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Versions: (draft-mahy-sip-connect-reuse) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 5923

SIP WG                                                           R. Mahy
Internet-Draft                                               Plantronics
Updates: 3261 (if approved)                              V. Gurbani, Ed.
Expires: April 9, 2007                    Lucent Technologies, Inc./Bell
                                                            Laboratories
                                                                 B. Tate
                                                               BroadSoft
                                                         October 6, 2006


       Connection Reuse in the Session Initiation Protocol (SIP)
                    draft-ietf-sip-connect-reuse-07

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
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   This Internet-Draft will expire on April 9, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   When SIP entities use a connection oriented protocol to send a
   request, they typically originate their connections from an ephemeral
   port.  The SIP protocol includes mechanisms which ensure that
   responses to a request, and new requests sent in the original



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   direction reuse an existing connection.  However, new requests sent
   in the opposite direction are unlikely to reuse the existing
   connection.  For TLS transports, this will frequently cause a pair of
   SIP entities to use one connection for requests sent in each
   direction, resulting in potential scaling and performance problems.
   This document proposes that a TLS connection, once opened, can be
   used to send requests in either direction.  Unfortunately, TCP
   connections cannot be shared in the same manner due to the risk of
   service hijacking.  This document proposes an alternate way to
   perform TCP connection reuse.

Table of Contents

   1.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Applicability Statement  . . . . . . . . . . . . . . . . . .   3
   3.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.   Benefits of TLS Connection Reuse . . . . . . . . . . . . . .   4
   5.   Overview of Operation  . . . . . . . . . . . . . . . . . . .   5
     5.1  TLS Operations . . . . . . . . . . . . . . . . . . . . . .   6
     5.2  TCP Operations . . . . . . . . . . . . . . . . . . . . . .   8
   6.   Requirements . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.   Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . .  10
   8.   Normative Behavior . . . . . . . . . . . . . . . . . . . . .  10
     8.1  Client Behavior  . . . . . . . . . . . . . . . . . . . . .  11
     8.2  Server Behavior  . . . . . . . . . . . . . . . . . . . . .  12
   9.   Security Considerations  . . . . . . . . . . . . . . . . . .  13
     9.1  Authenticating TLS Connections: Client View  . . . . . . .  14
     9.2  Authenticating TLS Connections: Server View  . . . . . . .  14
     9.3  Security Considerations for the TCP Transport  . . . . . .  14
   10.  Support for Virtual Servers  . . . . . . . . . . . . . . . .  16
     10.1   Virtual Servers and TLS Connections  . . . . . . . . . .  16
   11.  Connection Reuse and SRV Interaction . . . . . . . . . . . .  17
   12.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  17
   13.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  18
   14.  References . . . . . . . . . . . . . . . . . . . . . . . . .  18
     14.1   Normative References . . . . . . . . . . . . . . . . . .  18
     14.2   Informational References . . . . . . . . . . . . . . . .  19
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  19
        Intellectual Property and Copyright Statements . . . . . . .  20












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1.  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 [2].

   Additional terminology used in this document:

   Advertised address: The address that occurs in the Via sent-by
      production rule, including the port number and transport.
   Alias: A transport layer connection associated with a resolved
      address.
   Resolved address: The network identifiers (IP address, port,
      transport) associated with a user agent as a result of executing
      RFC3263 [4] on a Uniform Resource Identifier (URI).

2.  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 firewall reasons -- reuse their connections using the
   mechanism described in [8].

3.  Introduction

   SIP [1] entities can communicate using either unreliable/
   connectionless (e.g., UDP) or reliable/connection-oriented (e.g.,
   TCP, SCTP [14]) 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, Entity A listens for SIP requests over TLS
   [3] on TCP port 5061 (the default port for SIP over TLS over TCP),
   but uses an ephemeral port (port 8293) for a new connection to Entity
   B. These entities could be SIP User Agents or SIP Proxy Servers.

          +-----------+ 8293 (UAC)      5061 (UAS) +-----------+
          |           |--------------------------->|           |
          |  Entity   |                            |  Entity   |
          |     A     |                            |     B     |
          |           | 5061 (UAS)                 |           |
          +-----------+                            +-----------+

   Figure 1: Uni-directional connection for requests from A to B.




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   The SIP protocol includes mechanisms which insure that responses to a
   request reuse the existing connection which is typically still
   available, and also includes provisions for reusing existing
   connections for other requests sent by the originator of the
   connection.  However, new requests sent in the opposite 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.

          +-----------+ 8293              5061 +-----------+
          |           |.......................>|           |
          |  Entity   |                        |  Entity   |
          |     A     | 5061              9741 |     B     |
          |           |<-----------------------|           |
          +-----------+                        +-----------+

   Figure 2: Two connections for requests between A and B.


   While this is adequate for TCP, and indeed is the only way to
   securely do connection reuse over that transport (see Section 9.3),
   TLS connections can be reused 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 the connection opened from Entity A
   to Entity B is shared.  When Entity A wants to send a request to
   Entity B, it will reuse this connection, and when Entity B wants to
   send a request to Entity A, it will reuse the same connection.

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 3-way handshake, a handful of
   round-trips to establish TLS, typically expensive asymmetric
   authentication and key generation algorithms, and certificate
   verification.  This may 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 [9]).

   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.







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                      Proxy A    Proxy B
                         |          |
     Create connection 1 +---INV--->|
                         |          |
                         |<---200---+ Response over connection 1
                         |          |
     Re-use 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 or BYE after an initial INVITE, or a NOTIFY after a
   SUBSCRIBE [13] or a REFER [12]), 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-INVITEs or UPDATE [10] requests are handled automatically and
   rapidly in order to avoid media and session state from being out of
   step.  If a re-INVITE requires a new TLS connection, the reINVITE
   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 number 4 in RFC3725 [11] use many reINVITEs).

   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, it should be clear that the same technique can be used
   for connection reuse between a UAC and an edge proxy, or between an
   edge proxy and a UAS, or between an UAC and an UAS.




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   P1 and P2 are proxies responsible for routing SIP requests through
   user agents that use them as edge proxies (see Figure 4).

                   P1 <===================> P2
              p1.example.com          p2.example.com
               (192.0.2.1)              (192.0.2.128)

        +---+                                    +---+
        |   |   0---0                   0---0    |   |
        |___|    /-\                     /-\     |___|
       /    /   +---+                   +---+   /    /
      +----+                                   +----+
      User Agents                       User Agents

   Figure 4: Proxy setup.


5.1  TLS Operations

   The act of reusing a connection is initiated by an user agent client
   (UAC) when it adds an "alias" parameter (defined later) to the Via
   header.  When a user agent server (UAS) receives the request, it
   examines the topmost Via header.  If the header contained an "alias"
   parameter, the UAS establishes a binding such that subsequent
   requests going to the UAC 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 opposite direction, it is important to note that the
   validation model for requests sent in this direction (i.e., P2 to P1)
   should be 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 must
   have the desired property that requests get delivered in the reverse
   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 must be 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.  User agents can
   consult the alias table for an existing connection before opening up
   a new one.

   P1 gets a request from one of its upstream user agents, and after
   performing RFC3263 server selection, arrives at a resolved address of



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   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" parameter to
   the topmost Via header (inserted by it) before sending the request to
   P2.  The value in the sent-by production rule of the Via header
   (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 P2s 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       DNS:p2.example.com,  25
                                       sips:example.com
   Figure 5: Alias table at the client.

   Subsequent requests that traverse from P1 to P2 will reuse this
   connection; i.e., the requests will be sent over the descriptor 25.

   There are three items of interest in the alias table created at the
   client:
   1.  The IP address, port and transport are a result of executing
       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 RFC3263 server resolution process that
           the request should be sent to P2 on port 5061 using TLS, and
       B.  The URI used for RFC3263 server resolution matches one of the
           identities stored in the cached certificate (fourth column).

   When the server, P2, receives the request, it examines the topmost
   Via to determine whether P1 supports aliased connections.  The Via at
   P2 now looks like (the "received" parameter is added by P2):



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   Via: SIP/2.0/TLS p1.example.com;branch=z9hG4bKa7c8dze;alias;
     received=192.0.2.1

   The presence of the "alias" parameter indicates that P1 does support
   aliasing.  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. P2's alias table looks
   like:

   Destination  Destination  Destination  Destination     Alias
   IP Address   Port         Transport    Identity        Descriptor
   ...
   192.0.2.1    5061             TLS   DNS:p1.example.com,    18
                                       sips:example.com
   Figure 6: Alias table at the server.

   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), 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).
   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 request should be sent to P1 on port 5061 using TLS, 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 should be the source address as seen by P2 (i.e., the
       "received" 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.

5.2  TCP Operations

   Connection reuse on the TCP transport is done differently from the
   TLS case.  This is to prevent a service hijacking security attack



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   outlined in Section 9.3.

   In TCP, connection reuse is accomplished by each host pro-actively
   opening up a TCP connection towards its neighbor.  Thus, two TCP
   connections will be needed between an adjacent pair of hosts, as
   depicted in Figure 2.

   The presence of the "alias" parameter in the topmost Via for a TCP
   transport is not required.

   From an operations point of view, the same data structure used to
   maintain TLS connections can be used for TCP connections as well.
   For TCP connections, the contents of this table will be slightly
   different in two ways: first, the "Destination Transport" will be
   "TCP", and second, the "Destination Identity" is null, or empty.

   With reference to Figure 4, P1 gets a request from one of its
   upstream user agents, and after performing RFC3263 server selection,
   arrives at the resolved address of P2.  P1 populates the alias table
   with the IP address, port number, and transport of P2 as determined
   through RFC3263 server selection.  The value of the sent-by
   production rule of the Via header (including the port number), and
   the transport over which the request was sent becomes the advertised
   address of P1:

   Via: SIP/2.0/TCP p1.example.com;branch=z9hG4bKa7c8dze

   Assuming that P1 does not already have an existing entry with P2's
   resolved address in the alias table, P1 now opens up a new TCP
   connection with P2.  When P2 accepts the connection, 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  5060         TCP              -               32

    Figure 7: Alias table at the client for TCP transport.

   Because this same TCP connection cannot be used to send requests from
   P2 to P1, P2 will not update its alias table.  Instead, at a later
   time, if a request from P2 is destined towards P1, P2 will actively
   open up a new TCP connection towards P1, and update its alias table
   accordingly.  Once this is done, P1 and P2 will reuse the same
   connections that they established proactively for subsequent
   requests.





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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 4234 [5].  This document extends the
   via-params to include a new via-alias defined below.

      via-params =/ via-alias
      via-alias  =  "alias"


8.  Normative Behavior

   This document specifies how to reuse connections.  It is RECOMMENDED
   that servers keep connections up unless they need to reclaim
   resources, and that clients 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.  SIP entities
   therefore SHOULD NOT automatically drop connections on completion of
   a transaction or termination of a dialog.

   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.






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      Note that 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 that the INVITE request was sent over.  However, if
      between the sending of the INVITE and subsequent sending of the
      CANCEL or ACK to a non-2xx response, the connection was reclaimed,
      then the client SHOULD open a new connection to the resolved
      address and send the CANCEL or ACK there instead.  The newly
      opened connection MAY be inserted into the alias table.

8.1  Client Behavior

   For the TCP transport, when the client executes the RFC3263 server
   selection mechanism to arrive at an IP address, port, and transport
   tuple to send the request to, it updates the alias table with this
   information.  Subsequent requests that resolve to the same IP
   address, port, and transport tuple MUST reuse the same connection.
   The client must keep the connection open for as long as the resources
   on the operating system allow it to.  It MUST only accept responses
   over this connection and MUST NOT accept any requests over this
   connection.

   For TCP transports, the client MUST NOT update the topmost Via that
   it inserts with the "alias" parameter.

   The rest of the discussion below applies to only the TLS transport.

   For TLS transports, the proposed mechanism uses a new Via header
   field parameter.  The "alias" 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).

   The implications of placing an "alias" parameter in the topmost Via
   header of a request must be understood by the client.  Specifically,
   this means that the client MUST keep the connection open for as long
   as the resources on the host operating system allow it to, and that
   it MUST accept requests over this connection -- in addition to the
   default listening port -- from its downstream peer.  And furthermore,
   it MUST reuse the connection when subsequent requests in the same or
   different transactions are destined to the same resolved address.

      Note that RFC3261 states that a response should arrive 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



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   that it even supports this specification.  Thus, clients MUST NOT
   assume that the acceptance of a request by a server automatically
   enables connection aliasing.  They MUST continue receiving requests
   on their default port.

   For TLS connections, 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 they appear in the X.509 certificate subjectAlternativeName
   extension field.  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 socket descriptor 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, it should reuse 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.

8.2  Server Behavior

   A TCP connection accepted at the server is used by the server to only
   send responses upstream.  It MUST NOT be used to send requests.
   Furthermore, if the topmost Via header of a request that arrived over
   TCP had the "alias" parameter in it, the server MUST NOT accord any
   semantics to this parameter.

   The rest of the discussion below applies to only the TLS transport.

   When a server receives a request over TLS whose topmost Via header
   contains an "alias" 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 can reuse this
   connection.

   Whether or not to honor an aliased connection ultimately depends on
   the policies of the server.  It MAY choose to honor it, and thereby
   send subsequent requests over the aliased connection.  If the server



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   chooses not to honor an aliased connection, it MUST allow the request
   to proceed as though the "alias" parameter was not present in the
   topmost Via header.

      This assures interoperability with RFC3261 server behavior.
      Clients should feel comfortable including the "alias" 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 may happen if the peer ran out of operating
   system resources and had to close the connection.  In such a case, a
   new connection MUST be opened to the resolved address and the alias
   table updated accordingly.

   If the Via sent-by contains a port, it MUST be used as a destination
   port.  Otherwise the default port is the destination port.

   Servers must authenticate the connection before forming an alias.
   Section 9.2 discusses the authentication steps in more detail.

   The server, if it decides to accept 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 must also populate the destination IP address,
   port and transport in the alias table from the topmost Via header
   (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 must populate the alias descriptor
   field with the socket descriptor 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 should reuse 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.

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.



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   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 5 of [7].

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 6 of [7].

   If the client does not present a certificate, the server MUST proceed
   as if the "alias" parameter was not present in the topmost Via. In
   this case, the alias table MUST NOT be updated.

9.3  Security Considerations for the TCP Transport

   The mechanism for reusing TLS connections MUST NOT be used to reuse
   TCP connections because there isn't any way to perform the
   authentication step.  Instead, it is RECOMMENDED that TCP peers that
   want to avail of connection reuse do so such that each peer actively
   opens up a TCP connection in the direction of the other (as depicted
   in Figure 2).  This manner of opening connections, while still not
   secure, is at least much more apparent and direct than using the
   connection reuse mechanism over TCP in an unauthenticated fashion.

   Connection reuse over TCP is inherently insecure.  Because the nature
   of the aliasing mechanism is such that it redirects requests destined
   for one port at a host to another port, service hi-jacking can result
   if adequate care is not taken to ensure that the redirected port is
   indeed authorized to receive the requests that would normally have
   gone to another, authorized port.  Consider the following scenario to
   understand the service hi-jacking attack that can be mounted when
   using connection reuse over TCP.

   A TCP server receives a request with the topmost Via header as
   follows (the "received" parameter is added by the server after
   getting the request):

   Via: SIP/2.0/TCP uac.example.com;branch=z9hG4bKa7c8dze;
      received=192.0.4.33



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   If we were to allow TCP connection reuse in the same manner as TLS
   connection reuse, then the server would update its alias table such
   that whenever a request is destined to 192.0.4.33, port 5060, it will
   instead be sent to the peer at the end of the aliased connection.  A
   security attack can now be mounted as follows: assume a malware
   program is running on a multi-user computer.  The malware program
   knows that a user on the computer runs a SIP user agent, but the SIP
   user agent is currently not active (possibly by scanning ports on the
   local machine to seek a busy port 5060).  Note that the malware
   program does not need to wait until the legitimate user agent was not
   running, however, doing so increases the chances that the server will
   not reject the malware program's request.  Once the malware program
   decides that a legitimate user agent is not running, it sends a
   request to the server with an "alias" parameter.  The server believes
   it is accepting a request from a legitimate user agent and sends
   subsequent requests to the aliased connection.  The SIP service on
   the computer has now effectively been hi-jacked for the default port.
   The malware program does not need administrative privileges to
   execute, and in fact, can masquerade as any user (legitimate or not)
   of the computer.

   Later on, when the legitimate user agent is started, it may also send
   a request with an "alias" parameter to the server, which may detect
   that it now has two aliased connections.  Making matters much worse,
   it cannot determine which of the two is the legitimate one and may
   well reject the request from the legitimate user.

   In another form of this attack, the legitimate user agent may not
   support connection aliasing, but the malware program may use the
   mechanism to usurp the SIP service on the computer.

   In yet another form of an attack, the malware program uses the
   aliasing mechanism to shortcut registering with a proxy to receive
   requests.  In this case, it sends a request to the edge proxy (who
   may also substitute as the inbound proxy with access to a location
   service for that domain).  In the request is a bogus request URI that
   will cause the edge proxy to fail the request, however, the edge
   proxy keeps the connection open and any subsequent requests destined
   to that host on the default port are instead sent to the malware
   program.  Registration is thus not needed in order to receive
   incoming requests.

   HTTP Digest is useful to mitigate only a subset of these attacks over
   TCP.  For instance, HTTP Digest helps in authenticating a user agent
   to a proxy server before the alias table is updated.  However, HTTP
   Digest is of no help when one proxy desires to enter an aliasing
   agreement with another downstream proxy.




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10.  Support for Virtual Servers

   Virtual servers present special considerations for connection reuse.
   Under the name-based virtual server scheme, one SIP proxy may host
   many virtual domains.  If adequate defenses are not put in place, a
   connection opened to a downstream server on behalf of one domain can
   be usurped by a malicious user in another domain.  The Destination
   Identity column in the alias table has been added to aid in such
   defenses.  If an implementation does not support virtual SIP servers,
   it MAY omit caching the identities in the alias table; however, if an
   implementation supports virtual SIP servers, then it MUST cache the
   identities in the alias table.

10.1  Virtual Servers and TLS Connections

   To understand the specific problem associated with hijacking a TLS
   connection when virtual servers are used, consider a proxy P1 that
   hosts two domains: atlanta.example.com and chicago.example.org.  Also
   assume that the physical IP address of P1 is 192.168.0.1.  Incoming
   requests to all the domains that P1 hosts arrive on port 5061.

   A user, bob@atlanta.example.com, sends an instant message to a user
   Alice in a domain not hosted by P1.  Alice's proxy establishes an
   alias to P1, thereby resulting in the following alias table (note: to
   illustrate the connection hijacking problem associated with virtual
   servers, the alias table below does not contain the Destination
   Identity column).

   Destination  Destination  Destination  Alias
   IP Address   Port         Transport    Descriptor
   ...
   192.168.0.1  5061         TLS          25

   Figure 8: Alias table at Alice's Proxy.

   At some later time, a user hosted by another virtual domain in P1,
   bob@chicago.example.org, sends an instant message to Alice.  Alice's
   proxy will get the network identifiers from the topmost Via, and note
   that they are already in the alias table.  Thinking that the newly
   arrived request is intended to replace the old (possibly stale)
   alias, it may update its alias table with the new descriptor.

   Some time after that, Alice wants to send an instant message to
   sips:bob@atlanta.example.com.  When RFC3263 resolution is done on
   sips:atlanta.example.com, the resolved address will match an entry in
   the alias table.  But that entry is now aliased to a connection with
   bob@chicago.example.org.  The end result of all this is that an
   instant message intended for bob@atlanta.example.com ends up in the



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   inbox of bob@chicago.example.org.

   It is to alleviate this very problem that the identities from the
   X.509 certificates are stored in the alias table and used to
   determine whether or not to reuse a connection.  Saving the
   identities in the alias table mitigates this problem because Alice's
   proxy will actually form two aliased connections to P1: one row in
   the table will contain the resolved address of P1 but with an
   identity corresponding to atlanta.example.com and a second row will
   contain the same resolved address but with an identity corresponding
   to chicago.example.org.  Now, when Alice's proxy wants to send a
   request in the backwards direction, it will match the URI used to do
   RFC3263 resolution to the appropriate identity before reusing the
   connection.

11.  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 9: Load balancing.

   Here, the proxy uses 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.

12.  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 [6].  The required
   information is:








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

   RFC XXXX [NOTE TO RFC-EDITOR: Please replace with final RFC number of
   this specification.]


13.  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.

14.  References

14.1  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]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, March 1997.

   [3]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
        RFC 2246, January 1999.

   [4]  Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
        (SIP): Locating SIP Servers", RFC 3263, June 2002.

   [5]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 4234, October 2005.

   [6]  Camarillo, G., "The Internet Assigned Numbers Authority (IANA)
        Header  Field Paramater Registry for the Session Initiation
        Protocol (SIP)", BCP 98, RFC 3968, December 2004.

   [7]  Gurbani, V., Jeffrey, A., and S. Lawrence, "Domain Certificates
        in the Session Initiation Protocol (SIP)",
        draft-gurbani-sip-domain-certs-03 (work in progress),
        August 2006.






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14.2  Informational References

   [8]   Jennings, C. and R. Mahy, "Managing Client Initiated
         Connections in the Session Initiation  Protocol (SIP)",
         draft-ietf-sip-outbound-04.txt (work in progress), June 2006.

   [9]   Rescorla, E., "SSL and TLS: Designing and Building Secure
         Systems", Addison-Wesley Publishing , 2001.

   [10]  Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, September 2002.

   [11]  Rosenberg, J., Peterson, J., Schulzrinne, H., and H. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", RFC 3725, April 2004.

   [12]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
         Method", RFC 3515, April 2003.

   [13]  Roach, A., "The Session Initiation Protocol (SIP)-Specific
         Event Notification", RFC 3265, June 2002.

   [14]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V.
         Paxson, "The Session Initiation Protocol (SIP)-Specific Event
         Notification", RFC 2960, October 2000.


Authors' Addresses

   Rohan Mahy
   Plantronics

   Email: rohan@ekabal.com


   Vijay K. Gurbani (editor)
   Lucent Technologies, Inc./Bell Laboratories

   Email: vkg at acm dot org


   Brett Tate
   BroadSoft

   Email: brett@broadsoft.com





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