[pjsip] SCTP advice request

Net Dabbler netdabbler at gmail.com
Thu Apr 10 21:21:52 EDT 2008

Hi all,

We have been trying to interface PJSIP with the University of Essen's SCTP
implementation.  Can anyone offer advice on our approach?

In order to add SCTP to the SIP side of things we have copied the
functionality of sip_transport_tcp.c to sip_transport_sctp.c, renamed all
references to tcp in the new file to sctp, and specified IPPROTO_SCTP in all
the sock_socket calls.

SCTP is then initialised by calling pjsip_sctp_transport_start.

To decide which type of socket to create, sock_bsd.c checks the protocol
(TCP/SCTP) and calls either the system [Linux OpenSuse 10.3] or SCTP
library. The socket fd is kept in a table so that future calls to socket
methods are pointed to the appropriate library. Luckily, the SCTP library
uses socket fd values that start at 1023 and count down while the system
starts low and counts up.

This seems to work well so far, but of course the media side is still using
the old library because it doesn't use the transport manager setup on the
SIP side.

Because the media side is less modular than the SIP side - and in
particular, there is no TCP code we can simply duplicate and alter - we are
thinking of changing the media socket calls from UDP-datagram to
SCTP-seqpacket. Our hope is that this will work transparently for existing

All of this is aimed at getting standard PJSIP applications to run using the
SCTP library. We have yet to consider what problems we will encounter when
we try to incorporate multihoming using bindx etc.

What we would like to know is: does anyone (dis)agree with the approach we
have taken, if anyone can foresee problems we may encounter, and indeed if
anyone has any advice on how to proceed?

One other small matter you could also help us with is as follows. The system
and SCTP socket fds are not interchangeable. For this reason, whenever a
socket call is made, we first check the type of socket in our table before
despatching it. This is done in sock_bsd, which handles most socket calls.
Unfortunately there are other socket calls that bypass sock_bsd. ioctl is
one. We have replaced each occurrence of ioctl in PJSIP with a test for
socket type before allowing it to proceed. Our problem is that we do not
have control over third party libraries bundled with PJSIP.

The question is: do these third party libraries (Speex etc) have access to
the socket fd?

If they have access we cannot guarantee that the system library will not be
called with an SCTP fd.

Net Dabbler
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