Subject: Question about availability of SIP Servers and DNS SRV RRs

From: Sudipto Mukherjee <[email-masked]>
Date: Fri, 03 Sep 1999 10:54:12 -0400

Refering to the Section D of the SIP RFC2543 - "Using SRV DNS Records"

" A client may cache a successful DNS query result. A successful
query is one which contained records in the answer and a server
was contacted at one of the addresses from the answer. When the
client wishes to send a request to the same host, it starts the
search as if it had just received this answer from the name
server. The server uses the procedure specified in RFC1035 regarding
cache invalidation when the time-to-live of the DNS result
expires."

This scenario is applicable for UDP based SIP UA -

My question is, say for TTL value of 3600 (1 Hour), DNS Server
caches in the results. However during this time, one or all
the SIP (Redirect or Proxy) servers goes down. During a SIP Call,
the SIP UAC might end up selecting the server which is down.

This shall result in timing out after 6 retries of INVITE,
before  trying another SIP server. This might not be
acceptable as the call setup times increase.
The calls using the OOS server will continue to have high
call setup time, till the DNS entries gets refreshed.

Setting to TTL to ZERO which prevents caching of entries, might help
but has performance issues.

Is there any means to purge a DNS Resource Record (RR) from the
DNS Server, for a Server which has gone Out Of Service (OOS) ?
Is it possible to refresh the DNS cache before the TTL has expired ?

This will help because the SIP UAC will not select the OOS
SIP Server.

Thanks
Sudipto