Dialogic 6.2 manual Contact, November 250

	Understanding the SIP Protocol
Contact	Contains a SIP or SIPS URI that represents a direct route to contact
	Caller A, usually composed of a username at a fully qualified domain
	name (FQDN). While an FQDN is preferred, many end systems do
	not have registered domain names, so IP addresses are permitted.
	While the Via header field tells other elements where to send the
	response, the Contact header field tells other elements where to send
	future requests.
Max-Forwards	Serves to limit the number of hops a request can make on the way to
	its destination. It consists of an integer that is decremented by one at
	each hop.
Content-Type	Contains a description of the message body (not shown).
Content-Length	Contains an octet (byte) count of the message body.
	The details of the session, such as the type of media, codec, or
	sampling rate, are not described using SIP. Rather, the body of a SIP
	message contains a description of the session, encoded in some other
	protocol format. One such format is the Session Description Protocol
	(SDP) (RFC 2327 [1]). This SDP message (not shown in the example)
	is carried by the SIP message in a way that is analogous to a
	document attachment being carried by an email message, or a web
	page being carried in an HTTP message.
	Since the softphone does not know the location of Caller B or the SIP
	server in the biloxi.com domain, the softphone sends the INVITE to
	the SIP server that serves Caller A's domain, atlanta.com. The
	address of the atlanta.com SIP server could have been configured in
	Caller A's softphone, or it could have been discovered by the
	Dynamic Host Configuration Protocol (DHCP), for example.
	The atlanta.com SIP server is a type of SIP server known as a proxy
	server. A proxy server receives SIP requests and forwards them on
	behalf of the requestor. In this example, the proxy server receives
	the INVITE request and sends a 100 (Trying) response back to
	Caller A's softphone. The 100 (Trying) response indicates that the
	INVITE has been received and that the proxy is working on
	Caller A’s behalf to route the INVITE to the destination. Responses
	in SIP use a three-digit code followed by a descriptive phrase. This
	response contains the same To, From, Call-ID, CSeq and branch
	parameter in the Via as the INVITE, which allows Caller A's
	softphone to correlate this response to the sent INVITE. The
	atlanta.com proxy server locates the proxy server at biloxi.com,
	possibly by performing a particular type of DNS (Domain Name
	Service) lookup to find the SIP server that serves the biloxi.com
	domain. This is described in [4]. As a result, it obtains the IP address

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