One Server Per City: Using TCP for Very Large SIP Servers

The transport protocol for SIP can be chosen based on the requirements of services and network conditions. How does the choice of TCP affect the scalability and performance compared to UDP? We experimentally analyze the impact of using TCP as a transport protocol for a SIP server. We first investigate scalability of a TCP echo server, then compare performance of a SIP registrar server for two TCP connection lifetimes: transaction and persistent. Our results show that a Linux machine can establish 400,000+ TCP connections and maintaining connections does not affect the transaction response time. This is applicable to other servers with very large TCP connection counts. Additionally, the transaction response times using the two TCP connection lifetimes and UDP show no significant difference at 2,500 registration requests/second in our SIP server implementation. However, sustainable request rate is lower for TCP than for UDP, since using TCP requires more message processing, which causes longer delays at the thread queue for the server implementing a thread-pool model. Finally, we suggest how to reduce the impact of TCP for a scalable SIP server especially under overload