rfc1077.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   may see the network as a message-passing system, or as memory.  At
   the same time, the network may use classic packets, wavelength
   division, or space division switching.

   A number of basic functions must be rethought to provide an
   architecture that is not dependent on the underlying switching model.
   For example, our transport protocols assume that data will be lost in
   units of a packet.  If part of a packet is lost, we discard the whole
   thing.  And if several packets are systematically lost in sequence,
   we may not recover effectively.  There must be a host-level unit of
   error recovery that is independent of the network.  This sort of
   abstraction must be applied to all the aspects of service
   specification: error recovery, flow control, addressing, and so on.


   3.1.6.  Network Operations, Monitoring, and Control


   There is a hierarchy of progressively more effective and
   sophisticated techniques for network management that applies
   regardless of network bandwidth and application considerations:

      1.  Reactive problem management

      2.  Reactive resource management

      3.  Proactive problem management

      4.  Proactive resource management.

   Today's network management strategies are primarily reactive rather
   than proactive:  Problem management is initiated in response to user
   complaints about service outages; resource allocation decisions are
   made when users complain about deterioration of quality of service.
   Today's network management systems are stuck at step 1 or perhaps
   step 2 of the hierarchy.

   Future network management systems will provide proactive problem
   management---problem diagnosis and restoral of service before users
   become aware that there was a problem; and proactive resource
   management---dynamic allocation of network bandwidth and switching
   resources to ensure that an acceptable level of service is
   continuously maintained.

   The GN management system should be expected to provide proactive
   problem and resource management capabilities.  It will have to do so
   while contending with three important changes in the managed network
   environment:



Gigabit Working Group                                          [Page 16]

RFC 1077                                                   November 1988


      1.  More complicated devices under management

      2.  More diverse types of devices

      3.  More variety of application protocols.

   Performance under these conditions will require that we seriously
   re-think how a network management system handles the expected high
   volumes of raw management-related data.  It will become especially
   important for the system to provide thresholding, filtering, and
   alerting mechanisms that can save the human operator from drowning in
   data, while still permitting access to details when diagnostic or
   fault isolation modes are invoked.

   The presence of expert assistant capabilities for early fault
   detection, diagnosis, and problem resolution will be mandatory.
   These capabilities are highly desirable today, but they will be
   essential to contend with the complexity and diversity of devices and
   applications in the Gigabit Network.

   In addition to its role in dealing with complexity, automation
   provides the only hope of controlling and reducing the high costs of
   daily management and operation of a GN.

   Proactive resource management in GNs must be better understood and
   practiced, initially as an effort requiring human intervention and
   direction.  Once this is achieved, it too must become automated to a
   high degree in the GN.


   3.1.7.  Naming and Addressing Strategies


   Current networks, both voice (telephone) and data, use addressing
   structures which closely tie the address to the physical location on
   the network.  That is, the address identifies a physical access
   point, rather than the higher-level entity (computer, process, human)
   attached to that access point.  In future networks, this physical
   aspect of addressing must be removed.

   Consider, for example, finding the desired party in the telephone
   network of today.  For a person not at his listed number, finding the
   number of the correct telephone may require preliminary calls, in
   which advice is given to the person placing the call.  This works
   well when a human is placing the call, since humans are well equipped
   to cope with arbitrary conversations.  But if a computer is placing
   the call, the process of obtaining the correct address will have to
   be incorporated in the architecture as a core service of the network.



Gigabit Working Group                                          [Page 17]

RFC 1077                                                   November 1988


   Since it is reasonable to expect mobile hosts, hosts that are
   connected to multiple networks, and replicated hosts, the issue of
   mapping to the physical address must be properly resolved.

   To permit the network to maintain the dynamic mapping to current
   physical address, it is necessary that high-level entities have a
   name (or logical address) that identifies them independently of
   location.  The name is maintained by the network, and mapped to the
   current physical location as a core network service.  For example,
   mobile hosts, hosts that are connected to multiple networks, and
   replicated hosts would have static names whose mapping to physical
   addresses (many-to-one, in some cases) would change with time.

   Hosts are not the only entities whose physical location varies.
   Users' electronic mail addresses change.  Within distributed systems,
   processes and files migrate from host to host.  In a computing
   environment where robustness and survivability are important, entire
   applications may move about, or they may be redundant.

   The needed function must be considered in the context of the mobility
   and address resolution rates if all addresses in a global data
   network were of this sort.  The distributed network directory
   discussed elsewhere in this report should be designed to provide the
   necessary flexibility, and responsiveness.  The nature and
   administration of names must also be considered.

   Names that are arbitrary or unwieldy would be barely better than the
   addresses used now.  The name space should be designed so that it can
   easily be partitioned among the agencies that will assign names.  The
   structure of names should facilitate, rather than hinder, the mapping
   function.  For example, it would be hard to optimize the mapping
   function if names were flat and unstructured.


   3.2.  High-Speed Switching


   The term "high-speed switching" refers to changing the switching at a
   high rate, rather than switching high-speed links, because the latter
   is not difficult at low speeds.  (Consider, for example, manual
   switching of fiber connections).  The switching regime chosen for the
   network determines various aspects of its performance, its charging
   policies, and even its effective capabilities.  As an example of the
   latter, it is difficult to expect a circuit-switched network to
   provide strong multicast support.

   A major area of debate lies in the choice between packet switching
   and circuit switching.  This is a key research issue for the GN,



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RFC 1077                                                   November 1988


   considering also the possibility of there being combinations of the
   two approaches that are feasible.


   3.2.1.  Unit of Management vs. Multiplexing


   With very high data rates, either the unit of management and
   switching must be larger or the speed of the processor elements for
   management and switching must be faster.  For example, at a gigabit,
   a 576 byte packet takes roughly 5 microseconds to be received so a
   packet switch must act extremely fast to avoid being the dominant
   delay in packet times.  Moreover, the storage time for the packet in
   a conventional store and forward implementation also becomes a
   significant component of the delay.  Thus, for packet switching to
   remain attractive in this environment, it appears necessary to
   increase the size of packets (or switch on packet groups), do so-
   called virtual cut-through and use high-speed routing techniques,
   such as high-speed route caches and source routing.

   Alternatively, for circuit switching to be attractive, it must
   provide very fast circuit setup and tear-down to support the bursty
   nature of most computer communication.  This problem is rendered
   difficult (and perhaps impossible for certain traffic loads) because
   the delay across the country is so large relative to the data rate.
   That is, even with techniques such as so-called fast select,
   bandwidth is reserved by the circuit along the path for almost twice
   the propagation time before being used.

   With gigabit circuit switching, because it is not feasible to
   physically switch channels, the low-level switching is likely doing
   FTDM on micro-packets, as is currently done in telephony.  Performing
   FTDM at gigabit data rates is a challenging research problem if the
   skew introduced by wide-area communication is to be handled with
   reasonable overhead for spacing of this micro-packets.  Given the
   lead and resources of the telephone companies, this area of
   investigation should, if pursued, be pursued cooperatively.


   3.2.2.  Bandwidth Reservation Algorithms


   Some applications, such as real-time video, require sustained high
   data rate streams over a significant period of time, such as minutes
   if not hours.  Intuitively, it is appealing for such applications to
   pre-allocate the bandwidth they require to minimize the switching
   load on the network and guarantee that the required bandwidth is
   available.  Research is required to determine the merits of bandwidth



Gigabit Working Group                                          [Page 19]

RFC 1077                                                   November 1988


   reservation, particular in conjunction with the different switching
   technologies.  There is some concern to raise that bandwidth
   reservation may require excessive intelligence in the network,
   reducing the performance and reliability of the network.  In
   addition, bandwidth reservation opens a new option for denial of
   service by an intruder or malicious user.  Thus, investigations in
   this area need to proceed in concert with work on switching
   technologies and capabilities and security and reliability
   requirements.


   3.2.3.  Multicast Capabilities


   It is now widely accepted that multicast should be provided as a
   user-level service, as described in RFC 1054 for IP, for example.
   However, further research is required to determine the best way to
   support this facility at the network layer and lower.  It is fairly
   clear that the GN will be built from point-to-point fiber links that
   do not provide multicast/broadcast for free.  At the most
   conservative extreme, one could provide no support and require that
   each host or gateway simulate multicast by sending multiple,
   individually addressed packets.  However, there are significant
   advantages to providing very low level multicast support (besides the
   obvious performance advantages).  For example, multicast routing in a
   flooding form provides the most fault-tolerant, lowest-delay form of
   delivery which, if reserved for very high priority messages, provides
   a good emergency facility for high-stress network applications.
   Multicast may also be useful as an approach to defeat traffic
   analysis.

   Another key issue arises with the distinction between so-called open
   group multicast and closed group multicast.  In the former, any host
   can multicast to the group, whereas in the latter, only members of
   the group can multicast to it.  The latter is easier to support and
   adequate for conferencing, for example.  However, for more client-
   server structured applications, such as using file/database server,
   computation servers, etc. as groups, open multicast is required.
   Research is needed to address both forms of multicast.  In addition,
   security issues arise in controlling the membership of multicast
   groups.  This issue should be addressed in concert with work on
   secure forms of routing in general.









Gigabit Working Group                                          [Page 20]

RFC 1077                                                   November 1988


   3.2.4.  Gateway Technologies