Network Working Group C. Bormann Request for Comments: 5049 Universitaet Bremen TZI Category: Standards Track Z. Liu Nokia Research Center R. Price EADS Defence and Security Systems Limited G. Camarillo, Ed. Ericsson December 2007 Applying Signaling Compression (SigComp) to the Session Initiation Protocol (SIP) Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
AbstractThis document describes some specifics that apply when Signaling Compression (SigComp) is applied to the Session Initiation Protocol (SIP), such as default minimum values of SigComp parameters, compartment and state management, and a few issues on SigComp over TCP. Any implementation of SigComp for use with SIP must conform to this document and SigComp, and in addition, support the SIP and Session Description Protocol (SDP) static dictionary.
1. Introduction ....................................................3 2. Terminology .....................................................3 3. Compliance with This Specification ..............................3 4. Minimum Values of SigComp Parameters for SIP/SigComp ............3 4.1. decompression_memory_size (DMS) for SIP/SigComp ............4 4.2. state_memory_size (SMS) for SIP/SigComp ....................4 4.3. cycles_per_bit (CPB) for SIP/SigComp .......................5 4.4. SigComp_version (SV) for SIP/SigComp .......................5 4.5. locally available state (LAS) for SIP/SigComp ..............5 5. Delimiting SIP Messages and SigComp Messages on the Same Port ...5 6. Continuous Mode over TCP ........................................6 7. Too-Large SIP Messages ..........................................7 8. SIP Retransmissions .............................................7 9. Compartment and State Management for SIP/SigComp ................7 9.1. Remote Application Identification ..........................8 9.2. Identifier Comparison Rules ...............................10 9.3. Compartment Opening and Closure ...........................11 9.4. Lack of a Compartment .....................................13 10. Recommendations for Network Administrators ....................13 11. Private Agreements ............................................14 12. Backwards Compatibility .......................................14 13. Interactions with Transport Layer Security (TLS) ..............14 14. Example .......................................................15 15. Security Considerations .......................................17 16. IANA Considerations ...........................................17 17. Acknowledgements ..............................................17 18. References ....................................................18 18.1. Normative References .....................................18 18.2. Informative References ...................................19
RFC3320] is a solution for compressing messages generated by application protocols. Although its primary driver is to compress SIP [RFC3261] messages, the solution itself has been intentionally designed to be application agnostic so that it can be applied to any application protocol; this is denoted as ANY/SigComp. Consequently, many application-dependent specifics are left out of the base standard. It is intended that a separate specification be used to describe those specifics when SigComp is applied to a particular application protocol. This document binds SigComp and SIP; this is denoted as SIP/SigComp. RFC2119]. RFC3320]. Additionally, it must support the SIP/SDP static dictionary, as specified in [RFC3485], and the mechanism for discovering SigComp support at the SIP layer, as specified in [RFC3486]. Section 3.3 of [RFC3320]). For each parameter, [RFC3320] specifies a minimum value that any SigComp endpoint MUST support for ANY/SigComp. Those minimum values were determined with the consideration of all imaginable devices in which SigComp may be implemented. Scalability was also considered as a key factor. However, some of the minimum values specified in [RFC3320] are too small to allow good performance for SIP message compression. Therefore, they are increased for SIP/SigComp as specified in the following sections. For completeness, those parameters that are the same for SIP/SigComp as they are for ANY/SigComp are also listed. The new minimum values are specific to SIP/SigComp and, thus, do not apply to any other application protocols. A SIP/SigComp endpoint MAY offer additional resources over and above the minimum values
specified in this document if available; these resources can be advertised to remote endpoints as described in Section 9.4.9 of [RFC3320]. Section 3.3.1 of [RFC3320]. Minimum value for SIP/SigComp: 8192 bytes. Reason: a DMS of 2048 bytes is too small for SIP message compression as it seriously limits the compression ratio and even makes compression impossible for certain messages. For example, the condition set by [RFC3320] for SigComp over UDP means: C + 2*B + R + 2*S + 128 < DMS (each term is described below). Therefore, if DMS is too small, at least one of C, B, R, or S will be severely restricted. On the other hand, DMS is memory that is only temporarily needed during decompression of a SigComp message (the memory can be reclaimed when the message has been decompressed). Therefore, a requirement of 8 KB should not cause any problems for an endpoint that already implements SIP, SigComp, and applications that use SIP. C size of compressed application message, depending on R B size of bytecode. Note: two copies -- one as part of the SigComp message and one in UDVM (Universal Decompressor Virtual Machine) memory. R size of circular buffer in UDVM memory S any additional state uploaded other than that created from the content of the circular buffer at the end of decompression (similar to B, two copies of S are needed) 128 the smallest address in UDVM memory to copy bytecode to Section 3.3.1 of [RFC3320]. Minimum value for SIP/SigComp: 2048 bytes. Reason: a non-zero SMS allows an endpoint to upload a state in the first SIP message sent to a remote endpoint without the uncertainty of whether the remote endpoint will have enough memory to store such a state. A non-zero SMS obviously requires the SIP/SigComp implementation to keep state. Based on the observation that there is little gain from stateless SigComp compression, the assumption is that purely stateless SIP implementations are unlikely to provide a
SigComp function. Stateful implementations should have little problem to keep 2K additional state for each compartment (see Section 9). Note: SMS is a parameter that applies to each individual compartment. An endpoint MAY offer different SMS values for different compartments as long as the SMS value is not less than 2048 bytes. Section 3.3.1 of [RFC3320]. Minimum value for SIP/SigComp: 16 (same as above). Section 3.3.2 of [RFC3320]. For SIP/SigComp: >= 0x02 (at least SigComp + NACK). Note that this implies that the provisions of [RFC4077] apply. That is, decompression failures result in SigComp NACK messages sent back to the originating compressor. It also implies that the compressor need not make use of the methods detailed in Section 2.4 of [RFC4077] (Detecting Support for NACK); for example, it can use optimistic compression methods right from the outset. Section 3.3.3 of [RFC3320]. Minimum LAS for SIP/SigComp: the SIP/SDP static dictionary as defined in [RFC3485]. Note that, since support for the static SIP/SDP dictionary is mandatory, it does not need to be advertised.
checks the first octet of the UDP/SCTP payload to determine whether the message has been compressed using SigComp. If the MSBs (Most Significant Bits) of the octet are "11111", then the message is considered to be a SigComp message and is parsed as per [RFC3320]. If the MSBs of the octet take any other value, then the message is assumed to be an uncompressed SIP message, and it is passed directly to the application with no further effect on the SigComp layer. For a stream-based transport such as TCP, distinguishing between SigComp and non-SigComp messages has to be done per connection. The receiving endpoint checks the first octet of the TCP data stream to determine whether the stream has been compressed using SigComp. If the MSBs of the octet are "11111", then the stream is considered to contain SigComp messages and is parsed as per [RFC3320]. If the MSBs of the octet take any other value, then the stream is assumed to contain uncompressed SIP messages, and it is passed directly to the application with no further effect on the SigComp layer. Note that SigComp message delimiters MUST NOT be used if the stream contains uncompressed SIP messages. Applications MUST NOT mix SIP messages and SigComp messages on a single TCP connection. If the TCP connection is used to carry SigComp messages, then all messages sent over the connection MUST have a SigComp header and be delimited by the use of 0xFFFF, as described in [RFC3320]. Section 11 of [RFC4896] details a simple set of bytecodes, intended to be "well-known", that implement a null decompression algorithm. These bytecodes effectively allow SigComp peers to send selected SigComp messages with uncompressed data. If a SIP implementation has reason to send both compressed and uncompressed SIP messages on a single TCP connection, the compressor can be instructed to use these bytecodes to send uncompressed SIP messages that are also valid SigComp messages.
RFC4122] allows for non-centralized computation of a URN based on time, unique names (such as a Media Access Control (MAC) address), or a random number generator. If a URN scheme other than UUID is used, the URN MUST be selected such that the application can be certain that no other SIP/SigComp application would choose the same URN value. Note that the definition of SIP/SigComp identifier is similar to the definition of instance identifier in [OUTBOUND]. One difference is that instance identifiers are only required to be unique within their AoR (Address of Record) while SIP/SigComp identifiers are required to be globally unique. Even if instance identifiers are only required to be unique within their AoR, devices may choose to generate globally unique instance identifiers. A device with a globally unique instance identifier SHOULD use its instance identifier as its SIP/SigComp identifier. Note: Using the same value for an entity's instance and SIP/SigComp identifiers improves the compression ratio of header fields that carry both identifiers (e.g., a Contact header field in a REGISTER request). Server farms that share SIP/SigComp state across servers MUST use the same SIP/SigComp identifier for all their servers. SIP/SigComp identifiers are carried in the 'sigcomp-id' SIP URI (Uniform Resource Identifier) or Via header field parameter. The 'sigcomp-id' SIP URI parameter is a 'uri-parameter', as defined by the SIP ABNF (Augmented Backus-Naur Form, Section 25.1 of [RFC3261]). The following is its ABNF [RFC4234]: uri-sip-sigcomp-id = "sigcomp-id=" 1*paramchar
The SIP URI 'sigcomp-id' parameter MUST contain a URN [RFC2141]. The Via 'sigcomp-id' parameter is a 'via-extension', as defined by the SIP ABNF (Section 25.1 of [RFC3261]). The following is its ABNF [RFC4234]: via-sip-sigcomp-id = "sigcomp-id" EQUAL LDQUOT *( qdtext / quoted-pair ) RDQUOT The Via 'sigcomp-id' parameter MUST contain a URN [RFC2141]. The following is an example of a 'sigcomp-id' SIP URI parameter: sigcomp-id=urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128 The following is an example of a Via header field with a 'sigcomp-id' parameter: Via: SIP/2.0/UDP server1.example.com:5060 ;branch=z9hG4bK87a7 ;comp=sigcomp ;sigcomp-id="urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128" The following is an example of a REGISTER request that carries 'sigcomp-id' parameters in a Via entry and in the Contact header field. Additionally, it also carries a '+sip.instance' Contact header field parameter. REGISTER sip:example.net SIP/2.0 Via: SIP/2.0/UDP 192.0.2.247:2078;branch=z9hG4bK-et736vsjirav; rport;sigcomp-id="urn:uuid:2e5fdc76-00be-4314-8202-1116fa82a473" From: "Joe User" <sip:firstname.lastname@example.org>;tag=6to4gh7t5j To: "Joe User" <sip:email@example.com> Call-ID: 3c26700c1adb-lu1lz5ri5orr CSeq: 215196 REGISTER Max-Forwards: 70 Contact: <sip:firstname.lastname@example.org:2078; sigcomp-id=urn:uuid:2e5fdc76-00be-4314-8202-1116fa82a473>; q=1.0; expires=3600; +sip.instance="<urn:uuid:2e5fdc76-00be-4314-8202-1116fa82a473>" Content-Length: 0 SIP messages are matched with remote application identifiers as follows: Outgoing requests: the remote application identifier is the SIP/ SigComp identifier of the URI to which the request is sent. If the URI does not contain a SIP/SigComp identifier, the remote
application identifier is the IP address plus port of the datagram carrying the request for connectionless transport protocols, and the transport connection (e.g., a TCP connection) carrying the request for connection-oriented transport protocols (this is to support legacy SIP/SigComp applications). Incoming responses: the remote application identifier is the same as that of the previously sent request that initiated the transaction to which the response belongs. Incoming requests: the remote application identifier is the SIP/ SigComp identifier of the top-most Via entry. If the Via header field does not contain a SIP/SigComp identifier, the remote application identifier is the source IP address plus port of the datagram carrying the request for connectionless transport protocols, and the transport connection (e.g., a TCP connection) carrying the request for connection-oriented transport protocols (this is to support legacy SIP/SigComp applications). Outgoing responses: the remote application identifier is the same as that of the previously received request that initiated the transaction to which the response belongs. Note that, due to standard SIP Via header field processing, this identifier will be present in the top-most Via entry in such responses (as long as it was present in the top-most Via entry of the previously received request). A SIP/SigComp application placing its URI with the 'comp=sigcomp' parameter in a header field MUST add a 'sigcomp-id' parameter with its SIP/SigComp identifier to that URI. A SIP/SigComp application generating its own Via entry containing the 'comp=sigcomp' parameter MUST add a 'sigcomp-id' parameter with its SIP/SigComp identifier to that Via entry. A given remote application identifier is mapped to a particular SigComp compartment ID following the rules given in Section 9.3. RFC2141]. Lexical equality may result in two URNs being considered unequal when they are actually equal. In this specific usage of URNs, the only element that provides the URN is the SIP/SigComp application identified by
that URN. As a result, the SIP/SigComp application SHOULD provide lexically equivalent URNs in each registration it generates. This is likely to be normal behavior in any case; applications are not likely to modify the value of their SIP/SigComp identifiers so that they remain functionally equivalent yet lexicographically different from previous identifiers. Section 6.2 of [RFC3320], a logical deletion can become a physical deletion only when no compartment continues to exist that created the (same) state. A SigComp endpoint may offer to keep a state created upon request from a SigComp peer endpoint beyond the default lifetime of a compartment (i.e., beyond the duration of its associated registration). This may be used to improve compression efficiency of subsequent SIP messages generated by the same remote application at the SigComp peer endpoint. To indicate that such state will continue to be available, the SigComp endpoint can inform its peer SigComp endpoint by announcing the (partial) state ID in the returned SigComp parameters at the end of the registration that was supposed to limit the lifetime of the SigComp state. That signals the state will be maintained. The mandatory support for the SigComp Negative
Acknowledgement (NACK) Mechanism [RFC4077] in SIP/SigComp ensures that it is possible to recover from synchronization errors regarding compartment lifetimes. As an operational concern, bugs in the compartment management implementation are likely to lead to sporadic, hard-to-diagnose failures. Decompressors may therefore want to cache old state and, if still available, allow access while logging diagnostic information. Both compressors and decompressors use the SigComp Negative Acknowledgement (NACK) Mechanism [RFC4077] to recover from situations where such old state may no longer be available. A REGISTER transaction causes an application to open a new compartment to be valid for the duration of the registration established by the REGISTER transaction. A SIP application that needs to send a compressed SIP REGISTER (i.e., a user agent generating a REGISTER or a proxy server relaying one to its next hop) SHOULD open a compartment for the request's remote application identifier. A SIP application that receives a compressed SIP REGISTER (i.e., the registrar or a proxy relaying the REGISTER to its next-hop) SHOULD open a compartment for the request's remote application identifier. These compartments MAY be closed if the REGISTER request is responded with a non-2xx final response, or when the registration expires or is canceled. However, applications MAY also choose to keep these compartments open for a longer period of time, as discussed previously. For a given successful registration, applications SHOULD NOT close their associated compartments until the registration is over. Note: A SIP network can be configured so that regular SIP traffic to and from a user agent traverses a different set of proxies than the initial REGISTER transaction. The path the REGISTER transaction follows is typically determined by configuration data. The path subsequent requests traverse is determined by the Path [RFC3327] and the Service-Route [RFC3308] header fields in the REGISTER transaction and by the Record-Route and the Route header fields in dialog-creating transactions. Previous revisions of this document supported the use of different paths for different types of traffic. However, for simplicity reasons, this document now assumes that networks using compression will be configured so that subsequent requests follow the same path as the initial REGISTER transaction in order to achieve the best possible compression. Section 10 provides network administrators with recommendations so that they can configure the networks properly.
If, following the rules above, a SIP application is supposed to open a compartment for a remote application identifier for which it already has a compartment (e.g., the SIP application registers towards a second registrar using the same edge proxy server as for its registration towards its first registrar), the SIP application MUST use the already existing compartment. That is, the SIP application MUST NOT open a new compartment. Section 5 describes how a SIP application can send compressed and uncompressed messages over the same TCP connection. Note that RFC 3486 [RFC3486] states the following: "If the next-hop URI contains the parameter comp=sigcomp, the client SHOULD compress the request using SigComp". Experience since RFC 3486 [RFC3486] was written has shown that stateless compression is, in most cases, not worthwhile. That is why it is not recommended to use it any longer.
When a user agent's registration expires, proxy servers performing compression may close their associated SIP/SigComp compartment. If the user agent is involved in a dialog that was established before the registration expired, subsequent requests within the dialog may not be compressed any longer. In order to avoid this situation, it is RECOMMENDED that user agents are registered as long as they are involved in a dialog. Section 4). o Use of continuous mode (Section 6). o Compartment definition (Section 9). RFC4346] connection can use the compression provided by TLS. Two endpoints exchanging SIP/ SigComp traffic over a TLS connection that provides compression need to first compress the SIP messages using SigComp and then pass them to the TLS layer, which will compress them again. When receiving data, the processing order is reversed.
However, compressing messages this way twice does not typically bring significant gains. Once a message is compressed using SigComp, TLS is not usually able to compress it further. Therefore, TLS will normally only be able to compress SigComp code sent between compressor and decompressor. Since the gain of having SigComp code compressed should be minimal in most cases, it is NOT RECOMMENDED to use TLS compression when SigComp compression is being used. Figure 1 shows an example message flow where the user agent and the outbound proxy exchange compressed SIP traffic. Compressed messages are marked with a (c). User Agent Outbound Proxy Registrar |(1) REGISTER (c) | | |---------------->| | | |(2) REGISTER | | |---------------->| | |(3) 200 OK | | |<----------------| |(4) 200 OK (c) | | |<----------------| | |(5) INVITE (c) | | |---------------->| | | |(6) INVITE | | |------------------------------> | |(7) 200 OK | | |<------------------------------ |(8) 200 OK (c) | | |<----------------| | |(9) ACK (c) | | |---------------->| | | |(10) ACK | | |------------------------------> |(11) BYE (c) | | |---------------->| | | |(12) BYE | | |------------------------------> | |(13) 200 OK | | |<------------------------------ |(14) 200 OK (c) | | |<----------------| | Figure 1: Example Message Flow
The user agent in Figure 1 is initially configured (e.g., using the SIP configuration framework [CONFIG]) with the URI of its outbound proxy. That URI contains the outbound proxy's SIP/SigComp identifier, referred to as 'Outbound-id', in a 'sigcomp-id' parameter. When the user agent sends an initial REGISTER request (1) to the outbound proxy's URI, the user agent opens a new compartment for 'Outbound-id'. This compartment will be valid for the duration of the registration, at least. On receiving this REGISTER request (1), the outbound proxy opens a new compartment for the SIP/SigComp identifier that appears in the 'sigcomp-id' parameter of the top-most Via entry. This identifier, which is the user agent's SIP/SigComp identifier, is referred to as 'UA-id'. The compartment opened by the outbound proxy will be valid for the duration of the registration, at least. The outbound proxy adds a Path header field with its own URI, which contains the 'Outbound-id' SIP/SigComp identifier, to the REGISTER request and relays it to the registrar (2). When the registrar receives the REGISTER request (2), it constructs the route future incoming requests (to the user agent) will follow using the Contact and the Path header fields. Future incoming requests will traverse the outbound proxy before reaching the user agent. The registrar also constructs the route future outgoing requests (from the user agent) will follow and places it in a Service-Route header field in a 200 (OK) response (3). Future outgoing requests will always traverse the outbound proxy. The registrar has ensured that the outbound proxy performing compression handles both incoming and outgoing requests. When the outbound proxy receives a 200 (OK) response (3), it inspects the top-most Via entry. This entry's SIP/SigComp identifier 'UA-id' matches that of the compartment created before. Therefore, the outbound proxy uses that compartment to compress it and relay it to the user agent. On receiving the 200 (OK) response (4), the user agent stores the Service-Route header field in order to use it to send future outgoing requests. The Service-Route header field contains the outbound proxy's URI, which contains the 'Outbound-id' SIP/SigComp identifier. At a later point, the user agent needs to send an INVITE request (5). According to the Service-Route header field received previously, the user agent sends the INVITE request (5) to the outbound proxy's URI.
Since this URI's SIP/SigComp identifier 'Outbound-id' matches that of the compartment created before, this compartment is used to compress the INVITE request. On receiving the INVITE request (5), the outbound proxy Record Routes and relays the INVITE request (6) forward. The outbound proxy Record Routes to ensure that all SIP messages related to this new dialog are routed through the outbound proxy. Finally, the dialog is terminated by a BYE transaction (11) that also traverses the outbound proxy. RFC3320] apply to this document. Note that keeping SigComp states longer than the duration of a SIP dialog should not pose new security risks because the state has been allowed to be created in the first place. Section 9.1, under the Header Field Parameters and Parameter Values subregistry within the SIP Parameters registry: Predefined Header Field Parameter Name Values Reference ---------------------------- --------------- --------- --------- Via sigcomp-id No [RFC5049] The IANA has registered the 'sigcomp-id' SIP URI parameter, which is defined in Section 9.1, under the SIP/SIPS URI Parameters subregistry within the SIP Parameters registry: Parameter Name Predefined Values Reference -------------- ----------------- --------- sigcomp-id No [RFC5049]
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3308] Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer Two Tunneling Protocol (L2TP) Differentiated Services Extension", RFC 3308, November 2002. [RFC3320] Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu, Z., and J. Rosenberg, "Signaling Compression (SigComp)", RFC 3320, January 2003. [RFC3327] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP) Extension Header Field for Registering Non-Adjacent Contacts", RFC 3327, December 2002. [RFC3485] Garcia-Martin, M., Bormann, C., Ott, J., Price, R., and A. Roach, "The Session Initiation Protocol (SIP) and Session Description Protocol (SDP) Static Dictionary for Signaling Compression (SigComp)", RFC 3485, February 2003. [RFC3486] Camarillo, G., "Compressing the Session Initiation Protocol (SIP)", RFC 3486, February 2003. [RFC4077] Roach, A., "A Negative Acknowledgement Mechanism for Signaling Compression", RFC 4077, May 2005. [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, July 2005. [RFC4234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4896] Surtees, A., West, M., and A. Roach, "Signaling Compression (SigComp) Corrections and Clarifications", RFC 4896, June 2007. [CONFIG] Petrie, D. and S. Channabasappa, "A Framework for Session Initiation Protocol User Agent Profile Delivery", Work in Progress, June 2007. [OUTBOUND] Jennings, C. and R. Mahy, "Managing Client Initiated Connections in the Session Initiation Protocol (SIP)", Work in Progress, March 2007.
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