how to make SIP protocol more reliable using UDP

Question

Actually We are doing thesis work where we need to make 10 voip phones which are SIP based connected with each other.So they can call and talk among each other.Also we want to add video calls access.Another question is it possible video calls on SIP.

Answer 1

To make it reliable you need to emulate the following two features:

For Calls

1. You need to sequence the packets.
2. One end needs to tell the other end that a sequenced packet is missing if this happens, and you probably want take jitter into account -- i.e., wait a small amount of time before you request a missing packet.

For protocol commands

1. You need to ackknowledge command packets -- if a command is not acknowledged it has to be sent again.

Answer 2

SIP already has built in reliability measures, most of which are specifically to cope with unreliable transports such as UDP. You should read the section in the SIP RFC on Transactions to gain an understanding of how it works. One aspect missing from the SIP RFC is reliability for provisional responses and the supplementary RFC3262 deals with that.

SIP is agnostic to the type of sessions, such as voice or video, it sets up so yes it can be used to set up video calls. There are heaps of readily available SIP softphones around that already provide video, one example being x-lite.