Design, Implementation, and Performance of a Load Balancer for SIP Server Clusters

This paper introduces several novel load-balancing algorithms for distributing Session Initiation Protocol (SIP) requests to a cluster of SIP servers. Our load balancer improves both throughput and response time versus a single node while exposing a single interface to external clients. We present the design, implementation, and evaluation of our system using a cluster of Intel x86 machines running Linux. We compare our algorithms to several well-known approaches and present scalability results for up to 10 nodes.

Our best algorithm, Transaction Least-Work-Left (TLWL), achieves its performance by integrating several features: knowledge of the SIP protocol, dynamic estimates of back-end server load, distinguishing transactions from calls, recognizing variability in call length, and exploiting differences in processing costs for different SIP transactions. By combining these features, our algorithm provides finer-grained load balancing than standard approaches, resulting in throughput improvements of up to 24% and response-time improvements of up to two orders of magnitude. We present a detailed analysis of occupancy to show how our algorithms significantly reduce response time.

Existing System:

THE SESSION Initiation Protocol (SIP) is a general-purpose signaling protocol used to control various types of media sessions. SIP is a protocol of growing importance, with uses in Voice over IP (VoIP), instant messaging, IPTV, voice conferencing, and video conferencing. Wireless providers are standardizing on SIP as the basis for the IP Multimedia System (IMS) standard for the Third Generation Partnership Project (3GPP). Third-party VoIP providers use SIP (e.g., Vonage, Gizmo), as do digital voice offerings from existing legacy telecommunications companies (telcos) (e.g., AT&T, Verizon) as well as their cable competitors (e.g., Comcast, Time-Warner).

Proposed System:

SIP is a transaction-based protocol designed to establish and tear down media sessions, frequently referred to as calls.

The session-oriented nature of SIP has important implications for load balancing.

Transactions corresponding to the same call must be routed to the same server; otherwise, the server will not recognize the call. Session-aware request assignment (SARA) is the process where a system assigns requests to servers such that sessions are properly recognized by that server, and subsequent requests corresponding to that same session are assigned to the same server.

We introduce new algorithms that outperform existing ones. Our work is relevant not just to SIP, but also for other systems where it is advantageous for the load balancer to maintain sessions in which requests corresponding to the same session are sent by the load balancer to the same server.


  • Session Initiation Protocol
  • User agents
  • Transaction-Least-Work-Left
  • Load balancer

Tools Used:

Front End : Java