SIP WG S. Lawrence Internet-Draft Pingtel Corp. Updates: 3261 (if approved) A. Hawrylyshen Expires: December
2,4, 2006 Ditech Communications Corp.Networks Inc. R. Sparks Estacado Systems May 31,June 02, 2006 Diagnostic Responses for SIP Hop Limit Errors draft-ietf-sip-hop-limit-diagnostics-01draft-ietf-sip-hop-limit-diagnostics-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 2,4, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract The SIP protocol imposes a limit on the number of hops a request may make from a sender to the recipient. When this limit is reached, a 483 error response is returned. The present form of the 483 response does not provide enough information for the sender or proxies on the path to diagnose failures whose symptom is that the hop limit is reached. This document proposes additional diagnostic information to be returned in a 483 response. Comments are solicited, and should be directed to the SIP working group list at 'firstname.lastname@example.org'. Table of Contents 1. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 2. Diagnosing Hop Limit Exceeded Failures . . . . . . . . . . . . 3 2.1. Limitations of the 483 Error Response . . . . . . . . . . 3 2.2. Returning Useful Diagnostic Information in 483 Responses . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Pruning Responses . . . . . . . . . . . . . . . . . . . . 8 3. Implementation Experience . . . . . . . . . . . . . . . . . . 9 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.1. Normative References . . . . . . . . . . . . . . . . . . . 10 6.2. Informative References . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12 1. Conventions and Definitions 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 RFC-2119 [RFC2119]. 2. Diagnosing Hop Limit Exceeded Failures The SIP protocol imposes a limit on the number of hops a request may make from a sender to the recipient. The number of hops remaining for the request is carried in the Max-Forwards header, and is decremented each time the request is forwarded. When a SIP User Agent receives a request whose Max-Forwards is zero, it returns a 483 error response to indicate that the limit was reached. The 483 response alone does not provide enough information, for the sender to determine where the problem lies. In the authors experience, the problem is rarely that the target of the request was actually further away than the Max-Forwards limit allowed. The problem is usually incorrect routing; often a routing loop. 2.1. Limitations of the 483 Error Response Section 20.22 of RFC 3261 [RFC3261] says: The Max-Forwards header field must be used with any SIP method to limit the number of proxies or gateways that can forward the request to the next downstream server. This can also be useful when the client is attempting to trace a request chain that appears to be failing or looping in mid-chain. In practice, there is too little information returned in a 483 response for it to be of much use as a diagnostic. When a request has traversed a series of proxies, the response follows the Vias back to the requester; in the case of a typical 483 response it can be difficult to determine even what server the response came from. Even when the rejecting server does identify itself, it can be difficult to figure out why the request got there. The following is an actual example request; the IP addresses and domain names have been changed, but it is otherwise complete (it was intentionally sent without SDP for brevity): INVITE sip:email@example.com SIP/2.0 Via: SIP/2.0/TCP 10.1.1.20:59449 ;branch=z9hG4bK-56ec69968c31f498c9a5573a00c8fc04 To: sip:firstname.lastname@example.org From: Sip Send <sip:email@example.com>;tag=08e2f515 Call-ID: firstname.lastname@example.org Cseq: 1 INVITE Max-Forwards: 1 User-Agent: sipsend/0.02 Date: Wed, 12 Oct 2005 20:09:29 GMT Content-Length: 0 This request was sent with the Max-Forwards value set to only 1 to force the error response: it should traverse only the first outbound proxy, and then be rejected by the next system that it encounters. The response received in this case was: SIP/2.0 483 Too Many Hops Via: SIP/2.0/TCP 10.1.1.20:59449 ;branch=z9hG4bK-56ec69968c31f498c9a5573a00c8fc04 To: sip:email@example.com;tag=-1574266585 From: Sip Send <sip:firstname.lastname@example.org>;tag=08e2f515 Call-ID: email@example.com Cseq: 1 INVITE Content-Length: 0 There is no indication in the response of what server returned the error. Even with the error only one hop beyond the first proxy, there is no way to determine if that first proxy has routed the request incorrectly. 2.2. Returning Useful Diagnostic Information in 483 Responses In some ways, the Max-Forwards mechanism is analogous to the Time To Live (TTL) field in an IP datagram. The TTL field was originally [RFC0791] intended to be the maximum number of seconds that a datagram should remain in the network. In practice, IP TTL has evolved into a hop count, since each system forwarding a datagram was (is) required to decrement the TTL by (at least) one. As an aid to diagnosing problems, the Internet Control Message Protocol [RFC0792] defines a "Time Exceeded Message" to be sent by any system that discards an IP datagram because it was received with a TTL value of zero. The Time Exceeded Message is sent to the source address of the discarded datagram, and includes a field that carries the "Internet Header + 64 bits of Original Data Datagram". This allows the sender to see the datagram as it appeared where it was discarded. The 'traceroute' tool determines the route followed between a given pair of IP addresses by sending a series of IP packets from the source to the destination with gradually increasing TTL values. As each packet reaches its limit, an ICMP Time Exceeded Message is returned by the router that is discarding it; some checks on the route can be made by examining the original packet as it arrived at each hop. As an aid to diagnosing problems that result in 483 responses, it would be useful to know how the failed request arrived at the rejecting system; both what path it followed to get there, and what the request looked like when it ran out of hops. One way to accomplish this is to return the SIP header of the rejected message to the sender. Doing so is already allowed by existing rules: RFC 3261 (section 7.4) says: "All responses MAY include a body.". RFC 3420 [RFC3420] defines the Content-Type "message/sipfrag" to "allow SIP entities to make assertions about a subset of a SIP message". This document proposes the following new rule for all SIP implementations: Any 483 response SHOULD be constructed with both: A message body of type message/sipfrag containing as much as possible of the SIP header from the rejected request. A Warning header with a warn-code of 399 that identifies the system returning the error. 2.3. Example We will use this proposed change to diagnose an example routing problem. Here is a request sent to a proxy that implements the suggested content in a 483 response. INVITE sip:firstname.lastname@example.org SIP/2.0 Via: SIP/2.0/TCP 10.1.1.20:40221 ;branch=z9hG4bK-931ea14405e9da8c95cf4ed60a71f59f To: sip:email@example.com From: Sip Send <sip:firstname.lastname@example.org>;tag=612f37e7 Call-ID: email@example.com Cseq: 1 INVITE Max-Forwards: 9 User-Agent: sipsend/0.02 Date: Fri, 14 Oct 2005 15:35:53 GMT Content-Length: 0 The target user '9999' is one that has been deliberately configured to go into a forwarding loop alternating between two addresses (neither of them the original target); a situation that is currently difficult to diagnose. A relatively low Max-Forwards value of 9 was chosen to improve readability. The response received was: SIP/2.0 483 Too many hops Warning: 399 192.0.2.162:5080 Too Many Hops From: Sip Send <sip:firstname.lastname@example.org>;tag=612f37e7 To: sip:email@example.com Call-Id: firstname.lastname@example.org Cseq: 1 INVITE Via: SIP/2.0/TCP 10.1.1.20:40221 ;branch=z9hG4bK-931ea14405e9da8c95cf4ed60a71f59f Content-Type: message/sipfrag Content-Length: 1014 Date: Fri, 14 Oct 2005 15:27:47 GMT INVITE sip:InfiniteLoop@interop.pingtel.com SIP/2.0 Record-Route: <sip:192.0.2.162:5080 ;lr;a;t=612f37e7;s=96e09e8e8c93a8c60bf460029847f4b1> Via: SIP/2.0/TCP 192.0.2.162 ;branch=z9hG4bK-42e47bba67559bd9a3da1934a70bbc37 Via: SIP/2.0/TCP 192.0.2.162:5080 ;branch=z9hG4bK-50d909f1209f7a820de85c7831846330 Via: SIP/2.0/TCP 192.0.2.162 ;branch=z9hG4bK-994f162bc179fb75093166fabbfd13c7 Via: SIP/2.0/TCP 192.0.2.162:5080 ;branch=z9hG4bK-708842ad6ea22f8fa6e39c503d3d803e Via: SIP/2.0/TCP 192.0.2.162 ;branch=z9hG4bK-50b581a06ca023ebcddbc82c5221149c Via: SIP/2.0/TCP 192.0.2.14:1084 ;branch=z9hG4bK994220327571023745d7996c13560a11.0 Via: SIP/2.0/TCP 192.0.2.14:40221 ;branch=z9hG4bK-931ea14405e9da8c95cf4ed60a71f59f To: sip:email@example.com From: Sip Send <sip:firstname.lastname@example.org>;tag=612f37e7 Call-Id: email@example.com Cseq: 1 INVITE Max-Forwards: 0 User-Agent: sipsend/0.02 Date: Fri, 14 Oct 2005 15:35:53 GMT Content-Length: 0 The Warning header in this response identifies the server returning the error (192.0.2.162:5080). The Via headers of the returned message/sipfrag body show the path the failed message took. The returned request line also shows that the target URI has been changed to the user 'InfiniteLoop'. Resending the request with a hop limit one less than before (8), shows that at that hop the request URI is to user 'LoopForever': INVITE sip:LoopForever@interop.pingtel.com SIP/2.0 Record-Route: <sip:192.0.2.162:5080 ;lr;a;t=4f18a30b;s=a9711e1704ccd5273955589c5fe94745> Via: SIP/2.0/TCP 192.0.2.162 ;branch=z9hG4bK-9b7f69455266f7cccd4ae8a285c0417c Via: SIP/2.0/TCP 192.0.2.162:5080 ;branch=z9hG4bK-b9d3e2aff65e68497849a2609bf8c373 Via: SIP/2.0/TCP 192.0.2.162 ;branch=z9hG4bK-a178f4c5a3b8bbf35f979bc6c6d33022 Via: SIP/2.0/TCP 192.0.2.162:5080 ;branch=z9hG4bK-3c098b8d58e4b6ce98fca3495263e795 Via: SIP/2.0/TCP 192.0.2.162 ;branch=z9hG4bK-e7ceb06ed917d59024c905b3ee60e4cc Via: SIP/2.0/TCP 192.0.2.14:1085 ;branch=z9hG4bK8d685f52450e87da45d7996c13560a11.0 Via: SIP/2.0/TCP 192.0.2.14:56114 ;branch=z9hG4bK-4ae5a563b0cbc76aef7be17115836dea To: sip:firstname.lastname@example.org From: Sip Send <sip:email@example.com>;tag=4f18a30b Call-Id: firstname.lastname@example.org Cseq: 1 INVITE Max-Forwards: 0 User-Agent: sipsend/0.02 Date: Fri, 14 Oct 2005 15:42:21 GMT Content-Length: 0 Route: <sip:192.0.2.162:5070;transport=tcp;lr> Reducing the limit one at a time (or starting from 1 and working forward), the sender can determine that the InfiniteLoop/LoopForever forwarding loop exists (in reality, of course, the user names would rarely be such good hints), and where in the forwarding sequence the original '9999' was changed to enter the loop. Without the returned request headers, the 483 response does not help the request originator (or any proxy administrator on the path) diagnose why the error has occurred. With it, in this case a diagnostic application running as a User Agent is able to at least identify that there is a routing problem and which proxy is misrouting the request. 2.4. Pruning Responses A server may be unable or unwilling to return the full request message in every 483 response. The returned message may exceed the maximum message size it can handle, or may include security-sensitive information. It is suggested in this case that at least all of the Route and Via headers from the request be returned in the message/sipfrag body. In the example (Section 2.3), this would at least enable the end user to determine which proxies were in the routing loop and how the request arrived there, but not the specific address transformations that caused the loop. If including all Via and Route headers is still too large, the implementation SHOULD remove the oldest Vias (those nearest the message originator) until the size is acceptable. This way, the originator can detect that some Vias were removed (because the one that it put on is missing). 3. Implementation Experience One open source implementation has been generating 483 responses roughly as recommended above, this has been explicitly tested both at SIPit and in production use for any interoperability problems it might cause. No problems have been observed, except with some implementations that cannot reassemble fragmented UDP packets (these implementations tend to have problems with long paths in any case). 4. IANA Considerations None. 5. Security Considerations The proposed mechanism provides a means by which topology and some routing information about a set of SIP systems can be discovered. The mechanism is very similar to that provided for IP routing by the traceroute tool. Some systems may not want to expose as much information as is available in the full set of SIP request headers by returning them in the error response body. In this case, the system returning the error should prune the response as recommended in Section 2.4. 6. References 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 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. [RFC3420] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420, November 2002. 6.2. Informative References [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. Authors' Addresses Scott Lawrence Pingtel Corp. 400 West Cummings Park Suite 2200 Woburn, MA 01801 USA Phone: +1 781 938 5306 Email: email@example.com Alan Hawrylyshen Ditech Communications Corp. 602 - 11 Ave SWNetworks Inc. 1167 Kensington Rd NW Suite 310200 Calgary, Alberta T2R 1J8T2N 1X7 Canada Phone: +1 403 561 7313806 3366 Email: firstname.lastname@example.org@ditechnetworks.com Robert Sparks Estacado Systems 17210 Campbell Road Suite 250 Dallas, Texas 75254-4203 USA Email: RjS@nostrum.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. 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