rfc1077.txt

RFC 的详细文档！

   trunks are rising faster than the speeds of the switching elements.

   This change in the balance of speeds has manifested itself in several
   ways.  In most current designs for local area networks, where



Gigabit Working Group                                          [Page 10]

RFC 1077                                                   November 1988


   bandwidth is not expensive, the design decision was to trade off
   effective use of the bandwidth for a simplified switching technique.
   In particular, networks such as Ethernet use broadcast as the normal
   distribution method, which essentially eliminates the need for a
   switching element.

   As we look at still higher speed networks, and in particular networks
   in which the bandwidth is still the expensive component, we must
   design new options for switching which will permit effective use of
   bandwidth without the switch itself becoming the bottleneck.

   The central thrust of new research must thus be to explore new
   network architectures that are consistent with these very different
   speed assumptions.

   The development of computer communications has been tremendously
   distorted by the characteristics of wide-area networking: normally
   high cost, low speed, high error rate, large delay.  The time is ripe
   for a revolution in thinking, technology, and approaches, analogous
   to the revolution caused by VCR technology over 8 and 16 mm. film
   technology.

   Fiber optics is clearly the enabling technology for high-speed
   transmission, in fact, so much so that there is an expectation that
   the switching elements will now hold down the data rates.  Both
   conventional circuit switching and packet switching have significant
   problems at higher data rates.  For instance, circuit switching
   requires increasing delays for FTDM synchronization to handle skew.
   In the case of packet switching, traditional approaches require too
   much processing per packet to handle the tremendous data flow.  The
   problem for both switching regimes is the "intelligence" in the
   switches, which in turn requires electronics technology.

   Besides intelligence, another problem for wide-area networks is
   storage, both because it ties us to electronics (for the foreseeable
   future) and because it produces instabilities in a large-scale
   system.  (See, for instance, the work by Van Jacobson on self-
   organizing phenomena for self-destruction in the Internet.)
   Techniques are required to eliminate dependence on storage, such as
   cut-through routing.

   Overall, high-speed WANs are the greatest agents of change, the
   greatest catalyst both commercially and militarily, and the area ripe
   for revolution.  Judging by the attributes of current high-speed
   network research prototypes, WANs of the future will be photonic,
   multi-gigabit networks with enormous throughput, low delay, and low
   error rate.




Gigabit Working Group                                          [Page 11]

RFC 1077                                                   November 1988


   A zero-based budgeting approach is required to develop the new high-
   speed internetwork architecture.  That is, the time is ripe to
   significantly rethink the Internet, building on experience with this
   system.  Issues of concern are manageability, understanding
   evolvability and support for the new communication requirements,
   including remote procedure call, real-time, security and fault-
   tolerance.

   The GN must be able to deal with two sources of high-bandwidth
   requirements.  There will be some end devices (computers) connected
   more or less directly to the GN because of their individual
   requirements for high bandwidth (e.g., supercomputers needing to
   drive remote high-bandwidth graphics devices).  In addition, the
   aggregate traffic due to large numbers of moderate rate users
   (estimates are roughly up to a million potential users needing up to
   1 Mbit/s at any given time) results in a high-bandwidth requirement
   in total on the GN.  The statistics of such traffic are different and
   there are different possible technical approaches for dealing with
   them.  Thus, an architectural approach for dealing with both must be
   developed.

   Overall, the next-generation architecture has to be, first and
   foremost, a management architecture.  The directions in link speeds,
   processor speeds and memory solve the performance problems for many
   communication situations so well that manageability becomes the
   predominant concern.  (In fact, fast communication makes large
   systems more prone to performance, reliability, and security
   problems.)  In many ways, the management system of the internetwork
   is the ultimate distributed system.  The solution to this tough
   problem may well require the best talents from the communications,
   operating systems and distributed systems communities, perhaps even
   drawing on database and parallelism research.


   3.1.1.  High-Speed Internet using High-Speed Networks


   The GN will need to take advantage of a multitude of different and
   heterogeneous networks, all of high speed.  In addition to networks
   based on the technology of the GB, there will be high-speed LANs.  A
   key issue in the development of the GN will be the development of a
   strategy for interconnecting such networks to provide gigabit service
   on an end to end basis.  This will involve techniques for switching,
   interfacing, and management (as discussed in the sections below)
   coupled with an architecture that allows the GN to take full
   advantage of the performance of the various high-speed networks.





Gigabit Working Group                                          [Page 12]

RFC 1077                                                   November 1988


   3.1.2.  Network Organization


   The GN will need an architecture that supports the need to manage the
   system as well as obtain high performance.  We note that almost all
   human-engineered systems are hierarchically structured from the
   standpoint of control, monitoring, and information flow.  A
   hierarchical design may be the key to manageability in the next-
   generation architecture.

   One approach is to use a general three-level structure, corresponding
   to interadministrational, intraadministrational, and cluster
   networks.  The first level interconnects communication facilities of
   truly separate administrations where there is significant separation
   of security, accounting, and goals.  The second level interconnects
   subadministrations which exist for management convenience in large
   organizations.  For example, a research group within a university may
   function as a subadministration.  The cluster level consists of
   networks configured to provides maximal performance among hosts which
   are in frequent communication, such as a set of diskless workstations
   and their common file server.  These hosts are typically, but not
   necessarily, geographically collocated.  For example, two remote
   networks may be tightly coupled by a fiber optic link that bridges
   between the two physical networks, making them function as one.

   Research along these lines should study the interorganizational
   characteristics of communications, such as those being investigated
   by the IAB Task Force on Autonomous Networks.  Based on current
   results, we expect that such work would clearly demonstrate that
   considerable communication takes place between particular
   subadministrations in different administrations; communication
   patterns are not strictly hierarchical.  For example, there might be
   intense direct communication between the experimental physics
   departments of two independent universities, or between the computer
   support group of one company and the operating system development
   group of another.  In addition, (sub)administrations may well also
   require divisions into public information and private information.


   3.1.3.  Fault-Tolerant System


   Although the GN will be developed as part of an experimental research
   program, it will also serve as part of the infrastructure for
   researchers who are experimenting with applications which will use
   such a network.  The GN must have reasonably high availability to
   support these research activities.  In addition to facilitate the
   transfer of this technology to future operational military and



Gigabit Working Group                                          [Page 13]

RFC 1077                                                   November 1988


   commercial users, it will need to be designed to become highly
   reliable.  This can be accomplished through diversity of transmission
   paths, the development of fault-tolerant switches, use of a
   distributed control structure with self-correcting algorithms, and
   the protection of network control traffic.  The architecture of a GN
   should support and allow for all of these things.


   3.1.4.  Functional Division of Control Between Network Elements


   Current protocol architectures use the layered model of functional
   decomposition first developed in the early work on ARPANET protocols.
   The concept of layering has been a powerful concept which has allowed
   dramatic variation in network technologies without requiring the
   complete reimplementation of applications.  The concept of layering
   has had a first-order impact on the development of international
   standards for data communication---witness the ISO "Reference Model
   for Open Systems Interconnection."

   Unfortunately, however, the powerful concept of layering has been
   paired, both in the DoD Internet work and the ISO work, with an
   extremely weak concept of the interface between layers.  The
   interface designs are all organized around the idea of commands and
   responses plus an error indicator.  For example, the TCP service
   interface provides the user with commands to set up or close a TCP
   connection and commands to send and receive datagrams.  The user may
   well "know" whether they are using a file transfer service or a
   character-at-a- time virtual terminal, but can't tell the TCP.  The
   underlying network may "know" that failures have reduced the path to
   the user's destination to a single 9.6 kbit/s link, but it also can't
   tell the TCP implementation.

   All of the information that an analyst would consider crucial in
   diagnosing system performance is carefully hidden from adjacent
   layers.  One "solution" often discussed (but rarely implemented) is
   to condense all of this information into a few bits of "Type of
   Service" or "Quality of Service" request flowing in one direction
   only---from application to network.  It seems likely that this
   approach cannot succeed, both because it applies too much compression
   to the knowledge available and because it does not provide two-way
   flow.

   We believe it to be likely that the next-generation network will
   require a much richer interface between every pair of adjacent layers
   if adequate performance is to be achieved.  Research is needed into
   the conceptual mechanisms, both indicators and controls, that can be
   implemented at these interfaces and that, when used, will result in



Gigabit Working Group                                          [Page 14]

RFC 1077                                                   November 1988


   better performance.  If real differences in performance can be
   observed, then the implementors of every layer will have a strong
   incentive to make use of the mechanisms.

   We can observe the first glimmers of this sort of coordination
   between layers in current work.  For example, in the ISO work there
   are 5 classes of transport protocol which are supposed to provide a
   range of possible matches between application needs and network
   capabilities.  Unfortunately, it is the case today that the class of
   transport protocol is chosen statically, by the implementer, rather
   than dynamically.  The DARPA Wideband net offers a choice of stream
   or datagram service, but typically a given host uses all one or all
   the other---again, a static rather than a dynamic choice.  The
   research that we believe is needed, therefore, is not how to provide
   alternatives, but how to provide them and choose among them on a
   dynamic, real-time basis.


   3.1.5.  Different Switch Technologies


   One approach to high-performance networking is to design a technology
   that is expected to work as a stand-alone demonstration, without
   addressing the need for interconnection to other networks.  Such an
   experiment may be very valuable for rapid exploration of the design
   space.  However, our experience with the Internet project suggests
   that a primary research goal should be the development of a network
   architecture that permits the interconnection of a number of
   different switching technologies.

   The Internet project was successful to a large extent because it
   could incorporate a number of new and preexisting network
   technologies: various local area networks, store and forward
   switching networks, broadcast satellite nets, packet radio networks,
   and so on.  In this way, it decoupled the use of the protocols from a
   particular technology base.  In fact, the technology base evolved
   rapidly, but the Internet protocols themselves provided a stability
   that led to their success.

   The next-generation architecture must similarly deal with a diverse
   and evolving technology base.  We see "fast-packet" switching now