SIPPING Working Group J. Hautakorpi, Ed.
Internet-Draft G. Camarillo
Expires: September 7, 2006 Ericsson
M. Bhatia
NexTone Communications
R. Penfield
Acme Packet
A. Hawrylyshen
Ditech Communications Corporation
March 6, 2006
Requirements from SIP (Session Initiation Protocol) Session Border
Control Deployments
draft-camarillo-sipping-sbc-funcs-03.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 7, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This documents describes functions implemented in Session Initiation
Hautakorpi, et al. Expires September 7, 2006 [Page 1]
Internet-Draft Requirements from SBC Deployments March 2006
Protocol (SIP) intermediaries known as Session Border Controllers
(SBCs). Although the goal of this document is to describe all the
functions of SBCs, a special focus is given to those practices that
are viewed to be in conflict with SIP architectural principles. It
also explores the underlying requirements of network operators that
have led to the use of these functions and practices in order to
identify protocol requirements and determine whether those
requirements are satisfied by existing specifications or additional
standards work is required.
Hautakorpi, et al. Expires September 7, 2006 [Page 2]
Internet-Draft Requirements from SBC Deployments March 2006
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Background on SBCs . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Peering Scenario . . . . . . . . . . . . . . . . . . . . . 5
2.2. Access Scenario . . . . . . . . . . . . . . . . . . . . . 6
3. Functions of SBC . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Topology Hiding . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. General Information and Requirements . . . . . . . . . 7
3.1.2. Architectural Issues . . . . . . . . . . . . . . . . . 8
3.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Media Traffic Shaping . . . . . . . . . . . . . . . . . . 9
3.2.1. General Information and Requirements . . . . . . . . . 9
3.2.2. Architectural Issues . . . . . . . . . . . . . . . . . 10
3.2.3. Example . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Fixing Capability Mismatches . . . . . . . . . . . . . . . 11
3.3.1. General Information and Requirements . . . . . . . . . 11
3.3.2. Architectural Issues . . . . . . . . . . . . . . . . . 11
3.3.3. Example . . . . . . . . . . . . . . . . . . . . . . . 12
3.4. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 13
3.4.1. General Information and Requirements . . . . . . . . . 13
3.4.2. Architectural Issues . . . . . . . . . . . . . . . . . 13
3.4.3. Example . . . . . . . . . . . . . . . . . . . . . . . 13
3.5. Access Control . . . . . . . . . . . . . . . . . . . . . . 14
3.5.1. General Information and Requirements . . . . . . . . . 14
3.5.2. Architectural Issues . . . . . . . . . . . . . . . . . 15
3.5.3. Example . . . . . . . . . . . . . . . . . . . . . . . 15
3.6. Protocol Repair . . . . . . . . . . . . . . . . . . . . . 16
3.6.1. General Information and Requirements . . . . . . . . . 16
3.6.2. Architectural Issues . . . . . . . . . . . . . . . . . 16
3.6.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 16
3.7. Media Encryption . . . . . . . . . . . . . . . . . . . . . 17
3.7.1. General Information and Requirements . . . . . . . . . 17
3.7.2. Architectural Issues . . . . . . . . . . . . . . . . . 17
3.7.3. Example . . . . . . . . . . . . . . . . . . . . . . . 17
4. Derived Requirements (TODO) . . . . . . . . . . . . . . . . . 18
5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . . 19
9.2. Informational References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21
Hautakorpi, et al. Expires September 7, 2006 [Page 3]
Internet-Draft Requirements from SBC Deployments March 2006
1. Introduction
In the past few years there has been a rapid adoption of SIP [1] and
deployment of SIP-based communications networks. This has often out-
paced the development and implementation of protocol specifications
to meet network operator requirements. This has led to the
development of proprietary solutions. Often these proprietary
solutions are implemented in network intermediaries known in the
marketplace as Session Border Controllers (SBCs) because they
typically are deployed at the border between two networks. The
reason for this is that network policies are typically enforced at
the edge of the network.
Even though many SBCs currently break things like end-to-end security
and can impact feature negotiations, there is clearly a market for
them. Network operators need many of the features current SBCs
provide and many times there are no standard mechanisms available to
provide them in a better way. This document describes the most
common functions of current SBCs and the reasons that network
operators require them. It also describes the architectural issues
with these functions. Although this document focuses on functions
common to SBCs, many of the issues raised apply to other types of
B2BUAs.
2. Background on SBCs
The term SBC is pretty vague, since it is not standardized or defined
anywhere. Nodes that may be referred to as SBCs but do not implement
SIP are outside the scope of this document.
SBCs usually sit between two service provider networks in a peering
environment, or between an access network and a backbone network to
provide service to residential and/or enterprise customers. They
provide a variety of functions to enable or enhance session-based
multi-media services (e.g., Voice over IP). These functions include:
a) perimeter defense (access control, topology hiding, DoS
prevention, and detection); b) functionality not available in the
endpoints (NAT traversal, protocol interworking or repair); and c)
network management (traffic monitoring, shaping, and QoS). Some of
these functions may also get integrated into other SIP elements (like
pre-paid platforms, 3GPP P-CSCF, 3GPP I-CSCF etc).
SIP-based SBCs typically handle both signaling and media and
implement behavior which is equivalent to a "privacy service" (as
described in [3]) 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,
Hautakorpi, et al. Expires September 7, 2006 [Page 4]
Internet-Draft Requirements from SBC Deployments March 2006
B2BUAs (Back-to-Back User Agents). The transparency of these B2BUAs
varies depending on the functions they perform. For example, some
SBCs modify the session description carried in the message and insert
a Record-Route entry. Other SBCs replace the value of the Contact
header field with the SBCs address, and generate a new Call-ID and
new To and From tags.
+-----------------+
| SBC |
[signaling] | +-----------+ |
<------------|->| signaling |<-|---------->
outer | +-----------+ | inner
network | | | network
| +-----------+ |
<------------|->| media |<-|---------->
[media] | +-----------+ |
+-----------------+
Figure 1: SBC architecture
Figure 1 shows the logical architecture of an SBC, which includes a
signaling and a media component. In this document, the terms outer
and inner network are used for describing these two networks.
2.1. Peering Scenario
A typical peering scenario involves two network operators who
exchange traffic with each other. For example, in a toll bypass
application, a gateway in operator A's network sends an INVITE that
is routed to the softswitch (proxy) in operator B's network. The
proxy responds with a redirect (3xx) message back to the originating
gateway that points to the appropriate terminating gateway in
operator B's network. The originating gateway then sends the INVITE
to the terminating gateway.
Hautakorpi, et al. Expires September 7, 2006 [Page 5]
Internet-Draft Requirements from SBC Deployments March 2006
Operator A . Operator B
.
. 2) INVITE
+-----+ . /--------------->+-----+
| SSA | . / 3) 3xx (redir.) | SSB |
+-----+ . / /---------------+-----+
. / /
+-----+ 1) INVITE +-----+--/ / +-----+
|GW-A1|---------------->| SBC |<---/ 4) INVITE |GW-B1|
+-----+ +-----+---------------------->+-----+
.
+-----+ . +-----+
|GW-A2| . |GW-B2|
+-----+ . +-----+
Figure 2: Peering with SBC
Figure 2 illustrates the peering arrangement with a SBC where
Operator A is the outer network, and Operator B is the inner network.
Operator B uses the SBC to control access to its network, protect its
gateways and softswitches from unauthorized use and DoS attacks, and
monitor the signaling and media traffic. It also simplifies network
management by minimizing the number ACL (Access Control List) entries
in the gateways. The gateways do not need to be exposed to the peer
network, and they can restrict access (both media and signaling) to
the SBCs. The SBC guarantees that only media from valid sessions
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 backbone network (inner network) to control access to the
backbone network, protect its components (media servers, application
servers, gateways, etc.) from unauthorized use and DoS attacks, and
monitor the signaling and media traffic. Also, as a part of access
control, since the SBC is call stateful, it can prevent over
subscription of the access links. Endpoints are configured with the
SBC as their outbound proxy address. The SBC routes requests to one
or more proxies in the backbone.
Hautakorpi, et al. Expires September 7, 2006 [Page 6]
Internet-Draft Requirements from SBC Deployments March 2006
Access Network . Operator Backbone
.
+-----+ .
| UA1 |<---------\ .
+-----+ \ .
\ .
+-----+ \------->+-----+ +-------+
| UA2 |<-------------------->| SBC |<----->| proxy |<-- -
+-----+ /--->+-----+ +-------+
/ .
+-----+ +-----+ / .
| UA3 +---+ NAT |<---/ .
+-----+ +-----+ .
Figure 3: Access scenario with SBC
Some endpoints may be behind enterprise or residential NATs. In
cases where the access network is a private network, the SBC is the
NAT for all traffic. The proxy usually does authentication/
authorization for registrations and outbound calls. The SBC does
modify the REGISTER request so that subsequent requests to the
registered address-of-record is routed to the SBC. This is done
either with a Path header, or by modifying the Contact to point at
the SBC.
3. Functions of SBC
This section lists those functions that are used in SBC deployments
in current communication networks. Each subsection describes a
particular function or feature, operators' requirements for having
it, explanation on why it affects the SIP end-to-end model, and a
concrete example from its implementation. Each section also
discusses potential concerns specific to that particular way of
implementing it. Providing suggestions for alternative, more SIP-
friendly ways of implementing each of the functions is outside the
scope of this document.
All the examples given in this section are somewhat simplified
situations from the reality. Only the relevant header lines from SIP
and SDP [4] messages are displayed.
3.1. Topology Hiding
3.1.1. General Information and Requirements
Topology hiding consists of limiting the amount of topology
Hautakorpi, et al. Expires September 7, 2006 [Page 7]
Internet-Draft Requirements from SBC Deployments March 2006
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 [3]), which involves stripping Via and
Record-Route headers and replacing the Contact header. However, in
deployments which use IP addresses instead of domain names in headers
that cannot be removed (e.g. From and To headers), the SBC must
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 v/s an end-
to-end trust model. The messages are modified without subscriber
consent and could potentially modify or remove information about the
user's privacy, security requirements and higher layer applications
which are communicating end-to-end using SIP. Either users in an
end-to-end call may perceive this as a MitM (Man-in-the-Middle)
attack.
Modification of IP addresses in URIs in SIP headers can lead to
application failures when these URIs are communicated to other SIP
servers outside the current dialog. These URIs could appear in a
REFER request or in the body of NOTIFY request as part of an event
package. If these messages traverse the same SBC, it has the
opportunity to restore the original IP address. On the other hand,
if the REFER or NOTIFY message returns to the original network
through a different SBC that does not have access to the address
mapping, the recipient of the message will not see the original
address. This may cause the application function to fail.
3.1.3. Example
The current way of implementing topology hiding consists of having an
SBC act as a B2BUA (Back-to-Back User Agents) and remove all traces
of topology information (e.g., Record-Route and Via entries) from
outgoing messages.
Like a regular proxy server that inserts a Record-Route entry, the
Hautakorpi, et al. Expires September 7, 2006 [Page 8]
Internet-Draft Requirements from SBC Deployments March 2006
SBC handles every single message of a given SIP dialog. However,
unlike the proxy server, if the SBC loses state (e.g., the SBC
restarts for some reason), it will not be able to route messages
properly. For example, if the SBC removes Via entries from a request
and then restarts losing state, the SBC will not be able to route
responses to that request.
Let us imagine the following example scenario: The SBC
(p4.domain.example.com) is receiving an INVITE request from the inner
network, which in this case is an operator network. The received SIP
message is:
INVITE sip:callee@u2.domain.example.com SIP/2.0
Contact: sip:caller@u1.example.com
Record-Route: <sip:p3.middle.example.com>
Record-Route: <sip:p2.example.com;lr>
Record-Route: <sip:p1.example.com;lr>
Then the SBC performs a topology hiding function. In this imagined
situation the SBC removes and stores all existing Record-Route
headers, and then insert a Record-Route header field with its own
SIP-URI. After the topology hiding function, the message looks like:
INVITE sip:callee@u2.domain.example.com SIP/2.0
Contact: sip:caller@u1.example.com
Record-Route: <sip:p4.domain.example.com;lr>
This is only one example scenario from topology hiding, and SBCs can,
in some cases, modify other headers as well, like SIP Contact etc.
3.2. Media Traffic Shaping
3.2.1. General Information and Requirements
Media traffic shaping is the act of controlling media traffic.
Operators require this functionality, because they want to control
the traffic they carry on their network. Traffic shaping helps them
create different kinds of billing models (e.g., video telephony can
be priced differently than voice-only calls). Additionally, traffic
shaping can be used to implement intercept capabilities (e.g., lawful
intercept).
Since the path of the media through the network is independent of the
path of the signaling, the media may not traverse the operator's
network unless the SBC modifies the session description to force the
media to be sent thru the SBC.
Hautakorpi, et al. Expires September 7, 2006 [Page 9]
Internet-Draft Requirements from SBC Deployments March 2006
Some operators do not actually want to reshape the traffic, but only
to monitor it for collecting statistics and making sure that they are
able to meet any business level agreements with their subscribers
and/or partners. However, the SIP techniques needed for monitoring
media traffic are the same as for reshaping media traffic.
SBCs on the media path are also capable of dealing with the "lost
BYE" issue when either endpoint dies in the middle of the session.
The SBC can detect that the media has stopped flowing and issue a BYE
to the both sides to cleanup the state in the network.
One possible form of media traffic shaping is that SBCs terminate
media streams and SIP dialogs by generating BYE requests. This kind
of procedure can take place e.g., in a situation where subscriber
runs out of credits. In this scenario, the SBC may be perceived as
originating messages which the user may not be able to authenticate
as coming from the dialog peer or the SIP Registrar/Proxy.
3.2.2. Architectural Issues
The current way of implementing traffic shaping requires the SBC to
access and modify the session descriptions (i.e., offers and answers)
exchanged between the user agents. Consequently, this approach does
not work if user agents encrypt or integrity-protect their message
bodies end-to-end. Again, messages are modified without subscriber
consent, and user agents do not have any way to distinguish the SBC
actions from an attack by a MitM (Man-in-the-Middle).
3.2.3. Example
Currently, traffic shaping is performed in the following way. The
SBC behaves as a B2BUA and inserts itself, or some other entity under
the operator's control, in the media path. In practice, the SBC
modifies the session descriptions carried in the SIP messages. As a
result, the SBC receives media from one user agent and relays it to
the other in both directions.
An example of traffic shaping is codec restriction. The SBC
restricts the codec set negotiated in offer/answer [2] exchange
between the user agents. After modifying the session descriptions,
the SBC can check whether or not the media stream corresponds to what
was negotiated in the offer/answer exchange. If it differs, the SBC
has the ability to terminate the media stream.
Let us imagine the following example scenario: The SBC is receiving
an INVITE request from the outer network, which in this case is an
access network. The received SIP message contains the following SDP
session descriptor:
Hautakorpi, et al. Expires September 7, 2006 [Page 10]
Internet-Draft Requirements from SBC Deployments March 2006
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
c=IN IP4 126.16.64.4
m=audio 49230 RTP/AVP 96 98
a=rtpmap:96 L8/8000
a=rtpmap:98 L16/16000/2
Then the SBC performs a media traffic shaping function. In this
imagined situation the SBC rewrites the 'm' line, and removes one 'a'
line according to some policy. After the traffic shaping function,
the session descriptor looks like:
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
c=IN IP4 126.16.64.4
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
One problem of media traffic shaping is that the SBC needs to
understand the session description protocol and all the extensions
used by the user agents. This means that in order to use a new
extension (e.g., an extension to implement a new service) or a new
session description protocol, it is not enough with upgrading the
user agents; SBCs in the network need also to be upgraded. This fact
may slow down service innovation.
3.3. Fixing Capability Mismatches
3.3.1. General Information and Requirements
SBCs fixing capability mismatches enable communications between user
agents with different capabilities, SIP profiles or extensions. For
example, user agents on networks which implement different SIP
Profiles (for example 3GPP or Packet Cable etc) or those that support
different IP versions, different codecs, or that are in different
address realms. Operators have a requirement and a strong motivation
for performing capability mismatch fixing, so that they can provide
transparent communication across different domains. In some cases
different SIP extensions or methods to implement the same SIP
application (like monitoring session liveness, call history/diversion
etc) may also be interworked through the SBC.
3.3.2. Architectural Issues
SBCs fixing capability mismatches insert a media element in the media
path using the procedures described in Section 3.2. Therefore, these
SBCs have the same concerns as SBCs performing traffic shaping: the
Hautakorpi, et al. Expires September 7, 2006 [Page 11]
Internet-Draft Requirements from SBC Deployments March 2006
SBC modifies SIP messages without explicit consent from any of the
user agents. This may break end-to-end security and application
extensions negotiation.
Additionally, if the network is not engineered properly, an SBC may
make the wrong assumption about the capabilities of the user agents.
When this happens, user agents with compatible capabilities may end
up communicating via the SBC instead of doing it directly between
them (e.g., the SBC assumes that a dual-stack user agent only
supports IPv6).
3.3.3. Example
Let us imagine 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:callee@ipv6.domain.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4
Contact: sip:caller@u1.example.com
v=0
o=mhandley 2890844526 2890842807 IN IP4 192.0.2.4
c=IN IP4 192.0.2.4
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
Then the SBC performs a capability mismatch fixing function. In this
imagined situation the SBC inserts Record-Route and Via headers, and
rewrites the 'c' line from the sessions descriptor. After the
capability mismatch fixing function, the message look like:
INVITE sip:callee@ipv6.domain.com SIP/2.0
Record-Route: <sip:[2001:620:8:801:201:2ff:fe94:8e10];lr>
Via: SIP/2.0/UDP sip:[2001:620:8:801:201:2ff:fe94:8e10]
Via: SIP/2.0/UDP 192.0.2.4
Contact: sip:caller@u1.example.com
v=0
o=mhandley 2890844526 2890842807 IN IP4 192.0.2.4
c=IN IP6 2001:620:8:801:201:2ff:fe94:8e10
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
Now the SBC sends the modified message to the outer IPv6 network.
Hautakorpi, et al. Expires September 7, 2006 [Page 12]
Internet-Draft Requirements from SBC Deployments March 2006
3.4. NAT Traversal
3.4.1. General Information and Requirements
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
valid.
Operators implement this functionality in an SBC instead of in the
registrar for several reasons: (i) preventing packets from
unregistered users to prevent chances of DoS attack, (ii)
prioritization and/or re-routing of traffic (based on user or
service, like E911) as it enters the network. (iii) performing a load
balancing function or reducing the load on other network equipment.
3.4.2. Architectural Issues
This approach to NAT traversal does not work when end-to-end
confidentiality or integrity-protection is used. The SBC would be
seen as a MitM modifying the messages between the user agent and the
registrar.
3.4.3. Example
Let us imagine the following example scenario: The SBC resides
between the UA and Registrar. Previously the UA has sent a REGISTER
request to Registrar, and then the SBC is going to relay the
following SIP message to UA:
Hautakorpi, et al. Expires September 7, 2006 [Page 13]
Internet-Draft Requirements from SBC Deployments March 2006
SIP/2.0 200 OK
From: Bob <sip:bob@biloxi.example.com>;tag=a73kszlfl
To: Bob <sip:bob@biloxi.example.com>;tag=34095828jh
CSeq: 1 REGISTER
Contact: <sips:bob@client.biloxi.example.com>;expires=3600
Then the SBC performs a traversal function. In this imagined
situation the SBC rewrites the 'expires' parameter on the Contact
header field. After the NAT traversal function, the message look
like:
SIP/2.0 200 OK
From: Bob <sip:bob@biloxi.example.com>;tag=a73kszlfl
To: Bob <sip:bob@biloxi.example.com>;tag=34095828jh
CSeq: 1 REGISTER
Contact: <sips:bob@client.biloxi.example.com>;expires=60
Naturally also other measures needs to be taken in order to enable
the NAT traversal, but this example illustrated only the mechanism on
how NAT bindings can be kept alive.
3.5. Access Control
3.5.1. General Information and Requirements
It is pretty self evident that operators want to control what kind of
signaling and media traffic their network carries. So, they have a
strong motivation and a requirement to do access control on the edge
of their network. Access control can be based on, for example, IP
addresses or SIP identities.
This function is 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 behaves as a proxy server. Therefore, it
does not break any SIP architectural principle. If access control is
applied to both the signaling and media, then the SBC behaves as 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. In any case, since the SBC needs to
handle every single message, this function has a scalability
Hautakorpi, et al. Expires September 7, 2006 [Page 14]
Internet-Draft Requirements from SBC Deployments March 2006
implications. In addition, the SBC is a single point of failure from
the architectural point of view. Although, in practice, many current
SBCs have redundant configuration, which prevents the loss of calls/
sessions in the event of a failure.
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. Some SBCs
can contact a policy server to determine whether sufficient bandwidth
is available.
3.5.2. Architectural Issues
If access control is performed only on behalf of signaling, then the
SBC is not SIP friendly, 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
There could be a following scenario, where SBC is performing access
control for signaling and media:
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 |
|<======================>|<======================>|
| | |
Hautakorpi, et al. Expires September 7, 2006 [Page 15]
Internet-Draft Requirements from SBC Deployments March 2006
In this scenario, the SBC first identifies the caller, so it can
determine whether or not to give signaling access for the caller.
After that, the SBC modifies the session descriptors in INVITE and
200 OK messages in a way that the media is going to flow through SBC
itself. When the media starts flowing, the SBC can inspect whether
the callee and caller use the codec(s) that they had previously
agreed on.
3.6. Protocol Repair
3.6.1. General Information and Requirements
SBC are also used to repair protocol messages generated by not-fully-
standard clients. Operators have a requirement to support protocol
repair, if they want to support as many client as possible. It is
noteworthy, that this function affects only the signaling component
of SBC, and that protocol repair function is not the same as protocol
conversion.
3.6.2. Architectural Issues
In most cases, this function can be seen as SIP friendly, 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. In principle, such an SBC does not
break any architectural principle of SIP.
3.6.3. Examples
The SBC can, for example, receive the following INVITE message from a
not-fully-standard client:
INVITE
Via: SIP/2.0/UDP u1.example.com:5060
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
If the SBC does protocol repair, it can for example try to write the
request line based on To header field, and it also can remove excess
white spaces to make the SIP message more human readable.
Some other examples of "protocol repair" that have actually been
implemented in commercially available SBCs include:
Hautakorpi, et al. Expires September 7, 2006 [Page 16]
Internet-Draft Requirements from SBC Deployments March 2006
o Changing Content-Disposition from "signal" to "session". This was
required for a user agent which sent an incorrect Content-
Disposition header.
o Addition of userinfo to a Contact URI when none was present. This
was required for a softswitch/proxy that would reject requests if
the Contact URI had no user part.
o Addition of a to-tag to provisional or error responses.
o Re-ordering of Contact header values in a REGISTER response. This
was required for a user agent that would take the expires value
from the first Contact header value without matching it against
its Contact value.
o Correction of SDP syntax where the user agent used "annexb=" in
the fmtp attribute instead of the proper "annexb:".
o Correction of signaling errors (convert BYE to CANCEL) for
termination of early sessions.
o Repair of header parameters in 'archaic' or incorrect formats.
Some older stacks assume that parameters are always of the form
NAME=VALUE. For those elements, it is necessary to convert 'lr-
true' to 'lr' in order to interoperate with several commercially
available stacks and proxies.
3.7. Media Encryption
3.7.1. General Information and Requirements
SBCs are used to perform media encryption / decryption at the edge of
the network. This is the case when media encryption is used only on
the access network (outer network) side and the media is carried
unencrypted in the inner network. Operators can 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 media encryption function, SBCs need to be able to
inject either themselves, or some other entity to the media path.
Due to this, the SBCs have same architectural issues as explained in
Section 3.2.
3.7.3. Example
There could be a following scenario, where SBC is performing media
encryption:
Hautakorpi, et al. Expires September 7, 2006 [Page 17]
Internet-Draft Requirements from SBC Deployments March 2006
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 |
|<==================>|<- - - - - - - - ->|<==================>|
| | | |
First the caller send INVITE, 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 caller replies with 200 OK,
and when the SBCs receive it, they inject themselves also to the
returning media path. After signaling the media start flowing, and
both SBCs are performing media encryption and decryption.
4. Derived Requirements (TODO)
TODO: enumerate protocol requirements based on network operator
requirements and identify which are satisfied by existing work and
which may require new work.
5. Open Issues
Several domains and IP addresses in the examples within this document
still require NIT checking and changing to be in line with best
Hautakorpi, et al. Expires September 7, 2006 [Page 18]
Internet-Draft Requirements from SBC Deployments March 2006
practices for examples.
6. Security Considerations
Many of the functions this document describes have important security
and privacy implications. If the IETF decides to develop standard
mechanisms to address those functions, security and privacy-related
aspects will need to be taken into consideration.
7. IANA Considerations
This document has no IANA considerations.
8. Acknowledgements
The ad-hoc meeting about SBCs, held on Nov 9th 2004 at Washington DC
during the 61st IETF meeting, provided valuable input to this
document. Special thanks goes also to Sridhar Ramachandran, Gaurav
Kulshreshtha, and to Rakendu Devdhar.
9. References
9.1. Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[3] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
9.2. Informational References
[4] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
Hautakorpi, et al. Expires September 7, 2006 [Page 19]
Internet-Draft Requirements from SBC Deployments March 2006
Authors' Addresses
Jani Hautakorpi (editor)
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Jani.Hautakorpi@ericsson.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
Medhavi Bhatia
NexTone Communications
101 Orchard Ridge Drive
Gaithersburg, MD 20878
US
Email: mbhatia@nextone.com
Robert F. Penfield
Acme Packet
71 Third Avenue
Burlington, MA 01803
US
Email: bpenfield@acmepacket.com
Alan Hawrylyshen
Ditech Communications Corporation
Suite 200, 1167 Kensington Cres NW
Calgary, Alberta T2N 1X7
Canada
Email: ahawrylyshen@ditechcom.com
Hautakorpi, et al. Expires September 7, 2006 [Page 20]
Internet-Draft Requirements from SBC Deployments March 2006
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Hautakorpi, et al. Expires September 7, 2006 [Page 21]