Connected: An Internet Encyclopedia

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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.

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.

Exterior Gateway Protocol A protocol that distributes routing information to the gateways (routers) which connect autonomous systems. See IGP.

Exterior Gateway Protocol version 2 This is an EGP routing protocol developed to handle traffic between Autonomous Systems in the Internet.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.


Connected: An Internet Encyclopedia