ensungm的专栏

0117 Registrations1 Overview   SIP offers a discovery capability.  If a user wants to initiate a   session with another user, SIP must discover the current host(s) at   which the destination u

Responses are first processed by the transport layer and then passed   up to the transaction layer.  The transaction layer performs its   processing and then passes the response up to the TU.  T

xThe Via header field indicates the transport used for the transaction   and identifies the location where the response is to be sent.  A Via   header field value is added only after the transpo

 Any new request may receive 3xx responses themselves containing      the original URI as a contact.  Two locations can be configured to      redirect to each other.  Placing any given URI in th

    Certain 4xx response codes require specific UA processing,   independent of the method.   If a 401 (Unauthorized) or 407 (Proxy Authentication Required)   response is received, the UAC SHO

Local policy MAY specify an alternate set of destinations to attempt.   If the Request-URI contains a SIPS URI, any alternate destinations   MUST be contacted with TLS.  Beyond that, there are n

Assuming the UAS decides that it is the proper element to process the   request, it examines the Require header field, if present.    The Require header field is used by a UAC to tell a UAS ab

     If the request has no tag in the To header field, the UAS core MUST   check the request against ongoing transactions.  If the From tag,   Call-ID, and CSeq exactly match those associated

 Assuming the UAS understands any extensions required by the client,   the UAS examines the body of the message, and the header fields that   describe it.  If there are any bodies whose type (in

   A UAS that wishes to apply some extension when generating the   response MUST NOT do so unless support for that extension is   indicated in the Supported header field in the request.  If the

    In some architectures it may be desirable to reduce the processing   load on proxy servers that are responsible for routing requests, and   improve signaling path robustness, by relying on r

    When a UAS wishes to construct a response to a request, it follows   the general procedures detailed in the following subsections.   Additional behaviors specific to the response code in que

   The From field of the response MUST equal the From header field of   the request.  The Call-ID header field of the response MUST equal the   Call-ID header field of the request.  The CSeq hea

The Call-ID header field acts as a unique identifier to group   together a series of messages.  It MUST be the same for all requests   and responses sent by either UA in a dialog.  It SHOULD be

   A valid SIP request formulated by a UAC MUST, at a minimum, contain   the following header fields: To, From, CSeq, Call-ID, Max-Forwards,   and Via; all of these header fields are mandatory i

 Status-Line  =  SIP-Version SP Status-Code SP Reason-Phrase CRLF例如：        SIP/2.0       200          OK header  =  "header-name" HCOLON header-value *(COMMA header-value)例如：From: Alice ;

There are many ways by which the contents of the location service can   be established.  One way is administratively.  In the above example,   Bob is known to be a member of the engineering depa

   REGISTER requests add, remove, and query bindings.  A REGISTER   request can add a new binding between an address-of-record and one or   more contact addresses.  Registration on behalf of a p

0115 Client Behavior2Note that both the transaction corresponding to the original request   and the CANCEL transaction will complete independently.  However, a   UAC canceling a request cannot r

0130 Discovering a Registrar   UAs can use three ways to determine the address to which to send   registrations:  by configuration, using the address-of-record, and   multicast.  A UA can be con

 SIP is rather a component that can be used with other IETF protocols to   build a complete multimedia architecture.  Typically, these   architectures will include protocols such as the Real-tim

486 忙The callees end system was contacted successfully, but the callee is   currently not willing or able to take additional calls at this end   system.  The response MAY indicate a better ti

The other party sends a 200 (OK) to accept the change.   The requestor responds to the 200 (OK) with an ACK.  If the other   party does not accept the change, he sends an error response such as

 Before forwarding the request,   the atlanta.com proxy server adds an additional Via header field   value that contains its own address (the INVITE already contains   Alices address in the f

    SIP is structured as a layered protocol, which means that its   behavior is described in terms of a set of fairly independent   processing stages with only a loose coupling between each stag

saction layer is called the transaction user   (TU).  Each of the SIP entities, except the stateless proxy, is a   transaction user.  When a TU wishes to send a request, it creates a   client

No ACK is sent - an ACK is only sent in response to a response to an INVITE requestRegistration is another common operation in SIP.  Registration is one   way that the biloxi.com server can lear

 Both types of messages consist of a start-line, one or more header fields, an   empty line indicating the end of the header fields, and an optional   message-body.          generic-message 

A stateless UAS is a UAS that does not maintain transaction state.   It replies to requests normally, but discards any state that would   ordinarily be retained by a UAS after a response has bee