Requirements for Internet Gateways

Status of this Memo

This document is a formal statement of the requirements to be met by gateways used in the Internet system. As such, it is an official specification for the Internet community. Distribution of this memo is unlimited.

This RFC summarizes the requirements for gateways to be used between networks supporting the Internet protocols. While it was written specifically to support National Science Foundation research programs, the requirements are stated in a general context and are applicable throughout the Internet community.

The purpose of this document is to present guidance for vendors offering gateway 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. As other specifications are updated this document will be revised. Vendors are encouraged to maintain contact with the Internet research community.

1. Introduction

The following material is intended as an introduction and background for those unfamiliar with the 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], see also [19, 28, 30, 31].

The Internet protocol architecture was originally developed under DARPA sponsorship to meet both military and civilian communication requirements [32]. The Internet system presently supports a variety of government and government-sponsored operational and research activities. In particular, the National Science Foundation (NSF) is building a major extension to the Internet to provide user access to

RFC 1009 - Requirements for Internet Gateways June 1987

national supercomputer centers and other national scientific resources, and to provide a computer networking capability to a large number of universities and colleges.

In this document there are many terms that may be obscure to one unfamiliar with the Internet protocols. There is not much to be done about that but to learn, so dive in. There are a few terms that are much abused in general discussion but are carefully and intentionally used in this document. These few terms are defined here.

      Packet      A packet is the unit of transmission on a physical
      
                  network.
      
      Datagram    A datagram is the unit of transmission in the IP
                  protocol.  To cross a particular network a datagram is
                  encapsulated inside a packet.
      
      Router      A router is a switch that receives data transmission
                  units from input interfaces and, depending on the
                  addresses in those units, routes them to the
                  appropriate output interfaces.  There can be routers
                  at different levels of protocol.  For example,
                  Interface Message Processors (IMPs) are packet-level
                  routers.
      
      Gateway     In the Internet documentation generally, and in this
                  document specifically, a gateway is an IP-level
                  router.  In the Internet community the term has a long
                  history of this usage [32].

1.1. The DARPA Internet Architecture

1.1.1. Internet Protocols

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 Transmission Control Protocol (TCP), and application protocols depending upon them [22].

All Internet protocols use IP as the basic data transport mechanism. IP [1,31] is a datagram, or connectionless, internetwork service and includes provision for addressing, type-of-service specification, fragmentation and reassembly, and security information. ICMP [2] is considered an integral

RFC 1009 - Requirements for Internet Gateways June 1987

part of IP, although it is architecturally layered upon IP. ICMP provides error reporting, flow control and first-hop gateway redirection.

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

1.1.2. Networks and Gateways

The constituent networks of the Internet system are required only to provide packet (connectionless) transport. This requires only delivery of individual packets. According to the IP service specification, datagrams can be delivered out of order, be lost or duplicated and/or contain errors. Reasonable performance of the protocols that use IP (e.g., TCP) requires an IP datagram loss rate of less than 5%. 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 necessary for Internet operation.

Constituent networks may generally be divided into two classes:

• Local-Area Networks (LANs)

LANs may have a variety of designs, typically based upon buss, ring, or star topologies. 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.

• 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-routers (typified by ARPANET), or they may be as simple as point-to-point lines.

In the Internet model, constituent networks are connected together by IP datagram forwarders which are called "gateways" or "IP routers". In this document, every use of the term "gateway" is equivalent to "IP router". In current practice, gateways are normally realized with packet-switching software

RFC 1009 - Requirements for Internet Gateways June 1987

executing on a general-purpose CPU, but special-purpose

hardware may also be used (and may be required for future

higher-throughput gateways).

A gateway is connected to two or more networks, appearing to each of these networks as a connected host. Thus, it has a physical interface and an IP address on each of the connected networks. Forwarding an IP datagram generally requires the gateway to choose the address of the next-hop gateway or (for the final hop) the destination host. This choice, called "routing", depends upon a routing data-base within the gateway. This routing data-base should be maintained dynamically to reflect the current topology of the Internet system; a gateway normally accomplishes this by participating in distributed routing and reachability algorithms with other gateways. Gateways 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.

Routing devices may also operate at the network level; in this memo we will call such devices MAC routers (informally called "level-2 routers", and also called "bridges"). The name derives from the fact that MAC routers base their routing decision on the addresses in the MAC headers; e.g., in IEEE 802.3 networks, a MAC router bases its decision on the 48-bit addresses in the MAC header. Network segments which are connected by MAC routers share the same IP network number, i.e., they logically form a single IP network.

Another variation on the simple model of networks connected with gateways sometimes occurs: a set of gateways may be interconnected with only serial lines, to effectively form a network in which the routing is performed at the internetwork (IP) level rather than the network level.

1.1.3. Autonomous Systems

For technical, managerial, and sometimes political reasons, the gateways of the Internet system are grouped into collections called "autonomous systems" [35]. The gateways included in a single autonomous system (AS) are expected to:

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

• Employ common routing protocols among themselves, to maintain their routing data-bases dynamically.

RFC 1009 - Requirements for Internet Gateways June 1987

A number of different dynamic routing protocols have been developed (see Section 4.1); the particular choice of routing protocol within a single AS is generically called an interior gateway protocol or IGP.

An IP datagram may have to traverse the gateways of two or more ASs to reach its destination, and the ASs must provide each other with topology information to allow such forwarding. The Exterior Gateway Protocol (EGP) is used for this purpose, between gateways of different autonomous systems.

1.1.4. 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 gateway 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 two-level hierarchical routing structure for the Internet system. The subnet extension, described in RFC-950 [21], 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 ::=

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

The interconnected LANs of an organization will be given the same network number but different subnet numbers. The distinction between the subnets of such a subnetted network must not be visible outside that network. Thus, wide-area routing in the rest of the Internet will be based only upon the <Network-number> part of the IP destination address; gateways outside the network will lump <Subnet-number> and <Host-number>

RFC 1009 - Requirements for Internet Gateways June 1987

together to form an uninterpreted "rest" part of the 32-bit IP address. Within the subnetted network, the local gateways must route on the basis of an extended network number:

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

The bit positions containing this extended network number are indicated by a 32-bit mask called the "subnet mask" [21]; 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 (all one bits).

Flexible use of the available address space will be increasingly important in coping with the anticipated growth of the Internet. Thus, we allow a particular subnetted network to use more than one subnet mask. Several campuses with very large LAN configurations are also creating nested hierarchies of subnets, sub-subnets, etc.

There are special considerations for the gateway when a 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

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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).

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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.

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.

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

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,

RFC 1009 - Requirements for Internet Gateways June 1987

a gateway must be highly robust and able to operate, possibly in a degraded state, under conditions of extreme congestion or failure of network resources.

Even though the Internet system is not fully-interconnected, many parts of the system do need to have redundant connectivity. A rich connectivity allows reliable service despite failures of communication lines and gateways, and it can also improve service by shortening Internet paths and by providing additional capacity. The engineering tradeoff between cost and reliability must be made for each component of the Internet system.

RFC 1009 - Requirements for Internet Gateways June 1987

2. Protocols Required in Gateways

The Internet architecture uses datagram gateways to interconnect constituent networks. This section describes the various protocols which a gateway needs to implement.

2.1. Internet Protocol (IP)

IP is the basic datagram protocol used in the Internet system [19, 31]. It is described in RFC-791 [1] and also in MIL-STD-1777 [5] as clarified by RFC-963 [36] ([1] and [5] are intended to describe the same standard, but in quite different words). The subnet extension is described in RFC-950 [21].

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

It is important for gateways to implement both the Loose and Strict Source Route options. The Record Route and Timestamp options are useful diagnostic tools and must be supported in all gateways.

The Internet model requires that a gateway be able to fragment datagrams as necessary to match the MTU of the network to which they are being forwarded, but reassembly of fragmented datagrams is generally left to the destination hosts. Therefore, a gateway will not perform reassembly on datagrams it forwards.

However, a gateway will generally receive some IP datagrams addressed to itself; for example, these may be ICMP Request/Reply messages, routing update messages (see Sections 2.3 and 2.6), or for monitoring and control (see Section 5). For these datagrams, the gateway will be functioning as a destination host, so it must implement IP reassembly in case the datagrams have been fragmented by some transit gateway. The destination gateway must have a reassembly buffer which is at least as large as the maximum of the MTU values for its network interfaces and 576. Note also that it is possible for a particular protocol implemented by a host or gateway to require a lower bound on reassembly buffer size which is larger than 576. Finally, a datagram which is addressed to a gateway may use any of that gateway's IP addresses as destination address, regardless of which interface the datagram enters.

      There are five classes of IP addresses:  Class A through
      Class E [23].  Of these, Class D and Class E addresses are

RFC 1009 - Requirements for Internet Gateways June 1987

reserved for experimental use. A gateway which is not participating in these experiments must ignore all datagrams with a Class D or Class E destination IP address. ICMP Destination Unreachable or ICMP Redirect messages must not result from receiving such datagrams.

There are certain special cases for IP addresses, defined in the latest Assigned Numbers document [23]. These special cases can be concisely summarized using the earlier notation for an IP address:

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

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

if we also use the notation "-1" to mean the field contains all 1 bits. Some common special cases are as follows:

         (a)   {0, 0}

This host on this network. Can only be used as a source address (see note later).

         (b)   {0, <Host-number>}

Specified host on this network. Can only be used as a source address.

         (c)   { -1, -1}

Limited broadcast. Can only be used as a destination address, and a datagram with this address must never be forwarded outside the (sub-)net of the source.

         (d)   {<Network-number>, -1}

Directed broadcast to specified network. Can only be used as a destination address.

         (e)   {<Network-number>, <Subnet-number>, -1}

Directed broadcast to specified subnet. Can only be used as a destination address.

         (f)   {<Network-number>, -1, -1}

RFC 1009 - Requirements for Internet Gateways June 1987

Directed broadcast to all subnets of specified subnetted network. Can only be used as a destination address.

         (g)   {127, <any>}

Internal host loopback address. Should never appear outside a host.

The following two are conventional notation for network numbers, and do not really represent IP addresses. They can never be used in an IP datagram header as an IP source or destination address.

         (h)   {<Network-number>, 0}

Specified network (no host).

         (i)   {<Network-number>, <Subnet-number>, 0}

Specified subnet (no host).

Note also that the IP broadcast address, which has primary application to Ethernets and similar technologies that support an inherent 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 not in conformance with the specification [23, 49, 50].

2.2. Internet Control Message Protocol (ICMP)

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

We will discuss issues arising from gateway handling of particular ICMP messages. The ICMP messages are grouped into two classes: error messages and information messages. ICMP error messages are never sent about ICMP error messages, nor about broadcast or multicast datagrams.

The ICMP error messages are: Destination Unreachable, Redirect, Source Quench, Time Exceeded, and Parameter Problem.

The ICMP information messages are: Echo, Information, Timestamp, and Address Mask.

RFC 1009 - Requirements for Internet Gateways June 1987

2.2.1. Destination Unreachable

The distinction between subnets of a subnetted network, which depends on the address mask described in RFC-950 [21], must not be visible outside that network. This distinction is important in the case of the ICMP Destination Unreachable message.

The ICMP Destination Unreachable message is sent by a gateway in response to a datagram which it cannot forward because the destination is unreachable or down. The gateway chooses one of the following two types of Destination Unreachable messages to send:

• Net Unreachable

• Host Unreachable

Net unreachable implies that an intermediate gateway was unable to forward a datagram, as its routing data-base gave no next hop for the datagram, or all paths were down. Host Unreachable implies that the destination network was reachable, but that a gateway on that network was unable to reach the destination host. This might occur if the particular destination network was able to determine that the desired host was unreachable or down. It might also occur when the destination host was on a subnetted network and no path was available through the subnets of this network to the destination. Gateways should send Host Unreachable messages whenever other hosts on the same destination network might be reachable; otherwise, the source host may erroneously conclude that ALL hosts on the network are unreachable, and that may not be the case.

      2.2.2.  Redirect

The ICMP Redirect message is sent by a gateway to a host on the same network, in order to change the gateway used by the host for routing certain datagrams. A choice of four types of Redirect messages is available to specify datagrams destined for a particular host or network, and possibly with a particular type-of-service.

If the directly-connected network is not subnetted, a gateway can normally send a network Redirect which applies to all hosts on a specified remote network. Using a network rather than a host Redirect may economize slightly on network traffic and on host routing table storage. However, the saving is not significant, and subnets create an ambiguity about the subnet

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mask to be used to interpret a network Redirect. In a general subnet environment, it is difficult to specify precisely the cases in which network Redirects can be used.

Therefore, it is recommended that a gateway send only host (or host and type-of-service) Redirects.

2.2.3. Source Quench

All gateways must contain code for sending ICMP Source Quench messages when they are forced to drop IP datagrams due to congestion. Although the Source Quench mechanism is known to be an imperfect means for Internet congestion control, and research towards more effective means is in progress, Source Quench is considered to be too valuable to omit from production gateways.

There is some argument that the Source Quench should be sent before the gateway is forced to drop datagrams [62]. For example, a parameter X could be established and set to have Source Quench sent when only X buffers remain. Or, a parameter Y could be established and set to have Source Quench sent when only Y per cent of the buffers remain.

Two problems for a gateway sending Source Quench are: (1) the consumption of bandwidth on the reverse path, and (2) the use of gateway CPU time. To ameliorate these problems, a gateway must be prepared to limit the frequency with which it sends Source Quench messages. This may be on the basis of a count (e.g., only send a Source Quench for every N dropped datagrams overall or per given source host), or on the basis of a time (e.g., send a Source Quench to a given source host or overall at most once per T millseconds). The parameters (e.g., N or T) must be settable as part of the configuration of the gateway; furthermore, there should be some configuration setting which disables sending Source Quenches. These configuration parameters, including disabling, should ideally be specifiable separately for each network interface.

Note that a gateway itself may receive a Source Quench as the result of sending a datagram targeted to another gateway. Such datagrams might be an EGP update, for example.

2.2.4. Time Exceeded

The ICMP Time Exceeded message may be sent when a gateway discards a datagram due to the TTL being reduced to zero. It

RFC 1009 - Requirements for Internet Gateways June 1987

may also be sent by a gateway if the fragments of a datagram addressed to the gateway itself cannot be reassembled before the time limit.

2.2.5. Parameter Problem

The ICMP Parameter Problem message may be sent to the source host for any problem not specifically covered by another ICMP message.

2.2.6. Address Mask

Host and gateway implementations are expected to support the ICMP Address Mask messages described in RFC-950 [21].

      2.2.7.  Timestamp

The ICMP Timestamp message has proven to be useful for diagnosing Internet problems. The preferred form for a timestamp value, the "standard value", is in milliseconds since midnight GMT. However, it may be difficult to provide this value with millisecond resolution. For example, many systems use clocks which update only at line frequency, 50 or 60 times per second. Therefore, some latitude is allowed in a "standard" value:

• The value must be updated at a frequency of at least 30 times per second (i.e., at most five low-order bits of the value may be undefined).

• The origin of the value must be within a few minutes of midnight, i.e., the accuracy with which operators customarily set CPU clocks.

To meet the second condition for a stand-alone gateway, it will be necessary to query some time server host when the gateway is booted or restarted. It is recommended that the UDP Time Server Protocol [44] be used for this purpose. A more advanced implementation would use NTP (Network Time Protocol) [45] to achieve nearly millisecond clock synchronization; however, this is not required.

Even if a gateway is unable to establish its time origin, it ought to provide a "non-standard" timestamp value (i.e., with the non-standard bit set), as a time in milliseconds from system startup.

RFC 1009 - Requirements for Internet Gateways June 1987

New gateways, especially those expecting to operate at T1 or higher speeds, are expected to have at least millisecond clocks.

2.2.8. Information Request/Reply

The Information Request/Reply pair was intended to support self-configuring systems such as diskless workstations, to allow them to discover their IP network numbers at boot time. However, the Reverse ARP (RARP) protocol [15] provides a better mechanism for a host to use to discover its own IP address, and RARP is recommended for this purpose. Information Request/Reply need not be implemented in a gateway.

2.2.9. Echo Request/Reply

A gateway must implement ICMP Echo, since it has proven to be an extremely useful diagnostic tool. A gateway must be prepared to receive, reassemble, and echo an ICMP Echo Request datagram at least as large as the maximum of 576 and the MTU's of all of the connected networks. See the discussion of IP reassembly in gateways, Section 2.1.

The following rules resolve the question of the use of IP source routes in Echo Request and Reply datagrams. Suppose a gateway D receives an ICMP Echo Request addressed to itself from host S.

1. If the Echo Request contained no source route, D should send an Echo Reply back to S using its normal routing rules. As a result, the Echo Reply may take a different path than the Request; however, in any case, the pair will sample the complete round-trip path which any other higher-level protocol (e.g., TCP) would use for its data and ACK segments between S and D.

2. If the Echo Request did contain a source route, D should send an Echo Reply back to S using as a source route the return route built up in the source-routing option of the Echo Request.

RFC 1009 - Requirements for Internet Gateways June 1987

   2.3.  Exterior Gateway Protocol (EGP)

EGP is the protocol used to exchange reachability information between Autonomous Systems of gateways, and is defined in RFC-904 [11]. See also RFC-827 [51], RFC-888 [46], and RFC-975 [27] for background information. The most widely used EGP implementation is described in RFC-911 [13].

When a dynamic routing algorithm is operated in the gateways of an Autonomous System (AS), the routing data-base must be coupled to the EGP implementation. This coupling should ensure that, when a net is determined to be unreachable by the routing algorithm, the net will not be declared reachable to other ASs via EGP. This requirement is designed to minimize spurious traffic to "black holes" and to ensure fair utilization of the resources on other systems.

The present EGP specification defines a model with serious limitations, most importantly a restriction against propagating "third party" EGP information in order to prevent long-lived routing loops [27]. This effectively limits EGP to a two-level hierarchy; the top level is formed by the "core" AS, while the lower level is composed of those ASs which are direct neighbor gateways to the core AS. In practice, in the current Internet, nearly all of the "core gateways" are connected to the ARPANET, while the lower level is composed of those ASs which are directly gatewayed to the ARPANET or MILNET.

RFC-975 [27] suggested one way to generalize EGP to lessen these topology restrictions; it has not been adopted as an official specification, although its ideas are finding their way into the new EGP developments. There are efforts underway in the research community to develop an EGP generalization which will remove these restrictions.

In EGP, there is no standard interpretation (i.e., metric) for the distance fields in the update messages, so distances are comparable only among gateways of the same AS. In using EGP data, a gateway should compare the distances among gateways of the same AS and prefer a route to that gateway which has the smallest distance value.

The values to be announced in the distance fields for particular networks within the local AS should be a gateway configuration parameter; by suitable choice of these values, it will be possible to arrange primary and backup paths from other AS's. There are other EGP parameters, such as polling intervals, which also need to be set in the gateway configuration.

RFC 1009 - Requirements for Internet Gateways June 1987

When routing updates become large they must be transmitted in parts. One strategy is to use IP fragmentation, another is to explicitly send the routing information in sections. The Internet Engineering Task Force is currently preparing a recommendation on this and other EGP engineering issues.

   2.4.  Address Resolution Protocol (ARP)

ARP is an auxiliary protocol used to perform dynamic address translation between LAN hardware addresses and Internet addresses, and is described in RFC-826 [4].

ARP depends upon local network broadcast. In normal ARP usage, the initiating host broadcasts an ARP Request carrying a target IP address; the corresponding target host, recognizing its own IP address, sends back an ARP Reply containing its own hardware interface address.

A variation on this procedure, called "proxy ARP", has been used by gateways attached to broadcast LANs [14]. The gateway sends an ARP Reply specifying its interface address in response to an ARP Request for a target IP address which is not on the directly-connected network but for which the gateway offers an appropriate route. By observing ARP and proxy ARP traffic, a gateway may accumulate a routing data-base [14].

Proxy ARP (also known in some quarters as "promiscuous ARP" or "the ARP hack") is useful for routing datagrams from hosts which do not implement the standard Internet routing rules fully -- for example, host implementations which predate the introduction of subnetting. Proxy ARP for subnetting is discussed in detail in RFC-925 [14].

Reverse ARP (RARP) allows a host to map an Ethernet interface address into an IP address [15]. RARP is intended to allow a self-configuring host to learn its own IP address from a server at boot time.

2.5. Constituent Network Access Protocols

See Section 3.

RFC 1009 - Requirements for Internet Gateways June 1987

2.6. Interior Gateway Protocols

Distributed routing algorithms continue to be the subject of research and engineering, and it is likely that advances will be made over the next several years. A good algorithm needs to respond rapidly to real changes in Internet connectivity, yet be stable and insensitive to transients. It needs to synchronize the distributed data-base across gateways of its Autonomous System rapidly (to avoid routing loops), while consuming only a small fraction of the available bandwidth.

Distributed routing algorithms are commonly broken down into the following three components:

A. An algorithm to assign a "length" to each Internet path.

The "length" may be a simple count of hops (1, or infinity if the path is broken), or an administratively-assigned cost, or some dynamically-measured cost (usually an average delay).

In order to determine a path length, each gateway must at least test whether each of its neighbors is reachable; for this purpose, there must be a "reachability" or "neighbor up/down" protocol.

B. An algorithm to compute the shortest path(s) to a given

destination.

C. A gateway-gateway protocol used to exchange path length and

routing information among gateways.

The most commonly-used IGPs in Internet gateways are as follows.

      2.6.1.  Gateway-to-Gateway Protocol (GGP)

GGP was designed and implemented by BBN for the first experimental Internet gateways [41]. It is still in use in the BBN LSI/11 gateways, but is regarded as having serious drawbacks [58]. GGP is based upon an algorithm used in the early ARPANET IMPs and later replaced by SPF (see below).

GGP is a "min-hop" algorithm, i.e., its length measure is simply the number of network hops between gateway pairs. It implements a distributed shortest-path algorithm, which requires global convergence of the routing tables after a change in topology or connectivity. Each gateway sends a GGP

RFC 1009 - Requirements for Internet Gateways June 1987

routing update only to its neighbors, but each update includes an entry for every known network, where each entry contains the hop count from the gateway sending the update.

      2.6.2.  Shortest-Path-First (SPF) Protocols

SPF [40] is the name for a class of routing algorithms based on a shortest-path algorithm of Dijkstra. The current ARPANET routing algorithm is SPF, and the BBN Butterfly gateways also use SPF. Its characteristics are considered superior to GGP [58].

Under SPF, the routing data-base is replicated rather than distributed. Each gateway will have its own copy of the same data-base, containing the entire Internet topology and the lengths of every path. Since each gateway has all the routing data and runs a shortest-path algorithm locally, there is no problem of global convergence of a distributed algorithm, as in GGP. To build this replicated data-base, a gateway sends SPF routing updates to ALL other gateways; these updates only list the distances to each of the gateway's neighbors, making them much smaller than GGP updates. The algorithm used to distribute SPF routing updates involves reliable flooding.

2.6.3. Routing Information (RIP)

RIP is the name often used for a class of routing protocols based upon the Xerox PUP and XNS routing protocols. These are relatively simple, and are widely available because they are incorporated in the embedded gateway code of Berkeley BSD systems. Because of this simplicity, RIP protocols have come the closest of any to being an "Open IGP", i.e., a protocol which can be used between different vendors' gateways. Unfortunately, there is no standard, and in fact not even a good document, for RIP.

As in GGP, gateways using RIP periodically broadcast their

routing data-base to their neighbor gateways, and use a

hop-count as the metric.

A fixed value of the hop-count (normally 16) is defined to be "infinity", i.e., network unreachable. A RIP implementation must include measures to avoid both the slow-convergence phenomen called "counting to infinity" and the formation of routing loops. One such measure is a "hold-down" rule. This rule establishes a period of time (typically 60 seconds) during which a gateway will ignore new routing information about a given network, once the gateway has learned that network is

RFC 1009 - Requirements for Internet Gateways June 1987

unreachable (has hop-count "infinity"). The hold-down period must be settable in the gateway configuration; if gateways with different hold-down periods are using RIP in the same Autonomous System, routing loops are a distinct possibility. In general, the hold-down period is chosen large enough to allow time for unreachable status to propagate to all gateways in the AS.

      2.6.4.  Hello

The "Fuzzball" software for an LSI/11 developed by Dave Mills incorporated an IGP called the "Hello" protocol [39]. This IGP is mentioned here because the Fuzzballs have been widely used in Internet experimentation, and because they have served as a testbed for many new routing ideas.

2.7. Monitoring Protocols

See Section 5 of this document.

2.8. Internet Group Management Protocol (IGMP)

An extension to the IP protocol has been defined to provide Internet-wide multicasting, i.e., delivery of copies of the same IP datagram to a set of Internet hosts [47, 48]. This delivery is to be performed by processes known as "multicasting agents", which reside either in a host on each net or (preferably) in the gateways.

The set of hosts to which a datagram is delivered is called a "host group", and there is a host-agent protocol called IGMP, which a host uses to join, leave, or create a group. Each host group is distinguished by a Class D IP address.

This multicasting mechanism and its IGMP protocol are currently experimental; implementation in vendor gateways would be premature at this time. A datagram containing a Class D IP address must be dropped, with no ICMP error message.

RFC 1009 - Requirements for Internet Gateways June 1987

3. Constituent Network Interface

This section discusses the rules used for transmission of IP datagrams on the most common types of constituent networks. A gateway must be able to send and receive IP datagrams of any size up to the MTU of any constituent network to which it is connected.

3.1. Public data networks via X.25

The formats specified for public data networks accessed via X.25 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. Link-level retransmission, resequencing and flow control are performed by the network for each virtual circuit and by the LAPB link-level protocol. Note that a single X.25 virtual circuit may be used to multiplex all IP traffic between a pair of hosts. However, multiple parallel virtual circuits may be used in order to improve the utilization of the subscriber access line, in spite of small X.25 window sizes; this 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 the future. The table of the hosts on the Public Data Network is in the Assigned Numbers [23].

The normal MTU is 576; however, the two DTE's (hosts or gateways) can use X.25 packet size negotiation to increase this value [8].

3.2. ARPANET via 1822 LH, DH, or HDH

The formats specified for ARPANET networks using 1822 access are described in BBN Report 1822 [3], which includes the procedures for several subscriber access methods. The Distant Host (DH) method is used when the host and IMP (the Defense Communication Agency calls it a Packet Switch Node or PSN) are separated by not more than about 2000 feet of cable, while the HDLC Distant Host (HDH) is used for greater distances where a modem is required. Under HDH, retransmission, resequencing and flow control are performed by the network and by the HDLC link-level protocol.

The IP encapsulation format is simply to include the IP datagram as the data portion of an 1822 message. In addition, the high-order 8 bits of the Message Id field (also known as the "link" field") should be set to 155 [23]. The MTU is 1007 octets.

RFC 1009 - Requirements for Internet Gateways June 1987

While the ARPANET 1822 protocols are widely used at present, they are expected to be eventually overtaken by the DDN Standard X.25 protocol (see Section 3.3). The original IP address mapping (RFC-796 [38]) is in the process of being replaced by a new interface specification called AHIP-E; see RFC-1005 [61] for the proposal.

Gateways connected to ARPANET or MILNET IMPs using 1822 access must incorporate features to avoid host-port blocking (i.e., RFNM counting) and to detect and report as ICMP Unreachable messages the failure of destination hosts or gateways (i.e., convert the 1822 error messages to the appropriate ICMP messages).

In the development of a network interface it will be useful to review the IMP end-to-end protocol described in RFC-979 [29].

3.3. 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], which describes two sets of procedures: the DDN Basic X.25, and the DDN Standard X.25. Only DDN Standard X.25 provides the functionality required for interoperability assumptions of the Internet protocol.

The DDN Standard X.25 procedures are similar to the public data network 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. Multiple parallel virtual circuits may be used in order to improve the utilization of the subscriber access line; this can result in random resequencing.

Gateways connected to ARPANET or MILNET using Standard X.25 access must detect and report as ICMP Unreachable messages the failure of destination hosts or gateways (i.e., convert the X.25 diagnostic codes to the appropriate ICMP messages).

To achieve compatibility with 1822 interfaces, the effective MTU for a Standard X.25 interface is 1007 octets.

RFC 1009 - Requirements for Internet Gateways June 1987

3.4. Ethernet and IEEE 802

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 (the type field values are listed in the Assigned Numbers [23]). 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 link-level retransmission, resequencing or flow control, although most hardware interfaces will retransmit automatically in case of collisions on the cable.

The IEEE 802 networks use a Link Service Access Point (LSAP) field in much the same way the ARPANET uses the "link" field. Further, there is an extension of the LSAP header called the Sub-Network Access Protocol (SNAP).

The 802.2 encapsulation is used on 802.3, 802.4, and 802.5 network by using the SNAP with an organization code indicating that the following 16 bits specify the Ether-Type code [23].

Headers:

         ...--------+--------+--------+
          MAC Header|      Length     |                  802.{3/4/5} MAC
         ...--------+--------+--------+
         
         +--------+--------+--------+
         | DSAP=K1| SSAP=K1| control|                          802.2 SAP
         +--------+--------+--------+
         
         +--------+--------+--------+--------+--------+
         |protocol id or org code=K2|    Ether-Type   |       802.2 SNAP
         +--------+--------+--------+--------+--------+

The total length of the SAP Header and the SNAP header is 8-octets, making the 802.2 protocol overhead come out on a 64-bit boundary.

K1 is 170. The IEEE likes to talk about things in bit transmission order and specifies this value as 01010101. In big-endian order, as used in the Internet specifications, this becomes 10101010 binary, or AA hex, or 170 decimal. K2 is 0 (zero).

The use of the IP LSAP (K1 = 6) is reserved for future development.

RFC 1009 - Requirements for Internet Gateways June 1987

The assigned values for the Ether-Type field are the same for either this IEEE 802 encapsulation or the basic Ethernet encapsulation [10].

In either Ethernets or IEEE 802 nets, the IP datagram is the data portion of the packet immediately following the Ether-Type.

The MTU for an Ethernet or its IEEE-standard equivalent (802.3) is 1500 octets.

3.5. Serial-Line Protocols

In some configurations, gateways may be interconnected with each other 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 single Internet standard for this protocol, it is suggested that one of the following protocols be used.

         *  X.25 LAPB  (Synchronous Lines)

This is the link-level protocol used for X.25 network

access. It includes HDLC "bit-stuffing" as well as

rotating-window flow control and reliable delivery.

A gateway must be configurable to play the role of either the DCE or the DTE.

         *  HDLC Framing  (Synchronous Lines)

This is just the bit-stuffing and framing rules of LAPB. It is the simplest choice, although it provides no flow control or reliable delivery; however, it does provide error detection.

* Xerox Synchronous Point-to-Point (Synchronous Lines)

This Xerox protocol is an elaboration upon HDLC framing that includes negotiation of maximum packet sizes, dial-up or dedicated circuits, and half- or full-duplex operation [12].

* Serial Line Framing Protocol (Asynchronous Lines)

This protocol is included in the MIT PC/IP package for an IBM PC and is defined in Appendix I to the manual for that system [20].

RFC 1009 - Requirements for Internet Gateways June 1987

It will be important to make efficient use of the bandwidth available on a serial line between gateways. For example, it is desirable to provide some form of data compression. One possible standard compression algorithm, "Thinwire II", is described in RFC-914 [42]. This and similar algorithms are tuned to the particular types of redundancy which occur in IP and TCP headers; however, more work is necessary to define a standard serial-line compression protocol for Internet gateways. Until a standard has been adopted, each vendor is free to choose a compression algorithm; of course, the result will only be useful on a serial line between two gateways using the same compression algorithm.

Another way to ensure maximum use of the bandwidth is to avoid unnecessary retransmissions at the link level. For some kinds of IP traffic, low delay is more important than reliable delivery. The serial line driver could distinguish such datagrams by their IP TOS field, and place them on a special high-priority, no-retransmission queue.

A serial point-to-point line between two gateways may be considered to be a (particularly simple) network, a "null net". Considered in this way, a serial line requires no special considerations in the routing algorithms of the connected gateways, but does need an IP network number. To avoid the wholesale consumption of Internet routing data-base space by null nets, we strongly recommend that subnetting be used for null net numbering, whenever possible.

For example, assume that network 128.203 is to be constructed of gateways joined by null nets; these nets are given (sub-)net numbers 128.203.1, 128.203.2, etc., and the two interfaces on each end of null net 128.203.s might have IP addresses 128.203.s.1 and 128.203.s.2.

An alternative model of a serial line is that it is not a network, but rather an internal communication path joining two "half gateways". It is possible to design an IGP and routing algorithm that treats a serial line in this manner [39, 52].

RFC 1009 - Requirements for Internet Gateways June 1987

4. Gateway Algorithms

   Gateways are general packet-switches that forward packets according
   to the IP address, i.e., they are IP routers.   While it is beyond
   the scope of this document to specify the details of the mechanisms
   used in any particular, perhaps proprietary, gateway architecture,
   there are a number of basic algorithms which must be provided by any
   acceptable design.

4.1. Routing Algorithm

The routing mechanism is fundamental to Internet operation. In all but trivial network topologies, robust Internet service requires some degree of routing dynamics, whether it be effected by manual or automatic means or by some combination of both. In particular, if routing changes are made manually, it must be possible to make these routing changes from a remote Network Operation Center (NOC) without taking down the gateway for reconfiguration. If static routes are used, there must be automatic fallback or rerouting features.

Handling unpredictable changes in Internet connectivity must be considered the normal case, so that systems of gateways will normally be expected to have a routing algorithm with the capability of reacting to link and other gateway failures and changing the routing automatically.

This document places no restriction on the type of routing algorithm, e.g., node-based, link-based or any other algorithm, or on the routing distance metric, e.g., delay or hop-count. However, the following features are considered necessary for a successful gateway routing algorithm:

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

2. The algorithm must suppress routing loops between neighbor gateways and must contain provisions to avoid or suppress routing loops that may form between non-neighbor gateways. A design objective is for no loop to persist for longer than an interval greater than the typical TCP user time-out.

3. The control traffic necessary to operate the routing algorithm must not significantly degrade or disrupt normal

RFC 1009 - Requirements for Internet Gateways June 1987

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 incremental changes broadcast over a wide-area.

5. The size of the routing data-base must not be allowed to exceed a constant, independent of network topology, times the number of nodes times the mean connectivity (average number of incident links). An advanced design might not require that the entire routing data-base be kept in any particular gateway, so that discovery and caching techniques would be necessary.

6. Reachability and delay metrics, if used, must not depend on direct connectivity to all other gateways or on the use of network-specific broadcast mechanisms. Polling procedures (e.g., for consistency checking) must be used only sparingly and in no case introduce an overhead exceeding a constant, independent of network topology, times the longest non-looping path.

7. Default routes (generally intended as a means to reduce the size of the routing data-base) must be used with care, because of the many problems with multiple paths, loops, and mis-configurations which routing defaults have caused.

The most common application of defaults is for routing within an Internet region which is connected in a strictly hierarchical fashion and is a stub from the rest of the Internet system. In this case, the default is used for routing "up" the tree. Unfortunately, such restricted topology seldom lasts very long, and defaults cease to work.

More generally, defaults could be used for initial routing guesses, with final routes to be discovered and cached from external or internal data-bases via the routing algorithm or EGP.

RFC 1009 - Requirements for Internet Gateways June 1987

4.2. Subnets and Routing

We will call a gateway "subnetted" if at least one of its interfaces is connected to a subnet; the set of gateways directly connected to subnets of the same network will be referred to as a "subnet cluster". For example, in the following diagram, network 2 is subnetted, with subnets 2.1 and 2.2, but network 1 is not; gateways 1, 2, and 3 are subnetted and are members of the same subnet cluster.

         (Net 1) === [Gwy 1] === (Net 2.1) === [Gwy 2] === (Net 2.2)
            |                                                   |
            |                                                   |
             =================== [Gwy 3] =======================

Subnets have the following effects on gateway routing:

A. Non-subnetted gateways are not affected at all.

B. The routing data-base in a subnetted gateway must consider

the address mask for subnet entries.

C. Routing updates among the gateways in the same subnet

cluster must include entries for the various subnets. The corresponding address mask(s) may be implicit, but for full generality the mask needs to be given explicitly for each entry. Note that if the routing data-base included a full 32-bit mask for every IP network, the gateway could deal with networks and subnets in a natural way. This would also handle the case of multiple subnet masks for the same subnetted network.

D. Routing updates from a subnetted gateway to a gateway

outside the cluster can contain nets, never subnets.

E. If a subnetted gateway (e.g., gateway 2 above) is unable to

forward a datagram from one subnet to another subnet of the same network, then it must return a Host Unreachable, not a Net Unreachable, as discussed in Section 2.2.1.

When considering the choice of routing protocol, a gateway builder must consider how that protocol generalizes for subnets. For some routing protocols it will be possible to use the same procedures in a regular gateway and a subnetted gateway, with only a change of parameters (e.g., address masks).

A different subnet address mask must be configurable for each

RFC 1009 - Requirements for Internet Gateways June 1987

interface of a given gateway. This will allow a subnetted gateway to connect to two different subnetted networks, or to connect two subnets of the same network with different masks.

   4.3   Resource Allocation

In order to perform its basic datagram-forwarding functions, a gateway must allocate resources; its packet buffers and CPU time must be allocated to packets it receives from connected networks, while the bandwidth to each of the networks must also be allocated for sending packets. The choice of allocation strategies will be critical when a particular resource is scarce. The most obvious allocation strategy, first-come-first-served (FCFS), may not be appropriate under overload conditions, for reasons which we will now explore.

A first example is buffer allocation. It is important for a gateway to allocate buffers fairly among all of its connected networks, even if these networks have widely varying bandwidths. A high-speed interface must not be allowed to starve slower interfaces of buffers. For example, consider a gateway with a 10 Mbps Ethernet connection and two 56 Kbps serial lines. A buggy host on the Ethernet may spray that gateway interface with packets at high speed. Without careful algorithm design in the gateway, this could tie up all the gateway buffers in such a way that transit traffic between the serial lines would be completely stopped.

Allocation of output bandwidth may also require non-FCFS strategies. In an advanced gateway design, allocation of output bandwidth may depend upon Type-of-Service bits in the IP headers. A gateway may also want to give priority to datagrams for its own up/down and routing protocols.

Finally, Nagle [24] has suggested that gateways implement "fair queueing", i.e., sharing output bandwidth equitably among the current traffic sources. In his scheme, for each network interface there would be a dynamically-built set of output queues, one per IP source address; these queues would be serviced in a round-robin fashion to share the bandwidth. If subsequent research shows fair queueing to be desirable, it will be added to a future version of this document as a universal requirement.

RFC 1009 - Requirements for Internet Gateways June 1987

4.4. Special Addresses and Filters

Section 2.1 contained a list of the 32-bit IP addresses which have special meanings. They do not in general represent unique IP addresses of Internet hosts, and there are restrictions on their use in IP headers.

We can distinguish two classes of these special cases. The first class (specifically, cases (a), (b), (c), (g), (h), and (i) in section 2.1) contains addresses which should never appear in the destination address field of any IP datagram, so a gateway should never be asked to route to one of these addresses. However, in the real world of imperfect implementations and configuration errors, such bad destination addresses do occur. It is the responsibility of a gateway to avoid propagating such erroneous addresses; this is especially important for gateways included in the global interconnect system. In particular, a gateway which receives a datagram with one of these forbidden addresses should:

1. Avoid inserting that address into its routing database, and avoid including it in routing updates to any other gateway.

2. Avoid forwarding a datagram containing that address as a destination.

To enforce these restrictions, it is suggested that a gateway include a configurable filter for datagrams and routing updates. A typical filter entry might consist of a 32-bit mask and value pair. If the logical AND of the given address with the mask equals the value, a match has been found. Since filtering will consume gateway resources, it is vital that the gateway configuration be able to control the degree of filtering in use.

There is a second class of special case addresses (cases (d), (e), and (f) in section 2.1), the so-called "directed broadcasts". A directed broadcast is a datagram to be forwarded normally to the specified destination (sub-)net and then broadcast on the final hop. An Internet gateway is permitted, but not required, to filter out directed broadcasts destined for any of its locally-connected networks. Hence, it should be possible to configure the filter to block the delivery of directed broadcasts.

Finally, it will also be useful for Internet O&M to have a configurable filter on the IP source address. This will allow a network manager to temporarily block traffic from a particular misbehaving host, for example.

RFC 1009 - Requirements for Internet Gateways June 1987

   4.5.  Redirects

The ICMP Redirect message is specified only for use by a gateway to update the routing table of a host on the same connected net. However, the Redirect message is sometimes used between gateways, due to the following considerations:

The routing function in a host is very much like that in a "dumb gateway" (i.e., a gateway having only static routes). It is desirable to allow the routing tables of a dumb gateway to be changed under the control of a dynamic gateway (i.e., a gateway with full dynamic routing) on the same network. By analogy, it is natural to let the dynamic gateway send ICMP Redirect messages to dumb gateway.

      The use of ICMP Redirect between gateways in this fashion may be
      considered to be part of the IGP (in fact, the totality of the
      IGP, as far as the dumb gateway is concerned!) in the particular
      Autonomous System.   Specification of an IGP is outside the scope
      of this document, so we only note the possibility of using
      Redirect in this fashion.  Gateways are not required to receive
      and act upon redirects, and in fact dynamic gateways must ignore
      them.  We also note that considerable experience shows that dumb
      gateways often create problems resulting in "black holes"; a full
      routing gateway is always preferable.

Routing table entries established by redirect messages must be removed automatically, either by a time-out or when a use count goes to zero.

4.6. Broadcast and Multicast

A host which is connected to a network (generally a LAN) with an intrinsic broadcast capability may want to use this capability to effect multidestination delivery of IP datagrams. The basic Internet model assumes point-to-point messages, and we must take some care when we incorporate broadcasting. It is important to note that broadcast addresses may occur at two protocol levels: the local network header and the IP header.

Incorrect handling of broadcasting has often been the cause of packet avalanches (sometimes dubbed "meltdown") in LANs. These avalanches are generally caused by gratuitous datagram-forwarding by hosts, or by hosts sending ICMP error messages when they discard broadcast datagrams.

Gateways have a responsibility to prevent avalanches, or datagrams which can trigger avalanches, from escaping into another network.

RFC 1009 - Requirements for Internet Gateways June 1987

In general, a gateway must not forward a datagram which arrives via local network broadcast, and must not send an ICMP error message when dropping the datagram. A discussion of the rules will be found in Appendix A; see also [50].

As noted in Section 4.4, a gateway is permitted to filter out directed broadcasts. Hence, directed broadcasts will only be useful in limited Internet regions (e.g., the within the subnets of a particular campus) in which delivery is supported by the gateway administrators. Host group multicasting (see Sections 2.8 and 4.6) will soon provide a much more efficient mechanism than directed broadcasting. Gateway algorithms for host group multicasting will be specified in future RFC's.

4.7. 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 messages across each link. This function might be intrinsic to the link-level protocols used (e.g., LAPB). However, it is in general ill-advised to assume a host or gateway is operating correctly even 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.

   4.8.  Time-To-Live

The Time-to-Live (TTL) field of the IP header is defined to be a timer limiting the lifetime of a datagram in the Internet. It is an 8-bit field and the units are seconds. This would imply that for a maximum TTL of 255 a datagram would time-out after about 4 and a quarter minutes. Another aspect of the definition requires each gateway (or other module) that handles a datagram to decrement the TTL by at least one, even if the elapsed time was much less than a second. Since this is very often the case, the TTL effectively becomes a hop count limit on how far a datagram can propagate through the Internet.

RFC 1009 - Requirements for Internet Gateways June 1987

As the Internet grows, the number of hops needed to get from one edge to the opposite edge increases, i.e., the Internet diameter grows.

If a gateway holds a datagram for more than one second, it must decrement the TTL by one for each second.

If the TTL is reduced to zero, the datagram must be discarded, and the gateway may send an ICMP Time Exceeded message to the source. A datagram should never be received with a TTL of zero.

When it originates a datagram, a gateway is acting in the role of a host and must supply a realistic initial value for the TTL.

RFC 1009 - Requirements for Internet Gateways June 1987

5. Operation and Maintenance

   5.1.  Introduction

Facilities to support operation and maintenance (O&M) activities form an essential part of any gateway implementation. The following kinds of activity are included under gateway O&M:

• Diagnosing hardware problems in the gateway processor, in its network interfaces, or in the connected networks, modems, or communication lines.

* Installing a new version of the gateway software.

* Restarting or rebooting a gateway after a crash.

• Configuring (or reconfiguring) the gateway.

• Detecting and diagnosing Internet problems such as congestion, routing loops, bad IP addresses, black holes, packet avalanches, and misbehaved hosts.

         *  Changing network topology, either temporarily (e.g., to
            diagnose a communication line problem) or permanently.

• Monitoring the status and performance of the gateways and the connected networks.

• Collecting traffic statistics for use in (Inter-)network planning.

Gateways, packet-switches, and their connected communication lines are often operated as a system by a centralized O&M organization. This organization will maintain a (Inter-)network operation center, or NOC, to carry out its O&M functions. It is essential that gateways support remote control and monitoring from such a NOC, through an Internet path (since gateways might not be connected to the same network as their NOC). Furthermore, an IP datagram traversing the Internet will often use gateways under the control of more than one NOC; therefore, Internet problem diagnosis will often involve cooperation of personnel of more than one NOC. In some cases, the same gateway may need to be monitored by more than one NOC.

The tools available for monitoring at a NOC may cover a wide range of sophistication. Proposals have included multi-window, dynamic displays of the entire gateway system, and the use of AI techniques for automatic problem diagnosis.

RFC 1009 - Requirements for Internet Gateways June 1987

Gateway O&M facilities discussed here are only a part of the large and difficult problem of Internet management. These problems encompass not only multiple management organizations, but also multiple protocol layers. For example, at the current stage of evolution of the Internet architecture, there is a strong coupling between host TCP implementations and eventual IP-level congestion in the gateway system [9]. Therefore, diagnosis of congestion problems will sometimes require the monitoring of TCP statistics in hosts. Gateway algorithms also interact with local network performance, especially through handling of broadcast packets and ARP, and again diagnosis will require access to hosts (e.g., examining ARP caches). However, consideration of host monitoring is beyond the scope of this RFC.

There are currently a number of R&D efforts in progress in the area of Internet management and more specifically gateway O&M. It is hoped that these will lead quickly to Internet standards for the gateway protocols and facilities required in this area. This is also an area in which vendor creativity can make a significant contribution.

   5.2.   Gateway O&M Models

There is a range of possible models for performing O&M functions on a gateway. At one extreme is the local-only model, under which the O&M functions can only be executed locally, e.g., from a terminal plugged into the gateway machine. At the other extreme, the fully-remote model allows only an absolute minimum of functions to be performed locally (e.g., forcing a boot), with most O&M being done remotely from the NOC. There intermediate models, e.g., one in which NOC personnel can log into the gateway as a host, using the Telnet protocol, to perform functions which can also be invoked locally. The local-only model may be adequate in a few gateway installations, but in general remote operation from a NOC will be required, and therefore remote O&M provisions are required for most gateways.

Remote O&M functions may be exercised through a control agent (program). In the direct approach, the gateway would support remote O&M functions directly from the NOC using standard Internet protocols (e.g., UDP or TCP); in the indirect approach, the control agent would support these protocols and control the gateway itself using proprietary protocols. The direct approach is preferred, although either approach is acceptable. The use of specialized host hardware and/or software requiring significant additional investment is 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

RFC 1009 - Requirements for Internet Gateways June 1987

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.

It is desirable that a control agent and any other NOC software tools which a vendor provides operate as user programs in a standard operating system. The use of the standard Internet protocols UDP and TCP for communicating with the gateways should facilitate this.

Remote gateway monitoring and (especially) remote gateway control present important access control problems which must be addressed. Care must also be taken to ensure control of the use of gateway resources for these functions. It is not desirable to let gateway monitoring take more than some limited fraction of the gateway CPU time, for example. On the other hand, O&M functions must receive priority so they can be exercised when the gateway is congested, i.e., when O&M is most needed.

There are no current Internet standards for the control and monitoring protocols, although work is in progress in this area. The Host Monitoring Protocol (HMP) [7] could be used as a model until a standard is developed; however, it is strongly recommended that gateway O&M protocol be built on top of one of the standard Internet end-to-end protocols UDP or TCP. An example of a very simple but effective approach to gateway monitoring is contained in RFC-996 [43].

   5.3.   Gateway O&M Functions

The following O&M functions need to be performed in a gateway:

         A.  Maintenance -- Hardware Diagnosis

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 be able to run diagnostics or dump the gateway via the network in case of fault. Means should be provided to allow remote control from the NOC of of modems attached to the gateway. The most important modem control capability is entering and leaving loopback mode, to diagnose line problems.

RFC 1009 - Requirements for Internet Gateways June 1987

         B.  Control -- Dumping and Rebooting

It must be possible to dump and reboot a stand-alone gateway upon command from the NOC. In addition, a stand-alone gateway must include a watchdog timer that either initiates a reboot automatically or signals a remote control site if not reset periodically by the software. It is desirable that the boot data involved reside at an Internet host (e.g., the NOC host) and be transmitted via the net; however, the use of local devices at the gateway site is acceptable.

         C.  Control -- Configuring the Gateway

Every gateway will have a number of configuration parameters which must be set (see the next section for examples). It must be possible to update the parameters without rebooting the gateway; at worst, a restart may be required.

         D.  Monitoring -- Status and Performance

A mechanism must be provided for retrieving status and statistical information from a gateway. A gateway must supply such information in response to a polling message from the NOC. In addition, it may be desirable to configure a gateway to transmit status spontaneously and periodically to a NOC (or set of NOCs), for recording and display.

Examples of interesting status information include: link status, queue lengths, buffer availability, CPU and memory utilization, the routing data-base, error counts, and packet counts. Counts should be kept for dropped datagrams, separated by reason. Counts of ICMP datagrams should be kept by type and categorized into those originating at the gateway, and those destined for the gateway. It would be useful to maintain many of these statistics by network interface, by source/destination network pair, and/or by source/destination host pair.

Note that a great deal of useful monitoring data is often to be found in the routing data-base. It is therefore useful to be able to tap into this data-base from the NOC.

         E.  Monitoring -- Error Logging

A gateway should be capable of asynchronously sending exception ("trap") reports to one or more specified Internet addresses, one of which will presumably be the NOC host.

RFC 1009 - Requirements for Internet Gateways June 1987

There must also be a mechanism to limit the frequency of such trap reports, and the parameters controlling this frequency must be settable in the gateway configuration.

Examples of conditions which should result in traps include: datagrams discarded because of TTL expiration (an indicator of possible routing loops); resource shortages; or an interface changing its up/down status.

   5.4.   Gateway Configuration Parameters

Every gateway will have a set of configuration parameters controlling its operation. It must be possible to set these parameters remotely from the NOC or locally at any time, without taking the gateway down.

The following is a partial but representative list of possible configuration parameters for a full-function gateway. The items marked with "(i)" should be settable independently for each network interface.

         * (i)  IP (sub-) network address
         
         * (i)  Subnet address mask

• (i) MTU of local network

         * (i)  Hardware interface address

* (i) Broadcast compatibility option (0s or 1s)

         *      EGP parameters -- neighbors, Autonomous System number,
                and polling parameters
         
         *      Static and/or default routes, if any
         
         *      Enable/Disable Proxy ARP
         
         *      Source Quench parameters
         
         *      Address filter configuration
         
         *      Boot-host address
         
         *      IP address of time server host
         
         *      IP address(es) of logging host(s)

RFC 1009 - Requirements for Internet Gateways June 1987

         *      IP address(es) of hosts to receive traps
         
         *      IP address(es) of hosts authorized to issue control
                commands
         
         *      Error level for logging
         
         *      Maximum trap frequency
         
         *      Hold-down period (if any)

RFC 1009 - Requirements for Internet Gateways June 1987

Appendix A. Technical Details

This Appendix collects a number of technical details and rules concerning datagram forwarding by gateways and datagram handling by hosts, especially in the presence of broadcasting and subnets.

A.1. Rules for Broadcasting

The following rules define how to handle broadcasts of packets and datagrams [50]:

a. Hosts (which do not contain embedded gateways) must NEVER

forward any datagrams received from a connected network, broadcast or not.

When a host receives an IP datagram, if the destination address identifies the host or is an IP broadcast address, the host passes the datagram to its appropriate higher-level protocol module (possibly sending ICMP protocol unreachable, but not if the IP address was a broadcast address). Any other IP datagram must simply be discarded, without an ICMP error message. Hosts never send redirects.

b. All packets containing IP datagrams which are sent to the

local-network packet broadcast address must contain an IP broadcast address as the destination address in their IP header. Expressed in another way, a gateway (or host) must not send in a local-network broadcast packet an IP datagram that has a specific IP host address as its destination field.

c. A gateway must never forward an IP datagram that arrives

addressed to the IP limited broadcast address {-1,-1}. Furthermore, it must must not send an ICMP error message about discarding such a datagram.

d. A gateway must not forward an IP datagram addressed to

network zero, i.e., {0, *}.

e. A gateway may forward a directed broadcast datagram, i.e.,

a datagram with the IP destination address:

{ <Network-number>, -1}.

However, it must not send such a directed broadcast out the same interface it came in, if this interface has <Network-number> as its network number. If the code in the

RFC 1009 - Requirements for Internet Gateways June 1987

gateway making this decision does not know what interface the directed-broadcast datagram arrived on, the gateway cannot support directed broadcast to this connected network at all.

f. A gateway is permitted to protect its connected networks by

discarding directed broadcast datagrams.

A gateway will broadcast an IP datagram on a connected network if it is a directed broadcast destined for that network. Some gateway-gateway routing protocols (e.g., RIP) also require broadcasting routing updates on the connected networks. In either case, the datagram must have an IP broadcast address as its destination.

         Note:  as observed earlier, some host implementations (those
         based on Berkeley 4.2BSD) use zero rather than -1 in the host
         field.  To provide compatibility during the period until these
         systems are fixed or retired, it may be useful for a gateway to
         be configurable to send either choice of IP broadcast address
         and accept both if received.

A.2. ICMP Redirects

A gateway will generate an ICMP Redirect if and only if the destination IP address is reachable from the gateway (as determined by the routing algorithm) and the next-hop gateway is on the same (sub-)network as the source host. Redirects must not be sent in response to an IP network or subnet broadcast address or in response to a Class D or Class E IP address.

A host must discard an ICMP Redirect if the destination IP address is not its own IP address, or the new target address is not on the same (sub-)network. An accepted Redirect updates the routing data-base for the old target address. If there is no route 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.

RFC 1009 - Requirements for Internet Gateways June 1987

Appendix B. NSFNET Specific Requirements

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.  Proprietary and Extensibility Issues

Although hosts, gateways and networks supporting Internet technology have been in continuous operation for several years, vendors users and operators must understand that not all networking issues are fully resolved. 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 or augment existing ones. 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 new or augmented protocols.

NSF systems procurements may favor those vendors who 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, 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.2. Interconnection Technology

In order to ensure network-level interoperability of different vendor's gateways within the NSFNET context, we specify that a gateway must at a minimum support Ethernet connections and serial line protocol connections.

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.

RFC 1009 - Requirements for Internet Gateways June 1987

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, optical fiber, or other media may also be considered.

B.3. Routing Interoperability

The Internet does not currently have an "open IGP" standard, i.e., a common IGP which would allow gateways from different vendors to form a single Autonomous System. Several approaches to routing interoperability are currently in use among vendors and the NSF networking community.

• Proprietary IGP

At least one gateway vendor has implemented a proprietary IGP and uses EGP to interface to the rest of the Internet.

• RIP

Although RIP is undocumented and various implementations of it

differ in subtle ways, it has been used successfully for

interoperation among multiple vendors as an IGP.

• Gateway Daemon

The NSF networking community has built a "gateway daemon" program which can mediate among multiple routing protocols to create a mixed-IGP Autonomous System. In particular, the prototype gateway daemon executes on a 4.3BSD machine acting as a gateway and exchanges routing information with other gateways, speaking both RIP and Hello protocols; in addition, it supports EGP to other Autonomous Systems.

RFC 1009 - Requirements for Internet Gateways June 1987

B.4. Multi-Protocol Gateways

The present NSF gateway requirements specify only the Internet protocol IP. However, in a few years the Internet will begin a gradual transition to the functionally-equivalent subset of the ISO protocols [17]. In particular, an increasing percentage of the traffic will use the ISO Connectionless Mode Network Service (CLNS, but commonly called "ISO IP") [33] in place of IP. It is expected that the ISO suite will eventually become the dominant one; however, it is also expected that requirements to support Internet IP will continue, perhaps indefinitely.

To support the transition to ISO protocols and the coexistence stage, it is highly desirable that a gateway design provide for future extensions to support more than one protocol simultaneous, and in particular both IP and CLNS [18].

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 up through the application level [16]. 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.

Internet gateways function as intermediate systems (IS) with respect to the ISO connectionless network model and incorporate defined packet formats, routing algorithms and related procedures [33, 34]. The ISO ES-IS [37] provides the functions of ARP and ICMP Redirect.

   B.5.  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 subsequent edition of this document. Vendors are encouraged to plan for the introduction of these mechanisms into their products. While at this 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:

RFC 1009 - Requirements for Internet Gateways June 1987

1. The primary access control and accounting 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 mechanisms may be necessary in the gateways, 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 cannot be specified in advance.

3. NSF gateways may be required to incorporate access control and accounting mechanisms based on datagram source/destination address, as well as other fields in the IP header.

4. NSF gateways may be required to enforce policies on access to gateway and communication resources. These policies may be based upon equity ("fairness") or upon inequity ("priority").

RFC 1009 - Requirements for Internet Gateways June 1987

Acknowledgments

An earlier version of this document (RFC-985) [60] was prepared by Dave Mills in behalf of 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. This effort was chaired by Dave Mills, and contributed to by many people.

The authors of current document have also received assistance from many people in the NSF and ARPA networking community. We thank you, one and all.

RFC 1009 - Requirements for Internet Gateways June 1987

References and Bibliography

   Many of these references are  available from the DDN Network
   Information Center, SRI International, 333 Ravenswood Avenue, Menlo
   Park, California 94025 (telephone: 800-235-3155).
   
   [1]   Postel, J., "Internet Protocol", RFC-791, USC Information
         Sciences Institute, September 1981.
   
   [2]   Postel, J., "Internet Control Message Protocol", RFC-792, USC
         Information Sciences Institute, September 1981.
   
   [3]   BBN, "Interface Message Processor - Specifications for the
         Interconnection of a Host and an IMP", Report 1822, Bolt
         Beranek and Newman, December 1981.
   
   [4]   Plummer, D., "An Ethernet Address Resolution Protocol",
         RFC-826, Symbolics, September 1982.
   
   [5]   DOD, "Military Standard Internet Protocol", Military Standard
         MIL-STD-1777, United States Department of Defense, August 1983.
   
   [6]   BBN, "Defense Data Network X.25 Host Interface Specification",
         Report 5476, Bolt Beranek and Newman, December 1983.
   
   [7]   Hinden, R., "A Host Monitoring Protocol", RFC-869, BBN
         Communications, December 1983.
   
   [8]   Korb, J.T., "A Standard for the Transmission of IP Datagrams
         over Public Data Networks", RFC-877, Purdue University,
         September 1983.
   
   [9]   Nagle, J., "Congestion Control in IP/TCP Internetworks",
         RFC-896, Ford Aerospace, January 1984.
   
   [10]  Hornig, C., "A Standard for the Transmission of IP Datagrams
         over Ethernet Networks", RFC-894, Symbolics, April 1984.
   
   [11]  Mills, D.L., "Exterior Gateway Formal Specification", RFC-904,
         M/A-COM Linkabit, April 1984.
   
   [12]  Xerox, "Xerox Synchronous Point-to-Point Protocol", Xerox
         System Integration Standard 158412, December 1984.
   
   [13]  Kirton, P., "EGP Gateway under Berkeley UNIX 4.2", RFC-911, USC
         Information Sciences Institute, August 1984.

RFC 1009 - Requirements for Internet Gateways June 1987

   [14]  Postel, J., "Multi-LAN Address Resolution", RFC-925, USC
         Information Sciences Institute, October 1984.
   
   [15]  Finlayson, R., T. Mann, J. Mogul, and M. Theimer, "A Reverse
         Address Resolution Protocol", RFC-904, Stanford University,
         June 1984.
   
   [16]  NRC, "Transport Protocols for Department of Defense Data
         Networks", RFC-942, National Research Council, March 1985.
   
   [17]  Postel, J., "DOD Statement on NRC Report", RFC-945, USC
         Information Sciences Institute, April 1985.
   
   [18]  ISO, "Addendum to the Network Service Definition Covering
         Network Layer Addressing", 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, IEEE,
         Washington DC, March 1985.  Also in: IEEE Communications
         Magazine, March 1985.  Also available as ISI-RS-85-153.
   
   [20]  Romkey, J., "PC/IP Programmer's Manual", MIT Laboratory for
         Computer Science, pp. 57-59, April 1986.
   
   [21]  Mogul, J., and J. Postel, "Internet Standard Subnetting
         Procedure", RFC-950, Stanford University, August 1985.
   
   [22]  Reynolds, J., and J. Postel, "Official Internet Protocols",
         RFC-1011, USC Information Sciences Institute, May 1987.
   
   [23]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC-1010, USC
         Information Sciences Institute, May 1987.
   
   [24]  Nagle, J., "On Packet Switches with Infinite Storage", RFC-970,
         Ford Aerospace, December 1985.
   
   [25]  SRI, "DDN Protocol Handbook", NIC-50004, NIC-50005, NIC-50006,
         (three volumes), SRI International, December 1985.
   
   [26]  SRI, "ARPANET Information Brochure", NIC-50003, SRI
         International, December 1985.
   
   [27]  Mills, D.L., "Autonomous Confederations", RFC-975, M/A-COM
         Linkabit, February 1986.
   
   [28]  Jacobsen, O., and J. Postel, "Protocol Document Order
         Information",  RFC-980, SRI International, March 1986.

RFC 1009 - Requirements for Internet Gateways June 1987

   [29]  Malis, A.G., "PSN End-to-End Functional Specification",
         RFC-979, BBN Communications, March 1986.
   
   [30]  Postel, J, "Internetwork Applications using the DARPA Protocol
         Suite", Proceedings INFOCOM 85, IEEE, Washington DC,
         March 1985.  Also available as ISI-RS-85-151.
   
   [31]  Postel, J, C. Sunshine, and D. Cohen, "The ARPA Internet
         Protocol", Computer Networks, Vol. 5, No. 4, July 1981.
   
   [32]  Cerf, V., and R. Kahn, "A Protocol for Packet Network
         Intercommunication", IEEE Transactions on Communication,
         May 1974.
   
   [33]  ISO, "Protocol for Providing the Connectionless-mode Network
         Service", RFC-994, DIS-8473, International Standards
         Organization, March 1986.
   
   [34]  ANSI, "Draft Network Layer Routing Architecture", ANSI X3S3.3,
         86-215R, April 1987.
   
   [35]  Rosen, E., "Exterior Gateway Protocol (EGP)", RFC-827, Bolt
         Beranek and Newman, October 1982.
   
   [36]  Sidhu, D., "Some Problems with the Specification of the
         Military Standard Internet Protocol", RFC-963, Iowa State
         University, November 1985.
   
   [37]  ISO, "End System to Intermediate System Routing Exchange
         Protocol for use in conjunction with ISO 8473", RFC-995,
         April 1986.
   
   [38]  Postel, J., "Address Mappings", RFC-796, USC/Information
         Sciences Institute, September 1981.
   
   [39]  Mills, D., "DCN Local Network Protocols", RFC-891, M/A-COM
         Linkabit, December 1983.
   
   [40]  McQuillan, J. M., I. Richer, and E. C. Rosen, "The New Routing
         Algorithm for the ARPANET",  IEEE Transactions on
         Communications, May 1980.
   
   [41]  Hinden, R., and A. Sheltzer, "The DARPA Internet Gateway",
         RFC-823, Bolt Beranek and Newman, September 1982.
   
   [42]  Farber, D., G. Delp, and T. Conte, "A Thinwire Protocol for
         Connecting Personal Computers to the Internet", RFC-914,
         University of Delaware, September 1984.

RFC 1009 - Requirements for Internet Gateways June 1987

   [43]  Mills, D., "Statistics Server", RFC-996, University Of
         Delaware, February 1987.
   
   [44]  Postel, J. and K. Harrenstien, "Time Protocol", RFC-868,
         May 1983.
   
   [45]  Mills, D., "Network Time Protocol (NTP)", RFC-958, M/A-Com
         Linkabit, September 1985.
   
   [46]  Seamonson, L., and E. Rosen, "Stub Exterior Gateway Protocol",
         RFC-888, Bolt Beranek And Newman, January 1984.
   
   [47]  Deering, S., and D. Cheriton, "Host Groups: A Multicast
         Extension to the Internet Protocol", RFC-966, Stanford
         University, December 1985.
   
   [48]  Deering, S., "Host Extensions for IP Multicasting", RFC-988,
         Stanford University, July 1986.
   
   [49]  Mogul, J., "Broadcasting Internet Datagrams", RFC-919, Stanford
         University, October 1984.
   
   [50]  Mogul, J., "Broadcasting Internet Datagrams in the Presence of
         Subnets", RFC-922, Stanford University, October 1984.
   
   [51]  Rosen, E., "Exterior Gateway Protocol", RFC-827, Bolt Beranek
         and Newman, October 1982.
   
   [52]  Rose, M., "Low Tech Connection into the ARPA Internet: The Raw
         Packet Split Gateway", Technical Report 216, Department of
         Information and Computer Science, University of California,
         Irvine, February 1984.
   
   [53]  Rosen, E., "Issues in Buffer Management", IEN-182, Bolt Beranek
         and Newman, May 1981.
   
   [54]  Rosen, E., "Logical Addressing", IEN-183, Bolt Beranek and
         Newman, May 1981.
   
   [55]  Rosen, E., "Issues in Internetting - Part 1: Modelling the
         Internet", IEN-184, Bolt Beranek and Newman, May 1981.
   
   [56]  Rosen, E., "Issues in Internetting - Part 2: Accessing the
         Internet", IEN-187, Bolt Beranek and Newman, June 1981.
   
   [57]  Rosen, E., "Issues in Internetting - Part 3: Addressing",
         IEN-188, Bolt Beranek and Newman, June 1981.

RFC 1009 - Requirements for Internet Gateways June 1987

   [58]  Rosen, E., "Issues in Internetting - Part 4: Routing", IEN-189,
         Bolt Beranek and Newman, June 1981.
   
   [59]  Sunshine, C., "Comments on Rosen's Memos", IEN-191, USC
         Information Sciences Institute, July 1981.
   
   [60]  NTAG, "Requirements for Internet Gateways -- Draft", RFC-985,
         Network Technical Advisory Group, National Science Foundation,
         May 1986.
   
   [61]  Khanna, A., and Malis, A., "The ARPANET AHIP-E Host Access
         Protocol (Enhanced AHIP)", RFC-1005, BBN Communications,
         May 1987
   
   [62]  Nagle, J., "Congestion Control in IP/TCP Internetworks", ACM
         Computer Communications Review, Vol.14, no.4, October 1984.

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