Network Working Group R. Braden
Request for Comments: 1009 J. Postel
Obsoletes: 985 ISI
June 1987
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
Braden & Postel [Page 1]
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
Braden & Postel [Page 2]
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
Braden & Postel [Page 3]
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.
Braden & Postel [Page 4]
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>
Braden & Postel [Page 5]
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
Braden & Postel [Page 6]
RFC 1009 - Requirements for Internet Gateways June 1987
connected network the gateway must implement the
functions required by that network. These functions
typically include:
a. encapsulating and decapsulating the IP datagrams with
the connected network framing (e.g., an Ethernet
header and checksum);
b. sending and receiving IP datagrams up to the maximum
size supported by that network, this size is the
network's "Maximum Transmission Unit" or "MTU";
c. translating the IP destination address into an
appropriate network-level address for the connected
network (e.g., an Ethernet hardware address);
d. responding to the network flow control and error
indication, if any.
See Section 3 (Constituent Network Interface), for
details on particular constituent network interfaces.
3. Receives and forwards Internet datagrams. Important
issues are buffer management, congestion control, and
fairness. See Section 4 (Gateway Algorithms).
a. Recognizes various error conditions and generates
ICMP error and information messages as required.
b. Drops datagrams whose time-to-live fields have
reached zero.
c. Fragments datagrams when necessary to fit into the
MTU of the next network.
4. Chooses a next-hop destination for each IP datagram,
based on the information in its routing data-base. See
Section 4 (Gateway Algorithms).
5. Supports an interior gateway protocol (IGP) to carry out
distributed routing and reachability algorithms with the
other gateways in the same autonomous system. In
addition, some gateways will need to support the
Exterior Gateway Protocol (EGP) to exchange topological
information with other autonomous systems. See
Section 4 (Gateway Algorithms).
Braden & Postel [Page 7]
RFC 1009 - Requirements for Internet Gateways June 1987
6. Provides system support facilities, including loading,
debugging, status reporting, exception reporting and
control. See Section 5 (Operation and Maintenance).
1.2.2. Embedded Gateways
A gateway may be a stand-alone computer system, dedicated to
its IP router functions. Alternatively, it is possible to
embed gateway functionality within a host operating system
which supports connections to two or more networks. The
best-known example of an operating system with embedded gateway
code is the Berkeley BSD system. The embedded gateway feature
seems to make internetting easy, but it has a number of hidden
pitfalls:
1. If a host has only a single constituent-network
interface, it should not act as a gateway.
For example, hosts with embedded gateway code that
gratuitously forward broadcast packets or datagrams on
the same net often cause packet avalanches.
2. If a (multihomed) host acts as a gateway, it must
implement ALL the relevant gateway requirements
contained in this document.
For example, the routing protocol issues (see Sections
2.6 and 4.1) and the control and monitoring problems are
as hard and important for embedded gateways as for
stand-alone gateways.
Since Internet gateway requirements and
specifications may change independently of operating
system changes, an administration that operates an
embedded gateway in the Internet is strongly advised
to have an ability to maintain and update the gateway
code (e.g., this might require gateway code source).
3. Once a host runs embedded gateway code, it becomes part
of the Internet system. Thus, errors in software or
configuration of such a host can hinder communication
between other hosts. As a consequence, the host
administrator must lose some autonomy.
In many circumstances, a host administrator will need to
disable gateway coded embedded in the operating system,
and any embedded gateway code must be organized so it
can be easily disabled.
Braden & Postel [Page 8]
RFC 1009 - Requirements for Internet Gateways June 1987
4. If a host running embedded gateway code is concurrently
used for other services, the O&M (operation and
maintenance) requirements for the two modes of use may
be in serious conflict.
For example, gateway O&M will in many cases be performed
remotely by an operations center; this may require
privileged system access which the host administrator
would not normally want to distribute.
1.2.3. Transparent Gateways
The basic idea of a transparent gateway is that the hosts on
the local-area network behind such a gateway share the address
space of the wide-area network in front of the gateway. In
certain situations this is a very useful approach and the
limitations do not present significant drawbacks.
The words "in front" and "behind" indicate one of the
limitations of this approach: this model of interconnection is
suitable only for a geographically (and topologically) limited
stub environment. It requires that there be some form of
logical addressing in the network level addressing of the
wide-area network (that is, all the IP addresses in the local
environment map to a few (usually one) physical address in the
wide-area network, in a way consistent with the { IP address
<-> network address } mapping used throughout the wide-area
network).
Multihoming is possible on one wide-area network, but may
present routing problems if the interfaces are geographically
or topologically separated. Multihoming on two (or more)
wide-area networks is a problem due to the confusion of
addresses.
The behavior that hosts see from other hosts in what is
apparently the same network may differ if the transparent
gateway cannot fully emulate the normal wide-area network
service. For example, if there were a transparent gateway
between the ARPANET and an Ethernet, a remote host would not
receive a Destination Dead message [3] if it sent a datagram to
an Ethernet host that was powered off.
Braden & Postel [Page 9]
RFC 1009 - Requirements for Internet Gateways June 1987
1.3. Gateway Characteristics
Every Internet gateway must perform the functions listed above.
However, a vendor will have many choices on power, complexity, and
features for a particular gateway product. It may be helpful to
observe that the Internet system is neither homogeneous nor
fully-connected. For reasons of technology and geography, it is
growing into a global-interconnect system plus a "fringe" of LANs
around the "edge".
* The global-interconnect system is comprised of a number of
wide-area networks to which are attached gateways of several
ASs; there are relatively few hosts connected directly to
it. The global-interconnect system includes the ARPANET,
the NSFNET "backbone", the various NSF regional and
consortium networks, other ARPA sponsored networks such as
the SATNET and the WBNET, and the DCA sponsored MILNET. It
is anticipated that additional networks sponsored by these
and other agencies (such as NASA and DOE) will join the
global-interconnect system.
* Most hosts are connected to LANs, and many organizations
have clusters of LANs interconnected by local gateways.
Each such cluster is connected by gateways at one or more
points into the global-interconnect system. If it is
connected at only one point, a LAN is known as a "stub"
network.
Gateways in the global-interconnect system generally require:
* Advanced routing and forwarding algorithms
These gateways need routing algorithms which are highly
dynamic and also offer type-of-service routing. Congestion
is still not a completely resolved issue [24]. Improvements
to the current situation will be implemented soon, as the
research community is actively working on these issues.
* High availability
These gateways need to be highly reliable, providing 24 hour
a day, 7 days a week service. In case of failure, they must
recover quickly.
* Advanced O&M features
These gateways will typically be operated remotely from a
regional or national monitoring center. In their
Braden & Postel [Page 10]
RFC 1009 - Requirements for Internet Gateways June 1987
interconnect role, they will need to provide sophisticated
means for monitoring and measuring traffic and other events
and for diagnosing faults.
* High performance
Although long-haul lines in the Internet today are most
frequently 56 Kbps, DS1 lines (1.5 Mbps) are of increasing
importance, and even higher speeds are likely in the future.
Full-duplex operation is provided at any of these speeds.
The average size of Internet datagrams is rather small, of
the order of 100 bytes. At DS1 line speeds, the
per-datagram processing capability of the gateways, rather
than the line speed, is likely to be the bottleneck. To
fill a DS1 line with average-sized Internet datagrams, a
gateway would need to pass -- receive, route, and send --
2,000 datagrams per second per interface. That is, a
gateway which supported 3 DS1 lines and and Ethernet
interface would need to be able to pass a dazzling 2,000
datagrams per second in each direction on each of the
interfaces, or a aggregate throughput of 8,000 datagrams per
second, in order to fully utilize DS1 lines. This is beyond
the capability of current gateways.
Note: some vendors count input and output operations
separately in datagrams per second figures; for these
vendors, the above example would imply 16,000 datagrams
per second !
Gateways used in the "LAN fringe" (e.g., campus networks) will
generally have to meet less stringent requirements for
performance, availability, and maintenance. These may be high or
medium-performance devices, probably competitively procured from
several different vendors and operated by an internal organization
(e.g., a campus computing center). The design of these gateways
should emphasize low average delay and good burst performance,
together with delay and type-of-service sensitive resource
management. In this environment, there will be less formal O&M,
more hand-crafted static configurations for special cases, and
more need for inter-operation with gateways of other vendors. The
routing mechanism will need to be very flexible, but need not be
so highly dynamic as in the global-interconnect system.
It is important to realize that Internet gateways normally operate
in an unattended mode, but that equipment and software faults can
have a wide-spread (sometimes global) effect. In any environment,
Braden & Postel [Page 11]
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.
Braden & Postel [Page 12]
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
Braden & Postel [Page 13]
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}
Braden & Postel [Page 14]
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.
Braden & Postel [Page 15]
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
Braden & Postel [Page 16]
RFC 1009 - Requirements for Internet Gateways June 1987
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
Braden & Postel [Page 17]
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.
Braden & Postel [Page 18]
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.
Braden & Postel [Page 19]
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.
Braden & Postel [Page 20]
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.
Braden & Postel [Page 21]
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
Braden & Postel [Page 22]
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
Braden & Postel [Page 23]
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.
Braden & Postel [Page 24]
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.
Braden & Postel [Page 25]
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.
Braden & Postel [Page 26]
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.
Braden & Postel [Page 27]
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].
Braden & Postel [Page 28]
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].
Braden & Postel [Page 29]
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
Braden & Postel [Page 30]
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.
Braden & Postel [Page 31]
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
Braden & Postel [Page 32]
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.
Braden & Postel [Page 33]
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.
Braden & Postel [Page 34]
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.
Braden & Postel [Page 35]
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.
Braden & Postel [Page 36]
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.
Braden & Postel [Page 37]
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.
Braden & Postel [Page 38]
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
Braden & Postel [Page 39]
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.
Braden & Postel [Page 40]
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.
Braden & Postel [Page 41]
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)
Braden & Postel [Page 42]
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)
Braden & Postel [Page 43]
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
Braden & Postel [Page 44]
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.
Braden & Postel [Page 45]
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.
Braden & Postel [Page 46]
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.
Braden & Postel [Page 47]
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:
Braden & Postel [Page 48]
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").
Braden & Postel [Page 49]
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.
Braden & Postel [Page 50]
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.
Braden & Postel [Page 51]
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.
Braden & Postel [Page 52]
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.
Braden & Postel [Page 53]
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.
Braden & Postel [Page 54]
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.
Braden & Postel [Page 55]
