[Docs] [txt|pdf|xml] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-briscoe-tsvwg-byte-pkt-mark) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 7141

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


                Byte and Packet Congestion Notification
                  draft-ietf-tsvwg-byte-pkt-congest-03

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
   RED AQM algorithm that drops fewer small packets should not be used.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   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.

Copyright Notice

   Copyright (c) 2010 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



Briscoe & Manner         Expires April 27, 2011                 [Page 1]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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.















































Briscoe & Manner         Expires April 27, 2011                 [Page 2]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


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
   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
     4.2.  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
       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
   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
     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.2.5.  Pkt-congestible resource, equal packet rates (Bii) . . 37
   Appendix B.  Byte-mode Drop Complicates Policing Congestion
                Response  . . . . . . . . . . . . . . . . . . . . . . 37



Briscoe & Manner         Expires April 27, 2011                 [Page 3]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   Appendix C.  Changes from Previous Versions  . . . . . . . . . . . 38


















































Briscoe & Manner         Expires April 27, 2011                 [Page 4]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


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
   packet sizes into account has exercised the minds of researchers and
   practitioners for as long as active queue management (AQM) has been
   discussed.  Indeed, one reason AQM was originally introduced was to
   reduce the lock-out effects that small packets can have on large
   packets in drop-tail queues.  This memo aims to state the principles
   we should be using and to come to conclusions on what these
   principles will mean for future protocol design, taking into account
   the deployments we have already.

   The byte vs. packet dilemma arises at three stages in the congestion
   notification process:

   Measuring congestion:  When the congested resource decides locally to
      measure how congested it is, should the queue measure its length
      in bytes or packets?

   Encoding congestion notification into the wire protocol:  When the
      congested network resource decides whether to notify the level of
      congestion by dropping or marking a particular packet, should its
      decision depend on the byte-size of the particular packet being
      dropped or marked?

   Decoding congestion notification from the wire protocol:  When the
      transport interprets the notification in order to decide how much
      to respond to congestion, should it take into account the byte-
      size of each missing or marked packet?

   Consensus has emerged over the years concerning the first stage:
   whether queues are measured in bytes or packets, termed byte-mode
   queue measurement or packet-mode queue measurement.  This memo
   records this consensus in the RFC Series.  In summary the choice
   solely depends on whether the resource is congested by bytes or
   packets.

   The controversy is mainly around the last two stages: whether to
   allow for the size of the specific packet notifying congestion i)
   when the network encodes or ii) when the transport decodes the
   congestion notification.




Briscoe & Manner         Expires April 27, 2011                 [Page 5]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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.

   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
   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.

   This memo intentionally includes a non-negligible amount of material



Briscoe & Manner         Expires April 27, 2011                 [Page 6]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   on the subject.  A busy reader can jump right into Section 3 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
      will give a working definition of what congestion notification
      should be taken to mean for this document.  Congestion
      notification is a changing signal that aims to communicate the
      ratio E/L. E is the instantaneous excess load offered to a
      resource that it is either incapable of serving or unwilling to
      serve.  L is the instantaneous offered load.

      The phrase `unwilling to serve' is added, because AQM systems
      (e.g.  RED, PCN [RFC5670]) set a virtual limit smaller than the
      actual limit to the resource, then notify when this virtual limit
      is exceeded in order to avoid congestion of the actual capacity.

      Note that the denominator is offered load, not capacity.
      Therefore congestion notification is a real number bounded by the
      range [0,1].  This ties in with the most well-understood measure
      of congestion notification: drop probability (often loosely called
      loss rate).  It also means that congestion has a natural
      interpretation as a probability; the probability of offered
      traffic not being served (or being marked as at risk of not being
      served).

   Explicit and Implicit Notification:  The byte vs. packet dilemma
      concerns congestion notification irrespective of whether it is
      signalled implicitly by drop or using explicit congestion
      notification (ECN [RFC3168] or PCN [RFC5670]).  Throughout this
      document, unless clear from the context, the term marking will be
      used to mean notifying congestion explicitly, while congestion
      notification will be used to mean notifying congestion either
      implicitly by drop or explicitly by marking.

   Bit-congestible vs. Packet-congestible:  If the load on a resource
      depends on the rate at which packets arrive, it is called packet-
      congestible.  If the load depends on the rate at which bits arrive
      it is called bit-congestible.

      Examples of packet-congestible resources are route look-up engines
      and firewalls, because load depends on how many packet headers



Briscoe & Manner         Expires April 27, 2011                 [Page 7]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


      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.

      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-
      congestible.  This memo focuses on congestion of connectionless
      resources, but the same principles may be applicable for
      congestion notification protocols controlling per-flow and per-
      session processing or state.

   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?

   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:

   1.  The IETF pre-congestion notification (PCN) working group is
       standardising the external behaviour of a PCN congestion
       notification (AQM) algorithm [RFC5670];





Briscoe & Manner         Expires April 27, 2011                 [Page 8]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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;

   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).

   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;

   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;

   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].

   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).







Briscoe & Manner         Expires April 27, 2011                 [Page 9]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


2.  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

   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
   other (B).  Of course, because the load is the same, there will be
   proportionately more packets in the small packet flow (B).

   If a congestion control scales with packet size it should respond in
   the same way to the same congestion excursion, irrespective of the
   size of the packets that the bytes causing congestion happen to be
   broken down into.

   A bit-congestible queue suffering a congestion excursion has to drop
   or mark the same excess bytes whether they are in a few large packets
   (A) or many small packets (B).  So for the same congestion excursion,
   the same amount of bytes have to be shed to get the load back to its
   operating point.  But, of course, for smaller packets (B) more
   packets will have to be discarded to shed the same bytes.

   If all the transports interpret each drop/mark as a single loss event
   irrespective of the size of the packet dropped, those with smaller
   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

   TCP congestion control ensures that flows competing for the same
   resource each maintain the same number of segments in flight,



Briscoe & Manner         Expires April 27, 2011                [Page 10]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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
   network in order to fix one transport, no matter how prominent it is.
   Effectively, favouring small packets is reverse engineering of
   network equipment around one particular transport protocol (TCP),
   contrary to the excellent advice in [RFC3426], which asks designers
   to question "Why are you proposing a solution at this layer of the
   protocol stack, rather than at another layer?"

   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

   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:

   Malicious transports:  A queue that gives an advantage to small
      packets can be used to amplify the force of a flooding attack.  By
      sending a flood of small packets, the attacker can get the queue
      to discard more traffic in large packets, allowing more attack
      traffic to get through to cause further damage.  Such a queue
      allows attack traffic to have a disproportionately large effect on
      regular traffic without the attacker having to do much work.

   Non-malicious transports:  Even if a transport is not actually
      malicious, if it finds small packets go faster, over time it will
      tend to act in its own interest and use them.  Queues that give
      advantage to small packets create an evolutionary pressure for
      transports to send at the same bit-rate but break their data
      stream down into tiny segments to reduce their drop rate.



Briscoe & Manner         Expires April 27, 2011                [Page 11]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


      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).

   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

   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).







Briscoe & Manner         Expires April 27, 2011                [Page 12]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


2.5.  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).



Briscoe & Manner         Expires April 27, 2011                [Page 13]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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;

   o  updating host-based congestion control protocols to take account
      of packet size




Briscoe & Manner         Expires April 27, 2011                [Page 14]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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.

3.4.  Recommended Future Research

   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.

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
   further work, it was stated that no recommendation had been made on
   whether the queue size should be measured in bytes or packets, but
   noted that the difference could be significant.

   When RED was recommended for general deployment in 1998 [RFC2309],
   the two modes were mentioned implying the choice between them was a
   question of performance, referring to a 1997 email [pktByteEmail] for
   advice on tuning.  A later addendum to this email introduced the
   insight that there are in fact two orthogonal choices:

   o  whether to measure queue length in bytes or packets (Section 4.1)

   o  whether the drop probability of an individual packet should depend
      on its own size (Section 4.2).




Briscoe & Manner         Expires April 27, 2011                [Page 15]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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.

   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.

   NOTE: Congestion in some legacy bit-congestible buffers is only
   measured in packets not bytes.  In such cases, the operator has to
   set the thresholds mindful of a typical mix of packets sizes.  Any
   AQM algorithm on such a buffer will be oversensitive to high
   proportions of small packets, e.g. a DoS attack, and undersensitive
   to high proportions of large packets.  However, there is no need to
   make allowances for the possibility of such legacy in future protocol
   design.  This is safe because any undersensitivity during unusual
   traffic mixes cannot lead to congestion collapse given the buffer
   will eventually revert to tail drop, discarding proportionately more
   large packets.

4.1.1.  Fixed Size Packet Buffers

   Although the question of whether to measure queues in bytes or
   packets is fairly well understood these days, measuring congestion is
   not straightforward when the resource is bit congestible but the
   queue is packet congestible or vice versa.  This section outlines the
   approach to take.  There is no controversy over what should be done,
   you just need to be expert in probability to work it out.  And, even
   if you know what should be done, it's not always easy to find a
   practical algorithm to implement it.

   Some, mostly older, queuing hardware sets aside fixed sized buffers
   in which to store each packet in the queue.  Also, with some
   hardware, any fixed sized buffers not completely filled by a packet
   are padded when transmitted to the wire.  If we imagine a theoretical
   forwarding system with both queuing and transmission in fixed, MTU-
   sized units, it should clearly be treated as packet-congestible,
   because the queue length in packets would be a good model of



Briscoe & Manner         Expires April 27, 2011                [Page 16]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   congestion of the lower layer link.

   If we now imagine a hybrid forwarding system with transmission delay
   largely dependent on the byte-size of packets but buffers of one MTU
   per packet, it should strictly require a more complex algorithm to
   determine the probability of congestion.  It should be treated as two
   resources in sequence, where the sum of the byte-sizes of the packets
   within each packet buffer models congestion of the line while the
   length of the queue in packets models congestion of the queue.  Then
   the probability of congesting the forwarding buffer would be a
   conditional probability--conditional on the previously calculated
   probability of congesting the line.

   In systems that use fixed size buffers, it is unusual for all the
   buffers used by an interface to be the same size.  Typically pools of
   different sized buffers are provided (Cisco uses the term 'buffer
   carving' for the process of dividing up memory into these pools
   [IOSArch]).  Usually, if the pool of small buffers is exhausted,
   arriving small packets can borrow space in the pool of large buffers,
   but not vice versa.  However, it is easier to work out what should be
   done if we temporarily set aside the possibility of such borrowing.
   Then, with fixed pools of buffers for different sized packets and no
   borrowing, the size of each pool and the current queue length in each
   pool would both be measured in packets.  So an AQM algorithm would
   have to maintain the queue length for each pool, and judge whether to
   drop/mark a packet of a particular size by looking at the pool for
   packets of that size and using the length (in packets) of its queue.

   We now return to the issue we temporarily set aside: small packets
   borrowing space in larger buffers.  In this case, the only difference
   is that the pools for smaller packets have a maximum queue size that
   includes all the pools for larger packets.  And every time a packet
   takes a larger buffer, the current queue size has to be incremented
   for all queues in the pools of buffers less than or equal to the
   buffer size used.

   We will return to borrowing of fixed sized buffers when we discuss
   biasing the drop/marking probability of a specific packet because of
   its size in Section 4.2.1.  But here we can give a at least one
   simple rule for how to measure the length of queues of fixed buffers:
   no matter how complicated the scheme is, ultimately any fixed buffer
   system will need to measure its queue length in packets not bytes.

4.1.2.  Congestion Measurement without a Queue

   AQM algorithms are nearly always described assuming there is a queue
   for a congested resource and the algorithm can use the queue length
   to determine the probability that it will drop or mark each packet.



Briscoe & Manner         Expires April 27, 2011                [Page 17]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   But not all congested resources lead to queues.  For instance,
   wireless spectrum is bit-congestible (for a given coding scheme),
   because interference increases with the rate at which bits are
   transmitted.  But wireless link protocols do not always maintain a
   queue that depends on spectrum interference.  Similarly, power
   limited resources are also usually bit-congestible if energy is
   primarily required for transmission rather than header processing,
   but it is rare for a link protocol to build a queue as it approaches
   maximum power.

   Nonetheless, AQM algorithms do not require a queue in order to work.
   For instance spectrum congestion can be modelled by signal quality
   using target bit-energy-to-noise-density ratio.  And, to model radio
   power exhaustion, transmission power levels can be measured and
   compared to the maximum power available.  [ECNFixedWireless] proposes
   a practical and theoretically sound way to combine congestion
   notification for different bit-congestible resources at different
   layers along an end to end path, whether wireless or wired, and
   whether with or without queues.

4.2.  Congestion Notification Advice

4.2.1.  Network Bias when Encoding

   The previously mentioned email [pktByteEmail] referred to by
   [RFC2309] advised that most scarce resources in the Internet were
   bit-congestible, which is still believed to be true (Section 1.1).
   But it went on to give advice we now disagree with.  It said that
   drop probability should depend on the size of the packet being
   considered for drop if the resource is bit-congestible, but not if it
   is packet-congestible.  The argument continued that if packet drops
   were inflated by packet size (byte-mode dropping), "a flow's fraction
   of the packet drops is then a good indication of that flow's fraction
   of the link bandwidth in bits per second".  This was consistent with
   a referenced policing mechanism being worked on at the time for
   detecting unusually high bandwidth flows, eventually published in
   1999 [pBox].  However, the problem could and should have been solved
   by making the policing mechanism count the volume of bytes randomly
   dropped, not the number of packets.

   A few months before RFC2309 was published, an addendum was added to
   the above archived email referenced from the RFC, in which the final
   paragraph seemed to partially retract what had previously been said.
   It clarified that the question of whether the probability of
   dropping/marking a packet should depend on its size was not related
   to whether the resource itself was bit congestible, but a completely
   orthogonal question.  However the only example given had the queue
   measured in packets but packet drop depended on the byte-size of the



Briscoe & Manner         Expires April 27, 2011                [Page 18]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   packet in question.  No example was given the other way round.

   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.

   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
   can often lead to drop probabilities of greater than 1 (which should
   ring alarm bells hinting that there's a mistake in the theory
   somewhere).

   On 10-Nov-2004, this variant of byte-mode packet drop was made the
   default in the ns2 simulator.  None of the responses to our
   admittedly limited survey of implementers (Section 4.2.5) found any
   variant of byte-mode drop had been implemented.  Therefore any
   conclusions based on ns2 simulations that use RED without disabling
   byte-mode drop are likely to be highly questionable.

   The byte-mode drop variant of RED is, of course, not the only
   possible bias towards small packets in queueing systems.  We have
   already mentioned that tail-drop queues naturally tend to lock-out
   large packets once they are full.  But also queues with fixed sized
   buffers reduce the probability that small packets will be dropped if
   (and only if) they allow small packets to borrow buffers from the
   pools for larger packets.  As was explained in Section 4.1.1 on fixed
   size buffer carving, borrowing effectively makes the maximum queue
   size for small packets greater than that for large packets, because
   more buffers can be used by small packets while less will fit large
   packets.

   In itself, the bias towards small packets caused by buffer borrowing
   is perfectly correct.  Lower drop probability for small packets is



Briscoe & Manner         Expires April 27, 2011                [Page 19]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   legitimate in buffer borrowing schemes, because small packets
   genuinely congest the machine's buffer memory less than large
   packets, given they can fit in more spaces.  The bias towards small
   packets is not artificially added (as it is in RED's byte-mode drop
   algorithm), it merely reflects the reality of the way fixed buffer
   memory gets congested.  Incidentally, the bias towards small packets
   from buffer borrowing is nothing like as large as that of RED's byte-
   mode drop.

   Nonetheless, fixed-buffer memory with tail drop is still prone to
   lock-out large packets, purely because of the tail-drop aspect.  So a
   good AQM algorithm like RED with packet-mode drop should be used with
   fixed buffer memories where possible.  If RED is too complicated to
   implement with multiple fixed buffer pools, the minimum necessary to
   prevent large packet lock-out is to ensure smaller packets never use
   the last available buffer in any of the pools for larger packets.

4.2.2.  Transport Bias when Decoding

   The above proposals to alter the network equipment to bias towards
   smaller packets have largely carried on outside the IETF process
   (unless one counts a reference in an informational RFC to an archived
   email!).  Whereas, within the IETF, there are many different
   proposals to alter transport protocols to achieve the same goals,
   i.e. either to make the flow bit-rate take account of packet size, or
   to protect control packets from loss.  This memo argues that altering
   transport protocols is the more principled approach.

   A recently approved experimental RFC adapts its transport layer
   protocol to take account of packet sizes relative to typical TCP
   packet sizes.  This proposes a new small-packet variant of TCP-
   friendly rate control [RFC3448] called TFRC-SP [RFC4828].
   Essentially, it proposes a rate equation that inflates the flow rate
   by the ratio of a typical TCP segment size (1500B including TCP
   header) over the actual segment size [PktSizeEquCC].  (There are also
   other important differences of detail relative to TFRC, such as using
   virtual packets [CCvarPktSize] to avoid responding to multiple losses
   per round trip and using a minimum inter-packet interval.)

   Section 4.5.1 of this TFRC-SP spec discusses the implications of
   operating in an environment where queues have been configured to drop
   smaller packets with proportionately lower probability than larger
   ones.  But it only discusses TCP operating in such an environment,
   only mentioning TFRC-SP briefly when discussing how to define
   fairness with TCP.  And it only discusses the byte-mode dropping
   version of RED as it was before Cnodder et al pointed out it didn't
   sufficiently bias towards small packets to make TCP independent of
   packet size.



Briscoe & Manner         Expires April 27, 2011                [Page 20]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   So the TFRC-SP spec doesn't address the issue of which of the network
   or the transport _should_ handle fairness between different packet
   sizes.  In its Appendix B.4 it discusses the possibility of both
   TFRC-SP and some network buffers duplicating each other's attempts to
   deliberately bias towards small packets.  But the discussion is not
   conclusive, instead reporting simulations of many of the
   possibilities in order to assess performance but not recommending any
   particular course of action.

   The paper originally proposing TFRC with virtual packets (VP-TFRC)
   [CCvarPktSize] proposed that there should perhaps be two variants to
   cater for the different variants of RED.  However, as the TFRC-SP
   authors point out, there is no way for a transport to know whether
   some queues on its path have deployed RED with byte-mode packet drop
   (except if an exhaustive survey found that no-one has deployed it!--
   see Section 4.2.4).  Incidentally, VP-TFRC also proposed that byte-
   mode RED dropping should really square the packet size compensation
   factor (like that of Cnodder's RED_5, but apparently unaware of it).

   Pre-congestion notification [RFC5670] is a proposal to use a virtual
   queue for AQM marking for packets within one Diffserv class in order
   to give early warning prior to any real queuing.  The proposed PCN
   marking algorithms have been designed not to take account of packet
   size when forwarding through queues.  Instead the general principle
   has been to take account of the sizes of marked packets when
   monitoring the fraction of marking at the edge of the network, as
   recommended here.

4.2.3.  Making Transports Robust against Control Packet Losses

   Recently, two RFCs have defined changes to TCP that make it more
   robust against losing small control packets [RFC5562] [RFC5690].  In
   both cases they note that the case for these two TCP changes would be
   weaker if RED were biased against dropping small packets.  We argue
   here that these two proposals are a safer and more principled way to
   achieve TCP performance improvements than reverse engineering RED to
   benefit TCP.

   Although no proposals exist as far as we know, it would also be
   possible and perfectly valid to make control packets robust against
   drop by explicitly requesting a lower drop probability using their
   Diffserv code point [RFC2474] to request a scheduling class with
   lower drop.

   Although not brought to the IETF, a simple proposal from Wischik
   [DupTCP] suggests that the first three packets of every TCP flow
   should be routinely duplicated after a short delay.  It shows that
   this would greatly improve the chances of short flows completing



Briscoe & Manner         Expires April 27, 2011                [Page 21]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   quickly, but it would hardly increase traffic levels on the Internet,
   because Internet bytes have always been concentrated in the large
   flows.  It further shows that the performance of many typical
   applications depends on completion of long serial chains of short
   messages.  It argues that, given most of the value people get from
   the Internet is concentrated within short flows, this simple
   expedient would greatly increase the value of the best efforts
   Internet at minimal cost.

4.2.4.  Congestion Notification: Summary of Conflicting Advice

   +-----------+----------------+-----------------+--------------------+
   | transport |  RED_1 (packet |  RED_4 (linear  | RED_5 (square byte |
   |        cc |   mode drop)   | byte mode drop) |     mode drop)     |
   +-----------+----------------+-----------------+--------------------+
   |    TCP or |    s/sqrt(p)   |    sqrt(s/p)    |      1/sqrt(p)     |
   |      TFRC |                |                 |                    |
   |   TFRC-SP |    1/sqrt(p)   |    1/sqrt(sp)   |    1/(s.sqrt(p))   |
   +-----------+----------------+-----------------+--------------------+

     Table 1: Dependence of flow bit-rate per RTT on packet size s and
   drop rate p when network and/or transport bias towards small packets
                            to varying degrees

   Table 1 aims to summarise the potential effects of all the advice
   from different sources.  Each column shows a different possible AQM
   behaviour in different queues in the network, using the terminology
   of Cnodder et al outlined earlier (RED_1 is basic RED with packet-
   mode drop).  Each row shows a different transport behaviour: TCP
   [RFC5681] and TFRC [RFC3448] on the top row with TFRC-SP [RFC4828]
   below.

   Let us assume that the goal is for the bit-rate of a flow to be
   independent of packet size.  Suppressing all inessential details, the
   table shows that this should either be achievable by not altering the
   TCP transport in a RED_5 network, or using the small packet TFRC-SP
   transport (or similar) in a network without any byte-mode dropping
   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



Briscoe & Manner         Expires April 27, 2011                [Page 22]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   _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
   that, having checked the code-base for each of the product ranges,
   Cisco has not implemented any discrimination based on packet size in
   any AQM algorithm in any of its products.  Also an individual
   approach to Alcatel-Lucent drew a confirmation that it was very
   likely that none of their products contained RED code that
   implemented any packet-size bias.

   Turning to our more formal survey (Table 2), about 19% of those
   surveyed have replied so far, giving a sample size of 16.  Although
   we do not have permission to identify the respondents, we can say
   that those that have responded include most of the larger vendors,
   covering a large fraction of the market.  They range across the large
   network equipment vendors at L3 & L2, firewall vendors, wireless
   equipment vendors, as well as large software businesses with a small
   selection of networking products.  So far, all those who have
   responded have confirmed that they have not implemented the variant
   of RED with drop dependent on packet size (2 were fairly sure they
   had not but needed to check more thoroughly).  We have established
   that Linux does not implement RED with packet size drop bias,
   although we have not investigated a wider range of open source code.

   +-------------------------------+----------------+-----------------+
   |                      Response | No. of vendors | %age of vendors |
   +-------------------------------+----------------+-----------------+
   |               Not implemented |             14 |             17% |
   |    Not implemented (probably) |              2 |              2% |
   |                   Implemented |              0 |              0% |
   |                   No response |             68 |             81% |
   | Total companies/orgs surveyed |             84 |            100% |
   +-------------------------------+----------------+-----------------+

    Table 2: Vendor Survey on byte-mode drop variant of RED (lower drop
                      probability for small packets)

   Where reasons have been given, the extra complexity of packet bias
   code has been most prevalent, though one vendor had a more principled
   reason for avoiding it--similar to the argument of this document.




Briscoe & Manner         Expires April 27, 2011                [Page 23]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   Finally, we repeat that RED's byte mode drop SHOULD be disabled, but
   active queue management such as RED SHOULD be enabled wherever
   possible if we are to eradicate bias towards small packets--without
   any AQM at all, tail-drop tends to lock-out large packets very
   effectively.

   Our survey was of vendor implementations, so we cannot be certain
   about operator deployment.  But we believe many queues in the
   Internet are still tail-drop.  The company of one of the co-authors
   (BT) has widely deployed RED, but many tail-drop queues are there are
   bound to still exist, particularly in access network equipment and on
   middleboxes like firewalls, where RED is not always available.

   Routers using a memory architecture based on fixed size buffers with
   borrowing may also still be prevalent in the Internet.  As explained
   in Section 4.2.1, these also provide a marginal (but legitimate) bias
   towards small packets.  So even though RED byte-mode drop is not
   prevalent, it is likely there is still some bias towards small
   packets in the Internet due to tail drop and fixed buffer borrowing.

5.  Outstanding Issues and Next Steps

5.1.  Bit-congestible World

   For a connectionless network with nearly all resources being bit-
   congestible we believe the recommended position is now unarguably
   clear--that the network should not make allowance for packet sizes
   and the transport should.  This leaves two outstanding issues:

   o  How to handle any legacy of AQM with byte-mode drop already
      deployed;

   o  The need to start a programme to update transport congestion
      control protocol standards to take account of packet size.

   The sample of returns from our vendor survey Section 4.2.4 suggest
   that byte-mode packet drop seems not to be implemented at all let
   alone deployed, or if it is, it is likely to be very sparse.
   Therefore, we do not really need a migration strategy from all but
   nothing to nothing.

   A programme of standards updates to take account of packet size in
   transport congestion control protocols has started with TFRC-SP
   [RFC4828], while weighted TCPs implemented in the research community
   [WindowPropFair] could form the basis of a future change to TCP
   congestion control [RFC5681] itself.





Briscoe & Manner         Expires April 27, 2011                [Page 24]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


5.2.  Bit- & Packet-congestible World

   Nonetheless, the position is much less clear-cut if the Internet
   becomes populated by a more even mix of both packet-congestible and
   bit-congestible resources.  If we believe we should allow for this
   possibility in the future, this space contains a truly open research
   issue.

   We develop the concept of an idealised congestion notification
   protocol that supports both bit-congestible and packet-congestible
   resources in Appendix A.  This congestion notification requires at
   least two flags for congestion of bit-congestible and packet-
   congestible resources.  This hides a fundamental problem--much more
   fundamental than whether we can magically create header space for yet
   another ECN flag in IPv4, or whether it would work while being
   deployed incrementally.  Distinguishing drop from delivery naturally
   provides just one congestion flag--it is hard to drop a packet in two
   ways that are distinguishable remotely.  This is a similar problem to
   that of distinguishing wireless transmission losses from congestive
   losses.

   This problem would not be solved even if ECN were universally
   deployed.  A congestion notification protocol must survive a
   transition from low levels of congestion to high.  Marking two states
   is feasible with explicit marking, but much harder if packets are
   dropped.  Also, it will not always be cost-effective to implement AQM
   at every low level resource, so drop will often have to suffice.

   We should also note that, strictly, packet-congestible resources are
   actually cycle-congestible because load also depends on the
   complexity of each look-up and whether the pattern of arrivals is
   amenable to caching or not.  Further, this reminds us that any
   solution must not require a forwarding engine to use excessive
   processor cycles in order to decide how to say it has no spare
   processor cycles.

   Recently, the dual resource queue (DRQ) proposal [DRQ] has been made
   on the premise that, as network processors become more cost
   effective, per packet operations will become more complex
   (irrespective of whether more function in the network is desirable).
   Consequently the premise is that CPU congestion will become more
   common.  DRQ is a proposed modification to the RED algorithm that
   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



Briscoe & Manner         Expires April 27, 2011                [Page 25]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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
   congestion might become common and what to do if it does to the IRTF
   (the 'Small Packets' challenge in [I-D.irtf-iccrg-welzl]).

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
   are trying to behave to discover that they can go faster by using
   smaller packets.

   In practice, transports cannot all be trusted to respond to
   congestion.  So another reason for recommending that queues do not
   bias drop probability towards small packets is to avoid the
   vulnerability to small packet DDoS attacks that would otherwise
   result.  One of the benefits of implementing AQM was meant to be to
   remove drop-tail's DoS vulnerability to small packets, so we
   shouldn't add it back again.

   If most queues implemented AQM with byte-mode drop, the resulting
   network would amplify the potency of a small packet DDoS attack.  At
   the first queue the stream of packets would push aside a greater
   proportion of large packets, so more of the small packets would
   survive to attack the next queue.  Thus a flood of small packets
   would continue on towards the destination, pushing regular traffic
   with large packets out of the way in one queue after the next, but
   suffering much less drop itself.

   Appendix B explains why the ability of networks to police the
   response of _any_ transport to congestion depends on bit-congestible
   network resources only doing packet-mode not byte-mode drop.  In
   summary, it says that making drop probability depend on the size of
   the packets that bits happen to be divided into simply encourages the
   bits to be divided into smaller packets.  Byte-mode drop would
   therefore irreversibly complicate any attempt to fix the Internet's
   incentive structures.




Briscoe & Manner         Expires April 27, 2011                [Page 26]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


7.  Conclusions

   This memo strongly recommends that the size of an individual packet
   that is dropped or marked should only be taken into account when a
   transport reads this as a congestion indication, not when network
   equipment writes it.  The memo therefore strongly deprecates using
   RED's byte-mode of packet drop in network equipment.

   Whether network equipment should measure the length of a queue by
   counting bytes or counting packets is a different question to whether
   it should take into account the size of each packet being dropped or
   marked.  The answer depends on whether the network resource is
   congested respectively by bytes or by packets.  This means that RED's
   byte-mode queue measurement will often be appropriate even though
   byte-mode drop is strongly deprecated.

   At the transport layer the IETF should continue updating congestion
   control protocols to take account of the size of each packet that
   indicates congestion.  Also the IETF should continue to make
   transports less sensitive to losing control packets like SYNs, pure
   ACKs and DNS exchanges.  Although many control packets happen to be
   small, the alternative of network equipment favouring all small
   packets would be dangerous.  That would create perverse incentives to
   split data transfers into smaller packets.

   The memo develops these recommendations from principled arguments
   concerning scaling, layering, incentives, inherent efficiency,
   security and policability.  But it also addresses practical issues
   such as specific buffer architectures and incremental deployment.
   Indeed a limited survey of RED implementations is included, which
   shows there appears to be little, if any, installed base of RED's
   byte-mode drop.  Therefore it can be deprecated with little, if any,
   incremental deployment complications.

   The recommendations have been developed on the well-founded basis
   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



Briscoe & Manner         Expires April 27, 2011                [Page 27]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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
   <tsvwg@ietf.org>, 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.

   [RFC2309]                   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.

   [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.




Briscoe & Manner         Expires April 27, 2011                [Page 28]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


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, <http://doi.acm.org/
                               10.1145/997150.997162>.

   [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, <http://dx.doi.org/
                               10.1109/TNET.2007.900415>.

   [DupTCP]                    Wischik, D., "Short messages", Royal
                               Society workshop on networks: modelling
                               and control , September 2007, <http://
                               www.cs.ucl.ac.uk/staff/ucacdjw/Research/
                               shortmsg.html>.

   [ECNFixedWireless]          Siris, V., "Resource Control for Elastic
                               Traffic in CDMA Networks", Proc. ACM
                               MOBICOM'02 , September 2002, <http://
                               www.ics.forth.gr/netlab/publications/
                               resource_control_elastic_cdma.html>.

   [Evol_cc]                   Gibbens, R. and F. Kelly, "Resource
                               pricing and the evolution of congestion
                               control", Automatica 35(12)1969--1985,
                               December 1999, <http://
                               www.statslab.cam.ac.uk/~frank/evol.html>.

   [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.

   [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.irtf-iccrg-welzl]      Welzl, M., Scharf, M., Briscoe, B., and
                               D. Papadimitriou, "Open Research Issues
                               in Internet Congestion Control", draft-



Briscoe & Manner         Expires April 27, 2011                [Page 29]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


                               irtf-iccrg-welzl-congestion-control-open-
                               research-08 (work in progress),
                               September 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.

   [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, <http://www.cs.ucl.ac.uk/
                               staff/J.Crowcroft/hipparch/pricing.html>.

   [PktSizeEquCC]              Vasallo, P., "Variable Packet Size
                               Equation-Based Congestion Control", ICSI
                               Technical Report tr-00-008, 2000, <http:/
                               /http.icsi.berkeley.edu/ftp/global/pub/
                               techreports/2000/tr-00-008.pdf>.

   [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, <h
                               ttp://www.icir.org/floyd/papers/red/
                               red.html>.

   [REDbias]                   Eddy, W. and M. Allman, "A Comparison of
                               RED's Byte and Packet Modes", Computer
                               Networks 42(3) 261--280, June 2003, <http
                               ://www.ir.bbn.com/documents/articles/
                               redbias.ps>.

   [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, <http://
                               www.icir.org/floyd/red/Elloumi99.pdf>.

   [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.




Briscoe & Manner         Expires April 27, 2011                [Page 30]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   [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.

   [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.

   [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.

   [RFC5690]                   Floyd, S., Arcia, A., Ros, D., and J.
                               Iyengar, "Adding Acknowledgement
                               Congestion Control to TCP", RFC 5690,
                               February 2010.

   [Rate_fair_Dis]             Briscoe, B., "Flow Rate Fairness:
                               Dismantling a Religion", ACM
                               CCR 37(2)63--74, April 2007, <http://
                               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, <htt
                               p://www.ics.forth.gr/netgroup/
                               publications/
                               weighted_window_control.html>.

   [gentle_RED]                Floyd, S., "Recommendation on using the



Briscoe & Manner         Expires April 27, 2011                [Page 31]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


                               "gentle_" variant of RED", Web page ,
                               March 2000, <http://www.icir.org/floyd/
                               red/gentle.html>.

   [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, <h
                               ttp://www.aciri.org/floyd/
                               end2end-paper.html>.

   [pktByteEmail]              Floyd, S., "RED: Discussions of Byte and
                               Packet Modes", email , March 1997, <http:
                               //www-nrg.ee.lbl.gov/floyd/
                               REDaveraging.txt>.

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

   Congestion notification involves the congested resource coding a
   congestion notification signal into the packet stream and the
   transports decoding it.  The idealised protocol uses two different
   (imaginary) fields in each datagram to signal congestion: one for
   byte congestion and one for packet congestion.

   We are not saying two ECN fields will be needed (and we are not
   saying that somehow a resource should be able to drop a packet in one
   of two different ways so that the transport can distinguish which
   sort of drop it was!).  These two congestion notification channels
   are just a conceptual device.  They allow us to defer having to
   decide whether to distinguish between byte and packet congestion when
   the network resource codes the signal or when the transport decodes
   it.

   However, although this idealised mechanism isn't intended for
   implementation, we do want to emphasise that we may need to find a
   way to implement it, because it could become necessary to somehow
   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



Briscoe & Manner         Expires April 27, 2011                [Page 32]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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):

   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
       congestion' field in each packet with probability p_b, again
       irrespective of the packet's size.  Unlike before, the transport
       should take a packet with the byte congestion field marked to
       count as a mark on each byte in the packet.

   The worked examples in Appendix A.2 show that transports can extract
   sufficient and correct congestion notification from these protocols
   for cases when two flows with different packet sizes have matching
   bit rates or matching packet rates.  Examples are also given that mix
   these two flows into one to show that a flow with mixed packet sizes
   would still be able to extract sufficient and correct information.

   Sufficient and correct congestion information means that there is
   sufficient information for the two different types of transport
   requirements:

   Ratio-based:  Established transport congestion controls like TCP's
      [RFC5681] aim to achieve equal segment rates per RTT through the
      same bottleneck--TCP friendliness [RFC3448].  They work with the
      ratio of dropped to delivered segments (or marked to unmarked
      segments in the case of ECN).  The example scenarios show that
      these ratio-based transports are effectively the same whether
      counting in bytes or packets, because the units cancel out.
      (Incidentally, this is why TCP's bit rate is still proportional to
      packet size even when byte-counting is used, as recommended for
      TCP in [RFC5681], mainly for orthogonal security reasons.)

   Absolute-target-based:  Other congestion controls proposed in the
      research community aim to limit the volume of congestion caused to
      a constant weight parameter.  [MulTCP][WindowPropFair] are
      examples of weighted proportionally fair transports designed for
      cost-fair environments [Rate_fair_Dis].  In this case, the



Briscoe & Manner         Expires April 27, 2011                [Page 33]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


      transport requires a count (not a ratio) of dropped/marked bytes
      in the bit-congestible case and of dropped/marked packets in the
      packet congestible case.

A.2.  Example Scenarios

A.2.1.  Notation

   To prove our idealised wire protocol (Appendix A.1) is correct, we
   will compare two flows with different packet sizes, s_1 and s_2 [bit/
   pkt], to make sure their transports each see the correct congestion
   notification.  Initially, within each flow we will take all packets
   as having equal sizes, but later we will generalise to flows within
   which packet sizes vary.  A flow's bit rate, x [bit/s], is related to
   its packet rate, u [pkt/s], by

      x(t) = s.u(t).

   We will consider a 2x2 matrix of four scenarios:

   +-----------------------------+------------------+------------------+
   |           resource type and |   A) Equal bit   |   B) Equal pkt   |
   |            congestion level |       rates      |       rates      |
   +-----------------------------+------------------+------------------+
   |     i) bit-congestible, p_b |       (Ai)       |       (Bi)       |
   |    ii) pkt-congestible, p_p |       (Aii)      |       (Bii)      |
   +-----------------------------+------------------+------------------+

                                  Table 3

A.2.2.  Bit-congestible resource, equal bit rates (Ai)

   Starting with the bit-congestible scenario, for two flows to maintain
   equal bit rates (Ai) the ratio of the packet rates must be the
   inverse of the ratio of packet sizes: u_2/u_1 = s_1/s_2.  So, for
   instance, a flow of 60B packets would have to send 25x more packets
   to achieve the same bit rate as a flow of 1500B packets.  If a
   congested resource marks proportion p_b of packets irrespective of
   size, the ratio of marked packets received by each transport will
   still be the same as the ratio of their packet rates, p_b.u_2/p_b.u_1
   = s_1/s_2.  So of the 25x more 60B packets sent, 25x more will be
   marked than in the 1500B packet flow, but 25x more won't be marked
   too.

   In this scenario, the resource is bit-congestible, so it always uses
   our idealised bit-congestion field when it marks packets.  Therefore
   the transport should count marked bytes not packets.  But it doesn't
   actually matter for ratio-based transports like TCP (Appendix A.1).



Briscoe & Manner         Expires April 27, 2011                [Page 34]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   The ratio of marked to unmarked bytes seen by each flow will be p_b,
   as will the ratio of marked to unmarked packets.  Because they are
   ratios, the units cancel out.

   If a flow sent an inconsistent mixture of packet sizes, we have said
   it should count the ratio of marked and unmarked bytes not packets in
   order to correctly decode the level of congestion.  But actually, if
   all it is trying to do is decode p_b, it still doesn't matter.  For
   instance, imagine the two equal bit rate flows were actually one flow
   at twice the bit rate sending a mixture of one 1500B packet for every
   thirty 60B packets. 25x more small packets will be marked and 25x
   more will be unmarked.  The transport can still calculate p_b whether
   it uses bytes or packets for the ratio.  In general, for any
   algorithm which works on a ratio of marks to non-marks, either bytes
   or packets can be counted interchangeably, because the choice cancels
   out in the ratio calculation.

   However, where an absolute target rather than relative volume of
   congestion caused is important (Appendix A.1), as it is for
   congestion accountability [Rate_fair_Dis], the transport must count
   marked bytes not packets, in this bit-congestible case.  Aside from
   the goal of congestion accountability, this is how the bit rate of a
   transport can be made independent of packet size; by ensuring the
   rate of congestion caused is kept to a constant weight
   [WindowPropFair], rather than merely responding to the ratio of
   marked and unmarked bytes.

   Note the unit of byte-congestion-volume is the byte.

A.2.3.  Bit-congestible resource, equal packet rates (Bi)

   If two flows send different packet sizes but at the same packet rate,
   their bit rates will be in the same ratio as their packet sizes, x_2/
   x_1 = s_2/s_1.  For instance, a flow sending 1500B packets at the
   same packet rate as another sending 60B packets will be sending at
   25x greater bit rate.  In this case, if a congested resource marks
   proportion p_b of packets irrespective of size, the ratio of packets
   received with the byte-congestion field marked by each transport will
   be the same, p_b.u_2/p_b.u_1 = 1.

   Because the byte-congestion field is marked, the transport should
   count marked bytes not packets.  But because each flow sends
   consistently sized packets it still doesn't matter for ratio-based
   transports.  The ratio of marked to unmarked bytes seen by each flow
   will be p_b, as will the ratio of marked to unmarked packets.
   Therefore, if the congestion control algorithm is only concerned with
   the ratio of marked to unmarked packets (as is TCP), both flows will
   be able to decode p_b correctly whether they count packets or bytes.



Briscoe & Manner         Expires April 27, 2011                [Page 35]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   But if the absolute volume of congestion is important, e.g. for
   congestion accountability, the transport must count marked bytes not
   packets.  Then the lower bit rate flow using smaller packets will
   rightly be perceived as causing less byte-congestion even though its
   packet rate is the same.

   If the two flows are mixed into one, of bit rate x1+x2, with equal
   packet rates of each size packet, the ratio p_b will still be
   measurable by counting the ratio of marked to unmarked bytes (or
   packets because the ratio cancels out the units).  However, if the
   absolute volume of congestion is required, the transport must count
   the sum of congestion marked bytes, which indeed gives a correct
   measure of the rate of byte-congestion p_b(x_1 + x_2) caused by the
   combined bit rate.

A.2.4.  Pkt-congestible resource, equal bit rates (Aii)

   Moving to the case of packet-congestible resources, we now take two
   flows that send different packet sizes at the same bit rate, but this
   time the pkt-congestion field is marked by the resource with
   probability p_p.  As in scenario Ai with the same bit rates but a
   bit-congestible resource, the flow with smaller packets will have a
   higher packet rate, so more packets will be both marked and unmarked,
   but in the same proportion.

   This time, the transport should only count marks without taking into
   account packet sizes.  Transports will get the same result, p_p, by
   decoding the ratio of marked to unmarked packets in either flow.

   If one flow imitates the two flows but merged together, the bit rate
   will double with more small packets than large.  The ratio of marked
   to unmarked packets will still be p_p.  But if the absolute number of
   pkt-congestion marked packets is counted it will accumulate at the
   combined packet rate times the marking probability, p_p(u_1+u_2), 26x
   faster than packet congestion accumulates in the single 1500B packet
   flow of our example, as required.

   But if the transport is interested in the absolute number of packet
   congestion, it should just count how many marked packets arrive.  For
   instance, a flow sending 60B packets will see 25x more marked packets
   than one sending 1500B packets at the same bit rate, because it is
   sending more packets through a packet-congestible resource.

   Note the unit of packet congestion is a packet.







Briscoe & Manner         Expires April 27, 2011                [Page 36]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


A.2.5.  Pkt-congestible resource, equal packet rates (Bii)

   Finally, if two flows with the same packet rate, pass through a
   packet-congestible resource, they will both suffer the same
   proportion of marking, p_p, irrespective of their packet sizes.  On
   detecting that the pkt-congestion field is marked, the transport
   should count packets, and it will be able to extract the ratio p_p of
   marked to unmarked packets from both flows, irrespective of packet
   sizes.

   Even if the transport is monitoring the absolute amount of packets
   congestion over a period, still it will see the same amount of packet
   congestion from either flow.

   And if the two equal packet rates of different size packets are mixed
   together in one flow, the packet rate will double, so the absolute
   volume of packet-congestion will accumulate at twice the rate of
   either flow, 2p_p.u_1 = p_p(u_1+u_2).

Appendix B.  Byte-mode Drop Complicates Policing Congestion Response

   This appendix explains why the ability of networks to police the
   response of _any_ transport to congestion depends on bit-congestible
   network resources only doing packet-mode not byte-mode drop.

   To be able to police a transport's response to congestion when
   fairness can only be judged over time and over all an individual's
   flows, the policer has to have an integrated view of all the
   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.

   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



Briscoe & Manner         Expires April 27, 2011                [Page 37]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


   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
   mean that all the bytes in the packet are dropped/marked, a policer
   can remain robust against bits being re-divided into different size
   packets or across different size flows [Rate_fair_Dis].  Therefore
   policing would work naturally with just simple packet-mode drop in
   RED.

   In summary, making drop probability depend on the size of the packets
   that bits happen to be divided into simply encourages the bits to be
   divided into smaller packets.  Byte-mode drop would therefore
   irreversibly complicate any attempt to fix the Internet's incentive
   structures.

Appendix C.  Changes from Previous Versions

   To be removed by the RFC Editor on publication.

   Full incremental diffs between each version are available at
   <http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#byte-pkt-congest>
   or
   <http://tools.ietf.org/wg/tsvwg/draft-ietf-tsvwg-byte-pkt-congest/>
   (courtesy of the rfcdiff tool):

   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
            Recommendations

         +  Added more internal structure to Recommendations, so that
            recommendations specific to RED or to TCP are just
            corollaries of a more general recommendation, rather than
            being listed as a separate recommendation.

         +  Renamed "State of the Art" as "Critical Survey of Existing
            Advice" and retitled a number of subsections with more
            descriptive titles.




Briscoe & Manner         Expires April 27, 2011                [Page 38]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


         +  Split end of "Congestion Coding: Summary of Status" into a
            new subsection called "RED Implementation Status".

         +  Removed text that had been in the Appendix "Congestion
            Notification Definition: Further Justification".

      *  Reordered the intro text a little.

      *  Made it clearer when advice being reported is deprecated and
         when it is not.

      *  Described AQM as in network equipment, rather than saying "at
         the network layer" (to side-step controversy over whether
         functions like AQM are in the transport layer but in network
         equipment).

      *  Minor improvements to clarity throughout

   From -01 to -02:

      *  Restructured the whole document for (hopefully) easier reading
         and clarity.  The concrete recommendation, in RFC2119 language,
         is now in Section 7.

   From -00 to -01:

      *  Minor clarifications throughout and updated references

   From briscoe-byte-pkt-mark-02 to ietf-byte-pkt-congest-00:

      *  Added note on relationship to existing RFCs

      *  Posed the question of whether packet-congestion could become
         common and deferred it to the IRTF ICCRG.  Added ref to the
         dual-resource queue (DRQ) proposal.

      *  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.




Briscoe & Manner         Expires April 27, 2011                [Page 39]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


      *  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).

      *  Separated out issue of normalising TCP's bit rate from issue of
         preference to control packets (Section 2.4).

      *  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.

      *  Added tabulated results of vendor survey on byte-mode drop
         variant of RED (Table 2).

   From -00 to -01:

      *  Clarified applicability to drop as well as ECN.

      *  Highlighted DoS vulnerability.

      *  Emphasised that drop-tail suffers from similar problems to
         byte-mode drop, so only byte-mode drop should be turned off,
         not RED itself.

      *  Clarified the original apparent motivations for recommending
         byte-mode drop included protecting SYNs and pure ACKs more than



Briscoe & Manner         Expires April 27, 2011                [Page 40]

Internet-Draft   Byte and Packet Congestion Notification    October 2010


         equalising the bit rates of TCPs with different segment sizes.
         Removed some conjectured motivations.

      *  Added support for updates to TCP in progress (ackcc & ecn-syn-
         ack).

      *  Updated survey results with newly arrived data.

      *  Pulled all recommendations together into the conclusions.

      *  Moved some detailed points into two additional appendices and a
         note.

      *  Considerable clarifications throughout.

      *  Updated references

Authors' Addresses

   Bob Briscoe
   BT
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE
   UK

   Phone: +44 1473 645196
   EMail: bob.briscoe@bt.com
   URI:   http://bobbriscoe.net/


   Jukka Manner
   Aalto University
   Department of Communications and Networking (Comnet)
   P.O. Box 13000
   FIN-00076 Aalto
   Finland

   Phone: +358 9 470 22481
   EMail: jukka.manner@tkk.fi
   URI:   http://www.netlab.tkk.fi/~jmanner/










Briscoe & Manner         Expires April 27, 2011                [Page 41]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/