Internet DRAFT - draft-straw-sip-traceroute
draft-straw-sip-traceroute
STRAW Working Group H. Kaplan
Internet Draft Oracle
Intended status: Standards Track July 14, 2013
Expires: January 30, 2013
A Media-based Traceroute Function for
the Session Initiation Protocol (SIP)
draft-straw-sip-traceroute-00
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Abstract
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. Security Considerations.......................................5
5. IANA Considerations...........................................5
6. Acknowledgments...............................................6
7. References....................................................6
7.1. Normative References.....................................6
Author's Address..................................................6
1. 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 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
(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 TTL
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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
[RFC6849].
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.
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
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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
B2BUA.
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. A defined
[SDP] origin-line sess-id value will be in the SDP answer from each
B2BUA answering the INVITE, until the INVITE reaches the final UAS,
which does not use the defined sess-id value. (see Section 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
[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.
The SDP answer in the SIP response is also generated following
[RFC6849], with one exception: the B2BUA MUST also generate an [SDP]
origin ('o=') line <sess-id> value of "314159265". This not-quite-
randomly-chosen number is an indication that the media/SDP responder
is not the final target, but rather a B2BUA responding based on this
document's mechanism. This allows the call generator to detect that
it can try the INVITE again with a higher Max-Forwards value to go
beyond the B2BUA, and continue doing so until it reaches the final
target UAS, which MUST NOT respond with this <sess-id> value.
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; 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.
5. 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.
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6. 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).
7. References
7.1. Normative References
[RFC6849] Kaplan, H., et al, "An Extension to the Session
Description Protocol (SDP) for Media Loopback", RFC 6849,
February 2013.
[SDP] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
Description Protocol", RFC 4566, July 2006.
[draft-loop-detection] Kaplan, H., and Pascual, V., "Loop Detection
Mechanisms for Session Initiation Protocol (SIP) Back-to-Back
User Agents (B2BUAs)", draft-ietf-straw-b2bua-loop-detection-
00, April 2013.
Author's Address
Hadriel Kaplan
Oracle
Email: hadriel.kaplan@oracle.com
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