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Network Working Group                                      Eric C. Rosen
Internet Draft                                            Stewart Bryant
Expiration Date: March 2005                                  Bruce Davie
                                                     Cisco Systems, Inc.

                                                          September 2004


                   PWE3 Congestion Control Framework


                   draft-rosen-pwe3-congestion-02.txt

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 RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   http://www.ietf.org/ietf/1id-abstracts.txt.

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Abstract

   Insofar as 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
   procedures.








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Table of Contents

    1          Introduction  .......................................   2
    1.1        PWE3 and Congestion in IP Networks  .................   2
    1.2        Is This a Practical Problem?  .......................   4
    1.3        Why isn't this Easy?  ...............................   5
    1.4        The Goal of PW-specific Congestion Control  .........   6
    2          Detecting Congestion  ...............................   8
    3          Feedback from Receiver to Transmitter  ..............  11
    4          Responding to Congestion  ...........................  13
    5          Rate Control per Tunnel vs. per PW  .................  14
    6          Fixed Rate of Transmission Services  ................  15
    7          Mandatory vs. Optional  .............................  15
    8          References  .........................................  16
    9          Author's Addresses  .................................  16
   10          Intellectual Property Statement  ....................  17
   11          Full Copyright Statement  ...........................  17





1. Introduction

1.1. PWE3 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 TCP traffic.  TCP
   includes an elaborate congestion control mechanism which causes the
   endsystems to reduce their transmission rates when congestion occurs.




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   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
   delivery.

   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 MTU ("Maximum Transport
   Unit") 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
   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 endusers.  If pseudowires (PWs)
   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 endsystems 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



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   "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 retrofit TCP-like congestion
   control mechanisms to 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. Is This a Practical Problem?

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

     - "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 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.

     - "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



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       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.

     - "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 impossibility"

       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
       traffic.

     - "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.

       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
   compelling.


1.3. Why isn't this Easy?

   One easy solution 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
   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



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   direction, etc., etc.  In fact, we want to avoid acknowledgments
   altogether.  These same considerations lead us away from using e.g.,
   DCCP.

   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.


1.4. The Goal 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



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       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 consider."

   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.  RFC 3348, "TCP
   Friendly Rate Control (TFRC)" provides an alternative:

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

         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 rate."

   Note that the TFRC procedures require the transmitter to calculate a
   throughput equation.  For these procedures to be feasible in the as a
   means of PW congestion control, they must be computationally
   efficient.  Section 8 of RFC 3448 describes an implementation
   technique that appears to make it efficient to calculate the
   equation.



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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 misordered 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.  In fact, existing PW
   mechanisms and procedures 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 two evident methods for doing
   this:

      1. Trying to Detect Congestion Using PW Sequence Numbers

         When the optional sequencing feature is in use on a PW, 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
         "misordered 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



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

      2. Detecting Congestion Using Modified VCCV Packets

         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 [VCCV] packets into the PE data
         stream, where each VCCV packet carries:







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           - A sequence number, increasing by 1 for each successive VCCV
             packet.

           - 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.

         VCCVs 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 this off; that's why the
         different 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 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.




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         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.

      3. ECN

         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. However ECN is not widely deployed and the
         PEs must also be capable of operating in a network where packet
         loss is the only indicator of congestion.



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.

   There are a number of possible ways in which the feedback can be
   provided:

      1. Control Plane

         A control message can be sent periodically to indicate the
         presence or absence of congestion.  For example, when LDP is



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         the control protocol, 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.

      2. Reverse Data Traffic

         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



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         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 technique:

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

           - Sequencing of the data stream is required

           - The transmission of the congestion indications is not
             reliable

           - 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. 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.
   Everytime a new acknowledgment (i.e., not a duplicate acknowledgment)
   is received,  the rate is similarly increased (additive increase).



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   Thus TCP can adjust its transmit rate very rapidly, i.e., it responds
   on the order of a RTT.

   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.

   For PWs, the detection of congestion by the receiver is based on a
   periodic comparison of the number of packets received in an interval
   with the number transmitted.  Unless we are willing to sample at a
   rate of about half a RTT, PWE3 will have difficulty being as
   responsive.  The dynamic effects of sampling at a slow rate are
   difficult to understand.

   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, using a policing mechanism such as token bucket.

   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.



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, 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



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   may still be the best trade-off.


6. Fixed Rate of Transmission Services

   Some PW services may require a fixed rate of transmission, and it may
   be impossible to provide the service while 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. It is not known how such a prohibition could be
   enforced.

   One might try to be less draconian, by simply having the service
   turned off during periods of congestion.  The problem though is that
   there is no way to have it come up to speed slowly when the
   congestion disappears.

   If the fixed rate service is channelized, it may be possible to
   reduce the transmission rate by selectively shutting down channels,
   and to increase the transmission rate by adding back channels one at
   a time.

   In any event, the application of congestion control to fixed rate of
   transmission services is likely to be that all or part of the service
   gets shut down, an event which is likely to be made explicitly
   visible to the endusers.  This puts a premium on the ability to avoid
   "false alarms".


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.







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8. References

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

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

   RFC 2914, "Congestion Control Principles", S. Floyd. September 2000

   RFC 3168, "The Addition of Explicit Congestion Notification (ECN) to
   IP", K. Ramakrishnan, S. floyd, D. Black, September 2001

   RFC 3448, "TCP Friendly Rate Control (TFRC): Protocol Specification",
   M handley, S. Floyd, J. Padhye, J. Widmer, January 2003

   [VCCV] "Pseudo Wire (PW) Virtual Circuit Connection Verification
   (VCCV)", draft-ietf-pwe3-vccv-03.txt, Nadeau and Aggarwal, editors,
   June 2004


9. Author's Addresses


      Eric C. Rosen
      Cisco Systems, Inc.
      1414 Massachusetts Avenue
      Boxborough, MA 01719
      Email: erosen@cisco.com



      Bruce Davie
      Cisco Systems, Inc.
      1414 Massachusetts Avenue
      Boxborough, MA 01719
      Email: bdavie@cisco.com



      Stewart Bryant
      Cisco Systems,
      250, Longwater,
      Green Park,
      Reading, RG2 6GB,
      United Kingdom
      Email: stbryant@cisco.com




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10. Intellectual Property Statement

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11. Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
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