Internet Engineering Task Force SIP WG Internet Draft J. Rosenberg dynamicsoft J. Weinberger dynamicsoft H. Schulzrinne Columbia U. draft-ietf-sip-nat-02.txt July 1, 2002 Expires: January 2003 An Extension to the Session Initiation Protocol (SIP) for Symmetric Response Routing 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 The Session Initiation Protocol (SIP) operates over UDP and TCP. When used with UDP, responses to requests are returned to the source address the request came from, but from the port written into the topmost Via header of the request. This behavior is not desirable in many cases, most notably, when the client is behind a NAT. This extension defines a new parameter for the Via header, called rport, that allows a client to request that the server send the response back to the source IP address and port where the request came from. J. Rosenberg et. al. [Page 1] Internet Draft Response Routing July 1, 2002 Table of Contents 1 Introduction ........................................ 3 2 Terminology ......................................... 3 3 Client Behavior ..................................... 3 4 Server Behavior ..................................... 4 5 Syntax .............................................. 4 6 Example ............................................. 5 7 Security Considerations ............................. 6 8 IANA Considerations ................................. 6 9 Acknowledgements .................................... 6 10 Author's Addresses .................................. 7 11 Normative References ................................ 7 12 Informative References .............................. 7 J. Rosenberg et. al. [Page 2] Internet Draft Response Routing July 1, 2002 1 Introduction The Session Initiation Protocol (SIP) [1] operates over UDP and TCP. When used with UDP, responses to requests are returned to the source address the request came from, but from the port written into the topmost Via header of the request. This behavior is not desirable in many cases, most notably, when the client is behind a NAT. In that case, the response will not properly traverse the NAT, since it will not match the binding established with the request. Related to this, there is currently no way in SIP for a client to learn, from a response to its request, the source port that the server saw in the request. Currently, SIP does provide the client with the source IP address that the server saw in the request. This information is conveyed in the received parameter in the topmost Via header of the response. This information has proved useful for basic NAT traversal, debugging purposes, and support of multi-homed hosts. However, it is incomplete without the port information. This extension defines a new parameter for the Via header, called rport, that allows a client to request that the server send the response back to the source IP address and port where the request came from. The rport parameter is analagous to the received parameter, except rport contains a port number, not the IP address. 2 Terminology In this document, the key words "MUST", "MUSTNOT", "REQUIRED", "SHALL", "SHALLNOT", "SHOULD", "SHOULDNOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and indicate requirement levels for compliant SIP implementations. 3 Client Behavior The client behavior specified here affects the transport processing defined in Section 18.1 of SIP [1]. A client compliant to this specification (clients include UACs and proxies) MAY include an rport parameter in the top Via header of requests it generates. This parameter MUST have no value; it serves as a flag to indicate to the server that this extension is supported and requested for the transaction. When the client sends the request, if the request is sent using UDP, the client MUST be prepared to receive the response on the same socket the request was sent on. Specifically, it MUST be prepared to receive the response on the same IP address and port present in the source IP address and source port of the request. For backwards J. Rosenberg et. al. [Page 3] Internet Draft Response Routing July 1, 2002 compatibility, the client MUST still be prepared to receive a response on the port indicated in the sent-by field of the topmost Via, as specified in Section 18.1.1 of SIP [1]. In the case where there is a NAT between the client and server, in order for the response to always be received, the NAT binding must remain in existence for the duration of the transaction. Most UDP NAT bindings appear to have a timeout of about one minute. Therefore, non-INVITE transactions will have no problem. For INVITE transactions, the client may need to retransmit its INVITE every 20 seconds or so, even after receiving a provisional response, in order to keep the binding open to receive the final response. OPEN ISSUE: This is awful. Perhaps a further sign that the only real answer is TCP? Or, perhaps this belongs in sipping-nat-scenarios, not here. 4 Server Behavior The server behavior specified here affects the transport processing defined in Section 18.2 of SIP [1]. When a server compliant to this specification (which can be a proxy or UAS) receives a request, it examines the topmost Via header. If this Via header contains an rport parameter with no value, it MUST insert the port the request was received from as the value of this parameter. This is analagous to the way in which a server will insert the receieved parameter with the source IP address the request was received from. In fact, the server MUST insert a received parameter containing the source IP address that the request came from, even if it is identical to the value of the sent-by field. Note that this processing takes place independent of the transport protocol. When a server attempts to send a response over an unreliable unicast transport, such as UDP, and there is no Via maddr parameter present, but there is both a received parameter and an rport parameter, the response MUST be sent to the IP address listed in the received parameter, and the port in the rport parameter. This effectively adds a new processing step between bullets two and three in Section 18.2.2 of SIP [1]. 5 Syntax The syntax for the rport parameter is: response-port = "rport" [EQUAL 1*DIGIT] J. Rosenberg et. al. [Page 4] Internet Draft Response Routing July 1, 2002 This extends the existing definition of the Via header parameters, so that its BNF now looks like: via-params = via-ttl / via-maddr / via-received / via-branch / response-port / via-extension 6 Example Consider an example. A client sends an INVITE which looks like: INVITE sip:user@domain SIP/2.0 Via: SIP/2.0/UDP 10.1.1.1:4540;rport This INVITE is sent with a source port of 4540 and source IP address of 10.1.1.1. The request is natted, so that the source IP appears as 68.44.20.1 and the source port as 9988. This is received at a proxy. The proxy forwards the request, but not before appending a value to the rport parameter in the proxied request: INVITE sip:user@domain2 SIP/2.0 Via: SIP/2.0/UDP proxy.domain.com Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988 This request generates a response, which arrives at the proxy: SIP/2.0 200 OK Via: SIP/2.0/UDP proxy.domain.com Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988 The proxy strips its top Via, and then examines the next one. It contains both a received param, and an rport. The result is that the following response is sent to IP address 68.44.20.1, port 9988: SIP/2.0 200 OK J. Rosenberg et. al. [Page 5] Internet Draft Response Routing July 1, 2002 Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988 The NAT rewrites the destination address of this packet back to IP 10.1.1.1, port 4540, and is received by the client. 7 Security Considerations Since this extension merely adds source port information to the source address information already present in SIP, it does not appear to add any additional security considerations. 8 IANA Considerations There are no IANA Considerations associated with this specification. 9 Acknowledgements The authors would like to thank Rohan Mahy for his comments and contributions to this work. Full Copyright Statement Copyright (c) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION J. Rosenberg et. al. [Page 6] Internet Draft Response Routing July 1, 2002 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 10 Author's Addresses Jonathan Rosenberg dynamicsoft 72 Eagle Rock Avenue First Floor East Hanover, NJ 07936 email: jdrosen@dynamicsoft.com Joel Weinberger dynamicsoft 72 Eagle Rock Avenue First Floor East Hanover, NJ 07936 email: jweinberger@dynamicsoft.com Henning Schulzrinne Columbia University M/S 0401 1214 Amsterdam Ave. New York, NY 10027-7003 email: schulzrinne@cs.columbia.edu 11 Normative References [1] J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation protocol," Internet Draft, Internet Engineering Task Force, Feb. 2002. Work in progress. [2] S. Bradner, "Key words for use in RFCs to indicate requirement levels," RFC 2119, Internet Engineering Task Force, Mar. 1997. 12 Informative References Full Copyright Statement Copyright (c) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to J. Rosenberg et. al. [Page 7] Internet Draft Response Routing July 1, 2002 others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. J. Rosenberg et. al. [Page 8]