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/rfc5923.

Copyright Notice

Copyright © 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

1. Introduction

SIP entities can communicate using either unreliable/connectionless (e.g., UDP) or reliable/connection-oriented (e.g., TCP, SCTP [RFC4960]) 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 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 [RFC2119].

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 [RFC3261].  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 [RFC3263] on a Uniform Resource Identifier (URI).
   
   Shared connection:  See "Persistent connection".

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 [RFC5626].

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 [RFC3261] 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 [RFC5246]) 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.

                      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 [RFC3725] 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.

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.

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 RFC3263 [RFC3263] 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 RFC3263 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 RFC3263 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).

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 RFC3263 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 RFC3263 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.

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 RFC3263 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 RFC3263 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.

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 [RFC5234]. 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.

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 [RFC5922]. 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 RFC3263 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 RFC3263 resolution on a URI and arrives at a resolved address contained in the alias table, and

2. The URI used for RFC3263 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.

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 RFC3261 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

";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 RFC3263 resolution process on a URI and arrives at a resolved address contained in the alias table, and

2. The URI used for RFC3263 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 [RFC5922].

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 [RFC5922].

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. RFC3263 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:

   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 RFC3263 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 RFC3261 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.

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 [RFC3968]. The required information is:

   Header Field  Parameter Name  Predefined Values  Reference
   ___________________________________________________________________
   Via           alias                 No           RFC5923

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.

13. References

13.1. Normative References

   [RFC3261]  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.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.
   
   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.
   
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 5234, January 2008.
   
   [RFC5922]  Gurbani, V., Lawrence, S., and B. Laboratories, "Domain
              Certificates in the Session Initiation Protocol (SIP)",
              RFC 5922, June 2010.

13.2. Informative References

   [RFC3968]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Header Field Parameter Registry for the Session
              Initiation Protocol (SIP)", BCP 98, RFC 3968,
              December 2004.
   
   [RFC5626]  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.
   
   [RFC3725]  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.
   
   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

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