RFC 3261 excerpt

 

· If more than one contact issent in a register request,

Registering UA intends toassociate all of the URIs in these contact

Header field values with theaddress-of-record present in the

Field. This list can beprioritized with the "Q" parameter in

Contact Header field. The "Q" parameter indicates a relative

Preference for theparticular contact header field value compared

Other Bindings for thisaddress-of-record. Section 16.6 describes

How a proxy server uses thispreference indication.

--- 10.2.1.2preferences among contact addresses

 

How do I understand the "Q" parameter of contact?

 

 

· Independent of the method, if a request outside of a dialog generates

A non-2xx final response, any early dialogscreated through

Provisional responses to that request areterminated.
The mechanic

For terminating confirmed dialogs is methodspecific.
In this

Specification, the bye method terminates asession and the Dialog

Associated with it.
See section 15 for details.

--- 12.3 termination of a dialog

Note: For more information, see a UA must not send a bye outside of a dialog. Therefore, when cancel ends an earlier session, you must send a 487 request terminated message to end the session.

 

· If the invite requestcontained an offer, and the UAS had not yet

Sent an answer, the 2XX mustcontain an answer. If the invite did

Not contain an offer, the2xx must contain an offer if the UAS had

Not yet sent an offer.

--- 13.3.1.4the invite is accepted

Note: How to understand it?

· Therefore, it is necessary

To periodically pass theresponse directly to the transport until

Ack arrives. The 2 xxresponse is passed to the transport with

Interval that starts at t1seconds and doubles for each

Retransmission until itreaches T2 seconds (t1 and t2 are defined in

Section 17). responseretransmissions cease when an ACK request

The response is wrongly ed. Thisis independent of whatever transport

Protocols are used to sendthe response.

 

If the server retransmitsthe 2XX response for 64 * T1 seconds

Refreshing an ACK, The dialogis confirmed, but the session shocould be

Terminated. This isaccomplished with a bye, as described in Section

15.

--- 13.3.1.4the invite is accepted

It seems that 200 OK transmission is reliable.

 

 

 

· A stateful proxy has aserver

Transaction associated withone or more client transactions by

Higher layer proxyprocessing component (see figure 3), known as

Proxy core. An incomingrequest is processed by a server

Transaction. Requests fromthe server transaction are passed to

Proxy core. The proxy coredetermines where to route the request,

Choosing one or morenext-hop locations. An outgoing request

Each next-hop location isprocessed by its own associated Client

Transaction. The proxy corecollects the responses from the client

Transactions and uses themto send responses to the server

Transaction.

 

A stateful proxy creates anew server transaction for each new

Request received ed. anyretransmissions of the request will be

Handled by that servertransaction per section 17

--- 16.2 stateful proxy

Note: The UDP re-transmission mechanism can be used for reference.

 

 

· When the user hangs up, itindicates a desire

Terminate the attempt toestablish a session, and to terminate any

Sessions already created

--- 15 terminating a session

There are two types of sessions: uncreated and created

 

· Once the bye is constructed, the UAC core creates a new non-invite

Client Transaction, andpasses it the bye request. The UAC must

Consider the sessionterminated (and therefore stop sending or

Listening for media) as soonas the bye request is passed to

Client Transaction. If theresponse for the bye is a 481

(Call/transaction does notexist) or a 408 (request timeout) or no

Response at all is receivedfor the bye (that is, a timeout is

Returned by the clienttransaction), The UAC must consider

Session and the dialogterminated.

 

This rule means that a byesent without tags by a UAC will be

Rejected (A 481 call/transaction does not exist response

). This is a change from rfc2543, which allowed bye

Without tags.

--- 15.1.1 uacbehavior

 

· Sip proxies are elementsthat route SIP requests to user agent

Servers and SIP responses touser agent clients.

......

Responses will route throughthe same set of proxies

Traversed by the request inthe reverse order.

 

--- 16.1 Overview

SIP proxy Concept

 

· When

A connection is opened by the transportlayer, this index is set

Thedestination IP, port and transport.
Whenthe connection is

Accepted by the transport layer, this indexis set to the Source IP

Address, port number, and transport.

--- 18 transport

 

Note: source IP is for UAC, while destination IP is for UAS.

 

· Before a request is sent, the client transportmust insert a value

The "sent-by" field into the viaheader field.
This field contains

An IP address or host name, and port.

18.1.1 sendingrequests

Note: The modal message is must, but the package I caught has not seen
"Sent-by" Field

 

· When a Tel URL (RFC 2806 [9]) is converted to a SIP or sips uri,

Entire telephone-subscriber portion of thetel URL, including any

Parameters, is placed into the userinfo partof the SIP or sips uri.

 

Thus, Tel: + 358-555-1234567; postd = pp22 becomes

 

SIP: + 358-555-1234567; postd = pp22@foo.com; user = phone

 

Or

Sips: + 358-555-1234567; postd = pp22@foo.com; user = phone

 

Not

SIP: + 358-555-1234567@foo.com; postd = pp22; user = phone

 

Or

 

Sips: + 358-555-1234567@foo.com; postd = pp22; user = phone

19.1.6relating sip Uris and Tel URLs

 

· The CSeq Header field servesto order transactions

Within a dialog, to provide a means touniquely identify

Transactions, and to differentiate betweennew requests and request

Retransmissions.
Two CSeq Header fields are considered when if

Sequence Number and the request method areidentical

--- 20.16 CSeq

Note: CSeq remains unchanged if resend occurs.