Network Working Group Stewart Bryant Internet Draft Bruce Davie Expiration Date: December 2006 Luca Martini Eric C. Rosen Cisco Systems, Inc. June 2006 PWE3 Congestion Control Framework draft-rosen-pwe3-congestion-03.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 Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract Insofar as pseudo wires may be used to carry non-TCP data flows, it is necessary to provide pseudo wire-specific congestion control procedures. These procedures should ensure that pseudo wire traffic is "TCP-compatible", as defined in [RFC2914]. This document attempts to lay out the issues which must be considered when defining such procedures. Bryant, et al. [Page 1] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 Table of Contents 1 Introduction ....................................... 2 1.1 Conventions used in this document .................. 2 1.2 PWE3 and Congestion in IP Networks ................. 2 1.3 Is This a Practical Problem? ....................... 4 1.4 Why isn't this Easy? ............................... 6 1.5 The Goal of PW-specific Congestion Control ......... 6 1.6 Constant Bit Rate PWs .............................. 8 2 Detecting Congestion ............................... 9 2.1 ECN ................................................ 12 3 Feedback from Receiver to Transmitter .............. 12 4 Responding to Congestion ........................... 15 5 Rate Control per Tunnel vs. per PW ................. 16 6 Fixed Rate of Transmission Services ................ 16 7 Mandatory vs. Optional ............................. 17 8 Informative References ............................. 17 9 Author's Addresses ................................. 17 10 Intellectual Property Statement .................... 18 11 Full Copyright Statement ........................... 19 1. Introduction 1.1. Conventions used in this document 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 RFC-2119 [RFC2119]. 1.2. 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 Bryant, et al. [Page 2] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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 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 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 end-users. If pseudo wires (PWs) were to be used only for carrying TCP flows, there would be no Bryant, et al. [Page 3] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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 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. 1.3. 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. Bryant, et al. [Page 4] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 - "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. - "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. Bryant, et al. [Page 5] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 1.4. 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 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. 1.5. 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." Bryant, et al. [Page 6] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 "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 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. [RFC3448], "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). Bryant, et al. [Page 7] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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 [RFC3448] describes an implementation technique that appears to make it efficient to calculate the equation. 1.6. Constant Bit Rate PWs Some types of PW, for example SAToP, CESoPSN, TDMoIP, SONET/SDH and CBR ATM PWs represent an inelastic constant bit-rate (CBR) flow and although they cannot respond to congestion in a TCP-friendly manner prescribed by [RFC2914], the percentage of total bandwidth they consume remains constant. AIMD techniques are clearly no applicable to such services that are also much more sensitive to packet loss than connectionless packet PWs. Given the CBR services are not greedy, there is a case for allowing them greater latitude in ignoring such services during congestion peaks. Depending on the specific level of resilience to packet loss, CBR PWs may not be able to endure any packet loss without compromising the transported service, therefore in case of congestion such PWs MUST be shutdown when the level of congestion becomes excessive. At lower levels of congestion they should be allowed to continue to offer traffic to the network. Some CBR services are carried over connectionless packet PWs. An example of such a case would be an 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 an a priori CBR PE. However there is an issue of trust that needs to be resolved (section 7) Bryant, et al. [Page 8] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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. 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: -i. 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 "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 Bryant, et al. [Page 9] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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 sequencing, and it also introduces a fairly complicated interaction between the control processing and the data path. -ii. 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: Bryant, et al. [Page 10] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 - 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 Bryant, et al. [Page 11] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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. 2.1. 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: Bryant, et al. [Page 12] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 -i. Control Plane A control message can be sent periodically to indicate the presence or absence of congestion. For example, when LDP is 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 Bryant, et al. [Page 13] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 tunnel. -ii. 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 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). -iii. 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. Bryant, et al. [Page 14] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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. 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. Bryant, et al. [Page 15] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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 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". Bryant, et al. [Page 16] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 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. Informative References [RFC2001] RFC2001, "TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms", W. Stevens. January 1997 [RFC2581] RFC2581, "TCP Congestion Control", M. Allman, V. Paxson, R. Stevens, April 1999 [RFC2914] RFC2914, "Congestion Control Principles", S. Floyd. September 2000 [RFC3168] RFC3168, "The Addition of Explicit Congestion Notification (ECN) to IP", K. Ramakrishnan, S. floyd, D. Black, September 2001 [RFC3448] RFC3448, "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-09.txt, Nadeau and Aggarwal, editors, June 2006 9. Author's Addresses Eric C. Rosen Cisco Systems, Inc. 1414 Massachusetts Avenue Boxborough, MA 01719 Email: erosen@cisco.com Bryant, et al. [Page 17] Internet Draft draft-rosen-pwe3-congestion-03.txt June 2006 Luca Martini Cisco Systems, Inc. 9155 East Nichols Avenue, Suite 400 Englewood, CO, 80112 Email: lmartini@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 10. 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