rfc1009.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

         connected network provides a broadcast or multicast capability;
         these will be discussed later.

   1.2.  The Internet Gateway Model

      There are two basic models for interconnecting local-area networks
      and wide-area (or long-haul) networks in the Internet.  In the
      first, the local-area network is assigned a network number and all
      gateways in the Internet must know how to route to that network.
      In the second, the local-area network shares (a small part of) the
      address space of the wide-area network.  Gateways that support
      this second model are called "address sharing gateways" or
      "transparent gateways".  The focus of this memo is on gateways
      that support the first model, but this is not intended to exclude
      the use of transparent gateways.

      1.2.1.  Internet Gateways

         An Internet gateway is an IP-level router that performs the
         following functions:

            1.  Conforms to specific Internet protocols specified in
                this document, including the Internet Protocol (IP),
                Internet Control Message Protocol (ICMP), and others as
                necessary.  See Section 2 (Protocols Required).

            2.  Interfaces to two or more packet networks.  For each


Braden & Postel                                                 [Page 6]



RFC 1009 - Requirements for Internet Gateways                  June 1987


                connected network the gateway must implement the
                functions required by that network.  These functions
                typically include:

               a.  encapsulating and decapsulating the IP datagrams with
                   the connected network framing (e.g., an Ethernet
                   header and checksum);

               b.  sending and receiving IP datagrams up to the maximum
                   size supported by that network, this size is the
                   network's "Maximum Transmission Unit" or "MTU";

               c.  translating the IP destination address into an
                   appropriate network-level address for the connected
                   network (e.g., an Ethernet hardware address);

               d.  responding to the network flow control and error
                   indication, if any.

               See Section 3 (Constituent Network Interface), for
               details on particular constituent network interfaces.

            3.  Receives and forwards Internet datagrams.  Important
                issues are buffer management, congestion control, and
                fairness.  See Section 4 (Gateway Algorithms).

               a.  Recognizes various error conditions and generates
                   ICMP error and information messages as required.

               b.  Drops datagrams whose time-to-live fields have
                   reached zero.

               c.  Fragments datagrams when necessary to fit into the
                   MTU of the next network.

            4.  Chooses a next-hop destination for each IP datagram,
                based on the information in its routing data-base.  See
                Section 4 (Gateway Algorithms).

            5.  Supports an interior gateway protocol (IGP) to carry out
                distributed routing and reachability algorithms with the
                other gateways in the same autonomous system.  In
                addition, some gateways will need to support the
                Exterior Gateway Protocol (EGP) to exchange topological
                information with other autonomous systems.  See
                Section 4 (Gateway Algorithms).




Braden & Postel                                                 [Page 7]



RFC 1009 - Requirements for Internet Gateways                  June 1987


            6.  Provides system support facilities, including loading,
                debugging, status reporting, exception reporting and
                control.  See Section 5 (Operation and Maintenance).

      1.2.2.  Embedded Gateways

         A gateway may be a stand-alone computer system, dedicated to
         its IP router functions.  Alternatively, it is possible to
         embed gateway functionality within a host operating system
         which supports connections to two or more networks.  The
         best-known example of an operating system with embedded gateway
         code is the Berkeley BSD system.  The embedded gateway feature
         seems to make internetting easy, but it has a number of hidden
         pitfalls:

            1.  If a host has only a single constituent-network
                interface, it should not act as a gateway.

                For example, hosts with embedded gateway code that
                gratuitously forward broadcast packets or datagrams on
                the same net often cause packet avalanches.

            2.  If a (multihomed) host acts as a gateway, it must
                implement ALL the relevant gateway requirements
                contained in this document.

                For example, the routing protocol issues (see Sections
                2.6 and 4.1) and the control and monitoring problems are
                as hard and important for embedded gateways as for
                stand-alone gateways.

                   Since Internet gateway requirements and
                   specifications may change independently of operating
                   system changes, an administration that operates an
                   embedded gateway in the Internet is strongly advised
                   to have an ability to maintain and update the gateway
                   code (e.g., this might require gateway code source).

            3.  Once a host runs embedded gateway code, it becomes part
                of the Internet system.  Thus, errors in software or
                configuration of such a host can hinder communication
                between other hosts.  As a consequence, the host
                administrator must lose some autonomy.

                In many circumstances, a host administrator will need to
                disable gateway coded embedded in the operating system,
                and any embedded gateway code must be organized so it
                can be easily disabled.


Braden & Postel                                                 [Page 8]



RFC 1009 - Requirements for Internet Gateways                  June 1987


            4.  If a host running embedded gateway code is concurrently
                used for other services, the O&M (operation and
                maintenance) requirements for the two modes of use may
                be in serious conflict.

                For example, gateway O&M will in many cases be performed
                remotely by an operations center; this may require
                privileged system access which the host administrator
                would not normally want to distribute.

      1.2.3.  Transparent Gateways

         The basic idea of a transparent gateway is that the hosts on
         the local-area network behind such a gateway share the address
         space of the wide-area network in front of the gateway.  In
         certain situations this is a very useful approach and the
         limitations do not present significant drawbacks.

         The words "in front" and "behind" indicate one of the
         limitations of this approach: this model of interconnection is
         suitable only for a geographically (and topologically) limited
         stub environment.  It requires that there be some form of
         logical addressing in the network level addressing of the
         wide-area network (that is, all the IP addresses in the local
         environment map to a few (usually one) physical address in the
         wide-area network, in a way consistent with the { IP address
         <-> network address } mapping used throughout the wide-area
         network).

         Multihoming is possible on one wide-area network, but may
         present routing problems if the interfaces are geographically
         or topologically separated.  Multihoming on two (or more)
         wide-area networks is a problem due to the confusion of
         addresses.

         The behavior that hosts see from other hosts in what is
         apparently the same network may differ if the transparent
         gateway cannot fully emulate the normal wide-area network
         service.  For example, if there were a transparent gateway
         between the ARPANET and an Ethernet, a remote host would not
         receive a Destination Dead message [3] if it sent a datagram to
         an Ethernet host that was powered off.








Braden & Postel                                                 [Page 9]



RFC 1009 - Requirements for Internet Gateways                  June 1987


   1.3.  Gateway Characteristics

      Every Internet gateway must perform the functions listed above.
      However, a vendor will have many choices on power, complexity, and
      features for a particular gateway product.  It may be helpful to
      observe that the Internet system is neither homogeneous nor
      fully-connected.  For reasons of technology and geography, it is
      growing into a global-interconnect system plus a "fringe" of LANs
      around the "edge".

         *  The global-interconnect system is comprised of a number of
            wide-area networks to which are attached gateways of several
            ASs; there are relatively few hosts connected directly to
            it.  The global-interconnect system includes the ARPANET,
            the NSFNET "backbone", the various NSF regional and
            consortium networks, other ARPA sponsored networks such as
            the SATNET and the WBNET, and the DCA sponsored MILNET.  It
            is anticipated that additional networks sponsored by these
            and other agencies (such as NASA and DOE) will join the
            global-interconnect system.

         *  Most hosts are connected to LANs, and many organizations
            have clusters of LANs interconnected by local gateways.
            Each such cluster is connected by gateways at one or more
            points into the global-interconnect system.  If it is
            connected at only one point, a LAN is known as a "stub"
            network.

      Gateways in the global-interconnect system generally require:

         *  Advanced routing and forwarding algorithms

            These gateways need routing algorithms which are highly
            dynamic and also offer type-of-service routing.  Congestion
            is still not a completely resolved issue [24].  Improvements
            to the current situation will be implemented soon, as the
            research community is actively working on these issues.

         *  High availability

            These gateways need to be highly reliable, providing 24 hour
            a day, 7 days a week service.  In case of failure, they must
            recover quickly.

         *  Advanced O&M features

            These gateways will typically be operated remotely from a
            regional or national monitoring center.  In their


Braden & Postel                                                [Page 10]



RFC 1009 - Requirements for Internet Gateways                  June 1987


            interconnect role, they will need to provide sophisticated
            means for monitoring and measuring traffic and other events
            and for diagnosing faults.

         *  High performance

            Although long-haul lines in the Internet today are most
            frequently 56 Kbps, DS1 lines (1.5 Mbps) are of increasing
            importance, and even higher speeds are likely in the future.
            Full-duplex operation is provided at any of these speeds.

            The average size of Internet datagrams is rather small, of
            the order of 100 bytes.  At DS1 line speeds, the
            per-datagram processing capability of the gateways, rather
            than the line speed, is likely to be the bottleneck.  To
            fill a DS1 line with average-sized Internet datagrams, a
            gateway would need to pass -- receive, route, and send --
            2,000 datagrams per second per interface.  That is, a
            gateway which supported 3 DS1 lines and and Ethernet
            interface would need to be able to pass a dazzling 2,000
            datagrams per second in each direction on each of the
            interfaces, or a aggregate throughput of 8,000 datagrams per
            second, in order to fully utilize DS1 lines.  This is beyond
            the capability of current gateways.

               Note: some vendors count input and output operations
               separately in datagrams per second figures; for these
               vendors, the above example would imply 16,000 datagrams
               per second !

      Gateways used in the "LAN fringe" (e.g., campus networks) will
      generally have to meet less stringent requirements for
      performance, availability, and maintenance.  These may be high or
      medium-performance devices, probably competitively procured from
      several different vendors and operated by an internal organization
      (e.g., a campus computing center).  The design of these gateways
      should emphasize low average delay and good burst performance,
      together with delay and type-of-service sensitive resource
      management.  In this environment, there will be less formal O&M,
      more hand-crafted static configurations for special cases, and
      more need for inter-operation with gateways of other vendors.  The
      routing mechanism will need to be very flexible, but need not be
      so highly dynamic as in the global-interconnect system.

      It is important to realize that Internet gateways normally operate
      in an unattended mode, but that equipment and software faults can
      have a wide-spread (sometimes global) effect.  In any environment,