Network Working Group                                          S. Bryant
Internet-Draft                                                  B. Davie
Intended status: Standards Track                              L. Martini
Expires: August 6, 2007                                         E. Rosen
                                                     Cisco Systems, Inc.
                                                        February 2, 2007

                Pseudowire Congestion Control Framework

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   Given that pseudowires may be used to carry non-TCP data flows, it is
   necessary to provide pseudowire-specific congestion control
   procedures.  These procedures should ensure that pseudowire traffic
   is "TCP-compatible", as defined in RFC 2914.  This document attempts
   to lay out the issues which must be considered when defining such

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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Pseudowires and Congestion in IP Networks  . . . . . . . .  3
     1.2.  Arguments Against PW Congestion as a Practical Problem . .  4
     1.3.  Goals of PW-specific Congestion Control  . . . . . . . . .  6
     1.4.  Challenges for PW Congestion . . . . . . . . . . . . . . .  7
       1.4.1.  Scale  . . . . . . . . . . . . . . . . . . . . . . . .  7
       1.4.2.  Interaction among control loops  . . . . . . . . . . .  8
       1.4.3.  Constant Bit Rate PWs  . . . . . . . . . . . . . . . .  8
       1.4.4.  Determining the appropriate rate . . . . . . . . . . .  9
     1.5.  Goals and non-goals of this draft  . . . . . . . . . . . .  9
   2.  Detecting Congestion . . . . . . . . . . . . . . . . . . . . . 10
     2.1.  Using Sequence Numbers to Detect Congestion  . . . . . . . 11
     2.2.  Using VCCV to Detect Congestion  . . . . . . . . . . . . . 12
     2.3.  Explicit Congestion Notification . . . . . . . . . . . . . 13
   3.  Feedback from Receiver to Transmitter  . . . . . . . . . . . . 13
     3.1.  Control Plane Feedback . . . . . . . . . . . . . . . . . . 14
     3.2.  Using Reverse Data Packets for Feedback  . . . . . . . . . 15
     3.3.  Reverse VCCV Traffic . . . . . . . . . . . . . . . . . . . 15
   4.  Responding to Congestion . . . . . . . . . . . . . . . . . . . 15
     4.1.  Interaction with TCP . . . . . . . . . . . . . . . . . . . 16
   5.  Rate Control per Tunnel vs. per PW . . . . . . . . . . . . . . 17
   6.  Constant Bit Rate Services . . . . . . . . . . . . . . . . . . 17
   7.  Mandatory vs. Optional . . . . . . . . . . . . . . . . . . . . 18
   8.  Related Work: Pre-Congestion Notification  . . . . . . . . . . 18
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 22

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1.  Introduction

1.1.  Pseudowires and Congestion in IP Networks

   Congestion in an IP network occurs when the amount of traffic that
   needs to use a particular network resource exceeds the capacity of
   that resource.  This results first in long queues within the network,
   and then in packet loss.  If the amount of traffic is not then
   reduced, the packet loss rate will climb, potentially until it
   reaches 100%.

   To prevent this sort of "congestive collapse", there must be
   congestion control: a feedback loop by which the presence of
   congestion somewhere in the network forces the transmitters to reduce
   the amount of traffic being sent.  As a connectionless protocol, IP
   has no way to push back directly on the originator of the traffic.
   Procedures for (a) detecting congestion, (b) providing the necessary
   feedback to the transmitters, and (c) adjusting the transmission
   rates, are thus left to higher protocol layers such as TCP.

   The vast majority of traffic in IP networks is currently TCP traffic.
   TCP includes an elaborate congestion control mechanism which causes
   the end systems to reduce their transmission rates when congestion
   occurs.  For those readers not intimately familiar with the details
   of TCP congestion control, we give below a brief summary, greatly
   simplified and not entirely accurate, of TCP's very complicated
   feedback mechanism.  The details of TCP congestion control can be
   found in [RFC2581].  [RFC2001] is an earlier but more accessible
   discussion.  [RFC2914] articulates a number of general principles
   governing congestion control in the Internet.

   In TCP congestion control, a lost packet is considered to be an
   indication of congestion.  Roughly, TCP considers a given packet to
   be lost if that packet is not acknowledged within a specified time,
   or if three subsequent packets arrive at the receiver before the
   given packet.  The latter condition manifests itself at the
   transmitter as the arrival of three duplicate acks in a row.  The
   algorithm by which TCP detects congestion is thus highly dependent on
   the mechanisms used by TCP to ensure reliable and sequential

   Once a TCP transmitter becomes aware of congestion, it halves its
   transmission rate.  If congestion still occurs at the new rate, the
   rate is halved again.  When a rate is found at which congestion no
   longer occurs, the rate is increased by one MSS ("Maximum Segment
   Size") per RTT ("Round Trip Time").  The rate is increased each RTT
   until congestion is encountered again, or until something else limits
   it (e.g., the flow control window reached, or the application is

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   transmitting at its max desired rate, or at line rate).

   This sort of mechanism is known as an "Additive Increase,
   Multiplicative Decrease" (AIMD) mechanism.  Congestion causes
   relatively rapid decreases in the transmission rate, while the
   absence of congestion causes relatively slow increases in the allowed
   transmission rate.

   Currently, traffic in IP networks is predominantly TCP traffic.  Even
   the layer 2 tunneled traffic (e.g., PPP frames tunneled through L2TP)
   is predominantly TCP traffic from the end-users.  If pseudowires
   (PWs) [RFC3985] were to be used only for carrying TCP flows, there
   would be no need for any PW-specific congestion mechanisms.  The
   existing TCP congestion control mechanisms would be all that is
   needed, since any loss of packets on the PW would be detected as loss
   of packets on a TCP connection, and the TCP flow control mechanisms
   would ensure a reduction of transmission rate.  However, if a PW is
   carrying non-TCP traffic, then there is no feedback mechanism to
   cause the end-systems to reduce their transmission rates in response
   to congestion.  When congestion occurs, any TCP traffic that is
   sharing the congested resource with the non-TCP traffic will be
   throttled, and the non-TCP traffic may "starve" the TCP traffic.  If
   there is enough non-TCP traffic to congest the network all by itself,
   there is nothing to prevent congestive collapse.

   The non-TCP traffic in a PW can belong to any higher layer
   whatsoever, and there is no way to ensure that TCP-like congestion
   control mechanisms will be used by all those layers.  Hence it
   appears that there is a need for an edge-to-edge (i.e, PE-to-PE)
   feedback mechanism which forces a transmitting PE to reduce its
   transmission rate in the face of network congestion.

   As TCP uses window-based flow control, controlling the rate is really
   a matter of limiting the amount of traffic which can be "in flight"
   (i.e., transmitted but not yet acknowledged) at any one time.
   Obviously a different technique needs to be used to control the
   transmission rate of the non-windowed protocol used for transmitting
   data on PWs.

1.2.  Arguments Against PW Congestion as a Practical Problem

   One may argue that congestion due to non-TCP PW traffic is only a
   theoretical problem.

   o  "99.9% of all the traffic in PWs is really IP traffic"

      If this is the case, then the traffic is either TCP traffic, which
      is already congestion-controlled, or "other" IP traffic.  While

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      the congestion control issue may exist for the "other" IP traffic,
      it is a general issue which is not specific to PWs.

      Unfortunately, we cannot be sure that this is the case.  It may
      well be the case for the PW offerings of certain providers, but
      perhaps not for others.  It does appear that many providers want
      to be able to use PWs for transporting "legacy traffic" of various
      non-IP protocols.  Constant bit-rate services are an example of
      this, and raise particular issues for congestion control
      (discussed below).

   o  "PW traffic usually stays within one SP's network, and an SP
      always engineers its network carefully enough so that congestion
      is an impossibility"

      Perhaps this will be true of "most" PWs, but inter-provider PWs
      are certainly expected to have a significant presence.

      Even within a single provider's network, the provider might
      consider whether he is so confident of his network engineering
      that he does not need a feedback loop reducing the transmission
      rate in response to congestion.

      There is also the issue of keeping the network running (i.e., out
      of congestive collapse) after an unexpected reduction of capacity.

   o  "If one provider accepts PW traffic from another, policing will be
      done at the entry point to the second provider's network, so that
      the second provider is sure that the first provider is not sending
      too much traffic.  This policing, together with the second
      provider's careful network engineering, makes congestion an

      This could be the case given carefully controlled bilateral
      peering arrangements.  Note though that if the second provider is
      merely providing transit services for a PW whose endpoints are in
      other providers, it may be difficult for the transit provider to
      tell which traffic is the PW traffic and which is "ordinary" IP

   o  "The only time we really need a general congestion control
      mechanism is when traffic goes through the public Internet.
      Obviously this will never be the case for PW traffic."

      It is not at all difficult to imagine someone using an IPsec
      tunnel across the public Internet to transport a PW from one
      private IP network to another.

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      Nor is it difficult to imagine some enterprise implementing a PW
      and transporting it across some SP's backbone, e.g., if that SP is
      providing VPN service to that enterprise.

   The arguments that non-TCP traffic in PWs will never make any
   significant contribution to congestion thus do not seem to be totally

1.3.  Goals of PW-specific Congestion Control

   [RFC2914] defines the notion of a "TCP-compatible flow":

   "A TCP-compatible flow is responsive to congestion notification, and
   in steady-state uses no more bandwidth than a conformant TCP running
   under comparable conditions (drop rate, RTT [round trip time], MTU
   [maximum transmission unit], etc.)"

   TCP-compatible flows respond to congestion in much the way TCP does,
   so that they do not starve the TCP flows or otherwise obtain an
   unfair advantage.  [RFC2914] further points out:

   "any form of congestion control that successfully avoids a high
   sending rate in the presence of a high packet drop rate should be
   sufficient to avoid congestion collapse from undelivered packets."

   "This does not mean, however, that concerns about congestion collapse
   and fairness with TCP necessitate that all best-effort traffic deploy
   congestion control based on TCP's Additive-Increase Multiplicative-
   Decrease (AIMD) algorithm of reducing the sending rate in half in
   response to each packet drop."

   "However, the list of TCP-compatible congestion control procedures is
   not limited to AIMD with the same increase/ decrease parameters as
   TCP.  Other TCP-compatible congestion control procedures include
   rate-based variants of AIMD; AIMD with different sets of increase/
   decrease parameters that give the same steady-state behavior;
   equation-based congestion control where the sender adjusts its
   sending rate in response to information about the long-term packet
   drop rate ... and possibly other forms that we have not yet begun to

   The AIMD procedures are not mandated for non-TCP traffic, and might
   not be optimal for non-TCP PW traffic.  Choosing a proper set of
   procedures which are TCP-compatible while being optimized for a
   particular type of traffic is no simple task.  [RFC3448], "TCP
   Friendly Rate Control (TFRC)" provides an alternative:

   "TFRC is designed to be reasonably fair when competing for bandwidth

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   with TCP flows, where a flow is "reasonably fair" if its sending rate
   is generally within a factor of two of the sending rate of a TCP flow
   under the same conditions.  However, TFRC has a much lower variation
   of throughput over time compared with TCP, which makes it more
   suitable for applications such as telephony or streaming media where
   a relatively smooth sending rate is of importance."

   "For its congestion control mechanism, TFRC directly uses a
   throughput equation for the allowed sending rate as a function of the
   loss event rate and round-trip time.  In order to compete fairly with
   TCP, TFRC uses the TCP throughput equation, which roughly describes
   TCP's sending rate as a function of the loss event rate, round-trip
   time, and packet size."

   "Generally speaking, TFRC's congestion control mechanism works as

   o  The receiver measures the loss event rate and feeds this
      information back to the sender.

   o  The sender also uses these feedback messages to measure the round-
      trip time (RTT).

   o  The loss event rate and RTT are then fed into TFRC's throughput
      equation, giving the acceptable transmit rate.

   o  The sender then adjusts its transmit rate to match the calculated

   Note that the TFRC procedures require the transmitter to calculate a
   throughput equation.  For these procedures to be feasible as a means
   of PW congestion control, they must be computationally efficient.
   Section 8 of [RFC3448] describes an implementation technique that
   appears to make it efficient to calculate the equation.  It is not
   clear whether this is the case; this is an area for further
   consideration.  There are also issues in determining exactly what the
   fair rate for a PW should be, as discussed below.

1.4.  Challenges for PW Congestion

1.4.1.  Scale

   It might appear at first glance that an easy solution to PW
   congestion control would be to run the PWs through a TCP connection.
   This would provide congestion control automatically.  However, the
   overhead is prohibitive for the PW application.  The PWE3 data plane
   may be implemented in a microcoded hardware engine which needs to
   support thousands of PWs, and needs to do as little as possible for

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   each data packet; running a TCP state machine, and implementing TCP's
   flow control procedures, would impose too high a cost in this
   environment.  Nor do we want to add the large overhead of TCP to the
   PWs -- the large headers, the plethora of small acks in the reverse
   direction, etc., etc.  In fact, we want to avoid acknowledgments
   altogether.  These same considerations lead us away from using e.g.,
   DCCP [RFC4340].  Therefore we will investigate some PW-specific
   solutions for congestion control.

   We also want to minimize the amount of interaction between the data
   processing path (which is likely to be distributed among a set of
   line cards) and the control path; we need to be especially careful of
   interactions which might require atomic read/modify/write operations
   from the control path, or which might require atomic read/modify/
   write operations between different processors in a multiprocessing
   implementation, as such interactions can cause scaling problems.

   Thus, feasible solutions for PW-specific congestion will require
   scalable means to detect congestion and to reduce the amount of
   traffic sent into the network when congestion is detected.  These
   topics are discussed in more detail in subsequent sections.

1.4.2.  Interaction among control loops

   As noted above, much of the traffic that is carried on PWs is likely
   to be TCP traffic, and will therefore be subject the congestion
   control mechanisms of TCP.  It will typically be difficult for a PW
   endpoint to tell whether or not this is the case.  Thus there is the
   risk that the PE-PE congestion control mechanisms applied over the PW
   may interact in undesirable ways with the end-to-end congestion
   control mechanisms of TCP.  The PW-specific congestion control
   mechanisms should be designed to minimize the negative impact of such

1.4.3.  Constant Bit Rate PWs

   Some types of PW, for example SAToP (Structure Agnostic TDM over
   Packet) [RFC4553], CESoPSN (Circuit Emulation over Packet Switched
   Networks) [I-D.ietf-pwe3-cesopsn], TDM over IP
   [I-D.ietf-pwe3-tdmoip][I-D.ietf-pwe3-sonet], SONET/SDH and Constant
   Bit Rate ATM PWs represent an inelastic constant bit-rate (CBR) flow.
   Such PWs cannot respond to congestion in a TCP-friendly manner
   prescribed by [RFC2914]; the amount of total bandwidth consumed by
   such a PW remains constant.  AIMD or even more gradual TFRC
   techniques are clearly not applicable to such services; it is not
   feasible to reduce the rate of a CBR service without violating the
   service definition.  Such services are also frequently more sensitive
   to packet loss than connectionless packet PWs.  Given that CBR

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   services are not greedy (in the sense of trying to increase their
   share of a link, as TCP does), there may be a case for allowing them
   greater latitude during congestion peaks.  However, if some CBR PWs
   are not able to endure any significant packet loss or reduction in
   rate without compromising the transported service, such PWs must be
   shutdown when the level of congestion becomes excessive.  At suitably
   low levels of congestion they may be allowed to continue to offer
   traffic to the network.

   Some CBR services may be carried over connectionless packet PWs.  An
   example of such a case would be a CBR MPEG-2 video stream carried
   over over an Ethernet PW.  One could argue that such a service -
   provided the rate was policed at the ingress PE - should be offered
   the same latitude as a PW that explicitly provided a CBR service.
   Likewise, there may not be much value in trying to throttle such a
   service rather than cutting it off completely during severe
   congestion.  However, this clearly raises the issue of how to know
   that a PW is indeed carrying a CBR service.

1.4.4.  Determining the appropriate rate

   TCP tends to share the bandwidth of a bottleneck among flows somewhat
   evenly, although flows with shorter round-trip-times and larger MSS
   values will tend to get more throughput in the steady state than
   those with longer RTTs or smaller MSS.  TFRC simply tries to deliver
   the same rate to a flow that a TCP flow would obtain in the steady
   state under similar conditions (loss rate, RTT and MSS).  The
   challenge for the PWE environment is to determine what constitutes a
   "flow".  While it is tempting to consider a single PW to be
   equivalent to a "flow" for the purposes of fair rate estimation, it
   is clearly possible that one PW might be carrying a large number of
   flows, in which case it would seem quite unfair to throttle that PW
   down to the same rate that a single TCP flow would obtain.  This
   issue of what constitutes fairness and the perils of using the TCP
   flow as the basic unit of fairness have been explored at some length
   in [I-D.briscoe-tsvarea-fair].

1.5.  Goals and non-goals of this draft

   As a framework, this draft aims to explore the issues surrounding PW
   congestion and to lay out some of the design tradeoffs that will need
   to be made if a solution is to be developed.  It does not intend to
   propose a particular solution, but it does point out some of the
   problems that will arise with certain solution approaches.

   The over-riding technical goal of this work is to avoid scenarios in
   which the deployment of PWE technology leads to congestive collapse
   of the PSN over which the PWs are tunneled.  It is a non-goal of this

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   work to ensure that PWs receive a particular level of QoS.  While
   such an outcome may be desirable, it is beyond the scope of this

2.  Detecting Congestion

   In TCP, congestion is detected by the transmitter; the receipt of
   three successive duplicate TCP acks are taken to be indicative of
   congestion.  What this actually means is that the several packets in
   a row were received at the remote end, such that none of those
   packets had the next expected sequence number.  This is interpreted
   as meaning that the packet with the next expected sequence number was
   lost in the network, and the loss of a single packet in the network
   is taken as a sign of congestion.  (Naturally, the presence of
   congestion is also inferred if TCP has to retransmit a packet.)  Note
   that it is possible for mis-ordered packets to be misinterpreted as
   lost packets, if they do not arrive "soon enough".

   In TCP, a time-out while awaiting an ack is also interpreted as a
   sign of congestion.

   Since there are no acknowledgments on a PW, the PW-specific
   congestion control mechanism obviously cannot be based on either the
   presence of or the absence of acknowledgments.  Some types of
   pseudowire (the CBR PWs) have a single bit that indicates that a
   preset amount of data has been lost, but this is a non-quantitative
   indicator.  CBR PWs have the advantage that there is a constant two
   way data flow, while other PW types do not have the constant
   symmetric flow of payload on which to piggyback the congestion
   notification.  Most PW types therefore provide no way for a
   transmitter to determine (or even to make an educated guess as to)
   whether any data has been lost.

   Thus we need to add a mechanism for determining whether data packets
   on a PW have gotten lost.  There are several possible methods for
   doing this:

   o  Detect Congestion Using PW Sequence Numbers

   o  Detect Congestion Using Modified VCCV Packets [I-D.ietf-pwe3-vccv]

   o  Rely on Explicit Congestion Notification (ECN) [RFC3168]

   We discuss each option in turn in the following sections.

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2.1.  Using Sequence Numbers to Detect Congestion

   When the optional sequencing feature is in use on a PW [RFC4385], it
   is necessary for the receiver to maintain a "next expected sequence
   number" for the PW.  If a packet arrives with a sequence number that
   is earlier than the next expected (a "mis-ordered packet"), the
   packet is discarded; if it arrives with a sequence number that is
   greater than or equal to the next expected, the packet is delivered,
   and the next expected sequence number becomes the sequence number of
   the current packet plus 1.

   It is easy to tell when there is one or more missing packets (i.e.,
   there is a "gap" in the sequence space) -- that is the case when a
   packet arrives whose sequence number is greater than the next
   expected.  What is difficult to tell is whether any misordered
   packets that arrive after the gap are indeed the missing packets.
   One could imagine that the receiver remembers the sequence number of
   each missing packet for a period of time, and then checks off each
   such sequence number if a misordered packet carrying that sequence
   number later arrives.  The difficulty is doing this in a manner which
   is efficient enough to be done by the microcoded hardware handling
   the PW data path.  This approach does not really seem feasible.

   One could make certain simplifying assumptions, such as assuming that
   the presence of any gaps at all indicates congestion.  While this
   assumption makes it feasible to use the sequence numbers to "detect
   congestion", it also throttles the PW unnecessarily if there is
   really just misordering and no congestion.  Such an approach would be
   considerably more likely to misinterpret misordering as congestion
   than would TCP's approach.

   An intermediate approach would be to keep track of the number of
   missing packets and the number of misordered packets for each PW.
   One could "detect congestion" if the number of missing packets is
   significantly larger than the number of misordered packets over some
   sampling period.  However, gaps occurring near the end of a sampling
   period would tend to result in false indications of congestion.  To
   avoid this one might try to smooth the results over several sampling
   periods; While this would tend to decrease the responsiveness, it is
   inevitable that there will be a trade-off between the rapidity of
   responsiveness and the rate of false alarms.

   One would not expect the hardware or microcode to keep track of the
   sampling period; presumably software would read the necessary
   counters from hardware at the necessary intervals.

   Such a scheme would have the advantage of being based on existing PW
   mechanisms.  However, it has the disadvantage of requiring

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   sequencing, and it also introduces a fairly complicated interaction
   between the control processing and the data path.

2.2.  Using VCCV to Detect Congestion

   It is reasonable to suppose that the hardware keeps counts of the
   number of packets sent and received on each PW.  Suppose that the PW
   uses MPLS, and that the transmitter periodically inserts VCCV packets
   into the PE data stream, where each VCCV packet carries:

   o  A sequence number, increasing by 1 for each successive VCCV

   o  The current value of the transmission counter for the PW

   We assume that the size of the counter is such that it cannot wrap
   during the interval between n VCCV packets, for some n > 1.

   When the receiver gets one of these VCCV packets on a PW, he inserts
   into it his count of received packets for that PW, and delivers the
   packet to the software.  The receiving software can now compute, for
   the inter-VCCV intervals, the count of packets transmitted and the
   count of packets received.  The presence of congestion can be
   inferred if the count of packets transmitted is significantly greater
   than the count of packets received during the most recent interval.
   Even the loss rate could be calculated.  The loss rate calculated in
   this way could be used as input to the TFRC rate equation.

   VCCV messages would not need to be sent on a PW (for the purpose of
   detecting congestion) in the absence of traffic on that PW.

   Of course, misordered packets that are sent during one interval but
   arrive during the next will throw off the loss rate calculation;
   hence the difference between sent traffic and received traffic should
   be "significant" before the presence of congestion is inferred.  The
   value of "significance" can be made larger or smaller depending on
   the probability of misordering.

   Note that congestion can cause a VCCV packet to go missing, and
   anything that misorders packets can misorder a VCCV packet as well as
   any other.  One may not want to infer the presence of congestion if a
   single VCCV packet does not arrive when expected, as it may just be
   delayed in the network, even if it hasn't been misordered.  However,
   failure to receive a VCCV packet after a certain amount of time has
   elapsed since the last VCCV was received (on a particular PW) may be
   taken as evidence of congestion.  This scheme has the disadvantage of
   requiring periodic VCCV packets, and it requires VCCV packet formats
   to be modified to include the necessary counts.  However, the

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   interaction between the control path and the data path is very
   simple, as there is no polling of counters, no need for timers in the
   data path, and no need for the control path to do read-modify-write
   operations on the data path hardware.  A bigger disadvantage may
   arise from the possible inability to ensure that the transmit counts
   in the VCCVs are exactly correct.  The transmitting hardware may not
   be able to insert a packet count in the VCCV IMMEDIATELY before
   transmission of the VCCV on the wire, and if it cannot, the count of
   transmit packets will only be approximate.

   Neither scheme can provide the same type of continuous feedback that
   TCP gets.  TCP gets a continuous stream of acknowledgments, whereas
   the PW congestion detection mechanism would only be able to say
   whether congestion occurred during a particular interval.  If the
   interval is about 1 RTT, the PW congestion control would be
   approximately as responsive as TCP congestion control, and there does
   not seem to be any advantage to making it smaller.  However, sampling
   at an interval of 1 RTT might generate excessive amounts of overhead.
   Sampling at longer intervals would reduce responsiveness to
   congestion but would not necessarily render the congestion control
   mechanism "TCP-unfriendly".

2.3.  Explicit Congestion Notification

   In networks that support explicit congestion notification (ECN)
   [RFC3168] the ECN notification provides congestion information to the
   PEs before the onset of congestion discard.  This is particularly
   useful to PWs that are sensitive to packet loss, since it gives the
   PE the opportunity to intelligently reduce the offered load.  ECN
   marking rates of packets received on a PW could be used to calculate
   the TFRC rate for a PW.  However ECN is not widely deployed at the
   time of writing; hence it seems that PEs must also be capable of
   operating in a network where packet loss is the only indicator of

3.  Feedback from Receiver to Transmitter

   Given that the receiver can tell, for each sampling interval, whether
   or not a PW's traffic has encountered congestion, the receiver must
   provide this information as feedback to the transmitter, so that the
   transmitter can adjust its transmission rate appropriately.  The
   feedback could be as simple as a bit stating whether or not there was
   any packet loss during the specified interval.  Alternatively, the
   actual loss rate could be provided in the feedback, if that
   information turns out to be useful to the transmitter (e.g. to enable
   it to calculate a TCP-friendly rate at which to send).  There are a
   number of possible ways in which the feedback can be provided:

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   control plane, reverse data traffic, or VCCV messages.  We discuss
   each in turn below.

3.1.  Control Plane Feedback

   A control message can be sent periodically to indicate the presence
   or absence of congestion.  For example, when LDP is the control
   protocol [RFC4447], the control message would of course be delivered
   reliably by TCP.  (The same considerations apply for any protocol
   which has a reliable control channel.)  When congestion is detected,
   a control message can be sent indicating that fact.  No further
   congestion control messages would need to be sent until congestion is
   no longer detected.  If the loss rate is being sent, changes in the
   loss rate would need to be sent as well.  When there is no longer any
   congestion, a message indicating the absence of congestion would have
   to be sent.

   Since congestion in the reverse direction can prevent the delivery of
   these control messages, periodic "no congestion detected" messages
   would need to be sent whenever there is no congestion.  Failure to
   receive these in a timely manner would lead the control protocol peer
   to infer that there is congestion.  (Actually, there might or might
   not be congestion in the transmitting direction, but in the absence
   of any feedback one cannot assume that everything is fine.)  If
   control messages really cannot get through at all, control protocol
   keepalives will fail and the control connection will go down anyway.

   If the control messages simply say whether or not congestion was
   detected, then given a reliable control channel, periodic messages
   are not needed during periods of congestion.  Of course, if the
   control messages carry more data, such as the loss rate, then they
   need to be sent whenever that data changes.

   If it is desired to control congestion on a per-tunnel basis, these
   control messages will simply say that there was congestion on some PW
   (one or more) within the tunnel.  If it is desired to control
   congestion on a per-PW basis, the control message can list the PWs
   which have experienced congestion, most likely by listing the
   corresponding labels.  If the VCCV method of detecting congestion is
   used, one could even include the sent/received statistics for
   particular VCCV intervals.

   This method is very simple, as one does not have to worry about the
   congestion control messages themselves getting lost or out of
   sequence.  Feedback traffic is minimized, as a single control message
   relays feedback about an entire tunnel.

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3.2.  Using Reverse Data Packets for Feedback

   If a receiver detects congestion on a particular PW, it can set a bit
   in the data packets that are traveling on that PW in the reverse
   direction; when no congestion is detected, the bit would be clear.
   The bit would be ignored on any packet which is received out of
   sequence, of course.  There are several disadvantages to this

   o  There may be no (or insufficient) data traffic in the reverse

   o  Sequencing of the data stream is required

   o  The transmission of the congestion indications is not reliable

   o  The most one could hope to convey is one bit of information per PW
      (if there is even a bit available in the encapsulation).

3.3.  Reverse VCCV Traffic

   Congestion indications for a particular PW could be carried in VCCV
   packets traveling in the reverse direction on that PW.  Of course,
   this would require that the VCCV packets be sent periodically in the
   reverse direction whether or not there is reverse direction traffic.
   For congestion feedback purposes they might need to be sent more
   frequently than they'd need to be sent for OAM purposes.  It would
   also be necessary for the VCCVs to be sequenced (with respect to each
   other, not necessarily with respect to the datastream).  Since VCCV
   transmission is unreliable, one would want to send multiple VCCVs
   within whatever period we want to be able to respond in.  Further,
   this method provides no means of aggregating congestion information
   into information about the tunnel.

4.  Responding to Congestion

   In TCP, one tends to think of the transmission rate in terms of MTUs
   per RTT, which defines the maximum number of unacknowledged packets
   that TCP is allowed to maintain "in flight".  Upon detection of a
   lost packet, this rate is halved ("multiplicative decrease").  It
   will be halved again approximately every RTT until the missing data
   gets through.  Once all missing data has gotten through, the
   transmission rate is increased by one MTU per RTT.  Every time a new
   acknowledgment (i.e., not a duplicate acknowledgment) is received,
   the rate is similarly increased (additive increase).  Thus TCP can
   adjust its transmit rate very rapidly, i.e., it responds on the order
   of a RTT.  By contrast, TCP-friendly rate control adjusts its rate

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   rather more gradually.

   For simplicity, this discussion only covers the "congestion
   avoidance" phase of TCP congestion control.  The analogy of TCP's
   "slow start phase" would also be needed.

   TCP can easily estimate the RTT, since all its transmissions are
   acknowledged.  In PWE3, the best way to estimate the RTT might be via
   the control protocol.  In fact, if the control protocol is TCP-based,
   getting the RTT estimate from TCP might be a good option.

   TCP's rate control is window-based, expressed as a number of bytes
   that can be in flight.  PWE3's rate control would need to be rate-
   based.  The TFRC specification [RFC3448] provides the equation for
   the TCP-friendly rate for a given loss rate, RTT, and MTU.  Given
   some means of determining the loss rate, as described in Section 2,
   the TCP friendly rate for a PW or a tunnel can be calculated at the
   ingress PE.

   If the congestion detection mechanism only produces an approximate
   result, the probability of a "false alarm" (thinking that there is
   congestion when there really is not) for some interval becomes
   significant.  It would be better then to have some algorithm which
   smoothes the result over several intervals.  The TFRC procedures,
   which tend to generate a smoother and less abrupt change in the
   transmission rate than the AIMD procedures, may also be more
   appropriate in this case.

   Once a PE has determined the appropriate rate at which to transmit
   traffic on a given PW or tunnel, it needs some means to enforce that
   rate via policing, shaping, or selective shutting down of PWs.  There
   are tradeoffs to be made among these options, depending on various
   factors including the higher layer service that is carried.  The
   effect of different mechanisms when the higher layer traffic is
   already using TCP is discussed below.

4.1.  Interaction with TCP

   Ideally there should be no PW-specific congestion control mechanism
   used when the higher layer traffic is already running over TCP and is
   thus subject to TCP's existing congestion control.  However it may be
   difficult to determine what the higher layer is on any given PW.
   Thus, interaction between PW-specific congestion control and TCP's
   congestion control needs to be considered.

   As noted in Section 1.4.2, a PW-specific congestion control mechanism
   may interact poorly with the "outer" control loop of TCP if the PW
   carries TCP traffic.  A well-documented example of such poor

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   interaction is a token bucket policer that drops packets outside the
   token bucket.  TCP has difficulty finding the "bottleneck" bandwidth
   in such an environment and tends to overshoot, incurring heavy losses
   and consequent loss of throughput.

   A shaper that queues packets at the PE and only injects them into the
   network at the appropriate "TCP friendly" rate may be a better
   choice, but may still interact unpredictably with the "outer control
   loop" of TCP flows that happen to traverse the PW.  This issue
   warrants further study.

   Another possibility is simply to shut down a PW when the rate of
   traffic on the PW significantly exceeds the "TCP friendly" rate that
   has been determined for the PW.  While this might be viewed as
   draconian, it does ensure that any PW that is allowed to stay up will
   behave in a predictable manner.  Note that this would also be the
   most likely choice of action for CBR PWs (as discussed in Section 6).
   Thus all PWs would be treated alike and there would be no need to try
   to determine what sort of upper layer payload a PW is carrying.

5.  Rate Control per Tunnel vs. per PW

   Rate controls can be applied on a per-tunnel basis or on a per-PW
   basis.  Applying them on a per-tunnel basis (and obtaining congestion
   feedback on a per-tunnel basis) would seem to provide the most
   efficient and most scalable system.  Achieving fairness among the PWs
   then becomes a local issue for the transmitter.  However, if the
   different PWs follow different paths through the network (e.g.
   because of ECMP over the tunnel), it is possible that some PWs will
   encounter congestion while some will not.  If rate controls are
   applied on a per-tunnel basis, then if any PW in a tunnel is affected
   by congestion, all the PWs in the tunnel will be throttled.  While
   this is sub-optimal, it is not clear that this would be a significant
   problem in practice, and it may still be the best trade-off.

   Per-tunnel rate control also has some desirable properties if the
   action taken during congestion is to selectively shut down certain
   PWs.  Since a tunnel will typically carry many PWs, it will be
   possible to make relatively small adjustments in the total bandwidth
   consumed by the tunnel by selectively shutting down or bringing up
   one or more PWs.

6.  Constant Bit Rate Services

   As noted above, some PW services may require a fixed rate of
   transmission, and it may be impossible to provide the service while

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   throttling the transmission rate.  To provide such services, the
   network paths must be engineered so that congestion is impossible;
   providing such services over the Internet is thus not very likely.
   In fact, as congestion control cannot be applied to such services, it
   may be necessary to prohibit these services from being provided in
   the Internet, except in the case where the payload is known to
   consist of TCP connections or other traffic that is congestion-
   controlled by the end-points.  It is not clear how such a prohibition
   could be enforced.

   The only feasible mechanism for handling congestion affecting CBR
   services would appear to be to selectively turn off PWs when
   congestion occurs.  Clearly it is important to avoid "false alarms"
   in this case.  It is also important to avoid bringing PWs back up too
   quickly and re-introducing congestion.

   The idea of controlling rate per tunnel rather than PW, discussed
   above, seems particularly attractive when some of the PWs are CBR.
   First, it provides the possibility that non-CBR PWs could be
   throttled before it is necessary to shut down the CBR PWs.  Second,
   with the aggregation of multiple PWs on a single rate-controlled
   tunnel, it becomes possible to gradually increase or decrease the
   total offered load on the tunnel by selectively bringing up or
   shutting down PWs.  As noted above, local policies at a PE could be
   used to determine which PWs to shut down or bring up first.  Similar
   approaches would apply if the CBR PW offers a channelized service,
   with selected channels being shut down and brought up to control the
   total rate of the PW.

7.  Mandatory vs. Optional

   As discussed in section 1, there are a significant set of scenarios
   in which PW-specific congestion control is not necessary.  One might
   therefore argue that it doesn't seem to make sense to require PW-
   specific congestion control to be used on all PWs at all times.  On
   the other hand, if the option of turning off PW-specific congestion
   control is available, there is nothing to stop a provider from
   turning it off in inappropriate situations.  As this may contribute
   to congestive collapse outside the provider's own network, it may not
   be advisable to allow this.

8.  Related Work: Pre-Congestion Notification

   It has been suggested that Pre-congestion Notification (PCN)
   [I-D.briscoe-tsvwg-cl-architecture][I-D.briscoe-tsvwg-cl-phb] might
   provide a basis for addressing the PW congestion control problem.

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   Using PCN, it would potentially be possible to determine if the level
   of congestion currently existing between an ingress and an egress PE
   was sufficiently low to safely allow a new PW to be established.
   PCN's pre-emption mechanisms could be used to notify a PE that one or
   more PWs need to be brought down, which again could be coupled with
   local policies to determine exactly which PWs should be shut down
   first.  This approach certainly merits further examination, but we
   note that PCN is considerably further away from deployment in the
   Internet than ECN, and thus cannot be considered as a near-term
   solution to the problem of PW-induced congestion in the Internet.

9.  Informative References

              Briscoe, B., "Flow Rate Fairness: Dismantling a Religion",
              draft-briscoe-tsvarea-fair-00 (work in progress),
              October 2006.

              Briscoe, B., "An edge-to-edge Deployment Model for Pre-
              Congestion Notification: Admission  Control over a
              DiffServ Region", draft-briscoe-tsvwg-cl-architecture-04
              (work in progress), October 2006.

              Briscoe, B., "Pre-Congestion Notification marking",
              draft-briscoe-tsvwg-cl-phb-03 (work in progress),
              October 2006.

              Vainshtein, S., "Structure-aware TDM Circuit Emulation
              Service over Packet Switched Network  (CESoPSN)",
              draft-ietf-pwe3-cesopsn-07 (work in progress), May 2006.

              Malis, A., "SONET/SDH Circuit Emulation over Packet
              (CEP)", draft-ietf-pwe3-sonet-14 (work in progress),
              December 2006.

              Stein, Y., "TDM over IP", draft-ietf-pwe3-tdmoip-06 (work
              in progress), December 2006.

              Nadeau, T., "Pseudo Wire Virtual Circuit Connectivity
              Verification (VCCV)", draft-ietf-pwe3-vccv-12 (work in
              progress), January 2007.

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   [RFC2001]  Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
              Retransmit, and Fast Recovery Algorithms", RFC 2001,
              January 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, September 2000.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC3448]  Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 3448, January 2003.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, February 2006.

   [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
              Heron, "Pseudowire Setup and Maintenance Using the Label
              Distribution Protocol (LDP)", RFC 4447, April 2006.

   [RFC4553]  Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time
              Division Multiplexing (TDM) over Packet (SAToP)",
              RFC 4553, June 2006.

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Authors' Addresses

   Stewart Bryant
   Cisco Systems, Inc.
   250 Longwater
   Green Park, Reading  RG2 6GB


   Bruce Davie
   Cisco Systems, Inc.
   1414 Mass. Ave.
   Boxborough, MA  01719


   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400.
   Englewood, CO  80112


   Eric Rosen
   Cisco Systems, Inc.
   1414 Mass. Ave.
   Boxborough, MA  01719


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Full Copyright Statement

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