SIP Brief Intro
TERM
Session
Session is considered an exchange of data between anassociation of participants.
SIP
SIP is an application-layer control protocol that canestablish, modify, and terminate multimedia sessions that works independentlyof underlying transport protocols and without dependency on the type of session that is beingestablished.
Transaction
Each transaction consists of a request that invokes aparticular method, or function, on the server and at least one response.
SIP Layers
Syntax and Encoding Layer
Transport Layer --defines how a client sends requestand receives responses, and how a server receives request and sends response.
Transaction Layer --A transaction is a request sent bya client transaction (using the transport layer) to a server transaction, alongwith all responses to that request sent from the server transaction back to theclient.
TU -- The layerabove the transaction layer is called the transaction user
Dialog
A dialog is a peer-to-peer SIP relationship betweentwo user agents that persists for some time. The dialog facilitates sequencingof messages and proper routing of requests between the user agents. The INVITEmethod is the only way defined to establish a dialog.
B2BUA
A back-to-back user agent (B2BUA) is a logical entitythat receives a request and processes it as a user agent server (UAS). In orderto determine how the request should be answered, it acts as a user agent client(UAC) and generates requests. Unlike a proxy server, it maintains dialog stateand must participate in all requests sent on the dialogs it has established.
Downstream:
A direction of message forwarding within a transactionthat refers to the direction that requests flow from the user agent client touser agent server.
Upstream
Customer first, so client is up.
Invitee, Callee
same with Called Party
Ringback:
Ringback is the signaling tone produced by the callingparty’s application indicating that a called party is being alerted (ringing).
Characteristic
SIP does not provide services. Rather, SIP providesprimitives that can be used to implement different services.
Primitives
180 Ringing
200 OK.
re-INVITE -- change characteristics of sessions.
ACK --response 200 OK(during session accepted, not sessionterminated) with ACK
REGISTER
CANCEL --cancel a pending request
OPTIONS --used for determining the capabilities of a UA.
INVITE -- The INVITE method is the only way defined toestablish a dialog.
Headers
VIA/Contact
Via header field tells other elements where to send theresponse, the Contact header field tells other elements where to send futurerequests.
Record-Route
if the a proxy server wished to remain in the SIP messagingpath beyond the initial INVITE, it would add to the INVITE a required routingheader field known as Record-Route that contained a URI resolving to thehostname or IP address of the proxy.
Params
branch
Used to identify a transaction
Network Element
stateless/statefull proxy server
A proxy server receives SIP requests and forwards them onbehalf of the requestor.
registrar
UAS that handles a REGISTER is given the special name
location server
UAC
UAS
Redirect Server:
A redirect server is a user agent server that generates 3xxresponses to requests it receives, directing the client to contact an alternateset of URIs.
Related Spec
RTP RFC1889
RSTP RFC2326
MEGACO RFC2326
SDP RFC2327
SIP RFC3261
IPV4/IPV6
SIPp