Subject: [SIP] How to ping Prxoy Server ?

From: "rafi" <[email-masked]>
Date: Tue, 20 Jun 2000 14:51:07 +0530

Hi,

                  Consider a  UAC which  uses a default proxy server
(say proxy 1)  whenever it has to send SIP request. It may so happen
that though transport connection is available at server where proxy is
suppose to run but proxy  may not  be responding due to
some reason ( may be due to temporary failure).  UAC keeps retransmitting
 the request for a given number of time and when it decides to give up it 
sends  a CANCEL  message and then try other proxy server. Thus whenever
 a new SIP request is made UAC  first tries default proxy server  and  then everything 
 repeats once again. This may create some problem  because it increase the call set
 up time ( in worst case it may happen at every intermediate proxy server). Is there 
any method/way available in SIP using which status of proxy server can be known in advance ?.
 This  information helps the UAC in keeping track current active list of default 
proxy server(s).

Thank You
Rafi Assadi H.M.
Silicon Automation Sysytems
Bangalore, INDIA

---

From: "Vijay K. Gurbani" <[email-masked]>
Date: Tue, 20 Jun 2000 09:38:42 -0500

> rafi wrote:
> some reason ( may be due to temporary failure). UAC keeps 
> retransmitting the request for a given number of time and when it 
> decides to give up it sends  a CANCEL  message and then try other 
> proxy server. Thus whenever  a new SIP request is made UAC  first 
> tries default proxy server  and  then everything repeats once again. 
> This may create some problem  because it increase the call set
>  up time ( in worst case it may happen at every intermediate proxy 
> server). Is there any method/way available in SIP using which status 
> of proxy server can be known in advance ?.  This  information helps 
> the UAC in keeping track current active list of default proxy 
> server(s).

It appears that you are talking of some heart-beat mechanism; clearly,
if you bind your UDP socket to the destination address, the second
write should return ECONNREFUSED (see bis, Section 1.4.2).  On TCP
sockets, the connect() will fail.  However, it appears that you are
assuming that the transport is available between a UAC and the outbound
proxy, just that the outbound proxy is "in a wierd state" (for the lack
of a better description).

SIP does not talk about any such heart-beat mechanisms to detect the
health of a proxy from a UAC.  You can build such a mechanism into your
proxy and UAC, but it will only be useful within your domain.  If all
proxies in the world supported heart-beat across domains, I would 
presume that the resulting traffic would be something to contend with.
Not to mention security.  If a proxy in domain A wants to heart-beat a
proxy in domain B, how does it authenticate itself?  Is the heart-beat
message encrypted?  You run into a set formidable problems once you go
down this path.

Note that TCP provides a heart-beat mechanism via the SO_KEEPALIVE sock-
et option.  When this option is set for a TCP socket and no data has
been exchanged across the socket (in either direction) for 2 hours, TCP
sends out a probe to its peer, to which the peer must respond.  However,
this is of no help to your case unless your SIP transactions last for
such a long duration.

- vijay
-- 
Vijay K. Gurbani  [email-masked] [email-masked] [email-masked]
IN Architecture and Internet Software Group
Lucent Technologies/Bell Labs Innovations, 263 Shuman Blvd, Rm 1A-418
Naperville, Illinois 60566  Voice: +1 630 224 0216 Fax: +1 630 713 0184

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