Gordon Glas
CS 228

ROUTING
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Definition:
	The selection of paths through which a network sends
	packets.

This selection is usually done by the network. The source can
also specify the path if it has the necessary info.

The network must know the location of a destination before
selecting a path which leads to it. Networks learn that
certain paths are conjested or disabled and uses that
information to improve the path selection.

A user normally specifies a destination by name
(ogimaa.nmu.edu), rather than by giving the location of that
destination on the network. Since the name doesn't tell you
exactly where the computer is, the network translates the
name into the location.

Routing is physically taken care of in the Network Layer of
the OSI model. This layer routes packets from their sources
to their destinations, along paths made of successive packet
links, in the store-and-forward method.
(Figure 6.2 - Page 251)

Transport Methods:
- Virtual Circuits
	Path through which all packets of the same
	conversation are sent. Therefore, there's one routing
	decision per conversation.
- Datagram
	Makes routing decisions for each packet.

Routing Algorithm:
	Procedure that the nodes follow to select the paths
	of individual packets or of virtual circuits.

Routing Algorithm Properties:
- Robustness
	The algorithm must adjust the routing decisions when
	equipment fails and when traffic conditions change.
	A robust algorithm monitors the network constantly
	and updates the routing decisions either frequently
	or soon after a change is detected. Basically, it
	monitors the network, makes routing decisions, and
	implements these decisions. Routing decisions should
	be modified before congestion levels increase
	significantly. (Example on page 262)
- Stablity
	A routing algorithm is stable if the routing
	decisions adapt smoothly to changes in operating
	conditions. For example, a small change in operating
	conditions should result in a comparatively small
	change in routing decisions. Instability could result
	from the effect of routing decisions on the operating
	conditions. (Figure 6.9 - page 264)
- Fairness
	A routing algorithm is fair if it results in similar
	delays for the packets of different sources and
	destinations. For example, routing is fair if the
	average delay per packet is the same for two separate
	streams of packets being transmitted.
	(Example on page 267)
- Optimality
	A routing algorithm is optimal if it maximizes the
	network designer's objecting function while
	satisfying design contraints. (Example on page 269)

	Note: Optimality and Fairness aren't always
	compatible. Such incompatibility may be caused by an
	objective function that doesn't properly reflect the
	operating cost of the network.

Examples of widely used routing algorithms:
- Flooding
	Every node in the network transmits a copy of each
	packet it receives on every one of its transmission
	links, except on the link through which it received
	the packet.
- Spanning Tree Routing
	The transmission of packets along a tree of paths
	that leads to a specific destination.
	(Example on page 272)
- Static Routing
	Each node transmits a packet along a link selected
	according to some probabilities that depend on the
	destination.
- Bellman-Ford
	Determines the fastest path between nodes from the
	travel times along individual links.
- Hierarchical Routing
	Sizes of the routing tables are significantly
	reduced by using this algorithm. The nodes are
	grouped into domains, and each node keeps a list of
	the next node for each other domain and for each
	other node in its own domain.