General Internet Signaling Transport (GIST) over SCTP and Datagram TLS
draft-ietf-nsis-ntlp-sctp-08.txt

Abstract

The General Internet Signaling Transport (GIST) protocol currently uses TCP or TLS over TCP for connection mode operation. This document describes the usage of GIST over the Stream Control Transmission Protocol (SCTP) and Datagram Transport Layer Security (DTLS). The use of SCTP can take advantage of features provided by SCTP, namely streaming-based transport, support of multiple streams to avoid head of line blocking, the support of multi-homing to provide network level fault tolerance, as well as partial reliability extension for partially reliable data transmission. This document also specifies how to establish GIST security over datagram transport protocols using an extension to DTLS.

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Table of Contents

1.  Introduction
2.  Terminology and Abbreviations
3.  GIST Over SCTP
    3.1.  Message Association Setup
        3.1.1.  Overview
        3.1.2.  Protocol-Definition: Forwards-SCTP
    3.2.  Effect on GIST State Maintenance
    3.3.  PR-SCTP Support
    3.4.  API between GIST and NSLP
4.  Bit-Level Formats
    4.1.  MA-Protocol-Options
5.  Application of GIST over SCTP
    5.1.  Multi-homing support of SCTP
    5.2.  Streaming support in SCTP
6.  NAT Traversal Issue
7.  Use of DTLS with GIST
8.  Security Considerations
9.  IANA Considerations
10.  Acknowledgments
11.  References
    11.1.  Normative References
    11.2.  Informative References
§  Authors' Addresses

1.  Introduction

This document describes the usage of the General Internet Signaling Transport (GIST) protocol [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.) over the Stream Control Transmission Protocol (SCTP) [2] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).

GIST, in its initial specification for connection mode operation, runs on top of a byte-stream oriented transport protocol providing a reliable, in-sequence delivery, i.e., using the Transmission Control Protocol (TCP) [7] (Postel, J., “Transmission Control Protocol,” September 1981.) for signaling message transport. However, some NSLP context information has a definite lifetime, therefore, the GIST transport protocol could benefit from flexible retransmission, so stale NSLP messages that are held up by congestion can be dropped. Together with the head-of-line blocking issue and other issues with TCP, these considerations argue that implementations of GIST should support the Stream Control Transport Protocol (SCTP)[2] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) as an optional transport protocol for GIST, especially if deployment over the public Internet is contemplated. Like TCP, SCTP supports reliability, congestion control and fragmentation. Unlike TCP, SCTP provides a number of functions that are desirable for signaling transport, such as multiple streams and multiple IP addresses for path failure recovery. Furthermore, SCTP offers an advantage of message-oriented transport instead of using the byte stream oriented TCP where one has to provide its own framing mechanisms. In addition, its Partial Reliability extension (PR-SCTP) [3] (Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad, “Stream Control Transmission Protocol (SCTP) Partial Reliability Extension,” May 2004.) supports partial retransmission based on a programmable retransmission timer. Furthermore, Datagram Transport Layer Security (DTLS) [4] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) provides a viable solution for securing datagram transport protocols, e.g., by using DTLS over SCTP [5] (Tuexen, M., Seggelmann, R., and E. Rescorla, “Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP),” March 2010.).

This document defines the use of SCTP as a transport protocol and the use of DTLS as a security mechanism for GIST Messaging Associations and discusses the implications on GIST State Maintenance and API between GIST and NSLPs. Furthermore, this document shows how GIST SHOULD be used to provide the additional features offered by SCTP to deliver the GIST C-mode messages (which can in turn carry NSIS Signaling Layer Protocol (NSLP) [8] (Hancock, R., Karagiannis, G., Loughney, J., and S. Van den Bosch, “Next Steps in Signaling (NSIS): Framework,” June 2005.) messages as payload). More specifically:

• How to use the multiple streams feature of SCTP.
• How to use the PR-SCTP extension of SCTP.
• How to take advantage of the multi-homing support of SCTP.

The method described in this document does not require any changes of GIST or SCTP. However, SCTP implementations MUST support the optional feature of fragmentation of SCTP user messages.

Additionally, this document specifies the use of DTLS for securing GIST over datagram transport protocols such as SCTP.

2.  Terminology and Abbreviations

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 [6] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.). Other terminologies and abbreviations used in this document are taken from related specifications (e.g., [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.) and [2] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.)) as follows:

• SCTP - Stream Control Transmission Protocol
• PR-SCTP - SCTP Partial Reliability Extension
• MRM - Message Routing Method
• MRI - Message Routing Information
• MRS - Message Routing State
• SCD - Stack Configuration Data
• MA - A GIST Messaging Association is a single connection between two explicitly identified GIST adjacent peers on the data path. A messaging association may use a specific transport protocol and known ports. If security protection is required, it may use a specific network layer security association, or use a transport layer security association internally. A messaging association is bidirectional; signaling messages can be sent over it in either direction, and can refer to flows of either direction.
• SCTP Association - A protocol relationship between SCTP endpoints, composed of the two SCTP endpoints and protocol state information. An association can be uniquely identified by the transport addresses used by the endpoints in the association. All transport addresses used by an SCTP endpoint must use the same port number, but can use multiple IP addresses. A transport address used by an SCTP endpoint must not be used by another SCTP endpoint. In other words, a transport address is unique to an SCTP endpoint. Two SCTP endpoints MUST NOT have more than one SCTP association between them at any given time [2] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).
• Stream - A sequence of user messages that are to be delivered to the upper-layer protocol in order with respect to other messages within the same stream.

3.  GIST Over SCTP

3.1.  Message Association Setup

3.1.1.  Overview

The basic GIST protocol specification defines two possible protocols to be used in Messaging Associations, namely Forwards-TCP and TLS. This document adds Forwards-SCTP as another possible protocol. In Forwards-SCTP, analog to Forwards-TCP, connections between peers are opened in the forwards direction, from the querying node, towards the responder.

A new MA-Protocol-ID type, "Forwards-SCTP", is defined in this document for using SCTP as GIST transport protocol. A formal definition of Forwards-SCTP is given in the following section.

3.1.2.  Protocol-Definition: Forwards-SCTP

This MA-Protocol-ID denotes a basic use of SCTP between peers. Support for this protocol is OPTIONAL. If this protocol is offered, MA-protocol-options data MUST also be carried in the SCD object. The MA-protocol-options field formats are:

• in a Query: no information apart from the field header.
• in a Response: 2 byte port number at which the connection will be accepted, followed by 2 pad bytes.

The connection is opened in the forwards direction, from the querying node towards the responder. The querying node MAY use any source address and source port. The destination for establishing the message association MUST be derived from information in the Response: the address from the interface- address from the Network-Layer-Information object and the port from the SCD object as described above.

Associations using Forwards-SCTP can carry messages with the transfer attribute Reliable=True. If an error occurs on the SCTP connection such as a reset, as can be detected for example by a socket exception condition, GIST MUST report this to NSLPs as discussed in Section 4.1.2 of [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.).

3.2.  Effect on GIST State Maintenance

This document defines the use of SCTP as a transport protocol for GIST Messaging Associations. As SCTP provides additional functionality over TCP, this section dicusses the implications of using GIST over SCTP on GIST State Maintenance.

While SCTP defines uni-directional streams, for the purpose of this document, the concept of a bi-direction stream is used. Implementations MUST establish downstream and upstream (uni-directional) SCTP streams always together and use the same stream identifier in both directions. Thus, the two uni-directional streams (in opposite directions) form a bi-directional stream.

Due to the multi-streaming support of SCTP, it is possible to use different SCTP streams for different resources (e.g., different NSLP sessions), rather than maintaining all messages along the same transport connection/association in a correlated fashion as TCP (which imposes strict (re)ordering and reliability per transport level). However, there are limitations to the use of multi-streaming. All GIST messages for a particular session MUST be sent over the same SCTP stream to assure the NSLP assumption of in-order delivery. Multiple sessions MAY share the same SCTP stream based on local policy.

The GIST concept of Messaging Association re-use is not affected by this document or the use of SCTP. All rules defined in the GIST specification remain valid in the context of GIST over SCTP.

3.3.  PR-SCTP Support

A variant of SCTP, PR-SCTP [3] (Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad, “Stream Control Transmission Protocol (SCTP) Partial Reliability Extension,” May 2004.) provides a "timed reliability" service, which would be particular useful for delivering GIST Connection mode messages. It allows the user to specify, on a per message basis, the rules governing how persistent the transport service should be in attempting to send the message to the receiver. Because of the chunk bundling function of SCTP, reliable and partial reliable messages can be multiplexed over a single PR-SCTP association. Therefore, a GIST over SCTP implementation SHOULD attempt to establish a PR-SCTP association using "timed reliability" service instead of a standard SCTP association, if available, to support more flexible transport features for potential needs of different NSLPs.

In a standard SCTP, instead, if a node has sent the first transmission before the lifetime expires, then the message MUST be sent as a normal reliable message. During episodes of congestion this is particularly unfortunate, as retransmission wastes bandwidth that could have been used for other (non-lifetime expired) messages. The "timed reliability" service in PR-SCTP eliminates this issue and is hence RECOMMENDED to be used for GIST over PR-SCTP.

3.4.  API between GIST and NSLP

GIST specification defines an abstract API between GIST and NSLPs. While this document does not change the API itself, the semantics of some parameters have slightly different interpretation in the context of SCTP. This section only lists those primitives and parameters, that need special consideration when used in the context of SCTP. The relevant primitives from [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.) are as follows:

• The Timeout parameter in API "SendMessage": According to [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.), this parameter represents the "length of time GIST should attempt to send this message before indicating an error." When used with PR-SCTP, this parameter is used as the timeout for the "timed reliability" service of PR-SCTP.
• "NetworkNotification": According to [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.), this primitive "is passed from GIST to a signalling application. It indicates that a network event of possible interest to the signalling application occurred." Here, if SCTP detects a failure of the primary path, GIST SHOULD also indicate this event to the NSLP by calling this primitive with Network-Notification-Type "Routing Status Change". This notification should be done even if SCTP was able to remain an open connection to the peer due to its multi-homing capabilities.

4.  Bit-Level Formats

4.1.  MA-Protocol-Options

This section provides the bit-level format for the MA-protocol-options field that is used for SCTP protocol in the Stack-Configuration-Data object of GIST.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:       SCTP port number        |         Reserved              :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

SCTP port number  = Port number at which the responder will accept
                    SCTP connections

The SCTP port number is only supplied if sent by the responder.

5.  Application of GIST over SCTP

5.1.  Multi-homing support of SCTP

In general, the multi-homing support of SCTP can be used to improve fault-tolerance in case of a path- or link-failure. Thus, GIST over SCTP would be able to deliver NSLP messages between peers even if the primary path is not working anymore. However, for the Message Routing Methods (MRMs) defined in the basic GIST specification such a feature is only of limited use. The default MRM is path-coupled, which means, that if the primary path is failing for the SCTP association, it most likely is also for the IP traffic that is signaled for. Thus, GIST would need to perform a refresh anyway to cope with the route change. Nevertheless, the use of the multi-homing support of SCTP provides GIST and the NSLP with another source to detect route changes. Furthermore, for the time between detection of the route change and recovering from it, the alternative path offered by SCTP can be used by the NSLP to make the transition more smoothly. Finally, future MRMs might have different properties and therefore benefit from multi-homing more broadly.

5.2.  Streaming support in SCTP

Streaming support in SCTP is advantageous for GIST. It allows better parallel processing, in particular by avoiding head of line blocking issue in TCP. Since a same GIST MA may be reused by multiple sessions, using TCP as transport GIST signaling messages belonging to different sessions may be blocked if another message is dropped. In the case of SCTP, this can be avoided as different sessions having different requirements can belong to different streams, thus a message loss or reordering in a stream will only affect the delivery of messages within that particular stream, and not any other streams.

6.  NAT Traversal Issue

NAT traversal for GIST over SCTP will follow Section 7.2 of [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.) and the GIST extensibility capabilities defined in [9] (Manner, J., Bless, R., Loughney, J., and E. Davies, “Using and Extending the NSIS Protocol Family,” April 2010.). This specification does not define NAT traversal procedure for GIST over SCTP, although an approach for SCTP NAT traversal is described in [10] (Stewart, R., Tuexen, M., and I. Ruengeler, “Stream Control Transmission Protocol (SCTP) Network Address Translation,” December 2009.).

7.  Use of DTLS with GIST

The MA-Protocol-ID for DTLS denotes a basic use of datagram transport layer channel security, initially in conjunction with SCTP. It provides authentication, integrity and optionally replay protection for control packets. The use of DTLS for securing GIST over SCTP allows GIST to take the advantage of features provided by SCTP and its extensions. Note replay protection is not available for DTLS over SCTP [5] (Tuexen, M., Seggelmann, R., and E. Rescorla, “Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP),” March 2010.). The usage of DTLS for GIST over SCTP is similar to TLS for GIST as specified in [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.), where a stack-proposal containing both MA-Protocol-IDs for SCTP and DTLS during the GIST handshake phase.

GIST message associations using DTLS may carry messages with transfer attributes requesting confidentiality or integrity protection. The specific DTLS version will be negotiated within the DTLS layer itself, but implementations MUST NOT negotiate to protocol versions prior to DTLS v1.0 and MUST use the highest protocol version supported by both peers. GIST nodes supporting DTLS MUST be able to negotiate the DTLS NULL and block cipher ciphers and SHOULD be able to negotiate the new cipher suites. They MAY negotiate any mutually acceptable ciphersuite that provides authentication, integrity, and confidentiality. The same rules for negotiating TLS cipher suites as specified in Section 5.7.3 of [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.) apply.

No MA-protocol-options field is required for DTLS. The configuration information for the transport protocol over which DTLS is running (e.g. SCTP port number) is provided by the MA-protocol-options for that protocol.

8.  Security Considerations

The security considerations of [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.), [2] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) and [4] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) apply. Following [5] (Tuexen, M., Seggelmann, R., and E. Rescorla, “Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP),” March 2010.), replay detection of DTLS over SCTP is not supported.

The usage of DTLS [4] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) for securing GIST over datagram transport protocols MUST be implemented and SHOULD be used. An implementation of GIST over SCTP with no PR-SCTP support MAY use TLS for its channel security, when DTLS is not available between two GIST peers.

9.  IANA Considerations

This specification extends [1] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.) by introducing two additional MA-Protocol-IDs:

  +---------------------+------------------------------------------+
  | MA-Protocol-ID      | Protocol                                 |
  +---------------------+------------------------------------------+
  | 3                   | SCTP opened in the forwards direction    |
  |                     |                                          |
  | 4                   | DTLS initiated in the forwards direction |
  +---------------------+------------------------------------------+

10.  Acknowledgments

The authors would like to thank John Loughney, Robert Hancock, Andrew McDonald, Martin Stiemerling, Fang-Chun Kuo, Jan Demter, Lauri Liuhto, Michael Tuexen, and Roland Bless for their helpful suggestions.

11.  References

11.1. Normative References

11.2. Informative References

Authors' Addresses