[SIPForum-discussion] Invalid SDP line (no '=' delimiter) error !

[Jeff Poole]

Badri,

You clearly have a SIP ALG between the two devices (hence why the private
IP on the c= line was rewritten to a public one).  In my experience, they
are all terrible.  For an example, the SIP ALG in the Cisco/Comcast DPC3939
(a pretty common home router) will (silently) drop any SIP packet with a
hostname instead of an IP address in the o= line of the SDP (presumably,
because it doesn't know how to replace that).  So the best option would be
to turn off the SIP ALG functionality in whatever device is doing that, if
at all possible.  The next best option is to use something like SIPS so it
can't get into the encrypted packets to cause problems (sometimes just
using an alternate port for SIP can trick those devices).

The last option would be to modify your SDP to be less "confusing" and the
most obvious change there would be to change "m=audio 5000/1 RTP/AVP 8 101"
to "m=audio 5000 RTP/AVP 8 101", since the people who develop SIP ALGs seem
to do so by looking at some captured packets instead of actually reading
the RFCs, and the "number of ports" extension is not terribly common in the
real world (also, as it mauled your SDP, it dropped the /1, which tells me
that's likely part of the confusion).

Good luck...

Jeff


On Thu, May 28, 2015 at 7:16 AM, Badri Ranganathan <badri at arcatech.com>
wrote:

> Hi all,
>
>
>
> I am answering a SIP call made to my softphone. The softphone rings and
> also sends a 200 OK correctly. But the other end that made the call is
> seeing a failure in the SDP of the 200 OK packet.
>
>
>
> This error is seen in SDP header => Invalid SDP line (no '=' delimiter) error
>
>
>
> w.r.t w_frame4_public_ip.pcap It looks like the failure as seen in
> wireshark is because  line “audio 5000 RTP/AVP 8 101” should really be “m=
> audio 5000 RTP/AVP 8 101”
>
> so it doesn’t see the ‘=’ delimiter.
>
>
>
> But if you look at w_frame52_local_ip.pcap, the packets sent from the
> local IP has the contents correct.
>
>
>
> It is only after passing the public IP and at the network premises, that
> this failure is reported by the Session Border controller.
>
>
>
> I am not able to understand why a correctly formed packet sent out is seen
> “corrupted” at the network end.
>
>
>
> Any ideas really appreciated.
>
>
>
> Thanks,
>
> Badri.
>
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