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RFC 6314

 
 
 

NAT Traversal Practices for Client-Server SIP

Part 3 of 4, p. 27 to 48
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5.2.  Basic NAT Media Traversal

   This section provides example scenarios to demonstrate basic media
   traversal using the techniques outlined earlier in this document.

   In the flow diagrams, STUN messages have been annotated for
   simplicity as follows:

   o  The "Src" attribute represents the source transport address of the
      message.

   o  The "Dest" attribute represents the destination transport address
      of the message.

   o  The "Map" attribute represents the server reflexive (XOR-MAPPED-
      ADDRESS STUN attribute) transport address.

   o  The "Rel" attribute represents the relayed (RELAY-ADDRESS STUN
      attribute) transport address.

   The meaning of each STUN attribute is extensively explained in the
   core STUN [RFC5389] and TURN [RFC5766] specifications.

   A number of ICE SDP attributes have also been included in some of the
   examples.  Detailed information on individual attributes can be
   obtained from the core ICE specification [RFC5245].

   The examples also contain a mechanism for representing transport
   addresses.  It would be confusing to include representations of
   network addresses in the call flows and would make them hard to
   follow.  For this reason, network addresses will be represented using
   the following annotation.  The first component will contain the
   representation of the client responsible for the address.  For
   example, in the majority of the examples "L" (left client), "R"
   (right client), "NAT-PUB" (NAT public), "PRIV" (Private), and "STUN-
   PUB" (STUN public) are used.  To allow for multiple addresses from
   the same network element, each representation can also be followed by
   a number.  These can also be used in combination.  For example,
   "L-NAT-PUB-1" would represent a public network address of the left-
   hand side NAT while "R-NAT-PUB-1" would represent a public network
   address of the right-hand side of the NAT.  "L-PRIV-1" would
   represent a private network address of the left-hand side of the NAT
   while "R-PRIV-1" represents a private address of the right-hand side
   of the NAT.

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   It should also be noted that, during the examples, it might be
   appropriate to signify an explicit part of a transport address.  This
   is achieved by adding either the '.address' or '.port' tag on the end
   of the representation -- for example, 'L-PRIV-1.address' and 'L-PRIV-
   1.port'.

   The use of '$' signifies variable parts in example SIP messages.

5.2.1.  Endpoint-Independent NAT

   This section demonstrates an example of a client both initiating and
   receiving calls behind an Endpoint-Independent NAT.  An example is
   included for both STUN and ICE with ICE being the RECOMMENDED
   mechanism for media traversal.

   At this time, there is no reliable test to determine if a host is
   behind an Endpoint-Independent Filtering NAT or an Endpoint-
   Independent Mapping NAT [RFC5780], and the sort of failure that
   occurs in this situation is described in Section 5.2.2.1.  For this
   reason, ICE is RECOMMENDED over the mechanism described in this
   section.

5.2.1.1.  STUN Solution

   It is possible to traverse media through an Endpoint-Independent NAT
   using STUN.  The remainder of this section provides simplified
   examples of the 'Binding Discovery' STUN as defined in [RFC5389].
   The STUN messages have been simplified and do not include 'Shared
   Secret' requests used to obtain the temporary username and password.

5.2.1.1.1.  Initiating Session

   The following example demonstrates media traversal through a NAT with
   Endpoint-Independent Mapping properties using the STUN 'Binding
   Discovery' usage.  It is assumed in this example that the STUN client
   and SIP Client are co-located on the same physical machine.  Note
   that some SIP signaling messages have been left out for simplicity.

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     Client              NAT               STUN                [..]
                                          Server
       |                  |                  |                  |
       |(1) BIND Req      |                  |                  |
       |Src=L-PRIV-1      |                  |                  |
       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(2) BIND Req      |                  |
       |                  |Src=NAT-PUB-1     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(3) BIND Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-1    |                  |
       |                  |Map=NAT-PUB-1     |                  |
       |                  |                  |                  |
       |(4) BIND Resp     |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-1     |                  |                  |
       |Map=NAT-PUB-1     |                  |                  |
       |                  |                  |                  |
       |(5) BIND Req      |                  |                  |
       |Src=L-PRIV-2      |                  |                  |
       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(6) BIND Req      |                  |
       |                  |Src=NAT-PUB-2     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(7) BIND Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-2    |                  |
       |                  |Map=NAT-PUB-2     |                  |
       |                  |                  |                  |
       |(8) BIND Resp     |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-2     |                  |                  |
       |Map=NAT-PUB-2     |                  |                  |
       |                  |                  |                  |

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       |(9)SIP INVITE     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(10)SIP INVITE    |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |
       |                  |                  |(11)SIP 200 OK    |
       |                  |<------------------------------------|
       |                  |                  |                  |
       |(12)SIP 200 OK    |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |
       |========================================================|
       |>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>>>|
       |========================================================|
       |                                                        |
       |========================================================|
       |<<<<<<<<<<<<Incoming Media sent to NAT-PUB-1<<<<<<<<<<<<|
       |========================================================|
       |                                                        |
       |========================================================|
       |>>>>>>>>>>>>Outgoing RTCP sent from L-PRIV-2>>>>>>>>>>>>|
       |========================================================|
       |                                                        |
       |========================================================|
       |<<<<<<<<<<<<Incoming RTCP sent to NAT-PUB-2<<<<<<<<<<<<<|
       |========================================================|
       |                  |                  |                  |
       |(13)SIP ACK       |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(14) SIP ACK      |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |

             Figure 11: Endpoint-Independent NAT - Initiating

   o  On deciding to initiate a SIP voice session, the client starts a
      local STUN client on the interface and port that is to be used for
      media (send/receive).  The STUN client generates a standard
      'Binding Discovery' request as indicated in (1) from Figure 11
      that also highlights the source address and port for which the
      client device wishes to obtain a mapping.  The 'Binding Discovery'
      request is sent through the NAT towards the public Internet and
      STUN server.

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   o  Message (2) traverses the NAT and breaks out onto the public
      Internet towards the public STUN server.  Note that the source
      address of the 'Binding Discovery' request now represents the
      public address and port from the public side of the NAT.

   o  The STUN server receives the request and processes it
      appropriately.  This results in a successful 'Binding Discovery'
      response being generated and returned (3).  The message contains
      details of the XOR-mapped public address (contained in the STUN
      XOR-MAPPED-ADDRESS attribute) that is to be used by the
      originating client to receive media (see 'Map=NAT-PUB-1' from
      (3)).

   o  The 'Binding Discovery' response traverses back through the NAT
      using the path created by the 'Binding Discovery' request and
      presents the new XOR-mapped address to the client (4).  At this
      point, the process is repeated to obtain a second XOR-mapped
      address (as shown in (5)-(8)) for a second local address (the
      address has changed from "L-PRIV-1" to "L-PRIV-2") for an RTCP
      port.

   o  The client now constructs a SIP INVITE message (9).  Note that
      traversal of SIP is not covered in this example and is discussed
      in Section 5.1.  The INVITE request will use the addresses it has
      obtained in the previous STUN transactions to populate the SDP of
      the SIP INVITE as shown below:

      v=0
      o=test 2890844526 2890842807 IN IP4 $L-PRIV-1.address
      c=IN IP4 $NAT-PUB-1.address
      t=0 0
      m=audio $NAT-PUB-1.port RTP/AVP 0
      a=rtcp:$NAT-PUB-2.port

   o  Note that the XOR-mapped address obtained from the 'Binding
      Discovery' transactions are inserted as the connection address for
      the SDP (c=$NAT-PUB-1.address).  The Primary port for RTP is also
      inserted in the SDP (m=audio $NAT-PUB-1.port RTP/AVP 0).  Finally,
      the port gained from the additional 'Binding Discovery' is placed
      in the RTCP attribute (as discussed in Section 4.2.2) for
      traversal of RTCP (a=rtcp:$NAT-PUB-2.port).

   o  The SIP signaling then traverses the NAT and sets up the SIP
      session (9-12).  Note that the left-hand client transmits media as
      soon as the 200 OK to the INVITE arrives at the client (12).  Up
      until this point, the incoming media and RTCP to the left-hand

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      client will not pass through the NAT as no outbound association
      has been created with the far-end client.  Two-way media
      communication has now been established.

5.2.1.1.2.  Receiving Session Invitation

   Receiving a session for an Endpoint-Independent NAT using the STUN
   'Binding Discovery' usage is very similar to the example outlined in
   Section 5.2.1.1.1.  Figure 12 illustrates the associated flow of
   messages.

     Client              NAT               STUN                [..]
                                          Server
       |                  |                  | (1)SIP INVITE    |
       |                  |<------------------------------------|
       |                  |                  |                  |
       |(2) SIP INVITE    |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |
       |(3) BIND Req      |                  |                  |
       |Src=L-PRIV-1      |                  |                  |
       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(4) BIND Req      |                  |
       |                  |Src=NAT-PUB-1     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(5) BIND Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-1    |                  |
       |                  |Map=NAT-PUB-1     |                  |
       |                  |                  |                  |
       |(6) BIND Resp     |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-1     |                  |                  |
       |Map=NAT-PUB-1     |                  |                  |
       |                  |                  |                  |
       |(7) BIND Req      |                  |                  |
       |Src=L-PRIV-2      |                  |                  |
       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |

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       |                  |(8) BIND Req      |                  |
       |                  |Src=NAT-PUB-2     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(9) BIND Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-2    |                  |
       |                  |Map=NAT-PUB-2     |                  |
       |                  |                  |                  |
       |(10) BIND Resp    |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-2     |                  |                  |
       |Map=NAT-PUB-2     |                  |                  |
       |                  |                  |                  |
       |(11)SIP 200 OK    |                  |                  |
       |----------------->|                  |                  |
       |                  |(12)SIP 200 OK    |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |
       |========================================================|
       |>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>>>|
       |========================================================|
       |                  |                  |                  |
       |========================================================|
       |<<<<<<<<<<<<<Incoming Media sent to L-PRIV-1<<<<<<<<<<<<|
       |========================================================|
       |                  |                  |                  |
       |========================================================|
       |>>>>>>>>>>>>Outgoing RTCP sent from L-PRIV-2>>>>>>>>>>>>|
       |========================================================|
       |                  |                  |                  |
       |========================================================|
       |<<<<<<<<<<<<<Incoming RTCP sent to L-PRIV-2<<<<<<<<<<<<<|
       |========================================================|
       |                  |                  |                  |
       |                  |                  |(13)SIP ACK       |
       |                  |<------------------------------------|
       |                  |                  |                  |
       |(14)SIP ACK       |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |

              Figure 12: Endpoint-Independent NAT - Receiving

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   o  On receiving an invitation to a SIP voice session (SIP INVITE
      request), the User Agent starts a local STUN client on the
      appropriate port on which it is to receive media.  The STUN client
      generates a standard 'Binding Discovery' request as indicated in
      (3) from Figure 12 that also highlights the source address and
      port for which the client device wishes to obtain a mapping.  The
      'Binding Discovery' request is sent through the NAT towards the
      public Internet and STUN server.

   o  'Binding Discovery' message (4) traverses the NAT and breaks out
      onto the public Internet towards the public STUN server.  Note
      that the source address of the STUN requests now represents the
      public address and port from the public side of the NAT.

   o  The STUN server receives the request and processes it
      appropriately.  This results in a successful 'Binding Discovery'
      response being generated and returned (5).  The message contains
      details of the mapped public address (contained in the STUN XOR-
      MAPPED-ADDRESS attribute) that is to be used by the originating
      client to receive media (see 'Map=NAT-PUB-1' from (5)).

   o  The 'Binding Discovery' response traverses back through the NAT
      using the path created by the outgoing 'Binding Discovery' request
      and presents the new XOR-mapped address to the client (6).  At
      this point, the process is repeated to obtain a second XOR-mapped
      address (as shown in (7)-(10)) for a second local address (local
      port has now changed and is represented by L-PRIV-2 in (7)) for an
      RTCP port.

   o  The client now constructs a SIP 200 OK message (11) in response to
      the original SIP INVITE requests.  Note that traversal of SIP is
      not covered in this example and is discussed in Section 5.1.  SIP
      Provisional responses are also left out for simplicity.  The 200
      OK response will use the addresses it has obtained in the previous
      STUN transactions to populate the SDP of the SIP 200 OK as shown
      below:

      v=0
      o=test 2890844526 2890842807 IN IP4 $L-PRIV-1.address
      c=IN IP4 $NAT-PUB-1.address
      t=0 0
      m=audio $NAT-PUB-1.port RTP/AVP 0
      a=rtcp:$NAT-PUB-2.port

   o  Note that the XOR-mapped address obtained from the initial
      'Binding Discovery' transaction is inserted as the connection
      address for the SDP (c=NAT-PUB-1.address).  The Primary port for
      RTP is also inserted in the SDP (m=audio NAT-PUB-1.port RTP/AVP

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      0).  Finally, the port gained from the second 'Binding Discovery'
      is placed in the RTCP attribute (as discussed in Section 4.2.2)
      for traversal of RTCP (a=rtcp:NAT-PUB-2.port).

   o  The SIP signaling then traverses the NAT and sets up the SIP
      session (11-14).  Note that the left-hand client transmits media
      as soon as the 200 OK to the INVITE is sent to the User Agent
      Client (UAC) (11).  Up until this point, the incoming media from
      the right-hand client will not pass through the NAT as no outbound
      association has been created with the far-end client.  Two-way
      media communication has now been established.

5.2.1.2.  ICE Solution

   The preferred solution for media traversal of NAT is using ICE, as
   described in Section 4.2.3.3, regardless of the NAT type.  The
   following examples illustrate the traversal of an Endpoint-
   Independent NAT when initiating the session.  The example only covers
   ICE in association with the 'Binding Discovery' and TURN.  It is
   worth noting that the TURN server provides both STUN functions (to
   learn your public mapping) and TURN functions (media relaying).  It
   is also worth noting that in the example described in
   Section 5.2.1.2.1, both SIP clients L and R are contacting the same
   TURN server.  This is not necessary for ICE, STUN, TURN to function;
   all that is necessary is that the STUN and TURN server(s) be in the
   same addressing domain that is accessible on the Internet.

5.2.1.2.1.  Initiating Session

   The following example demonstrates an initiating traversal through an
   Endpoint-Independent NAT using ICE.

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   L               NAT              STUN             NAT               R
                                   Server
   |                |                |                |                |
   |(1) Alloc Req   |                |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=TURN-PUB-1 |                |                |                |
   |--------------->|                |                |                |
   |                |                |                |                |
   |                |(2) Alloc Req   |                |                |
   |                |Src=L-NAT-PUB-1 |                |                |
   |                |Dest=TURN-PUB-1 |                |                |
   |                |--------------->|                |                |
   |                |                |                |                |
   |                |(3) Alloc Resp  |                |                |
   |                |<---------------|                |                |
   |                |Src=TURN-PUB-1  |                |                |
   |                |Dest=L-NAT-PUB-1|                |                |
   |                |Map=L-NAT-PUB-1 |                |                |
   |                |Rel=TURN-PUB-2  |                |                |
   |                |                |                |                |
   |(4) Alloc Resp  |                |                |                |
   |<---------------|                |                |                |
   |Src=TURN-PUB-1  |                |                |                |
   |Dest=L-PRIV-1   |                |                |                |
   |Map=L-NAT-PUB-1 |                |                |                |
   |Rel=TURN-PUB-2  |                |                |                |
   |                |                |                |                |
   |(5) Alloc Req   |                |                |                |
   |Src=L-PRIV-2    |                |                |                |
   |Dest=TURN-PUB-1 |                |                |                |
   |--------------->|                |                |                |
   |                |                |                |                |
   |                |(6) Alloc Req   |                |                |
   |                |Src=L-NAT-PUB-2 |                |                |
   |                |Dest=TURN-PUB-1 |                |                |
   |                |--------------->|                |                |
   |                |                |                |                |
   |                |(7) Alloc Resp  |                |                |
   |                |<---------------|                |                |
   |                |Src=TURN-PUB-1  |                |                |
   |                |Dest=NAT-PUB-2  |                |                |
   |                |Map=NAT-PUB-2   |                |                |
   |                |Rel=TURN-PUB-3  |                |                |
   |                |                |                |                |

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   |(8) Alloc Resp  |                |                |                |
   |<---------------|                |                |                |
   |Src=TURN-PUB-1  |                |                |                |
   |Dest=L-PRIV-2   |                |                |                |
   |Map=L-NAT-PUB-2 |                |                |                |
   |Rel=TURN-PUB-3  |                |                |                |
   |                |                |                |                |
   |(9) SIP INVITE  |                |                |                |
   |------------------------------------------------->|                |
   |                |                |                |                |
   |                |                |                |(10) SIP INVITE |
   |                |                |                |--------------->|
   |                |                |                |                |
   |                |                |                |(11) Alloc Req  |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=TURN-PUB-1 |
   |                |                |                |                |
   |                |                |(12) Alloc Req  |                |
   |                |                |<---------------|                |
   |                |                |Src=R-NAT-PUB-1 |                |
   |                |                |Dest=TURN-PUB-1 |                |
   |                |                |                |                |
   |                |                |(13) Alloc Res  |                |
   |                |                |--------------->|                |
   |                |                |Src=TURN-PUB-1  |                |
   |                |                |Dest=R-NAT-PUB-1|                |
   |                |                |Map=R-NAT-PUB-1 |                |
   |                |                |Rel=TURN-PUB-4  |                |
   |                |                |                |                |
   |                |                |                |(14) Alloc Res  |
   |                |                |                |--------------->|
   |                |                |                |Src=TURN-PUB-1  |
   |                |                |                |Dest=R-PRIV-1   |
   |                |                |                |Map=R-NAT-PUB-1 |
   |                |                |                |Rel=TURN-PUB-4  |
   |                |                |                |                |
   |                |                |                |(15) Alloc Req  |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-2    |
   |                |                |                |Dest=TURN-PUB-1 |
   |                |                |                |                |
   |                |                |(16) Alloc Req  |                |
   |                |                |<---------------|                |
   |                |                |Src=R-NAT-PUB-2 |                |
   |                |                |Dest=TURN-PUB-1 |                |
   |                |                |                |                |

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   |                |                |(17) Alloc Res  |                |
   |                |                |--------------->|                |
   |                |                |Src=TURN-PUB-1  |                |
   |                |                |Dest=R-NAT-PUB-2|                |
   |                |                |Map=R-NAT-PUB-2 |                |
   |                |                |Rel=TURN-PUB-5  |                |
   |                |                |                |                |
   |                |                |                |(18) Alloc Res  |
   |                |                |                |--------------->|
   |                |                |                |Src=TURN-PUB-1  |
   |                |                |                |Dest=R-PRIV-2   |
   |                |                |                |Map=R-NAT-PUB-2 |
   |                |                |                |Rel=TURN-PUB-5  |
   |                |                |                |                |
   |                |                |                |(19) SIP 200 OK |
   |                |<-------------------------------------------------|
   |                |                |                |                |
   |(20) SIP 200 OK |                |                |                |
   |<---------------|                |                |                |
   |                |                |                |                |
   |(21) SIP ACK    |                |                |                |
   |------------------------------------------------->|                |
   |                |                |                |                |
   |                |                |                |(22) SIP ACK    |
   |                |                |                |--------------->|
   |                |                |                |                |
   |(23) Bind Req   |                |                |                |
   |------------------------>x       |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=R-PRIV-1   |                |                |                |
   |                |                |                |                |
   |(24) Bind Req   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=R-NAT-PUB-1|                |                |                |
   |                |                |                |                |
   |                |(25) Bind Req   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-1 |                |                |
   |                |Dest=R-NAT-PUB-1|                |                |
   |                |                |                |                |
   |                |                |                |(26) Bind Req   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-1 |
   |                |                |                |Dest=R-PRIV-1   |
   |                |                |                |                |

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   |                |                |                |(27) Bind Res   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=L-NAT-PUB-1|
   |                |                |                |Map=L-NAT-PUB-1 |
   |                |                |                |                |
   |                |                |(28) Bind Res   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-1 |                |
   |                |                |Dest=L-NAT-PUB-1|                |
   |                |                |Map=L-NAT-PUB-1 |                |
   |                |                |                |                |
   |(29) Bind Res   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-1 |                |                |                |
   |Dest=L-PRIV-1   |                |                |                |
   |Map=L-NAT-PUB-1 |                |                |                |
   |                |                |                |                |
   |===================================================================|
   |>>>>>>>>>>>>>>>>>>Outgoing RTP sent from L-PRIV-1 >>>>>>>>>>>>>>>>>|
   |===================================================================|
   |                |                |                |                |
   |                |                |                |(30) Bind Req   |
   |                |                |        x<-----------------------|
   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=L-PRIV-1   |
   |                |                |                |                |
   |                |                |                |(31) Bind Req   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=L-NAT-PUB-1|
   |                |                |                |                |
   |                |                |(32) Bind Req   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-1 |                |
   |                |                |Dest=L-NAT-PUB-1|                |
   |                |                |                |                |
   |(33) Bind Req   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-1 |                |                |                |
   |Dest=L-PRIV-1   |                |                |                |
   |                |                |                |                |
   |(34) Bind Res   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=R-NAT-PUB-1|                |                |                |
   |Map=R-NAT-PUB-1 |                |                |                |
   |                |                |                |                |

Top      Up      ToC       Page 40 
   |                |(35) Bind Res   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-1 |                |                |
   |                |Dest=R-NAT-PUB-1|                |                |
   |                |Map=R-NAT-PUB-1 |                |                |
   |                |                |                |                |
   |                |                |                |(36) Bind Res   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-1 |
   |                |                |                |Dest=R-PRIV-1   |
   |                |                |                |Map=R-NAT-PUB-1 |
   |                |                |                |                |
   |===================================================================|
   |<<<<<<<<<<<<<<<<<<Outgoing RTP sent from R-PRIV-1 <<<<<<<<<<<<<<<<<|
   |===================================================================|
   |(37) Bind Req   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=R-NAT-PUB-1|                |                |                |
   |USE-CANDIDATE   |                |                |                |
   |                |                |                |                |
   |                |(38) Bind Req   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-1 |                |                |
   |                |Dest=R-NAT-PUB-1|                |                |
   |                |USE-CANDIDATE   |                |                |
   |                |                |                |                |
   |                |                |                |(39) Bind Req   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-1 |
   |                |                |                |Dest=R-PRIV-1   |
   |                |                |                |USE-CANDIDATE   |
   |                |                |                |                |
   |                |                |                |(40) Bind Res   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=L-NAT-PUB-1|
   |                |                |                |Map=L-NAT-PUB-1 |
   |                |                |                |                |
   |                |                |(41) Bind Res   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-1 |                |
   |                |                |Dest=L-NAT-PUB-1|                |
   |                |                |Map=L-NAT-PUB-1 |                |
   |                |                |                |                |

Top      Up      ToC       Page 41 
   |(42) Bind Res   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-1 |                |                |                |
   |Dest=L-PRIV-1   |                |                |                |
   |Map=L-NAT-PUB-1 |                |                |                |
   |                |                |                |                |
   |(43) Bind Req   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-2    |                |                |                |
   |Dest=R-NAT-PUB-2|                |                |                |
   |                |                |                |                |
   |                |(44) Bind Req   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-2 |                |                |
   |                |Dest=R-NAT-PUB-2|                |                |
   |                |                |                |                |
   |                |                |                |(45) Bind Req   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-2 |
   |                |                |                |Dest=R-PRIV-2   |
   |                |                |                |                |
   |                |                |                |(46) Bind Res   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-2    |
   |                |                |                |Dest=L-NAT-PUB-2|
   |                |                |                |Map=L-NAT-PUB-2 |
   |                |                |                |                |
   |                |                |(47) Bind Res   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-2 |                |
   |                |                |Dest=L-NAT-PUB-2|                |
   |                |                |Map=L-NAT-PUB-2 |                |
   |                |                |                |                |
   |(48) Bind Res   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-2 |                |                |                |
   |Dest=L-PRIV-2   |                |                |                |
   |Map=L-NAT-PUB-2 |                |                |                |
   |                |                |                |                |
   |===================================================================|
   |>>>>>>>>>>>>>>>>>>Outgoing RTCP sent from L-PRIV-2 >>>>>>>>>>>>>>>>|
   |===================================================================|
   |                |                |                |                |
   |                |                |                |(49) Bind Req   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-2    |
   |                |                |                |Dest=L-NAT-PUB-2|
   |                |                |                |                |

Top      Up      ToC       Page 42 
   |                |                |(50) Bind Req   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-2 |                |
   |                |                |Dest=L-NAT-PUB-2|                |
   |                |                |                |                |
   |(51) Bind Req   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-2 |                |                |                |
   |Dest=L-PRIV-2   |                |                |                |
   |                |                |                |                |
   |(52) Bind Res   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-2    |                |                |                |
   |Dest=R-NAT-PUB-2|                |                |                |
   |Map=R-NAT-PUB-2 |                |                |                |
   |                |                |                |                |
   |                |(53) Bind Res   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-2 |                |                |
   |                |Dest=R-NAT-PUB-2|                |                |
   |                |Map=R-NAT-PUB-2 |                |                |
   |                |                |                |                |
   |                |                |                |(54) Bind Res   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-2 |
   |                |                |                |Dest=R-PRIV-2   |
   |                |                |                |Map=R-NAT-PUB-2 |
   |                |                |                |                |
   |===================================================================|
   |<<<<<<<<<<<<<<<<<<Outgoing RTCP sent from R-PRIV-2<<<<<<<<<<<<<<<<<|
   |===================================================================|
   |(55) Bind Req   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-2    |                |                |                |
   |Dest=R-NAT-PUB-2|                |                |                |
   |USE-CANDIDATE   |                |                |                |
   |                |                |                |                |
   |                |(56) Bind Req   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-2 |                |                |
   |                |Dest=R-NAT-PUB-2|                |                |
   |                |USE-CANDIDATE   |                |                |
   |                |                |                |                |
   |                |                |                |(57) Bind Req   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-2 |
   |                |                |                |Dest=R-PRIV-2   |
   |                |                |                |USE-CANDIDATE   |

Top      Up      ToC       Page 43 
   |                |                |                |                |
   |                |                |                |(58) Bind Res   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-2    |
   |                |                |                |Dest=L-NAT-PUB-2|
   |                |                |                |Map=L-NAT-PUB-2 |
   |                |                |                |                |
   |                |                |(59) Bind Res   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-2 |                |
   |                |                |Dest=L-NAT-PUB-2|                |
   |                |                |Map=L-NAT-PUB-2 |                |
   |                |                |                |                |
   |(60) Bind Res   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-2 |                |                |                |
   |Dest=L-PRIV-2   |                |                |                |
   |Map=L-NAT-PUB-2 |                |                |                |
   |                |                |                |                |
   |                |                |                |                |
   |(61) SIP INVITE |                |                |                |
   |------------------------------------------------->|                |
   |                |                |                |                |
   |                |                |                |(62) SIP INVITE |
   |                |                |                |--------------->|
   |                |                |                |                |
   |                |                |                |(63) SIP 200 OK |
   |                |<-------------------------------------------------|
   |                |                |                |                |
   |(64) SIP 200 OK |                |                |                |
   |<---------------|                |                |                |
   |                |                |                |                |
   |(65) SIP ACK    |                |                |                |
   |------------------------------------------------->|                |
   |                |                |                |                |
   |                |                |                |(66) SIP ACK    |
   |                |                |                |--------------->|
   |                |                |                |                |

               Figure 13: Endpoint-Independent NAT with ICE

   o  On deciding to initiate a SIP voice session, the SIP client L
      starts a local STUN client.  The STUN client generates a TURN
      Allocate request as indicated in (1) from Figure 13 that also
      highlights the source address and port combination for which the
      client device wishes to obtain a mapping.  The Allocate request is
      sent through the NAT towards the public Internet.

Top      Up      ToC       Page 44 
   o  The Allocate message (2) traverses the NAT to the public Internet
      towards the public TURN server.  Note that the source address of
      the Allocate request now represents the public address and port
      from the public side of the NAT (L-NAT-PUB-1).

   o  The TURN server receives the Allocate request and processes it
      appropriately.  This results in a successful Allocate response
      being generated and returned (3).  The message contains details of
      the server reflexive address that is to be used by the originating
      client to receive media (see 'Map=L-NAT-PUB-1') from (3)).  It
      also contains an appropriate TURN-relayed address that can be used
      at the STUN server (see 'Rel=TURN-PUB-2').

   o  The Allocate response traverses back through the NAT using the
      binding created by the initial Allocate request and presents the
      new mapped address to the client (4).  The process is repeated and
      a second STUN derived set of addresses is obtained, as illustrated
      in (5)-(8) in Figure 13.  At this point, the User Agent behind the
      NAT has pairs of derived external server reflexive and relayed
      representations.  The client can also gather IP addresses and
      ports via other mechanisms (e.g., NAT-PMP [NAT-PMP], UPnP IGD
      [UPnP-IGD]) or similar.

   o  The client now constructs a SIP INVITE message (9).  The INVITE
      request will use the addresses it has obtained in the previous
      STUN/TURN interactions to populate the SDP of the SIP INVITE.
      This should be carried out in accordance with the semantics
      defined in the ICE specification [RFC5245], as shown below in
      Figure 14:

Top      Up      ToC       Page 45 
 v=0
 o=test 2890844526 2890842807 IN IP4 $L-PRIV-1
 c=IN IP4 $L-PRIV-1.address
 t=0 0
 a=ice-pwd:$LPASS
 a=ice-ufrag:$LUNAME
 m=audio $L-PRIV-1.port RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 a=rtcp:$L-PRIV-2.port
 a=candidate:$L1 1 UDP 2130706431 $L-PRIV-1.address $L-PRIV-1.port
             typ host
 a=candidate:$L1 2 UDP 2130706430 $L-PRIV-2.address $L-PRIV-2.port
             typ host
 a=candidate:$L2 1 UDP 1694498815 $L-NAT-PUB-1.address $L-NAT-PUB-1.port
             typ srflx raddr $L-PRIV-1.address rport $L-PRIV-1.port
 a=candidate:$L2 2 UDP 1694498814 $L-NAT-PUB-2.address $L-NAT-PUB-2.port
             typ srflx raddr $L-PRIV-1.address rport $L-PRIV-2.port
 a=candidate:$L3 1 UDP 16777215 $STUN-PUB-2.address $STUN-PUB-2.port
             typ relay raddr $L-PRIV-1.address rport $L-PRIV-1.port
 a=candidate:$L3 2 UDP 16777214 $STUN-PUB-3.address $STUN-PUB-3.port
             typ relay raddr $L-PRIV-1.address rport $L-PRIV-2.port

                         Figure 14: ICE SDP Offer

   o  The SDP has been constructed to include all the available
      candidates that have been assembled.  The first set of candidates
      (as identified by Foundation $L1) contains two local addresses
      that have the highest priority.  They are also encoded into the
      connection (c=) and media (m=) lines of the SDP.  The second set
      of candidates, as identified by Foundation $L2, contains the two
      server reflexive addresses obtained from the STUN server for both
      RTP and RTCP traffic (identified by candidate-id $L2).  This entry
      has been given a priority lower than the pair $L1 by the client.
      The third and final set of candidates represents the relayed
      addresses (as identified by $L3) obtained from the STUN server.
      This pair has the lowest priority and will be used as a last
      resort if both $L1 and $L2 fail.

   o  The SIP signaling then traverses the NAT and sets up the SIP
      session (9)-(10).  On advertising a candidate address, the client
      should have a local STUN server running on each advertised
      candidate address.  This is for the purpose of responding to
      incoming STUN connectivity checks.

   o  On receiving the SIP INVITE request (10) client R also starts
      local STUN servers on appropriate address/port combinations and
      gathers potential candidate addresses to be encoded into the SDP
      (as the originating client did).  Steps (11-18) involve client R

Top      Up      ToC       Page 46 
      carrying out the same steps as client L. This involves obtaining
      local, server reflexive, and relayed addresses.  Client R is now
      ready to generate an appropriate answer in the SIP 200 OK message
      (19).  The example answer follows in Figure 15:

 v=0
 o=test 3890844516 3890842803 IN IP4 $R-PRIV-1
 c=IN IP4 $R-PRIV-1.address
 t=0 0
 a=ice-pwd:$RPASS
 m=audio $R-PRIV-1.port RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 a=rtcp:$R-PRIV-2.port
 a=candidate:$L1 1 UDP 2130706431 $R-PRIV-1.address $R-PRIV-1.port
             typ host
 a=candidate:$L1 2 UDP 2130706430 $R-PRIV-2.address $R-PRIV-2.port
             typ host
 a=candidate:$L2 1 UDP 1694498815 $R-NAT-PUB-1.address $R-NAT-PUB-1.port
             typ srflx raddr $R-PRIV-1.address rport $R-PRIV-1.port
 a=candidate:$L2 2 UDP 1694498814 $R-NAT-PUB-2.address $R-NAT-PUB-2.port
             typ srflx raddr $R-PRIV-1.address rport $R-PRIV-1.port
 a=candidate:$L3 1 UDP 16777215 $STUN-PUB-2.address $STUN-PUB-4.port
             typ relay raddr $R-PRIV-1.address rport $R-PRIV-1.port
 a=candidate:$L3 2 UDP 16777214 $STUN-PUB-3.address $STUN-PUB-5.port
             typ relay raddr $R-PRIV-1.address rport $R-PRIV-1.port

                         Figure 15: ICE SDP Answer

   o  The two clients have now exchanged SDP using offer/answer and can
      now continue with the ICE processing -- User Agent L assuming the
      role controlling agent, as specified by ICE.  The clients are now
      required to form their Candidate check lists to determine which
      will be used for the media streams.  In this example, User Agent
      L's Foundation 1 is paired with User Agent R's Foundation 1, User
      Agent L's Foundation 2 is paired with User Agent R's Foundation 2,
      and finally User Agent L's Foundation 3 is paired with User Agent
      R's Foundation 3.  User Agents L and R now have a complete
      candidate checklist.  Both clients now use the algorithm provided
      in ICE to determine candidate pair priorities and sort into a list
      of decreasing priorities.  In this example, both User Agents L and
      R will have lists that firstly specify the host address
      (Foundation $L1), then the server reflexive address (Foundation
      $L2), and lastly the relayed address (Foundation $L3).  All
      candidate pairs have an associate state as specified in ICE.  At
      this stage, all of the candidate pairs for User Agents L and R are
      initialized to the 'Frozen' state.  The User Agents then scan the
      list and move the candidates to the 'Waiting' state.  At this
      point, both clients will periodically, starting with the highest

Top      Up      ToC       Page 47 
      candidate pair priority, work their way down the list issuing STUN
      checks from the local candidate to the remote candidate (of the
      candidate pair).  As a STUN check is attempted from each local
      candidate in the list, the candidate pair state transitions to
      'In-Progress'.  As illustrated in (23), client L constructs a STUN
      connectivity check in an attempt to validate the remote candidate
      address received in the SDP of the 200 OK (20) for the highest
      priority in the checklist.  As a private address was specified in
      the active address in the SDP, the STUN connectivity check fails
      to reach its destination causing a STUN failure.  Client L
      transitions the state for this candidate pair to 'Failed'.  In the
      meantime, client L is attempting a STUN connectivity check for the
      second candidate pair in the returned SDP with the second highest
      priority (24).  As can be seen from messages (24) to (29), the
      STUN Bind request is successful and returns a positive outcome for
      the connectivity check.  Client L is now free to send media to the
      peer using the candidate pair.  Immediately after sending its 200
      OK, client R also carries out the same set of binding requests.
      It firstly (in parallel) tries to contact the active address
      contained in the SDP (30) which results in failure.

   o  In the meantime, a successful response to a STUN connectivity
      check by User Agent R (27) results in a tentative check in the
      reverse direction -- this is illustrated by messages (31) to (36).
      Once this check has succeeded, User Agent R can transition the
      state of the appropriate candidate to 'Succeeded', and media can
      be sent (RTP).  The previously (31-36) described check confirm on
      both sides (User Agents L and R) that connectivity can be achieved
      using the appropriate candidate pair.  User Agent L, as the
      controlling client now sends another connectivity check for the
      candidate pair, this time including the 'USE-CANDIDATE' attribute
      as specified in ICE to signal the chosen candidate.  This exchange
      is illustrated in messages (37) to (42).

   o  As part of the process in this example, both L and R will now
      complete the same connectivity checks for part 2 of the component
      named for the favored 'Foundation' selected for use with RTCP.
      The connectivity checks for part 2 of the candidate component are
      shown in L (43-48) and R (49-54).  Once this has succeeded, User
      Agent L as the controlling client sends another connectivity check
      for the candidate pair.  This time the 'USE-CANDIDATE' attribute
      is again specified to signal the chosen candidate for component 2.

   o  The candidates have now been fully verified (and selected), and as
      they are the highest priority, an updated offer (61-62) is now
      sent from the offerer (client L) to the answerer (client R)
      representing the new active candidates.  The new offer would look
      as follows:

Top      Up      ToC       Page 48 
 v=0
 o=test 2890844526 2890842808 IN IP4 $L-PRIV-1
 c=IN IP4 $L-NAT-PUB-1.address
 t=0 0
 a=ice-pwd:$LPASS
 a=ice-ufrag:$LUNAME
 m=audio $L-NAT-PUB-1.port RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 a=rtcp:$L-NAT-PUB-2.port
 a=candidate:$L2 1 UDP 2203948363 $L-NAT-PUB-1.address $L-NAT-PUB-1.port
             typ srflx raddr $L-PRIV-1.address rport $L-PRIV-1.port
 a=candidate:$L2 2 UDP 2172635342 $L-NAT-PUB-2.address $L-NAT-PUB-2.port
             typ srflx raddr $L-PRIV-1.address rport $L-PRIV-2.port

                     Figure 16: ICE SDP Updated Offer

   o  The resulting answer (63-64) for R would look as follows:

 v=0
 o=test 3890844516 3890842804 IN IP4 $R-PRIV-1
 c=IN IP4 $R-PRIV-1.address
 t=0 0
 a=ice-pwd:$RPASS
 a=ice-ufrag:$RUNAME
 m=audio $R-PRIV-1.port RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 a=rtcp:$R-PRIV-2.port
 a=candidate:$L2 1 UDP 2984756463 $R-NAT-PUB-1.address $R-NAT-PUB-1.port
             typ srflx raddr $R-PRIV-1.address rport $R-PRIV-1.port
 a=candidate:$L2 2 UDP 2605968473 $R-NAT-PUB-2.address $R-NAT-PUB-2.port
             typ srflx raddr $R-PRIV-1.address rport $R-PRIV-2.port

                     Figure 17: ICE SDP Updated Answer



(page 48 continued on part 4)

Next RFC Part