rfc1716.txt

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         although it is architecturally layered upon IP, i.e., it uses
         IP to carry its data end-to-end.  ICMP provides error
         reporting, congestion reporting, and first-hop router
         redirection.

         The Internet Group Management Protocol (IGMP) is an Internet
         layer protocol used for establishing dynamic host groups for IP
         multicasting.

         The Internet layer protocols IP, ICMP, and IGMP are discussed
         in chapter 4.

      o  Link Layer
         To communicate on its directly-connected network, a host must
         implement the communication protocol used to interface to that
         network.  We call this a Link Layer layer protocol.


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RFC 1716          Towards Requirements for IP Routers      November 1994


         Some older Internet documents refer to this layer as the
         Network Layer, but it is not the same as the Network Layer in
         the OSI Reference Model.

         This layer contains everything below the Internet Layer.

         Protocols in this Layer are generally outside the scope of
         Internet standardization; the Internet (intentionally) uses
         existing standards whenever possible.  Thus, Internet Link
         Layer standards usually address only address resolution and
         rules for transmitting IP packets over specific Link Layer
         protocols.  Internet Link Layer standards are discussed in
         chapter 3.

2.2.2  Networks

      The constituent networks of the Internet system are required to
      provide only packet (connectionless) transport.  According to the
      IP service specification, datagrams can be delivered out of order,
      be lost or duplicated, and/or contain errors.

      For reasonable performance of the protocols that use IP (e.g.,
      TCP), the loss rate of the network should be very low.  In
      networks providing connection-oriented service, the extra
      reliability provided by virtual circuits enhances the end-end
      robustness of the system, but is not necessary for Internet
      operation.

      Constituent networks may generally be divided into two classes:

        o  Local-Area Networks (LANs)
           LANs may have a variety of designs.  In general, a LAN will
           cover a small geographical area (e.g., a single building or
           plant site) and provide high bandwidth with low delays.  LANs
           may be passive (similar to Ethernet) or they may be active
           (such as ATM).

        o  Wide-Area Networks (WANs)
           Geographically-dispersed hosts and LANs are interconnected by
           wide-area networks, also called long-haul networks.  These
           networks may have a complex internal structure of lines and
           packet-switches, or they may be as simple as point-to-point
           lines.





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RFC 1716          Towards Requirements for IP Routers      November 1994


2.2.3  Routers

      In the Internet model, constituent networks are connected together
      by IP datagram forwarders which are called routers or IP routers.
      In this document, every use of the term router is equivalent to IP
      router.  Many older Internet documents refer to routers as
      gateways.

      Historically, routers have been realized with packet-switching
      software executing on a general-purpose CPU.  However, as custom
      hardware development becomes cheaper and as higher throughput is
      required, but special-purpose hardware is becoming increasingly
      common.  This specification applies to routers regardless of how
      they are implemented.

      A router is connected to two or more networks, appearing to each
      of these networks as a connected host.  Thus, it has (at least)
      one physical interface and (at least) one IP address on each of
      the connected networks (this ignores the concept of un-numbered
      links, which is discussed in section [2.2.7]).  Forwarding an IP
      datagram generally requires the router to choose the address of
      the next-hop router or (for the final hop) the destination host.
      This choice, called routing, depends upon a routing database
      within the router.  The routing database is also sometimes known
      as a routing table or forwarding table.

      The routing database should be maintained dynamically to reflect
      the current topology of the Internet system.  A router normally
      accomplishes this by participating in distributed routing and
      reachability algorithms with other routers.

      Routers provide datagram transport only, and they seek to minimize
      the state information necessary to sustain this service in the
      interest of routing flexibility and robustness.

      Packet switching devices may also operate at the Link Layer; such
      devices are usually called bridges. Network segments which are
      connected by bridges share the same IP network number, i.e., they
      logically form a single IP network.  These other devices are
      outside of the scope of this document.

      Another variation on the simple model of networks connected with
      routers sometimes occurs: a set of routers may be interconnected
      with only serial lines, to form a network in which the packet
      switching is performed at the Internetwork (IP) Layer rather than
      the Link Layer.


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RFC 1716          Towards Requirements for IP Routers      November 1994


2.2.4  Autonomous Systems

      For technical, managerial, and sometimes political reasons, the
      routers of the Internet system are grouped into collections called
      autonomous systems.  The routers included in a single autonomous
      system (AS) are expected to:

      o  Be under the control of a single operations and maintenance
         (O&M) organization;

      o  Employ common routing protocols among themselves, to
         dynamically maintain their routing databases.

      A number of different dynamic routing protocols have been
      developed (see Section [7.2]); the routing protocol within a
      single AS is generically called an interior gateway protocol or
      IGP.

      An IP datagram may have to traverse the routers of two or more ASs
      to reach its destination, and the ASs must provide each other with
      topology information to allow such forwarding.  An exterior
      gateway protocol (generally BGP or EGP) is used for this purpose.

2.2.5  Addresses and Subnets

      An IP datagram carries 32-bit source and destination addresses,
      each of which is partitioned into two parts - a constituent
      network number and a host number on that network.  Symbolically:

         IP-address  ::=  { <Network-number>, <Host-number> }

      To finally deliver the datagram, the last router in its path must
      map the Host-number (or rest) part of an IP address into the
      physical address of a host connection to the constituent network.

      This simple notion has been extended by the concept of subnets,
      which were introduced in order to allow arbitrary complexity of
      interconnected LAN structures within an organization, while
      insulating the Internet system against explosive growth in network
      numbers and routing complexity.  Subnets essentially provide a
      multi-level hierarchical routing structure for the Internet
      system.  The subnet extension, described in [INTERNET:2], is now a
      required part of the Internet architecture.  The basic idea is to
      partition the <Host-number> field into two parts: a subnet number,
      and a true host number on that subnet:

         IP-address  ::=


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RFC 1716          Towards Requirements for IP Routers      November 1994


           { <Network-number>, <Subnet-number>, <Host-number> }

      The interconnected physical networks within an organization will
      be given the same network number but different subnet numbers.
      The distinction between the subnets of such a subnetted network is
      normally not visible outside of that network.  Thus, routing in
      the rest of the Internet will be based only upon the <Network-
      number> part of the IP destination address; routers outside the
      network will combine <Subnet-number> and <Host-number> together to
      form an uninterpreted rest part of the 32-bit IP address.  Within
      the subnetted network, the routers must route on the basis of an
      extended network number:

         { <Network-number>, <Subnet-number> }

      Under certain circumstances, it may be desirable to support
      subnets of a particular network being interconnected only via a
      path which is not part of the subnetted network.  Even though many
      IGP's and no EGP's currently support this configuration
      effectively, routers need to be able to support this configuration
      of subnetting (see Section [4.2.3.4]).  In general, routers should
      not make assumptions about what are subnets and what are not, but
      simply ignore the concept of Class in networks, and treat each
      route as a { network, mask }-tuple.

      DISCUSSION:
         It is becoming clear that as the Internet grows larger and
         larger, the traditional uses of Class A, B, and C networks will
         be modified in order to achieve better use of IP's 32-bit
         address space.  Classless Interdomain Routing (CIDR)
         [INTERNET:15] is a method currently being deployed in the
         Internet backbones to achieve this added efficiency.  CIDR
         depends on the ability of assigning and routing to networks
         that are not based on Class A, B, or C networks.  Thus, routers
         should always treat a route as a network with a mask.

      Furthermore, for similar reasons, a subnetted network need not
      have a consistent subnet mask through all parts of the network.
      For example, one subnet may use an 8 bit subnet mask, another 10
      bit, and another 6 bit.  Routers need to be able to support this
      type of configuration (see Section [4.2.3.4]).

      The bit positions containing this extended network number are
      indicated by a 32-bit mask called the subnet mask; it is
      recommended but not required that the <Subnet-number> bits be
      contiguous and fall between the <Network-number> and the <Host-
      number> fields.  No subnet should be assigned the value zero or -1


Almquist & Kastenholz                                          [Page 19]

RFC 1716          Towards Requirements for IP Routers      November 1994


      (all one bits).

      Although the inventors of the subnet mechanism probably expected
      that each piece of an organization's network would have only a
      single subnet number, in practice it has often proven necessary or
      useful to have several subnets share a single physical cable.

      There are special considerations for the router when a connected
      network provides a broadcast or multicast capability; these will
      be discussed later.

2.2.6  IP Multicasting

      IP multicasting is an extension of Link Layer multicast to IP
      internets.  Using IP multicasts, a single datagram can be
      addressed to multiple hosts. This collection of hosts is called a
      multicast group.  Each multicast group is represented as a Class D
      IP address.  An IP datagram sent to the group is to be delivered
      to each group member with the same best-effort delivery as that
      provided for unicast IP traffic.  The sender of the datagram does
      not itself need to be a member of the destination group.

      The semantics of IP multicast group membership are defined in
      [INTERNET:4].  That document describes how hosts and routers join
      and leave multicast groups.  It also defines a protocol, the
      Internet Group Management Protocol (IGMP), that monitors IP
      multicast group membership.

      Forwarding of IP multicast datagrams is accomplished either
      through static routing information or via a multicast routing
      protocol.  Devices that forward IP multicast datagrams are called
      multicast routers. They may or may not also forward IP unicasts.
      In general, multicast datagrams are forwarded on the basis of both
      their source and destination addresses.  Forwarding of IP