RFC 2884 (rfc2884) - Page 2 of 18

Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks

RFC 2884                   ECN in IP Networks                  July 2000


   timeout. The sending TCP then moves into a congestion avoidance state
   where it carefully probes the network by sending at a slower rate
   (which goes up until another packet loss is detected).  Traditionally
   a router reacts to congestion by dropping a packet in the absence of
   buffer space. This is referred to as Tail Drop. This method has a
   number of drawbacks (outlined in Section 2). These drawbacks coupled
   with the limitations of end-to-end congestion control have led to
   interest in introducing smarter congestion control mechanisms in
   routers.  One such mechanism is Random Early Detection (RED) [9]
   which detects incipient congestion and implicitly signals the
   oversubscribing flow to slow down by dropping its packets. A RED-
   enabled router detects congestion before the buffer overflows, based
   on a running average queue size, and drops packets probabilistically
   before the queue actually fills up. The probability of dropping a new
   arriving packet increases as the average queue size increases above a
   low water mark minth, towards higher water mark maxth. When the
   average queue size exceeds maxth all arriving packets are dropped.

   An extension to RED is to mark the IP header instead of dropping
   packets (when the average queue size is between minth and maxth;
   above maxth arriving packets are dropped as before). Cooperating end
   systems would then use this as a signal that the network is congested
   and slow down. This is known as Explicit Congestion Notification
   (ECN).  In this paper we study an ECN implementation on Linux for
   both the router and the end systems in a live network.  The memo is
   organized as follows. In Section 2 we give an overview of queue
   management in routers. Section 3 gives an overview of ECN and the
   changes required at the router and the end hosts to support ECN.
   Section 4 defines the experimental testbed and the terminologies used
   throughout this memo. Section 5 introduces the experiments that are
   carried out, outlines the results and presents an analysis of the
   results obtained.  Section 6 concludes the paper.

2. Queue Management in routers

   TCP's congestion control and avoidance algorithms are necessary and
   powerful but are not enough to provide good service in all
   circumstances since they treat the network as a black box. Some sort
   of control is required from the routers to complement the end system
   congestion control mechanisms. More detailed analysis is contained in
   [19].  Queue management algorithms traditionally manage the length of
   packet queues in the router by dropping packets only when the buffer
   overflows.  A maximum length for each queue is configured. The router
   will accept packets till this maximum size is exceeded, at which
   point it will drop incoming packets. New packets are accepted when
   buffer space allows. This technique is known as Tail Drop. This
   method has served the Internet well for years, but has the several
   drawbacks.  Since all arriving packets (from all flows) are dropped



Salim & Ahmed                Informational