Internet DRAFT - draft-hilt-sipping-consider-policy
draft-hilt-sipping-consider-policy
SIPPING V. Hilt
Internet-Draft Bell Labs
Expires: November 12, 2004 G. Camarillo
Ericsson
May 14, 2004
Considerations for Session-specific SIP Session Policies
draft-hilt-sipping-consider-policy-00
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This draft intends to trigger discussions within the SIP community on
how to implement session-specific policies in SIP. In particular, we
discuss why the piggyback model, which was proposed previously, does
not meet important requirements.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements which are not Met by the Piggyback Model . . . . 3
3. Sending Intructions . . . . . . . . . . . . . . . . . . . . . 4
4. Disclosing Information . . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . 7
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1. Introduction
The concept of session-specific SIP session policies has been around
for some time. However, it has been proven that the mechanisms for
establishing session-specific policies are non-trivial and most
likely require to sacrifice some of the requirements defined in [1].
Session-specific policies involve two main operations:
1. A UA disclose information about the offer/answer exchange to a
proxy.
2. The proxy sends instructions to the UA.
Some types of policies do not involve sending intructions, but only
information disclosure (e.g., a proxy opening pin holes in a firewall
for media streams). Still, a general session-specific policy
mechanism needs to support both operations.
The same way, some proxies only need to inspect the offer, but not
the answer. Nevertheless, a general mechanism needs to consider
proxies which need to inspect both.
2. Requirements which are not Met by the Piggyback Model
The piggyback model, which was proposed some time ago, attemped to
perform both operations described previously (disclosing information
and sending instructions) by piggybacking information into the
INVITE-200 OK-ACK messages.
The piggyback model has two major drawbacks that make it unsuitable
as a general mechanism for session-specific policies:
1. It assumes that both UAs understand session-specific policies.
That is, the UAS needs to apply the policies that the UAC's proxy
proposes.
2. It does not support policy changes for ongoing sessions. That is,
proxies can only send information to the UAs when these happen to
exchange a SIP message for some reason. The proxy cannot initiate
a transaction towars the UAs (e.g., you can now use G.711 if you
want).
In other words, the piggyback model does not meet two important
requirements: support by one of the UAs should be enough and dynamic
policy changes should be supported.
The next sections discuss some alternatives to perform both
operations, (disclosing information and sending instructions).
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3. Sending Intructions
In order to support policy changes during ongoing sessions, the proxy
and its UA need to establish a separate channel. The proxy would use
this channel to send instructions to the UA.
Having the UA SUBSCRIBE to the proxy's policy event package seems
like a sensible solution. We would use the same mechanism as we
already use for session-independent policies, and we would meet the
requirement above.
If the UA is already subscribed to session-independent policies, it
could receive the session-specific policies over the same
subscription. If the UA is not subscribed, the proxy needs to provide
the UA with a URI to send the SUBSCRIBE.
The proxy could use a header field or a body part in a 4xx response
to the INVITE (for the UAC) or in the INVITE itself (for the UAS). We
would need m2e security for the UAS case.
4. Disclosing Information
There are four possible scenarios regarding information disclosure:
1. Figure 1: offer in INVITE and B does not support session
policies. That is, P provides policies for A.
2. Figure 1: offer in INVITE and A does not support session
policies. That is, P provides policies for B.
3. Figure 2: empty INVITE and B does not support session policies.
That is, P provides policies for A.
4. Figure 2: empty INVITE and A does not support session policies.
That is, P provides policies for B.
A P B
| INVITE (offer) | |
|------------------->| INVITE (offer) |
| |------------------->|
| | 200 OK (answer) |
| 200 OK (answer) |<-------------------|
|<-------------------| |
| ACK | |
|------------------->| ACK |
| |------------------->|
Figure 1
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A P B
| INVITE | |
|------------------->| INVITE |
| |------------------->|
| | 200 OK (offer) |
| 200 OK (offer) |<-------------------|
|<-------------------| |
| ACK (answer) | |
|------------------->| ACK (answer) |
| |------------------->|
Figure 2
The discussion here is whether we want to use the piggyback model to
disclose information to the proxy or we want to use a separate
channel as well.
In scenario 2 (offer in INVITE and A does not support session
policies), if we use the piggyback model B needs to generate a 200 OK
with a dummy answer and an object intended for P. P will receive the
200 OK, open the object, get the information it needs, and send
instructions to B using the separate channel (e.g., a NOTIFY). B,
then, would need to re-INVITE or UPDATE.
If we used a separate channel instead, B would contact P using the
separate channel to send P the information it needs.
At this point, a good discussion topic is: if we decide to use a
separate channel, what could that channel be? Adding information to a
SUBSCRIBE request to P's URI could do the job, but it does not seem a
nice solution. A different protocol than SIP?
In scenario 4 (empty INVITE and A does not support session policies),
using a separate channel would allow B to contact P before generating
the offer. Additionally, if B needs to disclose part of the answer to
P, it could do it using the separate channel without needing to
re-INVITE or UPDATE.
5. Acknowledgements
Many thanks to Jonathan Rosenberg who initiated the policy work. A
big thanks also to Allison Mankin and Markus Hofmann for their
contributions to the policy discussion.
6 References
[1] Rosenberg, J., "Requirements for Session Policy for the Session
Initiation Protocol (SIP)",
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draft-ietf-sipping-session-policy-req-01 (work in progress),
February 2004.
Authors' Addresses
Volker Hilt
Bell Labs
101 Crawfords Corner Rd
Holmdel, NJ 07733
USA
EMail: volkerh@bell-labs.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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