rfc1633.txt

<VC++网络游戏建摸与实现>源代码

      are not considered part of the flow from the perspective of
      admission control, since there is no commitment that they will be
      delivered.

   3.4 Usage Feedback

      Another important issue in the service is the model for usage
      feedback, also known as "accounting", to prevent abuse of network
      resources.   The link-sharing service described earlier can be
      used to provide administratively-imposed limits on usage.
      However, a more free-market model of network access will require
      back-pressure on users for the network resources they reserve.
      This is a highly contentious issue, and we are not prepared to say
      more about it at this time.

   3.5 Reservation Model

      The "reservation model" describes how an application negotiates
      for a QoS level.  The simplest model is that the application asks
      for a particular QoS and the network either grants it or refuses.
      Often the situation will be more complex.  Many applications will
      be able to get acceptable service from a range of QoS levels, or
      more generally, from anywhere within some region of the multi-
      dimensional space of a flowspec.

      For example, rather than simply refusing the request, the network
      might grant a lower resource level and inform the application of
      what QoS has been actually granted.  A more complex example is the
      "two-pass" reservation model, In this scheme, an "offered"
      flowspec is propagated along the multicast distribution tree from
      each sender Si to all receivers Rj.  Each router along the path
      records these values and perhaps adjusts them to reflect available
      capacity.  The receivers get these offers, generate corresponding
      "requested" flowspecs, and propagate them back along the same
      routes to the senders.  At each node, a local reconciliation must
      be performed between the offered and the requested flowspec to
      create a reservation, and an appropriately modified requested
      flowspec is passed on.  This two-pass scheme allows extensive
      properties like allowed delay to be distributed across hops in the
      path [Tenet90, ST2-90].  Further work is needed to define the
      amount of generality, with a corresponding level of complexity,
      that is required in the reservation model.









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RFC 1633            Integrated Services Architecture           June 1994


4. Traffic Control Mechanisms

   We first survey very briefly the possible traffic control mechanisms.
   Then in Section 4.2 we apply a subset of these mechanisms to support
   the various services that we have proposed.

   4.1 Basic Functions

      In the packet forwarding path, there is actually a very limited
      set of actions that a router can take.  Given a particular packet,
      a router must select a route for it; in addition the router can
      either forward it or drop it, and the router may reorder it with
      respect to other packets waiting to depart.  The router can also
      hold the packet, even though the link is idle.  These are the
      building blocks from which we must fashion the desired behavior.

      4.1.1 Packet Scheduling

         The basic function of packet scheduling is to reorder the
         output queue.  There are many papers that have been written on
         possible ways to manage the output queue, and the resulting
         behavior.  Perhaps the simplest approach is a priority scheme,
         in which packets are ordered by priority, and highest priority
         packets always leave first.  This has the effect of giving some
         packets absolute preference over others; if there are enough of
         the higher priority packets, the lower priority class can be
         completely prevented from being sent.

         An alternative scheduling scheme is round-robin or some
         variant, which gives different classes of packets access to a
         share of the link. A variant called Weighted Fair Queueing, or
         WFQ, has been demonstrated to allocate the total bandwidth of a
         link into specified shares.

         There are more complex schemes for queue management, most of
         which involve observing the service objectives of individual
         packets, such as delivery deadline, and ordering packets based
         on these criteria.

      4.1.2 Packet Dropping

         The controlled dropping of packets is as important as their
         scheduling.

         Most obviously, a router must drop packets when its buffers are
         all full.  This fact, however, does not determine which packet
         should be dropped.  Dropping the arriving packet, while simple,
         may cause undesired behavior.



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RFC 1633            Integrated Services Architecture           June 1994


         In the context of today's Internet, with TCP operating over
         best effort IP service, dropping a packet is taken by TCP as a
         signal of congestion and causes it to reduce its load on the
         network.  Thus, picking a packet to drop is the same as picking
         a source to throttle.  Without going into any particular
         algorithm, this simple relation suggests that some specific
         dropping controls should be implemented in routers to improve
         congestion control.

         In the context of real-time services, dropping more directly
         relates to achieving the desired quality of service.  If a
         queue builds up, dropping one packet reduces the delay of all
         the packets behind it in the queue.  The loss of one can
         contribute to the success of many.  The problem for the
         implementor is to determine when the service objective (the
         delay bound) is in danger of being violated.  One cannot look
         at queue length as an indication of how long packets have sat
         in a queue.  If there is a priority scheme in place, packets of
         lower priority can be pre-empted indefinitely, so even a short
         queue may have very old packets in it.  While actual time
         stamps could be used to measure holding time, the complexity
         may be unacceptable.

         Some simple dropping schemes, such as combining all the buffers
         in a single global pool, and dropping the arriving packet if
         the pool is full, can defeat the service objective of a WFQ
         scheduling scheme.  Thus, dropping and scheduling must be
         coordinated.

      4.1.3 Packet Classification

         The above discussion of scheduling and dropping presumed that
         the packet had been classified into some flow or sequence of
         packets that should be treated in a specified way.  A
         preliminary to this sort of processing is the classification
         itself.  Today a router looks at the destination address and
         selects a route.  The destination address is not sufficient to
         select the class of service a packet must receive; more
         information is needed.

         One approach would be to abandon the IP datagram model for a
         virtual circuit model, in which a circuit is set up with
         specific service attributes, and the packet carries a circuit
         identifier.  This is the approach of ATM as well as protocols
         such as ST-II [ST2-90].  Another model, less hostile to IP, is
         to allow the classifier to look at more fields in the packet,
         such as the source address, the protocol number and the port
         fields.  Thus, video streams might be recognized by a



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RFC 1633            Integrated Services Architecture           June 1994


         particular well-known port field in the UDP header, or a
         particular flow might be recognized by looking at both the
         source and destination port numbers.  It would be possible to
         look even deeper into the packets, for example testing a field
         in the application layer to select a subset of a
         hierarchically-encoded video stream.

         The classifier implementation issues are complexity and
         processing overhead.  Current experience suggests that careful
         implementation of efficient algorithms can lead to efficient
         classification of IP packets.  This result is very important,
         since it allows us to add QoS support to existing applications,
         such as Telnet, which are based on existing IP headers.

         One approach to reducing the overhead of classification would
         be to provide a "flow-id" field in the Internet-layer packet
         header.  This flow-id would be a handle that could be cached
         and used to short-cut classification of the packet.  There are
         a number of variations of this concept, and engineering is
         required to choose the best design.

      4.1.4 Admission Control

         As we stated in the introduction, real-time service depends on
         setting up state in the router and making commitments to
         certain classes of packets.  In order to insure that these
         commitments can be met, it is necessary that resources be
         explicitly requested, so that the request can be refused if the
         resources are not available.  The decision about resource
         availability is called admission control.

         Admission control requires that the router understand the
         demands that are currently being made on its assets.  The
         approach traditionally proposed is to remember the service
         parameters of past requests, and make a computation based on
         the worst-case bounds on each service.  A recent proposal,
         which is likely to provide better link utilization, is to
         program the router to measure the actual usage by existing
         packet flows, and to use this measured information as a basis
         of admitting new flows [JCSZ92]. This approach is subject to
         higher risk of overload, but may prove much more effective in
         using bandwidth.

         Note that while the need for admission control is part of the
         global service model, the details of the algorithm run in each
         router is a local matter.  Thus, vendors can compete by
         developing and marketing better admission control algorithms,
         which lead to higher link loadings with fewer service



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RFC 1633            Integrated Services Architecture           June 1994


         overloads.

   4.2 Applying the Mechanisms

      The various tools described above can be combined to support the
      services which were discussed in section 3.

      o    Guaranteed delay bounds

           A theoretical result by Parekh [Parekh92] shows that if the
           router implements a WFQ scheduling discipline, and if the
           nature of the traffic source can be characterized (e.g. if it
           fits within some bound such as a token bucket) then there
           will be an absolute upper bound on the network delay of the
           traffic in question.  This simple and very powerful result
           applies not just to one switch, but to general networks of
           routers.  The result is a constructive one; that is, Parekh
           displays a source behavior which leads to the bound, and then
           shows that this behavior is the worst possible.  This means
           that the bound he computes is the best there can be, under
           these assumptions.

      o    Link sharing

           The same WFQ scheme can provide controlled link sharing.  The
           service objective here is not to bound delay, but to limit
           overload sh