Explicit and Implicit Loss Notification for Error-prone Links
Johan Garcia · 2003
Abstract In certain wireless environments the common transport protocol assump-tion that all packet losses are due to congestion may not hold true. If transport proto-cols react identically to both congestion and non-congestion related losses, perfor-mance will degrade. To avoid this performance problem, transport protocols must be able to differentiate between losses that are due to congestion and losses that are due to wireless link errors. Various loss differentiation techniques exist, some based on the sender side and some on the receiver side. If the loss differentiation is performed at the receiver, loss notification is needed to inform the sender of the loss cause. The sender use the loss notification information to adapt its retransmission and congestion avoidance behavior. The work in this paper examines the effectiveness of two different notification schemes that are to be used in conjunction with receiver based loss differ-entiation. The first scheme uses a TCP option to explicitly convey the a loss counter and the sequence number of the last corrupted packet. The second scheme uses the generation of additional dup-acks as a way to implicitly influence the retransmission and congestion behavior of the sender. The advantage of the second scheme is that it requires no changes at the sender side TCP implementation. However, since the sec-ond scheme can not change the basic loss behavior of halving the congestion window of TCP, its performance is reduced in relation to the first scheme. This paper provides a description of the design of these two loss notification schemes. Additionally, an experimental evaluation is presented, based on a FreeBSD kernel implementation of the two notification schemes in conjunction with checksum based loss differentiation. 1