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RAW Working Group                                           H. Kaplan
Internet Draft                                                   Oracle
Intended status: Standards Track                          June 15, 2013
Expires: December 30, 2013

                   A Media-based Traceroute Function for
                   the Session Initiation Protocol (SIP)

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   Section 4.e of the Trust Legal Provisions and are provided without
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   SIP already provides the ability to perform hop-by-hop traceroute
   for SIP messages using the Max-Forwards header field, in order 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 which 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, in order to test the media path when SIP
   sessions go through media-relaying B2BUAs.

Table of Contents

   1. Terminology...................................................2
   2. Introduction..................................................3
   3. The SIP Traceroute Mechanism..................................4
      3.1. Processing a Received Max-Forwards Header Field..........4
      3.2. Answering the INVITE.....................................5
   4. Open Issues...................................................5
   5. Security Considerations.......................................5
   6. IANA Considerations...........................................5
   7. Acknowledgments...............................................5
   8. References....................................................6
      8.1. Normative References.....................................6
   Author's Address..................................................6

1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.  The
   terminology in this document conforms to RFC 2828, "Internet
   Security Glossary".

   B2BUA: a SIP Back-to-Back User Agent, which is the logical
   combination of a User Agent Server (UAS) and User Agent Client

   UAS: a SIP User Agent Server.

   UAC: a SIP User Agent Client.

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   Traceroute: a mechanism to trace a path of hops from an originator
   to a destination.  For IP, this is typically done using the 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.

   It is assumed the reader is already familiar with media-loopback

2. Introduction

   In many deployments, the media for SIP-created sessions does not
   flow directly from the originating user's UAC to the answering
   user's UAS.  Often, SIP B2BUAs in the SIP signaling path participate
   in the media plane, 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, SRTP termination, media steering, etc.

   As more and more SIP domains get deployed and interconnect, 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
   B2BUA's both in the SIP signaling plane as well as media plane.

   If failures or degradation occurs in the media plane, it is
   difficult to determine where in the media path they occur.  In order
   to aid managing and troubleshooting SIP-based sessions and media
   crossing such B2BUAs, it would be useful to be able to test the
   media path to each B2BUA separately from the source.  A mechanism to
   perform media-loopback test sessions has been defined in [RFC6849],
   but it would be difficult to use the mechanism 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
   crossed 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, because she
   doesn't know what 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.

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   This document defines how such a mechanism can be employed, using
   the [RFC6849] mechanism along with the Max-Forwards SIP header field
   such that a SIP User Agent can make multiple test calls, each
   reaching a B2BUA further downstream.  Each B2BUA in the path that
   supports this mechanism would answer the media-loopback call, and
   thus the originating SIP UA can test the media path up to that

3. The SIP Traceroute Mechanism

   The Max-Forwards header field can already be used to perform a
   simple SIP-request traceroute mechanism 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, thereby reaching SIP devices further and
   further downstream and receiving 483 from each of them.

   The mechanism described in this document uses such a Max-Forwards
   style traceroute to perform media-loopback testing.  To perform a
   SIP media-plane traceroute, the originating UAC generates a SIP
   INVITE to a target AoR, with SDP based on [RFC6849].  When the
   request reaches the first B2BUA that supports this mechanism, if the
   B2BUA allows such testing from the requesting UAC, then the B2BUA
   will answer the INVITE to establish the dialog and create a media-
   loopback session.

   The originating UAC can then generate another INVITE to the same
   target AoR with a B2bua-Hops header value of 1, which will reach the
   second B2BUA that supports this mechanism, and so on.

   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
   [draft-loop-detection], 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 such behavior in order to succeed, therefore a B2BUA
   supporting the traceroute mechanism defined in this document MUST
   also comply with [draft-loop-detection].

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3.2. Answering the INVITE

   If a SIP B2BUA or UAS 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/UAS allow it to answer
   such a request, then it is answered as if the original target of the
   request were the local SIP B2BUA/UAS.  The normal procedures of SIP
   apply, as well as [RFC6849], as if the request had been targeted at
   the local B2BUA device all along.

   [Open Issue: how does the UAC know the request reached a B2BUA vs.
   the final UAS? (e.g., how does it know when to stop testing?)]

4. Open Issues

   -  How does the UAC know when the request finally reached the
     ultimate UAS; or conversely that it has not yet reached the
     ultimate UAS? (e.g., insert a param somewhere?)

5. 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; therefore,
   B2BUAs should have some means of controlling who can make such test
   calls, how many concurrent calls can be established and maintained,
   and for how long.  Such policies are typically vendor-specific based
   on local policies, and do not need to be defined in this document.

6.   IANA Considerations

   This document makes no request of IANA yet - if a new parameter or
   field needs to be inserted when answering the INVITE, then it will
   be registered in IANA.

7.   Acknowledgments

   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, Patrick MeLampy, and others.  Other people
   that have contributed to the topic over the years since then: Zaid
   Ally, Dianna Stiller, Jon Boone, and several others whom I have lost
   the names of since.

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).

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8.   References

8.1. Normative References

   [RFC6849]  Kaplan, H., et al, "An Extension to the Session
   Description Protocol (SDP) for Media Loopback", RFC 6849, February

Author's Address

   Hadriel Kaplan
   Email: hadriel.kaplan@oracle.com

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