operation.
Constituent networks may generally be divided into two classes:
o Local-Area Networks (LANs)
LANs may have a variety of designs. In general, a LAN will
cover a small geographical area (e.g., a single building or
plant site) and provide high bandwidth with low delays. LANs
may be passive (similar to Ethernet) or they may be active
(such as ATM).
o Wide-Area Networks (WANs)
Geographically-dispersed hosts and LANs are interconnected by
wide-area networks, also called long-haul networks. These
networks may have a complex internal structure of lines and
packet-switches, or they may be as simple as point-to-point
lines.
2.2.3 Routers
In the Internet model, constituent networks are connected together
by IP datagram forwarders which are called routers or IP routers.
In this document, every use of the term router is equivalent to IP
router. Many older Internet documents refer to routers as
gateways.
Historically, routers have been realized with packet-switching
software executing on a general-purpose CPU. However, as custom
hardware development becomes cheaper and as higher throughput is
required, but special-purpose hardware is becoming increasingly
common. This specification applies to routers regardless of how
they are implemented.
A router is connected to two or more networks, appearing to each
of these networks as a connected host. Thus, it has (at least)
one physical interface and (at least) one IP address on each of
the connected networks (this ignores the concept of un-numbered
links, which is discussed in section [2.2.7]). Forwarding an IP
datagram generally requires the router to choose the address of
the next-hop router or (for the final hop) the destination host.
This choice, called routing, depends upon a routing database
within the router. The routing database is also sometimes known
as a routing table or forwarding table.
The routing database should be maintained dynamically to reflect
the current topology of the Internet system. A router normally
accomplishes this by participating in distributed routing and
reachability algorithms with other routers.
Routers provide datagram transport only, and they seek to minimize
the state information necessary to sustain this service in the
interest of routing flexibility and robustness.
Packet switching devices may also operate at the Link Layer; such
devices are usually called bridges. Network segments which are
connected by bridges share the same IP network number, i.e., they
logically form a single IP network. These other devices are
outside of the scope of this document.
Another variation on the simple model of networks connected with
routers sometimes occurs: a set of routers may be interconnected
with only serial lines, to form a network in which the packet
switching is performed at the Internetwork (IP) Layer rather than
the Link Layer.
2.2.4 Autonomous Systems
For technical, managerial, and sometimes political reasons, the
routers of the Internet system are grouped into collections called
autonomous systems. The routers included in a single autonomous
system (AS) are expected to:
o Be under the control of a single operations and maintenance
(O&M) organization;
o Employ common routing protocols among themselves, to
dynamically maintain their routing databases.
A number of different dynamic routing protocols have been
developed (see Section [7.2]); the routing protocol within a
single AS is generically called an interior gateway protocol or
IGP.
An IP datagram may have to traverse the routers of two or more ASs
to reach its destination, and the ASs must provide each other with
topology information to allow such forwarding. An exterior
gateway protocol (generally BGP or EGP) is used for this purpose.
2.2.5 Addresses and Subnets
An IP datagram carries 32-bit source and destination addresses,
each of which is partitioned into two parts - a constituent
network number and a host number on that network. Symbolically:
IP-address ::= { <Network-number>, <Host-number> }
To finally deliver the datagram, the last router in its path must
map the Host-number (or rest) part of an IP address into the
physical address of a host connection to the constituent network.
This simple notion has been extended by the concept of subnets,
which were introduced in order to allow arbitrary complexity of
interconnected LAN structures within an organization, while
insulating the Internet system against explosive growth in network
numbers and routing complexity. Subnets essentially provide a
multi-level hierarchical routing structure for the Internet
system. The subnet extension, described in [INTERNET:2], is now a
required part of the Internet architecture. The basic idea is to
partition the <Host-number> field into two parts: a subnet number,
and a true host number on that subnet:
IP-address ::=
{ <Network-number>, <Subnet-number>, <Host-number> }
The interconnected physical networks within an organization will
be given the same network number but different subnet numbers.
The distinction between the subnets of such a subnetted network is
normally not visible outside of that network. Thus, routing in
the rest of the Internet will be based only upon the <Network-
number> part of the IP destination address; routers outside the
network will combine <Subnet-number> and <Host-number> together to
form an uninterpreted rest part of the 32-bit IP address. Within
the subnetted network, the routers must route on the basis of an
extended network number:
{ <Network-number>, <Subnet-number> }
Under certain circumstances, it may be desirable to support
subnets of a particular network being interconnected only via a
path which is not part of the subnetted network. Even though many
IGP's and no EGP's currently support this configuration
effectively, routers need to be able to support this configuration
of subnetting (see Section [4.2.3.4]). In general, routers should
not make assumptions about what are subnets and what are not, but
simply ignore the concept of Class in networks, and treat each
route as a { network, mask }-tuple.
DISCUSSION:
It is becoming clear that as the Internet grows larger and
larger, the traditional uses of Class A, B, and C networks will
be modified in order to achieve better use of IP's 32-bit
address space. Classless Interdomain Routing (CIDR)
[INTERNET:15] is a method currently being deployed in the
Internet backbones to achieve this added efficiency. CIDR
depends on the ability of assigning and routing to networks
that are not based on Class A, B, or C networks. Thus, routers
should always treat a route as a network with a mask.
Furthermore, for similar reasons, a subnetted network need not
have a consistent subnet mask through all parts of the network.
For example, one subnet may use an 8 bit subnet mask, another 10
bit, and another 6 bit. Routers need to be able to support this
type of configuration (see Section [4.2.3.4]).
The bit positions containing this extended network number are
indicated by a 32-bit mask called the subnet mask; it is
recommended but not required that the <Subnet-number> bits be
contiguous and fall between the <Network-number> and the <Host-
number> fields. No subnet should be assigned the value zero or -1
(all one bits).
Although the inventors of the subnet mechanism probably expected
that each piece of an organization's network would have only a
single subnet number, in practice it has often proven necessary or
useful to have several subnets share a single physical cable.
There are special considerations for the router when a connected
network provides a broadcast or multicast capability; these will
be discussed later.
2.2.6 IP Multicasting
IP multicasting is an extension of Link Layer multicast to IP
internets. Using IP multicasts, a single datagram can be
addressed to multiple hosts. This collection of hosts is called a
multicast group. Each multicast group is represented as a Class D
IP address. An IP datagram sent to the group is to be delivered
to each group member with the same best-effort delivery as that
provided for unicast IP traffic. The sender of the datagram does
not itself need to be a member of the destination group.
The semantics of IP multicast group membership are defined in
[INTERNET:4]. That document describes how hosts and routers join
and leave multicast groups. It also defines a protocol, the
Internet Group Management Protocol (IGMP), that monitors IP
multicast group membership.
Forwarding of IP multicast datagrams is accomplished either
through static routing information or via a multicast routing
protocol. Devices that forward IP multicast datagrams are called
multicast routers. They may or may not also forward IP unicasts.
In general, multicast datagrams are forwarded on the basis of both
their source and destination addresses. Forwarding of IP
multicast packets is described in more detail in Section [5.2.1].
Appendix D discusses multicast routing protocols.
2.2.7 Unnumbered Lines and Networks and Subnets
Traditionally, each network interface on an IP host or router has
its own IP address. Over the years, people have observed that
this can cause inefficient use of the scarce IP address space,
since it forces allocation of an IP network number, or at least a
subnet number, to every point-to-point link.
To solve this problem, a number of people have proposed and
implemented the concept of unnumbered serial lines. An unnumbered
serial line does not have any IP network or subnet number
associated with it. As a consequence, the network interfaces
connected to an unnumbered serial line do not have IP addresses.
Because the IP architecture has traditionally assumed that all
interfaces had IP addresses, these unnumbered interfaces cause
some interesting dilemmas. For example, some IP options (e.g.
Record Route) specify that a router must insert the interface
address into the option, but an unnumbered interface has no IP
address. Even more fundamental (as we shall see in chapter 5) is
that routes contain the IP address of the next hop router. A
router expects that that IP address will be on an IP (sub)net that
the router is connected to. That assumption is of course violated
if the only connection is an unnumbered serial line.
To get around these difficulties, two schemes have been invented.
The first scheme says that two routers connected by an unnumbered
serial line aren't really two routers at all, but rather two
half-routers which together make up a single (virtual) router.
The unnumbered serial line is essentially considered to be an
internal bus in the virtual router. The two halves of the virtual
router must coordinate their activities in such a way that they
act exactly like a single router.
This scheme fits in well with the IP architecture, but suffers
from two important drawbacks. The first is that, although it
handles the common case of a single unnumbered serial line, it is
not readily extensible to handle the case of a mesh of routers and
unnumbered serial lines. The second drawback is that the
interactions between the half routers are necessarily complex and
are not standardized, effectively precluding the connection of
equipment from different vendors using unnumbered serial lines.
Because of these drawbacks, this memo has adopted an alternative
scheme, which has been invented multiple times but which is
probably originally attributable to Phil Karn. In this scheme, a
router which has unnumbered serial lines also has a special IP
address, called a router-id in this memo. The router-id is one of
the router's IP addresses (a router is required to have at least
one IP address). This router-id is used as if it is the IP
address of all unnumbered interfaces.
2.2.8 Notable Oddities
2.2.8.1 Embedded Routers
A router may be a stand-alone computer system, dedicated to its
IP router functions. Alternatively, it is possible to embed
router functions within a host operating system which supports
connections to two or more networks. The best-known example of
an operating system with embedded router code is the Berkeley
BSD system. The embedded router 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 router.
For example, hosts with embedded router code that
gratuitously forward broadcast packets or datagrams on the
same net often cause packet avalanches.
(2) If a (multihomed) host acts as a router, it must implement
ALL the relevant router requirements contained in this
document.
For example, the routing protocol issues and the router
control and monitoring problems are as hard and important
for embedded routers as for stand-alone routers.
Since Internet router requirements and specifications may
change independently of operating system changes, an
administration that operates an embedded router in the
Internet is strongly advised to have the ability to
maintain and update the router code (e.g., this might
require router code source).
(3) Once a host runs embedded router code, it becomes part of
the Internet system. Thus, errors in software or
configuration 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 router code embedded in the operating system, and
any embedded router code must be organized so that it can
be easily disabled.
(4) If a host running embedded router 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, router 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.
2.2.8.2 Transparent Routers
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 routers 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.
Routers that support this second model are called address
sharing routers or transparent routers. The focus of this memo
is on routers that support the first model, but this is not
intended to exclude the use of transparent routers.
The basic idea of a transparent router is that the hosts on the
local-area network behind such a router share the address space
of the wide-area network in front of the router. 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. All of the IP addresses in the local environment map
to a few (usually one) physical address in the wide-area
network. This mapping occurs 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
router cannot fully emulate the normal wide-area network
service. For example, the ARPANET used a Link Layer protocol
that provided a Destination Dead indication in response to an
attempt to send to a host which was powered off. However, if
there were a transparent router between the ARPANET and an
Ethernet, a host on the ARPANET would not receive a Destination
Dead indication if it sent a datagram to a host that was
powered off and was connected to the ARPANET via the
transparent router instead of directly.
2.3 Router Characteristics
An Internet router 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.
(2) Interfaces to two or more packet networks. For each connected
network the router must implement the functions required by that
network. These functions typically include:
o Encapsulating and decapsulating the IP datagrams with the
connected network framing (e.g., an Ethernet header and
checksum),
o 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,
o Translating the IP destination address into an appropriate
network-level address for the connected network (e.g., an
Ethernet hardware address), if needed, and
o Responding to the network flow control and error indication,
if any.
See chapter 3 (Link Layer).
(3) Receives and forwards Internet datagrams. Important issues in
this process are buffer management, congestion control, and
fairness.
o Recognizes various error conditions and generates ICMP error
and information messages as required.
o Drops datagrams whose time-to-live fields have reached zero.
o Fragments datagrams when necessary to fit into the MTU of the
next network.
See chapter 4 (Internet Layer - Protocols) and chapter 5
(Internet Layer - Forwarding) for more information.
(4) Chooses a next-hop destination for each IP datagram, based on
the information in its routing database. See chapter 5
(Internet Layer - Forwarding) for more information.
(5) (Usually) supports an interior gateway protocol (IGP) to carry
out distributed routing and reachability algorithms with the
other routers in the same autonomous system. In addition, some
routers will need to support an exterior gateway protocol (EGP)
to exchange topological information with other autonomous
systems. See chapter 7 (Application Layer - Routing Protocols)
for more information.
(6) Provides network management and system support facilities,
including loading, debugging, status reporting, exception
reporting and control. See chapter 8 (Application Layer -
Network Management Protocols) and chapter 10 (Operation and
Maintenance) for more information.
A router vendor will have many choices on power, complexity, and
features for a particular router 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. More and more these fringe LANs are becoming richly
interconnected, thus making them less out on the fringe and more
demanding on router requirements.
o The global interconnect system is comprised of a number of wide-
area networks to which are attached routers of several Autonomous
Systems (AS); there are relatively few hosts connected directly to
the system.
o Most hosts are connected to LANs. Many organizations have
clusters of LANs interconnected by local routers. Each such
cluster is connected by routers 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.
Routers in the global interconnect system generally require:
o Advanced Routing and Forwarding Algorithms
These routers need routing algorithms which are highly dynamic and
also offer type-of-service routing. Congestion is still not a
completely resolved issue (see Section [5.3.6]). Improvements in
these areas are expected, as the research community is actively
working on these issues.
o High Availability
These routers need to be highly reliable, providing 24 hours a
day, 7 days a week service. Equipment and software faults can
have a wide-spread (sometimes global) effect. In case of failure,
they must recover quickly. In any environment, a router must be
highly robust and able to operate, possibly in a degraded state,
under conditions of extreme congestion or failure of network
resources.
o Advanced O&M Features
Internet routers normally operate in an unattended mode. They
will typically be operated remotely from a centralized monitoring
center. They need to provide sophisticated means for monitoring
and measuring traffic and other events and for diagnosing faults.
o High Performance
Long-haul lines in the Internet today are most frequently 56 Kbps,
DS1 (1.4Mbps), and DS3 (45Mbps) speeds. LANs are typically
Ethernet (10Mbps) and, to a lesser degree, FDDI (100Mbps).
However, network media technology is constantly advancing and even
higher speeds are likely in the future. Full-duplex operation is
provided at all of these speeds.
The requirements for routers used in the LAN fringe (e.g., campus
networks) depend greatly on the demands of the local networks. 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
routers should emphasize low average latency and good burst
performance, together with delay and type-of-service sensitive
resource management. In this environment there may be less formal O&M
but it will not be less important. The need for the routing
mechanism to be highly dynamic will become more important as networks
become more complex and interconnected. Users will demand more out
of their local connections because of the speed of the global
interconnects.
As networks have grown, and as more networks have become old enough
that they are phasing out older equipment, it has become increasingly
imperative that routers interoperate with routers from other vendors.
Even though the Internet system is not fully interconnected, many
parts of the system need to have redundant connectivity. Rich
connectivity allows reliable service despite failures of
communication lines and routers, and it can also improve service by
shortening Internet paths and by providing additional capacity.
Unfortunately, this richer topology can make it much more difficult
to choose the best path to a particular destination.
2.4 Architectural Assumptions
The current Internet architecture is based on a set of assumptions
about the communication system. The assumptions most relevant to
routers are as follows:
o The Internet is a network of networks.
Each host is directly connected to some particular network(s); its
connection to the Internet is only conceptual. Two hosts on the
same network communicate with each other using the same set of
protocols that they would use to communicate with hosts on distant
networks.
o Routers don't keep connection state information.
To improve the robustness of the communication system, routers are
designed to be stateless, forwarding each IP packet independently
of other packets. As a result, redundant paths can be exploited
to provide robust service in spite of failures of intervening
routers and networks.
All state information required for end-to-end flow control and
reliability is implemented in the hosts, in the transport layer or
in application programs. All connection control information is
thus co-located with the end points of the communication, so it
will be lost only if an end point fails. Routers effect flow
control only indirectly, by dropping packets or increasing network
delay.
Note that future protocol developments may well end up putting
some more state into routers. This is especially likely for
resource reservation and flows.
o Routing complexity should be in the routers.
Routing is a complex and difficult problem, and ought to be
performed by the routers, not the hosts. An important objective
is to insulate host software from changes caused by the inevitable
evolution of the Internet routing architecture.
o The system must tolerate wide network variation.
A basic objective of the Internet design is to tolerate a wide
range of network characteristics - e.g., bandwidth, delay, packet
loss, packet reordering, and maximum packet size. Another
objective is robustness against failure of individual networks,
routers, and hosts, using whatever bandwidth is still available.
Finally, the goal is full open system interconnection: an Internet
router must be able to interoperate robustly and effectively with
any other router or Internet host, across diverse Internet paths.
Sometimes implementors have designed for less ambitious goals.
For example, the LAN environment is typically much more benign
than the Internet as a whole; LANs have low packet loss and delay
and do not reorder packets. Some vendors have fielded
implementations that are adequate for a simple LAN environment,
but work badly for general interoperation. The vendor justifies
such a product as being economical within the restricted LAN
market. However, isolated LANs seldom stay isolated for long;
they are soon connected to each other, to organization-wide
internets, and eventually to the global Internet system. In the
end, neither the customer nor the vendor is served by incomplete
or substandard routers.
The requirements spelled out in this document are designed for a
full-function router. It is intended that fully compliant routers
will be usable in almost any part of the Internet.
3. LINK LAYER
Although [INTRO:1] covers Link Layer standards (IP over foo, ARP,
etc.), this document anticipates that Link-Layer material will be
covered in a separate Link Layer Requirements document. A Link-Layer
requirements document would be applicable to both hosts and routers.
Thus, this document will not obsolete the parts of [INTRO:1] that deal
with link-layer issues.
3.1 INTRODUCTION
Routers have essentially the same Link Layer protocol requirements as
other sorts of Internet systems. These requirements are given in
chapter 3 of Requirements for Internet Gateways [INTRO:1]. A router
MUST comply with its requirements and SHOULD comply with its
recommendations. Since some of the material in that document has
become somewhat dated, some additional requirements and explanations
are included below.
DISCUSSION:
It is expected that the Internet community will produce a
Requirements for Internet Link Layer standard which will supersede
both this chapter and chapter 3 of [INTRO:1].
3.2 LINK/INTERNET LAYER INTERFACE
Although this document does not attempt to specify the interface
between the Link Layer and the upper layers, it is worth noting here
that other parts of this document, particularly chapter 5, require
various sorts of information to be passed across this layer boundary.
This section uses the following definitions:
o Source physical address
The source physical address is the Link Layer address of the host
or router from which the packet was received.
o Destination physical address
The destination physical address is the Link Layer address to
which the packet was sent.
The information that must pass from the Link Layer to the
Internetwork Layer for each received packet is:
(1) The IP packet [5.2.2],
(2) The length of the data portion (i.e., not including the Link-
Layer framing) of the Link Layer frame [5.2.2],
(3) The identity of the physical interface from which the IP packet
was received [5.2.3], and
(4) The classification of the packet's destination physical address
as a Link Layer unicast, broadcast, or multicast [4.3.2],
[5.3.4].
In addition, the Link Layer also should provide:
(5) The source physical address.
The information that must pass from the Internetwork Layer to the
Link Layer for each transmitted packet is:
(1) The IP packet [5.2.1]
(2) The length of the IP packet [5.2.1]
(3) The destination physical interface [5.2.1]
(4) The next hop IP address [5.2.1]
In addition, the Internetwork Layer also should provide:
(5) The Link Layer priority value [5.3.3.2]
The Link Layer must also notify the Internetwork Layer if the packet
to be transmitted causes a Link Layer precedence-related error
[5.3.3.3].
3.3 SPECIFIC ISSUES
3.3.1 Trailer Encapsulation
Routers which can connect to 10Mb Ethernets MAY be able to receive
and forward Ethernet packets encapsulated using the trailer
encapsulation described in [LINK:1]. However, a router SHOULD NOT
originate trailer encapsulated packets. A router MUST NOT
originate trailer encapsulated packets without first verifying,
using the mechanism described in section 2.3.1 of [INTRO:2], that
the immediate destination of the packet is willing and able to
accept trailer-encapsulated packets. A router SHOULD NOT agree
(using these same mechanisms) to accept trailer-encapsulated
packets.
3.3.2 Address Resolution Protocol - ARP
Routers which implement ARP MUST be compliant and SHOULD be
unconditionally compliant with the requirements in section 2.3.2
of [INTRO:2].
The link layer MUST NOT report a Destination Unreachable error to
IP solely because there is no ARP cache entry for a destination.
A router MUST not believe any ARP reply which claims that the Link
Layer address of another host or router is a broadcast or
multicast address.
3.3.3 Ethernet and 802.3 Coexistence
Routers which can connect to 10Mb Ethernets MUST be compliant and
SHOULD be unconditionally compliant with the requirements of
Section [2.3.3] of [INTRO:2].
3.3.4 Maximum Transmission Unit - MTU
The MTU of each logical interface MUST be configurable.
Many Link Layer protocols define a maximum frame size that may be
sent. In such cases, a router MUST NOT allow an MTU to be set
which would allow sending of frames larger than those allowed by
the Link Layer protocol. However, a router SHOULD be willing to
receive a packet as large as the maximum frame size even if that
is larger than the MTU.
DISCUSSION:
Note that this is a stricter requirement than imposed on hosts
by [INTRO:2], which requires that the MTU of each physical
interface be configurable.
If a network is using an MTU smaller than the maximum frame
size for the Link Layer, a router may receive packets larger
than the MTU from hosts which are in the process of
initializing themselves, or which have been misconfigured.
In general, the Robustness Principle indicates that these
packets should be successfully received, if at all possible.
3.3.5 Point-to-Point Protocol - PPP
Contrary to [INTRO:1], the Internet does have a standard serial
line protocol: the Point-to-Point Protocol (PPP), defined in
[LINK:2], [LINK:3], [LINK:4], and [LINK:5].
A serial line interface is any interface which is designed to send
data over a telephone, leased, dedicated or direct line (either 2
or 4 wire) using a standardized modem or bit serial interface
(such as RS-232, RS-449 or V.35), using either synchronous or
asynchronous clocking.
A general purpose serial interface is a serial line interface
which is not solely for use as an access line to a network for
which an alternative IP link layer specification exists (such as
X.25 or Frame Relay).
Routers which contain such general purpose serial interfaces MUST
implement PPP.
PPP MUST be supported on all general purpose serial interfaces on
a router. The router MAY allow the line to be configured to use
serial line protocols other than PPP, all general purpose serial
interfaces MUST default to using PPP.
3.3.5.1 Introduction
This section provides guidelines to router implementors so that
they can ensure interoperability with other routers using PPP
over either synchronous or asynchronous links.
It is critical that an implementor understand the semantics of
the option negotiation mechanism. Options are a means for a
local device to indicate to a remote peer what the local device
will *accept* from the remote peer, not what it wishes to send.
It is up to the remote peer to decide what is most convenient
to send within the confines of the set of options that the
local device has stated that it can accept. Therefore it is
perfectly acceptable and normal for a remote peer to ACK all
the options indicated in an LCP Configuration Request (CR) even
if the remote peer does not support any of those options.
Again, the options are simply a mechanism for either device to
indicate to its peer what it will accept, not necessarily what
it will send.
3.3.5.2 Link Control Protocol (LCP) Options
The PPP Link Control Protocol (LCP) offers a number of options
that may be negotiated. These options include (among others)
address and control field compression, protocol field
compression, asynchronous character map, Maximum Receive Unit
(MRU), Link Quality Monitoring (LQM), magic number (for
loopback detection), Password Authentication Protocol (PAP),
Challenge Handshake Authentication Protocol (CHAP), and the
32-bit Frame Check Sequence (FCS).
A router MAY do address/control field compression on either
synchronous or asynchronous links. A router MAY do protocol
field compression on either synchronous or asynchronous links.
A router MAY indicate that it can accept these compressions,
but MUST be able to accept uncompressed PPP header information
even if it has indicated a willingness to receive compressed
PPP headers.
DISCUSSION:
These options control the appearance of the PPP header.
Normally the PPP header consists of the address field (one
byte containing the value 0xff), the control field (one byte
containing the value 0x03), and the two-byte protocol field
that identifies the contents of the data area of the frame.
If a system negotiates address and control field compression
it indicates to its peer that it will accept PPP frames that
have or do not have these fields at the front of the header.
It does not indicate that it will be sending frames with
these fields removed. The protocol field may also be
compressed from two to one byte in most cases.
IMPLEMENTATION:
Some hardware does not deal well with variable length header
information. In those cases it makes most sense for the
remote peer to send the full PPP header. Implementations
may ensure this by not sending the address/control field and
protocol field compression options to the remote peer. Even
if the remote peer has indicated an ability to receive
compressed headers there is no requirement for the local
router to send compressed headers.
A router MUST negotiate the Async Control Character Map (ACCM)
for asynchronous PPP links, but SHOULD NOT negotiate the ACCM
for synchronous links. If a router receives an attempt to
negotiate the ACCM over a synchronous link, it MUST ACKnowledge
the option and then ignore it.
DISCUSSION:
There are implementations that offer both sync and async
modes of operation and may use the same code to implement
the option negotiation. In this situation it is possible
that one end or the other may send the ACCM option on a
synchronous link.
A router SHOULD properly negotiate the maximum receive unit
(MRU). Even if a system negotiates an MRU smaller than 1,500
bytes, it MUST be able to receive a 1,500 byte frame.
A router SHOULD negotiate and enable the link quality
monitoring (LQM) option.
DISCUSSION:
This memo does not specify a policy for deciding whether the
link's quality is adequate. However, it is important (see
Section [3.3.6]) that a router disable failed links.
A router SHOULD implement and negotiate the magic number option
for loopback detection.
A router MAY support the authentication options (PAP - password
authentication protocol, and/or CHAP - challenge handshake
authentication protocol).
A router MUST support 16-bit CRC frame check sequence (FCS) and
MAY support the 32-bit CRC.
3.3.5.3 IP Control Protocol (ICP) Options
A router MAY offer to perform IP address negotiation. A router
MUST accept a refusal (REJect) to perform IP address
negotiation from the peer.
A router SHOULD NOT perform Van Jacobson header compression of
TCP/IP packets if the link speed is in excess of 64 Kbps.
Below that speed the router MAY perform Van Jacobson (VJ)
header compression. At link speeds of 19,200 bps or less the
router SHOULD perform VJ header compression.
3.3.6 Interface Testing
A router MUST have a mechanism to allow routing software to
determine whether a physical interface is available to send
packets or not. A router SHOULD have a mechanism to allow routing
software to judge the quality of a physical interface. A router
MUST have a mechanism for informing the routing software when a
physical interface becomes available or unavailable to send
packets because of administrative action. A router MUST have a
mechanism for informing the routing software when it detects a
Link level interface has become available or unavailable, for any
reason.
DISCUSSION:
It is crucial that routers have workable mechanisms for
determining that their network connections are functioning
properly, since failure to do so (or failure to take the proper
actions when a problem is detected) can lead to black holes.
The mechanisms available for detecting problems with network
connections vary considerably, depending on the Link Layer
protocols in use and also in some cases on the interface
hardware chosen by the router manufacturer. The intent is to
maximize the capability to detect failures within the Link-
Layer constraints.
4. INTERNET LAYER - PROTOCOLS
4.1 INTRODUCTION
This chapter and chapter 5 discuss the protocols used at the Internet
Layer: IP, ICMP, and IGMP. Since forwarding is obviously a crucial
topic in a document discussing routers, chapter 5 limits itself to
the aspects of the protocols which directly relate to forwarding.
The current chapter contains the remainder of the discussion of the
Internet Layer protocols.
4.2 INTERNET PROTOCOL - IP
4.2.1 INTRODUCTION
Routers MUST implement the IP protocol, as defined by
[INTERNET:1]. They MUST also implement its mandatory extensions:
subnets (defined in [INTERNET:2]), and IP broadcast (defined in
[INTERNET:3]).
A router MUST be compliant, and SHOULD be unconditionally
compliant, with the requirements of sections 3.2.1 and 3.3 of
[INTRO:2], except that:
o Section 3.2.1.1 may be ignored, since it duplicates
requirements found in this memo.
o Section 3.2.1.2 may be ignored, since it duplicates
requirements found in this memo.
o Section 3.2.1.3 should be ignored, since it is superseded by
Section [4.2.2.11] of this memo.
o Section 3.2.1.4 may be ignored, since it duplicates
requirements found in this memo.
o Section 3.2.1.6 should be ignored, since it is superseded by
Section [4.2.2.4] of this memo.
o Section 3.2.1.8 should be ignored, since it is superseded by
Section [4.2.2.1] of this memo.
In the following, the action specified in certain cases is to
silently discard a received datagram. This means that the
datagram will be discarded without further processing and that the
router will not send any ICMP error message (see Section [4.3]) as
a result. However, for diagnosis of problems a router SHOULD
provide the capability of logging the error (see Section [1.3.3]),
including the contents of the silently-discarded datagram, and
SHOULD record the event in a statistics counter.
4.2.2 PROTOCOL WALK-THROUGH
RFC791 is [INTERNET:1], the specification for the Internet
Protocol.
4.2.2.1 Options: RFC-791 Section 3.2
In datagrams received by the router itself, the IP layer MUST
interpret those IP options that it understands and preserve the
rest unchanged for use by higher layer protocols.
Higher layer protocols may require the ability to set IP
options in datagrams they send or examine IP options in
datagrams they receive. Later sections of this document
discuss specific IP option support required by higher layer
protocols.
DISCUSSION:
Neither this memo nor [INTRO:2] define the order in which a
receiver must process multiple options in the same IP
header. Hosts and routers originating datagrams containing
multiple options must be aware that this introduces an
ambiguity in the meaning of certain options when combined
with a source-route option.
Here are the requirements for specific IP options:
(a) Security Option
Some environments require the Security option in every
packet originated or received. Routers SHOULD IMPLEMENT
the revised security option described in [INTERNET:5].
DISCUSSION:
Note that the security options described in
[INTERNET:1] and RFC1038 ([INTERNET:16]) are obsolete.
(b) Stream Identifier Option
This option is obsolete; routers SHOULD NOT place this
option in a datagram that the router originates. This
option MUST be ignored in datagrams received by the
router.
(c) Source Route Options
A router MUST be able to act as the final destination of a
source route. If a router receives a packet containing a
completed source route (i.e., the pointer points beyond
the last field and the destination address in the IP
header addresses the router), the packet has reached its
final destination; the option as received (the recorded
route) MUST be passed up to the transport layer (or to
ICMP message processing).
In order to respond correctly to source-routed datagrams
it receives, a router MUST provide a means whereby
transport protocols and applications can reverse the
source route in a received datagram and insert the
reversed source route into datagrams they originate (see
Section 4 of [INTRO:2] for details).
Some applications in the router MAY require that the user
be able to enter a source route.
A router MUST NOT originate a datagram containing multiple
source route options. What a router should do if asked to
forward a packet containing multiple source route options
is described in Section [5.2.4.1].
When a source route option is created, it MUST be
correctly formed even if it is being created by reversing
a recorded route that erroneously includes the source host
(see case (B) in the discussion below).
DISCUSSION:
Suppose a source routed datagram is to be routed from
source S to destination D via routers G1, G2, ... Gn.
Source S constructs a datagram with G1's IP address as
its destination address, and a source route option to
get the datagram the rest of the way to its
destination. However, there is an ambiguity in the
specification over whether the source route option in a
datagram sent out by S should be (A) or (B):
(A): {>>G2, G3, ... Gn, D} <--- CORRECT
(B): {S, >>G2, G3, ... Gn, D} <---- WRONG
(where >> represents the pointer). If (A) is sent, the
datagram received at D will contain the option: {G1,
G2, ... Gn >>}, with S and D as the IP source and
destination addresses. If (B) were sent, the datagram
received at D would again contain S and D as the same
IP source and destination addresses, but the option
would be: {S, G1, ...Gn >>}; i.e., the originating host
would be the first hop in the route.
(d) Record Route Option
Routers MAY support the Record Route option in datagrams
originated by the router.
(e) Timestamp Option
Routers MAY support the timestamp option in datagrams
originated by the router. The following rules apply:
o When originating a datagram containing a Timestamp
Option, a router MUST record a timestamp in the option
if
- Its Internet address fields are not pre-specified or
- Its first pre-specified address is the IP address of
the logical interface over which the datagram is
being sent (or the router's router-id if the
datagram is being sent over an unnumbered
interface).
o If the router itself receives a datagram containing a
Timestamp Option, the router MUST insert the current
timestamp into the Timestamp Option (if there is space
in the option to do so) before passing the option to
the transport layer or to ICMP for processing.
o A timestamp value MUST follow the rules given in
Section [3.2.2.8] of [INTRO:2].
IMPLEMENTATION:
To maximize the utility of the timestamps contained in
the timestamp option, it is suggested that the
timestamp inserted be, as nearly as practical, the time
at which the packet arrived at the router. For
datagrams originated by the router, the timestamp
inserted should be, as nearly as practical, the time at
which the datagram was passed to the Link Layer for
transmission.
4.2.2.2 Addresses in Options: RFC-791 Section 3.1
When a router inserts its address into a Record Route, Strict
Source and Record Route, Loose Source and Record Route, or
Timestamp, it MUST use the IP address of the logical interface
on which the packet is being sent. Where this rule cannot be
obeyed because the output interface has no IP address (i.e., is
an unnumbered interface), the router MUST instead insert its
router-id. The router's router-id is one of the router's IP
addresses. Which of the router's addresses is used as the
router-id MUST NOT change (even across reboots) unless changed
by the network manager or unless the configuration of the
router is changed such that the IP address used as the router-
id ceases to be one of the router's IP addresses. Routers with
multiple unnumbered interfaces MAY have multiple router-id's.
Each unnumbered interface MUST be associated with a particular
router-id. This association MUST NOT change (even across
reboots) without reconfiguration of the router.
DISCUSSION:
This specification does not allow for routers which do not
have at least one IP address. We do not view this as a
serious limitation, since a router needs an IP address to
meet the manageability requirements of Chapter [8] even if
the router is connected only to point-to-point links.
IMPLEMENTATION:
One possible method of choosing the router-id that fulfills
this requirement is to use the numerically smallest (or
greatest) IP address (treating the address as a 32-bit
integer) that is assigned to the router.
4.2.2.3 Unused IP Header Bits: RFC-791 Section 3.1
The IP header contains two reserved bits: one in the Type of
Service byte and the other in the Flags field. A router MUST
NOT set either of these bits to one in datagrams originated by
the router. A router MUST NOT drop (refuse to receive or
forward) a packet merely because one or more of these reserved
bits has a non-zero value.
DISCUSSION:
Future revisions to the IP protocol may make use of these
unused bits. These rules are intended to ensure that these
revisions can be deployed without having to simultaneously
upgrade all routers in the Internet.
4.2.2.4 Type of Service: RFC-791