Internet Engineering Task Force SIP WG Internet Draft G. Camarillo Ericsson draft-camarillo-sip-compression-00.txt April 22, 2002 Expires: October 2002 Compressing the Session Initiation Protocol STATUS OF THIS MEMO This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Abstract This document describes a mechanism to signal that compression is desired for one or more SIP messages. It also states when it is appropriate to send compressed SIP messages to a SIP entity. G. Camarillo [Page 1] Internet Draft SIP April 22, 2002 Table of Contents 1 Introduction ........................................ 3 2 Overview of operation ............................... 4 3 Sending a Request to a Server ....................... 4 4 Sending a Response to a Client ...................... 5 5 Error Situations .................................... 6 6 Augmented BNF ....................................... 6 7 Security Considerations ............................. 6 8 Authors' Addresses .................................. 6 9 Bibliography ........................................ 7 G. Camarillo [Page 2] Internet Draft SIP April 22, 2002 1 Introduction A SIP [1] client sending a request to a SIP server typically performs a DNS lookup for the domain name of the server. When NAPTR [2] or SRV [3] records are available for the server, the client can specify the type of service it wants. The service in this context is the transport protocol to be used by SIP (e.g., UDP, TCP or SCTP). A SIP server that support, for instance, three different transport protocols, will have three different DNS entries. Since it is foreseen that the number of transport protocols supported by a particular application layer protocol is not going to grow dramatically, having a DNS entry per transport seems like a scalable enough solution. However, sometimes it is necessary to include new layers between the transport protocol and the application layer protocol. Examples of these layers are transport layer security and compression. If DNS was used to discover the availability of these layers for a particular server, the number of DNS entries needed for that server would grow dramatically. A server that, for example, supported TCP and SCTP as transports, TLS for transport security and Sigcomp for signaling compression, would need the 8 DNS entries listed below: 1. TCP, no security, no compression 2. TCP, no security, SigComp 3. TCP, TLS, no compression 4. TCP, TLS, SigComp 5. SCTP, no security, no compression 6. SCTP, no security, SigComp 7. SCTP, TLS, no compression 8. SCTP, TLS, SigComp It is clear that this way of using DNS is not scalable. Therefore, an application layer mechanism to express support of signalling compression is needed. Note that for historical reasons both HTTP and SIP use a different port for TLS on top of TCP than for TCP alone, G. Camarillo [Page 3] Internet Draft SIP April 22, 2002 although at present this solution is not considered scalable any longer. 2 Overview of operation There are two types of SIP messages; SIP requests and SIP responses. Clients send SIP requests to the host part of a URI and servers send responses to the host in the sent-by parameter of the Via header field. We define two parameters, one for SIP URIs and the other for the Via header field. The format of both parameters is the same, as shown in the examples below: sip:alice@atlanta.com;comp=sigcomp Via: SIP/2.0/UDP server1.atlanta.com:5060;branch=z9hG4bK87asdks7;comp=sigcomp The presence of this parameter in a URI indicates that the request has to be compressed using Sigcomp, as defined in [4]. The presence of comp=sigcomp in a Via header field indicates that the response has to be compressed using Sigcomp. 3 Sending a Request to a Server A request is sent to the host part of a URI. This URI, referred to as the next-hop URI, is the Request-URI of the request or an entry in the Route header field. If the next-hop URI contains the parameter comp=sigcomp, the client SHOULD compress the request using Sigcomp as defined in [4]. The next-hop URI can contain such a parameter due to a number of reasons: o The URI was obtained from a Record-Route header field when the dialog was established. o The URI was previously registered in a registrar using a REGISTER request. o The URI was obtained from a Contact header field in a 3xx or 485 response to the request. o The URI was obtained out of band. If the next-hop URI does not contain the parameter comp=sigcomp but the client wishes to compress the request, the client can G. Camarillo [Page 4] Internet Draft SIP April 22, 2002 compress the request if, and only if, it knows that the server supports Sigcomp. This knowledge can be obtained in a number of ways: o In a previous transaction, the client obtained a URI for the same server with the parameter comp=sigcomp (e.g., a Contact header field in a 2xx response for a previous OPTIONS request or an entry in the route set of a previous dialog). o In a previous transaction, the server sent a request or a response using Sigcomp. o The client is statically configured to compress all the traffic to that particular server. A client SHOULD NOT send a compressed request to a server if it does not know whether or not the server supports Sigcomp. If a client does not know whether or not the server supports Sigcomp, but in case the server supported it, it would like to receive compressed responses, this client SHOULD add the parameter comp=sigcomp to the topmost entry of the Via header field. The request, however, as stated above, will not be compressed. Regardless of whether the request is sent compressed or not, if a client would like to receive subsequent requests within the same dialog compressed, this client SHOULD add the parameter comp=sigcomp to the URI in the Contact header field if it is a user agent client. If the client is a proxy, it SHOULD add the parameter comp=sigcomp to its URI in the Record-Route header field. 4 Sending a Response to a Client A response is sent to the host in the sent-by parameter of the Via header field. If the topmost Via header field contains the parameter comp=sigcomp, the response SHOULD be compressed. Otherwise, the response SHOULD NOT be compressed. A proxy performing Record-Route inspects the Record-Route and the Contact header fields in the response. It looks for the URI of the next upstream (closer to the user agent client) hop in the route set. If this URI contains the parameter comp=sigcomp, the proxy SHOULD add comp=sigcomp to its entry in the Record-Route header field. The same way, a user agent server SHOULD add comp=sigcomp to the Contact header field of the response if the URI of the next upstream hop in the route set contained the parameter comp=sigcomp. G. Camarillo [Page 5] Internet Draft SIP April 22, 2002 5 Error Situations If a compressed SIP request arrives to a SIP server that does not understand SigComp, the server will not have any means to indicate the error to the client. The message will be impossible to parse, and there will be no Via header field indicating an address to send the error response. If a SIP client sends a request and the client transaction times out without having received any response, the client SHOULD retry the same request without using compression. If the compressed request was sent over a TCP connection, the client SHOULD close that connection and open a new one to send the uncompressed request. Otherwise the server would not be able to detect the beginning of the new message. 6 Augmented BNF This section provides the augmented Backus-Naur Form (BNF) of both parameters described above. The compression URI parameter is a uri-parameter, as defined by the SIP ABNF: compression-param = "comp=" ("sigcomp" / other-compression) other-compression = token The Via compression parameter is a via-extension, as defined by the SIP ABNF: via-compression = "comp" EQUAL ("sigcomp" / other-compression) other-compression = token 7 Security Considerations A SIP entity receiving a compressed message has to decompress it and to parse it. This requires slightly more processing power than only parsing a message. This implies that a denial of service attack using compressed messages would be slightly worse than an attack with uncompressed messages. 8 Authors' Addresses Gonzalo Camarillo Ericsson G. Camarillo [Page 6] Internet Draft SIP April 22, 2002 Advanced Signalling Research Lab. FIN-02420 Jorvas Finland electronic mail: Gonzalo.Camarillo@ericsson.com 9 Bibliography [1] J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation protocol," Internet Draft, Internet Engineering Task Force, Feb. 2002. Work in progress. [2] M. Mealling and R. Daniel, "The naming authority pointer (NAPTR) DNS resource record," Request for Comments 2915, Internet Engineering Task Force, Sept. 2000. [3] A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)," Request for Comments 2782, Internet Engineering Task Force, Feb. 2000. [4] R. Price, J. Rosenberg, C. Bormann, H. Hannu, and Z. Liu, "Universal decompressor virtual machine (udvm)," Internet Draft, Internet Engineering Task Force, Jan. 2002. Work in progress. Full Copyright Statement Copyright (c) The Internet Society (2002). All Rights Reserved. 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