SIPPING Working Group J. Hautakorpi, Ed.
Internet-Draft G. Camarillo
Expires: March 30, 2006 Ericsson
M. Bhatia
NexTone Communications
R. Penfield
Acme Packet
A. Hawrylyshen
Ditech Communications Corporation
September 26, 2005
SIP (Session Initiation Protocol)-Unfriendly Functions in Current
Communication Architectures
draft-camarillo-sipping-sbc-funcs-02.txt
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Copyright (C) The Internet Society (2005).
Abstract
This document gives an overview to SIP-unfriendly functions in
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current communication architectures. These functions are generally
implemented in SBCs (Session Border Controllers). The purpose of
this document is to help the IETF community understand what SIP-
unfriendly functions can be found in existing networks so that the
appropriate working groups can decide whether or not new standard
solutions need to be developed to provide such functionality (or a
subset of it) in a standard, SIP-friendly way. Working groups may
also develop recommendations on how to use existing standard
mechanisms to provide such functionality. An additional goal of this
document is to describe how implementing certain functions in certain
ways breaks SIP and the disadvantages derived from doing so.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background on SBCs . . . . . . . . . . . . . . . . . . . . . . 3
3. SIP-Unfriendly Functions . . . . . . . . . . . . . . . . . . . 4
3.1. Topology Hiding . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. General Information . . . . . . . . . . . . . . . . . 4
3.1.2. SIP-Unfriendly Aspect . . . . . . . . . . . . . . . . 5
3.1.3. Concrete Example . . . . . . . . . . . . . . . . . . . 5
3.2. Media Traffic Shaping . . . . . . . . . . . . . . . . . . 6
3.2.1. General Information . . . . . . . . . . . . . . . . . 6
3.2.2. SIP-Unfriendly Aspect . . . . . . . . . . . . . . . . 6
3.2.3. Concrete Example . . . . . . . . . . . . . . . . . . . 6
3.3. Fixing Capability Mismatches . . . . . . . . . . . . . . . 7
3.3.1. General Information . . . . . . . . . . . . . . . . . 7
3.3.2. SIP-Unfriendly Aspect . . . . . . . . . . . . . . . . 8
3.3.3. Concrete Example . . . . . . . . . . . . . . . . . . . 8
3.4. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 9
3.4.1. General Information . . . . . . . . . . . . . . . . . 9
3.4.2. SIP-Unfriendly Aspect . . . . . . . . . . . . . . . . 9
3.4.3. Concrete Example . . . . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informational References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
This document gives an overview to SIP [1] unfriendly functions in
current communication architectures. These functions are generally
implemented in Session Border Controllers (SBCs). The reason for
this is that network policies are typically enforced at the edge of
the network, and SBCs are typically located there.
Of course, a typical SBC does not only implement SIP-unfriendly
functions. They usually implement a set of functions, some of which
are perfectly legal from the SIP point of view. However, this
document focuses on those functions that break SIP somehow.
Many existing SBCs use proprietary solutions because there is no
standard solution for a given issue or because the standard solution
does not fully meet the requirements of the network operator. This
document is intended to be taken as input by the IETF so that the
appropriate working groups can decide whether or not new standard
solutions need to be developed to provide the same functionality (or
a subset of it) in a SIP-friendly way. Working groups may also
decide to develop recommendations on how to use existing standard
mechanisms to provide such functionality.
2. Background on SBCs
The term SBC is pretty vague, since it is not standardized or defined
anywhere. Nodes that may be referred to as SBCs but do not implement
SIP are outside the scope of this document.
Even though many SBCs currently break things like end-to-end security
and can impact feature negotiations, there is clearly a market for
them. Network operators need many of the features current SBCs
provide and many times there are no standard mechanisms available to
provide them in a better way.
SIP-based SBCs typically handle both signaling and media, and often
modify the session descriptions contained in SIP messages.
Consequently, they are, by definition, B2BUAs (Back-to-Back User
Agents). The transparency of these B2BUAs varies depending on the
functions they perform. For example, some SBCs modify the session
description carried in the message and insert a Record-Route entry.
Other SBCs replace the value of the Contact header field with the
SBCs address, and generate a new Call-ID and new To and From tags.
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+-----------------+
| SBC |
[signaling] | +-----------+ |
<------------|->| signaling |<-|---------->
outer | +-----------+ | inner
network | | | network
| +-----------+ |
<------------|->| media |<-|---------->
[media] | +-----------+ |
+-----------------+
Figure 1: SBC architecture
Figure 1 shows the logical architecture of an SBC, which includes a
signaling and a media component. Typically SBCs are located between
two networks. In this document, the terms outer and inner network
are used for describing these two networks.
There are two typical deployment scenarios where SBCs are used. The
first one of them is a peering scenario, where the SBC is located
between different-operators' networks. The second deployment
scenario is an access scenario, where the SBC is located between the
access network and the operator's network.
3. SIP-Unfriendly Functions
This section examines SIP-unfriendly functions that are used in
current communication networks. Each subsection describes a
particular function or feature, what is the operator's motivation for
having it, explanation on why it breaks SIP, and a concrete example
from its implementation. Each section also discusses problems
specific to that particular way of implementing it. Providing
suggestions for alternative, more SIP-friendly ways of implementing
each of the functions is outside the scope of this document.
All the examples given in this section are somewhat simplified
situations from the reality. Only the relevant header lines from SIP
and SDP [3] messages are displayed.
3.1. Topology Hiding
3.1.1. General Information
Topology hiding consists of limiting the amount of topology
information given to external parties. Operators want to have this
functionality because they do not want the IP addresses of their
equipment (proxies, gateways, application servers, etc) to be exposed
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to outside parties. This may be because they do not want to expose
their equipment to DoS (Denial of Service) attacks or because they
may use other carriers for certain traffic and do not want their
customers to be aware of it. In some environments, the operator's
customers may wish to hide the addresses of their equipment or the
SIP messages may contain private, non-routable addresses.
3.1.2. SIP-Unfriendly Aspect
This functionality is SIP-unfriendly, because it breaks end-to-end
connectivity and modifies the SIP messages without user's explicit
consent. User does not have any way of knowing whether the SIP
messages are modified by the SBC, or is someone performing MitM (Man-
in-the-Middle) attack.
3.1.3. Concrete Example
The current way of implementing topology consists of having an SBC
act as a B2BUA (Back-to-Back User Agents) and remove all traces of
topology information (e.g., Record-Route and Vie entries) from
outgoing messages.
Like a regular proxy server that inserts a Record-Route entry, the
SBC handles every single message of a given SIP dialog. However,
unlike the proxy server, if the SBC loses state (e.g., the SBC
restarts for some reason), it will not be able to route messages
properly. For example, if the SBC removes Via entries from a request
and then restarts losing state, the SBC will not be able to route
responses to that request.
Let us imagine the following example scenario: The SBC
(p4.domain.com) is receiving an INVITE request from the inner
network, which in this case is an operator network. The received SIP
message is:
INVITE sip:callee@u2.domain.com SIP/2.0
Contact: sip:caller@u1.example.com
Record-Route: <sip:p3.middle.com>
Record-Route: <sip:p2.example.com;lr>
Record-Route: <sip:p1.example.com;lr>
Then the SBC performs a topology hiding function. In this imagined
situation the SBC removes and stores all existing Record-Route
headers, and then insert a Record-Route header field with its own
SIP-URI. After the topology hiding function, the message looks like:
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INVITE sip:callee@u2.domain.com SIP/2.0
Contact: sip:caller@u1.example.com
Record-Route: <sip:p4.domain.com;lr>
This is only one example scenario from topology hiding, and SBCs can,
in some cases, modify also other headers.
3.2. Media Traffic Shaping
3.2.1. General Information
Media traffic shaping is the act of controlling media traffic.
Operators have several reasons for having this functionality.
Operators want to control the traffic they carry on their network.
Traffic shaping helps them create different kinds of billing models
(e.g., video telephony can be priced differently than voice-only
calls). Additionally, traffic shaping can be used to implement
intercept capabilities (e.g., lawful intercept).
Some operators do not actually want to reshape the traffic, but only
to monitor it. However, the SIP techniques needed for monitoring
media traffic are the same as for reshaping media traffic.
One possible form of media traffic shaping is that SBCs terminate
media streams and SIP dialogs by generating BYE requests. This kind
of procedure can take place e.g., in a situation where subscriber
runs out of credits.
3.2.2. SIP-Unfriendly Aspect
The current way of implementing traffic shaping has some SIP-
unfriendly aspects. The SBC needs to access and modify the session
descriptions (i.e., offers and answers) exchanged between the user
agents. Consequently, this approach does not work if user agents
encrypt or integrity-protect their message bodies end-to-end. User
agents do not have any way to distinguish the SBC actions from an
attack by a MitM (Man-in-the-Middle).
3.2.3. Concrete Example
Currently, traffic shaping is performed in the following way. The
SBC behaves as a B2BUA and inserts itself, or some other entity under
the operator's control, in the media path. In practice, the SBC
modifies the session descriptions carried in the SIP messages. As a
result, the SBC receives media from one user agent and relays it to
the other in both directions.
An example of traffic shaping is codec restriction. The SBC
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restricts the codec set negotiated in offer/answer [2] exchange
between the user agents. After modifying the session descriptions,
the SBC can check whether or not the media stream corresponds to what
was negotiated in the offer/answer exchange. If it differs, the SBC
has the ability to terminate the media stream.
Let us imagine the following example scenario: The SBC is receiving
an INVITE request from the outer network, which in this case is an
access network. The received SIP message contains the following SDP
session descriptor:
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
c=IN IP4 126.16.64.4
m=audio 49230 RTP/AVP 96 98
a=rtpmap:96 L8/8000
a=rtpmap:98 L16/16000/2
Then the SBC performs a media traffic shaping function. In this
imagined situation the SBC rewrites the 'm' line, and removes one 'a'
line according to some policy. After the traffic shaping function,
the session descriptor looks like:
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
c=IN IP4 126.16.64.4
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
One problem of media traffic shaping is that the SBC needs to
understand the session description protocol and all the extensions
used by the user agents. This means that in order to use a new
extension (e.g., an extension to implement a new service) or a new
session description protocol, it is not enough with upgrading the
user agents; SBCs in the network need also to be upgraded. This fact
may slow down service innovation.
3.3. Fixing Capability Mismatches
3.3.1. General Information
SBCs fixing capability mismatches enable communications between user
agents with different capabilities. For example, user agents that
support different IP versions, different codecs, or that are in
different address realms.
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3.3.2. SIP-Unfriendly Aspect
SBCs fixing capability mismatches insert a media element in the media
path using the procedures described in Section 3.2. Therefore, these
SBCs have the same problems as SBCs performing traffic shaping: the
SBC modifies SIP messages without explicit consent from any of the
user agents. This breaks end-to-end security and extension
negotiation.
Additionally, SBCs may make wrong assumptions about the capabilities
of the user agents. When this happens, user agents with compatible
capabilities may end up communicating via the SBC instead of doing it
directly between them (e.g., the SBC assumes that a dual-stack user
agent only supports IPv6).
3.3.3. Concrete Example
Let us imagine the following example scenario: The SBC is receiving
an INVITE request from the inner network, which in this case is an
access network using IPv4. The received SIP message and session
descriptor is the following:
INVITE sip:callee@ipv6.domain.com SIP/2.0
Via: SIP/2.0/UDP 126.16.64.4
Contact: sip:caller@u1.example.com
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
c=IN IP4 126.16.64.4
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
Then the SBC performs a capability mismatch fixing function. In this
imagined situation the SBC inserts Record-Route and Via headers, and
rewrites the 'c' line from the sessions descriptor. After the
capability mismatch fixing function, the message look like:
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INVITE sip:callee@ipv6.domain.com SIP/2.0
Record-Route: <sip:[2001:620:8:801:201:2ff:fe94:8e10];lr>
Via: <sip:[2001:620:8:801:201:2ff:fe94:8e10];lr>
Via: SIP/2.0/UDP 126.16.64.4
Contact: sip:caller@u1.example.com
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
c=IN IP6 2001:620:8:801:201:2ff:fe94:8e10
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
Now the SBC sends the modified message to the outer network, which
uses IPv6.
3.4. NAT Traversal
3.4.1. General Information
An SBC performing a NAT (Network Address Translator) traversal
function for a user agent behind a NAT sits between the user agent
and the registrar of the domain. When the registrar receives a
REGISTER request from the user agent and responds with a 200 (OK)
response, the SBC modifies such a response decreasing the validity of
the registration (i.e., the registration expires sooner). This
forces the user agent to send a new REGISTER to refresh the
registration sooner that it would have done on receiving the original
response from the registrar. The REGISTER requests sent by the user
agent refresh the binding of the NAT before the binding expires.
Note that the SBC does not need to relay all the REGISTER requests
received from the user agent to the registrar. The SBC can generate
responses to REGISTER requests received before the registration is
about to expire at the registrar. Moreover, the SBC needs to
deregister the user agent if this fails to refresh its registration
in time, even if the registration at the registrar would still be
valid.
Operators implement this functionality in an SBC instead of in the
registrar because the SBC is supposed to have more information about
the access the user agent is using. Additionally, distributing this
functionality helps offload the registrar.
3.4.2. SIP-Unfriendly Aspect
This approach to NAT traversal does not work when end-to-end
confidentiality or integrity-protection is used. The SBC would be
seen as a MitM modifying the messages between the user agent and the
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registrar.
3.4.3. Concrete Example
Let us imagine the following example scenario: The SBC resides
between the UA and Registrar. Previously the UA has sent a REGISTER
request to Registrar, and then the SBC is going to relay the
following SIP message to UA:
SIP/2.0 200 OK
From: Bob <sip:bob@biloxi.example.com>;tag=a73kszlfl
To: Bob <sip:bob@biloxi.example.com>;tag=34095828jh
CSeq: 1 REGISTER
Contact: <sips:bob@client.biloxi.example.com>;expires=3600
Then the SBC performs a traversal function. In this imagined
situation the SBC rewrites the 'expires' parameter on the Contact
header field. After the NAT traversal function, the message look
like:
SIP/2.0 200 OK
From: Bob <sip:bob@biloxi.example.com>;tag=a73kszlfl
To: Bob <sip:bob@biloxi.example.com>;tag=34095828jh
CSeq: 1 REGISTER
Contact: <sips:bob@client.biloxi.example.com>;expires=60
Naturally also other measures needs to be taken in order to enable
the NAT traversal, but this example illustrated only the mechanism on
how NAT bindings can be kept alive.
4. Security Considerations
Many of the functions this document describes have important security
and privacy implications. If the IETF decides to develop standard
mechanisms to address those functions, security and privacy-related
aspects will need to be taken into consideration.
5. IANA Considerations
This document has no IANA considerations.
6. Acknowledgements
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The ad-hoc meeting about SBCs, held on Nov 9th 2004 at Washington DC
during the 61st IETF meeting, provided valuable input to this
document. Special thanks goes also to Sridhar Ramachandran, Gaurav
Kulshreshtha, and to Rakendu Devdhar.
7. References
7.1. Normative References
[1] 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.
[2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
7.2. Informational References
[3] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
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Authors' Addresses
Jani Hautakorpi (editor)
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Jani.Hautakorpi@ericsson.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
Medhavi Bhatia
NexTone Communications
101 Orchard Ridge Drive
Gaithersburg, MD 20878
US
Email: mbhatia@nextone.com
Robert F. Penfield
Acme Packet
71 Third Avenue
Burlington, MA 01803
US
Email: bpenfield@acmepacket.com
Alan Hawrylyshen
Ditech Communications Corporation
310, 602 - 11 Ave SW
Calgary, Alberta T2R 1J8
Canada
Email: alan@jasomi.com
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