path: root/doc/rfc/rfc985.txt

Network Working Group                   Network Technical Advisory Group
Request for Comments: 985                                            NSF
                                                                May 1986

              Requirements for Internet Gateways -- Draft


Status of this Memo

   This RFC summarizes the requirements for gateways to be used on
   networks supporting the DARPA Internet protocols.  While it applies
   specifically to National Science Foundation research programs, the
   requirements are stated in a general context and are believed
   applicable throughout the Internet community.  This document was
   prepared by the Gateway Requirements Subcommittee of the NSF Network
   Technical Advisory Group in cooperation with the Internet Activities
   Board, Internet Architecture Task Force and Internet Engineering Task
   Force.  It requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.

   The purpose of this document is to present guidance for vendors
   offering products that might be used or adapted for use in an
   Internet application.  It enumerates the protocols required and gives
   references to RFCs and other documents describing the current
   specifications.  In a number of cases the specifications are evolving
   and may contain ambiguous or incomplete information.  In these cases
   further discussion giving specific guidance is included in this
   document.  Specific policy issues relevant to the NSF scientific
   networking community are summarized in an Appendix.

   *********************************************************************

      This is a DRAFT edition of this statement of gateway requirements.
      Comments are sought on this document for consideration and
      possibly incorporated in the final edition.  Comments are
      especially sought from those actually developing gateways,
      particular vendors and potential vendors of gateways.  The period
      for comments is 90 days ending 15-Aug-86, at which time revised
      edition will be issued with a new RFC number.

   *********************************************************************

   Suggestions and comments on this document can be sent to the
   subcommittee chairman Dave Mills (mills@usc-isid.arpa), or NTAG
   committee chairman Dave Farber (farber@huey.udel.edu).  The
   subcommittee members, present affiliations and Internet mailboxes are
   as follows:

      Hank Dardy, NRL                 dardy@nrl.arpa
      Dave Farber, U Delaware         farber@huey.udel.edu
      Dennis Jennings, JVNC         jennings%pucc.bitnet@wiscvm.wisc.edu


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      Larry Landweber, U Wisconsin    landweber@rsch.wisc.edu
      Tony Lauck, DEC                 rhea!bergil!lauck@decwrl.arpa
      Dave Mills (Chairman), Linkabit mills@usc-isid.arpa
      Dennis Perry, DARPA/IPTO        perry@ipto.arpa

   The subcommittee wishes to thank the following additional
   contributors and invited referees:

      Len Bosack, Stanford U/CISCO    bosack@su-score.arpa
      Bob Braden, ISI                 braden@isi-braden.arpa
      Hans-Werner Braun, U Michigan   hwb@gw.umich.edu
      Noel Chiappa, MIT/Proteon       jnc@proteon.arpa
      Doug Comer, Purdue U            dec@cs.purdue.edu
      Ira Fuchs, Princeton U          fuchs%pucc.bitnet@wiscvm.wisc.edu
      Ed Krol, U Illinois            krol%uiucvmd.bitnet@wiscvm.wisc.edu
      Barry Leiner, RIACS             leiner@riacs.arpa
      Mike Muuss, BRL                 mike@brl.arpa
      Ron Natalie, BRL                ron@brl.arpa
      Harvey Newman, CIT              newman@cit-hex.arpa
      Jon Postel, ISI                 postel@usc-isib.arpa
      Marshall Rose, NRTC             mrose@nrtc-gremlin.northrop.com
      Jeff Schiller, MIT              jis@bitsy.mit.edu
      Lixia Zhang, MIT                lixia@xx.lcs.mit.edu

1.  Introduction

   The following sections are intended as an introduction and background
   for those unfamiliar with the DARPA Internet architecture and the
   Internet gateway model.  General background and discussion on the
   Internet architecture and supporting protocol suite can be found in
   the DDN Protocol Handbook [25] and ARPANET Information Brochure [26],
   both available from the Network Information Center, SRI
   International, Menlo Park, CA 94025.  Readers familiar with these
   concepts can proceed directly to Section 2.

   1.1.  The DARPA Internet Architecture

      The DARPA Internet system consists of a number of gateways and
      networks that collectively provide packet transport for hosts
      subscribing to the DARPA Internet protocol architecture.  These
      protocols include the Internet Protocol (IP), Internet Control
      Message Protocol (ICMP), Transmission Control Protocol (TCP) and
      application protocols depending upon them.  All protocols use IP
      as the basic packet-transport mechanism.  IP is a datagram, or
      connectionless, service and includes provision for service
      specification, fragmentation/reassembly and security information.
      ICMP is considered an integral part of IP, although it is


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      architecturally layered upon it.  ICMP provides error reporting,
      flow control and first-hop gateway redirection.  Reliable data
      delivery is provided in the protocol suite by TCP, which provides
      end-end retransmission, resequencing and connection control.
      Connectionless service is provided by the User Datagram Protocol
      (UDP).

      The Internet community presently includes several thousand hosts
      connected to over 400 networks with about 120 gateways.  There are
      now well over 2400 hosts registered in the ARPA domain alone and
      an unknown number registered in other domains, with the total
      increasing at about ten percent each month.  Many of the hosts,
      gateways and networks in the Internet community are administered
      by civil organizations, including universities, research
      laboratories and equipment manufacturers.  Most of the remainder
      are administered by the US DoD and considered part of the DDN
      Internet, which presently consists of three sets of networks: the
      experimental segment, or ARPANET, the unclassified segment, or
      MILNET, and the classified segment, which does not yet have a
      collective name.

      The Internet model includes constituent networks, called local
      networks to distinguish them from the Internet system as a whole,
      which are required only to provide datagram (connectionless)
      transport.  This requires only best-effort delivery of individual
      packets, or datagrams.  Each datagram carries 32-bit source and
      destination addresses, which are encoded in three formats
      providing a two-part address, one of which is the local-network
      number and the other the host number on that local net.  According
      to the Internet service specification, datagrams can be delivered
      out of order, be lost or duplicated and/or contain errors.  In
      those networks providing connection-oriented service the extra
      reliability provided by virtual circuits enhances the end-end
      robustness of the system, but is not strictly necessary.

      Local networks are connected together in the Internet model by
      means of Internet gateways.  These gateways provide datagram
      transport only and normally seek to minimize the state information
      necessary to sustain this service in the interest of routing
      flexibility and robustness.  In the conventional model the gateway
      has a physical interface and address on each of the local nets
      between which it provides forwarding services.  The gateway also
      participates in one or more distributed routing or reachability
      algorithm such as the Gateway-Gateway Protocol (GGP) or Exterior
      Gateway Protocol (EGP) in order to maintain its routing tables.




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   1.2.  The Internet Gateway Model

      An Internet gateway is a self-contained, stand-alone packet switch
      that performs the following functions:

         1.  Interfaces to two or more packet-switching networks,
             including encapsulation, address transformation and flow
             control.

         2.  Conforms to specific DARPA Internet protocols specified in
             this document, including the Internet Protocol (IP),
             Internet Control Message Protocol (ICMP), Exterior Gateway
             Protocol (EGP) and others as necessary.

         3.  Supports an interior gateway protocol (IGP) reachability or
             routing algorithm in cases of multiple gateways operating
             as a system.  Supports the EGP reachability algorithm to
             exchange routes between systems, in particular the DARPA
             "core" system operated by BBN.

         4.  Receives and forwards Internet datagrams consistent with
             good engineering practice in the management of resources,
             congestion control and fairness.  Recognizes various error
             conditions and generates ICMP error and information
             messages as required.

         5.  Provides system support facilities, including loading,
             debugging, status reporting, exception reporting and
             control.

      In some configurations gateways may be connected to
      packet-switching local nets that provide generic local-net
      routing, error-control and resource-management functions.  In
      others gateways may be directly connected via serial lines, so
      that these functions must be provided by the gateways themselves.

      There are three typical scenarios that should be addressed by
      gateway vendors:

         1.  National or regional network.  Gateways of this class
             should be capable of switching multiple continuous flows in
             the 1.5-Mbps range at rates to several thousand packets per
             second.  They will be high-performance, possibly redundant,
             multiple-processor devices, probably procured as a system
             and operated remotely from a regional or national
             monitoring center.  The design of these gateways should
             emphasize high aggregate throughput, throughput-sensitive


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             resource management and very high reliability.  The typical
             application would be an NSF backbone net or one of the
             consortium or regional nets.

         2.  Campus network.  Gateways of this class should be capable
             of switching some burst flows at 10-Mbps (Ethernets, etc.),
             together with some flows in the 64-Kbps range or lower, at
             rates to perhaps several thousand packets per second.  They
             will be medium-performance devices, probably competitively
             procured from different vendors for each campus and
             operated from 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.  Their chief function might
             be to interconnect various LANs and campus computing
             resources, including a high-speed interconnect to a
             national or regional net.  An important factor will be a
             very flexible routing mechanism, since these gateways may
             have to select among several backbone nets based on
             cost/performance considerations.

         3.  Department network.  Gateways of this class should be
             capable of switching a small number of burst flows at
             10-Mbps (Ethernets, etc.), together with a small number of
             flows in the range 64-Kbps or lower, at rates of a few
             hundred packets per second.  They will be
             medium-performance devices procured from a variety of
             vendors and used for protocol-matching, LAN repeaters and
             as general utility packet switches.  They will probably be
             locally maintained by the various users and not be used as
             transit switches.

      It is important to realize that Internet gateways normally operate
      in an unattended mode, but that equipment and software faults can
      affect the entire Internet.  While some of the above scenarios
      involve positive control of some gateways from a monitoring
      center, usually via a path involving other networks and Internet
      gateways, others may involve much less formal control procedures.
      Thus the gateways must be highly robust and be expected to
      operate, possibly in a degraded state, under conditions of extreme
      congestion or failure of network resources.








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2.  Protocols Required

   The Internet architecture uses datagram gateways to interconnect
   networks and subnetworks.  These gateways function as intermediate
   systems (IS) with respect to the ISO connectionless network model and
   incorporate defined packet formats, routing algorithms and related
   procedures.  In the following it is assumed the protocol
   implementation supports the full protocol, including all required
   options, with exceptions only as noted.

   2.1.  Internet Protocol (IP)

      This is the basic datagram protocol used in the Internet system.
      It is described in RFC-791 [1] and also MIL-STD-1777 [5], both of
      which are intended to describe the same standard, but in quite
      different words.

      With respect to current gateway requirements the following can be
      ignored, although they may be required in future:  Type of Service
      field, Security option, Stream ID option and Timestamp option.
      However, if recognized, the interpretation of these quantities
      must conform to the standard specification.

      Note that the Internet gateway model does not require that the
      gateway reassemble IP datagrams with destination address other
      than the gateway itself.  However, in the case of those protocols
      in which the gateway directly participates as a peer, including
      routing and monitor/control protocols, the gateway may have to
      reassemble datagrams addressed to it.  This consideration is most
      pertinent to EGP.

      Note that, of the five classes of IP addresses.  Class-A through
      Class-E, Class-D and Class-E addresses are reserved for
      experimental use.  A gateway which is not participating in these
      experiments should ignore all packets with a Class-D or Class-E
      destination IP address.  No ICMP Destination Unreachable or ICMP
      Redirect messages should result from receiving such packets.

   2.2.  Internet Control Message Protocol (ICMP)

      This is an auxiliary protocol used to convey advice and error
      messages and is described in RFC-792 [2].

      The distinction between subnets of a subnetted network, which
      depends on an arbitrary mask as described in RFC-950 [21], is in
      general not visible outside that network.  This distinction is
      important in the case of certain ICMP messages, including the ICMP


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      Destination Unreachable and ICMP Redirect messages.  The ICMP
      Destination Unreachable message is sent by a gateway in response
      to a datagram which cannot be forwarded because the destination is
      unreachable or down.  A choice of several types of these messages
      is available, including one designating the destination network
      and another the destination host. However, the span of addresses
      implied by the former is ill-defined unless the subnet mask is
      known to the sender, which is in general not the case.  It is
      recommended that use of the ICMP Destination Network Unreachable
      messages be avoided.  Instead, an ICMP Destination Host
      Unreachable message should be sent for each distinct unreachable
      IP address.

      The ICMP Redirect message is sent by a gateway to a host in order
      to change the address used by the host for a designated host or
      net.  A choice of four types of messages is available, depending
      on whether it applies to a particular host, network or service.
      As in the previous case, these distinctions may depend upon the
      subnet mask.  As in the above case, it is recommended that the use
      of ICMP messages implying a span of addresses (e.g.  net
      unreachable, net redirect) be avoided in favor of those implying
      specific addresses (e.g.  host unreachable, host redirect).

      The ICMP Source Quench message has been the subject of much
      controversy.  It is not considered realistic at this time to
      specify in detail the conditions under which this message is to be
      generated or interpreted by a host or gateway.

      New host and gateway implementations are expected to support the
      ICMP Address Mask messages described in RFC-950.  It is highly
      desirable, although not required, to provide correct data for ICMP
      Timestamp messages, which have been found useful in network
      debugging and maintenance.

   2.3.  Exterior Gateway Protocol (EGP)

      This is the basic protocol used to exchange information between
      gateway systems of the Internet and is described in RFC-904 [11].
      However, EGP as presently specified is an asymmetric protocol with
      only the "non-core" procedures defined in RFC-904.  There are at
      present no "core" procedures specified, which would be necessary
      for a stand-alone Internet.  RFC-975 [27] suggests certain
      modifications leading to a symmetric model;  however, this is not
      an official specification.

      In principle, a stand-alone Internet can be built with non-core
      EGP gateways using the EGP distance field to convey some metric


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      such as hop count.  However, the use of EGP in this way as a
      routing algorithm is discouraged, since typical implementations
      adapt very slowly to changing topology and have no loop-protection
      features.

      The EGP model requires each gateway belong to an autonomous system
      of gateways.  If a routing algorithm is operated in one or more
      gateways of an autonomous system, its data base must be coupled to
      the EGP implementation in such a way that, when a net is declared
      down by the routing algorithm, the net is also declared down via
      EGP to other autonomous systems.  This requirement is designed to
      minimize spurious traffic to "black holes" and insure fair
      utilization of the resources on other systems.

      There are no peer-discovery or authentication procedures defined
      in the present EGP specification and no defined interpretation of
      the distance fields in the update messages, although such
      procedures may be defined in future (see RFC-975).  There is
      currently no guidance on the selection of polling parameters and
      no specific recovery procedures in case of certain error messages
      (e.g.  "administratively prohibited").  It is recommended that EGP
      implementations include provisions to initialize these parameters
      as part of the monitoring and control procedures and that changing
      these procedures not require recompilation or rebooting the
      gateway.

   2.4.  Address Resolution Protocol (ARP)

      This is an auxiliary protocol used to manage the
      address-translation function between hardware addresses in a
      local-net environment and Internet addresses and described in
      RFC-826 [4].  However, there are a number of unresolved issues
      having to do with subnets and response to addresses not in the
      same subnet or net.  These issues, which are intertwined with ICMP
      and various gateway models, are discussed in Appendix A.

3.  Subnets

   The concept of subnets was 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.  The subnet architecture,
   described in RFC-950 [21], is intended to specify a standard approach
   that does not require reconfiguration for host implementations,
   regardless of subnetting scheme.  The document also specifies a new




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   ICMP Address Mask message, which a gateway can use to specify certain
   details of the subnetting scheme to hosts and is required in new host
   and gateway implementations.

   The current subnet specification RFC-950 does not describe the
   specific procedures to be used by the gateway, except by implication.
   It is recommended that a (sub)net address and address mask be
   provided for each network interface and that these values be
   established as part of the gateway configuration procedure.  It is
   not usually necessary to change these values during operation of any
   particular gateway; however, it should be possible to add new
   gateways and/or (sub)nets and make other configuration changes to a
   gateway without taking the entire network down.

4.  Local Network Interface

   The packet format used for transmission of datagrams on the various
   subnetworks is described in a number of documents summarized below.

   4.1.  Public data networks via X.25

      The formats specified for public data networks via X.25 access are
      described in RFC-877 [8].  Datagrams are transmitted over standard
      level-3 virtual circuits as complete packet sequences.  Virtual
      circuits are usually established dynamically as required and time
      out after a period of no traffic.  Retransmission, resequencing
      and flow control are performed by the network for each virtual
      circuit and by the LAPB link-level protocol.  Multiple parallel
      virtual circuits are often used in order to improve the
      utilization of the subscriber access line, which can result in
      random resequencing.  The correspondence between Internet and
      X.121 addresses is usually established by table-lookup.  It is
      expected that this will be replaced by some sort of directory
      procedure in future.

   4.2.  ARPANET via 1822 Local Host, Distant Host or HDLC Distant Host

      The formats specified for ARPANET networks via 1822 access are
      described in BBN Report 1822 [3], which includes the procedures
      for several subscriber access methods.  The Local Host (LH) and
      Very Distant Host (VDH) methods are not recommended for new
      implementations.  The Distant Host (DH) method is used when the
      host and IMP are separated by not more than about 2000 feet of
      cable, while the HDLC Distant Host is used for greater distances
      where a modem is required.  Retransmission, resequencing and flow
      control are performed by the network and by the HDLC link-level
      protocol, when used.  While the ARPANET 1822 protocols are widely


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      used at present, they are expected to be eventually overtaken by
      the DDN Standard X.25 protocol (see below) and the new PSN
      End-to-End Protocol described in RFC-979 [29].

      While the cited report gives details of the various ARPANET
      subscriber access methods, it specifies neither the IP packet
      encapsulation format nor address mappings.  While these are
      generally straightforward and easy to implement, the details
      involve considerations beyond the scope of readily accessable
      documentation. Potential vendors are encouraged to contact one of
      the individuals listed at the beginning of this document for
      further information.

      Gateways connected to ARPANET/MILNET IMPs must incorporate
      features to avoid host-port blocking (RFNM counting) and to detect
      and report (as ICMP Unreachable messages) the failure of
      destination hosts or gateways.

   4.3.  ARPANET via DDN Standard X.25

      The formats specified for ARPANET networks via X.25 are described
      in the Defense Data Network X.25 Host Interface Specification [6].
      This document describes two sets of procedures, the DDN Basic X.25
      and the DDN Standard X.25, but only the latter is suitable for use
      in the Internet system.  The DDN Standard X.25 procedures are
      similar to the public data subnetwork X.25 procedures, except in
      the address mappings. Retransmission, resequencing and flow
      control are performed by the network and by the LAPB link-level
      protocol.

   4.4.  Ethernets

      The formats specified for Ethernet networks are described in
      RFC-894 [10].  Datagrams are encapsulated as Ethernet packets with
      48-bit source and destination address fields and a 16-bit type
      field. Address translation between Ethernet addresses and Internet
      addresses is managed by the Address Resolution Protocol, which is
      required in all Ethernet implementations.  There is no explicit
      retransmission, resequencing or flow control.  although most
      hardware interfaces will retransmit automatically in case of
      collisions on the cable.

      It is expected that amendments will be made to this specification
      as the result of IEEE 802.3 evolution.  See RFC-948 [20] for
      further discussion and recommendations in this area.  Note also
      that the IP broadcast address, which has primary application to
      Ethernets and similar technologies that support an inherent


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      broadcast function, has an all-ones value in the host field of the
      IP address.  Some early implementations chose the all-zeros value
      for this purpose, which is presently not in conformance with the
      definitive specification RFC-950 [21].

      See Appendix A for further considerations.

   4.5.  Serial-Line Protocols

      Gateways may be used as packet switches in order to build
      networks. In some configurations gateways may be interconnected
      with each other and some hosts by means of serial asynchronous or
      synchronous lines, with or without modems.  When justified by the
      expected error rate and other factors, a link-level protocol may
      be required on the serial line. While there is no requirement that
      a particular standard protocol be used for this, it is recommended
      that standard hardware and protocols be used, unless a convincing
      reason to the contrary exists.  In order to support the greatest
      variety of configurations, it is recommended that some variation
      on full X.25 (i.e.  "symmetric mode") be used where resources
      permit;  however, X.25 LAPB would also be acceptable where
      requirements permit.  In the case of asynchronous lines no clear
      choice is apparent.

5.  Interoperability

   In order to assure interoperability between gateways procured from
   different vendors, it is necessary to specify points of protocol
   demarcation.  With respect to interoperability of the routing
   function, this is specified as EGP.  All gateway systems must include
   one or more gateways which support EGP with a core gateway, as
   described in RFC-904 [11].  It is desirable that these gateways be
   able to operate in a mode that does not require a core gateway or
   system.  Additional discussion on these issues can be found in
   RFC-975 [27].

   With respect to the interoperability at the network layer and below,
   two points of protocol demarcation are specified, one for Ethernets
   and the other for serial lines.  In the case of Ethernets the
   protocols are as specified in Section 4.4 and Appendix A of this
   document.  For serial lines between gateways of different vendors,
   the protocols are specified in Section 4.5 of this document.
   Exceptions to these requirements may be appropriate in some cases.






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6.  Subnetwork Architecture

   It is recognized that gateways may also function as general packet
   switches to build networks of modest size.  This requires additional
   functionality in order to manage network routing, control and
   configuration.  While it is beyond the scope of this document to
   specify the details of the mechanisms used in any particular, perhaps
   proprietary, architecture, there are a number of basic requirements
   which must be provided by any acceptable architecture.

   6.1.  Reachability Procedures

      The architecture must provide a robust mechanism to establish the
      operational status of each link and node in the network, including
      the gateways, the links connecting them and, where appropriate,
      the hosts as well.  Ordinarily, this requires at least a
      link-level reachability protocol involving a periodic exchange of
      hello messages across each link.  This function might be intrinsic
      to the link-level protocols used (e.g.  LAPB, DDCMP).  However, it
      is in general ill-advised to assume a host or gateway is operating
      correctly if its link-level reachability protocol is operating
      correctly.  Additional confirmation is required in the form of an
      operating routing algorithm or peer-level reachability protocol,
      such as used in EGP.

      Failure and restoration of a link and/or gateway are considered
      network events and must be reported to the control center.  It is
      desirable, although not required, that reporting paths not require
      correct functioning of the routing algorithm itself.

   6.2.  Routing Algorithm

      It has been the repeated experience of the Internet community
      participants that the routing mechanism, whether static or
      dynamic, is the single most important engineering issue in network
      design.  In all but trivial network topologies it is necessary
      that some degree of routing dynamics is vital to successful
      operation, whether it be affected by manual or automatic means or
      some combination of both.  In particular, if routing changes are
      made manually, the changes must be possible without taking down
      the gateways for reconfiguration and, preferably, be possible from
      a remote site such as a control center.

      It is not likely that all nets can be maintained from a
      full-service control center, so that automatic-fallback or
      rerouting features may be required.  This must be considered the
      normal case, so that systems of gateways operating as the only


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      packet switches in a network would normally be expected to have a
      routing algorithm with the capability of reacting to link and
      other gateway failures and changing the routing automatically.
      Following is a list of features considered necessary:

         1.  The algorithm must sense the failure or restoration of a
             link or other gateway and switch to appropriate paths
             within an interval less than the typical TCP user timeout
             (one minute is a safe assumption).

         2.  The algorithm must never form routing loops between
             neighbor gateways and must contain provisions to avoid and
             suppress routing loops that may form between non-neighbor
             gateways.  In no case should a loop persist for longer than
             an interval greater than the typical TCP user timeout.

         3.  The control traffic necessary to operate the routing
             algorithm must not significantly degrade or disrupt normal
             network operation. Changes in state which might momentarily
             disrupt normal operation in a local area must not cause
             disruption in remote areas of the network.

         4.  As the size of the network increases, the demand on
             resources must be controlled in an efficient way.  Table
             lookups should be hashed, for example, and data-base
             updates handled piecemeal, with only the changes broadcast
             over a wide area.  Reachability and delay metrics, if used,
             must not depend on direct connectivity to all other
             gateways or the use of network-specific broadcast
             mechanisms. Polling procedures (e.g.  for consistency
             checking) should be used only sparingly and in no case
             introduce an overhead exceeding a constant independent of
             network topology times the longest non-looping path.

         5.  The use of a default gateway as a means to reduce the size
             of the routing data base is strongly discouraged in view of
             the many problems with multiple paths, loops and
             mis-configuration vulnerabilities.  If used at all, it
             should be limited to a discovery function, with operational
             routes cached from external or internal data bases via
             either the routing algorithm or EGP.

         6.  This document places no restriction on the type of routing
             algorithm, such as node-based, link-based or any other
             algorithm, or metric, such as delay or hop-count.  However,
             the size of the routing data base must not be allowed to
             exceed a constant independent of network topology times the


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Requirements for Internet Gateways -- DRAFT


             number of nodes times the mean connectivity (average number
             of incident links).  An advanced design would not require
             that the entire routing data base be kept in any particular
             gateway, so that discovery and caching techniques would be
             necessary.

7.  Operation and Maintenance

   Gateways and packets switches are often operated as a system by some
   organization who agrees to operate and maintain the gateways, as well
   as to resolve link problems with the respective common carriers. It
   is important to note that the network control site may not be
   physically attached to the network being monitored.  In general, the
   following requirements apply:

      1.  Each gateway must operate as a stand-alone device for the
          purposes of local hardware maintenance.  Means must be
          available to run diagnostic programs at the gateway site using
          only on-site tools, which might be only a diskette or tape and
          local terminal.  It is desirable, although not required, to
          run diagnostics via the network and to automatically reboot
          and dump the gateway via the net in case of fault.  In
          general, this requires special hardware.

          The use of full-blown transport services such as TCP is in
          general ill-advised if required just to reboot and dump the
          gateway. Consideration should be given simple
          retransmission-overlay protocols based on UDP or specific
          monitoring protocols such as HMP described in RFC-869 [7].

      2.  It must be possible to reboot and dump the gateway manually
          from the control site.  Every gateway must include a watchdog
          timer that either initiates a reboot or signals a remote
          control site if not reset periodically by the software.  It is
          desirable that the data involved reside at the control site
          and be transmitted via the net; however, the use of local
          devices at the gateway site is acceptable. Nevertheless, the
          operation of initiating reboot or dump must be possible via
          the net, assuming a path is available and the connecting links
          are operating.

      3.  A mechanism must be provided to accumulate traffic statistics
          including, but not limited to, packet tallies, error-message
          tallies and so forth.  The preferred method of retrieving
          these data is by explicit, periodic request from the control
          site using a standard datagram protocol based on UDP or HMP.



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          The use of full-blown transport services such as TCP is in
          general ill-advised if required just to collect statistics
          from the gateway. Consideration should be given simple
          retransmission-overlay protocols based on UDP or HMP.

      4.  Exception reports ("traps") occuring as the result of hardware
          or software malfunctions should be transmitted immediately
          (batched to reduce packet overheads when possible) to the
          control site using a standard datagram protocol based on UDP
          or HMP.

      5.  A mechanism must be provided to display link and node status
          on a continuous basis at the control site.  While it is
          desirable that a complete map of all links and nodes be
          available, it is acceptable that only those components in use
          by the routing algorithm be displayed.  This information is
          usually available locally at the control site, assuming that
          site is a participant in the routing algorithm.

   The above functions require in general the participation of a control
   site or agent.  The preferred way to provide this is as a user
   program suitable for operation in a standard software environment
   such as Unix.  The program would use standard IP protocols such as
   TCP, UDP, and HMP to control and monitor the gateways.  The use of
   specialized host hardware and software requiring significant
   additional investment is strongly discouraged;  nevertheless, some
   vendors may elect to provide the control agent as an integrated part
   of the network in which the gateways are a part.  If this is the
   case, it is required that a means be available to operate the control
   agent from a remote site using Internet protocols and paths and with
   equivalent functionality with respect to a local agent terminal.

   Remote control of a gateway via Internet paths can involve either a
   direct approach, in which the gateway supports TCP and/or UDP
   directly, or an indirect approach, in which the control agent
   supports these protocols and controls the gateway itself using
   proprietary protocols. The former approach is preferred, although
   either approach is acceptable.











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RFC 985                                                         May 1986
Requirements for Internet Gateways -- DRAFT


8.  References and Bibliography

   [1]  Defense Advanced Research Projects Agency, "Internet Protocol",
        DARPA Network Working Group Report RFC-791, USC Information
        Sciences Institute, September 1981.

   [2]  Defense Advanced Research Projects Agency, "Internet Control
        Message Protocol", DARPA Network Working Group Report RFC-792,
        USC Information Sciences Institute, September 1981.

   [3]  Advanced Research Projects Agency, "Interface Message Processor
        - Specifications for the Interconnection of a Host and an IMP",
        BBN Report 1822, Bolt Beranek and Newman, December 1981.

   [4]  Plummer, D., "An Ethernet Address Resolution Protocol", DARPA
        Network Working Group Report RFC-826, Symbolics, September 1982.

   [5]  United States Department of Defense, "Military Standard Internet
        Protocol", Military Standard MIL-STD-1777, August 1983.

   [6]  Defense Communications Agency, "Defense Data Network X.25 Host
        Interface Specification", BBN Communications, December 1983.

   [7]  Hinden, R., "A Host Monitoring Protocol", DARPA Network Working
        Group Report RFC-869, BBN Communications, December 1983.

   [8]  Korb, J.T., "A Standard for the Transmission of IP Datagrams
        over Public Data Networks", DARPA Network Working Group Report
        RFC-877, Purdue University, September 1983.

   [9]  Nagle, J., "Congestion Control in IP/TCP Internetworks", DARPA
        Network Working Group Report RFC-896, Ford Aerospace,
        January 1984.

   [10] Hornig, C., "A Standard for the Transmission of IP Datagrams
        over Ethernet Networks", DARPA Network Working Group Report
        RFC-894, Symbolics, April 1984.

   [11] Mills, D.L., "Exterior Gateway formal Specification", DARPA
        Network Working Group Report RFC-904, M/A-COM Linkabit,
        April 1984.

   [12] Postel, J., and J. Reynolds., "ARPA-Internet Protocol Policy",
        DARPA Network Working Group Report RFC-902, USC Information
        Sciences Institute, July 1984.




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   [13] Kirton, P., "EGP Gateway under Berkeley UNIX 4.2", DARPA Network
        Working Group Report RFC-911, USC Information Sciences
        Institute, August 1984.

   [14] Postel, J., "Multi-LAN Address Resolution", DARPA Network
        Working Group Report RFC-925, USC Information Sciences
        Institute, October 1984.

   [15] International Standards Organization, "Protocol for Providing
        the Connectionless-Mode Network Services", DARPA Network Working
        Group Report RFC-926, International Standards Organization,
        December 1984.

   [16] National Research Council, "Transport Protocols for Department
        of Defense Data Networks", DARPA Network Working Group Report
        RFC-942, National Research Council, March 1985.

   [17] Postel, J., "DOD Statement on NRC Report", DARPA Network Working
        Group Report RFC-945, USC Information Sciences Institute,
        April 1985.

   [18] International Standards Organization, "Addendum to the Network
        Service Definition Covering Network Layer Addressing", DARPA
        Network Working Group Report RFC-941, International Standards
        Organization, April 1985.

   [19] Leiner, B., J. Postel, R. Cole and D. Mills, "The DARPA Internet
        Protocol Suite", Proceedings INFOCOM 85, Washington DC,
        March 1985]  Also in: IEEE Communications Magazine, March 1985.

   [20] Winston, I., "Two Methods for the Transmission of IP Datagrams
        over IEEE 802.3 Networks", DARPA Network Working Group Report
        RFC-948, University of Pennsylvania, June 1985.

   [21] Mogul, J., and J. Postel, "Internet Standard Subnetting
        Procedure", DARPA Network Working Group Report RFC-950, Stanford
        University, August 1985.

   [22] Reynolds, J., and J. Postel, "Official ARPA-Internet Protocols",
        DARPA Network Working Group Report RFC-961, USC Information
        Sciences Institute, October 1985.

   [23] Reynolds, J., and J. Postel, "Assigned Numbers", DARPA Network
        Working Group Report RFC-960, USC Information Sciences
        Institute, December 1985.




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   [24] Nagle, J., "On Packet Switches with Infinite Storage", DARPA
        Network Working Group Report RFC-970, Ford Aerospace,
        December 1985.

   [25] Defense Communications Agency, "DDN Protocol Handbook",
        NIC-50004, NIC-50005, NIC-50006, (three volumes), SRI
        International, December 1985.

   [26] Defense Communications Agency, "ARPANET Information Brochure",
        NIC-50003, SRI International, December 1985.

   [27] Mills, D.L., "Autonomous Confederations", DARPA Network Working
        Group Report RFC-975, M/A-COM Linkabit, February 1986.

   [28] Jacobsen, O., and J. Postel, "Protocol Document Order
        Information",  DARPA Network Working Group Report RFC-980, SRI
        International, March 1986.

   [29] Malis, A.G., "PSN End-to-End Functional Specification", DARPA
        Network Working Group Report RFC-979, BBN Communications,
        March 1986.




























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RFC 985                                                         May 1986
Requirements for Internet Gateways -- DRAFT


Appendix A.  Ethernet Management

   Following is a summary of procedures specified for use by hosts and
   gateways on an Ethernet.

   A.1.  Hardware

      A packet is accepted from the cable only if its destination
      Ethernet address matches either the assigned interface address or
      a broadcast/multicast address.  Presumably, this filtering is done
      by the interface hardware;  however, the software driver is
      expected to do this if the hardware does not.  Some hosts
      incorporate an optional feature that associates an assigned
      multicast address with a specific subnet in order to restrict
      access for testing, etc.  When this feature is activated, the
      assigned multicast address replaces the broadcast address.

   A.2.  IP datagram

      In case of broadcast/multicast (as determined from the destination
      Ethernet address) an IP datagram is discarded if the source IP
      address is not in the same subnet, as determined by the assigned
      host IP address and subnet mask.  It is desirable that this test
      be overridden by a configuration parameter, in order to support
      the infrequent cases where more than one subnet may coexist on the
      same cable.

   A.3.  ARP datagram

      An ARP reply is discarded if the destination IP address does not
      match the local host address.  An ARP request is discarded if the
      source IP address is not in the same subnet.  It is desirable that
      this test be overridden by a configuration parameter, in order to
      support the infrequent cases where more than one subnet may
      coexist on the same cable (see RFC-925 for examples).  An ARP
      reply is generated only if the destination protocol IP address is
      reachable from the local host (as determined by the routing
      algorithm) and the next hop is not via the same interface.  If the
      local host functions as a gateway, this may result in ARP replies
      for destinations not in the same subnet.

   A.4.  ICMP redirect

      An ICMP redirect is discarded if the destination IP address does
      not match the local host address or the new target address is not
      on the same subnet.  An accepted redirect updates the routing data
      base for the old target address.  If there is no route or


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      associated with the old target address, the redirect is ignored.
      If the old route is associated with a default gateway, a new route
      associated with the new target address is inserted in the data
      base.  Note that it is not possible to send a gratuitous redirect
      unless the sender is possessed of considerable imagination.

      When subnets are in use there is some ambiguity as to the scope of
      a redirect, unless all hosts and gateways involved have prior
      knowledge of the subnet masks.  It is recommended that the use of
      ICMP network-redirect messages be avoided in favor of ICMP
      host-redirect messages instead.  This requires the original sender
      (i.e.  redirect recipient) to support a general IP
      address-translation cache, rather than the usual network table.
      However, this is normally done anyway in the case of ARP.

      An ICMP redirect is generated only if the destination IP address
      is reachable from the local host (as determined by the routing
      algorithm) and the next hop is via the same interface and the
      target address is defined in the routing data base.  Redirects
      should never be sent in response to an IP net or subnet broadcast
      address or in response to a Class-D or Class-E IP address.

      ICMP redirects are never forwarded, regardless of destination
      address.  The source IP address of the ICMP redirect itself is not
      checked, since the sending gateway may use one of its addresses
      not on the common net.  The source IP address of the encapsulated
      IP datagram is not checked on the assumption the host or gateway
      sending the original IP datagram knows what it is doing.





















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RFC 985                                                         May 1986
Requirements for Internet Gateways -- DRAFT


Appendix B.  Policy Issues

   The following sections discuss certain issues of special concern to
   the NSF scientific networking community.  These issues have primary
   relevance in the policy area, but also have ramifications in the
   technical area.

   B.1.  Interconnection Technology

      Currently the most important common interconnection technology
      between Internet systems of different vendors is Ethernet.  Among
      the reasons for this are the following:

         1.  Ethernet specifications are well-understood and mature.

         2.  Ethernet technology is in almost all aspects vendor
             independent.

         3.  Ethernet-compatible systems are common and becoming more
             so.

      These advantages combined favor the use of Ethernet technology as
      the common point of demarcation between NSF network systems
      supplied by different vendors, regardless of technology.  It is a
      requirement of NSF gateways that, regardless of the possibly
      proprietary switching technology used to implement a given
      vendor-supplied network, its gateways must support an Ethernet
      attachment to gateways of other vendors.

      It is expected that future NSF gateway requirements will specify
      other interconnection technologies.  The most likely candidates
      are those based on X.25 or IEEE 802, but other technologies
      including broadband cable, fiber-optic or other protocols such as
      DDCMP may also be considered.

   B.2.  Proprietary and Extensible Issues

      Internet technology is a growing, adaptable technology.  Although
      hosts, gateways and networks supporting this technology have been
      in continuous operation for several years, vendors users and
      operators should understand that not all networking issues are
      fully understood. As a result, when new needs or better solutions
      are developed for use in the NSF networking community, it may be
      necessary to field new protocols.  Normally, these new protocols
      will be designed to interoperate in all practical respects with
      existing protocols; however, occasionally it may happen that
      existing systems must be upgraded to support these protocols.


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      NSF systems vendors should understand that they also undertake a
      commitment to remain aware of current Internet technology and be
      prepared to upgrade their products from time to time as
      appropriate.  As a result, these vendors are strongly urged to
      consider extensibility and periodic upgrades as fundamental
      characteristics of their products.  One of the most productive and
      rewarding ways to do this on a long-term basis is to participate
      in ongoing Internet research and development programs in
      partnership with the academic community.

   B.3.  Multi-Protocol Gateways

      Although the present requirements for an NSF gateway specify only
      the Internet protocol suite, it is highly desirable that gateway
      designs allow future extensions to support additional suites and
      allow simultaneous operation with more than a single one.
      Clearly, the ISO protocol suite is a prime candidate for one of
      these suites.  Other candidates include XNS and DECnet.

      Future requirements for NSF gateways may include provisions for
      other protocol suites in addition to Internet, as well as models
      and specifications to interwork between them, should that be
      appropriate.  For instance, it is expected that the ISO suite will
      eventually become the dominant one;  however, it is also expected
      that requirements to support other suites will continue, perhaps
      indefinitely.

      Present NSF gateway requirements do not include protocols above
      the network layer, such as TCP, unless necessary for network
      monitoring or control.  Vendors should recognize that future
      requirements to interwork between Internet and ISO applications,
      for example, may result in an opportunity to market gateways
      supporting multiple protocols at all levels through the
      application level.  It is expected that the network-level NSF
      gateway requirements summarized in this document will be
      incorporated in the requirements document for these
      application-level gateways.

   B.4.  Access Control and Accounting

      There are no requirements for NSF gateways at this time to
      incorporate specific access-control and accounting mechanisms in
      the design;  however, these important issues are currently under
      study and will be incorporated into a redraft of this document at
      an early date.  Vendors are encouraged to plan for the early
      introduction of these mechanisms in their products.  While at this



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RFC 985                                                         May 1986
Requirements for Internet Gateways -- DRAFT


      time no definitive common model for access control and accounting
      has emerged, it is possible to outline some general features such
      a model is likely to have, among them the following:

         1.  The primary access control and accounting executive
             mechanisms will be in the service hosts themselves, not the
             gateways, packet switches or workstations.

         2.  Agents acting on behalf of access control and accounting
             executive mechanisms may be necessary in the gateways,
             packet switches or workstations.  These may be used to
             collect data, enforce password protection or mitigate
             resource priority and fairness.  However, the architecture
             and protocols used by these agents may be a local matter
             and not possible to specify in advance.

         3.  NSF gateways may be required to incorporate access control
             and accounting mechanisms based on packet
             source/destination address, as well as other fields in the
             IP header, internal priority and fairness.  However, it is
             extremely unlikely that these mechanisms would involve a
             user-level login to the gateway itself.



























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