One document matched: draft-ietf-sipping-sbc-funcs-08.xml
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<rfc ipr="trust200811" docName="draft-ietf-sipping-sbc-funcs-08.txt" category="info">
<front>
<title abbrev="Requirements from SBC Deployments">
<!-- Functionality of Existing Session Border Controllers (SBCs) --> Requirements from SIP
(Session Initiation Protocol) Session Border Control Deployments </title>
<author initials="J." surname="Hautakorpi" fullname="Jani Hautakorpi" role="editor">
<organization>Ericsson</organization>
<address>
<postal>
<street>Hirsalantie 11</street>
<code>02420</code>
<city>Jorvas</city>
<country>Finland</country>
</postal>
<email>Jani.Hautakorpi@ericsson.com</email>
</address>
</author>
<author initials="G." surname="Camarillo" fullname="Gonzalo Camarillo">
<organization>Ericsson</organization>
<address>
<postal>
<street>Hirsalantie 11</street>
<code>02420</code>
<city>Jorvas</city>
<country>Finland</country>
</postal>
<email>Gonzalo.Camarillo@ericsson.com</email>
</address>
</author>
<author initials="R.F." surname="Penfield" fullname="Robert F. Penfield">
<organization>Acme Packet</organization>
<address>
<postal>
<street>71 Third Avenue</street>
<code>01803</code>
<city>Burlington</city>
<region>MA</region>
<country>US</country>
</postal>
<email>bpenfield@acmepacket.com</email>
</address>
</author>
<author fullname="Alan Hawrylyshen" initials="A." surname="Hawrylyshen">
<organization>Ditech Networks Inc.</organization>
<address>
<postal>
<street>825 E Middlefield Rd</street>
<city>Mountain View</city>
<region>CA</region>
<country>US</country>
</postal>
<email>alan.ietf@polyphase.ca</email>
</address>
</author>
<author initials="M." surname="Bhatia" fullname="Medhavi Bhatia">
<organization>3CLogic</organization>
<address>
<postal>
<street>9700 Great Seneca Hwy.</street>
<code>20850</code>
<city>Rockville</city>
<region>MD</region>
<country>US</country>
</postal>
<email>mbhatia@3clogic.com</email>
</address>
</author>
<date day="5" month="January" year="2009"/>
<area>Real-Time Applications and Infrastructure</area>
<workgroup>SIPPING Working Group</workgroup>
<keyword>SBC</keyword>
<keyword>SIP</keyword>
<abstract>
<t> 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.</t>
</abstract>
</front>
<middle>
<!-- ################################### -->
<section title="Introduction" anchor="sec-intro">
<t> In the past few years there has been a rapid adoption of the Session Initiation Protocol
(SIP) <xref target="RFC3261"/> 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.</t>
<t> 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.</t>
<t> 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.</t>
</section>
<!-- ################################### -->
<section title="Background on SBCs" anchor="sec-info">
<t> 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.</t>
<t>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 denial of service prevention and detection); b)
functionality not available in the endpoints (NAT traversal, protocol interworking or
repair); and c) traffic management (media monitoring and QoS). Some of these functions may
also get integrated into other SIP elements (like pre-paid platforms, 3GPP P-CSCF <xref
target="3GPP.23.228"/>, 3GPP I-CSCF, etc).</t>
<t> SIP-based SBCs typically handle both signaling and media and can implement behavior which
is equivalent to a "privacy service" (as described in<xref target="RFC3323"/>) 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.</t>
<figure title="SBC architecture" anchor="fig-SBC_placing">
<artwork><![CDATA[
+-----------------+
| SBC |
[signaling] | +-----------+ |
<------------|->| signaling |<-|---------->
outer | +-----------+ | inner
network | | | network
| +-----------+ |
<------------|->| media |<-|---------->
[media] | +-----------+ |
+-----------------+
]]></artwork>
</figure>
<t>
<xref target="fig-SBC_placing"/> 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. The SBC itself is configured and managed by the organization
operating the inner network.</t>
<t>In some scenarios SBCs operate with users' implicit consent and in others they operate
completely without users' consent. For example, if an SBC is at the edge of an enterprise
network performing topology hiding (see Section 3.1), it is in the same administrative
domain as the enterprise users, and the users may choose to route through it. If they choose
to route through the SBC, then the SBC can be seen as having an implicit consent from the
users. Another example is a scenario where a service provider has broken gateways and it
deploys an SBC in front of them for protocol repair (see Section 3.6) reasons, then users
may choose to configure the SBC as their gateway, and so the SBC can be seen as having an
implicit consent.</t>
<!-- ################################### -->
<section title="Peering Scenario" anchor="sec-info-peering">
<t>A typical peering scenario involves two network operators who exchange traffic with each
other. An example peering scenario is illustrated in <xref target="fig-SBC_peering"/>. An
originating gateway (GW) in operator A's network sends an INVITE that is routed to the SBC
in operator B's network. Then the SBC forward it to the softswitch (SS). The softswitch
responds with a redirect (3xx) message back to the SBC that points to the appropriate
terminating gateway in operator B's network. If operator B would not have an SBC, the
redirect message would go to the operator A's originating gateway. After receiving the
redirect message, the SBC sends the INVITE to the terminating gateway.</t>
<figure title="Peering with SBC" anchor="fig-SBC_peering">
<artwork><![CDATA[
Operator A . Operator B
.
. 2) INVITE
+-----+ . /--------------->+-----+
|SS-A | . / 3) 3xx (redir.) |SS-B |
+-----+ . / /---------------+-----+
. / /
+-----+ 1) INVITE +-----+--/ / +-----+
|GW-A1|---------------->| SBC |<---/ 4) INVITE |GW-B1|
+-----+ +-----+---------------------->+-----+
.
+-----+ . +-----+
|GW-A2| . |GW-B2|
+-----+ . +-----+
]]></artwork>
</figure>
<t> From SBC's perspective the 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 Denial of Service (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.</t>
</section>
<!-- ################################### -->
<section title="Access Scenario" anchor="sec-info-access">
<t> In an access scenario, presented in <xref target="fig-SBC_access"/>, 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 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.</t>
<figure title="Access scenario with SBC" anchor="fig-SBC_access">
<artwork><![CDATA[
Access Network Operator Network
+-----+
| UA1 |<---------\
+-----+ \
\
+-----+ \------->+-----+ +-------+
| UA2 |<-------------------->| SBC |<----->| proxy |<-- -
+-----+ /--->+-----+ +-------+
/
+-----+ +-----+ /
| UA3 +---+ NAT |<---/
+-----+ +-----+
]]></artwork>
</figure>
<t> 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). Despite where the SBC is
hosted, it is managed by the organization maintaining the operator network. </t>
<t> Some endpoints may be behind enterprise or residential NATs. In cases where the access
network is a private network, the SBC is 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.</t>
<t> 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.</t>
</section>
</section>
<!-- ################################### -->
<section title="Functions of SBCs" anchor="sec-func">
<t> 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.</t>
<t> All the examples given in this section are simplified; only the relevant header lines from
SIP and SDP <xref target="RFC4566"/> messages are displayed.</t>
<!-- ################################### -->
<section title="Topology Hiding" anchor="sec-func-topology">
<!-- ################################### -->
<section title="General Information and Requirements" anchor="sec-func-topology-info">
<t> 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 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.</t>
<t> The most common form of topology hiding is the application of header privacy (see
Section 5.1 of <xref target="RFC3323"/>), 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.</t>
<t>For a reference, there are also other ways of hiding topology information than
inserting an intermediary, like an SBC, to the signaling path. One of the ways is the
User Agent (UA) driven privacy mechanism <xref
target="I-D.ietf-sip-ua-privacy"/>, where the UA can facilitate the conceal of
topology information.</t>
</section>
<!-- ################################### -->
<section title="Architectural Issues" anchor="sec-func-topology-arch-issue">
<t> 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.</t>
<t> Topology hiding function does not work well with Authenticated Identity Management
<xref target="RFC4474"/> in scenarios where the SBC does not have any kind of consent
from the users. 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 <xref target="RFC4474"/>. Some forms of topology hiding are in
violation, because they are e.g., replacing the Contact header of a SIP message.</t>
</section>
<!-- ################################### -->
<section title="Example" anchor="sec-func-topology-example">
<t> The current way of implementing topology hiding consists of having an SBC act as a
B2BUA (Back-to-Back User Agent) and remove all traces of topology information (e.g.,
Via and Record-Route entries) from outgoing messages.</t>
<t> Imagine the following example scenario: The SBC (p4.domain.example.com) receives
an INVITE request from the inner network, which in this case is an operator network. The
received SIP message is shown in <xref target="pre-topo-hide"/>.</t>
<figure title="INVITE Request Prior to Topology Hiding" anchor="pre-topo-hide">
<artwork><![CDATA[
INVITE sip:callee@u2.domain.example.com SIP/2.0
Via: SIP/2.0/UDP p3.middle.example.com;branch=z9hG4bK48jq9w174131.1
Via: SIP/2.0/UDP p2.example.com;branch=z9hG4bK18an6i9234172.1
Via: SIP/2.0/UDP p1.example.com;branch=z9hG4bK39bn2e5239289.1
Via: SIP/2.0/UDP u1.example.com;branch=z9hG4bK92fj4u7283927.1
Contact: sip:caller@u1.example.com
Record-Route: <sip:p3.middle.example.com;lr>
Record-Route: <sip:p2.example.com;lr>
Record-Route: <sip:p1.example.com;lr>
]]></artwork>
</figure>
<t> 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 Via and
Record-Route header fields with its own SIP URI. After the topology hiding function, the
message could appear as shown in <xref target="post-topo-hide"/>.</t>
<figure title="INVITE Request After Topology Hiding" anchor="post-topo-hide">
<artwork><![CDATA[
INVITE sip:callee@u2.domain.example.com SIP/2.0
Via: SIP/2.0/UDP p4.domain.example.com;branch=z9hG4bK92es3w230129.1
Contact: sip:caller@u1.example.com
Record-Route: <sip:p4.domain.example.com;lr>
]]></artwork>
</figure>
<t> 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., SBC restarts for
some reason), it may not be able to route messages properly (note: some SBCs preserve
the state information also on restart). For example, if the SBC removes <spanx
style="verb">Via</spanx> 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.</t>
<t> This is only one example of topology hiding. 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 Contact header field values and other header
fields containing identity information. The header fields containing identity
information is listed in Section 4.1 of <xref target="RFC3323"/>. Since the publication
of <xref target="RFC3323"/>, the following header fields containing identity information
have been defined: <spanx style="verb">P-Asserted-Identity</spanx>, <spanx
style="verb">Referred-By</spanx>, <spanx style="verb">Identity</spanx>, and <spanx
style="verb">Identity-Info</spanx>.</t>
<!--
draft-munakata-sip-privacy-guideline-01 lists the following, additional headers that
carry identity information (draft-ietf-sip-ua-privacy-00 did not really have a list of
identity containing headers):
P-Asserted-Identity (RFC3325)
Referred-By (RFC3892)
Identity (RFC4474)
Identity-Info (RFC4474)
-->
</section>
</section>
<!-- ################################### -->
<section title="Media Traffic Management" anchor="sec-func-shape">
<!-- ################################### -->
<section title="General Information and Requirements" anchor="sec-func-shape-info">
<t>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.</t>
<t>One of the use cases for media traffic management is the implementation of intercept
capabilities where required to support audit or legal obligations. It is noteworthy that
the legal obligations mainly apply to operators providing voice services, and those
operators typically have infrastructure (e.g., SIP proxies acting as B2BUAs) for
providing intercept capabilities even without SBCs.</t>
<t>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, or to ensure that
subscribers are using only allowed codecs. It is noteworthy that the SBCs do not have
direct ties to routing topology and they do not, for example, change bandwidth
reservations on Traffic Engineering (TE) tunnels, nor they have direct interaction with
routing protocol.</t>
<t>Some operators do not want to manage 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, from the
SBC's viewpoint, needed for monitoring media traffic are the same as for managing media
traffic.</t>
<t>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.</t>
<t>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 to ensure that the subscriber cannot just ignore the BYE request generated by the
SBC and continue their media sessions.</t>
</section>
<!-- ################################### -->
<section title="Architectural Issues" anchor="sec-func-shape-arch-issue">
<t>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 <xref target="RFC4474"/>, see Section 3.1.2.</t>
<t>The insertion of a media relay can prevent "non-media" uses of media path, for example
media path key agreement. Sometimes this type of prevention is intentional, but it is
not always necessary. For example, if an SBC is used just for enabling media monitoring,
but not for interception.</t>
<t>There are some possible issues related to the media relaying. If the media relaying is
not done in a correct manner, it may break functions like Explicit Congestion
Notification (ECN) and Path MTU Discovery (PMTUD), for example. The media relays easily
break such IP and transport layer functionalities that rely on the correct handling of
the protocol fields. Some especially sensitive field are, for example, ECN and Type of
Service (TOS) fields, and Don't Fragment (DF) bit.</t>
<t>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.</t>
<t>If an SBC directs many media streams through a central point in the network, it is
likely to cause a significant amount of additional traffic to a path to that central
point. This might create possible bottleneck in the path.</t>
<t>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.</t>
</section>
<!-- ################################### -->
<section title="Example" anchor="sec-func-shape-example">
<t> 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.</t>
<t> As mentioned in <xref target="sec-func-shape-info"/>, codec restriction is a form of
traffic management. The SBC restricts the codec set negotiated in the offer/answer
exchange <xref target="RFC3264"/> 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).</t>
<t>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 <xref target="pre-media-shape"/>.</t>
<figure anchor="pre-media-shape" title="Request Prior to Media Management">
<artwork><![CDATA[
v=0
o=owner 2890844526 2890842807 IN IP4 192.0.2.4
c=IN IP4 192.0.2.4
m=audio 49230 RTP/AVP 96 98
a=rtpmap:96 L8/8000
a=rtpmap:98 L16/16000/2
]]></artwork>
</figure>
<t> 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. <xref
target="post-media-shape"/> shows the session description after the traffic management
function.</t>
<figure title="Request Body After Media Management" anchor="post-media-shape">
<artwork><![CDATA[
v=0
o=owner 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
]]></artwork>
</figure>
<t> 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 <xref
target="sec-func-topology"/>. 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.</t>
</section>
</section>
<!-- ################################### -->
<section title="Fixing Capability Mismatches" anchor="sec-func-mismatch">
<!-- ################################### -->
<section title="General Information and Requirements" anchor="sec-func-mismatch-info">
<t>SBCs fixing capability mismatches enable communications between user-agents with
different capabilities or extensions. For example, an SBC can enable a plain SIP <xref
target="RFC3261"/> user agent to connect to a 3GPP network, or enable a connection
between user agents 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.</t>
</section>
<!-- ################################### -->
<section title="Architectural Issues" anchor="sec-func-mismatch-arch-issue">
<t>SBCs that are fixing capability mismatches do it by insert a media element to the media
path using the procedures described in <xref target="sec-func-shape"/>. Therefore, these
SBCs have the same concerns as SBCs performing traffic management: the SBC may modify
SIP messages without consent from any of the user-agents. This may break end-to-end
security and application extensions negotiation.</t>
<t> 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.</t>
</section>
<!-- ################################### -->
<section title="Example" anchor="sec-func-mismatch-example">
<t> 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:</t>
<figure title="Request Prior to Capabilities Match" anchor="pre-caps-match">
<artwork><![CDATA[
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=owner 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
]]></artwork>
</figure>
<t> 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. <xref target="post-caps-match"/> shows the request after the
capability mismatch adjustment.</t>
<figure anchor="post-caps-match" title="Request After Capability Match">
<artwork><![CDATA[
INVITE sip:callee@ipv6.domain.com 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 192.0.2.4
Contact: sip:caller@u1.example.com
v=0
o=owner 2890844526 2890842807 IN IP4 192.0.2.4
c=IN IP6 2001:DB8::801:201:2ff:fe94:8e10
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
]]></artwork>
</figure>
<t> This message is then sent by the SBC to the onward IPv6 network.</t>
</section>
</section>
<!-- ################################### -->
<section title="Maintaining SIP-related NAT Bindings" anchor="sec-func-nats">
<!-- ################################### -->
<section title="General Information and Requirements" anchor="sec-func-nats-info">
<t> 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).</t>
<t> 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.</t>
<t> 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.</t>
<t> SBCs can also force traffic to go through a media relay for NAT traversal purposes (more
about media traffic management in <xref target="sec-func-shape"/>). 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).</t>
</section>
<!-- ################################### -->
<section title="Architectural Issues" anchor="sec-func-nats-arch-issue">
<t>This approach to NAT traversal does not work if end-to-end confidentiality or
integrity-protection 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.</t>
<t>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. Some SBCs do not have any
deterministic method for choosing a suitable value. However, SBCs can just use a
sub-optimal, relatively small value which usually works. An example from such value is
15 seconds (see <xref target="RFC5405"/>).</t>
<t>NAT Traversal for media using SBCs poses few issues as well. For example an SBC
normally guesses the recipient's public IP address on one of the media streams relayed
by the SBC by snooping on the source IP address of another media stream relayed by the
same SBC. This causes security and interoperability issues since the SBC can end up
associating wrong destination IP addresses on media streams it is relaying. For example,
an attacker may snoop on the local IP address and ports used by the SBC for media
relaying the streams and send a few packets from a malicious IP address to these
destinations. In most cases, this can cause media streams in the opposite directions to
divert traffic to the attacker resulting in a successful MitM or DoS attack. A similar
example of an interop issue is caused when an endpoint behind a NAT attempts to switch
the IP address of the media streams by using a re-INVITE. If any media packets are
re-ordered or delayed in the network, they can cause the SBC to block the switch from
happening even if the re-INVITE successfully goes through.</t>
</section>
<!-- ################################### -->
<section title="Example" anchor="sec-func-nats-example">
<t> 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 <xref target="pre-nats"/>. </t>
<figure anchor="pre-nats" title="Response Prior to NAT Maintenance Function">
<artwork><![CDATA[
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
]]></artwork>
</figure>
<t> 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. <xref target="post-nats"/> shows a possible modification of the response from
<xref target="pre-nats"/>. </t>
<figure title="Manipulated Response for NAT Traversal" anchor="post-nats">
<artwork><![CDATA[
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
]]></artwork>
</figure>
<t> Naturally also other measures could be taken in order to enable the NAT traversal
(e.g., non-SIP keepalive messages), but this example illustrates only one mechanism for
preserving the SIP-related NAT bindings.</t>
</section>
</section>
<!-- ################################### -->
<section title="Access Control" anchor="sec-func-access">
<!-- ################################### -->
<section title="General Information and Requirements" anchor="sec-func-access-info">
<t> 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.</t>
<t> 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.</t>
<t> 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 <xref target="sec-func-shape"/>. 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.</t>
<t> 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.</t>
<t> 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.</t>
</section>
<!-- ################################### -->
<section title="Architectural Issues" anchor="sec-func-access-arch-issue">
<t>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.</t>
<t> 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 <xref
target="sec-func-shape-arch-issue"/>.</t>
</section>
<!-- ################################### -->
<section title="Example" anchor="sec-func-access-example">
<t>
<xref target="pre-access-flow"/> shows a callflow where the SBC is providing both
signaling and media access control (ACKs omitted for brevity).</t>
<figure title="Example Access Callflow" anchor="pre-access-flow">
<artwork><![CDATA[
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 |
|<======================>|<======================>|
| | |
]]></artwork>
</figure>
<!-- This kind of authentication is used e.g. in IMS (2 x REGISTER > IPsec) -->
<t> 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.</t>
</section>
</section>
<!-- ################################### -->
<section title="Protocol Repair" anchor="sec-func-repair">
<!-- ################################### -->
<section title="General Information and Requirements" anchor="sec-func-repair-info">
<t> 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 (i.e., making translation between two
completely different protocols).</t>
</section>
<!-- ################################### -->
<section title="Architectural Issues" anchor="sec-func-repair-arch-issue">
<t>In many cases, doing protocol repair for SIP header fields 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 sends.</t>
<t>However, protocol repair may break security mechanism that do cryptographical
computations on SIP header values. Attempting protocol repair for SIP message bodies
(SDP) is incompatible with Authenticated Identity Management <xref target="RFC4474"/>
and end-to-end security mechanisms such as S/MIME.</t>
<t>A similar problem related to increasing complexity, as explained in <xref
target="sec-func-mismatch-arch-issue"/>, also affects protocol repair function.</t>
</section>
<!-- ################################### -->
<section title="Examples" anchor="sec-func-repair-example">
<t> The SBC can, for example, receive an INVITE message from a relatively new SIP UA as
illustrated in <xref target="pre-repair"/>.</t>
<figure title="Request from a relatively new client" anchor="pre-repair">
<artwork><![CDATA[
INVITE sip:callee@sbchost.example.com
Via: SIP/2.0/UDP u1.example.com:5060;lr
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
]]></artwork>
</figure>
<t> 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, badly implemented SIP
stacks. It could also remove excess white spaces to make the SIP message more human
readable.</t>
</section>
</section>
<!-- ################################### -->
<section title="Media Encryption" anchor="sec-func-medenc">
<!-- ################################### -->
<section title="General Information and Requirements" anchor="sec-func-medenc-info">
<t> 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. Some 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. One possible way to do this is to
perform media encryption function.</t>
</section>
<!-- ################################### -->
<section title="Architectural Issues" anchor="sec-func-medenc-arch-issue">
<t> 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 <xref target="sec-func-shape"/>.</t>
</section>
<!-- ################################### -->
<section title="Example" anchor="sec-func-medenc-example">
<t>
<xref target="medenc-callflow"/> shows an example where the SBC is performing media
encryption related functions (ACKs omitted for brevity).</t>
<figure title="Media Encryption Example" anchor="medenc-callflow">
<artwork><![CDATA[
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 |
|<==================>|<- - - - - - - - ->|<==================>|
| | | |
]]></artwork>
</figure>
<t> 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 and the SBCs inject themselves
in the returning media path. After signaling the media starts flowing, and both SBCs are
performing media encryption and decryption.</t>
</section>
</section>
</section>
<!-- ################################### -->
<section title="Derived Requirements for Future SIP Standardization Work"
anchor="proto-requirements">
<t>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:</t>
<t><list style="format Req-%d:">
<t>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 on
behalf of some header fields (see <xref target="sec-func-topology"/>). The topology hiding
is especially problematic in scenarios where an SBC does not have users' consent.</t>
<t>There should be a SIP-friendly way to direct media traffic through intermediaries.
Currently this is done by modifying session descriptors, which is against the principles of
SIP (see <xref target="sec-func-shape"/>, <xref target="sec-func-nats"/>, <xref
target="sec-func-access"/>, and <xref target="sec-func-medenc"/>). This is especially
problematic in scenarios where an SBC does not have users' consent.</t>
<t>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/SIP <xref
target="RFC3261"/> network mismatch. Currently this is done by modifying SIP messages, which
may violate end-to-end security (see <xref target="sec-func-mismatch"/> and <xref
target="sec-func-repair"/>), on behalf of some header fields. This is especially problematic
in scenarios where an SBC does not have users' consent.</t>
</list></t>
<t>Req-1 and Req-3 do not have an existing, standardized 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.</t>
<t>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 <xref target="sec-func-repair"/>)
functionality.</t>
<!-- I didn't add references to SIP session policies, TURN, and ICE, because they are still
on a draft-stage -->
</section>
<!-- ################################### -->
<section title="Security Considerations" anchor="sec-security">
<t> 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.</t>
<t> 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.</t>
<t> 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 attacks. However, on some cases, user agents can
establish encrypted media connections between each other. One example is a scenario where
SBC is used for enabling media monitoring, but not for interception.</t>
<t> 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-specific information makes the situation even worse. If the SBC crashes
(or is brought down by an attacker), ongoing sessions experience undetermined behavior.</t>
<t> If the IETF decides to develop standard mechanisms to address the requirements presented
in <xref target="proto-requirements"/>, the security and privacy-related aspects of those
mechanisms will, of course, need to be taken into consideration.</t>
</section>
<!-- ################################### -->
<section title="IANA Considerations" anchor="sec-iana">
<t> This document has no IANA considerations.</t>
</section>
<!-- ################################### -->
<section title="Acknowledgements" anchor="sec-acks">
<t> 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.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.3261"?>
<?rfc include="reference.RFC.3323"?>
<?rfc include="reference.RFC.3264"?>
<?rfc include="reference.RFC.4474"?>
</references>
<references title="Informational References">
<?rfc include="reference.RFC.4566"?>
<?rfc include="reference.3GPP.23.228"?>
<?rfc include="reference.RFC.5405"?>
<?rfc include="reference.I-D.draft-ietf-sip-ua-privacy-03"?>
</references>
</back>
</rfc>
<!-- LocalWords: xref PPR PPA SAA RTA RTR LIR LIA CDATA
-->
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