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.

   With the wide-area interconnection of local networks by the GN,
   gateways are expected to become a significant performance bottleneck
   unless significant advances are made in gateway performance.  In
   addition, many network management concerns suggest putting more
   functionality (such as access control) in the gateways, further
   increasing their load and the need for greater capacity.  This would
   then raise the issue of the trade-off between general-purpose
   hardware and special-purpose hardware.

   On the general-purpose side, it may be feasible to use a general-
   purpose multiprocessor based on high-end microprocessors (perhaps as
   exotic as the GaAs MIPS) in conjunction with a high-speed block
   transfer bus, as proposed as part of the FutureBus standard (which is
   extendible to higher speeds than currently commercially planned) and
   intelligent high-speed network adaptors.  This would also allow the
   direct use of hardware, operating systems, and software tools
   developed as part of other DARPA programs, such as Strategic
   Computing.  It also appears to make this gateway software more
   portable to commercial machines as they become available in this
   performance range.

   The specialized hardware approach is based on the assumption that
   general-purpose hardware, particularly the interconnection bus,
   cannot be fast enough to support the level of performance required.
   The expected emphasis is on various interconnection network
   techniques.  These approaches appear to require greater expense, less
   commercial availability and more specialized software.  They need to
   be critically evaluated with respect to the general-purpose gateway
   hardware approach, especially if the latter is using multiple buses
   for fault-tolerance as well as capacity extension (in the absence of
   failure).

   The same general-purpose vs. special-purpose contention is an issue
   with operating system software.  Conventionally, gateways run
   specialized run-time executives that are designed specifically for
   the gateway and gateway functions.  However, the growing
   sophistication of the gateway makes this approach less feasible.  It
   appears important to investigate the feasibility of using a standard
   operating system foundation on the gateways that is known to provide
   the required security and reliability properties (as well as real-
   time performance properties).







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RFC 1077                                                   November 1988


   3.2.5.  VLSI and Optronics Implementations


   It appears fairly clear that gigabit communication will use fiber
   optics for at least the near future.  Without major advances in
   optronics to allow effectively for optical computers, communication
   must cross the optical-electronic boundary two or more times.  There
   are significant cost, performance, reliability, and security benefits
   for minimizing the number of such crossings.  (As an example of a
   security benefit, optics is not prone to electronic surveillance or
   jamming while electronics clearly is, so replacing an optic-
   electronic-optic node with a pure optic node eliminates that
   vulnerability point.)

   The benefits of improved technology in optronics is so great that its
   application here is purely another motivation for an already active
   research area (that deserves strong continued support).  Therefore,
   we focus here in the issue of matching current (and near-term
   expected) optronics capabilities with network requirements.

   The first and perhaps greatest area of opportunity is to achieve
   totally (or largely) photonic switches in the network switching
   nodes.  That is, most packets would be switched without crossing the
   optics-electronics boundary at all.  For this to be feasible, the
   switch must use very simple switching logic, require very little
   storage and operate on packets of a significant size.  The source-
   routed packet switches with loopback on blockage of Blazenet
   illustrate the type of techniques that appear required to achieve
   this goal.

   Research is required to investigate the feasibility of optronic
   implementation of switches.  It appears highly likely that networks
   will at some point in the future be totally photonically switched,
   having the impact on networking comparable to the effect of
   integrated circuits on processors and memories.

   A next level of focus is to achieve optical switching in the common
   case in gateways.  One model is a multiprocessor with an optical
   interconnect.  Packets associated with established paths through the
   gateway are optically switched and processed through the
   interconnect.  Other packets are routed to the multiprocessor,
   crossing into the electronics domain.  Research is required to marry
   the networking requirements and technology with optronics technology,
   pushing the state of the art in both areas in the process.

   Given the long-term presence of the optic-electronic boundary,
   improvements in technology in this area are also important.  However,
   it appears that there is already enormous commercial research



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RFC 1077                                                   November 1988


   activity in this area, particularly within the telephone companies.
   This is another area in which collaborative investigation appears far
   better than an new independent research effort.

   VLSI technology is an established technology with active research
   support.  The GN effort does not appear to require major new
   initiatives in the VLSI area, yet one should be open to significant
   novel opportunities not identified here.


   3.2.6.  High-Speed Transfer Protocols


   To achieve the desired speeds, it will be necessary to rethink the
   form of protocols.

      1.  The simple idea of a stateless gateway must be replaced by a
          more complex model in which the gateway understands the
          desired function of the end point and applies suitable
          optimizations to the flow.

      2.  If multiplexing is done in the time domain, the elements of
          multiplexing are probably so small that no significant
          processing can be performed on each individually.  They must
          be processed as an aggregate.  This implies that the unit of
          multiplexing is not the same as the unit of processing.

      3.  The interfaces between the structural layers of the
          communication system must change from a simple
          command/response style to a richer system which includes
          indications and controls.

      4.  An approach must be developed that couples the memory
          management in the host and the structure of the transmitted
          data, to allow efficient transfers into host memory.

   The result of rethinking these problems will be a new style of
   communications and protocols, in which there is a much higher degree
   of shared responsibility among the components (hosts, switches,
   gateways).  This may have little resemblance to previous work either
   in the DARPA or commercial communities.


   3.3.  High-Speed Host Interfaces


   As networks get faster, the most significant bottleneck will turn out
   to be the packet processing overhead in the host.  While this does



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RFC 1077                                                   November 1988


   not restrict the aggregate rates we can achieve over trunks, it
   prevents delivery of high data rate flows to the host-based
   applications, which will prevent the development of new applications
   needing high bandwidth.  The host bottleneck is thus a serious
   impediment to networked use of supercomputers.

   To build a GN we need to create new ways for hosts and their high
   bandwidth peripherals to connect to networks.  We believe that
   pursuing research in the ways to most effectively isolate host and
   LAN development paths from the GN is the most productive way to
   proceed.  By decoupling the development paths, neither is restricted
   by the momentary performance of capability bottlenecks of the other.
   The best context in which to view this separation is with the notion
   of a network front end (NFE).  The NFE can take the electronic input
   data at many data rates and transform it into gigabit light data
   appropriately packetized to traverse the GN.  The NFE can accept
   inputs from many types of gateways, hosts, host peripherals, and LANS
   and provide arbitration and path set-up facilities as needed.  Most
   importantly, the NFE can perform protocol arbitration to retain
   upward compatibility with the existing Internet protocols while
   enabling those sophisticated network input sources to execute GN
   specific high-throughput protocols.  Of course, this introduces the
   need for research into high-speed NFEs to avoid the NFE becoming a
   bottleneck.


   3.3.1.  VLSI and Optronics Implementations


   In a host interface, unless the host is optical (an unlikely prospect
   in the near-term), the opportunities for optronic support are
   limited.  In fact, with a serial-to-parallel conversion on reception
   stepping the clock rate down by a factor of 32 (assuming a 32-bit
   data path on the host interface), optronic speeds are not required in
   the immediate future.

   One exception may be for encryption.  Current VLSI implementations of
   standard encryption algorithms run in the 10 Mbit/s range.  Optronic
   implementation of these encryption techniques and encryption
   techniques specifically oriented to, or taking advantage of, optronic
   capabilities appears to be an area of some potential (and enormous
   benefit if achieved).

   The potential of targeted VLSI research in this area appears limited
   for similar reasons discussed above with its application in high-
   speed switching.  The major benefits will arise from work that is
   well-motivated by other research (such as high-performance
   parallelism) and by strong commercial interest.  Again, we need to be



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RFC 1077                                                   November 1988


   open to imaginative opportunities not foreseen here while keeping
   ourselves from being diverted into low-impact research without
   further insights being put forward.


   3.3.2.  High-Performance Transport Protocols


   Current transport protocols exhibit some severe problems for maximal
   performance, especially for using hardware support.  For example, TCP
   places the checksum in the packet header, forcing the packet to be
   formed and read fully before transmission begins.  ISO TP4 is even
   worse, locating the checksum in a variable portion of the header at
   an indeterminate offset, making hardware implementation extremely
   difficult.

   The current Internet has thrived and grown due to the existence of
   TCP implementations for a wide variety of classes of host computers.
   These various TCP implementations achieve robust interoperability by
   a "least common denominator" approach to features and options.  Some
   applications have arisen in the current Internet, and analogs can be
   envisioned for the GN environment, which need qualities of service
   not generally supported by the ubiquitous generic TCP, and therefore
   special purpose transport protocols have been developed.  Examples
   include special purpose transport protocols such as UDP (user
   datagram protocol), RDP (reliable datagram protocol), LDP
   (loader/debugger protocol), NETBLT (high-speed block transfer
   protocol), NVP (network voice protocol) and PVP (packet video
   protocol).  Efforts are also under way to develop a new generic
   transport protocol VMTP (versatile message transaction protocol)
   which will remedy some of deficiencies of TCP, without the need to
   resort to special purpose protocols for some applications.  Research
   is needed in this area to understand how transport level protocols
   should be constructed for a GN which provide adequate qualities of
   service and ease of implementation.

   A new transport protocol of reasonable success can be expected to
   last for ten years more.  Therefore, a new protocol should not be
   over optimized for current networks and must not ignore the
   functional deficiencies of current protocols.  These deficiencies are
   essential to remedy before it is feasible to deploy even current
   distributed systems technology for military and commercial
   applications.

   Forward Error Correction (FEC) is a useful approach when the
   bandwidth/delay ratio of the physical medium is high, as can be
   expected in transcontinental photonic links.  A degenerate form of
   FEC is to simply transmit multiple copies of the data; this allows



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RFC 1077                                                   November 1988


   one to trade bandwidth for delay and reliability, without requiring
   much intelligence.  In fact, it is generally true that reliability,
   bandwidth, and delay are interrelated and an improvement in one
   generally comes at the expense of the others for a given technolo