TCP Performance Implications of Network Path Asymmetry

摘要

This document describes TCP performance problems that arise becauseof asymmetric effects. These problems arise in several accessnetworks, including bandwidth-asymmetric networks and packet radiosubnetworks, for different underlying reasons. However, the endresult on TCP performance is the same in both cases: performanceoften degrades significantly because of imperfection and variabilityin the ACK feedback from the receiver to the sender.This document details several mitigations to these effects, whichhave either been proposed or evaluated in the literature, or arecurrently deployed in networks. These solutions use a combinationof local link-layer techniques, subnetwork, and end-to-endmechanisms, consisting of: (i) techniques to manage the channel usedfor the upstream bottleneck link carrying the ACKs, typically usingheader compression or reducing the frequency of TCP ACKs, (ii)techniques to handle this reduced ACK frequency to retain the TCPsender's acknowledgment-triggered self-clocking and (iii) techniquesto schedule the data and ACK packets in the reverse direction toimprove performance in the presence of two-way traffic. Eachtechnique is described, together with known issues, andrecommendations for use. A table in section 8 provides a summary ofthe recommendations.