1 A Study of TCP Dynamics over HFC Networks Abstract New broadband access technologies such as Hybrid Fiber Coaxial (HFC) are likely to provide fast and cost effective sup- port to a variety of applications including Video on demand (VoD), interactive computer games, and internet-type applications such as Web browsing, ftp, email, and telephony. Since most of these applications, use TCP as the transport layer protocol, the key to their success largely depends on the effectiveness of the TCP protocol. We investigate the performance of TCP in terms of effective throughput in an HFC network environment using different load conditions and network buffer sizes. We find that TCP experiences low throughput as a result of the well known problem of ACK compression. An algorithm that controls ACK spacing is introduced to improve TCP performance. Keywords: TCP, HFC, ATM, Congestion avoidance, ACK compression. 1 Introduction The emergence of the HFC technology has a significant impact on already deployed Cable TV networks. As a return path from the stations to the headend becomes available, Cable network operators are able to add more ser- vices to television broadcast. A Medium Access Control (MAC) layer protocol is implemented at the root (or hea- dend) and at each of the cable network nodes (or stations) to allow various nodes to share resources in a multiaccess environment. It also controls the upstream (from the stations to the headend) and the downstream (from the headend to the stations) link transmissions. MAC protocol specifications are being drafted by the IEEE 802.14 working group to accomodate the needs of current and future network applications. The IEEE 802.14 Draft document [5] contains various MAC defining characteristics such as: frame format, station addressing, timing and synchronization proce- dures, and the ternary-tree mechanism to resolve collisions resulting from two or more stations transmitting at the same time. The MAC draft also provides the necessary “hooks” to support higher layer services such as CBR, VBR Hossam Afifi * Hossam.Afifi@enst-bretagne.fr * Ecole Nationale Supérieure des Télécommunications de Bretagne, Networks and Multimedia Department 2 rue de la Chataigneraie, BP 78, 35512 Cesson Sévigné, France Omar Elloumi * Omar.Elloumi@enst-bretagne.fr ** National Institute of Standards and Technology Gaithersburg, Maryland 20899, USA David Su ** dsu@isdn.ncsl.nist.gov Nada Golmie ** golmie@isdn.ncsl.nist.gov Omar Elloumi will serve as the corresponding author, Phone: (33) 2 99 12 70 42, Fax: (33) 2 99 12 70 30 This work was done while Omar Elloumi was a guest researcher at NIST in Spring 1997. A complete version of this paper is available via anonymous ftp from ftp.rennes.enst-bretagne.fr pub/reseau/elloumi/tcp_hfc.ps