tcp_ecn.pdf

This paper discusses the use of Explicit Congestion Noti- fication(ECN)mechanisms in the TCP/IP protocol.The first part proposes new guidelines for TCP’s response to ECN mechanisms(e.g.,Source Quench packets,ECN fields in packet headers).Next,using simulations,we explore the benefits and drawbacks of ECN in TCP/IP networks.Our simul ations use RED gateways modi- fied to set an ECN bit in the IP packet header as an indication of congestion,with Reno-style TCP modified to respond to ECN as well as to packet drops as indi- cations of congestion.The simulations show that one advantage of ECN mechanisms is in avoiding unneces- sary packet drops,and therefore avoiding unnecessary delay for packets from low-bandwidth delay-sensitive TCP connections.A second advantage of ECN mech- anisms is in networks(generally LANs)where the ef- fectiveness of TCP retransmit timers is limited by the coarse granularity of the TCP clock.The paper also dis- cusses some implementation issues concerning specific ECN mechanisms in TCP/IP networks. ations use RED gateways modi- fied to set an ECN bit in the IP packet header as an indication of congestion,with Reno-style TCP modified to respond to ECN as well as to packet drops as indi- cations of congestion.The simulations show that one advantage of ECN mechanisms is in avoiding unneces- sary packet drops,and therefore avoiding unnecessary delay for packets from low-bandwidth delay-sensitive TCP connections.A second advantage of ECN mech- anisms is in networks(generally LANs)where the ef- fectiveness of TCP retransmit timers is limited by the coarse granularity of the TCP clock.The paper also dis- cusses some implementation issues concerning specific ECN mechanisms in TCP/IP networks.