rfc2997.txt

<VC++网络游戏建摸与实现>源代码

Network Working Group                                            Y. Bernet
Request for Comments: 2997                                       Microsoft
Category: Standards Track                                         A. Smith
                                                          Allegro Networks
                                                                  B. Davie
                                                             Cisco Systems
                                                             November 2000


                 Specification of the Null Service Type

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   In the typical Resource Reservation Protocol (RSVP)/Intserv model,
   applications request a specific Intserv service type and quantify the
   resources required for that service.  For certain applications, the
   determination of service parameters is best left to the discretion of
   the network administrator.  For example, ERP applications are often
   mission critical and require some form of prioritized service, but
   cannot readily specify their resource requirements.  To serve such
   applications, we introduce the notion of the 'Null Service'.  The
   Null Service allows applications to identify themselves to network
   Quality of Service (QoS) policy agents, using RSVP signaling.
   However, it does not require them to specify resource requirements.
   QoS policy agents in the network respond by applying QoS policies
   appropriate for the application (as determined by the network
   administrator).  This mode of RSVP usage is particularly applicable
   to networks that combine differentiated service (diffserv) QoS
   mechanisms with RSVP signaling [intdiff].  In this environment, QoS
   policy agents may direct the signaled application's traffic to a
   particular diffserv class of service.








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1. Motivation

   Using standard RSVP/Intserv signaling, applications running on hosts
   issue requests for network resources by communicating the following
   information to network devices:

   1. A requested service level (Guaranteed or Controlled Load).
   2. The quantity of resources required at that service level.
   3. Classification information by which the network can recognize
      specific traffic (filterspec).
   4. Policy/identity information indicating the user and/or the
      application for which resources are required.

   In response, standard RSVP aware network nodes choose to admit or
   deny a resource request.  The decision is based on the availability
   of resources along the relevant path and on policies.  Policies
   define the resources that may be granted to specific users and/or
   applications.  When a resource request is admitted, network nodes
   install classifiers that are used to recognize the admitted traffic
   and policers that are used to assure that the traffic remains within
   the limits of the resources requested.

   The Guaranteed and Controlled Load Intserv services are not suitable
   for certain applications that are unable to (or choose not to)specify
   the resources they require from the network.  Diffserv services are
   better suited for this type of application.  Nodes in a diffserv
   network are typically provisioned to classify arriving packets to
   some small number of behavior aggregates (BAs) [diffarch].  Traffic
   is handled on a per-BA basis.  This provisioning tends to be 'top-
   down' with respect to end-user traffic flows in the sense that there
   is no signaling between hosts and the network.  Instead, the network
   administrator uses a combination of heuristics, measurement and
   experience to provision the network devices to handle aggregated
   traffic, with no deterministic knowledge of the volume of traffic
   that will arrive at any specific node.

   In applying diffserv mechanisms to manage application traffic,
   network administrators are faced with two challenges:

   1. Provisioning - network administrators need to anticipate the
      volume of traffic likely to arrive at each network node for each
      diffserv BA.  If the volume of traffic arriving is likely to
      exceed the capacity available for the BA claimed, the network
      administrator has the choice of increasing the capacity for the
      BA, reducing the volume of traffic claiming the BA, or
      compromising service to all traffic arriving for the BA.





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   2. Classification - diffserv nodes classify traffic to user and/or
      application, based on the diff-serv codepoint (DSCP) in each
      packet's IP header or based on other fields in the packet's IP
      header (source/destination address/port and protocol).  The latter
      method of classification is referred to as MF classification.
      This method of classification may be unreliable and imposes a
      management burden.

   By using RSVP signaling, the management of application traffic in
   diffserv networks can be significantly facilitated.  (Note that
   RSVP/diffserv interoperability has been discussed previously in the
   context of the Guaranteed and Controlled Load Intserv services.)
   This document focuses on RSVP/diffserv interoperability in the
   context of the Null Service.

2. Operational Overview

   In the proposed mechanism, the RSVP sender offers the new service
   type, 'Null Service Type' in the ADSPEC that is included with the
   PATH message.  A new Tspec corresponding to the new service type is
   added to the SENDER_TSPEC.  In addition, the RSVP sender will
   typically include with the PATH message policy objects identifying
   the user, application and sub application ID [identity, application].

   (Note that at this time, the new Tspec is defined only to carry the
   maximum packet size parameter (M), for the purpose of avoiding
   fragmentation.  No other parameters are defined.)

   Network nodes receiving these PATH messages interpret the service
   type to indicate that the application is requesting no specific
   service type or quantifiable resources.  Instead, network nodes
   manage the traffic flow based on the requesting user, the requesting
   application and the type of application sub-flow.

   This mechanism offers significant advantages over a pure diffserv
   network.  At the very least, it informs each network node which would
   be affected by the traffic flow (and which is interested in
   intercepting the signaling) of:

   1. The demand for resources in terms of number of flows of a
      particular type traversing the node.
   2. The binding between classification information and user,
      application and sub-application.








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   This information is particularly useful to policy enforcement points
   and policy decision points (PEPs and PDPs).  The network
   administrator can configure these elements of the policy management
   system to apply appropriate policy based on the identity of the user,
   the application and the specific sub application ID.

   PEPs and PDPs may be configured to return an RSVP RESV message to the
   sender.  The returned RESV message may include a DCLASS object
   [dclass].  The DCLASS object instructs the sender to mark packets on
   the corresponding flow with a specific DSCP (or set of DSCPs).  This
   mechanism allows PEP/PDPs to affect the volume of traffic arriving at
   a node for any given BA.  It enables the PEP/PDP to do so based on
   sophisticated policies.

3.1 Operational Notes

3.1.1 Scalability Issues

   In any network in which per-flow signaling is used, it is wise to
   consider scalability concerns.  The Null Service encourages signaling
   for a broader set of applications than that which would otherwise be
   signaled for.  However, RSVP signaling does not, in general, generate
   a significant amount of traffic relative to the actual data traffic
   associated with the session.  In addition, the Null Service does not
   encourage every application to signal.  It should be used by
   applications that are considered mission critical or needing QoS
   management by network administrators.

   Perhaps of more concern is the impact on processing resources at
   network nodes that process the signaling messages.  When considering
   this issue, it's important to point out that it is not necessary to
   process the signaling messages at each network node.  In fact, the
   combination of RSVP signaling with diff-serv networks may afford
   significant benefits even when the RSVP messages are processed only
   at certain key nodes (such as boundaries between network domains,
   first-hop routers, PEPs or any subset of these).  Individual nodes
   should be enabled or disabled for RSVP processing at the discretion
   of the network administrator.  See [intdiff] for a discussion of the
   impact of RSVP signaling on diff-serv networks.

   In any case, per-flow state is not necessarily required, even in
   nodes that apply per-flow processing.









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2.1.2 Policy Enforcement in Legacy Networks

   Network nodes that adhere to the RSVP spec should transparently pass
   signaling messages  for the Null Service.  As such, it is possible to
   introduce a small number of PEPs that are enabled for Null Service
   into a legacy network and to realize the benefits described in this
   document.

2.1.3 Combining Existing Intserv Services with the Null Service

   This document does not preclude applications from offering both a
   quantitative Intserv service (Guaranteed or Controlled Load)and the
   Null Service, at the same time.  An example of such an application
   would be a telephony application that benefits from the Guaranteed
   Service but is able to adapt to a less strict service.  By
   advertising its support for both, the application enables network
   policy to decide which service type to provide.

3. Signaling Details

3.1 ADSPEC Generation

   The RSVP sender constructs an initial RSVP ADSPEC object specifying
   the Null Service Type.  Since there are no service specific
   parameters associated with this service type, the associated ADSPEC
   fragment is empty and contains only the header word.  Network nodes
   may or may not supply valid values for bandwidth and latency general
   parameters.  As such, they may use the unknown values defined in
   [RFC2216].

   The ADSPEC is added to the RSVP PATH message created at the sender.

3.2 RSVP SENDER_TSPEC Object

   An additional Tspec is defined to correspond to the Null Service.  If
   only the Null Service is offered in the ADSPEC, then this is the only
   Tspec included in the SENDER_TSPEC object.  If guaranteed or
   controlled load services are also offered in the ADSPEC, then the new
   Tspec is appended following the standard Intserv token-bucket Tspec
   [RFC2210].

3.3 RSVP FLOWSPEC Object

   Receivers may respond to PATH messages by generating an RSVP RESV
   message including a FLOWSPEC object.  The FLOWSPEC object should
   specify that it is requesting the Null Service.  It is possible that,
   in the future, a specific Rspec may be defined to correspond to the
   new service type.



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4. Detailed Message Formats

4.1 Standard ADSPEC Format

   A standard RSVP ADSPEC object is described in [RFC2210].  It includes
   a message header and a default general parameters fragment.
   Following the default general parameters fragment are fragments for
   each supported service type.

4.2 ADSPEC for Null Service Type

   The Null Service ADSPEC includes the message header and the default
   general parameters fragment, followed by a single fragment denoting
   the Null Service.  The new fragment introduced for the Null Service
   is formatted as follows:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    6 (a)      |x| Reserved    |           0 (b)               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   a - indicates Null Service (6).
   x - is the break-bit.
   b - indicates zero words in addition to the header.




























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