[Search] [txt|pdf|bibtex] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01                                                         
   TSV working group
   Internet Draft                                        Naotaka MORITA
   Document: draft-morita-tsvwg-pps-01.txt              NTT Corporation
                                                        Gunnar KARLSSON
                                                                    KTH
   Expires: April 2004                                     October 2003


                  Framework of Priority Promotion Scheme


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   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 made obsolete 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.



Abstract

   The Priority Promotion Scheme (PPS) is a new scheme for traffic
   control; more specifically, PPS involves applying a kind of admission
   control to achieve end-to-end QoS for a series of packets on a
   packet-based network.  The main targets are interactive multimedia
   services such as VoIP, video chat, and video conferencing.  The
   scheme is based on end-to-end measurement of network resources by end
   systems.  Before a session is established or even during a session,
   the source end system senses, measures, or probes the availability of
   network resources by sending out packets with priority one level
   lower than that of normal packets.  The result is modification of the
   DiffServ Code Point (DSCP) value of the succeeding IP packets: the
   priority is raised or promoted to firmly establish the session,
   lowered to leave resources with existing sessions, or otherwise
   adjusted so that the amount of packets does not exceed the available
   capacity.  The network, i.e., output links of the routers or L2
   switches is only assumed to support the per-class form of priority


MORITA & KARLSSON        Expires - April 2004                 [Page 1]


                                 PPS                     October 2003


   control that accompanies the DiffServ architecture.  Having all end
   systems follow the above behavior achieves end-to-end QoS without the
   maintenance of per-flow state in each item of network equipment.

   This document describes the reasons for the end-to-end measurement-
   based approach and the general network architecture of PPS.


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 [2].

Table of Contents

   1. Introduction...................................................3
   2. The target service type - interactive multimedia...............3
   3. Motivation for the focus on an end-system-oriented measurement-
   based approach....................................................5
   4. Basic procedure for the Priority Promotion Scheme..............6
      4.1 Basic procedure for end systems............................6
      4.2 Router behavior............................................7
      4.3 Variation of measurement-based mechanisms..................7
      4.4 Monitoring of terminal behavior............................8
      4.5 Accommodation of variable-bit-rate sources.................9
   5. Service models provided by the PPS.............................9
      5.1 Admission control.........................................10
      5.2 Quality improvement.......................................10
      5.3 Available bit rate........................................10
      5.4 Bit-rate increase.........................................10
   6. The feasibility of probe-based admission control..............11
   7. Functional architecture of the Priority Promotion Scheme......11
   8. Requirements of the Priority Promotion Scheme.................11
      8.1 Routers...................................................11
      8.2 End systems...............................................12
      8.3 SIP proxies...............................................13
      8.4 Edge routers..............................................13
      8.5 Media monitoring servers..................................13
   9. Security Considerations.......................................14
   10. IANA Considerations..........................................14
   Acknowledgements.................................................14
   Authors' Addresses...............................................14
   References.......................................................14
   Appendix:  Probe-Based Admission Control (PBAC) - Current
   experimental results and obervations.............................16





Morita                   Expires - April 2004                 [Page 2]


                                 PPS                     October 2003


1. Introduction

   Emerging services such as VoIP, video chat, and video conferencing
   require session-based QoS.  A number of schemes for providing the
   required QoS control 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, which requires the allocation
   of large amounts of 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 the basic concept.  Here, we
   clarify the requirements for routers, introduce enhancements to
   session control using SIP, and show some alternative ways to
   implement the required monitoring of end-system behavior.  We refer
   to this scheme as the "Priority Promotion Scheme".

   One of the key functions of the Priority Promotion Scheme is the
   behavior of routers.  We introduce the MF-PHB (Measurable Forwarding
   Per Hop Behavior) as a new per-hop behavior that provides the
   required functionality.  Whether or not MF-PHB is feasible on given
   items of existing equipment will have to be verified. 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.  We make
   this proposal now because we feel that the only way to achieve a
   long-term solution for inter-domain QoS is to start putting intra-
   domain solutions into practice and then incrementally expand the
   scope of the work as more experience in deployment is gained.

   In this document, we introduce a framework for Priority Promotion.
   We describe the 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 procedures of the Priority
   Promotion Scheme are then explained in section 4.  In section 5,
   specific variant applications of the Priority Promotion Scheme are
   presented to show the scheme's potential.  The feasibility of a
   measurement-based approach is presented in the appendix to this
   document and section 6 states why the arguments in the appendix are
   applicable to the PPS.  The functional architecture of the scheme is
   described in section 7.  Finally, the requirements for individual
   functional entities are summarized in section 8.  MF-PHB (Measurable
   Forwarding) that is necessary to realize PPS is defined in [5] and
   the verification scenarios of MF-PHB is in [6].

2. The target service type - interactive multimedia



Morita                   Expires - April 2004                 [Page 3]


                                 PPS                     October 2003


   The major targets of the Priority Promotion Scheme are multimedia and
   interactive communications services provided through software tools
   running on PCs and operated by human beings.  We call such services
   interactive multimedia (IMM) services.  Typical examples of IMM are
   VoIP, video chat, and video conferencing.  Several characteristics
   differentiate IMM services from existing data services.  Web browsing
   and, in many cases, file retrieval are based on client/server models
   and the data transfers speeds required are not in general very high.
   In contrast to this, IMM services are any-to-any and require
   relatively high speeds in the range from less than 1 Mbps to several
   Mbps.  These IMM-inherent characteristics may cause large
   fluctuations in traffic patterns and may not be predictable in
   advance.

   Other important characteristics of IMM services are the QoS
   requirements: that is, the requirements for bandwidth guarantees and
   short delays.  The latter is because of the real-time nature of these
   services.  The former is because typical codecs are sensitive to
   fluctuations in bandwidth, which lead to degradation of the QoS.
   While several codecs adjust their information rates to suit the
   available bandwidth, they impose higher processing loads on the end
   systems; this approach also necessarily incurs noticeable and
   possibly annoying fluctuation in the perceived quality.  This implies
   that once a session has been established, the bandwidth has to be
   guaranteed until the end of the session.  In other words, the session
   should not be established unless the required bandwidth is available.
   Note that one desirable extended interpretation of this concept is to
   allow increases, but never decreases, in the bandwidth available to a
   session.  That is, improvement is acceptable but deterioration is not.
   This is why we have included "promotion" in the name of the scheme.

   Finally, a session of an IMM service is set up on-demand and may last
   for time of the order of minutes to tens of minutes.

   When we take the above-described characteristics and requirements of
   IMM into account, we see that explicit admission control on a per-
   flow basis is necessary.  A common argument is that simple over-
   provisioning is capable of meeting these requirements.  As was stated
   above, however, IMM combines the characteristics of relatively large
   bandwidth requirements and strict QoS needs in general with
   unpredictable traffic patterns.  Therefore, we need a form of
   session-based admission control to deliver QoS for IMM services.

   It should be emphasized that admission control has a completely
   different goal from the existing TCP core functionality.  The goal of
   admission control is to provide bandwidth guarantees with the
   appropriate QoS for a certain maximum number of sessions.  For
   example, if the network is able to carry 100 Mbps and 100 users
   request sessions with guarantees of 1 Mbps, nearly 100 sessions


Morita                   Expires - April 2004                 [Page 4]


                                 PPS                     October 2003


   should be established.  If 1000 users request the same 1-Mbps
   guarantees, only around 100 sessions should be established.  This is
   quite different from existing data services provided through the TCP.
   The idea of the TCP is to share network resources in a "fair" manner
   among the sessions requested at any time.  If the network is able to
   carry 100 Mbps and 100 users request sessions, 100 sessions should be
   established, each with roughly 1 Mbps throughput.  If 1000 users
   request sessions, all 1000 should be established, each with a
   throughput around 0.1 Mbps.  This is not suitable for IMM services.

   The SIP provides one suitable way to control IMM services.  Although
   we focus on the SIP in this description, session-control protocols
   for the PPS are not restricted in this way.

   The application of a QoS policy which includes differentiation based
   on the identity of the callers or callees in sessions has to be
   studied as a separate issue. Issues include competition between VIP
   calls and ordinary calls, or between preferential calls and ordinary
   calls in times of disaster.  If such a policy that caters for such
   situations is to be applied along with simple admission control based
   on resource availability, policy credential information from the SIP
   or another signaling method may have to be incorporate into the PPS
   framework.

3. Motivation for the focus on an end-system-oriented measurement-based
   approach

   As IP-based networks proliferate, overall network configurations
   become 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 such as the distance from the central office
   and interference among the 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 the same maximum throughput as is stated in the
   catalogue.  A customer's PC may impose similar restrictions.
   Furthermore, wireless access introduces further complications in
   terms of the access environment.  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 is able to see the actual
   conditions of communication.  In the Priority Promotion Scheme, the
   end systems measure, monitor, or probe levels of network resources so


Morita                   Expires - April 2004                 [Page 5]


                                 PPS                     October 2003


   that they are able, if possible, to set up and maintain media streams
   with required levels of QoS.  We focus on an end-to-end approach
   because only the end systems are able to judge the overall relevant
   network situation.

   We refer to the terminal points of the media stream, i.e. PCs or
   residential gateways and routers, as end systems.

4. Basic procedure for the Priority Promotion Scheme

   The Priority Promotion Scheme (PPS) is a new scheme for traffic
   control; specifically, the PPS achieves end-to-end QoS for
   interactive multimedia services by exercising admission control for
   series of packets on a packet-based network.  The scheme is based on
   end-to-end measurement of network resources through coordination of
   the end systems.

   In this context, "priority" means priority or precedence at the
   packet level as represented by the DiffServ Code Point (DSCP) in the
   IP layer.  If we apply the PPS in Layer 2, the priority is
   represented by the user_priority field specified in 802.1D and Q.  If
   MPLS is used as an underlying transport, EXP field corresponds to the
   code.

4.1 Basic procedure for end systems

   PPS largely relies on end-system behavior for sending the probe
   packets, which test the availability of network resources, and for
   decisions on whether or not the succeeding (higher priority) packets
   can in fact be sent.

   Before a session is established and even, under certain conditions,
   during sessions, the source-end system senses, measures, or probes to
   detect the availability of network resources. This is done by sending
   packets with priority one level lower than that of the non-probe
   packets, i.e. those for established streams.  Probe packets are given
   lower priority so that existing flows of packets are maintained and
   packet loss is confined to the probe packets; this gives a sharper
   focus to the loss characteristics.

   Criteria for successful receipt at the destination-end system can
   include loss, delay, and delay jitter.  The authors believe that loss
   will usually be the crucial parameter, but are willing to enlarge the
   scope of measurement to include the other two characteristics.

   The conditions of receipt determine how the DSCP value for the
   succeeding IP packets is adjusted: the priority is raised or promoted
   to firmly establish the session, lowered to leave resources with



Morita                   Expires - April 2004                 [Page 6]


                                 PPS                     October 2003


   existing sessions, or otherwise adjusted to control the amount of
   packets such that the traffic fits into the available capacity.

   The RTCP can be used to carry the report from the destination end
   system. Whether or not the probing packets can carry real media data
   depends on the required duration of measurement.  If measurement will
   take more than a couple of seconds, the probe packets should carry
   real media so that the customer does not have to wait for completion
   of the measurement period.

4.2 Router behavior

   The PPS in principle requires that the network, i.e. each output link
   of a router or Layer 2 switch, support per-class priority control.
   Prioritization allows the end systems to measure remaining resources
   without affecting existing streams.  In addition to the simple
   priority control required by the PPS in itself, existing classes
   (Per-Hop Behaviors or PHBs) such as EF, AF, and BE should be
   supported.  That is, we have to implement an extension to the
   DiffServ architecture.  To clarify the requirements specific to the
   PPS, we propose Measurable Forwarding as a new PHB (MF-PHB).  A
   detailed description of the MF-PHB has already been given [5].
   Whether or not current DiffServ implementations are capable of
   supporting this new PHB for the PPS without elaboration of the queue
   configuration is not clear.  However, having all end systems behave
   in the way described above and all network elements implement the MF-
   PHB ensures that the end-to-end QoS is achieved without having to
   maintain per-flow states in individual items of network equipment.

   A great advantage of the PPS is that it avoids persistent contention
   among real-time streams.  Note that we are talking about scheduling
   priority in the DiffServ scheduler as opposed to a policy perspective
   on call control preference or drop preference in a common queue.

4.3 Variation of measurement-based mechanisms

   Measurement-based approaches have many basic variants.  Any of the
   end systems - the media proxy or home gateway, the edge router at the
   ingress point of the network, or the border gateway - might be
   assigned the role of measurement and decision entity.

   The items for measurement from which we identify the remaining
   bandwidth are packet loss and/or delay.  Explicit congestion
   notification initiated by the network may also provide supplementary
   information.

   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
   mechanism for explicit indication from the network.


Morita                   Expires - April 2004                 [Page 7]


                                 PPS                     October 2003



   As we have previously noted, the above concept is not new.  It was
   originally proposed by Karlsson as probe-based admission control
   (PBAC) [3][4].  Based on Karlsson's proposal, we would like to extend
   the measurement-based approach to allow for various service models,
   to clarify the behavior required of routers, and to take into account
   monitoring of the correctness of end-terminal behavior.

4.4 Monitoring of terminal behavior

   How we monitor, check, or audit the behavior of end systems is an
   important issue for a commercial service.  Since the Priority
   Promotion Scheme is strongly reliant on the behavior of end systems,
   incorrect behavior, whether accidental or intentional, will affect
   the QoS for other customers.

   Here, the items to be monitored include whether or not flows have
   been given permission to enter or access the network, whether flows
   are at the correct priority level, and whether flows are at the bit
   rates indicated by probing or signaled by SIP.  These are the
   behaviors in the direction from source to destination.  The behavior
   in the direction from the destination to the source should also be
   correct, and feedback reports on e.g. correctness of the conditions
   of receipt might be included to monitor this.  Furthermore, the
   source behavior in response to such reports should be correct in
   terms of not promoting priority when the report indicates bad
   conditions.  One of the benefits of the PPS is the allocation of
   resource-management functions to the end systems, since this reduces
   the burden on the network.  If we implement functions of the kind
   just described to monitor the correctness of the behavior of end-
   systems, however, we place another burden on the network.  There is a
   tradeoff between the extent to which we should protect the network
   and the costs of doing so.

   The site of monitoring is another issue we face in designing the
   network.  One solution is to install checking mechanisms of the kind
   described above in every edge router and have them monitor every
   session.  This is perfect in terms of protecting the network from all
   kinds of incorrect behavior, but would cost too much.

   Another practical solution is to introduce two-stage monitoring of
   end-system behavior.  The intention here is to classify items for
   monitoring as either primary or secondary and having them checked at
   the appropriate places.  Primary monitoring may be implemented at the
   edge routers and is triggered by session initiation.  Secondary
   monitoring might be done by a dedicated media-monitoring server.  The
   primary monitor checks every PPS-controlled media stream it handles.
   Examples of items to check include whether the flow has been given
   permission to enter the network, whether the flow rate is no greater


Morita                   Expires - April 2004                 [Page 8]


                                 PPS                     October 2003


   than the probed bit rate, and the correctness of the usage of the
   DSCPs.  The secondary monitor checks the details of end-system
   behavior.  Whether or not the two monitoring stages are really used
   will depend on the specific network environment, but both should be
   available to allow flexibility in implementation.

4.5 Accommodation of variable-bit-rate sources

   Any measurement-based form of admission control is more suitable with
   constant bit rate (CBR) sources than with variable bit rate (VBR)
   sources.  CBR sources to which silence suppression is not applied are
   often used in public voice communications in Japan.  For interactive
   multimedia, on the other hand, it is important that we take VBR into
   account.

   Another approach is possible, relying on declared traffic parameters
   and deterministic capacity allocation rather than results of
   measurement.  The admission control system gets the declared
   parameters, estimates the equivalent bandwidth, and then judges
   whether or not admission is possible.  The drawbacks here are the
   difficulty of deriving truly representative parameters for each of
   the many popular codecs and of estimating the total required
   bandwidth when a new flow is offered.

   VBR has quite different implications for a measurement-based approach
   such as PPS.  PPS requires no parameters, no estimation, and no
   calculation.  In addition, utilization of bandwidth is ideal because
   measurement is of actual traffic.  There is, however, a trade off.
   The PPS depends on the usage of resources at the time of measurement.
   Measurement for a particular session may occur when the flows already
   present are at relatively low rates.  The new session may then suffer
   loss of QoS when the volume of flows returns to typical levels.

   The tuning of the PPS to support VBR sources thus has to reflect
   statistical variation, which can be done by probing over a longer
   time or by sending the probing packets at a higher rate than the non-
   probing packets.  A new (elastic) mode of PHB provides a way of
   avoiding such mechanisms and is introduced in the definition of the
   MF-PHB[5].

   Investigations with VBR sources including ON/OFF source have already
   been done by Prof. Karlsson as is indicated by the Appendix of the
   document.

5. Service models provided by the PPS

   The Priority Promotion Scheme can be viewed as a kind of admission
   control.  However, it is not limited to the kind of
   connection/session admission control we imagine if we think of the


Morita                   Expires - April 2004                 [Page 9]


                                 PPS                     October 2003


   legacy telephone network.  The probing can even be handled by the
   media packets themselves.  In this section, we examine the possible
   service models provided by the PPS.

5.1 Admission control

   Admission control alone is suitable for conventional service models
   such as legacy switched services.  The measurement is simply used for
   admission control when the session is established.  If the trial
   fails, the session is not established.  The user may retry, but the
   terminal behavior does not specify the extent to which this is
   possible.  PPS is quite effective in this role as long as the
   duration of probing is less than a couple of seconds.

5.2 Quality improvement

   The case of PPS where the media packets are used for probing is
   particularly applicable to quality improvement.  The source starts by
   sending media packets at probe level.  If the conditions of receipt
   are poor, the source stops sending the media packets at probe level,
   and recommences sending them as packets of another class.  After a
   while, the source returns to probing; if this succeeds, the packets
   are sent as packets of the higher (non-probing) MF-PHB class.

5.3 Available bit rate

   In the available-bit-rate service model, the transmitter uses the
   information on network conditions received in response to probing to
   estimate the actual available bandwidth, selects the closest
   bandwidth lower than the available bandwidth, and then sends the
   media at the higher MF-PHB priority level.  The transmission may be
   made to fit the available bit rate by sending the video data with
   less size or resolution than was originally desired or sending speech
   data alone rather than a mix of video and speech.  The quality of the
   session is then maintained.

   A further possible application of this approach is to send media data
   at the full rate but only assign the higher MF-PHB priority to the
   core part of the flow, which fits the available bit rate; the other
   parts are sent but assigned to another class.  This approach should
   work well with hierarchical coding (in MPEG for example, I frames
   would be sent with high priority and P or B frames with low priority).

5.4 Bit-rate increase

   This is an extension to the available-bit-rate service model.  If
   initial probing indicated that the requested bit rate is not
   available, the source sends at the lower rate than requested but
   retries probing from time to time.  When the requested rate becomes


Morita                   Expires - April 2004                [Page 10]


                                 PPS                     October 2003


   available, the source starts sending media packets at the requested
   rate.

6. The feasibility of probe-based admission control

   Karlsson has already investigated the characteristics of probe-based
   admission control (PBAC).  Although the overall system architecture
   of PBAC is slightly different from the PPS, the basic dynamics are
   the same and the analysis of PBAC is applicable to the PPS.  A
   summary of the analysis is thus given in the Appendix of this
   document.

7. 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 source and destination, the source-side edge router, the core
   routers, the SIP proxy, and the media-monitoring server.

                      SIP proxy      (Media-monitoring server)
                       |------|            |------|
             /---------|      |------------|      |
            /          |------|            |------|
           /              |               //
          /               |              //
     |------|         |------|      |------|      |------|      |------|
     |      |=========| Edge |======| Core |======| Edge |======|      |
     |------|         |------|      |------|      |------|      |------|
   End system                                                End system
   (Source)                                                (Destination)

   Figure 1. Functional architecture of the Priority Promotion Scheme

8. Requirements of the Priority Promotion Scheme

   In this section, we describe the requirements for the various
   functional entities.

8.1 Routers

   Although the end systems play an important role in the Priority
   Promotion Scheme, the scheme places a few other requirements on the
   network.  Specifically, the queuing mechanism or PHB (per-hop
   behavior) for the PPS creates new requirements for network elements.
   The Priority Promotion Scheme is intended to work with the existing
   Diffserv PHBs, as was indicated in the introduction.  However, to
   clearly explain how the scheme would be implemented in this context,
   we have to define a new PHB.  We refer to this as measurable
   forwarding (MF).  The essential requirements for MF are as follows.


Morita                   Expires - April 2004                [Page 11]


                                 PPS                     October 2003



   - 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, we have a total amount of MF-H and MF-M traffic as a
   limit rather than separate limits for the two sub-classes.  However,
   since MF-M traffic will always defer to MF-H traffic, MF-M traffic
   may experience markedly higher levels of jitter and loss than MF-H,
   while one would expect MF-H traffic to experience very low levels of
   jitter and loss.

   Another view of MF is that, if a given amount of MF-M traffic for a
   particular stream passes through a router, at least the same amount
   of MF-H traffic for that stream must also be able to pass through.
   In the absence of other DiffServ classes, configuring existing
   commercially available routers to implement the MF-PHB should be
   feasible.  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.

   The more detailed definition of MF-PHB and scenarios for its
   verification are available in [5][6].

8.2 End systems

   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
   results of reception.  Some improvements might be necessary to reduce
   the measurement period and to make quick decisions.  Actually, the
   minimum measurement period is the key factor that determines the
   usability of the Priority Promotion Scheme.  This determines whether
   or not the scheme is applicable to admission control, as was
   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 the conditions are not good, the transmitter gives up sending
   monitor packets and either 1) sends the remaining packets with


Morita                   Expires - April 2004                [Page 12]


                                 PPS                     October 2003


   another class such as BE, 2) stops sending any media data and, after
   a while, starts sending monitoring packets again, or 3) terminates
   the session.

   According to the service models described in section 5, further
   actions are necessary.

   Synchronization between the two directions of the media stream
   remains a subject for further study.

8.3 SIP proxies

   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 is
   capable of monitoring user streams, an SIP proxy can send commands to
   an edge router, requesting that it check on a particular end system's
   behavior.

   The specific signaling sequence may depend on the selected service
   model.

   If the policy is applied as was described in section 5, signaling is
   where the policy credentials are exchanged.

8.4 Edge routers

   As noted above, in some networks an SIP server might be available and
   is able to instruct edge routers to monitor the behavior of end
   systems.  An edge router might monitor the following items.

   - Packet-transmission rates: the transmitter should not send packets
   at rates above the peak bit rate offered in the monitoring phase.
   - Continuous sending of packets: if the transmitter pauses in the
   sending of packets, the other end systems overestimate the remaining
   network resources and incorrectly send higher-priority packets.
   Transmitters should thus not pause during sending.

8.5 Media monitoring servers

   In addition to primary monitoring by the edge routers, more detailed
   monitoring may be required.  The typical items to be monitored are as
   follows:
   - the accuracy of packet-reception information from receivers, and
   the correctness of reactions of transmitters to this information; and
   - if the received information indicates poor conditions, the
   transmitter stops sending high-priority packets; if a next trial is



Morita                   Expires - April 2004                [Page 13]


                                 PPS                     October 2003


   allowed, a certain time interval should be maintained between the
   initial trial and the next trial.

9. Security Considerations

   To be described.

10. IANA Considerations

   To be described.

Acknowledgements

   The authors would like to thank Fred Baker, David Oran, Glenn Reitsma
   and other technical experts at Cisco for some insightful suggestions.

Authors' Addresses

        Naotaka Morita
        Network Service Systems Laboratories
        NTT Corporation
        9-11, Midori-Cho 3-Chome,
        Musashino-Shi, Tokyo
        150-8585 Japan
        E-mail: morita.naotaka@lab.ntt.co.jp

        Gunnar KARLSSON
        KTH, Royal Institute of Technology
        Department of Microelectronics & Information Technology
        Laboratory of Communication Networks
        Isafjordsgatan 39
        P.O.Box Electrum 229
        SE-164 40 Kista, Sweden
        E-mail: gk@imit.kth.se

References


   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9,
      RFC 2026, October 1996.

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

   3  Karlsson, K., "Providing Quality for Internet Video Services," in
      Proc. of the CNIT/IEEE 10th International Tyrrhenian Workshop on
      Digital Communications, Ischia, Italy, September 15-18, 1998.




Morita                   Expires - April 2004                [Page 14]


                                 PPS                     October 2003



   4  Fodor, (nee Elek), V., Karlsson, G., and Roenngren, R., "Admission
      Control Based on End-to-End Measurements,"  in Proc. IEEE INFOCOM,
      Tel-Aviv, Israel, March 26-30, 2000.

   5  Morita, N., " Measurable Forwarding: A New per-Hop Behavior
      (PHB) ," Internet draft, October 2003.

   6  Morita, N., " Verification scenarios for Measurable Forwarding PHB
      (Per-Hop Behavior)," Internet draft, October 2003.









































Morita                   Expires - April 2004                [Page 15]


                                 PPS                     October 2003



Appendix:  Probe-Based Admission Control (PBAC) - Current experimental
results and obervations

   1. System definitions

   . Complete semantic definition of the probe-based admission control
     [A1, A2].

   . Multicast application of PBAC [A3]. The quality of service scheme
     for multicast traffic is based on admission control for both
     senders and receivers. The admission control is well suited to
     multicast sessions with a single multimedia stream or with several
     layered streams.

   . Simple security model to verify the end host identities and secure
     the probe phase and the admission decision [A4]. The scheme
     verifies the end user's identities and secures the transmission
     during the probing phase.

   2. Analytical models

   . Approximate mathematical model that relates probe and data packet
     loss rate, queue buffer sizes and achieved link utilization for
     the double queue system [A5]. The analysis is based on the
     following steps: First, computation of the probability of a single
     probe packet being successfully transmitted; second, computation
     of the acceptance probability as a binomial distribution; third,
     computation of the link utilization as a birth--death Markov
     chain; and fourth, computation of the data packet loss for a
     particular source type and the probe/data loss relationship.

   . Numerical results with figures for probe packet loss probability,
     acceptance probability as a function of the load on the system,
     link utilization and data packet loss probabilities. The results
     agree with the simulations and prove that the considered probe--
     based admission control leads to a stable link utilization and has
     a clear upper bound on the packet loss probability.

   3. Performance evaluation

   All the performance figures have been obtained with the NS-2
   simulator. Different source types and source rates have been used:
   sources with exponential and Pareto on--off holding times and traces
   of real MPEG-2 encoded videos, with peak rates from 64 kb/s to 10
   Mb/s. The sources are listed in Table 1. The following issues have
   been investigated:




Morita                   Expires - April 2004                [Page 16]


                                 PPS                     October 2003


   . Performance and comparison of the proposed queuing schemes for the
     controlled load service, a double queue system with two priorities
     and a single queue system with a discard threshold for probe
     packets [A2]. Both queue systems can be used with a proper buffer
     and threshold dimensioning.

   . The validity of the assumption of a normal distribution of the
     probe packet loss for the admission decision [A2]. Histograms of
     the probe packet loss prove the assumption valid.

   . Stress test with short sessions and sessions that keep silent for
     long periods of time [A2]. None of this special sessions have a
     serious effect unless they represent a substantial percentage of
     the link capacity (over 15 %). The performance of the system under
     heavy stress (many simultaneous probes or sessions that keep
     silent for periods of time longer than some probe lengths) is
     stable. In general, as the situation worsens, the admission
     control is conservative, allowing less ongoing sessions, but never
     failing to keep the data packet loss under the threshold for
     maximum session peak rates of less than 5% of the link capacity.

   . Relationship between probe packet loss and session data loss for
     different source types and peak rates [A1, A2]. Basically all
     source types show between half to one order of magnitude
     difference. All the figures show that there is a nearly linear
     relationship between the probe and the data packet loss.

   . Effect of multiple links scenarios with cross traffic [A1]. The
     simulations prove that the bottleneck link dominates the behavior.

   . Blocking and data packet loss probabilities and their relation to
     the probe length and the location of a multicast receiver [A3].
     The simulations prove that receivers in different branches of the
     multicast tree have different blocking probabilities, depending on
     the link loads on the different multicast branches.

   . Performance evaluation of an implementation of the security model
     proposed in [A4] with commodity hardware, focusing in the trade
     off between security level and setup delay. The simple solution
     does not require any change in the network nodes, just a
     cryptographic interface in the access gateways and the end nodes.

   Table 1: Parameters of the different test sources

   Source           On Time        Off Time         Peak Rate
   Exponential      20 and 325ms   35.5 and 650ms   64kb/s to 10Mb/s
   Pareto (fi=1.5)  20 and 325ms   35.5 and 650ms   64kb/s to 10Mb/s
   Mixed            20 and 325ms   35.5 and 650ms   64kb/s to 10Mb/s



Morita                   Expires - April 2004                [Page 17]


                                 PPS                     October 2003


   Video Traces                                     360kb/s
                                                      (64kb/s average)

   4. On-going work

   . Software implementation of PBAC for Linux. A library to provide
     the probing features is being developed, which will enable
     software generators or end applications to perform the probing
     before transmitting. The queuing system will be implemented using
     the QoS capabilities of the Linux kernel (iproute2 (1)).

   . A possible policing and metering tool for PBAC is under
     investigation using Netramet (2).

   References
   [A1] Viktoria Elek, G. Karlsson, and R. Roenngren, "Admission control
   based on end-to-end measurements," in Proc. of the 19th Infocom, (Tel
   Aviv, Israel), pp. 623--630, IEEE, March 2000.

   [A2] I. Mas Ivars and G. Karlsson, "PBAC: Probe--based admission
   control," in Proc. of QofIS 2001, vol. 2156 of LNCS, (Coimbra,
   Portugal), pp. 97--109, Springer, September 2001.

   [A3] I. Mas Ivars, V. Fodor, and G. Karlsson, "Probe--based admission
   control for multicast," in Proc. of the 10th IWQoS, (Miami Beach,
   Florida), pp. 99--105, IEEE, May 2002.

   [A4] M. Conte, I. Mas Ivars, V. Fodor, and G. Karlsson, "Policy
   enforcing for  probe--based admission control," in Proc. of NTS 16,
   (Espoo, Finland), pp. 45--55, Helsinki University of Technology,
   August 2002.

   [A5] I. Mas Ivars, V. Fodor, and G. Karlsson, "The performance of
   endpoint admission control based on packet loss," in Proc. of QofIS
   2003, vol. 2856 of LNCS, (Stockholm, Sweden), Springer, October 2003.


   (1) ftp://ftp.inr.ac.ru/ip-routing/
   (2) http://www.auckland.ac.nz/net/NeTraMet/












Morita                   Expires - April 2004                [Page 18]