Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 343)

Brocade Virtual ADX Server Load Balancing Guide 327
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SIP overview
7
SIP message headers
This section describes SIP message headers that you might find useful when making decisions
about SIP server load balancing.
Call-ID
The Call-ID is a header field that appears in all SIP requests and responses. This header field acts
as a unique identifier to group together all messages belonging to the same call. It must be the
same for all requests and responses sent by either the UAC or UAS in a dialog.
Call-ID is generated by the combination of a random string and the host name or IP address of a
particular UAC. There is no length restriction on Call-ID. In the first implementation, a real server is
selected based on the hash value of Call ID (stateless mode) or the value of Call ID (stateful mode).
• Record-Route: The Record-Route header field is inserted by a proxy in a request to force future
requests in the dialog to be routed through the proxy; for example, Record-Route: server10.example1.com; 1r>
• From: The From header field indicates the logical identity of the initiator of the request. It
contains a URI and, optionally, a display name. This field must contain a "tag" parameter,
chosen by the UAC.
IP addresses of the host on which the UAC is running should not be used as FROM URIs, as
these are not logical names.
• To: The To header field specifies the desired logical recipient of the request. This might not be
the ultimate recipient of the request. Normally, the initial To field is set to be the value of the
Request-URI. One exception is the REGISTER method.
• Via: The Via header field indicates the path taken by the request so far and indicates the path
that should be followed in routing responses. A Via header field value contains the transport
protocol used to send the message, the client's host name or network address, and possibly
the port number at which it wishes to receive responses. It is a mandatory field for the UAC or
UAS SIP proxies, and guarantees that the responses traverse through the same route as the
requests.
The branch ID parameter in the Via header field value serves as a transaction identifier, and is
used by proxies to detect loops.
• Max-Forwards: The Max-Forwards header field must be used with any SIP method to limit the
number of proxies or gateways that can forward the request to the next downstream server.
The Max-Forwards value is an integer from 1 through 255 indicating the remaining number of
times that a request message is allowed to be forwarded. The recommended initial value is 70.