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Sip Interview Question's Tat I Could Not Answer


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#1 karthikeyan2win

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Posted 22 August 2007 - 01:31 PM

Hi everybody,

In My recent interview i had qns which i have no clue of, any help or links would be appreciated :wacko:

1. What are Branch ID and Tag ID in SIP
2. How does SIP support Caller ID
3. How do we configure SIP Proxy Server
4. How do SIP recognize firewall

thanks in advance
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#2 karthikeyan2win

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Posted 26 August 2007 - 01:38 AM

no has got any clue abt these qns :unsure:
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#3 jhine

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Posted 26 August 2007 - 06:23 AM

no has got any clue abt these qns :unsure:


Some of your questions can be found here:
http://www.egyed.com/faq/sip_faq.html

Most of the people in this section is concentrated on cisco ccvp cert topics.
Unfortunately, SIP topic is not that in depth when it comes to CCVP. More information on SIP is outside of Cisco.com site also.
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#4 karthikeyan2win

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Posted 30 August 2007 - 09:54 AM

thanks for the link
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#5 Shweta

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Posted 25 April 2013 - 04:28 AM

I know its difficult to find good answers when it comes to SIP. I was also facing the same issue but recently I came through this site http://www.code2comp...-questions.html. It has some basic info regarding SIP.


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#6 Darby Weaver

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Posted 04 May 2013 - 02:20 AM

Hi everybody,

In My recent interview i had qns which i have no clue of, any help or links would be appreciated wacko.gif

1. What are Branch ID and Tag ID in SIP
2. How does SIP support Caller ID
3. How do we configure SIP Proxy Server
4. How do SIP recognize firewall

thanks in advance

 

Hmmm...

 

1. The branch parameter on a Via header needs to start with the magic cookie value of z9hG4bK and must also be unique to avoid the request getting classified as a duplicate.  It is a value used to help perform loop avoidance. 

 

Branch IDs allow proxies to match responses to forked requests. Without them, a proxy wouldn't be able to tell which branch a response corresponds to. Tags, in To headers, are of no help here since they are not known until responses arrive. Tags are used by the UAC to distinguish multiple final responses from different UAS.

 

A UAS has no reliable way of determining if the request has been forked or not. Proxies only insert tags into the final responses they generate themselves; they never insert tags into requests or responses they forward.

 

Since a request can be forked several times on its way to UAS, a single "tag" added to the request by one of the proxies is not sufficient for the next forking proxy along the chain to match responses on its own branches; every proxy that forked the request would need to add its own unique IDs to the branches it created. This is precisely what's being achieved by the branch parameter in the Via header.

 

2. Caller-ID is provided by the From SIP header containing the caller's name and "number". The number would most likely be placed in the user field of a SIP URL or appear in a tel: URL.

 

Since the callee generally does not know or trust the callee's server, only cryptographic signatures can be used to ensure that the information is valid. For example, the outgoing proxy might be operated by an ISP, enterprise or phone company and sign for the identity of the caller, using the signedby parameter, with the identity of the company verified by a public key certificate similar to those used by web sites.

 

3. http://www.cisco.com...ide/config.html - Know the major components involved and know you look up the rest if needed.

 

4. There are several possible approaches to SIP-capable firewalls. One of the difficulties is that, unlike for, say, HTTP, connections are originated both by hosts inside and outside the firewall. A likely arrangement is that a SIP proxy sits "on" the firewall and relays SIP requests between the Internet and the intranet. Thix proxy would also open up the necessary ports in the firewall to let audio and video flow through.

 

As an alternative, if a firewall or NAT allows outgoing TCP connections, the inside client can open up a TCP connection to an outside proxy. All outgoing and incoming calls would then be handled by that TCP connection. (The client would still have to use SOCKS or similar mechanism to convince the firewall to let RTP packets through.)

 

Reference: http://web.archive.o...q.html#firewall

 

Bottom line is the interviewer prepared the questions using a google search and probably does not know the answers too well themselves.

 

Just a guess...

 

Darby Weaver


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