Networks can differ in many ways. Some of the differences, such as different modulation techniques or frame formats, are in the physical and data link layers, these differences will not concern us here.
Some of the many ways networks can differ:
- Instead, in Fig.3.2 list some of the differences that can occur in the network layer.
- It is papering over these differences that makes internetworking more difficult than operating within a single network.
Figure 3.2 some of the many ways networks can differ
Problems between Networks interface:
When packets sent by a source on one network must transit one or more foreign networks before reaching the destination network many problems can occur at the interfaces between networks.
- To start with, when packets from a connection-oriented network must transit a connectionless one, they may be reordered, something the sender does not expect and the receiver is not prepared to deal with.
- Protocol conversions will often be needed, which can be difficult if the required functionality cannot be expressed.
- Address conversions will also be needed, which may require some kind of directory system.
- Passing multicast packets through a network that does not support multicasting requires generating separate packets for each destination.
- The differing maximum packet sizes used by different networks can be a major nuisance.
- Differing qualities of service is an issue when a packet that has real-time delivery constraints passes through a network that does not offer any real-time guarantees.
- Error, flow, and congestion control often differ among different networks.
- If the source and destination both expect all packets to be delivered in sequence without error but an intermediate network just discards packets whenever it smells congestion on the horizon, many applications will break.
- Also, if packets can wander around aimlessly for a while and then suddenly emerge and be delivered, trouble will occur if this behavior was not anticipated and dealt with.
- Different security mechanisms, parameter settings, and accounting rules, and even national privacy laws also can cause problems.