Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) User Manual
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Brocade Virtual ADX Server Load Balancing Guide
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SIP overview
7
SIP is based on an HTTP-like request-and-response transaction model. Each transaction consists of
a request that invokes a particular method, or function, on the server, and at least one response. In
this example, the transaction begins with User1's SIP phone sending an INVITE request addressed
to User2's SIP URI. The INVITE request contains a number of header fields. The fields present in an
INVITE request include a unique identifier for the call (Call-ID), the destination address, User1's
address, and information about the type of session that User1 wishes to establish with User2. The
INVITE (message F1 in
) would look like the following example:
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP pcuser1.brocade.com;branch=dkDKdkDKdkDK1111
Max-Forwards: 50
To: User2
From: User1
Call-ID: [email protected]
CSeq: 123456 INVITE
Contact:
Content-Type: application/sdp
Content-Length: 142
Because User1's SIP phone does not know the location of User2's SIP phone, it sends the INVITE
message to the SIP proxy server that is serving the brocade.com domain. The address of the
brocade.com proxy server is known to the SIP phone through static configuration or through
Dynamic Host Configuration Protocol (DHCP). The proxy server receives the INVITE request and
sends a 100 (Trying) response back to User1's SIP phone. This response contains the same To,
From, Call-ID, CSeq, and branch parameter in the Via field as the INVITE, which allows User1's SIP
phone to correlate this response to the previously sent INVITE. The proxy server consults a
database, generally called a location service, that contains the current IP address of User2. It then
forwards (or proxies) the INVITE request there. Before forwarding the request, the proxy server adds
an additional Via header field value with its own address (the INVITE already contains User1's
address in the first Via field).
User2's SIP phone receives the INVITE and alerts User2 of the incoming call from User1; that is,
User2's phone rings. User2's SIP phone indicates this by a 180 (Ringing) response, which is routed
back through the SIP proxy server in the reverse direction. When User1's SIP phone receives the
180 (Ringing) response, it passes this information to User1, using an audio ringback tone.
If User2 decides to answer the call (User2 picks up the handset), the SIP phone sends a 200 OK
response to indicate that the call has been answered. The 200 OK contains the Via, To, From,
Call-ID, and CSeq header fields that are copied from the INVITE request, and a message body with
the Session Description Protocol (SDP) media description of the type of session that User2 is willing
to establish with User1. The 200 OK (message F6 in
) would look like the following
example.
SIP/2.0 200 OK
Via: SIP/2.0/UDP pcproxy.brocade.com
;branch= dkDKdkDKdkDK2222;received=10.1.1.2
Via: SIP/2.0/UDP pcuser1.brocade.com
;branch= dkDKdkDKdkDK1111;received=10.1.1.1
To: User2
From: User1
Call-ID: [email protected]
CSeq: 123456 INVITE
Contact:
Content-Type: application/sdp
Content-Length: 131