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Versions: 00 01 02 03 04 05 draft-ietf-sipping-sbc-funcs

SIPPING Working Group                                 J. Hautakorpi, Ed.
Internet-Draft                                              G. Camarillo
Expires: April 26, 2007                                         Ericsson
                                                             R. Penfield
                                                             Acme Packet
                                                          A. Hawrylyshen
                                                    Ditech Networks Inc.
                                                               M. Bhatia
                                                  NexTone Communications
                                                        October 23, 2006


   Requirements from SIP (Session Initiation Protocol) Session Border
                          Control Deployments
                draft-camarillo-sipping-sbc-funcs-05.txt

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   This Internet-Draft will expire on April 26, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This documents describes functions implemented in Session Initiation



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   Protocol (SIP) intermediaries known as Session Border Controllers
   (SBCs).  Although the goal of this document is to describe all the
   functions of SBCs, a special focus is given to those practices that
   are viewed to be in conflict with SIP architectural principles.  It
   also explores the underlying requirements of network operators that
   have led to the use of these functions and practices in order to
   identify protocol requirements and determine whether those
   requirements are satisfied by existing specifications or additional
   standards work is required.










































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Background on SBCs . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Peering Scenario . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Access Scenario  . . . . . . . . . . . . . . . . . . . . .  6
   3.  Functions of SBCs  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Topology Hiding  . . . . . . . . . . . . . . . . . . . . .  7
       3.1.1.  General Information and Requirements . . . . . . . . .  7
       3.1.2.  Architectural Issues . . . . . . . . . . . . . . . . .  8
       3.1.3.  Example  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Media Traffic Shaping  . . . . . . . . . . . . . . . . . .  9
       3.2.1.  General Information and Requirements . . . . . . . . .  9
       3.2.2.  Architectural Issues . . . . . . . . . . . . . . . . . 10
       3.2.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 10
     3.3.  Fixing Capability Mismatches . . . . . . . . . . . . . . . 12
       3.3.1.  General Information and Requirements . . . . . . . . . 12
       3.3.2.  Architectural Issues . . . . . . . . . . . . . . . . . 12
       3.3.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 12
     3.4.  NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . 13
       3.4.1.  General Information and Requirements . . . . . . . . . 13
       3.4.2.  Architectural Issues . . . . . . . . . . . . . . . . . 14
       3.4.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 14
     3.5.  Access Control . . . . . . . . . . . . . . . . . . . . . . 15
       3.5.1.  General Information and Requirements . . . . . . . . . 15
       3.5.2.  Architectural Issues . . . . . . . . . . . . . . . . . 16
       3.5.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 16
     3.6.  Protocol Repair  . . . . . . . . . . . . . . . . . . . . . 17
       3.6.1.  General Information and Requirements . . . . . . . . . 17
       3.6.2.  Architectural Issues . . . . . . . . . . . . . . . . . 17
       3.6.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . 18
     3.7.  Media Encryption . . . . . . . . . . . . . . . . . . . . . 19
       3.7.1.  General Information and Requirements . . . . . . . . . 19
       3.7.2.  Architectural Issues . . . . . . . . . . . . . . . . . 19
       3.7.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 19
   4.  Derived Requirements . . . . . . . . . . . . . . . . . . . . . 20
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     8.2.  Informational References . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24







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1.  Introduction

   In the past few years there has been a rapid adoption of Session
   Initiation Protocol (SIP) [1] and deployment of SIP-based
   communications networks.  This has often out-paced the development
   and implementation of protocol specifications to meet network
   operator requirements.  This has led to the development of
   proprietary solutions.  Often these proprietary solutions are
   implemented in network intermediaries known in the marketplace as
   Session Border Controllers (SBCs) because they typically are deployed
   at the border between two networks.  The reason for this is that
   network policies are typically enforced at the edge of the network.

   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.  This document describes the most
   common functions of current SBCs and the reasons that network
   operators require them.  It also describes the architectural issues
   with these functions.  Although this document focuses on functions
   common to SBCs, many of the issues raised apply to other types of
   Back-to-Back User Agents (B2BUAs.)


2.  Background on SBCs

   The term SBC is relatively non-specific, 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.

   SBCs usually sit between two service provider networks in a peering
   environment, or between an access network and a backbone network to
   provide service to residential and/or enterprise customers.  They
   provide a variety of functions to enable or enhance session-based
   multi-media services (e.g., Voice over IP).  These functions include:
   a) perimeter defense (access control, topology hiding, DoS
   prevention, and detection); b) functionality not available in the
   endpoints (NAT traversal, protocol interworking or repair); and c)
   network management (traffic monitoring, shaping, and QoS).  Some of
   these functions may also get integrated into other SIP elements (like
   pre-paid platforms, 3GPP P-CSCF, 3GPP I-CSCF etc).

   SIP-based SBCs typically handle both signaling and media and can
   implement behavior which is equivalent to a "privacy service" (as
   described in[2]) performing both Header Privacy and Session Privacy.
   SBCs often modify certain SIP headers and message bodies that proxies
   are not allowed to modify.  Consequently, they are, by definition,



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   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.

                            +-----------------+
                            |       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.  In this document, the terms outer
   and inner network are used for describing these two networks.

2.1.  Peering Scenario

   A typical peering scenario involves two network operators who
   exchange traffic with each other.  For example, in a toll bypass
   application, a gateway in operator A's network sends an INVITE that
   is routed to the softswitch (proxy) in operator B's network.  The
   proxy responds with a redirect (3xx) message back to the originating
   gateway that points to the appropriate terminating gateway in
   operator B's network.  The originating gateway then sends the INVITE
   to the terminating gateway.

















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            Operator A           .                Operator B
                                 .
                                 .                2) INVITE
         +-----+                 .            /--------------->+-----+
         | SSA |                 .           / 3) 3xx (redir.) | SSB |
         +-----+                 .          /  /---------------+-----+
                                 .         /  /
         +-----+  1) INVITE      +-----+--/  /                 +-----+
         |GW-A1|---------------->| SBC |<---/     4) INVITE    |GW-B1|
         +-----+                 +-----+---------------------->+-----+
                                 .
         +-----+                 .                             +-----+
         |GW-A2|                 .                             |GW-B2|
         +-----+                 .                             +-----+


   Figure 2: Peering with SBC

   Figure 2 illustrates the peering arrangement with a SBC where
   Operator A is the outer network, and Operator B is the inner network.
   Operator B uses the SBC to control access to its network, protect its
   gateways and softswitches from unauthorized use and DoS attacks, and
   monitor the signaling and media traffic.  It also simplifies network
   management by minimizing the number ACL (Access Control List) entries
   in the gateways.  The gateways do not need to be exposed to the peer
   network, and they can restrict access (both media and signaling) to
   the SBCs.  The SBC helps ensure that only media from sessions the SBC
   authorizes will reach the gateway.

2.2.  Access Scenario

   In an access scenario, presented in Figure 3, the SBC is placed at
   the border between the access network (outer network) and the
   operator's network (inner network) to control access to the operator
   network, protect its components (media servers, application servers,
   gateways, etc.) from unauthorized use and DoS attacks, and monitor
   the signaling and media traffic.  Also, as a part of access control,
   since the SBC is call stateful, it can prevent over subscription of
   the access links.  Endpoints are configured with the SBC as their
   outbound proxy address.  The SBC routes requests to one or more
   proxies in the operator network.










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           Access Network             .    Operator Network
                                      .
         +-----+                      .
         | UA1 |<---------\           .
         +-----+           \          .
                            \         .
         +-----+             \------->+-----+       +-------+
         | UA2 |<-------------------->| SBC |<----->| proxy |<-- -
         +-----+                 /--->+-----+       +-------+
                                /     .
         +-----+   +-----+     /      .
         | UA3 +---+ NAT |<---/       .
         +-----+   +-----+            .


   Figure 3: Access scenario with SBC

   Some endpoints may be behind enterprise or residential NATs.  In
   cases where the access network is a private network, the SBC is the
   NAT for all traffic.  The proxy usually does authentication and/or
   authorization for registrations and outbound calls.  The SBC modifies
   the REGISTER request so that subsequent requests to the registered
   address-of-record are routed to the SBC.  This is done either with a
   Path header, or by modifying the Contact to point at the SBC.


3.  Functions of SBCs

   This section lists those functions that are used in SBC deployments
   in current communication networks.  Each subsection describes a
   particular function or feature, the operators' requirements for
   having it, explanation on any impact to the end-to-end SIP
   architecture, and a concrete implementation example.  Each section
   also discusses potential concerns specific to that particular
   implementation technique.  Suggestions for alternative implementation
   techniques that may be more architecturally compatible with SIP are
   outside the scope of this document.

   All the examples given in this section are simplified; only the
   relevant header lines from SIP and SDP [4] messages are displayed.

3.1.  Topology Hiding

3.1.1.  General Information and Requirements

   Topology hiding consists of limiting the amount of topology
   information given to external parties.  Operators have a requirement
   for this functionality because they do not want the IP addresses of



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   their equipment (proxies, gateways, application servers, etc) to be
   exposed to outside parties.  This may be because they do not want to
   expose their equipment to DoS (Denial of Service) attacks, they may
   use other carriers for certain traffic and do not want their
   customers to be aware of it or they may want to hide their internal
   network architecture from competitors or partners.  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.

   The most common form of topology hiding is the application of header
   privacy (see Section 5.1 of [2]), which involves stripping Via and
   Record-Route headers and replacing the Contact header.  However, in
   deployments which use IP addresses instead of domain names in headers
   that cannot be removed (e.g.  From and To headers), the SBC may
   replace these IP addresses with its own IP address or domain name.

3.1.2.  Architectural Issues

   This functionality is based on a hop-by-hop trust model as opposed to
   an end-to-end trust model.  The messages are modified without
   subscriber consent and could potentially modify or remove information
   about the user's privacy, security requirements and higher layer
   applications which are communicating end-to-end using SIP.  Either
   user in an end-to-end call may perceive this as a Man In The Middle
   (MitM) attack.

   Modification of IP addresses in Unifor Resource Indetifiers (URIs)
   within SIP headers can lead to application failures if these URIs are
   communicated to other SIP servers outside the current dialog.  These
   URIs could appear in a REFER request or in the body of NOTIFY request
   as part of an event package.  If these messages traverse the same
   SBC, it has the opportunity to restore the original IP address.  On
   the other hand, if the REFER or NOTIFY message returns to the
   original network through a different SBC that does not have access to
   the address mapping, the recipient of the message will not see the
   original address.  This may cause the application function to
   fail.[[Comment.1: Do we have a sane example of where this is a real
   problem?  It sounds somewhat contrived to me, but I agree it is a
   theoretical concern - Alan.]][[Comment.2: I personally would like to
   include this text, although it might be more of a theoretical
   concern. - Jani]]

3.1.3.  Example

   The current way of implementing topology hiding 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 Via entries) from



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   outgoing messages.

   Imagine the following example scenario: The SBC
   (p4.domain.example.com) is receiving an INVITE request from the inner
   network, which in this case is an operator network.  The received SIP
   message is shown in Figure 4.

     INVITE sip:callee@u2.domain.example.com SIP/2.0
     Contact: sip:caller@u1.example.com
     Record-Route: <sip:p3.middle.example.com>
     Record-Route: <sip:p2.example.com;lr>
     Record-Route: <sip:p1.example.com;lr>

   Figure 4: INVITE Request Prior to Topology Hiding

   Then the SBC performs a topology hiding function.  In this scenario,
   the SBC removes and stores all existing Record-Route headers, and
   then inserts a Record-Route header field with its own SIP URI.  After
   the topology hiding function, the message could appear as shown in
   Figure 5.

     INVITE sip:callee@u2.domain.example.com SIP/2.0
     Contact: sip:caller@u1.example.com
     Record-Route: <sip:p4.domain.example.com;lr>

   Figure 5: INVITE Request After Topology Hiding

   Like a regular proxy server that inserts a Record-Route entry, the
   SBC handles every single message of a given SIP dialog.  If the SBC
   loses state (e.g., the SBC restarts for some reason), it may 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
   may not be able to route responses to that request; depending on the
   information that was lost when the SBC restarted.  [[Comment.3: There
   are techniques to mitigate this problem, not all SBCs suffer from
   this.  Is this worth capturing in the text?  [Alan]]][[Comment.4: No,
   not all suffer from this, but some do, so I believe we shouldn't
   remove this text. - Jani]]

   This is only one example of topology hiding, in some cases, SBCs may
   modify other headers, including the Contact header field values.

3.2.  Media Traffic Shaping

3.2.1.  General Information and Requirements

   Media traffic shaping is the act of controlling media traffic.
   Network operators may require this functionality in order to control



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   the traffic being carried on their network on behalf of their
   subscibers.  Traffic shaping helps 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 where required to support audit or legal
   obligations.

   Since the media path is independent of the signaling path, the media
   may not traverse through the operator's network unless the SBC
   modifies the session description.  By modifying the session
   description the SBC can force the media to be sent through a media
   relay which may be co-located with the SBC.

   Some operators do not want to reshape the traffic, but only to
   monitor it for collecting statistics and making sure that they are
   able to meet any business service level agreements with their
   subscribers and/or partners.  The protocol techniques needed for
   monitoring media traffic are the same as for reshaping media traffic.

   SBCs on the media path are also capable of dealing with the "lost
   BYE" issue if either endpoint dies in the middle of the session.  The
   SBC can detect that the media has stopped flowing and issue a BYE to
   the both sides to cleanup any state in other intermediate elements
   and the endpoints.

   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.  Architectural Issues

   Implementing traffic shaping in this manner requires the SBC 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.  Again, messages are modified without subscriber
   consent, and user agents do not have any way to distinguish the SBC
   actions from an attack by a MitM (Man-in-the-Middle).

   In this application, the SBC may originate messages that the user may
   not be able to authenticate as coming from the dialog peer or the SIP
   Registrar/Proxy.

3.2.3.  Example

   Traffic shaping may be performed in the following way: The SBC
   behaves as a B2BUA and inserts itself, or some other entity under the



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   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
   user-agent and performs the identical operation with media traveling
   in the reverse direction.

   CODEC restriction is an example application of traffic shaping.  The
   SBC restricts the codec set negotiated in the offer/answer exchange
   [3] 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 or
   take other appropriate (configured) actions (e.g. alarming or
   reporting).

   Consider the following example scenario: The SBC receives an INVITE
   request from the one network, which in this case is an access
   network.  The received SIP message contains the SDP session
   descriptor shown in Figure 6.

     v=0
     o=mhandley 2890844526 2890842807 IN IP4 10.16.64.4
     c=IN IP4 10.16.64.4
     m=audio 49230 RTP/AVP 96 98
     a=rtpmap:96 L8/8000
     a=rtpmap:98 L16/16000/2

   Figure 6: Request Prior to Media Shaping

   In this example, the SBC performs the media traffic shaping function
   by rewritting the 'm' line, and removing one 'a' line according to
   some (external) policy.  Figure 7 shows the session description after
   the traffic shaping function.

     v=0
     o=mhandley 2890844526 2890842807 IN IP4 10.16.64.4
     c=IN IP4 10.16.64.4
     m=audio 49230 RTP/AVP 96
     a=rtpmap:96 L8/8000

   Figure 7: Request Body After Media Shaping

   One problem with media traffic shaping is that the SBC needs to
   understand the session description protocol and all 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, SBCs in the network may need to be upgraded in
   conjunction with the endpoints.  Certain extensions that do not



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   require active manipulation of the session descriptors to facilitate
   traffic shaping will be able to be deployed without upgrading
   existing SBCs, depending on the degree of transparency the SBC
   implementation affords.  In cases requiring an SBC modification to
   support the new protocol features, the rate of service deployment may
   be affected.  [[Comment.5: I do not think this will slow down
   innovation; innovation is a distinct phase of development and
   separable from operational network deployment. -Alan]][[Comment.6: I
   don't quite get what you are suggesting.  If you want to change the
   text, go ahead. - Jani]]

3.3.  Fixing Capability Mismatches

3.3.1.  General Information and Requirements

   SBCs fixing capability mismatches enable communications between user
   agents with different capabilities, SIP profiles or extensions.  For
   example, user agents on networks which implement different SIP
   Profiles (for example 3GPP or Packet Cable etc) or those that support
   different IP versions, different codecs, or that are in different
   address realms.  Operators have a requirement and a strong motivation
   for performing capability mismatch fixing, so that they can provide
   transparent communication across different domains.  In some cases
   different SIP extensions or methods to implement the same SIP
   application (like monitoring session liveness, call history/diversion
   etc) may also be interworked through the SBC.

3.3.2.  Architectural Issues

   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 concerns as SBCs performing traffic shaping: the
   SBC modifies SIP messages without explicit consent from any of the
   user agents.  This may break end-to-end security and application
   extensions negotiation.

   [[Comment.7: I have removed the network engineering concern; this is
   an unrealistic anti-Apple-Pie problem that could only arise through a
   fundamental bug in either configuration or SBC implementation.
   -Alan]][[Comment.8: Ok. - Jani]]

3.3.3.  Example

   Consider the following example scenario where the inner network is an
   access network using IPv4 and the outer network is using IPv6.  The
   SBC receives an INVITE request with a session description from the
   access network:




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     INVITE sip:callee@ipv6.domain.example.com SIP/2.0
     Via: SIP/2.0/UDP 192.0.2.4
     Contact: sip:caller@u1.example.com

     v=0
     o=mhandley 2890844526 2890842807 IN IP4 192.0.2.4
     c=IN IP4 192.0.2.4
     m=audio 49230 RTP/AVP 96
     a=rtpmap:96 L8/8000

   Figure 8: Request Prior to Capabilities Match

   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.  Figure 9 shows
   the request after the capability mismatch adjusment.

     INVITE sip:callee@ipv6.domain.com SIP/2.0
     Record-Route: <sip:[2001:620:8:801:201:2ff:fe94:8e10];lr>
     Via: SIP/2.0/UDP sip:[2001:620:8:801:201:2ff:fe94:8e10]
     Via: SIP/2.0/UDP 192.0.2.4
     Contact: sip:caller@u1.example.com

     v=0
     o=mhandley 2890844526 2890842807 IN IP4 192.0.2.4
     c=IN IP6 2001:620:8:801:201:2ff:fe94:8e10
     m=audio 49230 RTP/AVP 96
     a=rtpmap:96 L8/8000

   Figure 9: Request After Capability Match

   This message is then sent by the SBC to the onward IPv6 network.

3.4.  NAT Traversal

3.4.1.  General Information and Requirements

   NAT traversal in this instance refers to the specific message
   modifications required to assist a user-agent in maintaining SIP and
   media connectivity when there is a NAT device located between the
   user-agent and the proxy/registrar and, most likely, any other user-
   agent.

   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.  NATs are widely deployed in various
   access networks today, so operators have a requirement to support it.
   When the registrar receives a REGISTER request from the user agent



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   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 for several reasons: (i) preventing packets from
   unregistered users to prevent chances of DoS attack, (ii)
   prioritization and/or re-routing of traffic (based on user or
   service, like E911) as it enters the network, and (iii) performing a
   load balancing function or reducing the load on other network
   equipment.

   Also other measures, such as acting as a media relay by modifying SDP
   session descriptors (see Section 3.2), may be taken by SBC to ensure
   media connectivity.

3.4.2.  Architectural Issues

   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
   registrar.

   [[Comment.9: Is there something more specific to this function we can
   put here?  This is very generic and a general limitation of SBC
   architectures.]][[Comment.10: If you want to add something, please
   do. - Jani]]

3.4.3.  Example

   Consider the following example scenario: The SBC resides between the
   UA and Registrar.  Previously the UA has sent a REGISTER request to
   Registrar, and the SBC receives the registration response shown in
   Figure 10.





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     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

   Figure 10: Response Prior to NAT Maintenance Function

   When performing the NAT traversal function, the SBC may re-write the
   expiry time to coax the UA to re-register prior to the intermediating
   NAT deciding to close the pinhole.  Figure 11 shows a possible
   modification of the response from Figure 10.

     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

   Figure 11: Manipulated Response for NAT Traversal

   Naturally also other measures need to be taken in order to enable the
   NAT traversal, but this example illustrates only one mechanism for
   preserving the SIP related NAT bindings.

3.5.  Access Control

3.5.1.  General Information and Requirements

   Network operators may wish to control what kind of signaling and
   media traffic their network carries.  There is strong motivation and
   a requirement to do access control on the edge of an operator's
   network.  Access control can be based on, for example, IP addresses
   or SIP identities.

   This function can be implemented by protecting the inner network with
   firewalls and configuring them so that they only accept SIP traffic
   from the SBC.  This way, all the SIP traffic entering the inner
   network needs to be routed though the SBC, which only routes messages
   from authorized parties or traffic that meets a specific policy that
   is expressed in the SBC administratively.

   Access control can be applied either only to the signaling, or to
   both the signaling and media.  If it is applied only to the
   signaling, then the SBC might behave as a proxy server.  If access
   control is applied to both the signaling and media, then the SBC
   behaves in a similar manner as explained in Section 3.2.  A key part
   of media-layer access control is that only media for authorized



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   sessions is allowed to pass through the SBC and/or associated media
   relay devices.

   In environments where there is limited bandwidth on the access links,
   the SBC can compute the potential bandwidth usage by examining the
   codecs present in SDP offers and answers.  With this information, the
   SBC can reject sessions before the available bandwidth is exhausted
   to allow existing sessions to maintain acceptable quality of service.
   Otherwise, the link could become over subscribed and all sessions
   would experience a deterioration in quality of service.  SBCs may
   contact a policy server to determine whether sufficient bandwidth is
   available on a per-session basis.

3.5.2.  Architectural Issues

   Since the SBC needs to handle all SIP messages, this function has
   scalability implications.  In addition, the SBC is a single point of
   failure from an architectural point of view.  Although, in practice,
   many current SBCs have the capability to support redundant
   configuration, which prevents the loss of calls and/or sessions in
   the event of a failure on a single node.

   [[Comment.11: I am tempted to remove this paragraph; this is a
   general architectural problem that is not truly specific to SBCs.  A
   proxy configured into a SIP architecture that Record-Route'd requests
   would ALSO be a single point of failure.  Provisioning a network to
   deal with the outage of a single element is just good design.
   -Alan]][[Comment.12: I agree that this is not specific only to SBCs,
   but is specific also to SBCs.  I wouldn't like to remove this
   paragraph. - Jani]]

   If access control is performed only on behalf of signaling, then the
   SBC is compatible with general SIP architectural principles, but if
   it is performed for signaling and for media, then there are similar
   problems as described in Section 3.2.2.

3.5.3.  Example

   Figure 12 shows a callflow where the SBC is providing both signaling
   and media access control.











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        caller                    SBC                     callee
          |                        |                        |
          |  Identify the caller   |                        |
          |<- - - - - - - - - - - >|                        |
          |                        |                        |
          |      INVITE + SDP      |                        |
          |----------------------->|                        |
          |                [Modify the SDP]                 |
          |                        | INVITE + modified SDP  |
          |                        |----------------------->|
          |                        |                        |
          |                        |      200 OK + SDP      |
          |                        |<-----------------------|
          |                [Modify the SDP]                 |
          |                        |                        |
          | 200 OK + modified SDP  |                        |
          |<-----------------------|                        |
          |                        |                        |
          |       Media   [Media inspection]   Media        |
          |<======================>|<======================>|
          |                        |                        |

   Figure 12: Example Access Callflow

   In this scenario, the SBC first identifies the caller, so it can
   determine whether or not to give signaling access for the caller.
   Some SBCs may rely on the proxy to authenticate the user-agent
   placing the call.  After authentication, the SBC modifies the session
   descriptors in INVITE and 200 OK messages in a way that the media is
   going to flow through SBC itself.  When the media starts flowing, the
   SBC can inspect whether the callee and caller use the codec(s) that
   they had previously agreed on.

3.6.  Protocol Repair

3.6.1.  General Information and Requirements

   SBC are also used to repair protocol messages generated by not-fully-
   standard clients.  Operators may wish to support protocol repair, if
   they want to support as many client as possible.  It is noteworthy,
   that this function affects only the signaling component of SBC, and
   that protocol repair function is not the same as protocol conversion.

3.6.2.  Architectural Issues

   In most cases, this function can be seen as being compatible with SIP
   architectural principles, and it does not violate the end-to-end
   model of SIP.  The SBC repairing protocol messages behaves as a proxy



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   server that is liberal in what it accepts and strict in what it
   sends.

3.6.3.  Examples

   The SBC can, for example, receive the an INVITE message from a not-
   fully-standard client as illustrated in Figure 13.

     INVITE sip:callee@sbchost.example.com
     Via: SIP/2.0/UDP u1.example.com:5060;lr=1
     From: Caller <sip:caller@one.example.com>
     To: Callee <sip:callee@two.example.com>
     Call-ID: 18293281@u1.example.com
     CSeq: 1 INVITE
     Contact: sip:caller@u1.example.com

   Figure 13: Request from non-standard client

   If the SBC does protocol repair, it can re-write the request line (in
   this case the UAC did not put the target in the URI and instead put
   the SBC hostname as the host portion).  It could also remove excess
   white spaces to make the SIP message more human readable.

   Some other examples of "protocol repair" that have been implemented
   in commercially available SBCs include:

   o  Changing Content-Disposition from "signal" to "session".  This was
      required for a user agent which sent an incorrect Content-
      Disposition header.
   o  Addition of userinfo to a Contact URI when none was present.  This
      was required for a softswitch/proxy that would reject requests if
      the Contact URI had no user part.
   o  Addition of a to-tag to provisional or error responses.
   o  Re-ordering of Contact header values in a REGISTER response.  This
      was required for a user agent that would take the expires value
      from the first Contact header value without matching it against
      its Contact value.
   o  Correction of SDP syntax where the user agent used "annexb=" in
      the fmtp attribute instead of the proper "annexb:".
   o  Correction of signaling errors (convert BYE to CANCEL) for
      termination of early sessions.
   o  Repair of header parameters in 'archaic' or incorrect formats.
      Some older stacks assume that parameters are always of the form
      NAME=VALUE.  For those elements, it is necessary to convert
      'lr=true' or 'lr=1' to 'lr' in order to interoperate with several
      commercially available stacks and proxies.





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3.7.  Media Encryption

3.7.1.  General Information and Requirements

   SBCs are used to perform media encryption / decryption at the edge of
   the network.  This is the case when media encryption is used only on
   the access network (outer network) side and the media is carried
   unencrypted in the inner network.  Operators may have an obligation
   to provide the ability to do legal interception, while they still
   want to give their customers the ability to encrypt media in the
   access network.  This leads to a situation where operators have a
   requirement to perform media encryption function.

3.7.2.  Architectural Issues

   While performing a media encryption function, SBCs need to be able to
   inject either themselves, or some other entity to the media path.
   Due to this, the SBCs have the same architectural issues as explained
   in Section 3.2.

3.7.3.  Example

   Figure 14 shows an example where the SBC is performing media
   encryption related functions.



























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     caller              SBC#1                SBC#2              callee
      |                    |                    |                    |
      |   INVITE + SDP     |                    |                    |
      |------------------->|                    |                    |
      |             [Modify the SDP]            |                    |
      |                    |                    |                    |
      |                    | INVITE + mod. SDP  |                    |
      |                    |------------------->|                    |
      |                    |             [Modify the SDP]            |
      |                    |                    |                    |
      |                    |                    | INVITE + mod. SDP  |
      |                    |                    |------------------->|
      |                    |                    |                    |
      |                    |                    |     200 OK + SDP   |
      |                    |                    |<-------------------|
      |                    |             [Modify the SDP]            |
      |                    |                    |                    |
      |                    | 200 OK + mod. SDP  |                    |
      |                    |<-------------------|                    |
      |             [Modify the SDP]            |                    |
      |                    |                    |                    |
      |  200 OK + mod. SDP |                    |                    |
      |<-------------------|                    |                    |
      |                    |                    |                    |
      |    Encrypted       |         Plain      |         Encrypted  |
      |      media     [enc./dec.]   media   [enc./dec.]    media    |
      |<==================>|<- - - - - - - -  ->|<==================>|
      |                    |                    |                    |

   Figure 14: Media Encryption Example

   First the UAC sends an INVITE request , and the first SBC modifies
   the session descriptor in a way that it injects itself to the media
   path.  The same happens in the second SBC.  Then the UAS replies with
   a 200 OK response, the SBCs inject themselves in the returning media
   path.  After signaling the media start flowing, and both SBCs are
   performing media encryption and decryption.


4.  Derived Requirements

   Some of the functions listed in this document are more SIP-unfriendly
   than others.  This list requirements that are derived from the
   functions that break the principles of SIP in one way or the other.
   The derived requirements are:






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   Req-1: There should be a SIP-friendly way to hide network topology
          information.  Currently this is done e.g., by stripping and
          replacing header fields, which is against the principles of
          SIP.
   Req-2: There should be a SIP-friendly way to direct media traffic
          through intermediaries.  Currently this is done e.g., by
          modifying session descriptors, which is against the principles
          of SIP.
   Req-3: There should be a SIP-friendly way to fix capability
          mismatches in SIP messages.  Currently this is done by
          modifying SIP messages, which violates e.g., end-to-end
          security.

   All the above-mentioned requirements are such that they do not have
   an existing solution today.  Thus, future work is needed in order to
   develop solutions to these requirements.


5.  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.

   [[Comment.13: I wonder if it is worth classifying the specific type
   of security problems and assembling them here.  The remainder of this
   document can then refer to the specific problem a given operational
   activity has given today's typically implementation mechanisms.
   [Alan]]][[Comment.14: My gut feeling is that it would require a lot
   of work.  If you want to do it, go ahead, but at least I don't have
   the time to do it. - Jani]]


6.  IANA Considerations

   This document has no IANA considerations.


7.  Acknowledgements

   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.


8.  References



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8.1.  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]  Peterson, J., "A Privacy Mechanism for the Session Initiation
        Protocol (SIP)", RFC 3323, November 2002.

   [3]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        Session Description Protocol (SDP)", RFC 3264, June 2002.

8.2.  Informational References

   [4]  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


   Robert F. Penfield
   Acme Packet
   71 Third Avenue
   Burlington, MA  01803
   US

   Email: bpenfield@acmepacket.com


   Alan Hawrylyshen
   Ditech Networks Inc.
   Suite 200, 1167 Kensington Cres NW
   Calgary, Alberta  T2N 1X7
   Canada

   Email: ahawrylyshen@ditechnetworks.com


   Medhavi Bhatia
   NexTone Communications
   101 Orchard Ridge Drive
   Gaithersburg, MD  20878
   US

   Email: mbhatia@nextone.com






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