Datagram Congestion Control Protocol (DCCP) Encapsulation for NAT Traversal (DCCP-UDP)
draft-ietf-dccp-udpencap-03

Abstract

This document specifies an alternative encapsulation of the Datagram Congestion Control Protocol (DCCP), referred to as DCCP-UDP. This encapsulation allows DCCP to be carried through the current generation of Network Address Translation (NAT) middleboxes without modification of those middleboxes. This documents also updates the SDP information for DCCP defined in RFC 5762.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on June 11, 2011.

Copyright Notice

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Table of Contents

1.  Introduction
2.  Terminology
3.  DCCP-UDP
    3.1.  The UDP Header
    3.2.  The DCCP Generic Header
    3.3.  DCCP-UDP Checksum Procedures
        3.3.1.  Partial Checksums and the Minimum Checksum Coverage Feature
    3.4.  Network Layer Options
    3.5.  Explicit Congestion Notification
    3.6.  ICMP handling for messages relating to DCCP-UDP
    3.7.  Path Maximum Transmission Unit Discovery
    3.8.  Usage of the UDP port by DCCP-UDP
    3.9.  Service Codes and the DCCP Port Registry
4.  DCCP-UDP and Higher-Layer Protocols
5.  Signaling the Use of DCCP-UDP
    5.1.  SDP support for DCCP-UDP
6.  Security Considerations
7.  IANA Considerations
8.  Acknowledgments
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Authors' Addresses

1.  Introduction

The Datagram Congestion Control Protocol (DCCP), specified in [RFC4340] (Kohler, E., Handley, M., and S. Floyd, “Datagram Congestion Control Protocol (DCCP),” March 2006.), is a transport-layer protocol that provides upper layers with the ability to use non-reliable congestion-controlled flows. The current specification for DCCP [RFC4340] (Kohler, E., Handley, M., and S. Floyd, “Datagram Congestion Control Protocol (DCCP),” March 2006.) specifies a direct encapsulation in IPv4 or IPv6 packets.

[RFC5597] (Denis-Courmont, R., “Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol,” September 2009.) specifies how DCCP should be handled by devices that use Network Address Translation (NAT) or Network Address and Port Translation (NAPT). However, there is a significant installed base of NAT/NAPT devices that do not support [RFC5597] (Denis-Courmont, R., “Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol,” September 2009.). In the short term, it would be useful to have an encapsulation for DCCP that is compatible with this installed base of NAT/NAPT devices that supports [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.), but do not support [RFC5597] (Denis-Courmont, R., “Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol,” September 2009.). This document specifies that encapsulation, which is referred to as DCCP-UDP. For convenience, the standard encapsulation for DCCP [RFC4340] (Kohler, E., Handley, M., and S. Floyd, “Datagram Congestion Control Protocol (DCCP),” March 2006.) (including [RFC5596] (Fairhurst, G., “Datagram Congestion Control Protocol (DCCP) Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal,” September 2009.) as required) is referred to as DCCP-STD.

The document also provides an updated SDP specification for DCCP, and, in this respect only, it updates the method in [RFC5762] (Perkins, C., “RTP and the Datagram Congestion Control Protocol (DCCP),” April 2010.).

The DCCP-UDP encapsulation specified in this document supports all of the features contained in DCCP-STD, but with limited functionality for partial checksums.

2.  Terminology

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.  DCCP-UDP

The basic approach is to insert a UDP [RFC0768] (Postel, J., “User Datagram Protocol,” August 1980.) header between the IP header and the DCCP packet. Note that this is not a tunneling approach. The IP addresses of the communicating end systems are carried in the IP header. The method does not embed additional IP addresses.

The method is designed to support use when these addresses are modified by a device that implements NAT/NAPT. A NAT translates the IP addresses, which impacts the transport-layer checksum. A NAPT device may also translate the port values (usually the source port). In both cases, the outer transport header that includes these values would need to be updated by the NAT/NAPT.

Devices offering or using DCCP services via DCCP-UDP encapsulation listens on a UDP port (default port, XXX IANA PORT XXX), or may bind to a specified port utilising out-of-band signalling, such as the Session Description Protocol (SDP). The DCCP-UDP server accepts incoming packets over the UDP transport and passes the received packets to the DCCP protocol module, after removing the UDP encapsulation.

A DCCP implementation MAY allow services to be simultaneously offered over any or all combinations of DCCP-STD and DCCP-UDP encapsulations with IPv4 and IPv6.

The basic format of a DCCP-UDP packet is:

 +-----------------------------------+
 |     IP Header (IPv4 or IPv6)      |  Variable length
 +-----------------------------------+
 |            UDP Header             |  8 bytes
 +-----------------------------------+
 |       DCCP Generic Header         |  12 or 16 bytes
 +-----------------------------------+
 | Additional (type-specific) Fields |  Variable length (could be 0)
 +-----------------------------------+
 |           DCCP Options            |  Variable length (could be 0)
 +-----------------------------------+
 |      Application Data Area        |  Variable length (could be 0)
 +-----------------------------------+

3.1.  The UDP Header

The format of the UDP header is specified in [RFC0768] (Postel, J., “User Datagram Protocol,” August 1980.):

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |           Dest Port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Length            |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

For DCCP-UDP, the fields are interpreted as follows:

Source and Dest(ination) Ports: 16 bits each

These fields identify the UDP ports on which the source and destination (respectively) of the packet are listening for incoming DCCP-UDP packets (both may be the default port assigned by IANA). The UDP port values do not identify the DCCP source and destination ports.

Length: 16 bits

This field is the length of the UDP datagram, including the UDP header and the payload (for DCCP-UDP, the payload is a DCCP-UDP datagram).

Checksum: 16 bits

This field is the Internet checksum of a network-layer pseudoheader and Length bytes of the UDP packet [RFC0768]. The UDP checksum must not be zero for a UDP packet that carries DCCP-UDP.

3.2.  The DCCP Generic Header

The DCCP Generic Header [RFC4340] (Kohler, E., Handley, M., and S. Floyd, “Datagram Congestion Control Protocol (DCCP),” March 2006.) takes two forms, one with long sequence numbers (48 bits) and the other with short sequence numbers (24 bits).

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |           Dest Port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data Offset  | CCVal | CsCov |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     |       |X|               |                               .
   | Res | Type  |=|   Reserved    |  Sequence Number (high bits)  .
   |     |       |1|               |                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Sequence Number (low bits)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |           Dest Port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data Offset  | CCVal | CsCov |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     |       |X|                                               |
   | Res | Type  |=|   Sequence Number (low bits)                  |
   |     |       |0|                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

All generic header fields, except for Checksum field, have the meaning specified in [RFC4340] (Kohler, E., Handley, M., and S. Floyd, “Datagram Congestion Control Protocol (DCCP),” March 2006.) updated by [RFC5596] (Fairhurst, G., “Datagram Congestion Control Protocol (DCCP) Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal,” September 2009.).

3.3.  DCCP-UDP Checksum Procedures

DCCP-UDP employs a checksum at the UDP level and eliminates the use of the DCCP checksum. This approach was chosen to enable use of current NAT/NATP traversal methods developed for UDP. Such methods will generally be unaware whether DCCP is being encapuslated and hence do not update the inner checksum in the DCCP header. Use of UDP-checksum is mandated, although this was known to increase processing for lightweight systems, since standard DCCP processing requires protection of the DCCP header fields. In addition, UDP NAT traversal does not support partial checksums, and hence although this is still permitted end-to-end in the encapsulated DCCP datagram, links along the path will treat these as UDP packets and can not enable special partial checksum processing.

For DCCP-UDP, the function of the DCCP Checksum field is performed by the UDP checksum field. On transmit, the DCCP Checksum field SHOULD be set to zero. On receive, the DCCP Checksum field MUST be ignored.

The UDP checksum MUST NOT be zero for a UDP packet that is sent using DCCP-UDP. If the received UDP Checksum field is zero, the packet MUST be dropped.

If the UDP Length field is less than 20 (UDP Header length and minimum DCCP-UDP header length), the packet MUST be dropped.

If the UDP Checksum field, computed using standard UDP methods, is invalid, the packet MUST be dropped.

If the UDP Length field in a received packet is less than the length of the UDP header plus the entire DCCP-UDP header (including the generic header and type-specific fields and options, if present), or the UDP Length field is greater than the length of the packet from the beginning of the UDP header to the end of the packet, the packet MUST be dropped.

3.3.1.  Partial Checksums and the Minimum Checksum Coverage Feature

DCCP-UDP supports the syntax of partial checksums. It also supports negotiation of the Minimum Checksum Coverage feature and settings of the CsCov field. However, since the UDP checksum field in DCCP-UDP always covers the entire DCCP datagram, an application that enables this feature will experience a service that is functionally identical to using full checksum coverage.

3.4.  Network Layer Options

A DCCP-UDP implementations MAY transfer network-layer options intended for DCCP to the network-layer header of the encapsulating UDP packet.

A DCCP-UDP endpoint that receives IP-options for the encapsulating UDP packet MAY forward these to the DCCP protocol module. If teh endpoints forwards a specific network layer option to the DCCP module, it MUST also forward all susbequent packets with this option. Consistent forwarding is essential for correct operation of many end-to-end options.

3.5.  Explicit Congestion Notification

A DCCP-UDP endpoint SHOULD follow the procedures of DCCP-STD section 12 by setting the ECN fields in the IP Headers of outgoing packets and examining the values received in the ECN fields of incoming IP packets, relaying any packet markings to the DCCP module.

Implementations that do not support ECN MUST follow the procedures in DCCP-STD section 12.1 with regard to implementations that are not ECN capable.

3.6.  ICMP handling for messages relating to DCCP-UDP

To allow ICMP messages to be demultiplexed by the receiving endpoint, part of the original packet that resulted in the message is included in the payload of the ICMP error message. The receiving endpoint can therefore use this information to associate the ICMP error with the transport protocol instance that resulted in the ICMP message. When DCCP-UDP is used, the error message and the payload of the ICMP error message relate to the UDP transport.

DCCP-UDP endpoints SHOULD forward ICMP messages relating to a UDP packet that carry DCCP-UDP to the DCCP module. This may imply translation of the payload of the ICMP message into a form that is recognised by the DCCP stack. [ICMP] (Gont, “"ICMP attacks against TCP", IETF Work-in-Progress.,” .) describes precautions that are desirable before TCP acts on the receipt of an ICMP message. Similar precautions are desirable prior to forwarding by DCCP-UDP to the DCCP module.

3.7.  Path Maximum Transmission Unit Discovery

DCCP-UDP implementations SHOULD follow DCCP-STD section 14 with regard to determining the maximum packet size and the use of Path Maximum Transmission Unit Discovery (PMTUD).

3.8.  Usage of the UDP port by DCCP-UDP

A DCCP-UDP endpoint MAY use any UDP port number, providing the active endpoint knows a valid UDP Destination Port on the passive endpoint.

By default, the DCP-UDP client sets the source and destination ports to the default port number. UDP port number XXX IANA PORT XXX has been registered with IANA for this purpose.

A DCCP-UDP server (that is, an initially passive endpoint that wishes to receive DCCP-Request packets [RFC4340] over DCCP-UDP) binds a UDP port number for all encapsulated DCCP connections. If the DCCP-UDP server binds to this default port, it SHOULD accept datagrams from any UDP source port. For example, this would be needed if a NAPT along the path had translated the original UDP source port.

There is a risk that the same DCCP source port number will be used by two endpoints each behind a NAPT. A DCCP-UDP endpoint SHOULD therefore demultiplex a DCCP-UDP flow using both the UDP source and destination port numbers in addition to processing of the DCCP ports by the DCCP module. Hence, the endpoint identifier for a DCCP-UDP connection should be the 6-tuple <source address, UDP Source Port, DCCP Source Port, destination address, UDP Destination Port, DCCP Destination Port>, rather than a 4-tuple <source address, source port, destination address, destination port> defined by DCCP-STD.

3.9.  Service Codes and the DCCP Port Registry

This section clarifies the usage of DCCP Service Codes or the registration of server ports by DCCP-UDP. The section is not intended to update the procedures for allocating Service Codes or server ports.

There is one Service Code registry and one DCCP port registration that apply to all combinations of encapsulation and IP version. A DCCP Service Code specifies an application using DCCP regardless of the combination of DCCP encapsulation and IP version. An application may choose not to support some combinations of encapsulation and IP version, but its Service Code will remain registered for those combinations and the Service Code must not be used by other applications. An application should not register different Service Codes for different combinations of encapsulation and IP version. [RFC5595] (Fairhurst, G., “The Datagram Congestion Control Protocol (DCCP) Service Codes,” September 2009.) provides additional information about DCCP Service Codes.

Similarly, a port registration is applicable to all combinations of encapsulation and IP version. Again, an application may choose not to support some combinations of encapsulation and IP version on its registered port, although the port will remain registered for those combinations. Applications should not register different ports just for the purpose of using different combinations of encapsulation.

4.  DCCP-UDP and Higher-Layer Protocols

In general, the encapsulation of a higher-layer protocol within DCCP SHOULD be the same for both DCCP-STD and DCCP-UDP. Encapsulations of DTLS over DCCP is defined in [RFC5238] (Phelan, T., “Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP),” May 2008.) and RTP over DCCP is defined in [RFC5762] (Perkins, C., “RTP and the Datagram Congestion Control Protocol (DCCP),” April 2010.). This document does not update these encapsulations when using DCCP-UDP.

Higher-layer protocols that require a different encapsulation for DCCP-UDP MUST justify the reasons for the difference and MUST specify the encapsulations for both DCCP-STD and DCCP-UDP. If a document does not specify different encapsulations for DCCP-STD and DCCP-UDP, the specified encapsulation SHALL apply to both DCCP-STD and DCCP-UDP.

5.  Signaling the Use of DCCP-UDP

Applications often signal transport connection parameters through outside means, such as SDP. Applications that define such methods for DCCP MUST define how the DCCP encapsulation is chosen, and MUST allow either encapsulation to be signaled.

5.1.  SDP support for DCCP-UDP

[RFC5762] (Perkins, C., “RTP and the Datagram Congestion Control Protocol (DCCP),” April 2010.) defines SDP extensions for signaling RTP over DCCP connections. Since it predates this document, it does not define a method for determining the DCCP encapsulation type. This document updates [RFC5762] (Perkins, C., “RTP and the Datagram Congestion Control Protocol (DCCP),” April 2010.) to add a method for determining the DCCP encapsulation type.

A new SDP attribute "dccp-encap" is defined for signaling the DCCP encapsulation according to the following ABNF [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.):

    dccp-encap-attr = %x61 "=dccp-in-udp" [":" udp-port-num]

    udp-port-num    = *DIGIT

where *DIGIT is as defined in [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).

The presence of "a=dccp-in-udp" in an SDP offer indicates that the offerer is listening for DCCP-UDP connections on the indicated UDP port (if udp-port-num is included) or on the default port for the DCCP-UDP service if no port is included.

The absence of "a=dccp-in-udp" in an SDP offer indicates that the offerer is listening for DCCP-STD connections. The presence of "a=dccp-in-udp" conveys no information about whether or not the offerer is listening for DCCP-STD connections.

6.  Security Considerations

DCCP-UDP provides all of the security risk-mitigation measures present in DCCP-STD, and also all of the security risks.

The purpose of DCCP-UDP is to allow DCCP to pass through NAT/NAPT devices, and therefore it exposes DCCP to the risks associated with passing through NAT devices. It does not create any new risks with regard to NAT/NAPT devices.

The tunnel encapsulation recommends processing of ICMP messages received for packets swent using DCCP-UDP and translation to allow use by DCCP. [ICMP] (Gont, “"ICMP attacks against TCP", IETF Work-in-Progress.,” .) describes precautions that are desirable before TCP acts on receipt of ICMP messages. Similar precautions are desirable for endpoints processing ICMP for DCCP-UDP.

DCCP-UDP may also allow DCCP applications to pass through existing firewall devices, if the administrators of the devices so choose. A simple use may either allow all DCCP applications or allow none.

A firewall than interprets this specification could inspect the encapsualted DCCP header to filter based on DCCP information. Full control of DCCP conenctions by application will require enhancements to firewalls, as discussed in [RFC4340] (Kohler, E., Handley, M., and S. Floyd, “Datagram Congestion Control Protocol (DCCP),” March 2006.) and related RFCs (e.g. [RFC5595] (Fairhurst, G., “The Datagram Congestion Control Protocol (DCCP) Service Codes,” September 2009.)).

7.  IANA Considerations

This document requests IANA to allocate a UDP port for the dccp-udp service.

XXX Note: IANA is requested to replace all occurrances of "XXX IANA PORT XXX" by the allocated port value prior to publication. XXX

IANA is also requested to allocate the following new SDP attribute ("att-field"):

Contact name: Tom Phelan <tphelan@sonusnet.com>

Attribute name: dccp-in-udp

Long-form attribute name in English: DCCP in UDP Encapsulation

Type of attribute: Media level

Subject to charset attribute? No

Purpose of the attribute: See this document section Section 5.1 (SDP support for DCCP-UDP)

Allowed attribute values: See this document section Section 5.1 (SDP support for DCCP-UDP)

8.  Acknowledgments

This document was produced by the DCCP WG. The following contributed during the working group last call:

Andrew Lenvorski, Lloyd Wood, Pasi Sarolahti, Gerrit Renker, Eddie Kohler, Collin Perkins, Gorry Fairhurst and Tom Phelan.

9.  References

9.1. Normative References

9.2. Informative References

Authors' Addresses