This Appendix defines specific terms used in this memo. It also
defines some general purpose terms that may be of interest. See also
[INTRO:9] for a more general set of definitions.
- Autonomous System (AS)
-
An Autonomous System (AS) is a connected segment of a network
topology that consists of a collection of subnetworks (with
hosts attached) interconnected by a set of routes. The
subnetworks and the routers are expected to be under the control
of a single operations and maintenance (O&M) organization.
Within an AS routers may use one or more interior routing
protocols, and sometimes several sets of metrics. An AS is
expected to present to other ASs an appearence of a coherent
interior routing plan, and a consistent picture of the
destinations reachable through the AS. An AS is identified by
an Autonomous System number.
- Connected Network
-
A network prefix to which a router is interfaced is often known
as a local network or the subnetwork of that router. However,
these terms can cause confusion, and therefore we use the term
Connected Network in this memo.
- Connected (Sub)Network
-
A Connected (Sub)Network is an IP subnetwork to which a router
is interfaced, or a connected network if the connected network
is not subnetted. See also Connected Network.
- Datagram
-
The unit transmitted between a pair of internet modules. Data,
called datagrams, from sources to destinations. The Internet
Protocol does not provide a reliable communication facility.
There are no acknowledgments either end-to-end or hop-by-hop.
There is no error no retransmissions. There is no flow control.
See IP.
- Default Route
-
A routing table entry that is used to direct any data addressed
to any network prefixes not explicitly listed in the routing
table.
- Dense Mode
-
In multicast forwarding, two paradigms are possible: in Dense
Mode forwarding, a network multicast is forwarded as a data link
layer multicast to all interfaces except that on which it was
received, unless and until the router is instructed not to by a
multicast routing neighbor. See Sparse Mode.
- EGP
-
Exterior Gateway Protocol A protocol that distributes routing
information to the gateways (routers) which connect autonomous
systems. See IGP.
- EGP-2
-
Exterior Gateway Protocol version 2 This is an EGP routing
protocol developed to handle traffic between Autonomous Systems
in the Internet.
- Forwarder
-
The logical entity within a router that is responsible for
switching packets among the router's interfaces. The Forwarder
also makes the decisions to queue a packet for local delivery,
to queue a packet for transmission out another interface, or
both.
- Forwarding
-
Forwarding is the process a router goes through for each packet
received by the router. The packet may be consumed by the
router, it may be output on one or more interfaces of the
router, or both. Forwarding includes the process of deciding
what to do with the packet as well as queuing it up for
(possible) output or internal consumption.
- Forwarding Information Base (FIB)
-
The table containing the information necessary to forward IP
Datagrams, in this document, is called the Forwarding
Information Base. At minimum, this contains the interface
identifier and next hop information for each reachable
destination network prefix.
- Fragment
-
An IP datagram that represents a portion of a higher layer's
packet that was too large to be sent in its entirety over the
output network.
- General Purpose Serial Interface
-
A physical medium capable of connecting exactly two systems, and
therefore configurable as a point to point line, but also
configurable to support link layer networking using protocols
such as X.25 or Frame Relay. A link layer network connects
another system to a switch, and a higher communication layer
multiplexes virtual circuits on the connection. See Point to
Point Line.
- IGP
-
Interior Gateway Protocol A protocol that distributes routing
information with an Autonomous System (AS). See EGP.
- Interface IP Address
-
The IP Address and network prefix length that is assigned to a
specific interface of a router.
- Internet Address
-
An assigned number that identifies a host in an internet. It
has two parts: an IP address and a prefix length. The prefix
length indicates how many of the most specific bits of the
address constitute the network prefix.
- IP
-
Internet Protocol The network layer protocol for the Internet.
It is a packet switching, datagram protocol defined in RFC 791.
IP does not provide a reliable communications facility; that is,
there are no end-to-end of hop-by-hop acknowledgments.
- IP Datagram
-
An IP Datagram is the unit of end-to-end transmission in the
Internet Protocol. An IP Datagram consists of an IP header
followed by all of higher-layer data (such as TCP, UDP, ICMP,
and the like). An IP Datagram is an IP header followed by a
message.
An IP Datagram is a complete IP end-to-end transmission unit.
An IP Datagram is composed of one or more IP Fragments.
In this memo, the unqualified term Datagram should be understood
to refer to an IP Datagram.
- IP Fragment
-
An IP Fragment is a component of an IP Datagram. An IP Fragment
consists of an IP header followed by all or part of the higher-
layer of the original IP Datagram.
One or more IP Fragments comprises a single IP Datagram.
In this memo, the unqualified term Fragment should be understood
to refer to an IP Fragment.
- IP Packet
-
An IP Datagram or an IP Fragment.
In this memo, the unqualified term Packet should generally be
understood to refer to an IP Packet.
- Logical [network] interface
-
We define a logical [network] interface to be a logical path,
distinguished by a unique IP address, to a connected network.
- Martian Filtering
-
A packet that contains an invalid source or destination address
is considered to be martian and discarded.
- MTU (Maximum Transmission Unit)
-
The size of the largest packet that can be transmitted or
received through a logical interface. This size includes the IP
header but does not include the size of any Link Layer headers
or framing.
- Multicast
-
A packet that is destined for multiple hosts. See broadcast.
- Multicast Address
-
A special type of address that is recognizable by multiple
hosts.
A Multicast Address is sometimes known as a Functional Address
or a Group Address.
- Network Prefix
-
The portion of an IP Address that signifies a set of systems.
It is selected from the IP Address by logically ANDing a subnet
mask with the address, or (equivalently) setting the bits of the
address not among the most significant <prefix-length> bits of
the address to zero.
- Originate
-
Packets can be transmitted by a router for one of two reasons:
1) the packet was received and is being forwarded or 2) the
router itself created the packet for transmission (such as route
advertisements). Packets that the router creates for
transmission are said to originate at the router.
- Packet
-
A packet is the unit of data passed across the interface between
the Internet Layer and the Link Layer. It includes an IP header
and data. A packet may be a complete IP datagram or a fragment
of an IP datagram.
- Path
-
The sequence of routers and (sub-)networks that a packet
traverses from a particular router to a particular destination
host. Note that a path is uni-directional; it is not unusual to
have different paths in the two directions between a given host
pair.
- Physical Network
-
A Physical Network is a network (or a piece of an internet)
which is contiguous at the Link Layer. Its internal structure
(if any) is transparent to the Internet Layer.
In this memo, several media components that are connected using
devices such as bridges or repeaters are considered to be a
single Physical Network since such devices are transparent to
the IP.
- Physical Network Interface
-
This is a physical interface to a Connected Network and has a
(possibly unique) Link-Layer address. Multiple Physical Network
Interfaces on a single router may share the same Link-Layer
address, but the address must be unique for different routers on
the same Physical Network.
- Point to Point Line
-
A physical medium capable of connecting exactly two systems. In
this document, it is only used to refer to such a line when used
to connect IP entities. See General Purpose Serial Interface.
- router
-
A special-purpose dedicated computer that connects several
networks. Routers switch packets between these networks in a
process known as forwarding. This process may be repeated
several times on a single packet by multiple routers until the
packet can be delivered to the final destination - switching the
packet from router to router to router... until the packet gets
to its destination.
- RPF
-
Reverse Path Forwarding - A method used to deduce the next hops
for broadcast and multicast packets.
- Silently Discard
-
This memo specifies several cases where a router is to Silently
Discard a received packet (or datagram). This means that the
router should discard the packet without further processing, and
that the router will not send any ICMP error message (see
Section [4.3.2]) as a result. However, for diagnosis of
problems, the router should provide the capability of logging
the error (see Section [1.3.3]), including the contents of the
silently discarded packet, and should record the event in a
statistics counter.
- Silently Ignore
-
A router is said to Silently Ignore an error or condition if it
takes no action other than possibly generating an error report
in an error log or through some network management protocol, and
discarding, or ignoring, the source of the error. In
particular, the router does NOT generate an ICMP error message.
- Sparse Mode
-
In multicast forwarding, two paradigms are possible: in Sparse
Mode forwarding, a network layer multicast datagram is forwarded
as a data link layer multicast frame to routers and hosts that
have asked for it. The initial forwarding state is the inverse
of dense-mode in that it assumes no part of the network wants
the data. See Dense Mode.
- Specific-destination address
-
This is defined to be the destination address in the IP header
unless the header contains an IP broadcast or IP multicast
address, in which case the specific-destination is an IP address
assigned to the physical interface on which the packet arrived.
- subnet
-
A portion of a network, which may be a physically independent
network, which shares a network address with other portions of
the network and is distinguished by a subnet number. A subnet
is to a network what a network is to an internet.
- subnet number
-
A part of the internet address that designates a subnet. It is
ignored for the purposes internet routing, but is used for
intranet routing.
- TOS
-
Type Of Service A field in the IP header that represents the
degree of reliability expected from the network layer by the
transport layer or application.
- TTL
-
Time To Live A field in the IP header that represents how long a
packet is considered valid. It is a combination hop count and
timer value.