rfc1812.txt

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   When we say that a parameter must be configurable, we do not intend
   to require that its value be explicitly read from a configuration
   file at every boot time.  For many parameters, there is one value
   that is appropriate for all but the most unusual situations.  In such
   cases, it is quite reasonable that the parameter default to that
   value if not explicitly set.

   This memo requires a particular value for such defaults in some
   cases.  The choice of default is a sensitive issue when the
   configuration item controls accommodation of existing, faulty,
   systems.  If the Internet is to converge successfully to complete
   interoperability, the default values built into implementations must
   implement the official protocol, not misconfigurations to accommodate
   faulty implementations.  Although marketing considerations have led
   some vendors to choose misconfiguration defaults, we urge vendors to
   choose defaults that will conform to the standard.

   Finally, we note that a vendor needs to provide adequate
   documentation on all configuration parameters, their limits and
   effects.








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1.4 Algorithms

   In several places in this memo, specific algorithms that a router
   ought to follow are specified.  These algorithms are not, per se,
   required of the router.  A router need not implement each algorithm
   as it is written in this document.  Rather, an implementation must
   present a behavior to the external world that is the same as a
   strict, literal, implementation of the specified algorithm.

   Algorithms are described in a manner that differs from the way a good
   implementor would implement them.  For expository purposes, a style
   that emphasizes conciseness, clarity, and independence from
   implementation details has been chosen.  A good implementor will
   choose algorithms and implementation methods that produce the same
   results as these algorithms, but may be more efficient or less
   general.

   We note that the art of efficient router implementation is outside
   the scope of this memo.

2. INTERNET ARCHITECTURE

   This chapter does not contain any requirements.  However, it does
   contain useful background information on the general architecture of
   the Internet and of routers.

   General background and discussion on the Internet architecture and
   supporting protocol suite can be found in the DDN Protocol Handbook
   [ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and
   [ARCH:4].  The Internet architecture and protocols are also covered
   in an ever-growing number of textbooks, such as [ARCH:5] and
   [ARCH:6].

2.1 Introduction

   The Internet system consists of a number of interconnected packet
   networks supporting communication among host computers using the
   Internet protocols.  These protocols include the Internet Protocol
   (IP), the Internet Control Message Protocol (ICMP), the Internet
   Group Management Protocol (IGMP), and a variety transport and
   application protocols that depend upon them.  As was described in
   Section [1.2], the Internet Engineering Steering Group periodically
   releases an Official Protocols memo listing all the Internet
   protocols.

   All Internet protocols use IP as the basic data transport mechanism.
   IP is a datagram, or connectionless, internetwork service and
   includes provision for addressing, type-of-service specification,



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   fragmentation and reassembly, and security.  ICMP and IGMP are
   considered integral parts of IP, although they are architecturally
   layered upon IP.  ICMP provides error reporting, flow control,
   first-hop router redirection, and other maintenance and control
   functions.  IGMP provides the mechanisms by which hosts and routers
   can join and leave IP multicast groups.

   Reliable data delivery is provided in the Internet protocol suite by
   Transport Layer protocols such as the Transmission Control Protocol
   (TCP), which provides end-end retransmission, resequencing and
   connection control.  Transport Layer connectionless service is
   provided by the User Datagram Protocol (UDP).

2.2 Elements of the Architecture

2.2.1 Protocol Layering

   To communicate using the Internet system, a host must implement the
   layered set of protocols comprising the Internet protocol suite.  A
   host typically must implement at least one protocol from each layer.

   The protocol layers used in the Internet architecture are as follows
   [ARCH:7]:

   o Application Layer
      The Application Layer is the top layer of the Internet protocol
      suite.  The Internet suite does not further subdivide the
      Application Layer, although some application layer protocols do
      contain some internal sub-layering.  The application layer of the
      Internet suite essentially combines the functions of the top two
      layers - Presentation and Application - of the OSI Reference Model
      [ARCH:8].  The Application Layer in the Internet protocol suite
      also includes some of the function relegated to the Session Layer
      in the OSI Reference Model.

      We distinguish two categories of application layer protocols: user
      protocols that provide service directly to users, and support
      protocols that provide common system functions.  The most common
      Internet user protocols are:

      - Telnet (remote login)
      - FTP (file transfer)
      - SMTP (electronic mail delivery)

      There are a number of other standardized user protocols and many
      private user protocols.





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      Support protocols, used for host name mapping, booting, and
      management include SNMP, BOOTP, TFTP, the Domain Name System (DNS)
      protocol, and a variety of routing protocols.

      Application Layer protocols relevant to routers are discussed in
      chapters 7, 8, and 9 of this memo.

   o Transport Layer
      The Transport Layer provides end-to-end communication services.
      This layer is roughly equivalent to the Transport Layer in the OSI
      Reference Model, except that it also incorporates some of OSI's
      Session Layer establishment and destruction functions.

      There are two primary Transport Layer protocols at present:

      - Transmission Control Protocol (TCP)
      - User Datagram Protocol (UDP)

      TCP is a reliable connection-oriented transport service that
      provides end-to-end reliability, resequencing, and flow control.
      UDP is a connectionless (datagram) transport service.  Other
      transport protocols have been developed by the research community,
      and the set of official Internet transport protocols may be
      expanded in the future.

      Transport Layer protocols relevant to routers are discussed in
      Chapter 6.

   o Internet Layer
      All Internet transport protocols use the Internet Protocol (IP) to
      carry data from source host to destination host.  IP is a
      connectionless or datagram internetwork service, providing no
      end-to-end delivery guarantees.  IP datagrams may arrive at the
      destination host damaged, duplicated, out of order, or not at all.
      The layers above IP are responsible for reliable delivery service
      when it is required.  The IP protocol includes provision for
      addressing, type-of-service specification, fragmentation and
      reassembly, and security.

      The datagram or connectionless nature of IP is a fundamental and
      characteristic feature of the Internet architecture.

      The Internet Control Message Protocol (ICMP) is a control protocol
      that is considered to be an integral part of IP, although it is
      architecturally layered upon IP - it uses IP to carry its data
      end-to-end.  ICMP provides error reporting, congestion reporting,
      and first-hop router redirection.




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      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 a directly connected network, a host must
      implement the communication protocol used to interface to that
      network.  We call this a Link Layer protocol.

      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 and above
      the Physical Layer (which is the media connectivity, normally
      electrical or optical, which encodes and transports messages).
      Its responsibility is the correct delivery of messages, among
      which it does not differentiate.

      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.  LANs normally cover a small
        geographical area (e.g., a single building or plant site) and
        provide high bandwidth with low delays.  LANs may be passive



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        (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.

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