Transport Layer Loss Differentiation and Loss Notification

It is a well known problem that errors and losses caused by unfavorable wireless link conditions cause problems for congestion controlled transport protocols such as TCP. TCP assumes that all losses occur due to network congestion. To remedy the perceived congestion, congestion avoidance is employed, typically by the TCP sender halving the sending rate. When congestion avoidance is incorrectly applied also for wireless losses, performance is degraded. Protocols with similar congestion control, such as the Stream Control Transfer Protocol (SCTP) [11], share the problem. Many TCP-specific solutions have been proposed to improve the performance in a wireless environment. Examples include using split connections [1], TCP-aware local retransmissions (snoop) [2] or loss differentiation at the transport layer. Recent work addressing the problem of wireless over TCP include TCP Westwood [4], TCP Real [12] and WTCP [10]. Instead of applying TCPspecific solutions, the problem can be circumvented by using physical and link-layer solutions such as forward error correction (FEC) and/or link level retransmissions. This approach is used in many operational wireless networks. As perceived by the transport layer the unreliability of the wireless medium is now transformed into round-trip time variations caused by link-layer retransmissions. In some instances these variations can cause spurious timeouts, and subsequent unnecessary retransmissions. The TCP Eifel algorithm [8] has been proposed as a solution to improve performance for these cases. However, there are cases where link level retransmissions are unsuitable. These include interactive real-time applications, for example VoIP or video-conferencing. Due to their delay sensitivity, this type of applications does generally not use TCP but instead use UDP. One of the major problems with UDP is that it has no congestion control that ensures that it shares networks resources fairly. The new DCCP protocol [6] address this problem by providing a congestion controlled but unreliable transport service. However, this again introduces the discussed problem of inappropriate congestion behavior in the presence of non-congestion losses such as caused by wireless errors. In order to successfully perform congestion control the reason of a packet loss must be ascertained so that congestion avoidance is not performed for losses not caused by congestion. In the following, we use the term loss differentiation to mean the process of deciding the cause of a lost packet. Often the loss differentiation is performed at the receiver side and the congestion control at the sender side. This calls for the use of loss notification, i.e. the process of communicating the results of loss differentiation to the sender side. In this paper we present a simple checksumbased approach for loss differentiation together with two loss notification schemes in the context of TCP.