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).
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