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Network Working Group                                     J. Hadi SalimRequest for Comments: 2884                              Nortel NetworksCategory: Informational                                        U. Ahmed                                                    Carleton University                                                              July 2000Performance Evaluation of Explicit Congestion Notification (ECN)in IP NetworksStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2000).  All Rights Reserved.Abstract   This memo presents a performance study of the Explicit Congestion   Notification (ECN) mechanism in the TCP/IP protocol using our   implementation on the Linux Operating System. ECN is an end-to-end   congestion avoidance mechanism proposed by [6] and incorporated intoRFC 2481[7]. We study the behavior of ECN for both bulk and   transactional transfers. Our experiments show that there is   improvement in throughput over NON ECN (TCP employing any of Reno,   SACK/FACK or NewReno congestion control) in the case of bulk   transfers and substantial improvement for transactional transfers.   A more complete pdf version of this document is available at:http://www7.nortel.com:8080/CTL/ecnperf.pdf   This memo in its current revision is missing a lot of the visual   representations and experimental results found in the pdf version.1. Introduction   In current IP networks, congestion management is left to the   protocols running on top of IP. An IP router when congested simply   drops packets.  TCP is the dominant transport protocol today [26].   TCP infers that there is congestion in the network by detecting   packet drops (RFC 2581). Congestion control algorithms [11] [15] [21]   are then invoked to alleviate congestion.  TCP initially sends at a   higher rate (slow start) until it detects a packet loss. A packet   loss is inferred by the receipt of 3 duplicate ACKs or detected by aSalim & Ahmed                Informational                      [Page 1]

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 inSection 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. InSection 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 droppedSalim & Ahmed                Informational                      [Page 2]

RFC 2884                   ECN in IP Networks                  July 2000   when the buffer overflows, this interacts badly with the congestion   control mechanism of TCP. A cycle is formed with a burst of drops   after the maximum queue size is exceeded, followed by a period of   underutilization at the router as end systems back off. End systems   then increase their windows simultaneously up to a point where a   burst of drops happens again. This phenomenon is called Global   Synchronization. It leads to poor link utilization and lower overall   throughput [19] Another problem with Tail Drop is that a single   connection or a few flows could monopolize the queue space, in some   circumstances. This results in a lock out phenomenon leading to   synchronization or other timing effects [19].  Lastly, one of the   major drawbacks of Tail Drop is that queues remain full for long   periods of time. One of the major goals of queue management is to   reduce the steady state queue size[19].  Other queue management   techniques include random drop on full and drop front on full [13].2.1. Active Queue Management   Active queue management mechanisms detect congestion before the queue   overflows and provide an indication of this congestion to the end   nodes [7]. With this approach TCP does not have to rely only on   buffer overflow as the indication of congestion since notification   happens before serious congestion occurs. One such active management   technique is RED.2.1.1. Random Early Detection   Random Early Detection (RED) [9] is a congestion avoidance mechanism   implemented in routers which works on the basis of active queue   management. RED addresses the shortcomings of Tail Drop.  A RED   router signals incipient congestion to TCP by dropping packets   probabilistically before the queue runs out of buffer space. This   drop probability is dependent on a running average queue size to   avoid any bias against bursty traffic. A RED router randomly drops   arriving packets, with the result that the probability of dropping a   packet belonging to a particular flow is approximately proportional   to the flow's share of bandwidth. Thus, if the sender is using   relatively more bandwidth it gets penalized by having more of its   packets dropped.  RED operates by maintaining two levels of   thresholds minimum (minth) and maximum (maxth). It drops a packet   probabilistically if and only if the average queue size lies between   the minth and maxth thresholds. If the average queue size is above   the maximum threshold, the arriving packet is always dropped. When   the average queue size is between the minimum and the maximum   threshold, each arriving packet is dropped with probability pa, where   pa is a function of the average queue size. As the average queue   length varies between minth and maxth, pa increases linearly towards   a configured maximum drop probability, maxp. Beyond maxth, the dropSalim & Ahmed                Informational                      [Page 3]

RFC 2884                   ECN in IP Networks                  July 2000   probability is 100%.  Dropping packets in this way ensures that when   some subset of the source TCP packets get dropped and they invoke   congestion avoidance algorithms that will ease the congestion at the   gateway. Since the dropping is distributed across flows, the problem   of global synchronization is avoided.3. Explicit Congestion Notification   Explicit Congestion Notification is an extension proposed to RED   which marks a packet instead of dropping it when the average queue   size is between minth and maxth [7]. Since ECN marks packets before   congestion actually occurs, this is useful for protocols like TCP   that are sensitive to even a single packet loss. Upon receipt of a   congestion marked packet, the TCP receiver informs the sender (in the   subsequent ACK) about incipient congestion which will in turn trigger   the congestion avoidance algorithm at the sender.  ECN requires   support from both the router as well as the end hosts, i.e.  the end   hosts TCP stack needs to be modified. Packets from flows that are not   ECN capable will continue to be dropped by RED (as was the case   before ECN).3.1. Changes at the router   Router side support for ECN can be added by modifying current RED   implementations. For packets from ECN capable hosts, the router marks   the packets rather than dropping them (if the average queue size is   between minth and maxth).  It is necessary that the router identifies   that a packet is ECN capable, and should only mark packets that are   from ECN capable hosts. This uses two bits in the IP header.  The ECN   Capable Transport (ECT) bit is set by the sender end system if both   the end systems are ECN capable (for a unicast transport, only if   both end systems are ECN-capable). In TCP this is confirmed in the   pre-negotiation during the connection setup phase (explained inSection 3.2).  Packets encountering congestion are marked by the   router using the Congestion Experienced (CE) (if the average queue   size is between minth and maxth) on their way to the receiver end   system (from the sender end system), with a probability proportional   to the average queue size following the procedure used in RED   (RFC2309) routers.  Bits 10 and 11 in the IPV6 header are proposed   respectively for the ECT and CE bits. Bits 6 and 7 of the IPV4 header   DSCP field are also specified for experimental purposes for the ECT   and CE bits respectively.3.2. Changes at the TCP Host side   The proposal to add ECN to TCP specifies two new flags in the   reserved field of the TCP header. Bit 9 in the reserved field of the   TCP header is designated as the ECN-Echo (ECE) flag and Bit 8 isSalim & Ahmed                Informational                      [Page 4]

RFC 2884                   ECN in IP Networks                  July 2000   designated as the Congestion Window Reduced (CWR) flag.  These two   bits are used both for the initializing phase in which the sender and   the receiver negotiate the capability and the desire to use ECN, as   well as for the subsequent actions to be taken in case there is   congestion experienced in the network during the established state.   There are two main changes that need to be made to add ECN to TCP to   an end system and one extension to a router running RED.   1. In the connection setup phase, the source and destination TCPs   have to exchange information about their desire and/or capability to   use ECN. This is done by setting both the ECN-Echo flag and the CWR   flag in the SYN packet of the initial connection phase by the sender;   on receipt of this SYN packet, the receiver will set the ECN-Echo   flag in the SYN-ACK response. Once this agreement has been reached,   the sender will thereon set the ECT bit in the IP header of data   packets for that flow, to indicate to the network that it is capable   and willing to participate in ECN. The ECT bit is set on all packets   other than pure ACK's.   2. When a router has decided from its active queue management   mechanism, to drop or mark a packet, it checks the IP-ECT bit in the   packet header. It sets the CE bit in the IP header if the IP-ECT bit   is set. When such a packet reaches the receiver, the receiver   responds by setting the ECN-Echo flag (in the TCP header) in the next   outgoing ACK for the flow. The receiver will continue to do this in   subsequent ACKs until it receives from the sender an indication that   it (the sender) has responded to the congestion notification.   3. Upon receipt of this ACK, the sender triggers its congestion   avoidance algorithm by halving its congestion window, cwnd, and   updating its congestion window threshold value ssthresh. Once it has   taken these appropriate steps, the sender sets the CWR bit on the   next data outgoing packet to tell the receiver that it has reacted to   the (receiver's) notification of congestion.  The receiver reacts to   the CWR by halting the sending of the congestion notifications (ECE)   to the sender if there is no new congestion in the network.   Note that the sender reaction to the indication of congestion in the   network (when it receives an ACK packet that has the ECN-Echo flag   set) is equivalent to the Fast Retransmit/Recovery algorithm (when   there is a congestion loss) in NON-ECN-capable TCP i.e. the sender   halves the congestion window cwnd and reduces the slow start   threshold ssthresh. Fast Retransmit/Recovery is still available for   ECN capable stacks for responding to three duplicate acknowledgments.Salim & Ahmed                Informational                      [Page 5]

RFC 2884                   ECN in IP Networks                  July 20004. Experimental setup   For testing purposes we have added ECN to the Linux TCP/IP stack,   kernels version 2.0.32. 2.2.5, 2.3.43 (there were also earlier   revisions of 2.3 which were tested).  The 2.0.32 implementation   conforms toRFC 2481 [7] for the end systems only. We have also   modified the code in the 2.1,2.2 and 2.3 cases for the router portion   as well as end system to conform to the RFC. An outdated version of   the 2.0 code is available at [18].  Note Linux version 2.0.32   implements TCP Reno congestion control while kernels >= 2.2.0 default   to New Reno but will opt for a SACK/FACK combo when the remote end   understands SACK.  Our initial tests were carried out with the 2.0   kernel at the end system and 2.1 (pre 2.2) for the router part.  The   majority of the test results here apply to the 2.0 tests. We  did   repeat these tests on a different testbed (move from Pentium to   Pentium-II class machines)with faster machines for the 2.2 and 2.3   kernels, so the comparisons on the 2.0 and 2.2/3 are not relative.   We have updated this memo release to reflect the tests against SACK   and New Reno.4.1. Testbed setup                                             -----      ----                                            | ECN |    | ECN |                                            | ON  |    | OFF |          data direction ---->>              -----      ----                                              |          |      server                                  |          |       ----        ------        ------       |          |      |    |      |  R1  |      |  R2  |      |          |      |    | -----|      | ---- |      | ----------------------       ----        ------ ^      ------             |                          ^                         |                          |                        -----      congestion point ___|                       |  C  |                                                  |     |                                                   -----   The figure above shows our test setup.   All the physical links are 10Mbps ethernet.  Using Class Based   Queuing (CBQ) [22], packets from the data server are constricted to a   1.5Mbps pipe at the router R1. Data is always retrieved from the   server towards the clients labelled , "ECN ON", "ECN OFF", and "C".   Since the pipe from the server is 10Mbps, this creates congestion at   the exit from the router towards the clients for competing flows. The   machines labeled "ECN ON" and "ECN OFF"  are running the same versionSalim & Ahmed                Informational                      [Page 6]

RFC 2884                   ECN in IP Networks                  July 2000   of Linux and have exactly the same hardware configuration. The server   is always ECN capable (and can handle NON ECN flows as well using the   standard congestion algorithms). The machine labeled "C" is used to   create congestion in the network. Router R2 acts as a path-delay   controller.  With it we adjust the RTT the clients see.  Router R1   has RED implemented in it and has capability for supporting ECN   flows.  The path-delay router is a PC running the Nistnet [16]   package on a Linux platform. The latency of the link for the   experiments was set to be 20 millisecs.4.2. Validating the Implementation   We spent time validating that the implementation was conformant to   the specification inRFC 2481. To do this, the popular tcpdump   sniffer [24] was modified to show the packets being marked. We   visually inspected tcpdump traces to validate the conformance to the   RFC under a lot of different scenarios.  We also modified tcptrace   [25] in order to plot the marked packets for visualization and   analysis.   Both tcpdump and tcptrace revealed that the implementation was   conformant to the RFC.4.3. Terminology used   This section presents background terminology used in the next few   sections.   * Congesting flows: These are TCP flows that are started in the   background so as to create congestion from R1 towards R2. We use the   laptop labeled "C" to introduce congesting flows. Note that "C" as is   the case with the other clients retrieves data from the server.   * Low, Moderate and High congestion: For the case of low congestion   we start two congesting flows in the background, for moderate   congestion we start five congesting flows and for the case of high   congestion we start ten congesting flows in the background.   * Competing flows: These are the flows that we are interested in.   They are either ECN TCP flows from/to "ECN ON" or NON ECN TCP flows   from/to "ECN OFF".   * Maximum drop rate: This is the RED parameter that sets the maximum   probability of a packet being marked at the router. This corresponds   to maxp as explained inSection 2.1.Salim & Ahmed                Informational                      [Page 7]

RFC 2884                   ECN in IP Networks                  July 2000   Our tests were repeated for varying levels of congestion with varying   maximum drop rates. The results are presented in the subsequent   sections.   * Low, Medium and High drop probability: We use the term low   probability to mean a drop probability maxp of 0.02, medium   probability for 0.2 and high probability for 0.5. We also   experimented with drop probabilities of 0.05, 0.1 and 0.3.   * Goodput: We define goodput as the effective data rate as observed   by the user, i.e., if we transmitted 4 data packets in which two of   them were retransmitted packets, the efficiency is 50% and the   resulting goodput is 2*packet size/time taken to transmit.   * RED Region: When the router's average queue size is between minth   and maxth we denote that we are operating in the RED region.4.4. RED parameter selection   In our initial testing we noticed that as we increase the number of   congesting flows the RED queue degenerates into a simple Tail Drop   queue.  i.e. the average queue exceeds the maximum threshold most of   the times.  Note that this phenomena has also been observed by [5]   who proposes a dynamic solution to alleviate it by adjusting the   packet dropping probability "maxp" based on the past history of the   average queue size.  Hence, it is necessary that in the course of our   experiments the router operate in the RED region, i.e., we have to   make sure that the average queue is maintained between minth and   maxth. If this is not maintained, then the queue acts like a Tail   Drop queue and the advantages of ECN diminish. Our goal is to   validate ECN's benefits when used with RED at the router.  To ensure   that we were operating in the RED region we monitored the average   queue size and the actual queue size in times of low, moderate and   high congestion and fine-tuned the RED parameters such that the   average queue zones around the RED region before running the   experiment proper.  Our results are, therefore, not influenced by   operating in the wrong RED region.5. The Experiments   We start by making sure that the background flows do not bias our   results by computing the fairness index [12] inSection 5.1. We   proceed to carry out the experiments for bulk transfer presenting the   results and analysis in Section 5.2. InSection 5.3 the results for   transactional transfers along with analysis is presented.  More   details on the experimental results can be found in [27].Salim & Ahmed                Informational                      [Page 8]

RFC 2884                   ECN in IP Networks                  July 20005.1. Fairness   In the course of the experiments we wanted to make sure that our   choice of the type of background flows does not bias the results that   we collect.  Hence we carried out some tests initially with both ECN   and NON ECN flows as the background flows. We repeated the   experiments for different drop probabilities and calculated the   fairness index [12].  We also noticed (when there were equal number   of ECN and NON ECN flows) that the number of packets dropped for the   NON ECN flows was equal to the number of packets marked for the ECN   flows, showing thereby that the RED algorithm was fair to both kind   of flows.   Fairness index: The fairness index is a performance metric described   in [12].  Jain [12] postulates that the network is a multi-user   system, and derives a metric to see how fairly each user is treated.   He defines fairness as a function of the variability of throughput   across users. For a given set of user throughputs (x1, x2...xn), the   fairness index to the set is defined as follows:   f(x1,x2,.....,xn) = square((sum[i=1..n]xi))/(n*sum[i=1..n]square(xi))   The fairness index always lies between 0 and 1. A value of 1   indicates that all flows got exactly the same throughput.  Each of   the tests was carried out 10 times to gain confidence in our results.   To compute the fairness index we used FTP to generate traffic.   Experiment details: At time t = 0 we start 2 NON ECN FTP sessions in   the background to create congestion. At time t=20 seconds we start   two competing flows. We note the throughput of all the flows in the   network and calculate the fairness index. The experiment was carried   out for various maximum drop probabilities and for various congestion   levels.  The same procedure is repeated with the background flows as   ECN. The fairness index was fairly constant in both the cases when   the background flows were ECN and NON ECN indicating that there was   no bias when the background flows were either ECN or NON ECN.   Max     Fairness                Fairness   Drop    With BG                 With BG   Prob    flows ECN               flows NON ECN   0.02    0.996888                0.991946   0.05    0.995987                0.988286   0.1     0.985403                0.989726   0.2     0.979368                0.983342Salim & Ahmed                Informational                      [Page 9]

RFC 2884                   ECN in IP Networks                  July 2000   With the observation that the nature of background flows does not   alter the results, we proceed by using the background flows as NON   ECN for the rest of the experiments.5.2. Bulk transfers   The metric we chose for bulk transfer is end user throughput.   Experiment Details: All TCP flows used are RENO TCP. For the case of   low congestion we start 2 FTP flows in the background at time 0. Then   after about 20 seconds we start the competing flows, one data   transfer to the ECN machine and the second to the NON ECN machine.   The size of the file used is 20MB. For the case of moderate   congestion we start 5 FTP flows in the background and for the case of   high congestion we start 10 FTP flows in the background. We repeat   the experiments for various maximum drop rates each repeated for a   number of sets.   Observation and Analysis:   We make three key observations:   1) As the congestion level increases, the relative advantage for ECN   increases but the absolute advantage decreases (expected, since there   are more flows competing for the same link resource). ECN still does   better than NON ECN even under high congestion.  Infering a sample   from the collected results: at maximum drop probability of 0.1, for   example, the relative advantage of ECN increases from 23% to 50% as   the congestion level increases from low to high.   2) Maintaining congestion levels and varying the maximum drop   probability (MDP) reveals that the relative advantage of ECN   increases with increasing MDP. As an example, for the case of high   congestion as we vary the drop probability from 0.02 to 0.5 the   relative advantage of ECN increases from 10% to 60%.   3) There were hardly any retransmissions for ECN flows (except the   occasional packet drop in a minority of the tests for the case of   high congestion and low maximum drop probability).   We analyzed tcpdump traces for NON ECN with the help of tcptrace and   observed that there were hardly any retransmits due to timeouts.   (Retransmit due to timeouts are inferred by counting the number of 3   DUPACKS retransmit and subtracting them from the total recorded   number of retransmits).  This means that over a long period of time   (as is the case of long bulk transfers), the data-driven loss   recovery mechanism of the Fast Retransmit/Recovery algorithm is very   effective.  The algorithm for ECN on congestion notification from ECESalim & Ahmed                Informational                     [Page 10]

RFC 2884                   ECN in IP Networks                  July 2000   is the same as that for a Fast Retransmit for NON ECN. Since both are   operating in the RED region, ECN barely gets any advantage over NON   ECN from the signaling (packet drop vs. marking).   It is clear, however, from the results that ECN flows benefit in bulk   transfers.  We believe that the main advantage of ECN for bulk   transfers is that less time is spent recovering (whereas NON ECN   spends time retransmitting), and timeouts are avoided altogether.   [23] has shown that even with RED deployed, TCP RENO could suffer   from multiple packet drops within the same window of data, likely to   lead to multiple congestion reactions or timeouts (these problems are   alleviated by ECN). However, while TCP Reno has performance problems   with multiple packets dropped in a window of data, New Reno and SACK   have no such problems.   Thus, for scenarios with very high levels of congestion, the   advantages of ECN for TCP Reno flows could be more dramatic than the   advantages of ECN for NewReno or SACK flows.  An important   observation to make from our results is that we do not notice   multiple drops within a single window of data. Thus, we would expect   that our results are not heavily influenced by Reno's performance   problems with multiple packets dropped from a window of data.  We   repeated these tests with ECN patched newer Linux kernels. As   mentioned earlier these kernels would use a SACK/FACK combo with a   fallback to New Reno.  SACK can be selectively turned off (defaulting   to New Reno).  Our results indicate that ECN still improves   performance for the bulk transfers. More results are available in the   pdf version[27]. As in 1) above, maintaining a maximum drop   probability of 0.1 and increasing the congestion level, it is   observed that ECN-SACK improves performance from about 5% at low   congestion to about 15% at high congestion. In the scenario where   high congestion is maintained and the maximum drop probability is   moved from 0.02 to 0.5, the relative advantage of ECN-SACK improves   from 10% to 40%.  Although this numbers are lower than the ones   exhibited by Reno, they do reflect the improvement that ECN offers   even in the presence of robust recovery mechanisms such as SACK.5.3. Transactional transfers   We model transactional transfers by sending a small request and   getting a response from a server before sending the next request. To   generate transactional transfer traffic we use Netperf [17] with the   CRR (Connect Request Response) option.  As an example let us assume   that we are retrieving a small file of say 5 - 20 KB, then in effect   we send a small request to the server and the server responds by   sending us the file. The transaction is complete when we receive the   complete file. To gain confidence in our results we carry the   simulation for about one hour. For each test there are a few thousandSalim & Ahmed                Informational                     [Page 11]

RFC 2884                   ECN in IP Networks                  July 2000   of these requests and responses taking place.  Although not exactly   modeling HTTP 1.0 traffic, where several concurrent sessions are   opened, Netperf-CRR is nevertheless a close approximation.  Since   Netperf-CRR waits for one connection to complete before opening the   next one (0 think time), that single connection could be viewed as   the slowest response in the set of the opened concurrent sessions (in   HTTP).  The transactional data sizes were selected based on [2] which   indicates that the average web transaction was around 8 - 10 KB; The   smaller (5KB) size was selected to guestimate the size of   transactional processing that may become prevalent with policy   management schemes in the diffserv [4] context.  Using Netperf we are   able to initiate these kind of transactional transfers for a variable   length of time. The main metric of interest in this case is the   transaction rate, which is recorded by Netperf.   * Define Transaction rate as: The number of requests and complete   responses for a particular requested size that we are able to do per   second. For example if our request is of 1KB and the response is 5KB   then we define the transaction rate as the number of such complete   transactions that we can accomplish per second.   Experiment Details: Similar to the case of bulk transfers we start   the background FTP flows to introduce the congestion in the network   at time 0. About 20 seconds later we start the transactional   transfers and run each test for three minutes. We record the   transactions per second that are complete. We repeat the test for   about an hour and plot the various transactions per second, averaged   out over the runs. The experiment is repeated for various maximum   drop probabilities, file sizes and various levels of congestion.   Observation and Analysis   There are three key observations:   1) As congestion increases (with fixed drop probability) the relative   advantage for ECN increases (again the absolute advantage does not   increase since more flows are sharing the same bandwidth). For   example, from the results, if we consider the 5KB transactional flow,   as we increase the congestion from medium congestion (5 congesting   flows) to high congestion (10 congesting flows) for a maximum drop   probability of 0.1 the relative gain for ECN increases from 42% to   62%.   2) Maintaining the congestion level while adjusting the maximum drop   probability indicates that the relative advantage for ECN flows   increase.  From the case of high congestion for the 5KB flow weSalim & Ahmed                Informational                     [Page 12]

RFC 2884                   ECN in IP Networks                  July 2000   observe that the number of transactions per second increases from 0.8   to 2.2 which corresponds to an increase in relative gain for ECN of   20% to 140%.   3) As the transactional data size increases, ECN's advantage   diminishes because the probability of recovering from a Fast   Retransmit increases for NON ECN. ECN, therefore, has a huge   advantage as the transactional data size gets smaller as is observed   in the results.  This can be explained by looking at TCP recovery   mechanisms.  NON ECN in the short flows depends, for recovery, on   congestion signaling via receiving 3 duplicate ACKs, or worse by a   retransmit timer expiration, whereas ECN depends mostly on the TCP-   ECE flag. This is by design in our experimental setup.  [3] shows   that most of the TCP loss recovery in fact happens in timeouts for   short flows. The effectiveness of the Fast Retransmit/Recovery   algorithm is limited by the fact that there might not be enough data   in the pipe to elicit 3 duplicate ACKs.  TCP RENO needs at least 4   outstanding packets to recover from losses without going into a   timeout. For 5KB (4 packets for MTU of 1500Bytes) a NON ECN flow will   always have to wait for a retransmit timeout if any of its packets   are lost. ( This timeout could only have been avoided if the flow had   used an initial window of four packets, and the first of the four   packets was the packet dropped).  We repeated these experiments with   the kernels implementing SACK/FACK and New Reno algorithms. Our   observation was that there was hardly any difference with what we saw   with Reno. For example in the case of SACK-ECN enabling: maintaining   the maximum drop probability to 0.1 and increasing the congestion   level for the 5KB transaction we noticed that the relative gain for   the ECN enabled flows increases from 47-80%.  If we maintain the   congestion level for the 5KB transactions and increase the maximum   drop probabilities instead, we notice that SACKs performance   increases from 15%-120%.  It is fair to comment that the difference   in the testbeds (different machines, same topology) might have   contributed to the results; however, it is worth noting that the   relative advantage of the SACK-ECN is obvious.6. Conclusion   ECN enhancements improve on both bulk and transactional TCP traffic.   The improvement is more obvious in short transactional type of flows   (popularly referred to as mice).   * Because less retransmits happen with ECN, it means less traffic on   the network. Although the relative amount of data retransmitted in   our case is small, the effect could be higher when there are more   contributing end systems. The absence of retransmits also implies an   improvement in the goodput. This becomes very important for scenariosSalim & Ahmed                Informational                     [Page 13]

RFC 2884                   ECN in IP Networks                  July 2000   where bandwidth is expensive such as in low bandwidth links.  This   implies also that ECN lends itself well to applications that require   reliability but would prefer to avoid unnecessary retransmissions.   * The fact that ECN avoids timeouts by getting faster notification   (as opposed to traditional packet dropping inference from 3 duplicate   ACKs or, even worse, timeouts) implies less time is spent during   error recovery - this also improves goodput.   * ECN could be used to help in service differentiation where the end   user is able to "probe" for their target rate faster. Assured   forwarding [1] in the diffserv working group at the IETF proposes   using RED with varying drop probabilities as a service   differentiation mechanism.  It is possible that multiple packets   within a single window in TCP RENO could be dropped even in the   presence of RED, likely leading into timeouts [23]. ECN end systems   ignore multiple notifications, which help in countering this scenario   resulting in improved goodput. The ECN end system also ends up   probing the network faster (to reach an optimal bandwidth). [23] also   notes that RENO is the most widely deployed TCP implementation today.   It is clear that the advent of policy management schemes introduces   new requirements for transactional type of applications, which   constitute a very short query and a response in the order of a few   packets. ECN provides advantages to transactional traffic as we have   shown in the experiments.7. Acknowledgements   We would like to thank Alan Chapman, Ioannis Lambadaris, Thomas Kunz,   Biswajit Nandy, Nabil Seddigh, Sally Floyd, and Rupinder Makkar for   their helpful feedback and valuable suggestions.8. Security Considerations   Security considerations are as discussed insection 9 of RFC 2481.9. References   [1]  Heinanen, J., Finland, T., Baker, F., Weiss, W. and J.        Wroclawski, "Assured Forwarding PHB Group",RFC 2597, June 1999.   [2]  B.A. Mat. "An empirical model of HTTP network traffic."  In        proceedings INFOCOMM'97.Salim & Ahmed                Informational                     [Page 14]

RFC 2884                   ECN in IP Networks                  July 2000   [3]  Balakrishnan H., Padmanabhan V., Seshan S., Stemn M. and Randy        H. Katz, "TCP Behavior of a busy Internet Server: Analysis and        Improvements", Proceedings of IEEE Infocom, San Francisco, CA,        USA, March '98http://nms.lcs.mit.edu/~hari/papers/infocom98.ps.gz   [4]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W.        Weiss, "An Architecture for Differentiated Services",RFC 2475,        December 1998.   [5]  W. Feng, D. Kandlur, D. Saha, K. Shin, "Techniques for        Eliminating Packet Loss in Congested TCP/IP Networks", U.        Michigan CSE-TR-349-97, November 1997.   [6]  S. Floyd. "TCP and Explicit Congestion Notification." ACM        Computer Communications Review, 24, October 1994.   [7]  Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit        Congestion Notification (ECN) to IP",RFC 2481, January 1999.   [8]  Kevin Fall, Sally Floyd, "Comparisons of Tahoe, RENO and Sack        TCP", Computer  Communications Review, V. 26 N. 3, July 1996,        pp. 5-21   [9]  S. Floyd and V. Jacobson. "Random Early Detection Gateways for        Congestion Avoidance". IEEE/ACM Transactions on Networking,        3(1), August 1993.   [10] E. Hashem. "Analysis of random drop for gateway congestion        control." Rep. Lcs tr-465, Lav. Fot Comput. Sci., M.I.T., 1989.   [11] V. Jacobson. "Congestion Avoidance and Control." In Proceedings        of SIGCOMM '88, Stanford, CA, August 1988.   [12] Raj Jain, "The art of computer systems performance analysis",        John Wiley and sons QA76.9.E94J32, 1991.   [13] T. V. Lakshman, Arnie Neidhardt, Teunis Ott, "The Drop From        Front Strategy in TCP Over ATM and Its Interworking with Other        Control Features", Infocom 96, MA28.1.   [14] P. Mishra and H. Kanakia. "A hop by hop rate based congestion        control scheme." Proc. SIGCOMM '92, pp. 112-123, August 1992.   [15] Floyd, S. and T. Henderson, "The NewReno Modification to TCP's        Fast Recovery Algorithm",RFC 2582, April 1999.Salim & Ahmed                Informational                     [Page 15]

RFC 2884                   ECN in IP Networks                  July 2000   [16] The NIST Network Emulation Toolhttp://www.antd.nist.gov/itg/nistnet/   [17] The network performance toolhttp://www.netperf.org/netperf/NetperfPage.html   [18]ftp://ftp.ee.lbl.gov/ECN/ECN-package.tgz   [19] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, S.,        Estrin, D., Floyd, S., Jacobson, V., Minshall, G., Partridge,        C., Peterson, L., Ramakrishnan, K., Shenker, S., Wroclawski, J.        and L. Zhang, "Recommendations on Queue Management and        Congestion Avoidance in the Internet",RFC 2309, April 1998.   [20] K. K. Ramakrishnan and R. Jain. "A Binary feedback scheme for        congestion avoidance in computer networks." ACM Trans. Comput.        Syst.,8(2):158-181, 1990.   [21] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP        Selective Acknowledgement Options",RFC 2018, October 1996.   [22] S. Floyd and V. Jacobson, "Link sharing and Resource Management        Models for packet  Networks", IEEE/ACM Transactions on        Networking, Vol. 3 No.4, August 1995.   [23] Prasad Bagal, Shivkumar Kalyanaraman, Bob Packer, "Comparative        study of RED, ECN and TCP Rate Control".http://www.packeteer.com/technology/Pdf/packeteer-final.pdf   [24] tcpdump, the protocol packet capture & dumper program.ftp://ftp.ee.lbl.gov/tcpdump.tar.Z   [25] TCP dump file analysis tool:http://jarok.cs.ohiou.edu/software/tcptrace/tcptrace.html   [26] Thompson K., Miller, G.J., Wilder R., "Wide-Area Internet        Traffic Patterns and Characteristics". IEEE Networks Magazine,        November/December 1997.   [27]http://www7.nortel.com:8080/CTL/ecnperf.pdfSalim & Ahmed                Informational                     [Page 16]

RFC 2884                   ECN in IP Networks                  July 200010. Authors' Addresses   Jamal Hadi Salim   Nortel Networks   3500 Carling Ave   Ottawa, ON, K2H 8E9   Canada   EMail: hadi@nortelnetworks.com   Uvaiz Ahmed   Dept. of Systems and Computer Engineering   Carleton University   Ottawa   Canada   EMail: ahmed@sce.carleton.caSalim & Ahmed                Informational                     [Page 17]

RFC 2884                   ECN in IP Networks                  July 200011. Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Salim & Ahmed                Informational                     [Page 18]