Re: [Serusers] FCP support in SER: Modifying SDP - sr-users

24 Feb 2003

Thanks Jiri,

I'll have a look at the CVS version. I'm sure there will be more hacking to do
only to make it compile.... :(

However, got the replacement of the "c=.." part working. I'll test this as
soon
as I can.

Jaime

Jiri Kuthan &lt;jiri(a)iptel.org&gt; on 24/02/2003 13:20:00

To:   Jaime GILL/EN/HTLUK@HTLUK
      Nils Ohlmeier &lt;nils(a)ohlmeier.de&gt;
cc:   Jan Janak &lt;J.Janak(a)sh.cvut.cz&gt;
      serusers(a)lists.iptel.org

Subject:  Re: [Serusers] FCP support in SER: Modifying SDP

At 12:50 PM 2/24/2003, jaime.gill(a)orange.co.uk wrote:
[...]
...
 It think the problem is in the replacement of the SDP
information. The first
occurrence of the IP address in "v= " and the port in "m= " in the SDP
get
replaced, but the second IP in "c=" is not. 
nit: it's not the "v=" line, but "o=" ('owner') line which
you are replacing.
However, you are not probably worried so much about this one -- it maintains
primarily a (not widely utilizied) identification purpose. All "c=" occurences
do matter. (In addition to port numbers in "m=" lines.)

[...]

...
 I have been trying to understand how the proxy builds
the forwarded message from
...
 the old one, and realised that for the Via replacement
(or adding of more
params), I need to be using a string called add_to_branch_s and
add_to_branch_len (so ignore the replace_via implementation in the current
tar.gz). 
I suggest you used the mhomed option (available only on CVS). The issue is
you need to print the correct IP address in Via on multihomed host. With
mhomed enabled, IP routing is utilized to determine the right IP address.
Let me know if you need something more for getting Via right.

...
 But for the SDP, whenever I work with get_body, it does
not modify it
appropriately. So currently, I'm using msg->orig to get to the initial message,
search for certain IP4 and audio strings and replace them with the information
provided by the fcp server. That means, in the case of the SDP, 2 IP address
replacements (in v=.. and c=..) and 1 port replacement (in m=..). As I mentioned
...
 before, I only manged to change the v=.. and m=...
Whenever I try to replace
more than one appearance, strange things happen, like strings in non expected
places, like Via, and cannot work out why. So my question is an open one:): what
...
 is the best way to change the SDP part? 
I suggest here too -- use the CVS version. It has departed from the use of
the buffers (orig and buf) -- we have now just one buffer (buf) without any
zero termination. Previously, the two buffers and 0-termination caused lot
of issues, some of them possibly annoying you right now. Look at the
textops/replace_all action (only on CVS too) to see how to replace multiple
occurences of a string in SIP messages. (Caution: you will eventually need
to calculate new SDP body size and change content-length too.)

...
 The other of my questions is whether all this mess with
NAT's will get solved
when the proxy supports TCP, 
The major problem is media, which will keep using UDP.

...
 and whether this is the best approach to solve the
SIP through NAT/FW problem. 
As all NAT traversal methods -- none of them is perfect, each has cons and pros.
The benefit of FCP is that once fcpd works, maintenance of the SIP code is
easier
in user space. Also, you can better couple your pinhole policy with SER's
SIP-layer policy.

...
 For example, how about a nathelper module for
netfilter/iptables that gets this working, in the same manner as IRC or ftp
currently? Does anybody know about any work progressing this for linux/FreeBSD? 
I'm not aware of such. There is Billy Biggs masquerading module, but it is
pretty old and no longer maintained.

-Jiri

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