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rfc:rfc3665

Network Working Group A. Johnston Request for Comments: 3665 MCI BCP: 75 S. Donovan Category: Best Current Practice R. Sparks

                                                         C. Cunningham
                                                           dynamicsoft
                                                            K. Summers
                                                                 Sonus
                                                         December 2003
    Session Initiation Protocol (SIP) Basic Call Flow Examples

Status of this Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This document gives examples of Session Initiation Protocol (SIP)
 call flows.  Elements in these call flows include SIP User Agents and
 Clients, SIP Proxy and Redirect Servers.  Scenarios include SIP
 Registration and SIP session establishment.  Call flow diagrams and
 message details are shown.

Johnston, et al. Best Current Practice [Page 1] RFC 3665 SIP Basic Call Flow Examples December 2003

Table of Contents

 1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  General Assumptions. . . . . . . . . . . . . . . . . . .  3
     1.2.  Legend for Message Flows . . . . . . . . . . . . . . . .  3
     1.3.  SIP Protocol Assumptions . . . . . . . . . . . . . . . .  4
 2.  SIP Registration . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Successful New Registration. . . . . . . . . . . . . . .  5
     2.2.  Update of Contact List . . . . . . . . . . . . . . . . .  7
     2.3.  Request for Current Contact List . . . . . . . . . . . .  8
     2.4.  Cancellation of Registration . . . . . . . . . . . . . .  9
     2.5.  Unsuccessful Registration. . . . . . . . . . . . . . . . 10
 3.  SIP Session Establishment. . . . . . . . . . . . . . . . . . . 12
     3.1.  Successful Session Establishment . . . . . . . . . . . . 12
     3.2.  Session Establishment Through Two Proxies. . . . . . . . 15
     3.3.  Session with Multiple Proxy Authentication . . . . . . . 26
     3.4.  Successful Session with Proxy Failure. . . . . . . . . . 37
     3.5.  Session Through a SIP ALG. . . . . . . . . . . . . . . . 46
     3.6.  Session via Redirect and Proxy Servers with SDP in ACK . 54
     3.7.  Session with re-INVITE (IP Address Change) . . . . . . . 61
     3.8.  Unsuccessful No Answer . . . . . . . . . . . . . . . . . 67
     3.9.  Unsuccessful Busy. . . . . . . . . . . . . . . . . . . . 75
     3.10. Unsuccessful No Response from User Agent . . . . . . . . 80
     3.11. Unsuccessful Temporarily Unavailable . . . . . . . . . . 85
 4.  Security Considerations. . . . . . . . . . . . . . . . . . . . 91
 5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 91
     5.1.  Normative References . . . . . . . . . . . . . . . . . . 91
     5.2.  Informative References . . . . . . . . . . . . . . . . . 91
 6.  Intellectual Property Statement. . . . . . . . . . . . . . . . 91
 7.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 92
 8.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 93
 9.  Full Copyright Statement . . . . . . . . . . . . . . . . . . . 94

1. Overview

 The call flows shown in this document were developed in the design of
 a SIP IP communications network.  They represent an example minimum
 set of functionality.
 It is the hope of the authors that this document will be useful for
 SIP implementers, designers, and protocol researchers alike and will
 help further the goal of a standard implementation of RFC 3261 [1].
 These flows represent carefully checked and working group reviewed
 scenarios of the most basic examples as a companion to the
 specifications.

Johnston, et al. Best Current Practice [Page 2] RFC 3665 SIP Basic Call Flow Examples December 2003

 These call flows are based on the current version 2.0 of SIP in RFC
 3261 [1] with SDP usage described in RFC 3264 [2].  Other RFCs also
 comprise the SIP standard but are not used in this set of basic call
 flows.
 Call flow examples of SIP interworking with the PSTN through gateways
 are contained in a companion document, RFC 3666 [5].
 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 BCP 14, RFC 2119 [4].

1.1. General Assumptions

 A number of architecture, network, and protocol assumptions underlie
 the call flows in this document.  Note that these assumptions are not
 requirements.  They are outlined in this section so that they may be
 taken into consideration and to aid in the understanding of the call
 flow examples.
 The authentication of SIP User Agents in these example call flows is
 performed using HTTP Digest as defined in [1] and [3].
 Some Proxy Servers in these call flows insert Record-Route headers
 into requests to ensure that they are in the signaling path for
 future message exchanges.
 These flows show TCP, TLS, and UDP for transport.  See the discussion
 in RFC 3261 for details on the transport issues for SIP.

1.2. Legend for Message Flows

 Dashed lines (---) represent signaling messages that are mandatory to
 the call scenario.  These messages can be SIP or PSTN signaling.  The
 arrow indicates the direction of message flow.
 Double dashed lines (===) represent media paths between network
 elements.
 Messages with parentheses around their name represent optional
 messages.
 Messages are identified in the Figures as F1, F2, etc.  This
 references the message details in the list that follows the Figure.
 Comments in the message details are shown in the following form:
  /* Comments. */

Johnston, et al. Best Current Practice [Page 3] RFC 3665 SIP Basic Call Flow Examples December 2003

1.3. SIP Protocol Assumptions

 This document does not prescribe the flows precisely as they are
 shown, but rather the flows illustrate the principles for best
 practice.  They are best practices usages (orderings, syntax,
 selection of features for the purpose, handling of error) of SIP
 methods, headers and parameters.  IMPORTANT: The exact flows here
 must not be copied as is by an implementer due to specific incorrect
 characteristics that were introduced into the document for
 convenience and are listed below.  To sum up, the basic flows
 represent well-reviewed examples of SIP usage, which are best common
 practice according to IETF consensus.
 For simplicity in reading and editing the document, there are a
 number of differences between some of the examples and actual SIP
 messages.  For example, the HTTP Digest responses are not actual MD5
 encodings.  Call-IDs are often repeated, and CSeq counts often begin
 at 1.  Header fields are usually shown in the same order.  Usually
 only the minimum required header field set is shown, others that
 would normally be present such as Accept, Supported, Allow, etc are
 not shown.
 Actors:
 Element       Display Name   URI                         IP Address
 -------       ------------   ---                         ----------
 User Agent    Alice          alice@atlanta.example.com   192.0.2.101
 User Agent    Bob            bob@biloxi.example.com      192.0.2.201
 User Agent                   bob@chicago.example.com     192.0.2.100
 Proxy Server                 ss1.atlanta.example.com     192.0.2.111
 Proxy/Registrar              ss2.biloxi.example.com      192.0.2.222
 Proxy Server                 ss3.chicago.example.com     192.0.2.233
 ALG                          alg1.atlanta.example.com    192.0.2.128

2. SIP Registration

 Registration binds a particular device Contact URI with a SIP user
 Address of Record (AOR).

Johnston, et al. Best Current Practice [Page 4] RFC 3665 SIP Basic Call Flow Examples December 2003

2.1. Successful New Registration

  Bob                        SIP Server
   |                               |
   |          REGISTER F1          |
   |------------------------------>|
   |      401 Unauthorized F2      |
   |<------------------------------|
   |          REGISTER F3          |
   |------------------------------>|
   |            200 OK F4          |
   |<------------------------------|
   |                               |
 Bob sends a SIP REGISTER request to the SIP server.  The request
 includes the user's contact list.  This flow shows the use of HTTP
 Digest for authentication using TLS transport.  TLS transport is used
 due to the lack of integrity protection in HTTP Digest and the danger
 of registration hijacking without it, as described in RFC 3261 [1].
 The SIP server provides a challenge to Bob.  Bob enters her/his valid
 user ID and password.  Bob's SIP client encrypts the user information
 according to the challenge issued by the SIP server and sends the
 response to the SIP server.  The SIP server validates the user's
 credentials.  It registers the user in its contact database and
 returns a response (200 OK) to Bob's SIP client.  The response
 includes the user's current contact list in Contact headers.  The
 format of the authentication shown is HTTP digest.  It is assumed
 that Bob has not previously registered with this Server.
 Message Details
 F1 REGISTER Bob -> SIP Server
 REGISTER sips:ss2.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
 Max-Forwards: 70
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 Contact: <sips:bob@client.biloxi.example.com>
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 5] RFC 3665 SIP Basic Call Flow Examples December 2003

 F2 401 Unauthorized SIP Server -> Bob
 SIP/2.0 401 Unauthorized
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>;tag=1410948204
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 WWW-Authenticate: Digest realm="atlanta.example.com", qop="auth",
  nonce="ea9c8e88df84f1cec4341ae6cbe5a359",
  opaque="", stale=FALSE, algorithm=MD5
 Content-Length: 0
 F3 REGISTER Bob -> SIP Server
 REGISTER sips:ss2.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92
 Max-Forwards: 70
 From: Bob <sips:bob@biloxi.example.com>;tag=ja743ks76zlflH
 To: Bob <sips:bob@biloxi.example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 2 REGISTER
 Contact: <sips:bob@client.biloxi.example.com>
 Authorization: Digest username="bob", realm="atlanta.example.com"
  nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="",
  uri="sips:ss2.biloxi.example.com",
  response="dfe56131d1958046689d83306477ecc"
 Content-Length: 0
 F4 200 OK SIP Server -> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92
  ;received=192.0.2.201
 From: Bob <sips:bob@biloxi.example.com>;tag=ja743ks76zlflH
 To: Bob <sips:bob@biloxi.example.com>;tag=37GkEhwl6
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 2 REGISTER
 Contact: <sips:bob@client.biloxi.example.com>;expires=3600
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 6] RFC 3665 SIP Basic Call Flow Examples December 2003

2.2. Update of Contact List

 Bob                        SIP Server
   |                               |
   |          REGISTER F1          |
   |------------------------------>|
   |            200 OK F2          |
   |<------------------------------|
   |                               |
 Bob wishes to update the list of addresses where the SIP server will
 redirect or forward INVITE requests.
 Bob sends a SIP REGISTER request to the SIP server.  Bob's request
 includes an updated contact list.  Since the user already has
 authenticated with the server, the user supplies authentication
 credentials with the request and is not challenged by the server. The
 SIP server validates the user's credentials.  It registers the user
 in its contact database, updates the user's contact list, and returns
 a response (200 OK) to Bob's SIP client.  The response includes the
 user's current contact list in Contact headers.
 Message Details
 F1 REGISTER Bob -> SIP Server
 REGISTER sips:ss2.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
 Max-Forwards: 70
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 Contact: mailto:bob@biloxi.example.com
 Authorization: Digest username="bob", realm="atlanta.example.com",
  qop="auth", nonce="1cec4341ae6cbe5a359ea9c8e88df84f", opaque="",
  uri="sips:ss2.biloxi.example.com",
  response="71ba27c64bd01de719686aa4590d5824"
 Content-Length: 0
 F2 200 OK SIP Server -> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>;tag=34095828jh

Johnston, et al. Best Current Practice [Page 7] RFC 3665 SIP Basic Call Flow Examples December 2003

 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 Contact: <sips:bob@client.biloxi.example.com>;expires=3600
 Contact: <mailto:bob@biloxi.example.com>;expires=4294967295
 Content-Length: 0

2.3. Request for Current Contact List

 Bob                        SIP Server
   |                               |
   |          REGISTER F1          |
   |------------------------------>|
   |            200 OK F2          |
   |<------------------------------|
   |                               |
 Bob sends a register request to the Proxy Server containing no
 Contact headers, indicating the user wishes to query the server for
 the user's current contact list.  Since the user already has
 authenticated with the server, the user supplies authentication
 credentials with the request and is not challenged by the server.
 The SIP server validates the user's credentials.  The server returns
 a response (200 OK) which includes the user's current registration
 list in Contact headers.
 Message Details
 F1 REGISTER Bob -> SIP Server
 REGISTER sips:ss2.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
 Max-Forwards: 70
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 Authorization: Digest username="bob", realm="atlanta.example.com",
  nonce="df84f1cec4341ae6cbe5ap359a9c8e88", opaque="",
  uri="sips:ss2.biloxi.example.com",
  response="aa7ab4678258377c6f7d4be6087e2f60"
 Content-Length: 0
 F2 200 OK SIP Server -> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
  ;received=192.0.2.201

Johnston, et al. Best Current Practice [Page 8] RFC 3665 SIP Basic Call Flow Examples December 2003

 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>;tag=jqoiweu75
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 Contact: <sips:bob@client.biloxi.example.com>;expires=3600
 Contact: <mailto:bob@biloxi.example.com>;expires=4294967295
 Content-Length: 0

2.4. Cancellation of Registration

 Bob                         SIP Server
   |                               |
   |          REGISTER F1          |
   |------------------------------>|
   |            200 OK F2          |
   |<------------------------------|
   |                               |
 Bob wishes to cancel their registration with the SIP server.  Bob
 sends a SIP REGISTER request to the SIP server.  The request has an
 expiration period of 0 and applies to all existing contact locations.
 Since the user already has authenticated with the server, the user
 supplies authentication credentials with the request and is not
 challenged by the server.  The SIP server validates the user's
 credentials.  It clears the user's contact list, and returns a
 response (200 OK) to Bob's SIP client.
 Message Details
 F1 REGISTER Bob -> SIP Server
 REGISTER sips:ss2.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
 Max-Forwards: 70
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 Expires: 0
 Contact: *
 Authorization: Digest username="bob", realm="atlanta.example.com",
  nonce="88df84f1cac4341aea9c8ee6cbe5a359", opaque="",
  uri="sips:ss2.biloxi.example.com",
  response="ff0437c51696f9a76244f0cf1dbabbea"
 Content-Length: 0
 F2 200 OK SIP Server -> Bob

Johnston, et al. Best Current Practice [Page 9] RFC 3665 SIP Basic Call Flow Examples December 2003

 SIP/2.0 200 OK
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>;tag=1418nmdsrf
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 Content-Length: 0

2.5. Unsuccessful Registration

 Bob                        SIP Server
   |                               |
   |          REGISTER F1          |
   |------------------------------>|
   |      401 Unauthorized F2      |
   |<------------------------------|
   |          REGISTER F3          |
   |------------------------------>|
   |      401 Unauthorized F4      |
   |<------------------------------|
   |                               |
 Bob sends a SIP REGISTER request to the SIP Server.  The SIP server
 provides a challenge to Bob.  Bob enters her/his user ID and
 password.  Bob's SIP client encrypts the user information according
 to the challenge issued by the SIP server and sends the response to
 the SIP server.  The SIP server attempts to validate the user's
 credentials, but they are not valid (the user's password does not
 match the password established for the user's account).  The server
 returns a response (401 Unauthorized) to Bob's SIP client.
 Message Details
 F1 REGISTER Bob -> SIP Server
 REGISTER sips:ss2.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 Contact: <sips:bob@client.biloxi.example.com>
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 10] RFC 3665 SIP Basic Call Flow Examples December 2003

 F2 Unauthorized SIP Server -> Bob
 SIP/2.0 401 Unauthorized
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
 To: Bob <sips:bob@biloxi.example.com>;tag=1410948204
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 1 REGISTER
 WWW-Authenticate: Digest realm="atlanta.example.com", qop="auth",
  nonce="f1cec4341ae6ca9c8e88df84be55a359",
  opaque="", stale=FALSE, algorithm=MD5
 Content-Length: 0
 F3 REGISTER Bob -> SIP Server
 REGISTER sips:ss2.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92
 Max-Forwards: 70
 From: Bob <sips:bob@biloxi.example.com>;tag=JueHGuidj28dfga
 To: Bob <sips:bob@biloxi.example.com>
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 2 REGISTER
 Contact: <sips:bob@client.biloxi.example.com>
 Authorization: Digest username="bob", realm="atlanta.example.com",
  nonce="f1cec4341ae6ca9c8e88df84be55a359", opaque="",
  uri="sips:ss2.biloxi.example.com",
  response="61f8470ceb87d7ebf508220214ed438b"
 Content-Length: 0
 /*  The response above encodes the incorrect password */
 F4 401 Unauthorized SIP Server -> Bob
 SIP/2.0 401 Unauthorized
 Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92
  ;received=192.0.2.201
 From: Bob <sips:bob@biloxi.example.com>;tag=JueHGuidj28dfga
 To: Bob <sips:bob@biloxi.example.com>;tag=1410948204
 Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
 CSeq: 2 REGISTER
 WWW-Authenticate: Digest realm="atlanta.example.com", qop="auth",
  nonce="84f1c1ae6cbe5ua9c8e88dfa3ecm3459",
  opaque="", stale=FALSE, algorithm=MD5
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 11] RFC 3665 SIP Basic Call Flow Examples December 2003

3. SIP Session Establishment

 This section details session establishment between two SIP User
 Agents (UAs): Alice and Bob.  Alice (sip:alice@atlanta.example.com)
 and Bob (sip:bob@biloxi.example.com) are assumed to be SIP phones or
 SIP-enabled devices.  The successful calls show the initial
 signaling, the exchange of media information in the form of SDP
 payloads, the establishment of the media session, then finally the
 termination of the call.
 HTTP Digest authentication is used by Proxy Servers to authenticate
 the caller Alice.  It is assumed that Bob has registered with Proxy
 Server Proxy 2 as per Section 2 to be able to receive the calls via
 the Proxy.

3.1. Successful Session Establishment

 Alice                     Bob
   |                        |
   |       INVITE F1        |
   |----------------------->|
   |    180 Ringing F2      |
   |<-----------------------|
   |                        |
   |       200 OK F3        |
   |<-----------------------|
   |         ACK F4         |
   |----------------------->|
   |   Both Way RTP Media   |
   |<======================>|
   |                        |
   |         BYE F5         |
   |<-----------------------|
   |       200 OK F6        |
   |----------------------->|
   |                        |
 In this scenario, Alice completes a call to Bob directly.
 Message Details
 F1 INVITE Alice -> Bob
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>

Johnston, et al. Best Current Practice [Page 12] RFC 3665 SIP Basic Call Flow Examples December 2003

 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F2 180 Ringing Bob -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
 Content-Length: 0
 F3 200 OK Bob -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000

Johnston, et al. Best Current Practice [Page 13] RFC 3665 SIP Basic Call Flow Examples December 2003

 F4 ACK Alice -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bd5
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 /* RTP streams are established between Alice and Bob */
 /* Bob Hangs Up with Alice. Note that the CSeq is NOT 2, since
    Alice and Bob maintain their own independent CSeq counts.
    (The INVITE was request 1 generated by Alice, and the BYE is
    request 1 generated by Bob) */
 F5 BYE Bob -> Alice
 BYE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TCP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
 Max-Forwards: 70
 From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F6 200 OK Alice -> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 14] RFC 3665 SIP Basic Call Flow Examples December 2003

3.2. Session Establishment Through Two Proxies

 Alice           Proxy 1          Proxy 2            Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|                |                |
   |     407 F2     |                |                |
   |<---------------|                |                |
   |     ACK F3     |                |                |
   |--------------->|                |                |
   |   INVITE F4    |                |                |
   |--------------->|   INVITE F5    |                |
   |     100  F6    |--------------->|   INVITE F7    |
   |<---------------|     100  F8    |--------------->|
   |                |<---------------|                |
   |                |                |     180 F9     |
   |                |    180 F10     |<---------------|
   |     180 F11    |<---------------|                |
   |<---------------|                |     200 F12    |
   |                |    200 F13     |<---------------|
   |     200 F14    |<---------------|                |
   |<---------------|                |                |
   |     ACK F15    |                |                |
   |--------------->|    ACK F16     |                |
   |                |--------------->|     ACK F17    |
   |                |                |--------------->|
   |                Both Way RTP Media                |
   |<================================================>|
   |                |                |     BYE F18    |
   |                |    BYE F19     |<---------------|
   |     BYE F20    |<---------------|                |
   |<---------------|                |                |
   |     200 F21    |                |                |
   |--------------->|     200 F22    |                |
   |                |--------------->|     200 F23    |
   |                |                |--------------->|
   |                |                |                |
 In this scenario, Alice completes a call to Bob using two proxies
 Proxy 1 and Proxy 2.  The initial INVITE (F1) contains a pre-loaded
 Route header with the address of Proxy 1 (Proxy 1 is configured as a
 default outbound proxy for Alice).  The request does not contain the
 Authorization credentials Proxy 1 requires, so a 407 Proxy
 Authorization response is sent containing the challenge information.
 A new INVITE (F4) is then sent containing the correct credentials and
 the call proceeds.  The call terminates when Bob disconnects by
 initiating a BYE message.

Johnston, et al. Best Current Practice [Page 15] RFC 3665 SIP Basic Call Flow Examples December 2003

 Proxy 1 inserts a Record-Route header into the INVITE message to
 ensure that it is present in all subsequent message exchanges.  Proxy
 2 also inserts itself into the Record-Route header.  The ACK (F15)
 and BYE (F18) both have a Route header.
 Message Details
 F1 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74b43
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Proxy 1 challenges Alice for authentication */
 F2 407 Proxy Authorization Required Proxy 1 -> Alice
 SIP/2.0 407 Proxy Authorization Required
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74b43
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=3flal12sf
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 INVITE
 Proxy-Authenticate: Digest realm="atlanta.example.com", qop="auth",
  nonce="f84f1cec41e6cbe5aea9c8e88d359",
  opaque="", stale=FALSE, algorithm=MD5
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 16] RFC 3665 SIP Basic Call Flow Examples December 2003

 F3 ACK Alice -> Proxy 1
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74b43
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=3flal12sf
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 /* Alice responds be re-sending the INVITE with authentication
    credentials in it. */
 F4 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="wf84f1ceczx41ae6cbe5aea9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="42ce3cef44b22f50c6a6071bc8"
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Proxy 1 accepts the credentials and forwards the INVITE to Proxy
 2.  Client for Alice prepares to receive data on port 49172 from the
 network. */

Johnston, et al. Best Current Practice [Page 17] RFC 3665 SIP Basic Call Flow Examples December 2003

 F5 INVITE Proxy 1 -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F6 100 Trying Proxy 1 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 INVITE
 Content-Length: 0
 F7 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 68
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>

Johnston, et al. Best Current Practice [Page 18] RFC 3665 SIP Basic Call Flow Examples December 2003

 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F8 100 Trying Proxy 2 -> Proxy 1
 SIP/2.0 100 Trying
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 INVITE
 Content-Length: 0
 F9 180 Ringing Bob -> Proxy 2
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 3848276298220188511@atlanta.example.com
 Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
 CSeq: 2 INVITE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 19] RFC 3665 SIP Basic Call Flow Examples December 2003

 F10 180 Ringing Proxy 2 -> Proxy 1
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 3848276298220188511@atlanta.example.com
 Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
 CSeq: 2 INVITE
 Content-Length: 0
 F11 180 Ringing Proxy 1 -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 3848276298220188511@atlanta.example.com
 Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
 CSeq: 2 INVITE
 Content-Length: 0
 F12 200 OK Bob -> Proxy 2
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 INVITE

Johnston, et al. Best Current Practice [Page 20] RFC 3665 SIP Basic Call Flow Examples December 2003

 Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F13 200 OK Proxy 2 -> Proxy 1
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F14 200 OK Proxy 1 -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159

Johnston, et al. Best Current Practice [Page 21] RFC 3665 SIP Basic Call Flow Examples December 2003

 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F15 ACK Alice -> Proxy 1
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74b76
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>,
  <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 ACK
 Content-Length: 0
 F16 ACK Proxy 1 -> Proxy 2
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74b76
  ;received=192.0.2.101
 Max-Forwards: 69
 Route: <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 ACK
 Content-Length: 0
 F17 ACK Proxy 2 -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1

Johnston, et al. Best Current Practice [Page 22] RFC 3665 SIP Basic Call Flow Examples December 2003

 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74b76
  ;received=192.0.2.101
 Max-Forwards: 68
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 2 ACK
 Content-Length: 0
 /* RTP streams are established between Alice and Bob */
 /* Bob Hangs Up with Alice. */
 /* Again, note that the CSeq is NOT 3.  Alice and Bob maintain
    their own separate CSeq counts */
 F18 BYE Bob -> Proxy 2
 BYE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TCP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
 Max-Forwards: 70
 Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F19 BYE Proxy 2 -> Proxy 1
 BYE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/TCP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 Max-Forwards: 69
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 23] RFC 3665 SIP Basic Call Flow Examples December 2003

 F20 BYE Proxy 1 -> Alice
 BYE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/TCP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 Max-Forwards: 68
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F21 200 OK Alice -> Proxy 1
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/TCP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F22 200 OK Proxy 1 -> Proxy 2
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/TCP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.101
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 24] RFC 3665 SIP Basic Call Flow Examples December 2003

 F23 200 OK Proxy 2 -> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 3848276298220188511@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 25] RFC 3665 SIP Basic Call Flow Examples December 2003

3.3. Session with Multiple Proxy Authentication

   Alice        Proxy 1     Proxy 2         Bob
     |            |           |             |
     |  INVITE F1 |           |             |
     |----------->|           |             |
     |  407 Proxy Authorization Required F2 |
     |<-----------|           |             |
     |   ACK F3   |           |             |
     |----------->|           |             |
     |  INVITE F4 |           |             |
     |----------->|           |             |
     |   100 F5   |           |             |
     |<-----------| INVITE F6 |             |
     |            |---------->|             |
     |            |  407 Proxy Authorization Required F7
     |            |<----------|             |
     |            |   ACK F8  |             |
     |            |---------->|             |
     |  407 Proxy Authorization Required F9 |
     |<-----------|           |             |
     |   ACK F10  |           |             |
     |----------->|           |             |
     |  INVITE F11|           |             |
     |----------->|           |             |
     |   100 F12  |           |             |
     |<-----------| INVITE F13|             |
     |            |---------->|             |
     |            |  100 F14  |             |
     |            |<----------|  INVITE F15 |
     |            |           |------------>|
     |            |           | 200 OK F16  |
     |            | 200 OK F17|<------------|
     | 200 OK F18 |<----------|             |
     |<-----------|           |             |
     |   ACK F19  |           |             |
     |----------->|  ACK F20  |             |
     |            |---------->|   ACK F21   |
     |            |           |------------>|
     |           RTP Media Path             |
     |<====================================>|
 In this scenario, Alice completes a call to Bob using two proxies
 Proxy 1 and Proxy 2.  Alice has valid credentials in both domains.
 Since the initial INVITE (F1) does not contain the Authorization
 credentials Proxy 1 requires, so a 407 Proxy Authorization response
 is sent containing the challenge information. A new INVITE (F4) is

Johnston, et al. Best Current Practice [Page 26] RFC 3665 SIP Basic Call Flow Examples December 2003

 then sent containing the correct credentials and the call proceeds
 after Proxy 2 challenges and receives valid credentials.  The call
 terminates when Bob disconnects by initiating a BYE message.
 Proxy 1 inserts a Record-Route header into the INVITE message to
 ensure that it is present in all subsequent message exchanges.  Proxy
 2 also inserts itself into the Record-Route header.
 Message Details
 F1 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b03
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Proxy 1 challenges Alice for authentication */
 F2 407 Proxy Authorization Required Proxy 1 -> Alice
 SIP/2.0 407 Proxy Authorization Required
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b03
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=876321
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Proxy-Authenticate: Digest realm="atlanta.example.com", qop="auth",
  nonce="wf84f1cczx41ae6cbeaea9ce88d359",
  opaque="", stale=FALSE, algorithm=MD5
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 27] RFC 3665 SIP Basic Call Flow Examples December 2003

 F3 ACK Alice -> Proxy 1
 ACK sip:bob@biloxi.example.com SIP/2.0
 Max-Forwards: 70
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b03
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=876321
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 /* Alice responds be re-sending the INVITE with authentication
    credentials in it.  The same Call-ID is used, so the CSeq is
    increased. */
 F4 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b21
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="wf84f1ceczx41ae6cbe5aea9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="42ce3cef44b22f50c6a6071bc8"
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Proxy 1 accepts the credentials and forwards the INVITE to Proxy
 2.  Client for Alice prepares to receive data on port 49172 from the
 network. */

Johnston, et al. Best Current Practice [Page 28] RFC 3665 SIP Basic Call Flow Examples December 2003

 F5 100 Trying Proxy 1 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b21
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Content-Length: 0
 F6 INVITE Proxy 1 -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b21
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Proxy 2 challenges Alice for authentication */
 F7 407 Proxy Authorization Required Proxy 2 -> Proxy 1
 SIP/2.0 407 Proxy Authorization Required
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b21
  ;received=192.0.2.101

Johnston, et al. Best Current Practice [Page 29] RFC 3665 SIP Basic Call Flow Examples December 2003

 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=838209
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Proxy-Authenticate: Digest realm="biloxi.example.com", qop="auth",
  nonce="c1e22c41ae6cbe5ae983a9c8e88d359",
  opaque="", stale=FALSE, algorithm=MD5
 Content-Length: 0
 F8 ACK Proxy 1 -> Proxy 2
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b21
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=838209
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 ACK
 Content-Length: 0
 /* Proxy 1 forwards the challenge to Alice for authentication from
 Proxy 2 */
 F9 407 Proxy Authorization Required Proxy 1 -> Alice
 SIP/2.0 407 Proxy Authorization Required
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b21
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=838209
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Proxy-Authenticate: Digest realm="biloxi.example.com", qop="auth",
  nonce="c1e22c41ae6cbe5ae983a9c8e88d359",
  opaque="", stale=FALSE, algorithm=MD5
 Content-Length: 0
 F10 ACK Alice -> Proxy 1
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b21
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=838209
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com

Johnston, et al. Best Current Practice [Page 30] RFC 3665 SIP Basic Call Flow Examples December 2003

 CSeq: 2 ACK
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="wf84f1ceczx41ae6cbe5aea9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="42ce3cef44b22f50c6a6071bc8"
 Content-Length: 0
 /* Alice responds be re-sending the INVITE with authentication
 credentials for Proxy 1 AND Proxy 2.  */
 F11 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="wf84f1ceczx41ae6cbe5aea9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="42ce3cef44b22f50c6a6071bc8"
 Proxy-Authorization: Digest username="alice",
  realm="biloxi.example.com",
  nonce="c1e22c41ae6cbe5ae983a9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com", response="f44ab22f150c6a56071bce8"
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Proxy 1 finds its credentials and authorizes Alice, forwarding the
 INVITE to Proxy.  */

Johnston, et al. Best Current Practice [Page 31] RFC 3665 SIP Basic Call Flow Examples December 2003

 F12 100 Trying Proxy 1 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 INVITE
 Content-Length: 0
 F13 INVITE Proxy 1 -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="biloxi.example.com",
  nonce="c1e22c41ae6cbe5ae983a9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com", response="f44ab22f150c6a56071bce8"
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Proxy 2 finds its credentials and authorizes Alice, forwarding the
 INVITE to Bob.  */

Johnston, et al. Best Current Practice [Page 32] RFC 3665 SIP Basic Call Flow Examples December 2003

 F14 100 Trying Proxy 2 -> Proxy 1
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 INVITE
 Content-Length: 0
 F15 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK31972.1
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 68
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Bob answers the call immediately */

Johnston, et al. Best Current Practice [Page 33] RFC 3665 SIP Basic Call Flow Examples December 2003

 F16 200 OK Bob -> Proxy 2
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK31972.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=9103874
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F17 200 OK Proxy 2 -> Proxy 1
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=9103874
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-

Johnston, et al. Best Current Practice [Page 34] RFC 3665 SIP Basic Call Flow Examples December 2003

 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F18 200 OK Proxy 1 -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=9103874
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F19 ACK Alice -> Proxy 1
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b44
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>,
  <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=9103874
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 ACK
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="wf84f1ceczx41ae6cbe5aea9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="42ce3cef44b22f50c6a6071bc8"
 Proxy-Authorization: Digest username="alice",
  realm="biloxi.example.com",

Johnston, et al. Best Current Practice [Page 35] RFC 3665 SIP Basic Call Flow Examples December 2003

  nonce="c1e22c41ae6cbe5ae983a9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com", response="f44ab22f150c6a56071bce8"
 Content-Length: 0
 F20 ACK Proxy 1 -> Proxy 2
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b44
  ;received=192.0.2.101
 Max-Forwards: 69
 Route: <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=9103874
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 ACK
 Contact: <sip:bob@client.biloxi.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="biloxi.example.com",
  nonce="c1e22c41ae6cbe5ae983a9c8e88d359", opaque="",
  uri="sip:bob@biloxi.example.com", response="f44ab22f150c6a56071bce8"
 Content-Length: 0
 F21 ACK Proxy 2 -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK31972.1
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK230f2.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b44
  ;received=192.0.2.101
 Max-Forwards: 68
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=9103874
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 3 ACK
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 36] RFC 3665 SIP Basic Call Flow Examples December 2003

3.4. Successful Session with Proxy Failure

  Alice           Proxy 1          Proxy 2            Bob
    |                |                |                |
    |   INVITE F1    |                |                |
    |--------------->|                |                |
    |   INVITE F2    |                |                |
    |--------------->|                |                |
    |   INVITE F3    |                |                |
    |--------------->|                |                |
    |   INVITE F4    |                |                |
    |--------------->|                |                |
    |   INVITE F5    |                |                |
    |--------------->|                |                |
    |   INVITE F6    |                |                |
    |--------------->|                |                |
    |   INVITE F7    |                |                |
    |--------------->|                |                |
    |     INVITE F8                   |                |
    |-------------------------------->|                |
    |            407 F9               |                |
    |<--------------------------------|                |
    |             ACK F10             |                |
    |-------------------------------->|                |
    |           INVITE F11            |                |
    |-------------------------------->|   INVITE F12   |
    |             100  F13            |--------------->|
    |<--------------------------------|                |
    |                                 |     180 F14    |
    |             180 F15             |<---------------|
    |<--------------------------------|                |
    |                                 |     200 F16    |
    |             200 F17             |<---------------|
    |<--------------------------------|                |
    |             ACK F18             |                |
    |-------------------------------->|     ACK F19    |
    |                                 |--------------->|
    |                Both Way RTP Media                |
    |<================================================>|
    |                                 |     BYE F20    |
    |             BYE F21             |<---------------|
    |<--------------------------------|                |
    |             200 F22             |                |
    |-------------------------------->|     200 F23    |
    |                                 |--------------->|
    |                                 |                |

Johnston, et al. Best Current Practice [Page 37] RFC 3665 SIP Basic Call Flow Examples December 2003

 In this scenario, Alice completes a call to Bob via a Proxy Server.
 Alice is configured for a primary SIP Proxy Server Proxy 1 and a
 secondary SIP Proxy Server Proxy 2 (Or is able to use DNS SRV records
 to locate Proxy 1 and Proxy 2).  Alice has valid credentials for both
 domains.  Proxy 1 is out of service and does not respond to INVITEs
 (it is reachable, but unresponsive).  Alice then completes the call
 to Bob using Proxy 2.
 Message Details
 F1 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK465b6d
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F2 INVITE Alice -> Proxy 1
 Same as Message F1
 F3 INVITE Alice -> Proxy 1
 Same as Message F1
 F4 INVITE Alice -> Proxy 1
 Same as Message F1

Johnston, et al. Best Current Practice [Page 38] RFC 3665 SIP Basic Call Flow Examples December 2003

 F5 INVITE Alice -> Proxy 1
 Same as Message F1
 F6 INVITE Alice -> Proxy 1
 Same as Message F1
 F7 INVITE Alice -> Proxy 1
 Same as Message F1
 /* Alice gives up on the unresponsive proxy */
 F8 INVITE Alice -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b8a
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Proxy 2 challenges Alice for authentication */
 F9 407 Proxy Authorization Required Proxy 2 -> Alice
 SIP/2.0 407 Proxy Authorization Required
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b8a
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=2421452

Johnston, et al. Best Current Practice [Page 39] RFC 3665 SIP Basic Call Flow Examples December 2003

 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 1 INVITE
 Proxy-Authenticate: Digest realm="biloxi.example.com", qop="auth",
  nonce="1ae6cbe5ea9c8e8df84fqnlec434a359",
  opaque="", stale=FALSE, algorithm=MD5
 Content-Length: 0
 F10 ACK Alice -> Proxy 2
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b8a
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=2421452
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 /* Alice responds by re-sending the INVITE with authentication
 credentials in it.  */
 F11 INVITE Alice -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="biloxi.example.com",
  nonce="1ae6cbe5ea9c8e8df84fqnlec434a359", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="8a880c919d1a52f20a1593e228adf599"
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000

Johnston, et al. Best Current Practice [Page 40] RFC 3665 SIP Basic Call Flow Examples December 2003

 /* Proxy 2 accepts the credentials and forwards the INVITE to Bob.
 Client for Alice prepares to receive data on port 49172 from the
 network.
 */
 F12 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F13 100 Trying Proxy 2 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 2 INVITE
 Content-Length: 0
 F14 180 Ringing Bob -> Proxy 2
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222

Johnston, et al. Best Current Practice [Page 41] RFC 3665 SIP Basic Call Flow Examples December 2003

 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F15 180 Ringing Proxy 2 -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F16 200 OK Bob -> Proxy 2
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000

Johnston, et al. Best Current Practice [Page 42] RFC 3665 SIP Basic Call Flow Examples December 2003

 F17 200 OK Proxy 2 -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F18 ACK Alice -> Proxy 2
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b8g
 Max-Forwards: 70
 Route: <sip:ss2.biloxi.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 2 ACK
 Content-Length: 0
 F19 ACK Proxy 2 -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b8g
  ;received=192.0.2.101
 Max-Forwards: 69
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com

Johnston, et al. Best Current Practice [Page 43] RFC 3665 SIP Basic Call Flow Examples December 2003

 CSeq: 2 ACK
 Content-Length: 0
 /* RTP streams are established between Alice and Bob */
 /* Bob Hangs Up with Alice. */
 F20 BYE Bob -> Proxy 2
 BYE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
 Max-Forwards: 70
 Route: <sip:ss2.biloxi.example.com;lr>
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F21 BYE Proxy 2 -> Alice
 BYE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 Max-Forwards: 69
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F22 200 OK Alice -> Proxy 2
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 44] RFC 3665 SIP Basic Call Flow Examples December 2003

 F23 200 OK Proxy 2 -> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
  ;received=192.0.2.201
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 4Fde34wkd11wsGFDs3@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 45] RFC 3665 SIP Basic Call Flow Examples December 2003

3.5. Session Through a SIP ALG

 Alice             ALG           Proxy 2            Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|   INVITE F2    |                |
   |     100 F3     |--------------->|   INVITE F4    |
   |<---------------|     100 F5     |--------------->|
   |                |<---------------|      180 F6    |
   |                |     180 F7     |<---------------|
   |     180 F8     |<---------------|                |
   |<---------------|                |      200 F9    |
   |                |    200 F10     |<---------------|
   |     200 F11    |<---------------|                |
   |<---------------|                                 |
   |     ACK F12    |                                 |
   |--------------->|             ACK F13             |
   |                |-------------------------------->|
   |    RTP Media   |        Both Way RTP Media       |
   |<==============>|<===============================>|
   |     BYE F14    |                                 |
   |--------------->|             BYE F15             |
   |                |-------------------------------->|
   |                |             200 F16             |
   |     200 F17    |<--------------------------------|
   |<---------------|                                 |
   |                |                                 |
 Alice completes a call to Bob through a ALG (Application Layer
 Gateway) and a SIP Proxy.  The routing through the ALG is
 accomplished using a pre-loaded Route header in the INVITE F1.  Note
 that the media stream setup is not end-to-end - the ALG terminates
 both media streams and bridges them.  This is done by the ALG
 modifying the SDP in the INVITE (F1) and 200 OK (F10) messages, and
 possibly any 18x or ACK messages containing SDP.
 In addition to firewall traversal, this Back-to-Back User Agent
 (B2BUA) could be used as part of an anonymizer service (in which all
 identifying information on Alice would be removed), or to perform
 codec media conversion, such as mu-law to A-law conversion of PCM on
 an international call.
 Also note that Proxy 2 does not Record-Route in this call flow.

Johnston, et al. Best Current Practice [Page 46] RFC 3665 SIP Basic Call Flow Examples December 2003

 Message Details
 F1 INVITE Alice -> SIP ALG
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Route: <sip:alg1.atlanta.example.com;lr>
 Proxy-Authorization: Digest username="alice",
  realm="biloxi.example.com",
  nonce="85b4f1cen4341ae6cbe5a3a9c8e88df9", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="b3f392f9218a328b9294076d708e6815"
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* Client for Alice prepares to receive data on port 49172 from the
 network. */
 F2 INVITE SIP ALG -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="biloxi.example.com",

Johnston, et al. Best Current Practice [Page 47] RFC 3665 SIP Basic Call Flow Examples December 2003

  nonce="85b4f1cen4341ae6cbe5a3a9c8e88df9", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="b3f392f9218a328b9294076d708e6815"
 Content-Type: application/sdp
 Content-Length: 150
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.128
 t=0 0
 m=audio 2000 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F3 100 Trying SIP ALG -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 /* SIP ALG prepares to proxy data from port 192.0.2.128/2000 to
 192.0.2.101/49172.   Proxy 2 uses a Location Service function to
 determine where Bob is located. Based upon location analysis the call
 is forwarded to Bob */
 F4 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
  ;received=192.0.2.128
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 68
 Record-Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp

Johnston, et al. Best Current Practice [Page 48] RFC 3665 SIP Basic Call Flow Examples December 2003

 Content-Length: 150
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.128
 t=0 0
 m=audio 2000 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F5 100 Trying Proxy 2 -> SIP ALG
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
  ;received=192.0.2.128
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F6 180 Ringing Bob -> Proxy 2
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
  ;received=192.0.2.128
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F7 180 Ringing Proxy 2 -> SIP ALG
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
  ;received=192.0.2.128

Johnston, et al. Best Current Practice [Page 49] RFC 3665 SIP Basic Call Flow Examples December 2003

 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F8 180 Ringing SIP ALG -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F9 200 OK Bob -> Proxy 2
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
  ;received=192.0.2.128
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0

Johnston, et al. Best Current Practice [Page 50] RFC 3665 SIP Basic Call Flow Examples December 2003

 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F10 200 OK Proxy 2 -> SIP ALG
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
  ;received=192.0.2.128
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F11 200 OK SIP ALG -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.128
 t=0 0

Johnston, et al. Best Current Practice [Page 51] RFC 3665 SIP Basic Call Flow Examples December 2003

 m=audio 1734 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* The ALG prepares to proxy packets from 192.0.2.128/
    1734 to 192.0.2.201/3456 */
 F12 ACK Alice -> SIP ALG
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bhh
 Max-Forwards: 70
 Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 F13 ACK SIP ALG -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bhh
  ;received=192.0.2.101
 Max-Forwards: 69
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 /* RTP streams are established between Alice and the ALG and
 between the ALG and B*/
 /* Alice Hangs Up with Bob. */
 F14 BYE Alice -> SIP ALG
 BYE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74be5
 Max-Forwards: 70
 Route: <sip:alg1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com

Johnston, et al. Best Current Practice [Page 52] RFC 3665 SIP Basic Call Flow Examples December 2003

 CSeq: 2 BYE
 Content-Length: 0
 F15 BYE SIP ALG -> Bob
 BYE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74be5
  ;received=192.0.2.101
 Max-Forwards: 69
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 BYE
 Content-Length: 0
 F16 200 OK Bob -> SIP ALG
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP alg1.atlanta.example.com:5060;branch=z9hG4bK739578.1
  ;received=192.0.2.128
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74be5
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 BYE
 Content-Length: 0
 F17 200 OK SIP ALG -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74be5
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 53] RFC 3665 SIP Basic Call Flow Examples December 2003

3.6. Session via Redirect and Proxy Servers with SDP in ACK

 Alice        Redirect Server     Proxy 3             Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|                |                |
   |     302 F2     |                |                |
   |<---------------|                |                |
   |     ACK F3     |                |                |
   |--------------->|                |                |
   |     INVITE F4                   |                |
   |-------------------------------->|    INVITE F5   |
   |             100  F6             |--------------->|
   |<--------------------------------|      180 F7    |
   |             180 F8              |<---------------|
   |<--------------------------------|                |
   |                                 |     200 F9     |
   |             200 F10             |<---------------|
   |<--------------------------------|                |
   |             ACK F11             |                |
   |-------------------------------->|     ACK F12    |
   |                                 |--------------->|
   |                Both Way RTP Media                |
   |<================================================>|
   |                                 |     BYE F13    |
   |             BYE F14             |<---------------|
   |<--------------------------------|                |
   |             200 F15             |                |
   |-------------------------------->|     200 F16    |
   |                                 |--------------->|
   |                                 |                |
 In this scenario, Alice places a call to Bob using first a Redirect
 server then a Proxy Server.  The INVITE message is first sent to the
 Redirect Server.  The Server returns a 302 Moved Temporarily response
 (F2) containing a Contact header with Bob's current SIP address.
 Alice then generates a new INVITE and sends to Bob via the Proxy
 Server and the call proceeds normally.  In this example, no SDP is
 present in the INVITE, so the SDP is carried in the ACK message.
 The call is terminated when Bob sends a BYE message.

Johnston, et al. Best Current Practice [Page 54] RFC 3665 SIP Basic Call Flow Examples December 2003

 Message Details
 F1 INVITE Alice -> Redirect Server
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Length: 0
 F2 302 Moved Temporarily Redirect Proxy -> Alice
 SIP/2.0 302 Moved Temporarily
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=53fHlqlQ2
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@chicago.example.com;transport=tcp>
 Content-Length: 0
 F3 ACK Alice -> Redirect Server
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=53fHlqlQ2
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 F4 INVITE Alice -> Proxy 3
 INVITE sip:bob@chicago.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com

Johnston, et al. Best Current Practice [Page 55] RFC 3665 SIP Basic Call Flow Examples December 2003

 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Content-Length: 0
 F5 INVITE Proxy 3 -> Bob
 INVITE sip:bob@client.chicago.example.com SIP/2.0
 Via: SIP/2.0/TCP ss3.chicago.example.com:5060;branch=z9hG4bK721e.1
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss3.chicago.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Content-Length: 0
 F6 100 Trying Proxy 3 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Content-Length: 0
 F7 180 Ringing Bob -> Proxy 3
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP ss3.chicago.example.com:5060;branch=z9hG4bK721e.1
  ;received=192.0.2.233
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss3.chicago.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.chicago.example.com;transport=tcp>
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 56] RFC 3665 SIP Basic Call Flow Examples December 2003

 F8 180 Ringing Proxy 3 -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss3.chicago.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.chicago.example.com;transport=tcp>
 Content-Length: 0
 F9 200 OK Bob -> Proxy 3
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP ss3.chicago.example.com:5060;branch=z9hG4bK721e.1
  ;received=192.0.2.233
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss3.chicago.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.chicago.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 148
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.chicago.example.com
 s=-
 c=IN IP4 192.0.2.100
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F10 200 OK Proxy -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss3.chicago.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159

Johnston, et al. Best Current Practice [Page 57] RFC 3665 SIP Basic Call Flow Examples December 2003

 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 INVITE
 Contact: <sip:bob@client.chicago.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 148
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.chicago.example.com
 s=-
 c=IN IP4 192.0.2.100
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* ACK contains SDP of Alice since none present in INVITE */
 F11 ACK Alice -> Proxy 3
 ACK sip:bob@client.chicago.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bq9
 Max-Forwards: 70
 Route: <sip:ss3.chicago.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 ACK
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F12 ACK Proxy 3 -> Bob
 ACK sip:bob@client.chicago.example.com SIP/2.0
 Via: SIP/2.0/TCP ss3.chicago.example.com:5060;branch=z9hG4bK721e.1
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bq9
  ;received=192.0.2.101
 Max-Forwards: 69
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159

Johnston, et al. Best Current Practice [Page 58] RFC 3665 SIP Basic Call Flow Examples December 2003

 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 ACK
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /* RTP streams are established between Alice and Bob */
 /* Bob Hangs Up with Alice. */
 F13 BYE Bob -> Proxy 3
 BYE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TCP client.chicago.example.com:5060;branch=z9hG4bKfgaw2
 Max-Forwards: 70
 Route: <sip:ss3.chicago.example.com;lr>
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F14 BYE Proxy 3 -> Alice
 BYE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TCP ss3.chicago.example.com:5060;branch=z9hG4bK721e.1
  ;received=192.0.2.100
 Via: SIP/2.0/TCP client.chicago.example.com:5060;branch=z9hG4bKfgaw2
 Max-Forwards: 69
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F15 200 OK Alice -> Proxy 3
 SIP/2.0 200 OK

Johnston, et al. Best Current Practice [Page 59] RFC 3665 SIP Basic Call Flow Examples December 2003

 Via: SIP/2.0/TCP ss3.chicago.example.com:5060;branch=z9hG4bK721e.1
  ;received=192.0.2.233
 Via: SIP/2.0/TCP client.chicago.example.com:5060;branch=z9hG4bKfgaw2
  ;received=192.0.2.100
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0
 F16 200 OK Proxy 3 -> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/TCP client.chicago.example.com:5060;branch=z9hG4bKfgaw2
  ;received=192.0.2.100
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 60] RFC 3665 SIP Basic Call Flow Examples December 2003

3.7. Session with re-INVITE (IP Address Change)

   Alice                Proxy 2                Bob
      |   F1 INVITE        |                    |
      |------------------->|      F2 INVITE     |
      |   F3 100 Trying    |------------------->|
      |<-------------------|   F4 180 Ringing   |
      |   F5 180 Ringing   |<-------------------|
      |<-------------------|                    |
      |                    |    F6 200 OK       |
      |    F7 200 OK       |<-------------------|
      |<-------------------|                    |
      |                 F8  ACK                 |
      |---------------------------------------->|
      |      Both Way RTP Media Established     |
      |<=======================================>|
      |                                         |
      |           Bob changes IP address        |
      |                                         |
      |                 F9 INVITE               |
      |<----------------------------------------|
      |                F10 200 OK               |
      |---------------------------------------->|
      |                 F11  ACK                |
      |<----------------------------------------|
      |         New RTP Media Stream            |
      |<=======================================>|
      |                 F12 BYE                 |
      |---------------------------------------->|
      |               F13 200 OK                |
      |<----------------------------------------|
      |                                         |
 This example shows a session in which the media changes midway
 through the session.  When Bob's IP address changes during the
 session, Bob sends a re-INVITE containing a new Contact and SDP
 (version number incremented) information to A.  In this flow, the
 proxy does not Record-Route so is not in the SIP messaging path after
 the initial exchange.

Johnston, et al. Best Current Practice [Page 61] RFC 3665 SIP Basic Call Flow Examples December 2003

 Message Details
 F1 INVITE Alice -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F2 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000

Johnston, et al. Best Current Practice [Page 62] RFC 3665 SIP Basic Call Flow Examples December 2003

 F3 100 Trying Proxy 2 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F4 180 Ringing Bob -> Proxy 2
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F5 180 Ringing Proxy 2 -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F6 200 OK Bob -> Proxy 2
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl

Johnston, et al. Best Current Practice [Page 63] RFC 3665 SIP Basic Call Flow Examples December 2003

 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F7 200 OK Proxy 2 -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
 s=-
 c=IN IP4 192.0.2.201
 t=0 0
 m=audio 3456 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F8 ACK Alice -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74b7b
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 64] RFC 3665 SIP Basic Call Flow Examples December 2003

 /* RTP streams are established between Alice and Bob */
 /* Bob changes IP address and re-INVITEs Alice with new Contact and
 SDP */
 F9 INVITE Bob -> Alice
 INVITE sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/UDP client.chicago.example.com:5060;branch=z9hG4bKlkld5l
 Max-Forwards: 70
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 14 INVITE
 Contact: <sip:bob@client.chicago.example.com>
 Content-Type: application/sdp
 Content-Length: 149
 v=0
 o=bob 2890844527 2890844528 IN IP4 client.chicago.example.com
 s=-
 c=IN IP4 192.0.2.100
 t=0 0
 m=audio 47172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F10 200 OK Alice -> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.chicago.example.com:5060;branch=z9hG4bKlkld5l
  ;received=192.0.2.100
 Max-Forwards: 70
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 14 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 150
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0

Johnston, et al. Best Current Practice [Page 65] RFC 3665 SIP Basic Call Flow Examples December 2003

 m=audio 1000 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F11 ACK Bob -> Alice
 ACK sip:alice@client.atlanta.example.com SIP/2.0
 Via: SIP/2.0/UDP client.chicago.example.com:5060;branch=z9hG4bKlkldcc
 Max-Forwards: 70
 From: Bob <sip:bob@biloxi.example.com>;tag=314159
 To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 14 ACK
 Content-Length: 0
 /* New RTP stream established between Alice and Bob */
 /* Alice hangs up with Bob */
 F12 BYE Alice -> Bob
 BYE sip:bob@client.chicago.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bo4
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 BYE
 Content-Length: 0
 F13 200 OK Bob -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bo4
  ;received=192.0.2.101
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 2 BYE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 66] RFC 3665 SIP Basic Call Flow Examples December 2003

3.8. Unsuccessful No Answer

 Alice           Proxy 1          Proxy 2            Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|   INVITE F2    |                |
   |     100  F3    |--------------->|   INVITE F4    |
   |<---------------|     100  F5    |--------------->|
   |                |<---------------|                |
   |                |                |      180 F6    |
   |                |     180 F7     |<---------------|
   |     180 F8     |<---------------|                |
   |<---------------|                |                |
   |   CANCEL F9    |                |                |
   |--------------->|                |                |
   |     200 F10    |                |                |
   |<---------------|   CANCEL F11   |                |
   |                |--------------->|                |
   |                |     200 F12    |                |
   |                |<---------------|                |
   |                |                |   CANCEL F13   |
   |                |                |--------------->|
   |                |                |     200 F14    |
   |                |                |<---------------|
   |                |                |     487 F15    |
   |                |                |<---------------|
   |                |                |     ACK F16    |
   |                |     487 F17    |--------------->|
   |                |<---------------|                |
   |                |     ACK F18    |                |
   |     487 F19    |--------------->|                |
   |<---------------|                |                |
   |     ACK F20    |                |                |
   |--------------->|                |                |
   |                |                |                |
 In this scenario, Alice gives up on the call before Bob answers
 (sends a 200 OK response).  Alice sends a CANCEL (F9) since no final
 response had been received from Bob.  If a 200 OK to the INVITE had
 crossed with the CANCEL, Alice would have sent an ACK then a BYE to
 Bob in order to properly terminate the call.
 Note that the CANCEL message is acknowledged with a 200 OK on a hop
 by hop basis, rather than end to end.

Johnston, et al. Best Current Practice [Page 67] RFC 3665 SIP Basic Call Flow Examples December 2003

 Message Details
 F1 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="ze7k1ee88df84f1cec431ae6cbe5a359", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="b00b416324679d7e243f55708d44be7b"
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /*Client for Alice prepares to receive data on port 49172 from the
 network.*/
 F2 INVITE Proxy 1 -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151

Johnston, et al. Best Current Practice [Page 68] RFC 3665 SIP Basic Call Flow Examples December 2003

 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F3 100 Trying Proxy 1 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F4 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 Max-Forwards: 68
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000

Johnston, et al. Best Current Practice [Page 69] RFC 3665 SIP Basic Call Flow Examples December 2003

 F5 100 Trying Proxy 2 -> Proxy 1
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F6 180 Ringing Bob -> Proxy 2
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F7 180 Ringing Proxy 2 -> Proxy 1
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE

Johnston, et al. Best Current Practice [Page 70] RFC 3665 SIP Basic Call Flow Examples December 2003

 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F8 180 Ringing Proxy 1 -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F9 CANCEL Alice -> Proxy 1
 CANCEL sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Route: <sip:ss1.atlanta.example.com;lr>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 CANCEL
 Content-Length: 0
 F10 200 OK Proxy 1 -> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 CANCEL
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 71] RFC 3665 SIP Basic Call Flow Examples December 2003

 F11 CANCEL Proxy 1 -> Proxy 2
 CANCEL sip:alice@atlanta.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 CANCEL
 Content-Length: 0
 F12 200 OK Proxy 2 -> Proxy 1
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 CANCEL
 Content-Length: 0
 F13 CANCEL Proxy 2 -> Bob
 CANCEL sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 CANCEL
 Content-Length: 0
 F14 200 OK Bob -> Proxy 2
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 CANCEL
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 72] RFC 3665 SIP Basic Call Flow Examples December 2003

 F15 487 Request Terminated Bob -> Proxy 2
 SIP/2.0 487 Request Terminated
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F16 ACK Proxy 2 -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 F17 487 Request Terminated Proxy 2 -> Proxy 1
 SIP/2.0 487 Request Terminated
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 73] RFC 3665 SIP Basic Call Flow Examples December 2003

 F18 ACK Proxy 1 -> Proxy 2
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 F19 487 Request Terminated Proxy 1 -> Alice
 SIP/2.0 487 Request Terminated
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 F20 ACK Alice -> Proxy 1
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="ze7k1ee88df84f1cec431ae6cbe5a359", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="b00b416324679d7e243f55708d44be7b"
 CSeq: 1 ACK
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 74] RFC 3665 SIP Basic Call Flow Examples December 2003

3.9. Unsuccessful Busy

 Alice           Proxy 1          Proxy 2            Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|   INVITE F2    |                |
   |     100  F3    |--------------->|   INVITE F4    |
   |<---------------|     100  F5    |--------------->|
   |                |<---------------|                |
   |                |                |      486 F6    |
   |                |                |<---------------|
   |                |                |     ACK F7     |
   |                |      486 F8    |--------------->|
   |                |<---------------|                |
   |                |      ACK F9    |                |
   |     486 F10    |--------------->|                |
   |<---------------|                |                |
   |     ACK F11    |                |                |
   |--------------->|                |                |
   |                |                |                |
 In this scenario, Bob is busy and sends a 486 Busy Here response to
 Alice's INVITE.  Note that the non-2xx response is acknowledged on a
 hop-by-hop basis instead of end-to-end.  Also note that many SIP UAs
 will not return a 486 response, as they have multiple line and other
 features.
 Message Details
 F1 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="dc3a5ab2530aa93112cf5904ba7d88fa", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="702138b27d869ac8741e10ec643d55be"
 Content-Type: application/sdp
 Content-Length: 151

Johnston, et al. Best Current Practice [Page 75] RFC 3665 SIP Basic Call Flow Examples December 2003

 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /*Client for Alice prepares to receive data on port 49172 from the
 network.*/
 F2 INVITE Proxy 1 -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F3 100 Trying Proxy 1 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 76] RFC 3665 SIP Basic Call Flow Examples December 2003

 F4 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 68
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F5 100 Trying Proxy 2 -> Proxy 1
 SIP/2.0 100 Trying
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F6 486 Busy Here Bob -> Proxy 2
 SIP/2.0  486 Busy Here
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1

Johnston, et al. Best Current Practice [Page 77] RFC 3665 SIP Basic Call Flow Examples December 2003

  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F7 ACK Proxy 2 -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 F8 486 Busy Here Proxy 2 -> Proxy 1
 SIP/2.0  486 Busy Here
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F9 ACK Proxy 1 -> Proxy 2
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 78] RFC 3665 SIP Basic Call Flow Examples December 2003

 F10 486 Busy Here Proxy 1 -> Alice
 SIP/2.0  486 Busy Here
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F11 ACK Alice -> Proxy 1
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="dc3a5ab2530aa93112cf5904ba7d88fa", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="702138b27d869ac8741e10ec643d55be"
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 79] RFC 3665 SIP Basic Call Flow Examples December 2003

3.10. Unsuccessful No Response from User Agent

 Alice           Proxy 1          Proxy 2            Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|   INVITE F2    |                |
   |     100  F3    |--------------->|   INVITE F4    |
   |<---------------|     100  F5    |--------------->|
   |                |<---------------|   INVITE F6    |
   |                |                |--------------->|
   |                |                |   INVITE F7    |
   |                |                |--------------->|
   |                |                |   INVITE F8    |
   |                |                |--------------->|
   |                |                |   INVITE F9    |
   |                |                |--------------->|
   |                |                |   INVITE F10   |
   |                |                |--------------->|
   |                |                |   INVITE F11   |
   |                |     480 F12    |--------------->|
   |                |<---------------|                |
   |                |     ACK F13    |                |
   |     480 F14    |--------------->|                |
   |<---------------|                |                |
   |     ACK F15    |                |                |
   |--------------->|                |                |
   |                |                |                |
 In this example, there is no response from Bob to Alice's INVITE
 messages being re-transmitted by Proxy 2.  After the sixth
 re-transmission, Proxy 2 gives up and sends a 480 No Response to
 Alice.
 Message Details
 F1 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",

Johnston, et al. Best Current Practice [Page 80] RFC 3665 SIP Basic Call Flow Examples December 2003

  nonce="cf5904ba7d8dc3a5ab2530aa931128fa", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="7afc04be7961f053c24f80e7dbaf888f"
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /*Client for Alice prepares to receive data on port 49172 from the
 network.*/
 F2 INVITE Proxy 1 -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F3 100 Trying Proxy 1 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101

Johnston, et al. Best Current Practice [Page 81] RFC 3665 SIP Basic Call Flow Examples December 2003

 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F4 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 68
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
 <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F5 100 Trying Proxy 2 -> Proxy 1
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 82] RFC 3665 SIP Basic Call Flow Examples December 2003

 F6 INVITE Proxy 2 -> Bob
 Resend of Message F4
 F7 INVITE Proxy 2 -> Bob
 Resend of Message F4
 F8 INVITE Proxy 2 -> Bob
 Resend of Message F4
 F9 INVITE Proxy 2 -> Bob
 Resend of Message F4
 F10 INVITE Proxy 2 -> Bob
 Resend of Message F4
 F11 INVITE Proxy 2 -> Bob
 Resend of Message F4
 /* Proxy 2 gives up */
 F12 480 No Response Proxy 2 -> Proxy 1
 SIP/2.0 480 No Response
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 83] RFC 3665 SIP Basic Call Flow Examples December 2003

 F13 ACK Proxy 1 -> Proxy 2
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 F14 480 No Response Proxy 1 -> Alice
 SIP/2.0 480 No Response
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F15 ACK Alice -> Proxy 1
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="cf5904ba7d8dc3a5ab2530aa931128fa", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="7afc04be7961f053c24f80e7dbaf888f"
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 84] RFC 3665 SIP Basic Call Flow Examples December 2003

3.11. Unsuccessful Temporarily Unavailable

 Alice          Proxy 1          Proxy 2            Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|   INVITE F2    |                |
   |     100  F3    |--------------->|   INVITE F4    |
   |<---------------|     100  F5    |--------------->|
   |                |<---------------|      180 F6    |
   |                |     180 F7     |<---------------|
   |     180 F8     |<---------------|                |
   |<---------------|                |     480 F9     |
   |                |                |<---------------|
   |                |                |     ACK F10    |
   |                |     480 F11    |--------------->|
   |                |<---------------|                |
   |                |     ACK F12    |                |
   |     480 F13    |--------------->|                |
   |<---------------|                |                |
   |     ACK F14    |                |                |
   |--------------->|                |                |
   |                |                |                |
 In this scenario, Bob initially sends a 180 Ringing response to
 Alice, indicating that alerting is taking place.  However, then a
 480 Unavailable is then sent to Alice.  This response is
 acknowledged then proxied back to Alice.
 Message Details
 F1 INVITE Alice -> Proxy 1
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="aa9311cf5904ba7d8dc3a5ab253028fa", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="59a46a91bf1646562a4d486c84b399db"
 Content-Type: application/sdp

Johnston, et al. Best Current Practice [Page 85] RFC 3665 SIP Basic Call Flow Examples December 2003

 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 /*Client for Alice prepares to receive data on port 49172 from the
 network.*/
 F2 INVITE Proxy 1 -> Proxy 2
 INVITE sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 69
 Record-Route: <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F3 100 Trying Proxy 1 -> Alice
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE

Johnston, et al. Best Current Practice [Page 86] RFC 3665 SIP Basic Call Flow Examples December 2003

 Content-Length: 0
 F4 INVITE Proxy 2 -> Bob
 INVITE sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Max-Forwards: 68
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:alice@client.atlanta.example.com>
 Content-Type: application/sdp
 Content-Length: 151
 v=0
 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
 s=-
 c=IN IP4 192.0.2.101
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 F5 100 Trying Proxy 2 -> Proxy 1
 SIP/2.0 100 Trying
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 87] RFC 3665 SIP Basic Call Flow Examples December 2003

 F6 180 Ringing Bob -> Proxy 2
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F7 180 Ringing Proxy 2 -> Proxy 1
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>
 Content-Length: 0
 F8 180 Ringing Proxy 1 -> Alice
 SIP/2.0 180 Ringing
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 Record-Route: <sip:ss2.biloxi.example.com;lr>,
  <sip:ss1.atlanta.example.com;lr>
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Contact: <sip:bob@client.biloxi.example.com>

Johnston, et al. Best Current Practice [Page 88] RFC 3665 SIP Basic Call Flow Examples December 2003

 Content-Length: 0
 F9 480 Temporarily Unavailable Bob -> Proxy 2
 SIP/2.0 480 Temporarily Unavailable
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
  ;received=192.0.2.222
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F10 ACK Proxy 2 -> Bob
 ACK sip:bob@client.biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss2.biloxi.example.com:5060;branch=z9hG4bK721e4.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 F11 480 Temporarily Unavailable Proxy 2 -> Proxy 1
 SIP/2.0 480 Temporarily Unavailable
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
  ;received=192.0.2.111
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 89] RFC 3665 SIP Basic Call Flow Examples December 2003

 F12 ACK Proxy 1 -> Proxy 2
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 ACK
 Content-Length: 0
 F13 480 Temporarily Unavailable Proxy 1 -> Alice
 SIP/2.0 480 Temporarily Unavailable
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
  ;received=192.0.2.101
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 CSeq: 1 INVITE
 Content-Length: 0
 F14 ACK Alice -> Proxy 1
 ACK sip:bob@biloxi.example.com SIP/2.0
 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
 Max-Forwards: 70
 From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
 To: Bob <sip:bob@biloxi.example.com>;tag=314159
 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
 Proxy-Authorization: Digest username="alice",
  realm="atlanta.example.com",
  nonce="aa9311cf5904ba7d8dc3a5ab253028fa", opaque="",
  uri="sip:bob@biloxi.example.com",
  response="59a46a91bf1646562a4d486c84b399db"
 CSeq: 1 ACK
 Content-Length: 0

Johnston, et al. Best Current Practice [Page 90] RFC 3665 SIP Basic Call Flow Examples December 2003

4. Security Considerations

 Since this document contains examples of SIP session establishment,
 the security considerations in RFC 3261 [1] apply.  RFC 3261
 describes the basic threats including registration hijacking, server
 impersonation, message body tampering, session modifying or teardown,
 and denial of service and amplification attacks.  The use of HTTP
 Digest as shown in this document provides one-way authentication and
 protection against replay attacks.  TLS transport is used in
 registration scenarios due to the lack of integrity protection in
 HTTP Digest and the danger of registration hijacking without it, as
 described in RFC 3261 [1].  A full discussion of the weaknesses of
 HTTP Digest is provided in RFC 3261 [1].  The use of TLS and the
 Secure SIP (sips) URI scheme provides a better level of security
 including two-way authentication.  S/MIME can provide end-to-end
 confidentiality and integrity protection of message bodies, as
 described in RFC 3261.

5. References

5.1. Normative References

 [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
     Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
     Session Initiation Protocol", RFC 3261, June 2002.
 [2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
     SDP", RFC 3264, April 2002.
 [3] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach,
     P., Luotonen, A. and L. Stewart, "HTTP authentication: Basic and
     Digest Access Authentication", RFC 2617, June 1999.
 [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

5.2. Informative References

 [5] Johnston, A., Donovan, S., Sparks, R., Cunningham, C. and K.
     Summers, "Session Initiation Protocol (SIP) Public Switched
     Telephone Network (PSTN) Call Flows", BCP 76, RFC 3666, December
     2003.

6. Intellectual Property Statement

 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in

Johnston, et al. Best Current Practice [Page 91] RFC 3665 SIP Basic Call Flow Examples December 2003

 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11.  Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 obtain a general license or permission for the use of such
 proprietary rights by implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard.  Please address the information to the IETF Executive
 Director.

7. Acknowledgments

 This document is has been a group effort by the SIP and SIPPING WGs.
 The authors wish to thank everyone who has read, reviewed, commented,
 or made suggestions to improve this document.
 Thanks to Rohan Mahy, Adam Roach, Gonzalo Camarillo, Cullen Jennings,
 and Tom Taylor for their detailed comments during the final review.
 Thanks to Dean Willis for his early contributions to the development
 of this document.
 The authors wish to thank Kundan Singh for performing parser
 validation of messages.
 The authors wish to thank the following individuals for their
 participation in the review of this call flows document: Aseem
 Agarwal, Rafi Assadi, Ben Campbell, Sunitha Kumar, Jon Peterson, Marc
 Petit-Huguenin, Vidhi Rastogi, and Bodgey Yin Shaohua.
 The authors also wish to thank the following individuals for their
 assistance: Jean-Francois Mule, Hemant Agrawal, Henry Sinnreich,
 David Devanatham, Joe Pizzimenti, Matt Cannon, John Hearty, the whole
 MCI WorldCom IPOP Design team, Scott Orton, Greg Osterhout, Pat
 Sollee, Doug Weisenberg, Danny Mistry, Steve McKinnon, and Denise
 Ingram, Denise Caballero, Tom Redman, Ilya Slain, Pat Sollee, John
 Truetken, and others from MCI WorldCom, 3Com, Cisco, Lucent and
 Nortel.

Johnston, et al. Best Current Practice [Page 92] RFC 3665 SIP Basic Call Flow Examples December 2003

8. Authors' Addresses

 All listed authors actively contributed large amounts of text to this
 document.
 Alan Johnston
 MCI
 100 South 4th Street
 St. Louis, MO 63102
 USA
 EMail: alan.johnston@mci.com
 Steve Donovan
 dynamicsoft, Inc.
 5100 Tennyson Parkway
 Suite 1200
 Plano, Texas 75024
 USA
 EMail: sdonovan@dynamicsoft.com
 Robert Sparks
 dynamicsoft, Inc.
 5100 Tennyson Parkway
 Suite 1200
 Plano, Texas 75024
 USA
 EMail: rsparks@dynamicsoft.com
 Chris Cunningham
 dynamicsoft, Inc.
 5100 Tennyson Parkway
 Suite 1200
 Plano, Texas 75024
 USA
 EMail: ccunningham@dynamicsoft.com
 Kevin Summers
 Sonus
 1701 North Collins Blvd, Suite 3000
 Richardson, TX 75080
 USA
 EMail: kevin.summers@sonusnet.com

Johnston, et al. Best Current Practice [Page 93] RFC 3665 SIP Basic Call Flow Examples December 2003

9. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assignees.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Johnston, et al. Best Current Practice [Page 94]

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