Network software

Network software:

structured as a hierarchy of layers 

* each layer offers certain services to the higher layers, while hiding from the higher layers the details of how those services are implemented hierarchical modularity

* a particular layer in one network node communicates with the corresponding layer in another network node by using an agreed protocol for that layer

this communication may be actual or virtual

Network software 

•actual communication is “vertical” except in the physical medium
•peer entities are programmed as if data transmission were “horizontal”
•together, these peer entities execute distributed scripts
•interfaces between adjacent layers define which operations and services the lower layer offers to the higher layer
• minimise amount of information passed between layers
•allow different equivalent implementations of a layer
•encapsulation:
•at the sender,layer N may add control information to the data it receives from layer N+1 before passing the (increased) data to layer N-1; at the receiver,layer N-1 passes data to layer N, which can read, act upon, & remove this control information before passing the (reduced) data up to layer N+1
•layer N should not need to know which portion of layer N+1’s data is control information, or its meaning

•network architecture= set of layers & protocols
•doesn’t contain implementation details or interface specifications
•basic principle: each layer should perform a specific set of well-defined functions
•basic question: how many layers are needed ?
•we’ll see that different network architectures have different numbers of layers and/or different functions within their layers
•minimum of 2: one focused on (user) application issues, and one focused on network issues
•more realistically, the following general issues must be addressed:
􀂾physical signal transmission between directly connected nodes
􀂾digital transmission of a message between neighboring entities
􀂾end-to-end communication between a source and a destination
􀂾communications needed to run a distributed (user) application