Hello Gurminder, To add to the answer, Bob's "line" might have some services enabled which need help from a proxy. His IP address might be behind a firewall, which a trusted proxy can get to. Hence the service provider would proxies to act as the go-between.
One goal of using DNS is to locate the proxy host (one or more if proxies are in load share mode, see description of DNS SRV records) given a SIP URI (sip:[EMAIL PROTECTED] in your instance). The parameters obtained as a result of such a request are the host, protocol (e.g. TCP/UDP) and port number of the proxy that can handle the request, and the preference of which proxy to use. In some cases, sip URI rules exist that can help you deduce the destination without going through a DNS, using defaults for protocol and port number. Regards, Kedar On 5/23/06, Nataraju Basavaraju <[EMAIL PROTECTED]> wrote: > > comments inline... > > Regards, > Nataraju A.B. > > -----Original Message----- > From: [EMAIL PROTECTED] on behalf of Gurminder > Sent: Tue 5/23/2006 12:30 AM > To: [email protected] > Subject: [Sip-implementors] Rfc 3261 ch 4 > > Hi all, > > I have a small doubt ... > > As I was reading ch 4 in sip rfc.... > There it is said that first time Alice sends INVITE to Bob it will go > to its proxy(atlanta.com) which will forward it to Bob's > proxy(biloxy.com) which will in turn forward to Bob's phone.. > After this initial transaction both Alice and Bob will be able to > communicate directly because > in initial transaction they have also attached locations where they can > be contacted directly. > > > All said done..... > So the job of Alice's proxy is to make DNS lookup and find the > Bob's Domain's ip address.... > If Bob and Alice are able to communicate directly then obviously they > have public Ips > So when Alice's proxy did DNS lookup why did it performed for Bob's > proxy and not Bob's IP.. > If Bob has a public Ip why it needs to distribute its Sip ISP's domain. > > [ABN] DNS would be performed on Bob's domain name, not on BOb's IP > address. For better routing its preferred to use the domain names in all the > URI entries. > > Note: every user may not have the public IP allocated, in that case its > preferred to use that domain names, so that we can have hierarchial routing. > Hence Alice's proxy applies DNS query on Bob's domain name i.e., " > biloxi.com" > > One reason I thought of is may be Bob's IP is not registered to a > domain name and therefore Alice's proxy did DNS lookup for proxy and > not direct IP. > > [ABN] if we already have got a IP address in the incoming message, then we > will not perform DNS to get the IP address, but might to get the port > number. > > So that's it.......... > Give it your shots..... > > > Guri > > _______________________________________________ > Sip-implementors mailing list > [email protected] > https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors > > > The information contained in this message may be confidential to Kodiak > Networks, Inc. and its subsidiaries and protected from disclosure. If this > message did not reach the intended recipient, or an employee or agent > responsible for delivering it to the intended recipient, you are hereby > informed that any distribution or copying of this communication is > prohibited. If you have received this communication in error, please notify > us immediately by replying to the sender of the message and then delete the > message. Thank you. > _______________________________________________ > Sip-implementors mailing list > [email protected] > https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors > _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
