In the thesis, the focus will be limited to the set of protocol entities
relevant for the implementation of IP multicasting on top of ATM. With an
implementation, the location of the functionality within the layers is
important. Although already described in Section 
, the
protocol entities are shown here in the context of the physical components they
will be implemented on. A basic understanding of how the components are
connected together is also useful in understanding how the Internet Protocol
functions on a set of interconnected LANs. In this context, the term
nodes can be used to distinguish the IP-routers (intermediate nodes)
and IP-hosts (end nodes) from the interconnecting network.
In this and other documents about network protocols, the terms node, host and router are used often. The terms have a special meaning and a relation to each other.
A node is a physical entity connected to the network, which implements the protocol specified with it (e.g. ``IP node'' when IP is implemented or ``ATM node'' when ATM is implemented on that node). If the protocol is not specified, it should be clear from the context.
A router (or a switch, in the case of ATM) is a node that performs some forwarding function on the data carriers. E.g. IP routers forwards IP packets to the next hop in the path to their destination address.
A host is a node that can only operate as end-point of a communication at this protocol level and never performs routing functions at that protocol level.
Protocol entities are groups of functions that reside in a node. Peer
protocol entities in different nodes exchange information, using a
protocol across the network. Together the peer protocol entities form a protocol layer. As explained in
Section and shown in Figure , the IETF
groups different protocol entities in a single layer. Another way to
represent that
is to use a user 'plane' and a
control 'plane'. The protocols (UDP, TCP, IP) form the user plane, they
simply transport data for their Service Users. IGMP, ICMP, ARP and RARP
are support protocols that make it possible for the user plane protocols to function properly. Therefore, they are
part of the control plane.
The protocol entities are represented by boxes (within) the layered
representation (see Figure ). Not all protocol entities
shown in Figure have to be present in an implementation
of the TCP/IP protocol suite on a node. E.g. the IGMP protocol entity
is not needed if IP multicasting functionality is not used. This also
applies to the Network Interface layer, e.g. it may be possible to have
a Network Interface layer implementation without broadcast support.
In the rest of this thesis, the focus will be mostly on IP multicasting and the integration with ATM and Classical IP and ARP over ATM. The protocol entities most important in that context are: UDP, IGMP, IP and the entire Network Interface layer.
Figure: Architectural view of the protocol functions in the IP-nodes
Note that in Figure applications may sometimes access
IP layer functions directly. Two network management applications,
ping and traceroute, use ICMP and IP respectively.
Ping is used to check the reachability and operational status of
another host and traceroute uses some special features of the IP-packet
header values to find out which route is taken to reach another host.
It is assumed that IP multicasting applications initiate the joining
and leaving of host groups by talking directly to the IGMP
functionality.
The IP router has no need for transport or application protocols when it
only needs to transfer IP packets from one LAN to another. But an IP
host with two or more connections (to different LANs) can also function
as a router. The model of the IP router in Figure shows the
minimum number of layers for router functionality.
Of course, the same exceptions apply as in the IP host. Multicasting was added later, so a router may not have IGMP and multicast routing capabilities. The network interface layer can be described for both types of nodes equally. The network interface layer may not have support for multicasting and/or broadcasting.
An internet can be seen as a set of LANs connected to each other
using IP-routers. Each network can be based on one of a variety of networking
technologies. When two computers (IP-hosts) on different networks communicate,
their traffic is routed through the IP-routers across the intermediate
networks.
The interconnected structure of these networks is such that it is usually possible to choose from several routes between the two hosts. There are some options in the internet protocol to help select the best route. These options specify certain qualities of the network, like high throughput or high reliability. The routers may or may not have the ability to use these options. When a network or IP-router in one route to the destination host is not operational, an intermediate router can chose another route to reach the same destination IP-host.
In Figure (a part of) an internet is shown. The IP hosts
and IP routers correspond to the two architectural views shown in
Figure . The routers connect the LAN clouds. Two
connected IP hosts are shown (A and B), they are connected to LANs (A
to I, B to X). When a packet is routed from A to B,
the routers can choose from several possible paths. Route 1 in
Figure is: A--1--3--7--B. If router 1 should break
down, route 2 can be chosen as an alternative: A--4--5--6--B. Only
if routers 1 and 4 break down at the same time, host A is
completely cut off from the network.
