Communications Server for z/OS V1R7 TCP/IP Implementation, Volume 1: Base Functions, Connectivity, and Routing

140 Communications Server for z/OS V1R7 TCP/IP Implementation, Volume 1 - Base Functions, Connectivity, and Routing

5.1 Overview

When we talk about networks, one of the key issues is how to transport data across the

network. Based on the OSI reference model, the act of moving data traffic across a network

from a source to a destination can be accomplished either by bridging or routing this data

between the endpoints. Routing is often compared with bridging, which might seem to

accomplish precisely the same thing. However, the primary difference between the two is that

bridging occurs at Layer 2 (the data link control layer) of the OSI reference model, while

routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging

with different information to use in the process of moving information from source to

destination, so the two functions accomplish their tasks in different ways.

5.1.1 Basic concepts

To help understand the concepts described in this section, Table 5-1 lists some of the

common IP routing-related terms. Most of the functions or protocols listed are supported by

the z/OS Communications Server.

Table 5-1 IP routing terms

In order to route packets on the network, each network interface must have a unique IP

address assigned. Whenever a packet is sent, the destination and source IP addresses are

included in the packet’s header information. The network layer (Layer 3) of the TCP/IP stack

examines the destination IP address to determine how the packet should be forwarded. The

Term Definition

Routing The process used in an IP network to deliver a datagram to the

correct destination.

Static routing Routing that is manually configured and does not change

automatically in response to network topology changes.

Replaceable

static routes Static routes that can be replaced by OMPROUTE.

Dynamic routing Routing that is dynamically managed by a routing daemon and

automatically changes in response to network topology

changes.

Routing daemon A server process that manages the IP routing table.

Autonomous system (AS) A group of routers exchanging routing information through a

common routing protocol. A single AS may represent a large

number of IP networks.

Router A device or host that interprets protocols at the Internet

Protocol (IP) layer and forwards datagrams on a path towards

their correct destination.

Gateway A router that is placed between networks or subnetworks. The

term is used to represent routers between autonomous

systems.

Interior gateway protocols (IGP) Dynamic route update protocols that are used between routers

and hosts inside your own network and between your network

and a service provider.

Exterior gateway protocols (EGP) Dynamic route update protocols that are used between routers

that are placed between two or more Autonomous Systems.