rfc2386.txt

<VC++网络游戏建摸与实现>源代码

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RFC 2386           A Framework for QoS-based Routing         August 1998


    Domain A                    Domain B           Domain C
     ____________          ___________      ____________
    |            |        |           |    |            |
    |            B1------B2          B3---B4            |
    |            |        |           |    |            |
     ------------          -----------      ------------

   A problem with charging for a flow is the determination of the cost
   when the QoS promised for the flow was not actually delivered.
   Clearly, when a flow is routed via multiple domains, it must be
   determined whether each domain delivers the QoS it declares possible
   for traffic through it.

6. QOS-BASED MULTICAST ROUTING

   The goals of QoS-based multicast routing are as follows:

   - Scalability to large groups with dynamic membership

   - Robustness in the presence of topological changes

   - Support for receiver-initiated, heterogeneous reservations

   - Support for shared reservation styles, and

   - Support for "global" admission control, i.e., administrative
     control of resource consumption by the multicast flow.

   The RSVP multicast flow model is as follows. The sender of a
   multicast flow advertises the traffic characteristics periodically to
   the receivers.  On receipt of an advertisement, a receiver may
   generate a message to reserve resources along the flow path from the
   sender. Receiver reservations may be heterogeneous. Other multicast
   models may be considered.

   The multicast routing scheme attempts to determine a path from the
   sender to each receiver that can accommodate the requested
   reservation.  The routing scheme may attempt to maximize network
   resource utilization by minimizing the total bandwidth allocated to
   the multicast flow, or by optimizing some other measure.

6.1   Scalability, Robustness and Heterogeneity

   When addressing scalability, two aspects must be considered:

     1.  The overheads associated with receiver discovery. This overhead
         is incurred when determining the multicast tree for forwarding
         best-effort sender traffic characterization to receivers.



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     2.  The overheads associated with QoS-based multicast path
         computation.  This overhead is incurred when flow-specific
         state information has to be collected by a router to determine
         QoS-accommodating paths to a receiver.

   Depending on the multicast routing scheme, one or both of these
   aspects become important. For instance, under the present RSVP model,
   reservations are established on the same path over which sender
   traffic characterizations are sent, and hence there is no path
   computation overhead. On the other hand, under the proposed QOSPF
   model [ZSSC97] of multicast source routing, receiver discovery
   overheads are incurred by MOSPF [M94] receiver location broadcasts,
   and additional path computation overheads are incurred due to the
   need to keep track of existing flow paths. Scaling of QoS-based
   multicast depends on both these scaling issues. However, scalable
   best-effort multicasting is really not in the domain of QoS-based
   routing work (solutions for this are being devised by the IDMR WG
   [BCF94, DEFV94]). QoS-based multicast routing may build on these
   solutions to achieve overall scalability.

   There are several options for QoS-based multicast routing. Multicast
   source routing is one under which multicast trees are computed by the
   first-hop router from the source, based on sender traffic
   advertisements.  The advantage of this is that it blends nicely with
   the present RSVP signaling model. Also, this scheme works well when
   receiver reservations are homogeneous and the same as the maximum
   reservation derived from sender advertisement.  The disadvantages of
   this scheme are the extra effort needed to accommodate heterogeneous
   reservations and the difficulties in optimizing resource allocation
   based on shared reservations.

   In these regards, a receiver-oriented multicast routing model seems
   to have some advantage over multicast source routing. Under this
   model:

     1.  Sender traffic advertisements are multicast over a best-effort
         tree which can be different from the QoS-accommodating tree for
         sender data.

     2.  Receiver discovery overheads are minimized by utilizing a
         scalable scheme (e.g., PIM, CBT), to multicast sender traffic
         characterization.

     3.  Each receiver-side router independently computes a QoS-
         accommodating path from the source, based on the receiver
         reservation. This path can be computed based on unicast routing
         information only, or with additional multicast flow-specific
         state information. In any case, multicast path computation is



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RFC 2386           A Framework for QoS-based Routing         August 1998


         broken up into multiple, concurrent nunicast path computations.

     4.  Routers processing unicast reserve messages from receivers
         aggregate resource reservations from multiple receivers.

   Flow-specific state information may be limited in Step 3 to achieve
   scalability [RN98]. In general, limiting flow-specific information in
   making multicast routing decisions is important in any routing model.
   The advantages of this model are the ease with which heterogeneous
   reservations can be accommodated, and the ability to handle shared
   reservations. The disadvantages are the incompatibility with the
   present RSVP signaling model, and the need to rely on reverse paths
   when link state routing is not used. Both multicast source routing
   and the receiver-oriented routing model described above utilize per-
   source trees to route multicast flows. Another possibility is the
   utilization of shared, per-group trees for routing flows. The
   computation and usage of such trees require further work.

   Finally, scalability at the interdomain level may be achieved if
   QoS-based multicast paths are computed independently in each domain.
   This principle is illustrated by the QOSPF multicast source routing
   scheme which allows independent path computation in different OSPF
   areas. It is easy to incorporate this idea in the receiver-oriented
   model also. An evaluation of multicast routing strategies must take
   into account the relative advantages and disadvantages of various
   approaches, in terms of scalability features and functionality
   supported.

6.2    Multicast Admission Control

   Higher level admission control, as defined for unicast, prevents
   excessive resource consumption by flows when traffic load is high.
   Such an admission control strategy must be applied to multicast flows
   when the flow path computation is receiver-oriented or sender-
   oriented. In essence, a router computing a path for a receiver must
   determine whether the incremental resource allocation for the
   receiver is excessive under some administratively determined
   admission control policy. Other admission control criteria, based on
   the total resource consumption of a tree may be defined.

7.    QOS-BASED ROUTING AND RESOURCE RESERVATION PROTOCOLS

   There must clearly be a well-defined interface between routing and
   resource reservation protocols. The nature of this interface, and the
   interaction between routing and resource reservation has to be
   determined carefully to avoid incompatibilities. The importance of
   this can be readily illustrated in the case of RSVP.




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   RSVP has been designed to operate independent of the underlying
   routing scheme. Under this model, RSVP PATH messages establish the
   reverse path for RESV messages.  In essence, this model is not
   compatible with QoS-based routing schemes that compute paths after
   receiver reservations are received. While this incompatibility can be
   resolved in a simple manner for unicast flows, multicast with
   heterogeneous receiver requirements is a more difficult case.  For
   this, reconciliation between RSVP and QoS-based routing models is
   necessary. Such a reconciliation, however, may require some changes
   to the RSVP model depending on the QoS-based routing model [ZES97,
   ZSSC97, GOA97]. On the other hand, QoS-based routing schemes may be
   designed with RSVP compatibility as a necessary goal. How this
   affects scalability and other performance measures must be
   considered.

8. SECURITY CONSIDERATIONS

   Security issues that arise with routing in general are about
   maintaining the integrity of the routing protocol in the presence of
   unintentional or malicious introduction of information that may lead
   to protocol failure [P88]. QoS-based routing requires additional
   security measures both to validate QoS requests for flows and to
   prevent resource-depletion type of threats that can arise when flows
   are allowed to make arbitratry resource requests along various paths
   in the network. Excessive resource consumption by an errant flow
   results in denial of resources to legitimate flows. While these
   situations may be prevented by setting up proper policy constraints,
   charging models and policing at various points in the network, the
   formalization of such protection requires work [BCCH94].

9. RELATED WORK

   "Adaptive" routing, based on network state, has a long history,
   especially in circuit-switched networks. Such routing has also been
   implemented in early datagram and virtual circuit packet networks.
   More recently, this type of routing has been the subject of study in
   the context of ATM networks, where the traffic characteristics and
   topology are substantially different from those of circuit-switched
   networks [MMR96]. It is instructive to review the adaptive routing
   methodologies, both to understand the problems encountered and
   possible solutions.

   Fundamentally, there are two aspects to adaptive, network state-
   dependent routing:

     1.  Measuring and gathering network state information, and
     2.  Computing routes based on the available information.




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RFC 2386           A Framework for QoS-based Routing         August 1998


   Depending on how these two steps are implemented, a variety of
   routing techniques are possible. These differ in the following
   respects:

   -  what state information is used
   -  whether local or global state is used
   -  what triggers the propagation of state information
   -  whether routes are computed in a distributed or centralized manner
   -  whether routes are computed on-demand, pre-computed, or in a
      hybrid manner
   -  what optimization criteria, if any, are used in computing routes
   -  whether source routing or hop by hop routing is used, and
   -  how alternate route choices are explored

   It should be noted that most of the adaptive routing work has focused
   on unicast routing. Multicast routing is one of the areas that would
   be prominent with Internet QoS-based routing. We treat this
   separately, and the following review considers only unicast routing.
   This review is not exhaustive, but gives a brief overview of some of
   the approaches.

9.1 Optimization Criteria

   The most common optimization criteria used in adaptive routing is
   throughput maximization or delay minimization. A general formulation
   of the optimization problem is the one in which the network revenue
   is maximized, given that there is a cost associated with routing a
   flow over a given path [MMR96, K88]. In general, global optimization
   solutions are difficult to implement, and they rely on a number of
   assumptions on the characteristics of the traffic being routed
   [MMR96]. Thus, the practical approach has been to treat the routing
   of each flow (VC, circuit or packet stream to a given destination)
   independently of the routing of other flows. Many such routing
   schemes have been implemented.

9.2  Circuit Switched Networks

   Many adaptive routing concepts have been proposed for circuit-
   switched networks. An example of a simple adaptive routing scheme is
   sequential alternate routing [T88]. This is a hop-by-hop
   destination-based routing scheme where only local state information
   is utilized.  Under this scheme, a routing table is computed for each
   node, which lists multiple output link choices for each destination.
   When a call set-up request is received by a node, it tries each
   output link choice in sequence, until it finds one that can
   accommodate the call. Resources are reserved on this link, and the
   call set-up is forwarded to the next node. The set-up either reaches
   the destination, or is b