SIPPING Working Group                                 J. Hautakorpi, Ed.
Internet-Draft                                              G. Camarillo
Intended status: Informational                                  Ericsson
Expires: June 23, September 26, 2008                                  R. Penfield
                                                             Acme Packet
                                                          A. Hawrylyshen
                                                    Ditech Networks Inc.
                                                               M. Bhatia
                                                       December 21, 2007
                                                          March 25, 2008

   Requirements from SIP (Session Initiation Protocol) Session Border
                          Control Deployments

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Copyright Notice

   Copyright (C) The IETF Trust (2007). (2008).


   This document describes functions implemented in Session Initiation
   Protocol (SIP) intermediaries known as Session Border Controllers
   (SBCs).  The goal of this document is to describe the commonly
   provided functions of SBCs.  A special focus is given to those
   practices that are viewed to be in conflict with SIP architectural
   principles.  This document 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.

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  . . . . . . . . . . . . . . . . . . . . .  8
       3.1.1.  General Information and Requirements . . . . . . . . .  8
       3.1.2.  Architectural Issues . . . . . . . . . . . . . . . . .  8
       3.1.3.  Example  . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Media Traffic Management . . . . . . . . . . . . . . . . . 10
       3.2.1.  General Information and Requirements . . . . . . . . . 10
       3.2.2.  Architectural Issues . . . . . . . . . . . . . . . . . 11
       3.2.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 11
     3.3.  Fixing Capability Mismatches . . . . . . . . . . . . . . . 12
       3.3.1.  General Information and Requirements . . . . . . . . . 12
       3.3.2.  Architectural Issues . . . . . . . . . . . . . . . . . 13
       3.3.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 13
     3.4.  Maintaining SIP-related NAT Bindings . . . . . . . . . . . 14
       3.4.1.  General Information and Requirements . . . . . . . . . 14
       3.4.2.  Architectural Issues . . . . . . . . . . . . . . . . . 15
       3.4.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 15
     3.5.  Access Control . . . . . . . . . . . . . . . . . . . . . . 16
       3.5.1.  General Information and Requirements . . . . . . . . . 16
       3.5.2.  Architectural Issues . . . . . . . . . . . . . . . . . 17
       3.5.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 17
     3.6.  Protocol Repair  . . . . . . . . . . . . . . . . . . . . . 18
       3.6.1.  General Information and Requirements . . . . . . . . . 18
       3.6.2.  Architectural Issues . . . . . . . . . . . . . . . . . 18 19
       3.6.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . 18 19
     3.7.  Media Encryption . . . . . . . . . . . . . . . . . . . . . 19
       3.7.1.  General Information and Requirements . . . . . . . . . 19
       3.7.2.  Architectural Issues . . . . . . . . . . . . . . . . . 19 20
       3.7.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 19 20
   4.  Derived Requirements for Future SIP Standardization Work . . . 20 21
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 23
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 22 23
     8.2.  Informational References . . . . . . . . . . . . . . . . . 22 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24 25

1.  Introduction

   In the past few years there has been a rapid adoption of the Session
   Initiation Protocol (SIP) [1] and deployment of SIP-based
   communications networks.  This has often outpaced 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 behave badly in a sense that they
   break end-to-end security and impact feature negotiations, there is
   clearly a market for them.  Network operators need many of the
   features current SBCs provide and often there are no standard
   mechanisms available to provide them.

   The purpose of this document is to describe functions implemented in
   SBCs.  A special focus is given to those practices that are
   conflicting with SIP architectural principles in some way.  The
   document 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.

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, and DoS
   prevention and detection); b) functionality not available in the
   endpoints (NAT traversal, protocol interworking or repair); and c)
   network management (traffic monitoring, management, and QoS).  Some
   of these functions may also get integrated into other SIP elements
   (like pre-paid platforms, 3GPP P-CSCF [5], 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,
   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.  An SBC
   is logically associated to the inner network, and it typically
   provides functions such as controlling and protecting access to the
   inner network from the outer network.

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.

            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 an SBC where
   Operator A is the outer network, and Operator B is the inner network.
   Operator B can use the SBC, for example, 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's network, protect its components (media servers,
   application servers, gateways, etc.) from unauthorized use and DoS
   attacks, and monitor the signaling and media traffic.  Also, since
   the SBC is call stateful, it may provide access control functions to
   prevent over subscription 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.

           Access Network                  Operator Network

         | UA1 |<---------\
         +-----+           \
         +-----+             \------->+-----+       +-------+
         | UA2 |<-------------------->| SBC |<----->| proxy |<-- -
         +-----+                 /--->+-----+       +-------+
         +-----+   +-----+     /
         | UA3 +---+ NAT |<---/
         +-----+   +-----+

                    Figure 3: Access scenario with SBC

   The SBC may be hosted in the access network (e.g,. this is common
   when the access network is an enterprise network), or in the operator
   network (e.g., this is common when the access network is a
   residential or small business network).

   Some endpoints may be behind enterprise or residential NATs.  In
   cases where the access network is a private network, the SBC is the a NAT
   for all traffic.  It is noteworthy that SIP traffic may have to
   traverse more that one NAT.  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.

   The scenario presented in this section is a general one, and it
   applies also to other similar settings.  One example from a similar
   setting is the one where an access network is the open internet, and
   the operator network is the network of a SIP service provider.

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 of 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 [6] 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
   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, replacing the Contact header, and even changing
   Call-IDs.  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.  Neither
   user agent in an end-to-end call has any way to distinguish the SBC
   actions from a Man-In-The-Middle (MitM) attack.

   Topology hiding function does not work well with Authenticated
   Identity Management [3].  The Authenticated Identity Management
   mechanism is based on a hash value that is calculated from parts of
   From, To, Call-Id, CSeq, Date, and Contact header fields plus from
   the whole message body.  If the authentication service is not
   provided by the SBC itself, the modification of the forementioned
   header fields and the message body is in violation of [3].  Some
   forms of topology hiding are in violation, because they are e.g.,
   replacing the Contact header of a SIP message.

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) Agent) and remove all traces of
   topology information (e.g., Via and Record-Route entries) from
   outgoing messages.

   Imagine the following example scenario: The SBC
   ( receives 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/2.0
    Via: SIP/2.0/UDP;branch=z9hG4bK48jq9w9174131.1
    Via: SIP/2.0/UDP;branch=z9hG4bK18an6i9234172.1
    Via: SIP/2.0/UDP;branch=z9hG4bK39bn2e5239289.1
    Via: SIP/2.0/UDP;branch=z9hG4bK92fj4u7283927.1
    Record-Route: <> <;lr>
    Record-Route: <;lr>
    Record-Route: <;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 Via and Record-Route headers,
   and then inserts a Via and Record-Route header fields with its own SIP
   URI.  After the topology hiding function, the message could appear as
   shown in Figure 5.

    INVITE SIP/2.0
    Via: SIP/2.0/UDP;branch=z9hG4bK92es3w1230129.1
    Record-Route: <;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. properly (note: some SBCs preserve the state
   information also on restart).  For example, if the SBC removes "Via"
   entries from a request and then restarts, thus 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.

   This is only one example of topology hiding.  In some cases,  Besides topology hiding
   (i.e., information related to network elements is beeing hidden),
   SBCs may also do identity hiding (i.e., information related to
   identity of subscribers in beeing hidden).  While performing identity
   hiding, SBCs may modify other headers, including the Contact header field values. values and other header
   fields containing identity information.  The header fields containing
   identity information is listed in Section 4.1 of [2].  Since the
   publication of [2], the following header fields containing identity
   information have been defined: "P-Asserted-Identity", "Referred-By",
   "Identity", and "Identity-Info".

3.2.  Media Traffic Management

3.2.1.  General Information and Requirements

   Media traffic management is the function of controlling media
   traffic.  Network operators may require this functionality in order
   to control the traffic being carried on their network on behalf of
   their subscribers.  Traffic management helps the creation of
   different kinds of billing models (e.g., video telephony can be
   priced differently to voice-only calls) and it also makes it possible
   for operators to enforce the usage of selected codecs.  Additionally,
   traffic management 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.  This kind of traffic
   management can be done, for example, to ensure a certain QoS (Quality
   of Service) level. level, or to ensure that subscribers are using only
   allowed codecs.

   Some operators do not want to manage the traffic (e.g., allowing only
   certain codecs), 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 managing 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
   both sides to cleanup any state in other intermediate elements and
   the endpoints.

   One possible form of media traffic management is that SBCs terminate
   media streams and SIP dialogs by generating BYE requests.  This kind
   of procedure can take place, for example, in a situation where the
   subscriber runs out of credits.  Media management is needed for
   ensuring to ensure
   that the subscriber cannot just ignore the BYE request generated by
   the SBC and continue their media sessions.

3.2.2.  Architectural Issues

   Implementing traffic management 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.  Furthermore, this is in violation
   of Authenticated Identity Management [3], see Section 3.1.2.

   Media traffic management function also hinders innovations.  The
   reason for the hinderance is that in many cases SBCs need to be able
   to support new ways of communicating (e.g., extensions to the SDP
   protocol) before new services can be taken into use, and that slows
   down the adoption of innovations.

   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

3.2.3.  Example

   Traffic management may be 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
   user-agent and performs the identical operation with media traveling
   in the reverse direction.

   As mentioned in Section 3.2.1, codec restriction is a form of traffic
   management.  The SBC restricts the codec set negotiated in the offer/
   answer exchange [4] 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.
   raise an alarm).

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

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

                Figure 6: Request Prior to Media Management

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

     o=owner 2890844526 2890842807 IN IP4
     c=IN IP4
     m=audio 49230 RTP/AVP 96
     a=rtpmap:96 L8/8000

               Figure 7: Request Body After Media Management

   Media traffic management has a problem where 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.  It is noteworthy that a similar
   problem, but with header fields, applies to, for example, topology
   hiding function, see Section 3.1.  Certain extensions that do not
   require active manipulation of the session descriptors to facilitate
   traffic management 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.

3.3.  Fixing Capability Mismatches

3.3.1.  General Information and Requirements

   SBCs fixing capability mismatches enable communications between user-
   agents with different capabilities or extensions.  For example, user-
   agents on networks which implement SIP differently (for example 3GPP
   or Packet Cable SIP [1] network 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 management:
   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.

   The capability mismatch fixing is a fragile function in the long
   term.  The number of incompatibilities built into various network
   elements is increasing the fragility and complexity over time.  This
   might lead to a situation where SBCs need to be able to handle a
   large number of capability mismatches in parallel.

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:

     INVITE SIP/2.0
     Via: SIP/2.0/UDP

     o=owner 2890844526 2890842807 IN IP4
     c=IN IP4
     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
   scenario 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 adjustment.

     INVITE SIP/2.0
     Record-Route: <sip:[2001:DB8::801:201:2ff:fe94:8e10];lr>
     Via: SIP/2.0/UDP sip:[2001:DB8::801:201:2ff:fe94:8e10]
     Via: SIP/2.0/UDP

     o=owner 2890844526 2890842807 IN IP4
     c=IN IP6 2001:DB8::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.  Maintaining SIP-related NAT Bindings

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 a user-
   agent and a proxy/registrar and, possibly, any other user-agent.  The
   primary purpose of NAT traversal function is to keep up a control
   connection to user-agents behind NATs.  This can, for example, be
   achieved by generating periodic network traffic that keeps bindings
   in NATs alive.  SBCs' NAT traversal function is required in scenarios
   where the NAT is outside the SBC (i.e., not in cases where SBC itself
   acts as a NAT).

   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
   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

   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

   SBCs can also force traffic to go through a media relay for NAT
   traversal purposes (more about media traffic management in
   Section 3.2).  A typical call has media streams to two directions.
   Even though SBCs can force media streams from both directions to go
   through a media relay, in some cases it is enough to relay only the
   media from one direction (e.g., in a scenario where only the other
   endpoint is behind a NAT).

3.4.2.  Architectural Issues

   This approach to NAT traversal does not work when if end-to-end
   confidentiality or integrity-protection is used. mechanisms are used (e.g.,
   S/MIME).  The SBC would be seen as a MitM modifying the messages
   between the user-agent and the registrar.

   There is also a problem related to the method how SBCs choose the
   value for the validity of a registration period.  This value should
   be as high as possible, but it still needs to be low enough to
   maintain the NAT binding.  Typically SBCs do not have any
   deterministic method for choosing a suitable value.

3.4.3.  Example

   Consider the  However, SBCs
   can just use a sub-optimal, relatively small value which usually

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.

     SIP/2.0 200 OK
     From: Bob <>;tag=a73kszlfl
     To: Bob <>;tag=34095828jh
     CSeq: 1 REGISTER
     Contact: <>;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 <>;tag=a73kszlfl
     To: Bob <>;tag=34095828jh
     CSeq: 1 REGISTER
     Contact: <>;expires=60

             Figure 11: Manipulated Response for NAT Traversal

   Naturally also other measures need to could be taken in order to enable the
   NAT traversal, traversal (e.g., non-SIP keepalive messages), but this example
   illustrates only one mechanism for preserving the SIP related SIP-related NAT

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, link-layer
   identifiers, 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
   sessions is allowed to pass through the SBC and/or associated media
   relay devices.

   Operators implement some functionalities, like NAT traversal for
   example, in an SBC instead of other elements in the inner network 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.

   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 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.

   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 (ACKs omitted for brevity).

        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.
   This might be achieved using information gathered during
   registration, or by other means.  Some SBCs may rely on the proxy to
   authenticate the user-agent placing the call.  After identification,
   the SBC modifies the session descriptors in INVITE and 200 OK
   messages in a way so that the media is going to flow through the 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

   SBCs are also used to repair protocol messages generated by not-
   fully-standard compliant or badly implemented clients.  Operators may
   wish to support protocol repair, if they want to support as many
   clients as possible.  It is noteworthy, that this function affects
   only the signaling component of an SBC, and that the protocol repair
   function is not the same as protocol conversion. conversion (i.e., making
   translation between two completely different protocols).

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

   A similar problem related to increasing complexity, as explained in
   Section 3.3.2, also affects protocol repair function.

3.6.3.  Examples

   The SBC can, for example, receive an INVITE message from a relatively
   new SIP UA as illustrated in Figure 13.

     Via: SIP/2.0/UDP;lr
     From: Caller <>
     To:        Callee   <>
     CSeq: 1   INVITE

              Figure 13: Request from a relatively new client

   If the SBC does protocol repair, it can re-write the 'lr' parameter
   on the Via header field into the form 'lr=true', in order to support
   some older, non-standard badly implemented SIP stacks.  It could also remove
   excess white spaces to make the SIP message more human readable.

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 (e.g., Secure
   Real-time Transport Protocol (SRTP)) 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.  It
   must be noted that this kind of behavior is the same as a classical
   MitM attack.  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 (ACKs omitted for brevity).

     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, response and the SBCs inject themselves in the returning
   media path.  After signaling the media start starts flowing, and both SBCs
   are performing media encryption and decryption.

4.  Derived Requirements for Future SIP Standardization Work

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

   Req-1:  There should be a SIP-friendly way to hide network topology
           information.  Currently this is done by stripping and
           replacing header fields, which is against the principles of
           SIP (see Section 3.1).
   Req-2:  There should be a SIP-friendly way to direct media traffic
           through intermediaries.  Currently this is done without user
           consent by modifying session descriptors, which is against
           the principles of SIP. SIP (see Section 3.2, Section 3.4,
           Section 3.5, and Section 3.7).
   Req-3:  There should be a SIP-friendly way to fix capability
           mismatches in SIP messages.  This requirement is harder to
           fulfill on complex mismatch cases, like the 3GPP/Packet Cable 3GPP/SIP [1]
           network mismatch.  Currently this is done by modifying SIP
           messages, which violates end-to-end security. security (see Section 3.3
           and Section 3.6).

   Req-1 and Req-3 does do not have an exiting solution today.  There is
   ongoing work in the IETF for addressing Req-2, such as SIP session
   policies, Traversal Using Relays around NAT (TURN), and Interactive
   Connectivity Establishment (ICE).  Nonetheless, future work is needed
   in order to develop solutions to these requirements.

   It is noteworthy that a subset of the functions of SBCs will remain
   as non-standardized functions, because it is not reasonable, or
   feasible to develop a standardized solutions to replace them.
   Examples from this kind of functions are the ability to enforce the
   usage of a specific codec and the protocol repair (see Section 3.6)

5.  Security Considerations

   Many of the functions this document describes have important security
   and privacy implications.  One major security problem is that many
   functions implemented by SBCs (e.g., topology hiding and media
   traffic management) modify SIP messages and their bodies without the
   user agents' consent.  The result is that the user agents may
   interpret the actions taken by an SBC as a MitM attack.  SBCs modify
   SIP messages because it allows them to, for example, protect elements
   in the inner network from direct attacks.

   SBCs that place themselves (or another entity) on the media path can
   be used to eavesdrop on conversations.  Since, often, user agents
   cannot distinguish between the actions of an attacker and those of an
   SBC, users cannot know whether they are being eavesdropped or an SBC
   on the path is performing some other function.  SBCs place themselves
   on the media path because it allows them to, for example, perform
   legal interception.

   On a general level SBCs prevent the use of end-to-end authentication.
   This is because SBCs need to be able to perform actions that look
   like MitM attacks, and in order for user agents to communicate, they
   must allow those type of attacks.  It other words, user agents can
   not use end-to-end security.  This is especially harmful because also
   other network element, besides SBCs, are then able to do similar

   An SBC is a single point of failure from the architectural point of
   view.  This makes it an attractive target for DoS attacks.  The fact
   that some functions of SBCs require those SBCs to maintain session session-
   specific information makes the situation even worse.  If the SBC
   crashes (or is brought down by an attacker), ongoing sessions
   experience undetermined behavior.

   If the IETF decides to develop standard mechanisms to address the
   requirements presented in Section 4, the security and privacy-related
   aspects of those mechanisms will, of course, need to be taken into

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.  Authors would also like to thank Sridhar Ramachandran,
   Gaurav Kulshreshtha, and Rakendu Devdhar.  Reviewers Spencer Dawkins
   and Francois Audet also deserve special thanks.

8.  References

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]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
        Identity Management in the Session Initiation Protocol (SIP)",
        RFC 4474, August 2006.

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

8.2.  Informational References

   [5]  3GPP, "IP Multimedia Subsystem (IMS); Stage 2", 3GPP TS 23.228
        5.15.0, June 2006.

   [6]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
        Description Protocol", RFC 4566, July 2006.

Authors' Addresses

   Jani Hautakorpi (editor)
   Hirsalantie 11
   Jorvas  02420


   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420

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


   Alan Hawrylyshen
   Ditech Networks Inc.
   Suite 200, 1167 Kensington Cres NW
   Calgary, Alberta  T2N 1X7
   825 E Middlefield Rd
   Mountain View, CA


   Medhavi Bhatia
   9700 Great Seneca Hwy.
   Rockville, MD  20850


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