rfc1152.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   causal consistency, in which systems ensure that operations happen in
   the proper order, without synchronizing through a single system.

Session 6: Hosts and Host Interfaces (Gary Delp, Chair)

   Gary Delp (IBM Research) discussed several issues involved in the
   increase in speed of network attachment to hosts of increasing
   performance.  These issues included:

      -  Media Access - There are aspects of media access that are
         best handled by dedicated silicon, but there are also aspects
         that are best left to a general-purpose processor.

      -  Compression - Some forms of compression/expansion may belong
         on the network interface; most will be application-specific.

      -  Forward Error Correction - The predicted major packet loss
         mode is packet drops due to internal network congestion, rather
         than bit errors, so forward error correction internal to a
         packet may not be useful.  On the other hand, the latency cost
         of not being able to recover from bit errors is very high.
         Some proposals were discussed which suggest that FEC among
         packet groups, with dedicated hardware support, is the way
         to go.

      -  Encryption/Decryption - This is a computationally intensive
         task.  Most agree that if it is done with all traffic, some
         form of hardware support is helpful.  Where does it fit in the
         protocol stack?

      -  Application Memory Mapping - How much of the host memory
         structure should be exposed to the network interface?
         Virtual memory and paging complicate this issue considerably.

      -  Communication with Other Channel Controllers - Opinions were
         expressed that ranged from absolutely passive network
         interfaces to interfaces that run major portions of the
         operating system and bus arbitration codes.

      -  Blocking/Segmentation - The consensus is that B/S should



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         occur wherever the transport layer is processed.

      -  Routing - This is related to communications with other
         controllers.  A routing-capable interface can reduce the bus
         requirements by a factor of two.

      -  Intelligent participation in the host structure as a gateway,
         router, or bridge.

      -  Presentation Layer issues - All of the other overheads can be
         completely overshadowed by this issue if it is not solved well
         and integrated into the overall host architecture.  This points
         out the need for some standardization of representation (IEEE
         floating point, etc.)

   Eric Cooper (CMU) summarized some initial experience with Nectar, a
   high-speed fiber-optic LAN that has been built at Carnegie Mellon.
   Nectar consists of an arbitrary mesh of crossbar switches connected
   by means of 100 Mbps fiber-optic links.  Hosts are connected to
   crossbar switches via communication processor boards called CABs.
   The CAB presents a memory-mapped interface to user processes and
   off-loads all protocol processing from the host.

   Preliminary performance figures show that latency is currently
   limited by the number of VME operations required by the host-to-CAB
   shared memory interface in the course of sending and receiving a
   message.  The bottleneck in throughput is the speed of the VME
   interface: although processes running on the CABs can communicate
   over Nectar at 70 Mbps, processes on the hosts are limited to
   approximately 25 Mbps.

   Jeff Mogul (DEC Western Research Lab) made these observations:
   Although off-board protocol processors have been a popular means to
   connect a CPU to a network, they will be less useful in the future.
   In the hypothetical workstation of the late 1990s, with a 1000-MIPS
   CPU and a Gbps LAN, an off-board protocol processor will be of no
   use.  The bottleneck will not be the computation required to
   implement the protocol, but the cost of moving the packet data into
   the CPU's cache and the cost of notifying the user process that the
   data is available.  It will take far longer (hundreds of instruction
   cycles) to perform just the first cache miss (required to get the
   packet into the cache) than to perform all of the instructions
   necessary to implement IP and TCP (perhaps a hundred instructions).

   A high-speed network interface for a reasonably-priced system must be
   designed with this cost structure in mind; it should also eliminate
   as many CPU interrupts as possible, since interrupts are also very
   expensive.  It makes more sense to let a user process busy-wait on a



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   network-interface flag register than to suspend it and then take an
   interrupt; the normal CPU scheduling mechanism is more efficient than
   interrupts if the network interactions are rapid.

   David Greaves (Olivetti Research Ltd.) briefly described the need for
   a total functionality interface architecture that would allow the
   complete elimination of communication interrupts.  He described the
   Cambridge high-speed ring as an ATM cell-like interconnect that
   currently runs at 500-1000 MBaud, and claims that ATM at that speed
   is a done deal.   Dave Tennenhouse also commented that ATM at high
   speeds with parallel processors is not the difficult thing that
   several others have been claiming.

   Bob Beach (Ultra Technologies) started his talk with the observation
   that networking could be really fast if only we could just get rid of
   the hosts.   He then supported his argument with illustrations of
   80-MByte/second transfers to frame buffers from Crays that drop to
   half that speed when the transfer is host-to-host.  Using null
   network layers and proprietary MAC layers, the Ultra Net system can
   communicate application-to-application with ISO TP4 as the transport
   layer at impressive rates of speed.  The key to high-speed host
   interconnects has been found to be both large packets and large (on
   the order of one megabyte) channel transfer requests.  Direct DMA
   interfaces exhibit much smaller transfer latencies.

   Derek McAuley (University Cambridge Computer Laboratory) described
   work of the Fairisle project which is producing an ATM network based
   on fast packet switches.  A RISC processor (12 MIPS) is used in the
   host interface to do segmentation/reassembly/demultiplexing.  Line
   rates of up to 150 Mbps are possible even with this modest processor.
   Derek has promised that performance and requirement results from this
   system will be published in the spring.

   Bryan Lyles (XEROX PARC) volunteered to give an abbreviated talk in
   exchange for discussion rights.  He reported that Xerox PARC is
   interested in ATM technology and wants to install an ATM LAN at the
   earliest possible opportunity.  Uses will include such applications
   as video where guaranteed quality of service (QOS) is required.  ATM
   technology and the desire for guaranteed QOS places a number of new
   constraints on the host interface.  In particular, they believe that
   they will be forced towards rate-based congestion control.  Because
   of implementation issues and burst control in the ATM switches, the
   senders will be forced to do rate based control on a cell-by-cell
   basis.

   Don Tolmie (Los Alamos National Laboratory) described the High-
   Performance Parallel Interface (HPPI) of ANSI task group X3T9.3.  The
   HPPI is a standardized basic building block for implementing, or



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   connecting to, networks at the Gbps speeds, be they ring, hub,
   cross-bar, or long-haul based.  The HPPI physical layer operates at
   800 or 1600 Mbps over 25-meter twisted-pair copper cables in a
   point-to-point configuration.  The HPPI physical layer has almost
   completed the standards process, and a companion HPPI data framing
   standard is under way, and a Fiber Channel standard at comparable
   speeds is also being developed.  Major companies have completed, or
   are working on, HPPI interfaces for supercomputers, high-end
   workstations, fiber-optic extenders, and networking components.

   The discussion at the end of the session covered a range of topics.
   The appropriateness of outboard protocol processing was questioned.
   Several people agreed that outboarding on a Cray (or similar
   cost/performance) machines makes economic sense.  Van Jacobson
   contended that for workstations, a simple memory-mapped network
   interface that provides packets visible to the host processor may
   well be the ideal solution.

   Bryan Lyles reiterated several of his earlier points, asserting that
   when we talk about host interfaces and how to build them we should
   remember that we are really talking about process-to-process
   communication, not CPU-to-CPU communication.  Not all processes run
   on the central CPU, e.g., graphics processors and multimedia.
   Outboard protocol processing may be a much better choice for these
   architectures.

   This is especially true when we consider that memory/bus bandwidth is
   often a bottleneck.  When our systems run out of bandwidth, we are
   forced towards a NUMA model and multiple buses to localize memory
   traffic.

   Because of QOS issues, the receiver must be able to tell the sender
   how fast it can send.  Throwing away cells (packets) will not work
   because unwanted packets will still clog the receiver's switch
   interface, host interface, and requires processing to throw away.

Session 7: Congestion Control (Scott Shenker, Chair)

   The congestion control session had six talks.  The first two talks
   were rather general, discussing new approaches and old myths.  The
   other four talks discussed specific results on various aspects of
   packet (or cell) dropping: how to avoid drops, how to mitigate their
   impact on certain applications, a calculation of the end-to-end
   throughput in the presence of drops, and how rate-based flow control
   can reduce buffer usage.  Thumbnail sketches of the talks follow.

   In the first of the general talks, Scott Shenker (XEROX PARC)
   discussed how ideas from economics can be applied to congestion



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   control.  Using economics, one can articulate questions about the
   goals of congestion control, the minimal feedback necessary to
   achieve those goals, and the incentive structure of congestion
   control.  Raj Jain (DEC) then discussed eight myths related to
   congestion control in high-speed networks.  Among other points, Raj
   argued that (1) congestion problems will not become less important
   when memory, processors, and links become very fast and cheap, (2)
   window flow control is required along with rate flow control, and (3)
   source-based controls are required along with router-based control.

   In the first of the more specific talks, Isidro Castineyra (BBN
   Communications Corporation) presented a back-of-the-envelope
   calculation on the effect of cell drops on end-to-end throughput.
   While at extremely low drop rates the retransmission strategies of
   go-back-n and selective retransmission produced similar end-to-end
   throughput, at higher drop rates selective retransmission achieved
   much higher throughput.  Next, Tony DeSimone (AT&T) told us why
   high-speed networks are not just fast low-speed networks.   If the
   buffer/window ratio is fixed, the drop rate decreases as the network
   speed increases.  Also, data was presented which showed that adaptive
   rate control can greatly decrease buffer utilization.  Jamal
   Golestani (Bellcore) then presented his work on stop-and-go queueing.
   This is a simple stalling algorithm implemented at the switches which
   guarantees no dropped packets and greatly reduces delay jitter.  The
   algorithm requires prior bandwidth reservation and some flow control
   on sources, and is compatible with basic FIFO queues.  In the last
   talk, Victor Frost (University of Kansas) discussed the impact of
   different dropping policies on the perceived quality of a voice
   connection.  When the source marks the drop priority of cells and the
   switch drops low priority cells first, the perceived quality of the
   connection is much higher than when cells are dropped randomly.

Session 8: Switch Architectures (Dave Sincoskie, Chair)

   Dave Mills (University of Delaware) presented work on a project now
   under way at the University of Delaware to study architectures and
   protocols for a high-speed network and packet switch capable of
   operation to the gigabit regime over distances spanning the country.
   It is intended for applications involving very large, very fast, very
   bursty traffic typical of supercomputing, remote sensing, and
   visualizing applications.  The network is assumed to be composed of
   fiber trunks, while the switch architecture is based on a VLSI
   baseband crossbar design which can be configured for speeds from 25