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Internet Engineering Task Force (IETF)                         H. KaplanRequest for Comments: 7403                                        OracleCategory: Standards Track                                  November 2014ISSN: 2070-1721A Media-Based Traceroute Functionfor the Session Initiation Protocol (SIP)Abstract   SIP already provides the ability to perform hop-by-hop traceroute for   SIP messages using the Max-Forwards header field to determine the   reachability path of requests to a target.  A mechanism for media-   loopback calls has also been defined separately, which enables test   calls to be generated that result in media being looped back to the   originator.  This document describes a means of performing hop-by-hop   traceroute-style test calls using the media-loopback mechanism to   test the media path when SIP sessions go through media-relaying back-   to-back user agents (B2BUAs).Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7403.Kaplan                       Standards Track                    [Page 1]

RFC 7403                Media Traceroute for SIP           November 2014Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1. Introduction ....................................................22. Terminology .....................................................33. The SIP Traceroute Mechanism ....................................43.1. Processing a Received Max-Forwards Header Field ............43.2. Answering the INVITE .......................................54. Security Considerations .........................................55. Normative References ............................................6   Acknowledgments ....................................................7   Author's Address....................................................71.  Introduction   In many deployments, the media for SIP-created sessions does not flow   directly from the originating User Agent Client (UAC) to the   answering User Agent Server (UAS).  Often, SIP B2BUAs in the SIP   signaling path also insert themselves in the media plane path by   manipulating Session Description Protocol (SDP), either for injecting   media such as rich ringtones or music-on-hold or for relaying media   in order to provide functions such as transcoding, IPv4-IPv6   conversion, NAT traversal, Secure Realtime Transport Protocol (SRTP)   termination, media steering, etc.   As more SIP domains get deployed and interconnected, the odds of a   SIP session crossing such media-plane B2BUAs increases as well as the   number of such B2BUAs any given SIP session may go through.  In other   words, any given SIP session may cross any number of B2BUAs both in   the SIP signaling plane as well as the media plane.   When a failure or degradation occurs in the media plane, it is   difficult to determine where in the media path it occurred.  In order   to aid managing and troubleshooting SIP-based sessions and mediaKaplan                       Standards Track                    [Page 2]

RFC 7403                Media Traceroute for SIP           November 2014   traversing such B2BUAs, it would be useful to progressively test the   media path as it reaches successive B2BUAs with a test controlled   solely by the source User Agent (UA).  A mechanism to perform media-   loopback test sessions has been defined in [RFC6849], but it cannot   be used directly to test B2BUAs because, typically, the B2BUAs do not   have an Address of Record (AOR) to be targeted, nor is it known a   priori which B2BUAs will be traversed for any given session.   For example, suppose calls from Alice to Bob have media problems.   Alice would like to test the media path to each B2BUA in the path to   Bob separately, to determine which segment has the issues.  Alice   cannot target the B2BUAs directly for each test call; she doesn't   know which URIs to use to target them, nor would using such URIs   guarantee the same media path be used as a call to Bob.  A better   solution would be to make a test call targeted to Bob, but with a SIP   traceroute-type mechanism that makes the call terminate at the   B2BUAs, such that she can perform test sessions to test the media   path to each downstream B2BUA.   This document defines how such a mechanism can be employed, using the   mechanism in [RFC6849] along with the Max-Forwards SIP header field   such that a SIP UA can make multiple test calls, each reaching a   B2BUA further downstream.  Each B2BUA in the path that supports the   mechanism in [RFC6849] would answer the media-loopback call; thus,   the originating SIP UA can test the media path up to that B2BUA.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].   B2BUA:      a SIP Back-to-Back User Agent, which is the logical               combination of a User Agent Server (UAS) and User Agent               Client (UAC).   UAS:        a SIP User Agent Server   UAC:        a SIP User Agent Client   Traceroute: a mechanism to trace a path of hops from an originator to               a destination.  For IP, this is typically done using the               Time To Live (TTL) field of the IP header, starting at               the value 1 and incrementing by 1 as each IP hop responds               with an ICMP error.  For SIP, this can be done using Max-               Forwards header field starting with the value 0, in a               similar fashion to the TTL field.Kaplan                       Standards Track                    [Page 3]

RFC 7403                Media Traceroute for SIP           November 2014   It is assumed the reader is already familiar with media-loopback   [RFC6849].3.  The SIP Traceroute Mechanism   The Max-Forwards header field can already be used to generate a   simple SIP-request traceroute by generating a SIP request initially   using a Max-Forwards value of 0, receiving a 483 Too Many Hops   response from the next-hop, and then incrementing the value for   subsequent SIP requests; one would thereby reach SIP devices further   and further downstream, receiving 483 from each of them.   The mechanism described in this document uses such a traceroute of a   Max-Forwards style to perform media-loopback testing.  To perform a   SIP media-plane traceroute, the originating UAC (Alice) generates a   SIP INVITE to a target AOR (Bob), with a Max-Forwards header field   value of 0 and with SDP based on [RFC6849].  The SIP next-hop will   either reject the request with a 483 Too Many Hops response or, if   the next-hop is a B2BUA that supports this mechanism and if the B2BUA   allows such testing from the requesting UAC, the B2BUA will answer   the INVITE to establish the dialog and create a media-loopback   session.   The originating UAC can then end the media-loopback session, generate   another INVITE to the same target AOR with a Max-Forwards header   field value of 1, which will reach the second SIP next-hop, and so   on.   A SIP Reason header field cause value of '483' (as defined in   [RFC3326]) will be in the 200 answer from each B2BUA answering the   INVITE, until the INVITE reaches the final UAS (Bob), which does not   use the Reason cause value (seeSection 3.2 for details).   Using this mechanism, a SIP UAC can test the path from itself to each   successive B2BUA on the path to a target.  Such a mechanism could   also be useful for establishing a permanent test call between an   Enterprise and a Service Provider across a SIP Trunk, for example, or   for automated measurement systems to test the media path between   domains, etc.3.1.  Processing a Received Max-Forwards Header Field   As currently defined in [RFC3261], the UAS half of a B2BUA does not   technically need to inspect the Max-Forwards header field value for   received requests: only Proxies do.  This behavior was updated by   [RFC7332], such that a compliant B2BUA needs to both inspect the   value in order to prevent loops, as well as copy and decrement the   value as if it were a Proxy.  This document also requires suchKaplan                       Standards Track                    [Page 4]

RFC 7403                Media Traceroute for SIP           November 2014   behavior in order for the mechanism to succeed; therefore, a B2BUA   supporting the traceroute mechanism defined in this document MUST   also comply with [RFC7332].3.2.  Answering the INVITE   If a SIP B2BUA receives a dialog-creating INVITE request with a Max-   Forwards header value of 0, with SDP for media-loopback based on   [RFC6849], and the policies of the B2BUA allow it to answer such a   request, then it is answered as if the original target of the request   were the local SIP B2BUA.  The normal procedures of SIP apply, as   well as [RFC6849], as if the request had been targeted at the local   B2BUA device as the intended destination all along.   In the 200 response for the INVITE, the B2BUA MUST also add a Reason   header, per [RFC3326], with a protocol field value of "SIP", a cause   field value of "483", and a reason-text value of "Traceroute   Response".  The purpose of the Reason header is to indicate to the   UAC that the request is being answered due to reaching a Max-   Forwards of 0, rather than being answered due to reaching the final   UAS.  When the ultimate target UAS answers a loopback-based INVITE   with a Max-Forwards greater than or equal to 0, the Reason header   would not be added to the response and the UAC will know the   traceroute is complete.   If a B2BUA receives an INVITE with media-loopback SDP and a Max-   Forwards header field value of 0, as defined in this document, and it   does not accept the session (e.g., due to local policy), then it   SHOULD respond with a 483 Too Many Hops response, per the normal   rules of [RFC3261], as it would previously.  In other words, in such   a case, it behaves no differently than it would have if it did not   support this document's new behavior.4.  Security Considerations   There are security implications for the mechanism defined in this   document.  Answering media-loopback calls in a B2BUA consumes   resources on the B2BUA, and network bandwidth in between and, thus,   exposes a vector for denial-of-service (DoS) attacks; therefore,   B2BUAs should provide configuration options to control who can make   such test calls, how many concurrent calls can be established and   maintained, and how long calls can continue.  Entities that deploy   B2BUAs should set these options to values that reduce the DoS risk to   an acceptable level.  For example, a B2BUA might perform digest-   challenge authentication with specific credentials for such calls or   it might only allow specific sources to make such calls, at aKaplan                       Standards Track                    [Page 5]

RFC 7403                Media Traceroute for SIP           November 2014   specific time.  Such policies are typically vendor specific based on   local policies and deployment usage scenarios and cannot be   explicitly defined in this document.   The security considerations of [RFC6849] also apply to this document.   Since B2BUAs are not end-user devices, there is no human user to   monitor the loopback session activity on the B2BUA as recommended in   [RFC6849]; instead, B2BUAs should log such events or provide some   form of administrative notification.5.  Normative References   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997,               <http://www.rfc-editor.org/info/rfc2119>.   [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, <http://www.rfc-editor.org/info/rfc3261>.   [RFC3326]   Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason               Header Field for the Session Initiation Protocol (SIP)",RFC 3326, December 2002,               <http://www.rfc-editor.org/info/rfc3326>.   [RFC6849]   Kaplan, H., Ed., Hedayat, K., Venna, N., Jones, P., and               N. Stratton, "An Extension to the Session Description               Protocol (SDP) and Real-time Transport Protocol (RTP) for               Media Loopback",RFC 6849, February 2013,               <http://www.rfc-editor.org/info/rfc6849>.   [RFC7332]   Kaplan, H. and V. Pascual, "Loop Detection Mechanisms for               Session Initiation Protocol (SIP) Back-to-Back User               Agents (B2BUAs)",RFC 7332, August 2014,               <http://www.rfc-editor.org/info/rfc7332>.Kaplan                       Standards Track                    [Page 6]

RFC 7403                Media Traceroute for SIP           November 2014Acknowledgments   The general concept of performing media-loopback on a hop-by-hop   basis using a decrementing header traceroute-style approach came out   of discussions several years ago, between the author, Kaynam Hedayat,   Nagarjuna Venna, and Patrick MeLampy.  Other people that have   contributed to the topic over the years since then: Brett Tate, Paul   Kyzivat, Peter Dawes, Zaid Ally, Dianna Stiller, Jon Boone, and   several others whom I have lost the names of since.Author's Address   Hadriel Kaplan   Oracle   EMail: hadrielk@yahoo.comKaplan                       Standards Track                    [Page 7]