SRV Question

[Chris Thompson]

On Feb 11 2009, Johnny wrote:

>_sip._udp.as.host.com       as1.host.com   0 1 5060
>_sip._udp.as.host.com       as2.host.com   0 0 5060

That's the wrong syntax. I'll assume you mean

_sip._udp.as.host.com. SRV 0 1 5060 as1.host.com.
_sip._udp.as.host.com. SRV 0 0 5060 as2.host.com.

>Since "as1" has a higher weight, it will always be picked first by the
>SRV aware client?

Not because it has a higher weight, but because it has an *infinitely*
higher weight.

>I want the devices to ALWAYS go to "as1" and only go to "as2" if "as1"
>is not available.
>
>Do the above lines accomplish that?

Yes, but in a perverse way, IMNSHO. It's more natural to use priorities
rather than weights, if that's what you want:

_sip._udp.as.host.com. SRV 0 100 5060 as1.host.com.
_sip._udp.as.host.com. SRV 1 100 5060 as2.host.com.

That defines a strict ordering. If you had

_sip._udp.as.host.com. SRV 0 2 5060 as1.host.com.
_sip._udp.as.host.com. SRV 0 1 5060 as2.host.com.

then a client is meant to try as1 2/3 of the time, and as2 1/3 of
the time. The intended consequences of using a zero weight value
are spelled out in detail in RFC 2782, but there is no need to
rely on them.

-- 
Chris Thompson
Email: cet1 at cam.ac.uk