TSVWG Naotaka Morita Internet-Draft NTT Corporation Expires: December 22, 2003 Gunnar Karlsson KTH June 23, 2003 Framework of Priority Promotion Scheme draft-morita-tsvwg-pps-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 December 22, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document describes a framework of a new scheme for traffic control to achieve end-to-end QoS for interactive multimedia services. The scheme is based on end-to-end measurement of network resources by end systems. The network is assumed to fully support the priority control scheme specified in the Diffserv architecture for QoS and SIP [1] for session control. Since the scheme relies on the behavior of the end systems, this document also touches on mechanisms for monitoring end-system behavior. Conventions used in this document Morita & Karlsson Expires December 22, 2003 [Page 1] Internet-Draft Framework of Priority Promotion Scheme June 2003 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 [2]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. The target service - Interactive multimedia services . . . . . 4 3. Motivation to focus on an end-system oriented measurement-based approach . . . . . . . . . . . . . . . . . . 6 4. Basic concepts behind the Priority Promotion Scheme . . . . . 7 5. Variation of specific usage of the Priority Promotion Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6. Functional architecture of the Priority Promotion Scheme . . . 10 7. Requirements of the Priority Promotion Scheme . . . . . . . . 11 7.1 Routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7.2 End system . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.3 SIP proxy . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.4 Edge router . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.5 Media monitoring server . . . . . . . . . . . . . . . . . . . 13 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 14 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18 Intellectual Property and Copyright Statements . . . . . . . . 19 Morita & Karlsson Expires December 22, 2003 [Page 2] Internet-Draft Framework of Priority Promotion Scheme June 2003 1. Introduction Emerging services such as VoIP, video chat, and video conferencing require session-based QoS. A few schemes for providing the needed QoS have been put forward, but they either require per-flow management of routers within the network or handle the provision of QoS on a per-class basis by allocating high capacity resources. In this document a framework for a new QoS scheme is proposed. The scheme is suitable for session-based interactive multimedia and adds less complexity to the network than previous approaches, while delivering per-flow QoS. Karlsson [3][4] originally proposed this concept. Based on his ideas, we clarify the requirements to routers, introduce enhancements to session control using SIP, and show some alternatives to monitor end-system behavior. We refer to this scheme as the "Priority Promotion Scheme". One of the key functions of the Priority Promotion Scheme is routers behavior. We introduce a new MF-PHB (Measurable Forwarding Per Hop Behavior) to represent such function. MF-PHB should be verified whether it is feasible by exiting equipment. This framework is intended as a guide for device manufacturers, network administrators and operators who need a way to provide QoS for Interactive Multimedia services. It is not intended, in its current state, for use by the majority of networks in the Internet. The reason that this proposal is being made at this time is that we feel that the only way to achieve a long-term solution for inter-domain QoS is to start with practicing on intra-domain solutions, and incrementally expand the scope of the work as more experience is gained in deployment. In this document, we introduce a framework for such a Priority Promotion. We describe a target service category, which we refer to as "Interactive Multimedia Services", in section 2. In section 3, we explain our motivation in focusing on an end-system oriented measurement based approach. The basic concepts behind the Priority Promotion Scheme are then explained in section 4. In section 5, variations of specific usage of the Priority Promotion Scheme are presented to show the potential of this scheme. The functional architecture of the scheme is described in section 5, and finally requirements for the individual functional entities are summarized in section 6. Morita & Karlsson Expires December 22, 2003 [Page 3] Internet-Draft Framework of Priority Promotion Scheme June 2003 2. The target service - Interactive multimedia services The major targeted services of the Priority Promotion Scheme are for multimedia and interactive communication software tools running on PCs, and operated by human being. We call such services interactive multimedia (IMM) services. The typical examples of IMM are VoIP, video chat, and video conferencing. IMM services have several characteristics that differentiate them from existing data services. Web browsing and some of file retrieval are based on client/server models and the data transfers speeds required are not so high in general. On the contrary, IMM services are any-to-any and relatively high speed at the range of less than 1 Mbps to a few Mbps. These IMM-inherent characteristics may cause big fluctuations of traffic patterns and may not be predictable in advance. Other important characteristics of IMM services are the requirements for bandwidth guarantees and the real time nature in terms of QoS. This is because normal codecs are sensitive to the fluctuation of bandwidth and the degradation of QoS. There are several codecs that adjust their information rates according to the available bandwidth, but they impose high processing load on the end system and can never avoid noticeable and maybe also annoying fluctuation in the perceptual quality. This is why we need to assume that there will be bandwidth guarantees. This implies that once the session is established, the bandwidth is to be guaranteed. In other words, if the required bandwidth is not available, the session should not be established. It should be noted that a more extended interpretation of this concept is that once the bandwidth is guaranteed at a certain level, it should be maintained until the end of the session. Improvement is acceptable but degradation is not acceptable. Finally, IMM services are set up on-demand and may last for a period of time in the order of minutes. Taking into account the characteristics or requirements of IMM as described above, explicit admission control on per-flow basis becomes necessary. There is an argument that simple over-provisioning is capable of meeting these kinds of requirements. But, as described above, IMM has the characteristics of relatively high bandwidth, unpredictability of traffic pattern and strict QoS needs. Therefore, we need session based admission control to delivery QoS for IMM services. It should be emphasized that admission control has a completely different goal from existing TCP base functionality. The goal is to provide bandwidth guarantees with the appropriate QoS for a certain maximum number of sessions. For example, if the network resource is 100 Mbps and 100 users request sessions with guarantees of 1 Mbps, Morita & Karlsson Expires December 22, 2003 [Page 4] Internet-Draft Framework of Priority Promotion Scheme June 2003 nearly 100 sessions should be established. If 1000 users request the same 1 Mbps guarantees, still only around 100 sessions should be established. This is quite different from existing data services, typically operated using TCP. With TCP, the network resources are shared in a "fair" manner among existing sessions at that time. If the network resource is 100 Mbps and 100 users request sessions, 100 sessions should be established with 1 Mbps throughput. If 1000 users request, all 1000 sessions should be established with around 0.1 Mbps throughput. SIP has become a suitable way to control IMM services. Although we focus on SIP in this description, session-control protocols for the Priority Promotion Scheme are not restricted to it. The application of a QoS policy, which means any differentiation based on the identity of a caller or callee in the session, needs to be studied. There are issues such as the competition between a VIP call with an ordinary call, or between a preferential call and an ordinary call in case of a disaster. If such policy is applied along with simple admission control based on the resource availability, policy credential information from SIP or other signaling methods may needs to be incorporate into this framework. Morita & Karlsson Expires December 22, 2003 [Page 5] Internet-Draft Framework of Priority Promotion Scheme June 2003 3. Motivation to focus on an end-system oriented measurement-based approach As IP-based networks proliferate, the overall network configuration becomes increasingly complex. In terms of bandwidth available in the access network, DSL alone includes many variants. 12-Mbps ADSL is quite popular in Japan and higher speed ADSL services will be deployed in the near future, but the actual throughput is completely dependent on conditions that have nothing to do with the access network, such as the distance from the central office and interference among lines. Another point is the variations in the network configurations of customers, including broadband routers. The broadband routers initially offered for use with higher-speed access lines may not be capable of providing maximum throughput. A customer's PC may impose similar restrictions. The network to which the customer is connected adds a lot of variables. In such a complicated situation, end-to-end guarantees of QoS are difficult to achieve and the role of the end system becomes more important, because only the end system knows the actual communication conditions. In the Priority Promotion Scheme, the end systems measure, monitor, or probe network resources to set up and maintain a media stream with the required QoS. We refer to terminal points of the media stream, i.e. PCs or residential gateways, as end systems. Morita & Karlsson Expires December 22, 2003 [Page 6] Internet-Draft Framework of Priority Promotion Scheme June 2003 4. Basic concepts behind the Priority Promotion Scheme As is described in the previous section, we will take end-system oriented measurement-based approach. When it comes to the network, the routers and L2 switches within the network also have their roles, and the Diffserv architecture is very popular for these devices. Certain configurations of the priority mechanism provided by Diffserv give us very simple ways to get information on the availability of network resources. There are two examples. When expedited forwarding (EF) and best effort (BE) are applied on a certain link and the maximum-rate limit for EF is the same as the link capacity, the throughput of BE represents the bandwidth remaining after allocation of bandwidth to the EF traffic. Let us consider the other example. One assured forwarding (AF) class with two-drop precedence might be configured as, for example, af11 with a low drop precedence and af12 with a high drop precedence. In this case, the throughput of af12 represents the bandwidth remaining after allocation to af11. We can automatically achieve the required QoS by setting up communication between the devices that play the two roles above. In other words, the network provides two priority classes, which share the same capacity. The end system at the transmitter's side sends data before or during the first phase of the media stream as lower-priority traffic, and the other end system, at the receiver side, reports the condition of the received media to the transmitter side. The transmitter-side system uses this information to decide whether it will assign the higher priority to the packets for the remainder of the media stream, give up on this and send lower-priority packets, or stop setting up the media stream. The key is to promote and demote the priority between the monitoring phase and the real media transmission, or in the very early stage of stream delivery. Changing the packet priority in this way gives us a way to measure remaining bandwidth, while maintaining the QoS of established media streams. If all of the end systems in a network behave this way, per-flow QoS is inevitably achieved. We refer to this basic concept as end-system oriented, measurement-based connection/session admission control. It should be noted that we are talking about scheduling priority in the diff-serv scheduler as opposed to call control preference from a policy perspective or drop preference in a common queue. The measurement-based approach has many variants. Any of the end Morita & Karlsson Expires December 22, 2003 [Page 7] Internet-Draft Framework of Priority Promotion Scheme June 2003 systems-the proxy or home gateway, the edge router at the ingress point of the network, or the border gateway-might have the role of measurement entity. The items for measurement might be packet loss and/or delay, from which we can find the remaining bandwidth. Explicit indication by the network, e.g. of congestion, is another possible information to use in forming measurement results. When it comes to the media, the media characteristics are an important factor. If the media streams are all constant bit rate, the overall behavior of the system is quite simple. However, most actual media streams, particularly video, flow at variable rates. For the sake of simplicity, we would like to focus on an approach that is 1) end-system oriented, 2) loss-rate-based, 3) includes no explicit indications from the network, and has 4) streams which flow, at constant bit rates, for periods of the order of minutes. As we previously have noted, the above concept is not new. It was originally proposed by Karlsson [3][4]. We would like to extend Karlsson's proposal to meet the needs of a real service. How we check the end system's behavior is an important point for a real service. Since the Priority Promotion Scheme is completely reliant on knowledge of the end system's behavior, incorrect behavior, whether accidental or intentional, will affect the QoS for other customers. One possible solution for this is to introduce two-stage monitoring of end-system behavior. Primary monitoring may be implemented at the edge router and is triggered by session initiation. Secondary monitoring might be done by a dedicated media monitoring server. The primary monitor checks all media streams that it handles which are controlled by the Priority-Promotion-Scheme. Example items to check are whether the flow is allowed to enter the network and whether the flow is less than the declared peak bit rate. The secondary monitor checks end-systems behavior in detail. Whether or not the two monitoring stages are really used will depend on the specific network environment, but it should be possible to use both to good effect. Another solution is that such checking mechanism is installed in every edge routers. As we describe in the next section, The Priority Promotion Scheme as described in the following sections is quite suitable for constant- bit- rate traffic such as voice coded by G.711 without silence compression, but we should investigate the possibility of tackling variable bit-rate traffic in future work. Initial investigations have already been done by Karlsson. Morita & Karlsson Expires December 22, 2003 [Page 8] Internet-Draft Framework of Priority Promotion Scheme June 2003 5. Variation of specific usage of the Priority Promotion Scheme As is described above, the Priority Promotion Scheme can be used as a kind of admission control. However, it is not limited to the connection/session admission control as is imagined in the legacy telephone network. For example, if the initial trial fails, there are options for the next actions. One is to give up the connection establishment. This is like the ordinary admission control. Another one is to stop sending the real media at the low level, but then try to send it with another class. After a while, the transmitter will retry and if this try succeeds, the real media is sent with the high class. Another possibility is that depending on the received condition information, the transmitter estimates the actual available bandwidth, selects the closest bandwidth lower than the available bandwidth and then sends the media with high priority. Another possible action is to send media at the full rate but only the core part of the flow is sent with high priority, and the other parts are sent with low priority. If hierarchical coding is used, this approach may work well (for example, in MPEG, sending I frames with high priority and P or B frames with low priority). Morita & Karlsson Expires December 22, 2003 [Page 9] Internet-Draft Framework of Priority Promotion Scheme June 2003 6. Functional architecture of the Priority Promotion Scheme Figure 1 shows the functional architecture of the Priority Promotion Scheme. The main functional elements are the two end systems, i.e. the transmitter and receiver, the transmitter-side edge router, core routers, the SIP proxy, and media-monitoring server. SIP proxy (Media-monitoring server) +------+ +------+ /---------------| |------------| | / +------+ +------+ / | // / | // +------+ +------+ +------+ +------+ +------+ | |===============|Edge |======|Core |======| |======| | +------+ +------+ +------+ +------+ +------+ End system End system (Transmitter) (Receiver) Figure 1: Functional architecture of the Priority Promotion Scheme Morita & Karlsson Expires December 22, 2003 [Page 10] Internet-Draft Framework of Priority Promotion Scheme June 2003 7. Requirements of the Priority Promotion Scheme In this section, we describe the requirements for each functional entity. 7.1 Routers Although the end systems perform an important role in the Priority Promotion Scheme, there are a few requirements put on the network. More specifically, the queuing mechanism or the PHB (per-hop behavior) for the PPS creates new requirements for network elements. The Priority Promotion Scheme appears to work with the existing Diffserv PHB, as was indicated in the introduction. However, to clearly explain the scheme's requirements, we have to define a new PHB. We refer to this as measurable forwarding (MF). The essential requirements for MF are as follows. * MF has two sub classes, MF-High (MF-H) and MF-Middle (MF-M) * MF-H and MF-M share the same capacity * MF-H takes priority over MF-M In other words, the total amount of MF-H and MF-M traffic is set as a limit, rather than having separated limits for MF-H and MF-M traffic. However, since MF-M traffic will always defer to MF-H traffic, MF-M traffic may experience markedly higher jitter and loss than MF-H; in fact, one would expect MF-H traffic to experience very nominal jitter or loss. Another view of MF is that, if a given amount of MF-M traffic for a particular stream passes through a router, the same amount of MF-H traffic for that stream must also be able to pass through. In the absence of other classes, it appears feasible to configure existing commercially available routers to produce the desired MF-PHB. Further requirements are as follows. 1. The MF must co-exist with other PHBs, such as the EF, AF, and BE. Existing implementations may not be capable of satisfying this extended requirement. 2. MF should take priority over AF and BE. This is because the target services are IMM services, where real-time variations in traffic characteristics are crucially important. Morita & Karlsson Expires December 22, 2003 [Page 11] Internet-Draft Framework of Priority Promotion Scheme June 2003 7.2 End system The transmitter should send trial packets before or at the beginning of a session. The receiver should record the results of trial-packet reception and report this information to the transmitter. The RTCP would be the best candidate to handle reporting of the receiving result. Some improvements might be necessary to reduce the measurement period and to make quick decisions. Actually, the minimum measurement period is the key factor of the usability of the Priority Promotion Scheme. This determines the possible service scenarios as is described in section 5. The transmitter then decides on the next action. * If the conditions of reception are good, the transmitter sends the remaining packets with the higher priority. * If conditions are not good, the transmitter gives up sending monitor packets and either 1) sends the remaining packets with other classes such as BE, 2) stops sending any media data and, after a while, starts sending monitoring packets again, or 3) terminates the session. Synchronization between the two directions of the media stream remains a subject for further study. 7.3 SIP proxy In principle, SIP is not directly related to the Priority Promotion Scheme. However, for commercial applicability, the operator would have to be able to monitor the service subscription of the customer before establishing the call. Furthermore, if the edge router has the capability to monitor the user stream, the SIP proxy can send commands to the edge router asking it to check up on the end system's behavior. The specific signaling sequence may depend on the chosen service model. If the policy is applied as is described in the previous section, signaling is where the policy credentials can get exchanged. 7.4 Edge router As noted above, in some networks the SIP server is able to instruct Morita & Karlsson Expires December 22, 2003 [Page 12] Internet-Draft Framework of Priority Promotion Scheme June 2003 the edge router to monitor the end system's behavior. The edge router might monitor the following things: * The transmitter should not send packets at rates above the peak bit rate offered in the monitoring phase. * The transmitter should not pause while sending packets. This is because, if it does this, the other end systems overestimate the remaining network resources and incorrectly send higher-priority packets. 7.5 Media monitoring server In addition to primary monitoring by the edge routers, more detailed monitoring may be required. The typical items to monitor are as follows: * The accuracy of packet-reception information from receivers, and the correct reaction of transmitters to the information from receivers should be achieved. * If the received information indicates poor conditions, the transmitter stops sending high priority packets. If a next trial is allowed, a certain time interval should be maintained between the initial trial and next trial. Morita & Karlsson Expires December 22, 2003 [Page 13] Internet-Draft Framework of Priority Promotion Scheme June 2003 8. Conclusion With the architecture described above, the next step will be a detailed specification of each relevant functional entity's actions. Candidates to be specified include; 1. MF-based per-hop behavior; 2. A SIP signaling extension for the Priority Promotion Scheme; 3. The interface between an SIP proxy and edge router; Although the existing Diffserv architecture may already meet the requirements of a MF class, there is an urgent need to verify this. This is because, although new requirements may not seem like much, the MF PHB is essential to the realization of the Priority Promotion Scheme. Other items may be left to each implementation. Morita & Karlsson Expires December 22, 2003 [Page 14] Internet-Draft Framework of Priority Promotion Scheme June 2003 9. Security Considerations To be described. Morita & Karlsson Expires December 22, 2003 [Page 15] Internet-Draft Framework of Priority Promotion Scheme June 2003 10. IANA Considerations To be described. Morita & Karlsson Expires December 22, 2003 [Page 16] Internet-Draft Framework of Priority Promotion Scheme June 2003 11. Acknowledgements The authors would like to thank Fred Baker, David Oran, Glenn Reitsma and other Cisco technical experts for their insightful suggestions. Morita & Karlsson Expires December 22, 2003 [Page 17] Internet-Draft Framework of Priority Promotion Scheme June 2003 References [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [3] Karlsson, G., "Providing Quality for Internet Video Services", in Proc. of the CNIT/IEEE 10th International Tyrrhenian Workshop on Digital Communications, Ischia, Italy, September 1998. [4] Elek, V., Karlsson, G. and R. Ronngren, "Providing Quality for Internet Video Services", in Proc. IEEE INFOCOM Tel-Aviv, Israel, March 2000. Authors' Addresses Naotaka Morita Network Service Systems Laboratories NTT Corporation 9-11, Midori-Cho 3-Chome Musashino-shi, Tokyo 180-8585 Japan EMail: morita.naotaka@lab.ntt.co.jp Gunnar Karlsson KTH, Royal Institute of Technology Isafjordsgatan 39 P.O.Box Electrum 229 SE-164 40, Kista Sweden EMail: gk@imit.kth.se Morita & Karlsson Expires December 22, 2003 [Page 18] Internet-Draft Framework of Priority Promotion Scheme June 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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