CS335 Networking & Network Administration Tuesday, April 20, 2010

WAN’sWAN’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 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 Jitter

Protocols 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 Protocol suite OSI layer Model OSI layer

OSI Model

Multiple nested headers

Data Encapsulation in a layered architecture Your computerWeb 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 Physical Bit stream 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 Sliding Window algorithm

Sliding Window 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).