Quick Tech Tip: Overview Of The Network Layer

The network layer provides services to the transport layer through virtual circuits or datagrams. In both cases, its main job is routing packets from the source to the destination. In virtual circuit subnets, a routing decision is made when the virtual circuit is set up. In datagram subnets, it is made on every packet.
Many routing algorithms are used in computer networks. Static algorithms include shortest path routing, flooding, and flow-based routing. Dynamic algorithms include distance vector routing and link state routing. Most actual networks use one of these. Other important routing techniques are hierarchical routing, routing for mobile hosts, broadcast routing, and multicast routing.
Subnets can become congested, increasing the delay and lowering the throughput for packets. Techniques include traffic shaping, flow specifications, and bandwidth reservation. If congestion does occur, it must be dealt with. Choke packets can be sent back, load can be shed, and other methods applied.
Networks differ in various ways, so when multiple networks are connected together problems can occur. Sometimes problems can be finessed by tunneling a packet through a hostile network, but if the source and the destination networks are different, this approach fails. Fragmentation may be called for if different networks are having different maximum sizes.
The Internet has a rich variety of protocols related to the network layer. These include the data protocol, IP, but also the control protocols ICMP, ARP, and RARP, and the routing protocols OSPF and BGP. The Internet is rapidly running out of IP addresses, so a new version of IP, IPv6, has been developed.
Unlike the datagram-based Internet, ATM networks use virtual circuits inside. There must be a set up before data can be transferred and torn down after transmission is completed. Quality of service and congestion control are major issues with ATM networks.

Quick Tech Tip: Concatenated Virtual Circuits

Two styles of internetworking are common:

* a connection-oriented concatenation of virtual circuit subnets,
* a datagram internet style.

In the concatenated virtual circuit model, a connection to a host in a distant network is set up in a way similar to the way connections are normally established. The virtual circuit consists of concatenated virtual circuits between the routers or gateways along the way from the source node to the destination node. Each gateway maintains tables telling which virtual circuits pass through it, where they are to be routed, and what the new virtual circuit number is. This process continues until the destination host has been reached.



Once data packets begin flowing along the path, each gateway relays incoming packets, converting between packet formats and virtual circuit numbers as needed. Clearly, all data packets must traverse the same sequence of gateways , and thus arrive in order.
This scheme works best when all the networks have roughly the same properties.
Concatenated virtual circuits are also common in the transport layer. In particular, it is possible to build a bit pipe using OSI, which terminates in a gateway, and have a TCP connection go from the gateway to the next gateway. In this manner, an end-to-end virtual circuit can be built spanning different networks and protocols.