Ti SIP Protocol Structure

SIP Protocol Structure through an Example http://www.tech-invite.com

V2.2 – Nov 26, 2007InitiatingUA Proxy 1

(Stateful)

INVITE

100 Trying

INVITE

ACK

100 Trying

180 Ringing180 Ringing

180 Ringing200 OK

200 OK200 OK

Copyright © 2005-2008 Joël Repiquet. All Rights Reserved. 21 pages

Proxy 3(Stateful)

RespondingUA

Proxy 2(Stateless)

INVITE

180 Ringing

200 OK

INVITE407 Proxy

Authorization Required

INVITE

ACKACK

ACK

BYE

BYEBYE

200 OK200 OK

200 OK

Media Session

This example illustrates, as a slide show, the structure of the SIP protocol, as outlined in chapter 5 of RFC 3261:

"The lowest layer is the transport layer. It defines how a client sends requests and receives responses and how a server receives requests and sends responses over the network. All SIP elements contain a transport layer.

The second layer is the transaction layer. A transaction is a request sent by a client transaction (using the transport layer) to a server transaction, along with all responses to that request sent from the server transaction back to the client. Any task that a user agent client (UAC) accomplishes takes place using a series of transactions. Stateless proxies do not contain a transaction layer.

The layer above the transaction layer is called the transaction user (TU). Each of the SIP entities, except the stateless proxy, is a transaction user."

Note: we set aside the syntax and encoding as the lowest layer.

In this example, the rejection of the first INVITE request, followed by a valid INVITE request, enables the analysis of the processing of the ACK for these two situations.

It is assumed that both Proxy 1 and Proxy 3 stateful proxy servers are in the final signalling path because they requested it in the INVITE requests they routed on.

1

2

3

5

4

4

4

5

4

66

66

77

77

8

8

8

x

x

x

y

y

y

TCPSCTP

TLS

IP

UDP

RTP

TCP SCTP

TLS

IP

UDP

IP IP

server transaction

server transport

clienttransport

proxy coreUAScore

UACcore

client transaction

clienttransport

INVITErequest

request

Initiating UA

receive

request

Proxy 1(Stateful)Open

Session

INVITErequest

SDPOffer

send

server transport

Client Transport:- before sending the request: insert the 'sent-by' parameter in the Via header field

Server Transport:- when receiving the request: by examining the 'sent-by' parameter in the top Via header field, match it to the relevant server transaction and add the "received" parameter

The new "INVITE" client transaction (state="calling") is identified by the CSeq header field and the "branch" parameter of the Via header field. T1 timer is started (if UDP) before passing message request to transport.

The UAC (User Agent Client) TU:- creates the initial INVITE request- creates a new client transaction (in the transaction layer) and passes it the INVITE message, plus the IP address, port and transport

The new "INVITE" server transaction (state="proceeding") is created by the proxy core (not acting as TU). The server transaction transmits the INVITE request to the TU and somehow knows that this TU will generate a response within 200ms: it does not send back a 100 Trying response.

The proxy core TU first validates the INVITE request. It cannot authenticate the originator because no credentials are provided. It rejects the request by sending back (see next slide) a 407 (Proxy Authentication Required) response.

INV/br-a0 INV/br-a1

SIP Transport Layer

SIP Transaction Layer

SIP Transaction User (TU)

1

SIP Protocol Structure through an Example (1)http://www.tech-invite.comCopyright © 2005-2008 Joël Repiquet. All Rights Reserved.

TCP SCTP

TLS

IP

UDPTCPSCTP

TLS

IP

UDP

RTP

server transaction

server transport

clienttransport

proxy coreUAScore

UACcore

client transaction

clienttransport

ACK(request)

Initiating UA

receive

request

Proxy 1(Stateful)

ACK

response

send

server transport

Client Transport:- when receiving the response: match it to the relevant client transaction by examining 'sent-by' parameter in top Via header field- before sending the (ACK) request: insert 'sent-by' parameter in Via header field

Server Transport:- before sending the response: retrieve IP@ & port from 'sent-by' and "received"- when receiving the (ACK) request: by examining 'sent-by' parameter in top Via header field, match it to relevant server transaction + add "received" parameter

When receiving the 407 response, the client transitions to state="completed", passes the response up to the TU, generates an ACK, and passes it to transport. D timer started. When D timer fires, the client transaction is destroyed.

The UAC analyses the response and prepares a new INVITE request.

The server transaction enters the "completed" state and passes the 407 response to transport.When ACK (request) is received: passes to "confirmed" state and starts I timer. When I timer fires, the server transaction is destroyed.

The proxy core TU:sends back a 407 (Proxy Authentication Required) response

INV/br-a0 INV/br-a0

SIP Transport Layer



receive

response

407response

407response

2

response

send

3


IP IP

TCP SCTP

TLS

IP

UDPTCPSCTP

TLS

IP

UDP

RTP

server transaction

server transport

clienttransport

proxy coreUAScore

UACcore

client transaction

clienttransport

INVITErequest

request

Initiating UA

receive

request

Proxy 1(Stateful)

INVITErequest

receive

response

100(Trying)

100 (Trying) response

sendsend

server transport

The new "INVITE" client transaction (state="calling") is identified by CSeq header field and "branch" parameter of Via header field. T1 timer started (if UDP) before passing message request to transport. When receiving 1xx response: state="proceeding" and T1 reset.

The UAC (User Agent Client) TU:(1) creates the new INVITE request containing the correct credentials(2) creates a new client transaction and passes it the INVITE message, plus the IP address, port and transport

The new "INVITE" server transaction (identified by CSeq header field and "branch" parameter of Via header field) is created by proxy core (not acting as TU). The server transaction sends back a 100 Trying response and transmits the INVITE request to proxy core (acting as TU).

The proxy core TU:(1) validates the request(2) determines the target for the request(3) forwards the request (see next slide) towards the target

INV/br-a1 INV/br-a1

4

SIP Transport Layer



Client & Server Transport main behavior, whatever the system:

Server: (1) when receiving a request: by examining 'sent-by' parameter in top Via header field, match it to relevant server transaction + add "received" parameter; (2) before sending a response: retrieve IP@ and port from 'sent-by' and "received"

Client: (1) before sending a request: insert 'sent-by' parameter in Via header field (2) when receiving a response: match it to relevant client transaction by examining 'sent-by' parameter in top Via header field


IP IP

5

TCP SCTP

TLS

IP

UDPTCP SCTP

TLS

IP

UDPTCP SCTP

TLS

IP

UDP

proxy coreserver

transaction

clienttransport

server transport

server transport

clienttransport

clienttransport

server transport

proxy core

server transaction

client transaction

Proxy 1(Stateful)

Proxy 3(Stateful)

proxy core

receive

request

INVITErequest

request

send

Proxy 2(Stateless)

send

receive

INVITErequest

request

request

receivesend

response100 (Trying)

response

INV/br-x10 INV/br-x10INV/br-a1

For forwarding the INVITE request, the proxy core creates a new client transaction

The proxy core TU:(1) validates the request(2) determines the target for the request(3) forwards the request (see next slide) towards the target

There is no Transaction Management with a stateless

proxy


IP IP

4

5

TCP SCTP

TLS

IP

UDP

RTP

TCPSCTP

TLS

IP

UDP

IPIP

server transport

clienttransport

server transaction

UACcore

UAScore

proxy core

client transaction

server transaction

clienttransport

server transport

INVITErequest

request

send

Responding UA

Proxy 3(Stateful)

request

receive

INVITErequest

SDPOffer

response

send

response

180(Ringing)

receive

180 (Ringing)response

INV/br-x30 INV/br-x30INV/br-x10

For forwarding the INVITE request, the proxy core creates a new client transaction


4 6

TCP SCTP

TLS

IP

UDPTCP SCTP

TLS

IP

UDPTCP SCTP

TLS

IP

UDP

proxy coreclient

transactionserver

transaction

clienttransport

server transport

server transport

clienttransport

clienttransport

server transport

proxy core

server transaction

client transaction

Proxy 1(Stateful)

Proxy 3(Stateful)

proxy core

Proxy 2(Stateless)

receive

sendsend

response

responseresponse

receive

response

180(Ringing)

180(Ringing)

INV/br-x10 INV/br-x10INV/br-a1 INV/br-x30


IP IP

6

6

TCPSCTP

TLS

IP

UDP

RTP

TCP SCTP

TLS

IP

UDP

IP IP

client transaction

server transaction

server transport

clienttransport

proxy coreUAScore

UACcore

client transaction

clienttransport

Initiating UA

Proxy 1(Stateful)

receive

response

180(Ringing)

send

server transport

response

180(Ringing)

INV/br-a1INV/br-a1 INV/br-x10


6

TCP SCTP

TLS

IP

UDP

RTP

TCPSCTP

TLS

IP

UDP

IPIP

server transport

clienttransport

server transaction

UACcore

UAScore

proxy core

client transaction

server transaction

clienttransport

server transport

Responding UA

Proxy 3(Stateful)

response

send

response

200(OK)

receive

200 (OK)response

SDPAnswer

INV/br-x30INV/br-x30INV/br-x10

Retransmissions of 200 OK (whatever the UA transport protocol) are handled by the UAS core, not the transaction layer. The 200 OK response retransmission (if any) is passed to the transport with an interval that starts at T1 seconds and doubles for each retransmission until it reaches T2 seconds.

The INVITE server transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the transport.

The INVITE client transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the proxy core TU.

SIP Protocol Structure through an Example (8)

The callee answers the call, and an SDP answer is communicated to the UAS core

http://www.tech-invite.comCopyright © 2005-2008 Joël Repiquet. All Rights Reserved.

7

proxy coreserver

transaction

clienttransport

server transport

server transport

clienttransport

clienttransport

server transport

proxy core

server transaction

client transaction

TCP SCTP

TLS

IP

UDP TCP SCTP

TLS

IP

UDP TCP SCTP

TLS

IP

UDP

Proxy 1(Stateful)

Proxy 3(Stateful)

proxy core

Proxy 2(Stateless)

receive

sendsend

response

responseresponse

receive

IP IP

response

200(OK)

200(OK)

INV/br-x10INV/br-x10INV/br-a1 The INVITE server transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the transport.

The INVITE client transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the proxy core TU.


7

7

TCPSCTP

TLS

IP

UDP

RTP

TCP SCTP

TLS

IP

UDP

IP IP

server transaction

server transport

clienttransport

proxy coreUAScore

UACcore

client transaction

clienttransport

Initiating UA

Proxy 1(Stateful)

receive

response

200(OK)

send

server transport

response

200(OK)

INV/br-a1INV/br-a1

SDPAnswer

The INVITE server transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the transport.

The INVITE client transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the UAC core TU.


7

TCP SCTP

TLS

IP

UDPTCPSCTP

TLS

IP

UDP

RTP

server transport

clienttransport

proxy coreUAScore

UACcore

clienttransport

Initiating UA

Proxy 1(Stateful)

server transport

ACKrequest

send receive

ACKrequest


The ACK request (following a 2xx final response) is passed to the transport layer directly for transmission


IP IP

8

TCP SCTP

TLS

IP

UDP

clienttransport

server transport

clienttransport

server transport

proxy core

Proxy 1(Stateful)

Proxy 3(Stateful)

proxy core

ACKrequest

send receive

ACKrequest


Both Proxy 1 and Proxy 3 stateful proxy servers are in the final signalling path because they requested it in the INVITE request they routed on.


8

TCP SCTP

TLS

IP

UDP

IP IPIP

TCPSCTP

TLS

IP

UDP TCP SCTP

TLS

IP

UDP

RTP

server transport

clienttransport

UACcore

UAScore

proxy core

clienttransport

server transport

Responding UA

Proxy 3(Stateful)

ACKrequest

send receive

ACKrequest


IPIP

8

UAScore

UACcore

clienttransport

Initiating UA

server transport

server transport

clienttransport

UACcore

UAScore

Responding UA

Media part Media part


TCPSCTP

TLS

IP

UDP

RTP

IP IP

TCP SCTP

TLS

IP

UDP

RTP

TCP SCTP

TLS

IP

UDP

RTP

TCPSCTP

TLS

IP

UDP

server transport

clienttransport

UAScore

UACcore

client transaction

clienttransport

Responding UA

BYErequest

request

send

BYE/br-b1

proxy core

Proxy 3(Stateful)

server transaction

request

receive

BYErequest

BYE/br-b1


IPIP

server transport

x

TCP SCTP

TLS

IP

UDPTCP SCTP

TLS

IP

UDP

server transport

clienttransport

proxy core

Proxy 1(Stateful)

Proxy 3(Stateful)

proxy core

clienttransport

server transport

server transaction

BYE/br-b1

client transaction

BYE/br-b31

BYErequest

request

send

server transaction

BYE/br-b31

request

receive

BYErequest


IP IPIP

x

TCPSCTP

TLS

IP

UDP

RTP

TCP SCTP

TLS

IP

UDP

IP IP

clienttransport

server transport

proxy core

Initiating UA

Proxy 1(Stateful)

server transaction

BYE/br-b31

client transaction

BYE/br-b11

BYErequest

request

send

UACcore

UAScore

server transport

clienttransport

server transaction

BYE/br-b11

request

receive

BYErequest


x

TCPSCTP

TLS

IP

UDP

RTP

TCP SCTP

TLS

IP

UDP

IP IP

clienttransport

server transport

proxy core

Initiating UA

Proxy 1(Stateful)

server transaction

BYE/br-b31

client transaction

BYE/br-b11

UACcore

UAScore

server transport

clienttransport

server transaction

BYE/br-b11

response

send

200(OK)

response

receive

200(OK)


The BYE server transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the transport.

The BYE client transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the proxy core TU.


y

TCP SCTP

TLS

IP

UDP TCP SCTP

TLS

IP

UDP

server transport

clienttransport

proxy core

Proxy 1(Stateful)

Proxy 3(Stateful)

proxy core

clienttransport

server transport

server transaction

BYE/br-b1

client transaction

BYE/br-b31

server transaction

BYE/br-b31

response

send

200(OK)

response

receive

200(OK)



The BYE client transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the proxy core TU.


y

IP IPIP

TCP SCTP

TLS

IP

UDP

RTP

TCPSCTP

TLS

IP

UDP

IPIP

server transport

clienttransport

UAScore

UACcore

client transaction

clienttransport

Responding UA

BYE/br-b1

proxy core

Proxy 3(Stateful)

server transaction

BYE/br-b1

response

send

200(OK)

response

receive

200(OK)

SIP Protocol Structure through an Example (end)


The BYE client transaction is destroyed as soon as it receives the 200 (OK) response and passes it to the UAC core TU.


y

server transport

Documents

Ti SIP Protocol Structure