RFC 3261摘录

 

·        If more than one Contact issent in a REGISTER request, the

registering UA intends toassociate all of the URIs in these Contact

header field values with theaddress-of-record present in the To

field. This list can beprioritized with the "q" parameter in the

Contact header field. The"q" parameter indicates a relative

preference for theparticular Contact header field value compared to

other bindings for thisaddress-of-record. Section 16.6 describes

how a proxy server uses thispreference indication.

---10.2.1.2Preferences among Contact Addresses

 

Contact的"q"参数如何理解？

 

 

·        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 mechanism

   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

注：参考A UA MUST NOT send a BYE outside of a dialog理解，所以cancel结束早期会话的时候，需要再发送487 Request Terminated结束会话。

 

·        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

注：如何理解？

·        Therefore, it is necessary

to periodically pass theresponse directly to the transport until the

ACK arrives. The 2xxresponse is passed to the transport with an

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 for

the response is received. Thisis independent of whatever transport

protocols are used to sendthe response.

 

If the server retransmitsthe 2xx response for 64*T1 seconds without

receiving an ACK, the dialogis confirmed, but the session SHOULD be

terminated. This isaccomplished with a BYE, as described in Section

15.

---13.3.1.4The INVITE is Accepted

看起来200 OK传输是可靠的

 

 

 

·        A stateful proxy has aserver

transaction associated withone or more client transactions by a

higher layer proxyprocessing component (see figure 3), known as a

proxy core. An incomingrequest is processed by a server

transaction. Requests fromthe server transaction are passed to a

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

choosing one or morenext-hop locations. An outgoing request for

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. Anyretransmissions of the request will then be

handled by that servertransaction per Section 17   

---16.2Stateful Proxy                                      

注：走UDP的重发机制，可以借鉴使用

 

 

·        when the user hangs up, itindicates a desire to

terminate the attempt toestablish a session, and to terminate any

sessions already created

---15Terminating a Session

session分两种：尚未建立的以及已经建立

 

·        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 the

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 the

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.

 

---16.1Overview

SIP Proxy概念

 

·        When

   a connection is opened by the transportlayer, this index is set to

   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

 

注： source IP针对UAC而言，而Destination IP针对UAS而言

 

·         Before a request is sent, the client transportMUST insert a value of

   the "sent-by" field into the Viaheader field.  This field contains

   an IP address or host name, and port.

18.1.1 SendingRequests

注：语气词MUST，然而我抓的包还没看到via 的"sent-by" field

 

·        When a tel URL (RFC 2806[9]) is converted to a SIP or SIPS URI, the

   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 equalif the

   sequence number and the request method areidentical

---20.16 CSeq

注意：如果重发的话，CSeq不变