number of addresses in the array with addrcnt. Note that the wildcard addresses cannot be used in combination with non wildcard addresses on a socket with this function, doing so will result in an error.

On success, sctp_bindx() returns 0. On failure, sctp_bindx() returns -1, and sets errno to the appropriate error code.

For SCTP, the port given in each socket address must be the same, or sctp_bindx() will fail, setting errno to EINVAL.

The flags parameter is formed from the bitwise OR of zero or more of the following currently defined flags:

• SCTP_BINDX_ADD_ADDR
• SCTP_BINDX_REM_ADDR

SCTP_BINDX_ADD_ADDR directs SCTP to add the given addresses to the association, and SCTP_BINDX_REM_ADDR directs SCTP to remove the given addresses from the association. The two flags are mutually exclusive; if both are given, sctp_bindx() will fail with EINVAL. A caller may not remove all addresses from an association; sctp_bindx() will reject such an attempt with EINVAL.

An application can use sctp_bindx(SCTP_BINDX_ADD_ADDR) to associate additional addresses with an endpoint after calling bind(). Or use sctp_bindx(SCTP_BINDX_REM_ADDR) to remove some addresses a listening socket is associated with so that no new association accepted will be associated with those addresses. If the endpoint supports dynamic address a SCTP_BINDX_REM_ADDR or SCTP_BINDX_ADD_ADDR may cause a endpoint to send the appropriate message to the peer to change the peers address lists.

Adding and removing addresses from a connected association is optional functionality. Implementations that do not support this functionality should return EOPNOTSUPP.

sctp_bindx() can be called on an already bound socket or on an unbound socket. If the socket is unbound and the first port number in the addrs is zero, the kernel will chose a port number. All port numbers after the first one being 0 MUST also be zero. If the first port number is not zero, the following port numbers MUST be zero or have the same value as the first one. For an already bound socket, all port numbers provided MUST be the bound one or 0.

sctp_bindx() is an atomic operation. Therefore the binding will be either successful on all addresses or fail on all addresses. If multiple addresses are provided and the sctp_bindx() call fails there is no indication which address is responsible for the failure. The only way to get an specific error indication is to call sctp_bindx() with only one address sequentially.

9.2.  sctp_peeloff()

After an association is established on a one-to-many style socket, the application may wish to branch off the association into a separate socket/file descriptor.

This is particularly desirable when, for instance, the application wishes to have a number of sporadic message senders/receivers remain under the original one-to-many style socket but branch off those associations carrying high volume data traffic into their own separate socket descriptors.

The application uses sctp_peeloff() call to branch off an association into a separate socket (Note the semantics are somewhat changed from the traditional one-to-one style accept() call). Note that the new socket is a one-to-one style socket. Thus it will be confined to operations allowed for a one-to-one style socket.

The function prototype is

int sctp_peeloff(int sd,
                 sctp_assoc_t assoc_id);

and the arguments are

sd:

The original one-to-many style socket descriptor returned from the socket() system call (see Section 4.1.1 (socket())).

assoc_id:

the specified identifier of the association that is to be branched off to a separate file descriptor (Note, in a traditional one-to-one style accept() call, this would be an out parameter, but for the one-to-many style call, this is an in parameter).

The function returns a non-negative file descriptor representing the branched-off association, or -1 if an error occurred. The variable errno is then set appropriately.

9.3.  sctp_getpaddrs()

sctp_getpaddrs() returns all peer addresses in an association.

The function protoype is:

int sctp_getpaddrs(int sd,
                   sctp_assoc_t id,
                   struct sockaddr **addrs);

On return, addrs will point to an array dynamically allocated sockaddr structures of the appropriate type for the socket type. The caller should use sctp_freepaddrs() to free the memory. Note that the in/out parameter addrs must not be NULL.

If sd is an IPv4 socket, the addresses returned will be all IPv4 addresses. If sd is an IPv6 socket, the addresses returned can be a mix of IPv4 or IPv6 addresses.

For one-to-many style sockets, id specifies the association to query. For one-to-one style sockets, id is ignored.

On success, sctp_getpaddrs() returns the number of peer addresses in the association. If there is no association on this socket, sctp_getpaddrs() returns 0, and the value of *addrs is undefined. If an error occurs, sctp_getpaddrs() returns -1, and the value of *addrs is undefined.

9.4.  sctp_freepaddrs()

sctp_freepaddrs() frees all resources allocated by sctp_getpaddrs().

The function prototype is

void sctp_freepaddrs(struct sockaddr *addrs);

and addrs is the array of peer addresses returned by sctp_getpaddrs().

9.5.  sctp_getladdrs()

sctp_getladdrs() returns all locally bound address(es) on a socket.

The function prototype is

int sctp_getladdrs(int sd,
                   sctp_assoc_t id,
                   struct sockaddr **ss);

On return, addrs will point to a dynamically allocated array of sockaddr structures of the appropriate type for the socket type. The caller should use sctp_freeladdrs() to free the memory. Note that the in/out parameter addrs must not be NULL.

If sd is an IPv4 socket, the addresses returned will be all IPv4 addresses. If sd is an IPv6 socket, the addresses returned can be a mix of IPv4 or IPv6 addresses.

For one-to-many style sockets, id specifies the association to query. For one-to-one style sockets, id is ignored.

If the id field is set to the value '0' then the locally bound addresses are returned without regard to any particular association.

On success, sctp_getladdrs() returns the number of local addresses bound to the socket. If the socket is unbound, sctp_getladdrs() returns 0, and the value of *addrs is undefined. If an error occurs, sctp_getladdrs() returns -1, and the value of *addrs is undefined.

9.6.  sctp_freeladdrs()

sctp_freeladdrs() frees all resources allocated by sctp_getladdrs().

The function prototype is

void sctp_freeladdrs(struct sockaddr *addrs);

and addrs is the array of peer addresses returned by sctp_getladdrs().

9.7.  sctp_sendmsg()

An implementation may provide a library function (or possibly system call) to assist the user with the advanced features of SCTP.

The function prototype is

ssize_t sctp_sendmsg(int sd,
                     const void *msg,
                     size_t len,
                     const struct sockaddr *to,
                     socklen_t tolen,
                     uint32_t ppid,
                     uint32_t flags,
                     uint16_t stream_no,
                     uint32_t pr_value,
                     uint32_t context);

and the arguments are:

sd:

The socket descriptor

msg:

The message to be sent.

len:

Yhe length of the message.

to:

The destination address of the message.

tolen:

The length of the destination address.

ppid:

The same as sinfo_ppid (see Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)))

flags:

The same as sinfo_flags (see Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)))

stream_no:

The same as sinfo_stream (see Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)))

pr_value:

The same as sinfo_pr_value (see Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV))).

context:

The same as sinfo_context (see Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)))

The call returns the number of characters sent, or -1 if an error occurred. The variable errno is then set appropriately.

Sending a message using sctp_sendmsg() is atomic unless explicit EOR marking is enabled on the socket specified by sd.

Using sctp_sendmsg() on a non-connected one-to-one style socket for implicit connection setup may or may not work depending on the SCTP implementation.

9.8.  sctp_recvmsg()

An implementation may provide a library function (or possibly system call) to assist the user with the advanced features of SCTP. Note that in order for the sctp_sndrcvinfo structure to be filled in by sctp_recvmsg() the caller must enable the sctp_data_io_events with the SCTP_EVENTS option. Note that the setting of the SCTP_USE_EXT_RCVINFO will effect this function as well, causing the sctp_sndrcvinfo information to be extended.

The function prototype is

ssize_t sctp_recvmsg(int sd,
                     void *msg,
                     size_t len,
                     struct sockaddr *from,
                     socklen_t *fromlen
                     struct sctp_sndrcvinfo *sinfo
                     int *msg_flags);

and the arguments are

sd:

The socket descriptor.

msg:

The message buffer to be filled.

len:

The length of the message buffer.

from:

A pointer to a address to be filled with the sender of this messages address.

fromlen:

An in/out parameter describing the from length.

sinfo:

A pointer to a sctp_sndrcvinfo structure to be filled upon receipt of the message.

msg_flags:

A pointer to a integer to be filled with any message flags (e.g. MSG_NOTIFICATION). Note that this field is an in-out field. Options for the receive may also be passed into the value (e.g. MSG_PEEK). On return from the call, the msg_flags value will be different than what was sent in to the call. If implemented via a recvmsg() call, the msg_flags should only contain the value of the flags from the recvmsg() call.

The call returns the number of bytes received, or -1 if an error occurred. The variable errno is then set appropriately.

9.9.  sctp_connectx()

An implementation may provide a library function (or possibly system call) to assist the user with associating to an endpoint that is multi-homed. Much like sctp_bindx() this call allows a caller to specify multiple addresses at which a peer can be reached. The way the SCTP stack uses the list of addresses to set up the association is implementation dependent. This function only specifies that the stack will try to make use of all the addresses in the list when needed.

Note that the list of addresses passed in is only used for setting up the association. It does not necessarily equal the set of addresses the peer uses for the resulting association. If the caller wants to find out the set of peer addresses, it must use sctp_getpaddrs() to retrieve them after the association has been set up.

The function prototype is

int sctp_connectx(int sd,
                  struct sockaddr *addrs,
                  int addrcnt,
                  sctp_assoc_t *id);

and the arguments are:

sd:

The socket descriptor.

addrs:

An (packed) array of addresses.

addrcnt:

The number of addresses in the array.

id:

An output parameter that if passed in as a non-NULL will return the association identification for the newly created association (if successful).

The call returns 0 on success or -1 if an error occurred. The variable errno is then set appropriately.

9.10.  sctp_send()

An implementation may provide another alternative function or system call to assist an application with the sending of data without the use of the CMSG header structures.

The function prototype is

ssize_t sctp_send(int sd,
                  const void *msg,
                  size_t len,
                  const struct sctp_sndrcvinfo *sinfo,
                  int flags);

and the arguments are

sd:

The socket descriptor.

msg:

The message to be sent.

len:

The length of the message.

sinfo:

A pointer to a sctp_sndrcvinfo structure used as described in Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)) for a sendmsg call.

flags:

The same flags as used by the sendmsg call flags (e.g. MSG_DONTROUTE).

The call returns the number of bytes sent, or -1 if an error occurred. The variable errno is then set appropriately.

This function call may also be used to terminate an association using an association identification by setting the sinfo.sinfo_flags to SCTP_EOF and the sinfo.sinfo_assoc_id to the association that needs to be terminated. In such a case the len of the message would be zero.

Using sctp_send() on a non-connected one-to-one style socket for implicit connection setup may or may not work depending on the SCTP implementation.

Sending a message using sctp_send() is atomic unless explicit EOR marking is enabled on the socket specified by sd.

9.11.  sctp_sendx()

An implementation may provide another alternative function or system call to assist an application with the sending of data without the use of the CMSG header structures that also gives a list of addresses. The list of addresses is provided for implicit association setup. In such a case the list of addresses serves the same purpose as the addresses given in sctp_connectx() (see Section 9.9 (sctp_connectx())).

The function prototype is

ssize_t sctp_sendx(int sd,
                   const void *msg,
                   size_t len,
                   struct sockaddr *addrs,
                   int addrcnt,
                   struct sctp_sndrcvinfo *sinfo,
                   int flags);

and the arguments are:

sd:

The socket descriptor.

msg:

The message to be sent.

len:

The length of the message.

addrs:

is an array of addresses.

addrcnt:

The number of addresses in the array.

sinfo:

A pointer to a sctp_sndrcvinfo structure used as described in Section 6.2.2 (SCTP Header Information Structure (SCTP_SNDRCV)) for a sendmsg call.

flags:

The same flags as used by the sendmsg call flags (e.g. MSG_DONTROUTE).

The call returns the number of bytes sent, or -1 if an error occurred. The variable errno is then set appropriately.

Note that on return from this call the sinfo structure will have changed in that the sinfo_assoc_id will be filled in with the new association id.

This function call may also be used to terminate an association using an association identification by setting the sinfo.sinfo_flags to SCTP_EOF and the sinfo.sinfo_assoc_id to the association that needs to be terminated. In such a case the len of the message would be zero.

Sending a message using sctp_send() is atomic unless explicit EOR marking is enabled on the socket specified by sd.

Using sctp_sendx() on a non-connected one-to-one style socket for implicit connection setup may or may not work depending on the SCTP implementation.

9.12.  sctp_getaddrlen()

For application binary portability it is sometimes desirable to know what the kernel thinks is the length of a socket address family.

The function prototype is:

int sctp_getaddrlen(sa_family_t family);

This function, when called with a valid family type returns the length that the operating system uses in the specified family's socket address structure. In case of an error, -1 is returned an the variable errno is then set appropriately

10.  IANA Considerations

This document requires no actions from IANA.

11.  Security Considerations

Many TCP and UDP implementations reserve port numbers below 1024 for privileged users. If the target platform supports privileged users, the SCTP implementation SHOULD restrict the ability to call bind() or sctp_bindx() on these port numbers to privileged users.

Similarly unprivileged users should not be able to set protocol parameters which could result in the congestion control algorithm being more aggressive than permitted on the public Internet. These parameters are:

• struct sctp_rtoinfo

If an unprivileged user inherits a one-to-many style socket with open associations on a privileged port, it MAY be permitted to accept new associations, but it SHOULD NOT be permitted to open new associations. This could be relevant for the r* family of protocols.

Applications using the one-to-many style sockets and using the interleave level if 0 are subject to denial of service attacks as described in Section 8.1.20 (Get or Set Fragmented Interleave (SCTP_FRAGMENT_INTERLEAVE)).

12.  Acknowledgments

Special acknowledgment is given to Ken Fujita, Jonathan Woods, Qiaobing Xie, and La Monte Yarroll, who helped extensively in the early formation of this document.

The authors also wish to thank Kavitha Baratakke, Mike Bartlett, Jon Berger, Mark Butler, Scott Kimble, Renee Revis, Andreas Fink, Jonathan Leighton, and many others on the TSVWG mailing list for contributing valuable comments.

A special thanks to Phillip Conrad, for his suggested text, quick and constructive insights, and most of all his persistent fighting to keep the interface to SCTP usable for the application programmer.

13. Normative References

Appendix A.  One-to-One Style Code Example

The following code is a simple implementation of an echo server over SCTP. The example shows how to use some features of one-to-one style IPv4 SCTP sockets, including:

• Opening, binding, and listening for new associations on a socket
• Enabling ancillary data
• Enabling notifications
• Using ancillary data with sendmsg() and recvmsg()
• Using MSG_EOR to determine if an entire message has been read
• Handling notifications

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/sctp.h>
#include <sys/uio.h>

#define BUFLEN  100

static void
handle_event(void *buf)
{
    struct sctp_assoc_change *sac;
    struct sctp_send_failed *ssf;
    struct sctp_paddr_change *spc;
    struct sctp_remote_error *sre;
    union sctp_notification *snp;
    char addrbuf[INET6_ADDRSTRLEN];
    const char *ap;
    struct sockaddr_in *sin;
    struct sockaddr_in6 *sin6;

    snp = buf;

    switch (snp->sn_header.sn_type) {
    case SCTP_ASSOC_CHANGE:
          sac = &snp->sn_assoc_change;
          printf("^^^ assoc_change: state=%hu, error=%hu, instr=%hu "
              "outstr=%hu\n", sac->sac_state, sac->sac_error,
              sac->sac_inbound_streams, sac->sac_outbound_streams);
          break;
    case SCTP_SEND_FAILED:
          ssf = &snp->sn_send_failed;
          printf("^^^ sendfailed: len=%hu err=%d\n", ssf->ssf_length,
              ssf->ssf_error);
          break;

    case SCTP_PEER_ADDR_CHANGE:
          spc = &snp->sn_paddr_change;
          if (spc->spc_aaddr.ss_family == AF_INET) {
            sin = (struct sockaddr_in *)&spc->spc_aaddr;
            ap = inet_ntop(AF_INET, &sin->sin_addr,
                           addrbuf, INET6_ADDRSTRLEN);
          } else {
            sin6 = (struct sockaddr_in6 *)&spc->spc_aaddr;
            ap = inet_ntop(AF_INET6, &sin6->sin6_addr,
                           addrbuf, INET6_ADDRSTRLEN);
          }
          printf("^^^ intf_change: %s state=%d, error=%d\n", ap,
                 spc->spc_state, spc->spc_error);
          break;
    case SCTP_REMOTE_ERROR:
          sre = &snp->sn_remote_error;
          printf("^^^ remote_error: err=%hu len=%hu\n",
              ntohs(sre->sre_error), ntohs(sre->sre_length));
          break;
    case SCTP_SHUTDOWN_EVENT:
          printf("^^^ shutdown event\n");
          break;
    default:
          printf("unknown type: %hu\n", snp->sn_header.sn_type);
          break;
    };
}

static void *
mysctp_recvmsg(int fd, struct msghdr *msg, void *buf, size_t *buflen,
    ssize_t *nrp, size_t cmsglen)
{
    ssize_t nr = 0, nnr = 0;
    struct iovec iov[1];

    *nrp = 0;
    iov->iov_base = buf;
    iov->iov_len = *buflen;
    msg->msg_iov = iov;
    msg->msg_iovlen = 1;


    for (;;) {
#ifndef MSG_XPG4_2
#define MSG_XPG4_2 0
#endif
            msg->msg_flags = MSG_XPG4_2;
            msg->msg_controllen = cmsglen;

            nnr = recvmsg(fd, msg, 0);
            if (nnr <= 0) {
                    /* EOF or error */
                    *nrp = nr;
                    return (NULL);
            }
            nr += nnr;

            if ((msg->msg_flags & MSG_EOR) != 0) {
                    *nrp = nr;
                    return (buf);
            }

            /* Realloc the buffer? */
            if (*buflen == (size_t)nr) {
                    buf = realloc(buf, *buflen * 2);
                    if (buf == 0) {
                            fprintf(stderr, "out of memory\n");
                            exit(1);
                    }
                    *buflen *= 2;
            }
            /* Set the next read offset */
            iov->iov_base = (char *)buf + nr;
            iov->iov_len = *buflen - nr;
     }
}

static void
echo(int fd, int socketModeone_to_many)
{
    ssize_t nr;
    struct sctp_sndrcvinfo *sri;
    struct msghdr msg[1];
    struct cmsghdr *cmsg;
    char cbuf[sizeof (*cmsg) + sizeof (*sri)];
    char *buf;
    size_t buflen;
    struct iovec iov[1];
    size_t cmsglen = sizeof (*cmsg) + sizeof (*sri);
    /* Allocate the initial data buffer */
    buflen = BUFLEN;
    if (!(buf = malloc(BUFLEN))) {
            fprintf(stderr, "out of memory\n");
            exit(1);
    }

    /* Set up the msghdr structure for receiving */
    memset(msg, 0, sizeof (*msg));
    msg->msg_control = cbuf;
    msg->msg_controllen = cmsglen;
    msg->msg_flags = 0;
    cmsg = (struct cmsghdr *)cbuf;
    sri = (struct sctp_sndrcvinfo *)(cmsg + 1);

    /* Wait for something to echo */
    while (buf = mysctp_recvmsg(fd, msg,
             buf, &buflen, &nr, cmsglen)) {

            /* Intercept notifications here */
            if (msg->msg_flags & MSG_NOTIFICATION) {
                    handle_event(buf);
                    continue;
            }

            iov->iov_base = buf;
            iov->iov_len = nr;
            msg->msg_iov = iov;
            msg->msg_iovlen = 1;

            printf("got %u bytes on stream %hu:\n", nr,
                sri->sinfo_stream);
            write(0, buf, nr);

            /* Echo it back */
            msg->msg_flags = MSG_XPG4_2;
            if (sendmsg(fd, msg, 0) < 0) {
                    perror("sendmsg");
                    exit(1);
            }
    }

    if (nr < 0) {
            perror("recvmsg");
    }
    if(socketModeone_to_many == 0)
         close(fd);
}

int main()
{
    struct sctp_event_subscribe event;
    int lfd, cfd;
    int onoff = 1;
    struct sockaddr_in sin[1];
    if ((lfd = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP)) == -1) {
            perror("socket");
            exit(1);
    }

    sin->sin_family = AF_INET;
    sin->sin_port = htons(7);
    sin->sin_addr.s_addr = INADDR_ANY;
    if (bind(lfd, (struct sockaddr *)sin, sizeof (*sin)) == -1) {
            perror("bind");
            exit(1);
    }

    if (listen(lfd, 1) == -1) {
            perror("listen");
            exit(1);
    }

    /* Wait for new associations */
    for (;;) {
            if ((cfd = accept(lfd, NULL, 0)) == -1) {
                    perror("accept");
                    exit(1);
            }

            /* Enable all events */
            event.sctp_data_io_event = 1;
            event.sctp_association_event = 1;
            event.sctp_address_event = 1;
            event.sctp_send_failure_event = 1;
            event.sctp_peer_error_event = 1;
            event.sctp_shutdown_event = 1;
            event.sctp_partial_delivery_event = 1;
            event.sctp_adaptation_layer_event = 1;
            if (setsockopt(cfd, IPPROTO_SCTP,
                SCTP_EVENTS, &event,
                sizeof(event)) != 0) {
                perror("setevent failed");
                exit(1);
            }
            /* Echo back any and all data */
            echo(cfd,0);
    }
}

Appendix B.  One-to-Many Style Code Example

The following code is a simple implementation of an echo server over SCTP. The example shows how to use some features of one-to-many style IPv4 SCTP sockets, including:

• Opening and binding of a socket
• Enabling ancillary data
• Enabling notifications
• Using ancillary data with sendmsg() and recvmsg()
• Using MSG_EOR to determine if an entire message has been read
• Handling notifications

Note most functions defined in Appendix A (One-to-One Style Code Example) are reused in this example.

int main()
{
    int fd;
    int idleTime = 2;
    struct sockaddr_in sin[1];
    struct sctp_event_subscribe event;

    if ((fd = socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP)) == -1) {
      perror("socket");
      exit(1);
    }

    sin->sin_family = AF_INET;
    sin->sin_port = htons(7);
    sin->sin_addr.s_addr = INADDR_ANY;
    if (bind(fd, (struct sockaddr *)sin, sizeof (*sin)) == -1) {
      perror("bind");
      exit(1);
    }

    /* Enable all notifications and events */
    event.sctp_data_io_event = 1;
    event.sctp_association_event = 1;
    event.sctp_address_event = 1;
    event.sctp_send_failure_event = 1;
    event.sctp_peer_error_event = 1;
    event.sctp_shutdown_event = 1;
    event.sctp_partial_delivery_event = 1;
    event.sctp_adaptation_layer_event = 1;
    if (setsockopt(fd, IPPROTO_SCTP,
        SCTP_EVENTS, &event,
        sizeof(event)) != 0) {
        perror("setevent failed");
        exit(1);
    }
    /* Set associations to auto-close in 2 seconds of
     * inactivity
     */
    if (setsockopt(fd, IPPROTO_SCTP, SCTP_AUTOCLOSE,
                      &idleTime, 4) < 0) {
      perror("setsockopt SCTP_AUTOCLOSE");
      exit(1);
    }

    /* Allow new associations to be accepted */
    if (listen(fd, 1) < 0) {
      perror("listen");
      exit(1);
    }

    /* Wait for new associations */
    while(1){
      /* Echo back any and all data */
      echo(fd,1);
    }
}

Authors' Addresses