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IETF RFC 5923
Connection Reuse in the Session Initiation Protocol (SIP)
Last modified on Wednesday, June 23rd, 2010
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Internet Engineering Task Force (IETF) V. Gurbani, Ed.
Request for Comments: 5923 Bell Laboratories, Alcatel-Lucent
Category: Standards Track R. Mahy
ISSN: 2070-1721 Unaffiliated
B. Tate
BroadSoft
June 2010
Connection Reuse in the Session Initiation Protocol (SIP)
 Abstract
This document enables a pair of communicating proxies to reuse a
congestion-controlled connection between themselves for sending
requests in the forwards and backwards direction. Because the
connection is essentially aliased for requests going in the backwards
direction, reuse is predicated upon both the communicating endpoints
authenticating themselves using X.509 certificates through Transport
Layer Security (TLS). For this reason, we only consider connection
reuse for TLS over TCP and TLS over Stream Control Transmission
Protocol (SCTP). This document also provides guidelines on
connection reuse and virtual SIP servers and the interaction of
connection reuse and DNS SRV lookups in SIP.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 5923.
Gurbani, et al. Standards Track PAGE 1 
RFC 5923 SIP Connection Reuse June 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ...................................................3
2. Terminology ....................................................4
3. Applicability Statement ........................................5
4. Benefits of TLS Connection Reuse ...............................5
5. Overview of Operation ..........................................6
6. Requirements ..................................................10
7. Formal Syntax .................................................11
8. Normative Behavior ............................................11
8.1. Client Behavior ...........................................11
8.2. Server Behavior ...........................................13
8.3. Closing a TLS Connection ..................................14
9. Security Considerations .......................................14
9.1. Authenticating TLS Connections: Client View ...............14
9.2. Authenticating TLS Connections: Server View ...............15
9.3. Connection Reuse and Virtual Servers ......................15
10. Connection Reuse and SRV Interaction ..........................17
11. IANA Considerations ...........................................17
12. Acknowledgments ...............................................17
13. References ....................................................18
13.1. Normative References ......................................18
13.2. Informative References ....................................18
Gurbani, et al. Standards Track PAGE 2
RFC 5923 SIP Connection Reuse June 2010
1. Introduction
SIP entities can communicate using either unreliable/connectionless
(e.g., UDP) or reliable/connection-oriented (e.g., TCP, SCTP
[RFC 4960]) transport protocols. When SIP entities use a connection-
oriented protocol (such as TCP or SCTP) to send a request, they
typically originate their connections from an ephemeral port.
In the following example, A listens for SIP requests over TLS on TCP
port 5061 (the default port for SIP over TLS over TCP), but uses an
ephemeral port (port 49160) for a new connection to B. These
entities could be SIP user agents or SIP proxy servers.
+-----------+ 49160 (UAC) 5061 (UAS) +-----------+
| |--------------------------->| |
| Entity | | Entity |
| A | | B |
| | 5061 (UAS) | |
+-----------+ +-----------+
Figure 1: Uni-directional connection for requests from A to B
The SIP protocol includes the notion of a persistent connection
(defined in Section 2), which is a mechanisms to insure that
responses to a request reuse the existing connection that is
typically still available, as well as reusing the existing
connections for other requests sent by the originator of the
connection. However, new requests sent in the backwards direction --
in the example above, requests from B destined to A -- are unlikely
to reuse the existing connection. This frequently causes a pair of
SIP entities to use one connection for requests sent in each
direction, as shown below.
+-----------+ 49160 5061 +-----------+
| |.......................>| |
| Entity | | Entity |
| A | 5061 49170 | B |
| |<-----------------------| |
+-----------+ +-----------+
Figure 2: Two connections for requests between A and B
Unlike TCP, TLS connections can be reused to send requests in the
backwards direction since each end can be authenticated when the
connection is initially set up. Once the authentication step has
been performed, the situation can thought to resemble the picture in
Figure 1 except that A and B both use a single shared connection, for
Gurbani, et al. Standards Track PAGE 3
RFC 5923 SIP Connection Reuse June 2010
example, between port 49160 on A and port 5061 on B. When A wants to
send a request to B, it will reuse this connection, and when B wants
to send a request to A, it will reuse the same connection.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
Additional terminology used in this document:
Advertised address: The address that occurs in the Via header
field's sent-by production rule, including the port number and
transport.
Alias: Reusing an existing connection to send requests in the
backwards direction; i.e., A opens a connection to B to send a
request, and B uses that connection to send requests in the
backwards direction to A.
Connection reuse: See "Alias".
Persistent connection: The process of sending multiple, possibly
unrelated requests on the same connection, and receiving responses
on that connection as well. More succinctly, A opens a connection
to B to send a request, and later reuses the same connection to
send other requests, possibly unrelated to the dialog established
by the first request. Responses will arrive over the same
connection. Persistent connection behavior is specified in
Section 18 of RFC 3261 [RFC 3261]. Persistent connections do not
imply connection reuse.
Resolved address: The network identifiers (IP address, port,
transport) associated with a user agent as a result of executing
RFC 3263 [RFC 3263] on a Uniform Resource Identifier (URI).
Shared connection: See "Persistent connection".
Gurbani, et al. Standards Track PAGE 4
RFC 5923 SIP Connection Reuse June 2010
3. Applicability Statement
The applicability of the mechanism described in this document is for
two adjacent SIP entities to reuse connections when they are agnostic
about the direction of the connection, i.e., either end can initiate
the connection. SIP entities that can only open a connection in a
specific direction -- perhaps because of Network Address Translation
(NAT) and firewalls -- reuse their connections using the mechanism
described in the outbound document [RFC 5626].
This memo concerns connection reuse, not persistent connections (see
definitions of these in Section 2). Behavior for persistent
connections is specified in Section 18 of RFC 3261 [RFC 3261] and is
not altered by this memo.
This memo documents that it is good practice to only reuse those
connections where the identity of the sender can be verified by the
receiver. Thus, TLS (RFC 5246 [RFC 5246]) connections (over any
connection-oriented transport) formed by exchanging X.509
certificates can be reused because they authoritatively establish
identities of the communicating parties (see Section 5).
4. Benefits of TLS Connection Reuse
Opening an extra connection where an existing one is sufficient can
result in potential scaling and performance problems. Each new
connection using TLS requires a TCP three-way handshake, a handful of
round trips to establish TLS, typically expensive asymmetric
authentication and key generation algorithms, and certificate
verification. This can lead to a build up of considerable queues as
the server CPU saturates by the TLS handshakes it is already
performing (Section 6.19 of Rescorla [Book-Rescorla-TLS]).
Consider the call flow shown below where Proxy A and Proxy B use the
Record-Route mechanism to stay involved in a dialog. Proxy B will
establish a new TLS connection just to send a BYE request.
Gurbani, et al. Standards Track PAGE 5
RFC 5923 SIP Connection Reuse June 2010
Proxy A Proxy B
| |
Create connection 1 +---INV--->|
| |
|<---200---+ Response over connection 1
| |
Reuse connection 1 +---ACK--->|
| |
= =
| |
|<---BYE---+ Create connection 2
| |
Response over +---200--->|
connection 2
Figure 3: Multiple connections for requests
Setting up a second connection (from B to A above) for subsequent
requests, even requests in the context of an existing dialog (e.g.,
re-INVITE request or BYE request after an initial INVITE request, or
a NOTIFY request after a SUBSCRIBE request or a REFER request), can
also cause excessive delay (especially in networks with long round-
trip times). Thus, it is advantageous to reuse connections whenever
possible.
From the user expectation point of view, it is advantageous if the
re-INVITE requests or UPDATE requests are handled automatically and
rapidly in order to avoid media and session state from being out of
step. If a re-INVITE request requires a new TLS connection, the re-
INVITE request could be delayed by several extra round-trip times.
Depending on the round-trip time, this combined delay could be
perceptible or even annoying to a human user. This is especially
problematic for some common SIP call flows (for example, the
recommended example flow in Figure 4 in RFC 3725 [RFC 3725] uses many
re-INVITE requests).
The mechanism described in this document can mitigate the delays
associated with subsequent requests.
5. Overview of Operation
This section is tutorial in nature, and does not specify any
normative behavior.
Gurbani, et al. Standards Track PAGE 6
RFC 5923 SIP Connection Reuse June 2010
We now explain this working in more detail in the context of
communication between two adjacent proxies. Without any loss of
generality, the same technique can be used for connection reuse
between a User Agent Client (UAC) and an edge proxy, or between an
edge proxy and a UAS, or between an UAC and an UAS.
P1 and P2 are proxies responsible for routing SIP requests to user
agents that use them as edge proxies (see Figure 4).
P1 <===================> P2
p1.example.com p2.example.net
(192.0.2.1) (192.0.2.128)
+---+ +---+
| | 0---0 0---0 | |
|___| /-\ /-\ |___|
/ / +---+ +---+ / /
+----+ +----+
User Agents User Agents
example.com domain example.net domain
Figure 4: Proxy setup
For illustration purpose the discussion below uses TCP as a transport
for TLS operations. Another streaming transport -- such as SCTP --
can be used as well.
The act of reusing a connection is initiated by P1 when it adds an
"alias" header field parameter (defined later) to the Via header
field. When P2 receives the request, it examines the topmost Via
header field. If the Via header contained an "alias" header field
parameter, P2 establishes a binding such that subsequent requests
going to P1 will reuse the connection; i.e., requests are sent over
the established connection.
With reference to Figure 4, in order for P2 to reuse a connection for
requests in the backwards direction, it is important that the
validation model for requests sent in this direction (i.e., P2 to P1)
is equivalent to the normal "connection in each direction" model,
wherein P2 acting as client would open up a new connection in the
backwards direction and validate the connection by examining the
X.509 certificate presented. The act of reusing a connection needs
the desired property that requests get delivered in the backwards
direction only if they would have been delivered to the same
destination had connection reuse not been employed. To guarantee
this property, the X.509 certificate presented by P1 to P2 when a TLS
connection is first authenticated are cached for later use.
Gurbani, et al. Standards Track PAGE 7
RFC 5923 SIP Connection Reuse June 2010
To aid the discussion of connection reuse, this document defines a
data structure called the connection alias table (or simply, alias
table), which is used to store aliased addresses and is used by user
agents to search for an existing connection before a new one is
opened up to a destination. It is not the intent of this memo to
standardize the implementation of an alias table; rather, we use it
as a convenience to aid subsequent discussions.
P1 gets a request from one of its upstream user agents, and after
performing RFC 3263 [RFC 3263] server selection, arrives at a resolved
address of P2. P1 maintains an alias table, and it populates the
alias table with the IP address, port number, and transport of P2 as
determined through RFC 3263 server selection. P1 adds an "alias"
header field parameter to the topmost Via header field (inserted by
it) before sending the request to P2. The value in the sent-by
production rule of the Via header field (including the port number),
and the transport over which the request was sent becomes the
advertised address of P1:
Via: SIP/2.0/TLS p1.example.com;branch=z9hG4bKa7c8dze;alias
Assuming that P1 does not already have an existing aliased connection
with P2, P1 now opens a connection with P2. P2 presents its X.509
certificate to P1 for validation (see Section 9.1). Upon connection
authentication and acceptance, P1 adds P2 to its alias table. P1's
alias table now looks like:
Destination Destination Destination Destination Alias
IP Address Port Transport Identity Descriptor
...
192.0.2.128 5061 TLS sip:example.net 25
sip:p2.example.net
Subsequent requests that traverse from P1 to P2 will reuse this
connection; i.e., the requests will be sent over the descriptor 25.
The following columns in the alias table created at the client
warrant an explanation:
1. The IP address, port, and transport are a result of executing the
RFC 3263 server resolution process on a next-hop URI.
2. The entries in the fourth column consists of the identities of
the server as asserted in the X.509 certificate presented by the
server. These identities are cached by the client after the
server has been duly authenticated (see Section 9.1).
Gurbani, et al. Standards Track PAGE 8
RFC 5923 SIP Connection Reuse June 2010
3. The entry in the last column is the socket descriptor over which
P1, acting as a client, actively opened a TLS connection. At
some later time, when P1 gets a request from one of the user
agents in its domain, it will reuse the aliased connection
accessible through socket descriptor 25 if and only if all of the
following conditions hold:
A. P1 determines through the RFC 3263 server resolution process
that the {transport, IP-address, port} tuple of P2 to be
{TLS, 192.0.2.128, 5061}, and
B. The URI used for the RFC 3263 server resolution matches one of
the identities stored in the cached certificate (fourth
column).
When P2 receives the request, it examines the topmost Via header
field to determine whether P1 is willing to use this connection as an
aliased connection (i.e., accept requests from P2 towards P1). The
Via header field at P2 now looks like the following (the "received"
header field parameter is added by P2):
Via: SIP/2.0/TLS p1.example.com;branch=z9hG4bKa7c8dze;alias;
received=192.0.2.1
The presence of the "alias" Via header field parameter indicates that
P1 supports aliasing on this connection. P2 now authenticates the
connection (see Section 9.2) and if the authentication was
successful, P2 creates an alias to P1 using the advertised address in
the topmost Via header field. P2's alias table looks like the
following:
Destination Destination Destination Destination Alias
IP Address Port Transport Identity Descriptor
...
192.0.2.1 5061 TLS sip:example.com 18
sip:p1.example.com
There are a few items of interest here:
1. The IP address field is populated with the source address of the
client.
2. The port field is populated from the advertised address (topmost
Via header field), if a port is present in it, or 5061 if it is
not.
Gurbani, et al. Standards Track PAGE 9
RFC 5923 SIP Connection Reuse June 2010
3. The transport field is populated from the advertised address
(topmost Via header field).
4. The entries in the fourth column consist of the identities of the
client as asserted in the X.509 certificate presented by the
client. These identities are cached by the server after the
client has been duly authenticated (see Section 9.2).
5. The entry in the last column is the socket descriptor over which
the connection was passively accepted. At some later time, when
P2 gets a request from one of the user agents in its domain, it
will reuse the aliased connection accessible through socket
descriptor 18 if and only if all of the following conditions
hold:
A. P2 determines through RFC 3263 server resolution process that
the {transport, IP-address, port} tuple of P1 to be {TLS,
192.0.2.1, 5061}, and
B. The URI used for RFC 3263 server resolution matches one of the
identities stored in the cached certificate (fourth column).
6. The network address inserted in the "Destination IP Address"
column is the source address as seen by P2 (i.e., the "received"
header field parameter). It could be the case that the host name
of P1 resolves to different IP addresses due to round-robin DNS.
However, the aliased connection is to be established with the
original sender of the request.
6. Requirements
The following are the requirements that motivated this specification:
1. A connection sharing mechanism should allow SIP entities to reuse
existing connections for requests and responses originated from
either peer in the connection.
2. A connection sharing mechanism must not require clients to send
all traffic from well-know SIP ports.
3. A connection sharing mechanism must not require configuring
ephemeral port numbers in DNS.
4. A connection sharing mechanism must prevent unauthorized
hijacking of other connections.
5. Connection sharing should persist across SIP transactions and
dialogs.
Gurbani, et al. Standards Track PAGE 10
RFC 5923 SIP Connection Reuse June 2010
6. Connection sharing must work across name-based virtual SIP
servers.
7. There is no requirement to share a complete path for ordinary
connection reuse. Hop-by-hop connection sharing is more
appropriate.
7. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC 5234 [RFC 5234]. This document extends
the via-params to include a new via-alias defined below.
via-params =/ via-alias
via-alias = "alias"
8. Normative Behavior
8.1. Client Behavior
Clients SHOULD keep connections up as long as they are needed.
Connection reuse works best when the client and the server maintain
their connections for long periods of time. Clients, therefore,
SHOULD NOT automatically drop connections on completion of a
transaction or termination of a dialog.
The mechanism for connection reuse uses a new Via header field
parameter. The "alias" header field parameter is included in a Via
header field value to indicate that the client wants to create a
transport layer alias. The client places its advertised address in
the Via header field value (in the sent-by production).
If the client places an "alias" header field parameter in the topmost
Via header of the request, the client SHOULD keep the connection open
for as long as the resources on the host operating system allow it
to, and that the client MUST accept requests over this connection --
in addition to the default listening port -- from its downstream
peer. And furthermore, the client SHOULD reuse the connection when
subsequent requests in the same or different transactions are
destined to the same resolved address.
Note that RFC 3261 states that a response arrives over the same
connection that was opened for a request.
Gurbani, et al. Standards Track PAGE 11
RFC 5923 SIP Connection Reuse June 2010
Whether or not to allow an aliased connection ultimately depends on
the recipient of the request; i.e., the client does not get any
confirmation that its downstream peer created the alias, or indeed
that it even supports this specification. Thus, clients MUST NOT
assume that the acceptance of a request by a server automatically
enables connection aliasing. Clients MUST continue receiving
requests on their default port.
Clients MUST authenticate the connection before forming an alias;
Section 9.1 discusses the authentication steps in more detail. Once
the server has been authenticated, the client MUST cache, in the
alias table, the identity (or identities) of the server as determined
in Section 7.1 of RFC 5922 [RFC 5922]. The client MUST also populate
the destination IP address, port, and transport of the server in the
alias table; these fields are retrieved from executing RFC 3263 server
resolution process on the next-hop URI. And finally, the client MUST
populate the alias descriptor field with the connection handle (or
identifier) used to connect to the server.
Once the alias table has been updated with a resolved address, and
the client wants to send a new request in the direction of the
server, the client reuses the connection only if all of the following
conditions hold:
1. The client uses the RFC 3263 resolution on a URI and arrives at a
resolved address contained in the alias table, and
2. The URI used for RFC 3263 server resolution matches one of the
identities stored in the alias table row corresponding to that
resolved address.
Clients MUST be prepared for the case that the connection no longer
exists when they are ready to send a subsequent request over it. In
such a case, a new connection MUST be opened to the resolved address
and the alias table updated accordingly.
This behavior has an adverse side effect when a CANCEL request or an
ACK request for a non-2xx response is sent downstream. Normally,
these would be sent over the same connection over which the INVITE
request was sent. However, if between the sending of the INVITE
request and subsequent sending of the CANCEL request or ACK request
to a non-2xx response, the connection was closed, then the client
SHOULD open a new connection to the resolved address and send the
CANCEL request or ACK request there instead. The client MAY insert
the newly opened connection into the alias table.
Gurbani, et al. Standards Track PAGE 12
RFC 5923 SIP Connection Reuse June 2010
8.2. Server Behavior
Servers SHOULD keep connections up unless they need to reclaim
resources. Connection reuse works best when the client and the
server maintain their connections for long periods of time. Servers,
therefore, SHOULD NOT automatically drop connections on completion of
a transaction or termination of a dialog.
When a server receives a request over TLS whose topmost Via header
field contains an "alias" header field parameter, it signifies that
the upstream client will leave the connection open beyond the
transaction and dialog lifetime, and that subsequent transactions and
dialogs that are destined to a resolved address that matches the
identifiers in the advertised address in the topmost Via header field
can reuse this connection.
Whether or not to use in the reverse direction a connection marked
with the "alias" Via header field parameter ultimately depends on the
policies of the server. It can choose to honor it, and thereby send
subsequent requests over the aliased connection. If the server
chooses not to honor an aliased connection, the server MUST allow the
request to proceed as though the "alias" header field parameter was
not present in the topmost Via header.
This assures interoperability with RFC 3261 server behavior.
Clients can include the "alias" header field parameter without
fear that the server will reject the SIP request because of its
presence.
Servers MUST be prepared to deal with the case that the aliased
connection no longer exist when they are ready to send a subsequent
request over it. This can happen if the peer ran out of operating
system resources and had to close the connection. In such a case,
the server MUST open a new connection to the resolved address and the
alias table updated accordingly.
If the sent-by production of the Via header field contains a port,
the server MUST use it as a destination port. Otherwise, the default
port is the destination port.
Servers MUST follow the authentication steps outlined in Section 9.2
to authenticate the connection before forming an alias.
The server, if it decides to reuse the connection, MUST cache in the
alias table the identity (or identities) of the client as they appear
in the X.509 certificate subjectAlternativeName extension field. The
server also populates the destination IP address, port, and transport
in the alias table from the topmost Via header field (using the
Gurbani, et al. Standards Track PAGE 13
RFC 5923 SIP Connection Reuse June 2010
";received" parameter for the destination IP address). If the port
number is omitted, a default port number of 5061 is to be used. And
finally, the server populates the alias descriptor field with the
connection handle (or identifier) used to accept the connection from
the client (see Section 5 for the contents of the alias table).
Once the alias table has been updated, and the server wants to send a
request in the direction of the client, it reuses the connection only
if all of the following conditions hold:
1. The server, which acts as a client for this transaction, uses the
RFC 3263 resolution process on a URI and arrives at a resolved
address contained in the alias table, and
2. The URI used for RFC 3263 server resolution matches one of the
identities stored in the alias table row corresponding to that
resolved address.
8.3. Closing a TLS connection
Either the client or the server may terminate a TLS session by
sending a TLS closure alert. Before closing a TLS connection, the
initiator of the closure MUST either wait for any outstanding SIP
transactions to complete, or explicitly abandon them.
After the initiator of the close has sent a closure alert, it MUST
discard any TLS messages until it has received a similar alert from
its peer. The receiver of the closure alert MUST NOT start any new
SIP transactions after the receipt of the closure alert.
9. Security Considerations
This document presents requirements and a mechanism for reusing
existing connections easily. Unauthenticated connection reuse would
present many opportunities for rampant abuse and hijacking.
Authenticating connection aliases is essential to prevent connection
hijacking. For example, a program run by a malicious user of a
multiuser system could attempt to hijack SIP requests destined for
the well-known SIP port from a large relay proxy.
9.1. Authenticating TLS Connections: Client View
When a TLS client establishes a connection with a server, it is
presented with the server's X.509 certificate. Authentication
proceeds as described in Section 7.3 ("Client behavior") of RFC 5922
[RFC 5922].
Gurbani, et al. Standards Track PAGE 14
RFC 5923 SIP Connection Reuse June 2010
9.2. Authenticating TLS Connections: Server View
A TLS server conformant to this specification MUST ask for a client
certificate; if the client possesses a certificate, it will be
presented to the server for mutual authentication, and authentication
proceeds as described in Section 7.4 ("Server behavior") of RFC 5922
[RFC 5922].
If the client does not present a certificate, the server MUST proceed
as if the "alias" header field parameter was not present in the
topmost Via header. In this case, the server MUST NOT update the
alias table.
9.3. Connection Reuse and Virtual Servers
Virtual servers present special considerations for connection reuse.
Under the name-based virtual server scheme, one SIP proxy can host
many virtual domains using one IP address and port number. If
adequate defenses are not put in place, a connection opened to a
downstream server on behalf of one domain can be reused to send
requests in the backwards direction to a different domain. The
"Destination Identity" column in the alias table has been added to
aid in such defenses.
Virtual servers MUST only perform connection reuse for TLS
connections; virtual servers MUST NOT perform connection reuse for
other connection-oriented transports. To understand why this is the
case, note that the alias table caches not only which connections go
to which destination addresses, but also which connections have
authenticated themselves as responsible for which domains. If a
message is to be sent in the backwards direction to a new SIP domain
that resolves to an address with a cached connection, the cached
connection cannot be used because it is not authenticated for the new
domain.
As an example, consider a proxy P1 that hosts two virtual domains --
example.com and example.net -- on the same IP address and port.
RFC 3263 server resolution is set up such that a DNS lookup of
example.com and example.net both resolve to an {IP-address, port,
transport} tuple of {192.0.2.1, 5061, TLS}. A user agent in the
example.com domain sends a request to P1 causing it to make a
downstream connection to its peering proxy, P2, and authenticating
itself as a proxy in the example.com domain by sending it a X.509
certificate asserting such an identity. P2's alias table now looks
like the following:
Gurbani, et al. Standards Track PAGE 15
RFC 5923 SIP Connection Reuse June 2010
Destination Destination Destination Destination Alias
IP Address Port Transport Identity Descriptor
...
192.0.2.1 5061 TLS sip:example.com 18
At some later point in time, a user agent in P2's domain wants to
send a request to a user agent in the example.net domain. P2
performs an RFC 3263 server resolution process on sips:example.net to
derive a resolved address tuple {192.0.2.1, 5061, TLS}. It appears
that a connection to this network address is already cached in the
alias table; however, P2 cannot reuse this connection because the
destination identity (sip:example.com) does not match the server
identity used for RFC 3261 resolution (sips:example.net). Hence, P2
will open up a new connection to the example.net virtual domain
hosted on P1. P2's alias table will now look like:
Destination Destination Destination Destination Alias
IP Address Port Transport Identity Descriptor
...
192.0.2.1 5061 TLS sip:example.com 18
192.0.2.1 5061 TLS sip:example.net 54
The identities conveyed in an X.509 certificate are associated with a
specific TLS connection. Absent such a guarantee of an identity tied
to a specific connection, a normal TCP or SCTP connection cannot be
used to send requests in the backwards direction without a
significant risk of inadvertent (or otherwise) connection hijacking.
The above discussion details the impact on P2 when connection reuse
is desired for virtual servers. There is a subtle, but important
impact on P1 as well.
P1 should keep separate alias tables for the requests served from the
UAs in the example.com domain from those served by the UAs in the
example.net domain. This is so that the boundary between the two
domains is preserved; P1 MUST NOT open a connection on behalf of one
domain and reuse it to send a new request on behalf of another
domain.
Gurbani, et al. Standards Track PAGE 16
RFC 5923 SIP Connection Reuse June 2010
10. Connection Reuse and SRV Interaction
Connection reuse has an interaction with the DNS SRV load balancing
mechanism. To understand the interaction, consider the following
figure:
/+---- S1
+-------+/
| Proxy |------- S2
+-------+\
\+---- S3
Figure 5: Load balancing
Here, the proxy uses the DNS SRV to load balance across the three
servers, S1, S2, and S3. Using the connect reuse mechanism specified
in this document, over time the proxy will maintain a distinct
aliased connection to each of the servers. However, once this is
done, subsequent traffic is load balanced across the three downstream
servers in the normal manner.
11. IANA Considerations
This specification defines a new Via header field parameter called
"alias" in the "Header Field Parameters and Parameter Values" sub-
registry as per the registry created by RFC 3968 [RFC 3968]. The
required information is:
Header Field Parameter Name Predefined Values Reference
___________________________________________________________________
Via alias No RFC 5923
12. Acknowledgments
Thanks to Jon Peterson for helpful answers about certificate behavior
with SIP, Jonathan Rosenberg for his initial support of this concept,
and Cullen Jennings for providing a sounding board for this idea.
Other members of the SIP WG that contributed to this document include
Jeroen van Bemmel, Keith Drage, Matthew Gardiner, Rajnish Jain, Benny
Prijono, and Rocky Wang.
Dale Worley and Hadriel Kaplan graciously performed a WGLC review of
the document. The resulting revision has benefited tremendously from
their feedback.
Gurbani, et al. Standards Track PAGE 17
RFC 5923 SIP Connection Reuse June 2010
13. References
13.1. Normative References
[RFC 3261] 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.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC 3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC 5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[RFC 5922] Gurbani, V., Lawrence, S., and B. Laboratories, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, June 2010.
13.2. Informative References
[RFC 3968] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Header Field Parameter Registry for the Session
Initiation Protocol (SIP)", BCP 98, RFC 3968,
December 2004.
[RFC 5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol
(SIP)", RFC 5626, October 2009.
[Book-Rescorla-TLS]
Rescorla, E., "SSL and TLS: Designing and Building Secure
Systems", Addison-Wesley Publishing, 2001.
[RFC 3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
BCP 85, RFC 3725, April 2004.
[RFC 4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
Gurbani, et al. Standards Track PAGE 18
RFC 5923 SIP Connection Reuse June 2010
Authors' Addresses
Vijay K. Gurbani (editor)
Bell Laboratories, Alcatel-Lucent
EMail: vkg@alcatel-lucent.com
Rohan Mahy
Unaffiliated
EMail: rohan@ekabal.com
Brett Tate
BroadSoft
EMail: brett@broadsoft.com
Gurbani, et al. Standards Track PAGE 19
Connection Reuse in the Session Initiation Protocol (SIP)
RFC TOTAL SIZE: 41905 bytes
PUBLICATION DATE: Wednesday, June 23rd, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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