Measurement-Based Analysis of Head-of-Line Blocking for SIP over TCP

The Transmission Control Protocol (TCP) is a transport protocol which delivers packets strictly in order. For signaling protocols which send multiple independent requests within one single TCP session, the loss of a TCP packet may therefore lead to increased delay for later signaling requests. This phenomenon is known as Head-of-Line Blocking (HoLB) and for instance has led to the standardization of the Stream Control Transmission Protocol (SCTP) as a transport protocol for SS7 over IP. In this paper, we will discuss the effects of HoLB for another signaling protocol, i.e. the Session Initiation Protocol (SIP). We introduce a two layer measurement methodology which allows differentiating several types of delay (e.g. due to congestion control, transmission and HoLB), and discuss the resulting correlations between delay, loss and HoLB duration for TCP.