--- 1/draft-ietf-tsvwg-byte-pkt-congest-03.txt 2011-03-14 21:14:28.000000000 +0100 +++ 2/draft-ietf-tsvwg-byte-pkt-congest-04.txt 2011-03-14 21:14:28.000000000 +0100 @@ -1,19 +1,19 @@ Transport Area Working Group B. Briscoe Internet-Draft BT Updates: 2309 (if approved) J. Manner Intended status: Informational Aalto University -Expires: April 27, 2011 October 24, 2010 +Expires: September 15, 2011 March 14, 2011 Byte and Packet Congestion Notification - draft-ietf-tsvwg-byte-pkt-congest-03 + draft-ietf-tsvwg-byte-pkt-congest-04 Abstract This memo concerns dropping or marking packets using active queue management (AQM) such as random early detection (RED) or pre- congestion notification (PCN). We give three strong recommendations: (1) packet size should be taken into account when transports read congestion indications, (2) packet size should not be taken into account when network equipment creates congestion signals (marking, dropping), and therefore (3) the byte-mode packet drop variant of the @@ -27,87 +27,87 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on April 27, 2011. + This Internet-Draft will expire on September 15, 2011. Copyright Notice - Copyright (c) 2010 IETF Trust and the persons identified as the + Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Terminology and Scoping . . . . . . . . . . . . . . . . . 7 - 1.2. Why now? . . . . . . . . . . . . . . . . . . . . . . . . . 8 - 2. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 10 - 2.1. Scaling Congestion Control with Packet Size . . . . . . . 10 - 2.2. Transport-Independent Network . . . . . . . . . . . . . . 10 - 2.3. Avoiding Perverse Incentives to (Ab)use Smaller Packets . 11 - 2.4. Small != Control . . . . . . . . . . . . . . . . . . . . . 12 - 2.5. Implementation Efficiency . . . . . . . . . . . . . . . . 13 - 3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 13 - 3.1. Recommendation on Queue Measurement . . . . . . . . . . . 13 - 3.2. Recommendation on Notifying Congestion . . . . . . . . . . 13 - 3.3. Recommendation on Responding to Congestion . . . . . . . . 14 - 3.4. Recommended Future Research . . . . . . . . . . . . . . . 15 + 2. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 8 + 2.1. Recommendation on Queue Measurement . . . . . . . . . . . 8 + 2.2. Recommendation on Notifying Congestion . . . . . . . . . . 9 + 2.3. Recommendation on Responding to Congestion . . . . . . . . 10 + 2.4. Recommendation on Handling Congestion Indications when + Splitting or Merging Packets . . . . . . . . . . . . . . . 11 + 3. Motivating Arguments . . . . . . . . . . . . . . . . . . . . . 11 + 3.1. Scaling Congestion Control with Packet Size . . . . . . . 11 + 3.2. Transport-Independent Network . . . . . . . . . . . . . . 12 + 3.3. Avoiding Perverse Incentives to (Ab)use Smaller Packets . 13 + 3.4. Small != Control . . . . . . . . . . . . . . . . . . . . . 14 + 3.5. Implementation Efficiency . . . . . . . . . . . . . . . . 14 + 3.6. Why now? . . . . . . . . . . . . . . . . . . . . . . . . . 14 4. A Survey and Critique of Past Advice . . . . . . . . . . . . . 15 4.1. Congestion Measurement Advice . . . . . . . . . . . . . . 16 - 4.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 16 - 4.1.2. Congestion Measurement without a Queue . . . . . . . . 17 + 4.1.1. Fixed Size Packet Buffers . . . . . . . . . . . . . . 17 + 4.1.2. Congestion Measurement without a Queue . . . . . . . . 18 4.2. Congestion Notification Advice . . . . . . . . . . . . . . 18 4.2.1. Network Bias when Encoding . . . . . . . . . . . . . . 18 4.2.2. Transport Bias when Decoding . . . . . . . . . . . . . 20 4.2.3. Making Transports Robust against Control Packet - Losses . . . . . . . . . . . . . . . . . . . . . . . . 21 + Losses . . . . . . . . . . . . . . . . . . . . . . . . 22 4.2.4. Congestion Notification: Summary of Conflicting Advice . . . . . . . . . . . . . . . . . . . . . . . . 22 4.2.5. RED Implementation Status . . . . . . . . . . . . . . 23 5. Outstanding Issues and Next Steps . . . . . . . . . . . . . . 24 5.1. Bit-congestible World . . . . . . . . . . . . . . . . . . 24 5.2. Bit- & Packet-congestible World . . . . . . . . . . . . . 25 6. Security Considerations . . . . . . . . . . . . . . . . . . . 26 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 27 - 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 + 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28 9. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 28 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 10.1. Normative References . . . . . . . . . . . . . . . . . . . 28 10.2. Informative References . . . . . . . . . . . . . . . . . . 29 - Appendix A. Idealised Wire Protocol . . . . . . . . . . . . . . . 32 - A.1. Protocol Coding . . . . . . . . . . . . . . . . . . . . . 32 + Appendix A. Idealised Wire Protocol . . . . . . . . . . . . . . . 33 + A.1. Protocol Coding . . . . . . . . . . . . . . . . . . . . . 33 A.2. Example Scenarios . . . . . . . . . . . . . . . . . . . . 34 A.2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . 34 - A.2.2. Bit-congestible resource, equal bit rates (Ai) . . . . 34 - A.2.3. Bit-congestible resource, equal packet rates (Bi) . . 35 - A.2.4. Pkt-congestible resource, equal bit rates (Aii) . . . 36 + A.2.2. Bit-congestible resource, equal bit rates (Ai) . . . . 35 + A.2.3. Bit-congestible resource, equal packet rates (Bi) . . 36 + A.2.4. Pkt-congestible resource, equal bit rates (Aii) . . . 37 A.2.5. Pkt-congestible resource, equal packet rates (Bii) . . 37 Appendix B. Byte-mode Drop Complicates Policing Congestion - Response . . . . . . . . . . . . . . . . . . . . . . 37 - - Appendix C. Changes from Previous Versions . . . . . . . . . . . 38 + Response . . . . . . . . . . . . . . . . . . . . . . 38 + Appendix C. Changes from Previous Versions . . . . . . . . . . . 39 1. Introduction This memo is initially concerned with how we should correctly scale congestion control functions with packet size for the long term. But it also recognises that expediency may be necessary to deal with existing widely deployed protocols that don't live up to the long term goal. When notifying congestion, the problem of how (and whether) to take @@ -151,61 +151,61 @@ congestion notification. Currently, the RFC series is silent on this matter other than a paper trail of advice referenced from [RFC2309], which conditionally recommends byte-mode (packet-size dependent) drop [pktByteEmail]. Reducing drop of small packets certainly has some tempting advantages: i) it drops less control packets, which tend to be small and ii) it makes TCP's bit-rate less dependent on packet size. However, there are ways of addressing these issues at the transport layer, rather than reverse engineering network forwarding to fix the - problems of one specific transport. + problems of one specific transport, as byte-mode variant of RED was + designed to do. The primary purpose of this memo is to build a definitive consensus against deliberate preferential treatment for small packets in AQM algorithms and to record this advice within the RFC series. It recommends that (1) packet size should be taken into account when transports read congestion indications, (2) not when network equipment writes them. In particular this means that the byte-mode packet drop variant of RED should not be used to drop fewer small packets, because that creates a perverse incentive for transports to use tiny segments, consequently also opening up a DoS vulnerability. Fortunately all the RED implementers who responded to our survey (Section 4.2.4) have not followed the earlier advice to use byte-mode drop, so the consensus this memo argues for seems to already exist in implementations. However, at the transport layer, TCP congestion control is a widely - deployed protocol that we argue doesn't scale correctly with packet - size. To date this hasn't been a significant problem because most - TCPs have been used with similar packet sizes. But, as we design new + deployed protocol that doesn't scale correctly with packet size. To + date this hasn't been a significant problem because most TCPs have + been used with similar packet sizes. But, as we design new congestion controls, we should build in scaling with packet size rather than assuming we should follow TCP's example. This memo continues as follows. First it discusses terminology and - scoping and why it is relevant to publish this memo now. Section 2 - gives motivating arguments for the recommendations that are formally - stated in Section 3, which follows. We then critically survey the - advice given previously in the RFC series and the research literature - (Section 4), followed by an assessment of whether or not this advice - has been followed in production networks (Section 4.2.5). To wrap - up, outstanding issues are discussed that will need resolution both - to inform future protocols designs and to handle legacy (Section 5). - Then security issues are collected together in Section 6 before - conclusions are drawn in Section 7. The interested reader can find - discussion of more detailed issues on the theme of byte vs. packet in - the appendices. + scoping. Section 2 gives the concrete formal recommendations, + followed by motivating arguments in Section 3. We then critically + survey the advice given previously in the RFC series and the research + literature (Section 4), followed by an assessment of whether or not + this advice has been followed in production networks (Section 4.2.5). + To wrap up, outstanding issues are discussed that will need + resolution both to inform future protocols designs and to handle + legacy (Section 5). Then security issues are collected together in + Section 6 before conclusions are drawn in Section 7. The interested + reader can find discussion of more detailed issues on the theme of + byte vs. packet in the appendices. This memo intentionally includes a non-negligible amount of material - on the subject. A busy reader can jump right into Section 3 to read + on the subject. A busy reader can jump right into Section 2 to read a summary of the recommendations for the Internet community. 1.1. Terminology and Scoping The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Congestion Notification: Rather than aim to achieve what many have tried and failed, this memo will not try to define congestion. It @@ -249,23 +249,23 @@ they have to process. Examples of bit-congestible resources are transmission links, radio power and most buffer memory, because the load depends on how many bits they have to transmit or store. Some machine architectures use fixed size packet buffers, so buffer memory in these cases is packet-congestible (see Section 4.1.1). Currently a design goal of network processing equipment such as routers and firewalls is to keep packet processing uncongested even under worst case bit rates with minimum packet sizes. - Therefore, packet-congestion is currently rare - [I-D.irtf-iccrg-welzl; S.3.3], but there is no guarantee that it - will not become common with future technology trends. + Therefore, packet-congestion is currently rare [RFC6077; S.3.3], + but there is no guarantee that it will not become common with + future technology trends. Note that information is generally processed or transmitted with a minimum granularity greater than a bit (e.g. octets). The appropriate granularity for the resource in question should be used, but for the sake of brevity we will talk in terms of bytes in this memo. Coarser Granularity: Resources may be congestible at higher levels of granularity than bits or packets, for instance stateful firewalls are flow-congestible and call-servers are session- @@ -276,82 +276,163 @@ RED Terminology: In RED, whether to use packets or bytes when measuring queues is called respectively packet-mode queue measurement or byte-mode queue measurement. And whether the probability of dropping a packet is independent or dependent on its byte-size is called respectively packet-mode drop or byte-mode drop. The terms byte-mode and packet-mode should not be used without specifying whether they apply to queue measurement or to drop. -1.2. Why now? +2. Recommendations - Now is a good time to discuss whether fairness between different - sized packets would best be implemented in network equipment, or at - the transport, for a number of reasons: +2.1. Recommendation on Queue Measurement - 1. The IETF pre-congestion notification (PCN) working group is - standardising the external behaviour of a PCN congestion - notification (AQM) algorithm [RFC5670]; + Queue length is usually the most correct and simplest way to measure + congestion of a resource. To avoid the pathological effects of drop + tail, an AQM function can then be used to transform queue length into + the probability of dropping or marking a packet (e.g. RED's + piecewise linear function between thresholds). - 2. [RFC2309] says RED may either take account of packet size or not - when dropping, but gives no recommendation between the two, - referring instead to advice on the performance implications in an - email [pktByteEmail], which recommends byte-mode drop. Further, - just before RFC2309 was issued, an addendum was added to the - archived email that revisited the issue of packet vs. byte-mode - drop in its last paragraph, making the recommendation less clear- - cut; + If the resource is bit-congestible, the implementation SHOULD measure + the length of the queue in bytes. If the resource is packet- + congestible, the implementation SHOULD measure the length of the + queue in packets. No other choice makes sense, because the number of + packets waiting in the queue isn't relevant if the resource gets + congested by bytes and vice versa. - 3. Without the present memo, the only advice in the RFC series on - packet size bias in AQM algorithms would be a reference to an - archived email in [RFC2309] (including an addendum at the end of - the email to correct the original). + Corollaries: - 4. The IRTF Internet Congestion Control Research Group (ICCRG) - recently took on the challenge of building consensus on what - common congestion control support should be required from network - forwarding functions in future [I-D.irtf-iccrg-welzl]. The wider - Internet community needs to discuss whether the complexity of - adjusting for packet size should be in the network or in - transports; + 1. A RED implementation SHOULD use byte mode queue measurement for + measuring the congestion of bit-congestible resources and packet + mode queue measurement for packet-congestible resources. - 5. Given there are many good reasons why larger path max - transmission units (PMTUs) would help solve a number of scaling - issues, we don't want to create any bias against large packets - that is greater than their true cost; + 2. "An Admin SHOULD NOT be able to configure the way a queue + measures itself, because wether a queue is bit-congestible or + packet-congestible is a property of the resource." - 6. The IETF audio/video transport (AVT) working group is - standardising how the real-time protocol (RTP) should feedback - and respond to explicit congestion notification (ECN) - [I-D.ietf-avt-ecn-for-rtp]. + The recommended approach in less straightforward scenarios, such as + fixed size buffers, and resources without a queue, is discussed in + Section 4.1. - 7. The IETF has started to consider the question of fairness between - flows that use different packet sizes (e.g. in the small-packet - variant of TCP-friendly rate control, TFRC-SP [RFC4828]). Given - transports with different packet sizes, if we don't decide - whether the network or the transport should allow for packet - size, it will be hard if not impossible to design any transport - protocol so that its bit-rate relative to other transports meets - design guidelines [RFC5033] (Note however that, if the concern - were fairness between users, rather than between flows - [Rate_fair_Dis], relative rates between flows would have to come - under run-time control rather than being embedded in protocol - designs). +2.2. Recommendation on Notifying Congestion -2. Motivating Arguments + When notifying congestion, a network device SHOULD treat all packets + equally, regardless of their size. Therefore, the probability that + network equipment drops or marks a packet to notify congestion SHOULD + NOT depend on the size of the packet. For instance, to drop any bit + with probability 0.1% it is only necessary to drop every packet with + probability 0.1% without regard to the size of each packet. + + This means that the Internet's congestion notification protocols + (drop, ECN & PCN) SHOULD NOT take account of packet size when + congestion is notified by network equipment. Allowance for packet + size is only appropriate when the transport responds to congestion + (See Recommendation 2.3). This approach offers sufficient and + correct congestion information for all known and future transport + protocols and also ensures no perverse incentives are created that + would encourage transports to use inappropriately small packet sizes. + + Corollaries: + + 1. AQM algorithms such as RED SHOULD NOT use byte-mode drop, which + deflates RED's drop probability for smaller packet sizes. RED's + byte-mode drop has no enduring advantages. It is more complex, + it creates the perverse incentive to fragment segments into tiny + pieces and it reopens the vulnerability to floods of small- + packets that drop-tail queues suffered from and AQM was designed + to remove. + + 2. If a vendor has implemented byte-mode drop, and an operator has + turned it on, it is strongly RECOMMENDED that it SHOULD be turned + off. Note that RED as a whole SHOULD NOT be turned off, as + without it, a drop tail queue also biases against large packets. + But note also that turning off byte-mode drop may alter the + relative performance of applications using different packet + sizes, so it would be advisable to establish the implications + before turning it off. + + NOTE WELL that RED's byte-mode queue drop is completely + orthogonal to byte-mode queue measurement and should not be + confused with it. If a RED implementation has a byte-mode but + does not specify what sort of byte-mode, it is most probably + byte-mode queue measurement, which is fine. However, if in + doubt, the vendor should be consulted. + + The byte mode packet drop variant of RED was recommended in the past + (see Section 4.2.1 for how thinking evolved). However, our survey of + 84 vendors across the industry (Section 4.2.5) has found that none of + the 19% who responded have implemented byte mode drop in RED. Given + there appears to be little, if any, installed base it seems we can + deprecate byte-mode drop in RED with little, if any, incremental + deployment impact. + +2.3. Recommendation on Responding to Congestion + + When a transport detects that a packet has been lost or congestion + marked, it SHOULD consider the strength of the congestion indication + as proportionate to the size in octets of the missing or marked + packet. + + In other words, when a packet indicates congestion (by being lost or + marked) it can be considered conceptually as if there is a congestion + indication on every octet of the packet, not just one indication per + packet. + + Therefore, instead of network equipment biasing its congestion + notification in favour of small packets, the IETF transport area + should continue its programme of; + + o updating host-based congestion control protocols to take account + of packet size + + o making transports less sensitive to losing control packets like + SYNs and pure ACKs. + + Corollaries: + + 1. If two TCPs with different packet sizes are required to run at + equal bit rates under the same path conditions, this SHOULD be + done by altering TCP (Section 4.2.2), not network equipment, + which would otherwise affect other transports besides TCP. + + 2. If it is desired to improve TCP performance by reducing the + chance that a SYN or a pure ACK will be dropped, this should be + done by modifying TCP (Section 4.2.3), not network equipment. + +2.4. Recommendation on Handling Congestion Indications when Splitting + or Merging Packets + + Packets carrying congestion indications may be split or merged (e.g. + at a transcoder or during fragment reassembly). Splitting and + merging only make sense in the context of ECN, not loss. + + The general rule to follow is that the number of octets in packets + with congestion indications should be roughly the same before and + after merging or splitting. This is based on the principle used + above; that an indication of congestion on a packet can be considered + as an indication of congestion on each octet of the packet. + + One can think of a splitting or merging process as if all the + incoming congestion-marked octets increment a counter and all the + outgoing marked octets decrement the same counter. In order to + ensure that congestion indications remain timely, even the smallest + positive remainder in the conceptual counter should trigger the next + outgoing packet to be marked (causing the counter to go negative). + +3. Motivating Arguments In this section, we evaluate the topic of packet vs. byte based congestion notifications and motivate the recommendations given in this document. -2.1. Scaling Congestion Control with Packet Size +3.1. Scaling Congestion Control with Packet Size There are two ways of interpreting a dropped or marked packet. It can either be considered as a single loss event or as loss/marking of the bytes in the packet. Consider a bit-congestible link shared by many flows (bit-congestible is the more common case, see Section 1.1), so that each busy period tends to cause packets to be lost from different flows. Consider further two sources that have the same data rate but break the load into large packets in one application (A) and small packets in the @@ -375,21 +456,21 @@ packets (B) will respond more to the same congestion excursion. On the other hand, if they respond proportionately less when smaller packets are dropped/marked, overall they will be able to respond the same to the same congestion excursion. Therefore, for a congestion control to scale with packet size it should respond to dropped or marked bytes (as TFRC-SP [RFC4828] effectively does), instead of dropped or marked packets (as TCP does). -2.2. Transport-Independent Network +3.2. Transport-Independent Network TCP congestion control ensures that flows competing for the same resource each maintain the same number of segments in flight, irrespective of segment size. So under similar conditions, flows with different segment sizes will get different bit rates. Even though reducing the drop probability of small packets (e.g. RED's byte-mode drop) helps ensure TCPs with different packet sizes will achieve similar bit rates, we argue this correction should be made to any future transport protocols based on TCP, not to the @@ -402,21 +483,21 @@ RFC2309 refers to an email [pktByteEmail] for advice on how RED should allow for different packet sizes. The email says the question of whether a packet's own size should affect its drop probability "depends on the dominant end-to-end congestion control mechanisms". But we argue network equipment should not be specialised for whatever transport is predominant. No matter how convenient it is, we SHOULD NOT hack the network solely to allow for omissions from the design of one transport protocol, even if it is as predominant as TCP. -2.3. Avoiding Perverse Incentives to (Ab)use Smaller Packets +3.3. Avoiding Perverse Incentives to (Ab)use Smaller Packets Increasingly, it is being recognised that a protocol design must take care not to cause unintended consequences by giving the parties in the protocol exchange perverse incentives [Evol_cc][RFC3426]. Again, imagine a scenario where the same bit rate of packets will contribute the same to bit-congestion of a link irrespective of whether it is sent as fewer larger packets or more smaller packets. A protocol design that caused larger packets to be more likely to be dropped than smaller ones would be dangerous in this case: @@ -438,175 +518,115 @@ Encouraging a high volume of tiny packets might in turn unnecessarily overload a completely unrelated part of the system, perhaps more limited by header-processing than bandwidth. Imagine two unresponsive flows arrive at a bit-congestible transmission link each with the same bit rate, say 1Mbps, but one consists of 1500B and the other 60B packets, which are 25x smaller. Consider a scenario where gentle RED [gentle_RED] is used, along with the variant of RED we advise against, i.e. where the RED algorithm is configured to adjust the drop probability of packets in proportion to - each packet's size (byte mode packet drop). In this case, if RED - drops 25% of the larger packets, it will aim to drop 1% of the - smaller packets (but in practice it may drop more as congestion - increases [RFC4828; S.B.4]). Even though both flows arrive with the - same bit rate, the bit rate the RED queue aims to pass to the line - will be 750k for the flow of larger packet but 990k for the smaller - packets (but because of rate variation it will be less than this - target). + each packet's size (byte mode packet drop). In this case, RED aims + to drop 25x more of the larger packets than the smaller ones. Thus, + for example if RED drops 25% of the larger packets, it will aim to + drop 1% of the smaller packets (but in practice it may drop more as + congestion increases [RFC4828; S.B.4]). Even though both flows + arrive with the same bit rate, the bit rate the RED queue aims to + pass to the line will be 750Kbit for the flow of larger packet but + 990Kbit for the smaller packets (but because of rate variation it + will be less than this target). Note that, although the byte-mode drop variant of RED amplifies small packet attacks, drop-tail queues amplify small packet attacks even more (see Security Considerations in Section 6). Wherever possible neither should be used. -2.4. Small != Control +3.4. Small != Control It is tempting to drop small packets with lower probability to improve performance, because many control packets are small (TCP SYNs & ACKs, DNS queries & responses, SIP messages, HTTP GETs, etc) and dropping fewer control packets considerably improves performance. However, we must not give control packets preference purely by virtue of their smallness, otherwise it is too easy for any data source to get the same preferential treatment simply by sending data in smaller packets. Again we should not create perverse incentives to favour small packets rather than to favour control packets, which is what we intend. Just because many control packets are small does not mean all small packets are control packets. So again, rather than fix these problems in the network, we argue that the transport should be made more robust against losses of control packets (see 'Making Transports Robust against Control Packet Losses' in Section 4.2.3). -2.5. Implementation Efficiency +3.5. Implementation Efficiency Allowing for packet size at the transport rather than in the network ensures that neither the network nor the transport needs to do a multiply operation--multiplication by packet size is effectively achieved as a repeated add when the transport adds to its count of marked bytes as each congestion event is fed to it. This isn't a principled reason in itself, but it is a happy consequence of the other principled reasons. -3. Recommendations - -3.1. Recommendation on Queue Measurement - - Queue length is usually the most correct and simplest way to measure - congestion of a resource. To avoid the pathological effects of drop - tail, an AQM function can then be used to transform queue length into - the probability of dropping or marking a packet (e.g. RED's - piecewise linear function between thresholds). - - If the resource is bit-congestible, the implementation SHOULD measure - the length of the queue in bytes. If the resource is packet- - congestible, the implementation SHOULD measure the length of the - queue in packets. No other choice makes sense, because the number of - packets waiting in the queue isn't relevant if the resource gets - congested by bytes and vice versa. - - Corollaries: - - 1. Whether a resource is bit-congestible or packet-congestible is a - property of the resource, so an admin should not ever need to, or - be able to, configure the way a queue measures itself. - - 2. If RED is used, the implementation SHOULD use byte mode queue - measurement for measuring the congestion of bit-congestible - resources and packet mode queue measurement for packet- - congestible resources. - - The recommended approach in less straightforward scenarios, such as - fixed size buffers, and resources without a queue, is discussed in - Section 4.1. - -3.2. Recommendation on Notifying Congestion - - The Internet's congestion notification protocols (drop, ECN & PCN) - SHOULD NOT take account of packet size when congestion is notified by - network equipment. Allowance for packet size is only appropriate - when the transport responds to congestion (See Recommendation 3.3). - - This approach offers sufficient and correct congestion information - for all known and future transport protocols and also ensures no - perverse incentives are created that would encourage transports to - use inappropriately small packet sizes. - - Corollaries: - - 1. AQM algorithms such as RED SHOULD NOT use byte-mode drop, which - deflates RED's drop probability for smaller packet sizes. RED's - byte-mode drop has no enduring advantages. It is more complex, - it creates the perverse incentive to fragment segments into tiny - pieces and it reopens the vulnerability to floods of small- - packets that drop-tail queues suffered from and AQM was designed - to remove. - - 2. If a vendor has implemented byte-mode drop, and an operator has - turned it on, it is strongly RECOMMENDED that it SHOULD be turned - off. Note that RED as a whole SHOULD NOT be turned off, as - without it, a drop tail queue also biases against large packets. - But note also that turning off byte-mode drop may alter the - relative performance of applications using different packet - sizes, so it would be advisable to establish the implications - before turning it off. - - NOTE WELL that RED's byte-mode queue drop is completely - orthogonal to byte-mode queue measurement and should not be - confused with it. If a RED implementation has a byte-mode but - does not specify what sort of byte-mode, it is most probably - byte-mode queue measurement, which is fine. However, if in - doubt, the vendor should be consulted. - - The byte mode packet drop variant of RED was recommended in the past - (see Section 4.2.1 for how thinking evolved). However, our survey of - 84 vendors across the industry (Section 4.2.5) has found that none of - the 19% who responded have implemented byte mode drop in RED. Given - there appears to be little, if any, installed base it seems we can - deprecate byte-mode drop in RED with little, if any, incremental - deployment impact. - -3.3. Recommendation on Responding to Congestion - - Instead of network equipment biasing its congestion notification in - favour of small packets, the IETF transport area should continue its - programme of; +3.6. Why now? - o updating host-based congestion control protocols to take account - of packet size + Now is a good time to discuss whether fairness between different + sized packets would best be implemented in network equipment, or at + the transport, for a number of reasons: - o making transports less sensitive to losing control packets like - SYNs and pure ACKs. + 1. The IETF pre-congestion notification (PCN) working group is + standardising the external behaviour of a PCN congestion + notification (AQM) algorithm [RFC5670]; - Corollaries: + 2. [RFC2309] says RED may either take account of packet size or not + when dropping, but gives no recommendation between the two, + referring instead to advice on the performance implications in an + email [pktByteEmail], which recommends byte-mode drop. Further, + just before RFC2309 was issued, an addendum was added to the + archived email that revisited the issue of packet vs. byte-mode + drop in its last paragraph, making the recommendation less clear- + cut. RFC2309 is currently the only advice in the RFC series on + packet size bias in AQM algorithms; - 1. If two TCPs with different packet sizes are required to run at - equal bit rates under the same path conditions, this SHOULD be - done by altering TCP (Section 4.2.2), not network equipment, - which would otherwise affect other transports besides TCP. + 3. The IRTF Internet Congestion Control Research Group (ICCRG) + recently took on the challenge of building consensus on what + common congestion control support should be required from network + forwarding functions in future [RFC6077]. The wider Internet + community needs to discuss whether the complexity of adjusting + for packet size should be in the network or in transports; - 2. If it is desired to improve TCP performance by reducing the - chance that a SYN or a pure ACK will be dropped, this should be - done by modifying TCP (Section 4.2.3), not network equipment. + 4. Given there are many good reasons why larger path max + transmission units (PMTUs) would help solve a number of scaling + issues, we don't want to create any bias against large packets + that is greater than their true cost; -3.4. Recommended Future Research + 5. The IETF audio/video transport (AVT) working group is + standardising how the real-time protocol (RTP) should feedback + and respond to explicit congestion notification (ECN) + [I-D.ietf-avt-ecn-for-rtp]. - The above conclusions cater for the Internet as it is today with most - resources being primarily bit-congestible. A secondary conclusion of - this memo is that research is needed to determine whether there might - be more packet-congestible resources in the future. Then further - research would be needed to extend the Internet's congestion - notification (drop or ECN) so that it would be able to handle a more - even mix of bit-congestible and packet-congestible resources. + 6. The IETF has started to consider the question of fairness between + flows that use different packet sizes (e.g. in the small-packet + variant of TCP-friendly rate control, TFRC-SP [RFC4828]). Given + transports with different packet sizes, if we don't decide + whether the network or the transport should allow for packet + size, it will be hard if not impossible to design any transport + protocol so that its bit-rate relative to other transports meets + design guidelines [RFC5033] (Note however that, if the concern + were fairness between users, rather than between flows + [Rate_fair_Dis], relative rates between flows would have to come + under run-time control rather than being embedded in protocol + designs). 4. A Survey and Critique of Past Advice The original 1993 paper on RED [RED93] proposed two options for the RED active queue management algorithm: packet mode and byte mode. Packet mode measured the queue length in packets and dropped (or marked) individual packets with a probability independent of their size. Byte mode measured the queue length in bytes and marked an individual packet with probability in proportion to its size (relative to the maximum packet size). In the paper's outline of @@ -625,21 +645,22 @@ o whether the drop probability of an individual packet should depend on its own size (Section 4.2). The rest of this section is structured accordingly. 4.1. Congestion Measurement Advice The choice of which metric to use to measure queue length was left open in RFC2309. It is now well understood that queues for bit- congestible resources should be measured in bytes, and queues for - packet-congestible resources should be measured in packets. + packet-congestible resources should be measured in packets + [pktByteEmail]. Some modern queue implementations give a choice for setting RED's thresholds in byte-mode or packet-mode. This may merely be an administrator-interface preference, not altering how the queue itself is measured but on some hardware it does actually change the way it measures its queue. Whether a resource is bit-congestible or packet- congestible is a property of the resource, so an admin should not ever need to, or be able to, configure the way a queue measures itself. @@ -775,21 +795,21 @@ In 2000, Cnodder et al [REDbyte] pointed out that there was an error in the part of the original 1993 RED algorithm that aimed to distribute drops uniformly, because it didn't correctly take into account the adjustment for packet size. They recommended an algorithm called RED_4 to fix this. But they also recommended a further change, RED_5, to adjust drop rate dependent on the square of relative packet size. This was indeed consistent with one implied motivation behind RED's byte mode drop--that we should reverse engineer the network to improve the performance of dominant end-to- end congestion control mechanisms. But it is not consistent with the - present recommendations of Section 3. + present recommendations of Section 2. By 2003, a further change had been made to the adjustment for packet size, this time in the RED algorithm of the ns2 simulator. Instead of taking each packet's size relative to a `maximum packet size' it was taken relative to a `mean packet size', intended to be a static value representative of the `typical' packet size on the link. We have not been able to find a justification in the literature for this change, however Eddy and Allman conducted experiments [REDbias] that assessed how sensitive RED was to this parameter, amongst other things. No-one seems to have pointed out that this changed algorithm @@ -955,21 +975,21 @@ RED (top right and bottom left). Top left is the `do nothing' scenario, while bottom right is the `do-both' scenario in which bit- rate would become far too biased towards small packets. Of course, if any form of byte-mode dropping RED has been deployed on a subset of queues that congest, each path through the network will present a different hybrid scenario to its transport. Whatever, we can see that the linear byte-mode drop column in the middle considerably complicates the Internet. It's a half-way house that doesn't bias enough towards small packets even if one believes - the network should be doing the biasing. Section 3 recommends that + the network should be doing the biasing. Section 2 recommends that _all_ bias in network equipment towards small packets should be turned off--if indeed any equipment vendors have implemented it-- leaving packet size bias solely as the preserve of the transport layer (solely the leftmost, packet-mode drop column). 4.2.5. RED Implementation Status A survey has been conducted of 84 vendors to assess how widely drop probability based on packet size has been implemented in RED. Prior to the survey, an individual approach to Cisco received confirmation @@ -1103,25 +1123,25 @@ folds both bit congestion and packet congestion into one signal (either loss or ECN). The problem of signalling packet processing congestion is not pressing, as most Internet resources are designed to be bit- congestible before packet processing starts to congest (see Section 1.1). However, the IRTF Internet congestion control research group (ICCRG) has set itself the task of reaching consensus on generic forwarding mechanisms that are necessary and sufficient to support the Internet's future congestion control requirements (the - first challenge in [I-D.irtf-iccrg-welzl]). Therefore, rather than - not giving this problem any thought at all, just because it is hard - and currently hypothetical, we defer the question of whether packet + first challenge in [RFC6077]). Therefore, rather than not giving + this problem any thought at all, just because it is hard and + currently hypothetical, we defer the question of whether packet congestion might become common and what to do if it does to the IRTF - (the 'Small Packets' challenge in [I-D.irtf-iccrg-welzl]). + (the 'Small Packets' challenge in [RFC6077]). 6. Security Considerations This draft recommends that queues do not bias drop probability towards small packets as this creates a perverse incentive for transports to break down their flows into tiny segments. One of the benefits of implementing AQM was meant to be to remove this perverse incentive that drop-tail queues gave to small packets. Of course, if transports really want to make the greatest gains, they don't have to respond to congestion anyway. But we don't want applications that @@ -1192,229 +1212,244 @@ that most Internet resources are bit-congestible not packet- congestible. We need to know the likelihood that this assumption will prevail longer term and, if it might not, what protocol changes will be needed to cater for a mix of the two. These questions have been delegated to the IRTF. 8. Acknowledgements Thank you to Sally Floyd, who gave extensive and useful review comments. Also thanks for the reviews from Philip Eardley, Toby - Moncaster and Arnaud Jacquet as well as helpful explanations of - different hardware approaches from Larry Dunn and Fred Baker. I am - grateful to Bruce Davie and his colleagues for providing a timely and - efficient survey of RED implementation in Cisco's product range. - Also grateful thanks to Toby Moncaster, Will Dormann, John Regnault, - Simon Carter and Stefaan De Cnodder who further helped survey the - current status of RED implementation and deployment and, finally, - thanks to the anonymous individuals who responded. + Moncaster, Arnaud Jacquet and Mirja Kuehlewind as well as helpful + explanations of different hardware approaches from Larry Dunn and + Fred Baker. We are grateful to Bruce Davie and his colleagues for + providing a timely and efficient survey of RED implementation in + Cisco's product range. Also grateful thanks to Toby Moncaster, Will + Dormann, John Regnault, Simon Carter and Stefaan De Cnodder who + further helped survey the current status of RED implementation and + deployment and, finally, thanks to the anonymous individuals who + responded. Bob Briscoe and Jukka Manner are partly funded by Trilogy, a research project (ICT- 216372) supported by the European Community under its Seventh Framework Programme. The views expressed here are those of the authors only. 9. Comments Solicited Comments and questions are encouraged and very welcome. They can be addressed to the IETF Transport Area working group mailing list , and/or to the authors. 10. References 10.1. Normative References - [RFC2119] Bradner, S., "Key words for use in RFCs - to Indicate Requirement Levels", BCP 14, - RFC 2119, March 1997. + [RFC2119] Bradner, S., "Key words for use in + RFCs to Indicate Requirement Levels", + BCP 14, RFC 2119, March 1997. [RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, S., Estrin, D., - Floyd, S., Jacobson, V., Minshall, G., - Partridge, C., Peterson, L., + 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. + "Recommendations on Queue Management + and Congestion Avoidance in the + Internet", RFC 2309, April 1998. [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [RFC3426] Floyd, S., "General Architectural and Policy Considerations", RFC 3426, November 2002. - [RFC5033] Floyd, S. and M. Allman, "Specifying New - Congestion Control Algorithms", BCP 133, - RFC 5033, August 2007. + [RFC5033] Floyd, S. and M. Allman, "Specifying + New Congestion Control Algorithms", + BCP 133, RFC 5033, August 2007. 10.2. Informative References - [CCvarPktSize] Widmer, J., Boutremans, C., and J-Y. Le - Boudec, "Congestion Control for Flows - with Variable Packet Size", ACM CCR 34(2) - 137--151, 2004, . + [CCvarPktSize] Widmer, J., Boutremans, C., and J-Y. + Le Boudec, "Congestion Control for + Flows with Variable Packet Size", ACM + CCR 34(2) 137--151, 2004, . - [DRQ] Shin, M., Chong, S., and I. Rhee, "Dual- - Resource TCP/AQM for Processing- - Constrained Networks", IEEE/ACM - Transactions on Networking Vol 16, issue - 2, April 2008, . [DupTCP] Wischik, D., "Short messages", Royal - Society workshop on networks: modelling - and control , September 2007, . + Society workshop on networks: + modelling and control , + September 2007, . - [ECNFixedWireless] Siris, V., "Resource Control for Elastic - Traffic in CDMA Networks", Proc. ACM - MOBICOM'02 , September 2002, . [Evol_cc] Gibbens, R. and F. Kelly, "Resource - pricing and the evolution of congestion - control", Automatica 35(12)1969--1985, + pricing and the evolution of + congestion control", + Automatica 35(12)1969--1985, December 1999, . - - [I-D.conex-concepts-uses] Briscoe, B., Woundy, R., Moncaster, T., - and J. Leslie, "ConEx Concepts and Use - Cases", - draft-moncaster-conex-concepts-uses-01 - (work in progress), July 2010. + www.statslab.cam.ac.uk/~frank/ + evol.html>. - [I-D.ietf-avt-ecn-for-rtp] Westerlund, M., Johansson, I., Perkins, - C., and K. Carlberg, "Explicit Congestion - Notification (ECN) for RTP over UDP", - draft-ietf-avt-ecn-for-rtp-02 (work in - progress), July 2010. + [I-D.ietf-avt-ecn-for-rtp] Westerlund, M., Johansson, I., + Perkins, C., and K. Carlberg, + "Explicit Congestion Notification + (ECN) for RTP over UDP", + draft-ietf-avt-ecn-for-rtp-03 (work + in progress), October 2010. - [I-D.irtf-iccrg-welzl] Welzl, M., Scharf, M., Briscoe, B., and - D. Papadimitriou, "Open Research Issues - in Internet Congestion Control", draft- - irtf-iccrg-welzl-congestion-control-open- - research-08 (work in progress), - September 2010. + [I-D.ietf-conex-concepts-uses] Briscoe, B., Woundy, R., Moncaster, + T., and J. Leslie, "ConEx Concepts + and Use Cases", + draft-ietf-conex-concepts-uses-00 + (work in progress), November 2010. [IOSArch] Bollapragada, V., White, R., and C. Murphy, "Inside Cisco IOS Software Architecture", Cisco Press: CCIE - Professional Development ISBN13: 978-1- - 57870-181-0, July 2000. + Professional Development ISBN13: 978- + 1-57870-181-0, July 2000. [MulTCP] Crowcroft, J. and Ph. Oechslin, "Differentiated End to End Internet - Services using a Weighted Proportional - Fair Sharing TCP", CCR 28(3) 53--69, - July 1998, . + Services using a Weighted + Proportional Fair Sharing TCP", + CCR 28(3) 53--69, July 1998, . [PktSizeEquCC] Vasallo, P., "Variable Packet Size - Equation-Based Congestion Control", ICSI - Technical Report tr-00-008, 2000, . + Equation-Based Congestion Control", + ICSI Technical Report tr-00-008, + 2000, . - [RED93] Floyd, S. and V. Jacobson, "Random Early - Detection (RED) gateways for Congestion - Avoidance", IEEE/ACM Transactions on - Networking 1(4) 397--413, August 1993, . - [REDbias] Eddy, W. and M. Allman, "A Comparison of - RED's Byte and Packet Modes", Computer - Networks 42(3) 261--280, June 2003, . + [REDbias] Eddy, W. and M. Allman, "A Comparison + of RED's Byte and Packet Modes", + Computer Networks 42(3) 261--280, + June 2003, . [REDbyte] De Cnodder, S., Elloumi, O., and K. Pauwels, "RED behavior with different - packet sizes", Proc. 5th IEEE Symposium - on Computers and Communications - (ISCC) 793--799, July 2000, . + packet sizes", Proc. 5th IEEE + Symposium on Computers and + Communications (ISCC) 793--799, + July 2000, . - [RFC2474] Nichols, K., Blake, S., Baker, F., and D. - Black, "Definition of the Differentiated - Services Field (DS Field) in the IPv4 and - IPv6 Headers", RFC 2474, December 1998. + [RFC2474] Nichols, K., Blake, S., Baker, F., + and D. Black, "Definition of the + Differentiated Services Field (DS + Field) in the IPv4 and IPv6 Headers", + RFC 2474, December 1998. - [RFC3448] Handley, M., Floyd, S., Padhye, J., and - J. Widmer, "TCP Friendly Rate Control - (TFRC): Protocol Specification", - RFC 3448, January 2003. + [RFC3448] Handley, M., Floyd, S., Padhye, J., + and J. Widmer, "TCP Friendly Rate + Control (TFRC): Protocol + Specification", RFC 3448, + January 2003. [RFC3714] Floyd, S. and J. Kempf, "IAB Concerns - Regarding Congestion Control for Voice - Traffic in the Internet", RFC 3714, - March 2004. + Regarding Congestion Control for + Voice Traffic in the Internet", + RFC 3714, March 2004. - [RFC4828] Floyd, S. and E. Kohler, "TCP Friendly - Rate Control (TFRC): The Small-Packet - (SP) Variant", RFC 4828, April 2007. + [RFC4828] Floyd, S. and E. Kohler, "TCP + Friendly Rate Control (TFRC): The + Small-Packet (SP) Variant", RFC 4828, + April 2007. - [RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., - and K. Ramakrishnan, "Adding Explicit - Congestion Notification (ECN) Capability - to TCP's SYN/ACK Packets", RFC 5562, - June 2009. + [RFC5562] Kuzmanovic, A., Mondal, A., Floyd, + S., and K. Ramakrishnan, "Adding + Explicit Congestion Notification + (ECN) Capability to TCP's SYN/ACK + Packets", RFC 5562, June 2009. [RFC5670] Eardley, P., "Metering and Marking Behaviour of PCN-Nodes", RFC 5670, November 2009. - [RFC5681] Allman, M., Paxson, V., and E. Blanton, - "TCP Congestion Control", RFC 5681, - September 2009. + [RFC5681] Allman, M., Paxson, V., and E. + Blanton, "TCP Congestion Control", + RFC 5681, September 2009. [RFC5690] Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding Acknowledgement Congestion Control to TCP", RFC 5690, February 2010. + [RFC6077] Papadimitriou, D., Welzl, M., Scharf, + M., and B. Briscoe, "Open Research + Issues in Internet Congestion + Control", RFC 6077, February 2011. + [Rate_fair_Dis] Briscoe, B., "Flow Rate Fairness: Dismantling a Religion", ACM CCR 37(2)63--74, April 2007, . + portal.acm.org/ + citation.cfm?id=1232926>. - [WindowPropFair] Siris, V., "Service Differentiation and - Performance of Weighted Window-Based - Congestion Control and Packet Marking - Algorithms in ECN Networks", Computer - Communications 26(4) 314--326, 2002, . - [gentle_RED] Floyd, S., "Recommendation on using the - "gentle_" variant of RED", Web page , - March 2000, . + [gentle_RED] Floyd, S., "Recommendation on using + the "gentle_" variant of RED", Web + page , March 2000, . - [pBox] Floyd, S. and K. Fall, "Promoting the Use - of End-to-End Congestion Control in the - Internet", IEEE/ACM Transactions on - Networking 7(4) 458--472, August 1999, . - [pktByteEmail] Floyd, S., "RED: Discussions of Byte and - Packet Modes", email , March 1997, . Appendix A. Idealised Wire Protocol We will start by inventing an idealised congestion notification protocol before discussing how to make it practical. The idealised protocol is shown to be correct using examples later in this appendix. A.1. Protocol Coding @@ -1440,21 +1475,21 @@ distinguish between bit and packet congestion [RFC3714]. Currently, packet-congestion is not the common case, but there is no guarantee that it will not become common with future technology trends. The idealised wire protocol is given below. It accounts for packet sizes at the transport layer, not in the network, and then only in the case of bit-congestible resources. This avoids the perverse incentive to send smaller packets and the DoS vulnerability that would otherwise result if the network were to bias towards them (see the motivating argument about avoiding perverse incentives in - Section 2.3): + Section 3.3): 1. A packet-congestible resource trying to code congestion level p_p into a packet stream should mark the idealised `packet congestion' field in each packet with probability p_p irrespective of the packet's size. The transport should then take a packet with the packet congestion field marked to mean just one mark, irrespective of the packet size. 2. A bit-congestible resource trying to code time-varying byte- congestion level p_b into a packet stream should mark the `byte @@ -1664,25 +1697,25 @@ congestion an individual (not just one flow) has caused due to all traffic entering the Internet from that individual. This is termed congestion accountability. But a byte-mode drop algorithm has to depend on the local MTU of the line - an algorithm needs to use some concept of a 'normal' packet size. Therefore, one dropped or marked packet is not necessarily equivalent to another unless you know the MTU at the queue where it was dropped/marked. To have an integrated view of a user, we believe congestion policing has to be located at an individual's attachment - point to the Internet [I-D.conex-concepts-uses]. But from there it - cannot know the MTU of each remote queue that caused each drop/mark. - Therefore it cannot take an integrated approach to policing all the - responses to congestion of all the transports of one individual. - Therefore it cannot police anything. + point to the Internet [I-D.ietf-conex-concepts-uses]. But from there + it cannot know the MTU of each remote queue that caused each drop/ + mark. Therefore it cannot take an integrated approach to policing + all the responses to congestion of all the transports of one + individual. Therefore it cannot police anything. The security/incentive argument _for_ packet-mode drop is similar. Firstly, confining RED to packet-mode drop would not preclude bottleneck policing approaches such as [pBox] as it seems likely they could work just as well by monitoring the volume of dropped bytes rather than packets. Secondly packet-mode dropping/marking naturally allows the congestion notification of packets to be globally meaningful without relying on MTU information held elsewhere. Because we recommend that a dropped/marked packet should be taken to @@ -1701,20 +1734,26 @@ Appendix C. Changes from Previous Versions To be removed by the RFC Editor on publication. Full incremental diffs between each version are available at or (courtesy of the rfcdiff tool): + From -03 to -04: + + * Reordered Sections 2 and 3, and some clarifications here and + there based on feedback from Colin Perkins and Mirja + Kuehlewind. + From -02 to -03 (this version) * Structural changes: + Split off text at end of "Scaling Congestion Control with Packet Size" into new section "Transport-Independent Network" + Shifted "Recommendations" straight after "Motivating Arguments" and added "Conclusions" at end to reinforce @@ -1768,44 +1807,44 @@ * Changed PCN references from the PCN charter & architecture to the PCN marking behaviour draft most likely to imminently become the standards track WG item. From -01 to -02: * Abstract reorganised to align with clearer separation of issue in the memo. * Introduction reorganised with motivating arguments removed to - new Section 2. + new Section 3. * Clarified avoiding lock-out of large packets is not the main or only motivation for RED. * Mentioned choice of drop or marking explicitly throughout, rather than trying to coin a word to mean either. * Generalised the discussion throughout to any packet forwarding function on any network equipment, not just routers. * Clarified the last point about why this is a good time to sort out this issue: because it will be hard / impossible to design new transports unless we decide whether the network or the transport is allowing for packet size. * Added statement explaining the horizon of the memo is long term, but with short term expediency in mind. * Added material on scaling congestion control with packet size - (Section 2.1). + (Section 3.1). * Separated out issue of normalising TCP's bit rate from issue of - preference to control packets (Section 2.4). + preference to control packets (Section 3.4). * Divided up Congestion Measurement section for clarity, including new material on fixed size packet buffers and buffer carving (Section 4.1.1 & Section 4.2.1) and on congestion measurement in wireless link technologies without queues (Section 4.1.2). * Added section on 'Making Transports Robust against Control Packet Losses' (Section 4.2.3) with existing & new material included.