Transcript CS335 Networking & Network Administration

CS335
Networking &
Network Administration
Tuesday, April 20, 2010
WAN’s and Routing
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LAN’s
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MAN’s
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Local Area Network span a building or campus
Metropolitan Area Network spans a city
WAN’s
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Wide Area Network can span sites in multiple
cities, countries, or continents
Large corporations
Scalability
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A WAN must be able to grow as needed to
connect many sites across large distances
Support many simultaneous connections
Packet Switches
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Allows many switches to interconnect for
expansion
Packet Switched WAN
Physical addressing in a WAN
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Hierarchical addressing
Next-Hop forwarding
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Packet switch does not keep complete
information on how to reach all possible
destinations, just its next hop
Source independence
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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
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Forwarding a packet to its next hop is routing
Two-part hierarchical address has practical
consequences
Next hop routing table
Default route
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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
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Static routing
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A program computes and installs routes when the
packet switch boots; the routs do not change
Dynamic routing
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A program builds an initial routing table when booted;
the program alters the table as condition in the
network change
Shortest Path Computation
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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
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Least weight path
Distributed route computation
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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
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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:
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RIP – Routing information protocol
IGRP – interior gateway routing protocol
EIGRP – enhanced interior gateway routing protocol
OSPF – open shortest path first
Network performance
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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
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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
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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
User data
7-Application
http
A
User data
6-Presentation
unicode
5-Session
security
4-Transport
tcp
3-Network
ip
2-Data link
ethernet
1-Physical
Bit stream
PA
User data
SPA
User data
TSPA
User data
NTSPA
User data
DNTSPA
User data
100100010101010010100
7-Application
http
A
User data
6-Presentation
unicode
5-Session
security
4-Transport
tcp
3-Network
ip
2-Data link
ethernet
1-Physical
Bit stream
PA
User data
SPA
User data
TSPA
User data
NTSPA
User data
DNTSPA
User data
100100010101010010100
Retransmitting lost packets
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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
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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).