TSVWG S. Dudley Internet Draft Nortel Expires: January 2, 2006 July 2005 Simulation of RT-ECN based Admission Control and Preemption draft-dudley-rtecn-simulation-00.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/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 12, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document summarizes simulation results obtained from studies of a measurement based admission control and preemption scheme using Real-Time ECN semantics for SIP voice session setup. Conventions used in this document Some acronyms are used in this document for brevity and may refer to general concepts. Dudley Expires - January 2, 2006 [Page 1] Simulation of RT-ECN based Admission Control and Preemption SAC (Session Admission Control) may refer to either Admission Control, Preemption or both as required by the text TDM (Time Division Multiplexing) refers to traditional Voice Switching networks ECN (Explicit Congestion Notification) refers to either the concept of explicit congestion notification or to the bit field identified for use with Explicit Congestion Control per RFC 3168. RT (Real Time) refers to either Real-Time services or applications. Table of Contents 1. Introduction...................................................2 1.1 Motivation.................................................2 1.2 Approach...................................................3 2. Notes on Token Bucket Modifications............................5 3. Simulation Setup...............................................7 4. Qualitative Performance........................................8 4.1 Admission Control- Qualitative Analysis....................8 4.2 Preemption - Qualitative Analysis..........................9 5. Quantitative Performance......................................11 5.1 Admission Control - Quantitative Analysis.................12 5.2 Preemption - Quantitative Analysis........................14 5.3 Impact of Multiple Application Managers...................16 6. Conclusions...................................................18 Security Considerations..........................................19 References.......................................................19 Intellectual Property Statement........Error! Bookmark not defined. Disclaimer of Validity.................Error! Bookmark not defined. Copyright Statement..............................................20 Acknowledgments..................................................20 Author's Addresses...............................................20 1. Introduction This report summarizes simulations carried out on measurement based admission control and preemption based on Real-Time ECN semantics. 1.1 Motivation The investigation of admission control and preemption mechanisms is part of an effort to look at ways of managing session bandwidth on IP networks. The need to manage bandwidth may come either from a need to protect the network from traffic surges, re-route events, etc., or it may come from a need to prioritize session traffic on links of limited capacity. Real-Time ECN semantics have the advantage of taking action at the endpoints, rather than inside the network and so can avoid the need to track flows within the network. The general Dudley Expires - January 2006 [Page 2] Simulation of RT-ECN based Admission Control and Preemption approach also provides a wide latitude in choice of semantics that can permit independent action by endpoints or application managers at the edge of the network. Simulations were carried out to take a more detailed look at the capabilities, and issues, that would arise from this approach. One motivation for looking at admission control and preemption is the need to provide preferential treatment to some sessions on the network. Examples of networks where multiple precedence levels exist could include the U.S. Department of Defense DSN network which has 5 levels of precedence but could also include, under special circumstances, the existing TDM voice network where some endpoints are sometimes given special treatment. For example, the current TDM voice network, under periods of network stress or during emergency conditions, provides preferential treatment for fire, medical and emergency services endpoints. Emergency services, such as the ones listed above are deliberately wired to the switch in a way that guarantees that they are the last to be dropped when load shedding is needed, are the first to get service when a switch recovers from a failure, and can get admitted to the network when all non emergency callers are denied. The transition from a TDM voice network to an all VoIP network would currently involve losing these capabilities so mechanisms which could provide an equivalent capability have value. 1.2 Approach The approach taken for Real-Time ECN is summarized below. A more detailed description of this approach can be found in “Congestion Notification Process for Real-Time Traffic, draft-babiarz-tsvwg- rtecn-03”, Feb 2005i, and “RTP Payload Format for ECN Probing, draft- alexander-rtp-payload-for-ecn-probing”, May 2005ii. The semantics of the scheme are tailored for session based traffic such as is created by SIP, and the simulations summarized here used SIP endpoints. The basic notion of protecting the network is to ensure that the call arrival rate never permanently swamps the call departure rate. While admission control affects the arrival rate, preemption affects (increases) the departure rate. Network devices meter traffic and, based on thresholds, signal their level of congestion to the endpoints by setting bits in the ECN field of RTP packets emitted by endpoints. The endpoints react to that signaling to either make independent admission control decisions, or to notify an application manager that mediates the decision making process for session preemption. Decisions are made in a way that guarantees that , traffic of lower precedence is denied or preempted before traffic of higher precedence. Admission control is based on having endpoints send small RTP packets (probe packets) through the network during call setup. Probe packets Dudley Expires - January 2006 [Page 3] Simulation of RT-ECN based Admission Control and Preemption use the same IP address and port number as the media packets which would follow so the probing process was guaranteed to have the same congestion conditions as the media which would follow. The decision to admit is made at the endpoint, based on rules coded into the device. The admission decision is based on session precedence and level of congestion. Two levels of congestion are defined. Routine sessions (i.e. the lowest precedence session and the most common kind of session) call setup terminates with a network failure condition if either Level 1 or Level 2 congestion is indicated on the link during probing. Higher precedence sessions terminate call setup if Level 2 congestion is detected and persists for at least 500 milliseconds. If congestion clears before the 500 millisecond timer expires, the session is admitted. The 500 millisecond interval gives the network some time to clear congestion via its preemption mechanisms. There is a trade-off in how long to wait for the network to clear before making the final admission decision. Since it is possible, even though unlikely, that the congestion is caused entirely by high precedence sessions, the session is denied at some point in time if congestion does not clear. (i.e. The choice of a 500 millisecond interval is somewhat arbitrary.) Probing continues until the call is answered. For routine calls, if a Level 1 indication is received during that time, call setup terminates with a network failure condition. For high precedence sessions, if a Level 2 indication is received during that time, it follows the sequence of activities associated with preemption. (i.e. the high precedence session has already been granted admission to the network and should only be removed when other sessions of the same precedence are also being preempted.) The preemption process begins with all of the endpoints whose traffic traverses an affected link detecting Level 2 congestion. If an endpoint experiences congestion for a sufficiently long time, it sends a NOTIFY message to an application manager that contains contact information for both ends of the session. The length of time that it waits before sending a NOTIFY message is randomly selected from within a range dependent on the precedence of the session. Each precedence level has a separate time range, separated by a minimum length of time from other precedence ranges. The deliberate separation of notification intervals permits all sessions of a lower precedence to have their notifications reach the application manager before notifications coming from higher precedence sessions, thus ensuring that all session of lower precedence will become targets of preemption before sessions of higher precedence. In the implementation used for these simulations, the first NOTIFY results in the application manager preempting the session (i.e it sends BYE messages to both endpoints) and then setting a timer (500 Dudley Expires - January 2006 [Page 4] Simulation of RT-ECN based Admission Control and Preemption millisecond). No other preemptions are attempted until the timer expires. During that interval, the application manager continues to listen for congestion NOTIFY messages and builds a preemption list from the messages. The length of time that the application manager collects notifications without between preemptions represents the expected length of time it would take for the BYE message from the application manager to reach endpoints, for them to stop sending media packets, and for token buckets in the routers to decide whether the congestion condition has changed. For a large network, with longer transmission delays, it may need to be longer than 500 milliseconds, and for a small network it may be shorter. If a single preemption is sufficient to bring the congested link back below the threshold, all endpoints will begin receiving a new congestion indication. All endpoints that have sent a NOTIFY message to the application manager send another NOTIFY message indicating that congestion has cleared. The application manager removes them from the preemption list. Approach Notes: In selecting which network elements need to meter and mark the ECN field, it was noted that not all links on a given network can be congested. Many, because of their relation to other links on the network that always become congested first, can never become congested. The full benefits of the admission control and preemption scheme can be realized by implementing the metering and marking process on a subset of those routers. Management of the admission control and preemption decision at the edges allows multiple behavior systems to be established and co- exist. The initial simulation work described here only looks at voice sessions established using SIP. Extension of this work to include non-voice sessions (video conferencing) is underway. Lessons learned in this type of investigation will likely have merit in looking at other kinds of session based traffic. 2. Notes on Token Bucket Modifications For use in the metering process in the router, a token bucket algorithm is common and has many benefits for use in this scenario. It provides the benefit of ensuring that the average threshold rate has been exceeded for a sufficient time to reliably declare that the threshold has been exceeded. When used with randomly arriving packets it provides a very good random marking of a percentage of the packets. However, for non-randomly arriving packets, such as those coming form Real-Time sources, the marking behavior is also non- Dudley Expires - January 2006 [Page 5] Simulation of RT-ECN based Admission Control and Preemption random and cannot be guaranteed to mark fairly across sessions of different precedence. Part of the problem arises from the non-statistical nature of the inter-packet arrival times in a real-time flow like voice. Codecs emit packets on a fixed interval to support the needs of a voice application. When two codecs are emitting packets at the same fixed interval, the relative arrival time of the two packets is quite likely to be the same from frame to frame. This can cause a problem with a system based on marking only when the bucket is empty. If the spacing between packets is large enough in one frame to cause the bucket to fill slightly, and thus not mark the packet as experiencing congestion, it is quite likely to be so in the frame that follows as well. This would mean that even though the average rate of all flows exceeds the threshold, the particular flow that is lucky enough to have a long interval between it and the previous packet will almost always be marked as not experiencing congestion. The more desirable pattern is to start marking when a congestion condition is first detected and then not stop marking until we are certain that the condition has cleared. The mechanism chosen to modify the token bucket involved creating a two state marking process. Starting from the un-congested condition, no marking of packets is done. When the token bucket is emptied for the first time, marking of packets begins. Marking of packets continues until the token bucket fill level exceeds a defined threshold. At that time, the marking of packets stops, and the token bucket reverts back to its original state. The size of the token bucket, or alternatively its burst size, determines how quickly the token bucket responds to changes in throughput. A small burst size responds quickly to changes, whereas a large burst size requires a longer period of time to fill or empty. In creating the two state device, it was noted during initial simulations that if the burst size used for declaring the start of marking and the burst size used for declaring the end of marking were the same, that short burst sizes led to a risk of stopping the marking process too early and large burst sizes led to the problem of admitting more sessions before the endpoints start to receive indication of congestion. The solution to the issue was to make the burst size different for detecting when to start marking than for detecting when to stop marking. A small burst size for detecting the start of marking and a much larger burst size for detecting when to stop marking produced a very stable system, capable of reacting quickly to rapid onset events but yet not incorrectly declaring drops in throughput. The mechanics of how this was accomplished won’t be addressed in this report but can be obtained by consulting with the author. Dudley Expires - January 2006 [Page 6] Simulation of RT-ECN based Admission Control and Preemption 3. Simulation Setup The way that this simulation was undertaken was to take an existing simulation program (OPNET Modeler) and to extend the behaviors of the models available in it. A router model was extended to be able to meter traffic on its links and set ECN bits in packets traversing its links. A workstation model was extended to monitor ECN bits and to simulate important elements of SIP message behaviors including: INVITE, ACK, UPDATE, PRACK, CANCEL, BYE, and NOTIFY. The INVITE and non-INVITE Client and Server Transaction models as described in RFC 3261 and elements of the Pre-conditions model as described in RFC 3312 were also implemented. The same workstation model was extended in a different way to act as a Back to Back User Agent and as an application manager to mediate preemption decisions. The ECN metering process used by the router follows the recommendations in “Congestion Notification Process for Real-Time Traffic, draft-babiarz-tsvwg-rtecn-03”, Feb 2005. The pre-conditions model as described in RFC 3312 was used as the point of departure for the simulations. However, it was not followed exactly as described in RFC 3312 since the semantics there pertain to RSVP. At the point where RFC 3312 requires an indication of permission to continue with call setup, the simulation triggers the establishment of a flow of probe packets between the endpoints. (More details about the probing process can be acquired from “RTP Payload Format for ECN Probing, draft-alexander-rtp-payload-for-ecn- probing”, May 2005.) ECN bit marking on the probe packets is used to detect the level of congestion on the network, which is used by the endpoint to make the decision to proceed or to terminate call setup i.e. to carry out admission control. After the call is established, the ECN markings on RTP media packets are used to make preemption decisions. The partition of functions was carried out as follows. Who Meters Traffic Selected Routers on the network Detection of congestion: metering of links using a modified token bucket algorithm. Who is signaled: SIP endpoints How it is signaled: ECN bits are set with values 0,1 or 2 What is signaled Level 0 indicates no congestion, Level 1 indicates a first level of congestion and Level 2 indicated a second level of congestion. (No attempt was made to distinguish between ECN capable and not ECN capable flows or to mimic the exact bit patterns of RT-ECN) Dudley Expires - January 2006 [Page 7] Simulation of RT-ECN based Admission Control and Preemption Admit Action Endpoints make the decision based on the precedence of their session as indicated in the Approach section Preempt Action Endpoints notify a Back to Back User Agent (B2BUA), which fills the role of the application manager described in the Approach section, if congestion still persists when an internal randomly selected timer expires . The B2BUA maintains a preemption list and sends BYE messages to endpoints periodically (every 500 milliseconds). 4. Qualitative Performance Before examining the performance of the system from a numerical perspective, it is instructive to have a look at some of the qualitative response of the system 4.1 Admission Control- Qualitative Analysis The desired behavior of the admission control mechanisms is to deny admission to routine or high precedence sessions whenever the threshold for each is crossed on any link in the network, regardless of where in the network this event occurs. This was the observed behavior of the simulation. Getting the system to cross the second threshold to test admission control of high precedence sessions, however, proved to be difficult. Even with very high call arrival rates, the second level of congestion could only ever be reached if the volume of high precedence traffic, by itself, was sufficient to generate enough traffic to exceed the second threshold. This is not to say that traffic levels never exceed the first threshold. There is some latency in the detection mechanism, both coming from the token bucket filters, and from the latency of packet traversal across the network that can allow more than one “extra” call to be admitted above the Level 1 threshold. The closer together that calls arrive, i.e. the higher the instantaneous call arrival rate, the more sessions can be admitted before the congestion indication is acted on at the edges. For routine calls, continuous probing is very effective in reducing overshoot, even when call arrival rates are very high. After Level 1 ECN markings began to arrive at all endpoints, the behavior of the system was to stop admitting routine calls. As will be seen in the Quantitative Performance section of this document, the system was able to tolerate very high call arrival rates. We have not attempted to determine what call arrival rates are in line with a mass calling event but the overall performance suggests that it is Dudley Expires - January 2006 [Page 8] Simulation of RT-ECN based Admission Control and Preemption likely possible to engineer for protection against any level of mass calling event. The desired behavior of the admission control mechanism is to be insensitive to where in the network the congestion occurs, and to how many congestion events are present in the network. The results of the simulation confirm this behavior. Marking of Level 1 and Level 2 congestion states by routers in the network was established so that an incoming packet marked at Level 2 congestion from a previous link would not be re-marked to indicate a Level 1 congestion. The congestion indication received by the endpoint is always the worst case congestion event anywhere in the network. 4.2 Preemption - Qualitative Analysis Since the preemption control point (Level 2 Congestion indication) is higher than the Level 1 congestion point, the network is already denying all routine sessions by the time that preemption is required. At that point, the arrival rate of high precedence calls has to exceed the departure rate of all calls in order to reach Level 2 congestion. In the normal condition where high precedence calls are a small percentage of total calls, that can be made highly unlikely just by setting the Level 2 threshold far enough from the Level 1 threshold. In the core of a network where this condition can be reasonably expected to be true, preemption would only be necessary to handle the case of traffic re-route after a link failure. At the edges of the network, it may be possible for a sudden surge to reach a Level 2 congestion condition, and these are the cases that are investigated in the Qualitative section that follows. The desired behavior of the preemption process is to begin preempting sessions when a specific threshold is reached in the network, to remove only sessions that traverse the affected link, to always remove sessions with a lower precedence before removing sessions of a higher precedence and to stop removing sessions as soon as the congestion level drops below the specified threshold. The observed results conform to all of the requirements except for the effects of latency in the start and stop conditions. The latency comes in part from the token bucket which must either empty or fill a certain amount before beginning to change ECN markings, and in part from the transmission delay on the network. The overall result is that the performance of the system at high call arrival rates in slightly different than at low call arrival rates. The differences, however, are small as will be seen later.. For the Level 2 threshold, since the preemption control point (Level 2 threshold) is also the admission control point for high precedence traffic, the latency leads to a condition where high precedence calls can be admitted for a short time after the Level 2 threshold is Dudley Expires - January 2006 [Page 9] Simulation of RT-ECN based Admission Control and Preemption actually reached. Continuous probing does not have a significant impact on reducing the number of high precedence sessions admitted after the threshold is crossed because the behavior for high precedence sessions that receive a Level 2 congestion indication during ringing is based on the strategy of waiting to see if other routine sessions could be preempted to allow them to remain connected. The mediation of the preemption process through the application manager resulted in a small increase in signaling load. The increase was kept small by to the randomization of the notification process. This paced the delivery of NOTIFY messages. Not only did this prevent a sudden message surge but it also allowed the network to clear itself from small amounts of congestion before most endpoints needed to send a message. The simulation involved both network architectures with a single application manager managing the preemption process and architectures with multiple independent application managers. No coordination between application managers was necessary to implement the preemption process. Results indicated that the network was protected from oversubscription in all cases. The only difference between single application manager and multiple application managers occurred at high call arrival rates where there was a small difference in the number of sessions that were preempted for a single event. Having multiple application managers serving the same session, with one application manager associated with one endpoint of the sessions and a second application manager associated with the other endpoint, appeared to have no adverse affects on the way that the preemption mechanism worked. Dudley Expires - January 2006 [Page 10] Simulation of RT-ECN based Admission Control and Preemption 5. Quantitative Performance What follows is a summary of the results obtained through simulation. The network used in this simulation was set up to accommodate both a single application manager and multiple application managers. The diagram below illustrates the distances involved and the sites selected. At four of the sites (AK, WA, FL, DC) a bank of endpoints was located. The link chosen to be metered was the link between KS and WA. This selection is made not because we believe that the long distance links are the ones likely to be congested. We actually believe that these are the least likely to be congested. However, in order to both get the long latencies and multiple spans of control on a single link, it was simpler to use this link than any other. To view the chart please see pdf version of this memo draft-dudley-rtecn-simulation-00.pdf All of the links used in this simulation are Gigabit Ethernet links. The decision to do this is based partly on being able to hold all parameters as close as possible between simulations. Overall delay in the network is affected by both transmission delay, from the Dudley Expires - January 2006 [Page 11] Simulation of RT-ECN based Admission Control and Preemption distances involved, and serialization delay, from the time it takes to send packets of a given size. With all of the links being high speed links, there is essentially no serialization delay. The only delay simulated is distance delay. The thresholds for the link being metered were engineered to perform admission control at about 8 calls per minute. The surges applied to the network were in the order of 240 calls per minute (4 calls per second). Session Admission Control (SAC) illustrates admission control or preemption cases, as noted. 5.1 Admission Control - Quantitative Analysis To view the chart please see pdf version of this memo draft-dudley-rtecn-simulation-00.pdf Figure 1 Throughput Comparisons for 8 Calls per minute w/o SAC 10 Calls per Minute w/SAC This chart illustrates the base functionality of the ECN based Admission Control system. The first run (purple trace) is performed at 8 calls per minute without the Session Admission Control (SAC) scheme (8cpm_noSAC). The second run (blue trace) is performed at 10 Dudley Expires - January 2006 [Page 12] Simulation of RT-ECN based Admission Control and Preemption calls per minute with the Admission Control point for Routine calls (10cppm_SAC) at 1.755 Mbps (Ethernet Throughput) and the Preemption threshold at 2.34 Mbps (Admit, Preempt). The percentage of precedence calls in this example is 1% for both runs. The 10cpm_SAC run shows the traffic rising until it crosses the admission control threshold. At this point, new routine calls are denied. Since Precedence calls are such a small percentage of the overall call mix, the throughput ceases to rise. At some points, the departure of calls from the system allow it to fall back below the threshold for a while until a new routine call takes its place. To view the chart please see pdf version of this memo draft-dudley-rtecn-simulation-00.pdf Figure 2 Throughput Comparisons for 240 Calls per minute w/o SAC 240 Calls per Minute w/SAC Dudley Expires - January 2006 [Page 13] Simulation of RT-ECN based Admission Control and Preemption This chart shows two runs at 240 Calls per minute. One with SAC (240cpm_SAC) and one without (240cpm_noSAC). The Admission Control threshold for Routine calls is 1.755 Mbps and the Preempt threshold is 2.34 Mbps (Admit, Preempt). Both runs are at 1% Precedence. The network is configured to have a maximum of 800 calls connected. The run without SAC demonstrates that the number of calls rises continuously to reach that limit. The run with SAC, however, is limited at the Admission Control point. 5.2 Preemption - Quantitative Analysis In order to reach the 2nd level of congestion, it was necessary to increase the arrival rate of high precedence traffic so that it exceeds the second level threshold. This was done by making 100% of the traffic high precedence (called Precedence here). This is not the normal, or expected case, so it should be noted that the results shown here do not represent any expected behavior is a real network. The results shown here illustrate the behavior of the system if we choose to drive it completely with higher precedence traffic. It doesn’t represent the expected behavior of the system but can be used to illustrate the results of having a scenario where the admission control point and the preemption point are at the same level. i.e. since high precedence traffic ignores the Level 1 threshold, and these runs show only High Precedence traffic, the admission control and preemption control points are the same. To view the chart please see pdf version of this memo draft-dudley-rtecn-simulation-00.pdf Dudley Expires - January 2006 [Page 14] Simulation of RT-ECN based Admission Control and Preemption To view the chart please see pdf version of this memo draft-dudley-rtecn-simulation-00.pdf Figure 3 Throughput Comparisons for Preemption at 10 cpm and 240 cpm The two charts illustrate the preemption process in action to keep the throughput (measured here in terms of numbers of sessions) at or below the second level threshold point. The chart at 10 calls per minute (10 cpm) clearly show the impact of having two token bucket burst sizes, one for making the decision to start marking, and one to make the decision to stop marking. The larger of the two burst sizes is the decision to start marking so the excursions above the threshold are larger than the excursions below the threshold. The second chart appears, at times, not to make it back to the threshold. This is, in fact, an artifact of the charting process because the shortest time visible on the chart is a 3 second window. The maximum value over the 3 second window was used as the reference point for building the chart. Because of the high call arrival rate, the link throughput only stayed below the threshold momentarily. A second item to note form the charts is that the excursions above the threshold are higher for the 240 cpm case than for the 10 cpm case. This is a consequence of the latency in signaling the new threshold level to the endpoints. Of interest is the fact that even though the second call arrival rate is more than an order of magnitude higher than the first rate, the size of the excursions are Dudley Expires - January 2006 [Page 15] Simulation of RT-ECN based Admission Control and Preemption still small, indicating that the mechanism is quite robust to traffic surges. 5.3 Impact of Multiple Application Managers In our simulations, the application manager was a Back to Back User Agent (B2BUA) that was co-located with one of the SIP endpoints in the single application manager cases, and with each of the endpoint sites in the 4 application manager case.. The charts that follow compare the two cases. The reason for looking at the single application manager and multiple application manager cases is to examine whether the mechanism can scale to larger size networks. To view the chart please see pdf version of this memo draft-dudley-rtecn-simulation-00.pdf Figure 4 Comparison of 1 and 4 B2BUA Networks at 10 Calls/Min, 1% Precedence The above chart illustrates the behavior of the single (1B_P01) and four B2BUA (4B_P01) network cases when run at an arrival rate of 10 Calls/Minute. At low call arrival rates, the performance of the single application manager and multiple application manager cases is identical. The next chart looks at the higher call arrival rate of 240 calls/minute. It should be noted that 240 calls/minute, with a 5 minute average hold time, would result in 1200 calls on a link. Although this would not completely fill a Gigabit Ethernet link, it Dudley Expires - January 2006 [Page 16] Simulation of RT-ECN based Admission Control and Preemption is representative of the call levels that we might expect to see in a very large network. Remembering that the thresholds are set here not for the case where we have engineered the link for 240 calls/minute but where the link is engineered for 8 calls/minute. To view the chart please see pdf version of this memo draft-dudley-rtecn-simulation-00.pdf Figure 5 Comparison of 1 and 4 B2BUA Networks at 240 Calls/Min, 1% Precedence In this case, the overall appearance of the plots are similar, with one exception on the 4B2BUA case, which dips lower on one occasion that the 1 B2BUA case. Detailed analysis of the 4 B2BUA network showed that each of the B2BUA is acting independently so although each B2BUA is paced at 1 preemption per 500 milliseconds, the overall result is that 4 preemptions will occur every 500 milliseconds if the congestion event lasts long enough to incur multiple preemption events. In comparing this chart with the earlier chart illustrating the 10 call/minute scenario, we have kept the admission control and preemption points identical and the average number of sessions is Dudley Expires - January 2006 [Page 17] Simulation of RT-ECN based Admission Control and Preemption slightly higher in this case. Detailed analysis indicated that this is caused partly by latency in the detection process. With a higher call arrival rate, a larger number of sessions might be admitted between the time that the threshold is actually crossed at the network router and that the router can react and new markings arrive at the endpoints. 6. Conclusions This summary of test results illustrated the behaviors of an admission control and preemption scheme based on metering link traffic at network devices and using the ECN field to signal congestion information to the endpoints. Taking action at the endpoints was seen to be effective in limiting overall traffic through the congested links. Some of the accommodations in the metering process and the endpoint behaviors that are required to make this system work have been noted in this summary. For admission control, taking action at the endpoints themselves is very effective. For preemption, the mediation of a application manager and deliberate pacing of notifications to that server at the endpoints allows the scheme to accommodate multiple levels of precedence. Taking action at the endpoints allows the scheme to operate without the requirement to track flows at network devices. The mechanics of the scheme permit each endpoint to make independent admission control decisions. The mechanics also permit independent preemption action to be taken by multiple application managers so it is not necessary to track congestion state of the entire network in a single application manager either. The range of behaviors possible with this general approach is quite large. This simulation only looked at voice sessions and one set of behaviors that could be implemented for them. Since action is taken independently by each endpoint, it is feasible for a different set of behaviors to be defined for different types of applications on the same network. These different sets of behaviors could co-exist without harm to the network as long as they were based on the same ECN semantics and they provided a reasonable guarantee of providing admission control and preemption limits. Work is currently underway to investigate the performance of RT-ECN mechanism with video conferencing systems. The variability of packet size makes it feasible for natural variations in throughput to occasionally reach a Level 2 threshold, which suggest that the decision process in the endpoint for video traffic may need to be slightly different than that needed for constant bit rate voice. Dudley Expires - January 2006 [Page 18] Simulation of RT-ECN based Admission Control and Preemption Security Considerations The studies on which this summary is based did not consider security impacts of implementing admission control and preemption schemes. References i Babiarz, J. et al, “Congestion Notification Process for Real-Time Traffic, draft-babiarz-tsvwg-rtecn-03”, Feb 2005 ii Alexander, C., “RTP Payload Format for ECN Probing, draft- alexander-rtp-payload-for-ecn-probing”, May 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, Dudley Expires - January 2006 [Page 19] Simulation of RT-ECN based Admission Control and Preemption INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). 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. Acknowledgments Funding for the RFC Editor function is currently provided by the Internet Society. Author's Addresses Stephen Dudley Nortel 4001 E. Chapel Hill Nelson Highway P.O. Box 13010, ms 570-01-0V8 Research Triangle Park, NC 27709 Email: SMDudley@nortel.com Dudley Expires - January 2006 [Page 20]