CS335 Networking &

Network Administration

Tuesday, April 20, 2010

WAN’s and Routing

LAN’s Local Area Network span a building or campus

MAN’s Metropolitan Area Network spans a city

WAN’s Wide Area Network can span sites in multiple

cities, countries, or continents Large corporations

Scalability

A WAN must be able to grow as needed to connect many sites across large distances

Support many simultaneous connections

Packet Switches

Allows many switches to interconnect for expansion

Packet Switched WAN

Physical addressing in a WAN

Hierarchical addressing

Next-Hop forwarding

Packet switch does not keep complete information on how to reach all possible destinations, just its next hop

Source independence

Doesn’t matter where a packet comes from Next hop depends only on the destination This allows the forwarding mechanism to be

compact and efficient Because all packets follow the same path, only one

table is required Only the destination address needs to be extracted

from the packet A single mechanism handles forwarding uniformly

Routing table

Forwarding a packet to its next hop is routing Two-part hierarchical address has practical

consequences

Next hop routing table

Default route

Eliminates duplicate entries Uses default if doesn’t find an explicit entry Only one default route in a table, and has

lower priority than other entries

Routing table computation

Static routing A program computes and installs routes when the

packet switch boots; the routs do not change Dynamic routing

A program builds an initial routing table when booted; the program alters the table as condition in the network change

Shortest Path Computation

Dijkstra’s algorithm used to find the shortest path from a source to other nodes

Uses weights on edges* as a measure of distance

Path with fewest number of edges may not be the path with the least weight

* edges are connections between nodes * weights are assigned non-negative values

Shortest Path Computation

Least weight path

Distributed route computation

Packet switches send each other routing info Distance vector routing – packet switch

sends routing info across network and updates routes based on distance vector algorithm (13.2 in book)

Link state routing (known as SPF - Shortest path first)

Routed vs. routing protocols Routed protocols

Any network protocol that provides enough information in its network layer address to allow a packet to be forwarded. Routed protocols define the field formats within a packet. Uses the routing table to forward packets. IP is an example of a routed protocol.

Routing protocols Support a routed protocol by providing mechanisms for sharing

routing information. Routing protocol messages move between routers to update and maintain tables. TCP/IP examples are: RIP – Routing information protocol IGRP – interior gateway routing protocol EIGRP – enhanced interior gateway routing protocol OSPF – open shortest path first

Network performance

Delay – the time required to send a bit from one device to another

Throughput – the number of bits per second that can be transmitted across the network Throughput and delay are not independent. As

traffic (congestion) increases, delay increases. A network that operates at close to 100% of its throughput capacity has sever delay.

Jitter – a measure of the variance in delay

Protocols and layering

Protocols – a design that specifies the details of how computers interact, including the format of the messages they exchange and how errors are handled

Protocol suite – a set of protocols that work together to provide a seamless communication system. Each protocol handles a subset of all possible details – also called the protocol stack

OSI layer Model

OSI Model

Multiple nested headers

Data Encapsulation in a layered architecture

Your computer Web server

7-Application

http

7-Application

http

6-Presentation

unicode

6-Presentation

unicode

5-Session

security

5-Session

security

4-Transport

tcp

4-Transport

tcp

3-Network

ip

3-Network

ip

2-Data link

ethernet

2-Data link

ethernet

1-Physical

Bit stream 100100010101010010100

1-Physical

Bit stream 100100010101010010100

User data

A

User data

P A

User data

S P A

User data

T S P A

User data

N T S P A

User data

D N T S P A

User data

D N T S P A

User data

N T S P A

User data

T S P A

User data

S P A

User data

P A

User data

A

User data

Retransmitting lost packets

Reliable transfer requires protocols to use positive acknowledgement with retransmission

Can introduce duplicate packets when there is long delay

Flow control mechanisms to handle data overrun – stop and go sender waits for receiver to send an ACK

Inefficient use of network capacity

Sliding Window algorithm

Sliding Window algorithm

Technique a protocol can use to improve throughput by allowing a sender to transmit additional packets before receiving an acknowledgement. A receiver tells a sender how many packets can be sent at a time (called a window size).