[VoiceOps] acme nat traversal clue

milosz mewash at gmail.com
Fri Oct 2 19:08:37 EDT 2009

hi guys,

need a bit of help... have never touched an acme before so i'm not sure what
to tell the people i am dealing with, and they seem incapable of resolving
the problem themselves.  trying to explain the difference between layer 3
and layer 7 is not working.

setup: i have an ip pbx with a single interface at, and a
netscreen 50 with a static routable ip natted to the pbx at (sip alg
is off).  my itsp has an acme at

this is what an inbound call looks like right now: -> invite sip:xxxxxx at <sip%3Axxxxxx at> -> 100 trying -> 200 ok (contact:
sip:xxxxxx at<sip%3Axxxxxx at>
) -> ack sip:xxxxxx at <sip%3Axxxxxx at>

so obviously it breaks at this point.

according to them, they are unable to configure the acme to respond to the
originating routable ip instead of the nonroutable ip in the sip contact.

this is what someone at acme told them:

It’s not a configuration issue, the endpoint at has set it’s contact
header address to in it’s 200 OK response back to (SBC).
The SD will look at the Contact header in order to determine where to send
future in-dialog requests such as ACK, BYE, Re-INVITEs etc (according to
RFC3261). Please can you check why would be doing so?

this sounds irrelevant to me, since any sip endpoint will set its contact
header address to its own ip.  sounds like this guy is saying "well it's
natted so that won't work, tell them to fix it."  i certainly can't
configure my pbx to respond with a modified contact header.

can anyone provide some guidance here?  or maybe i am just wrong and it is
impossible to make this work.  but it looks like a basic hnt situation to


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