Review of key networking techniques: –Reliable communication over unreliable channels –Error detection and correction –Medium access control –routing –Congestion control

Reliable communication over unreliable channels: –Positive acknowledgement with retransmission (PAR) Sender progresses only after receiving positive ACK Sender retransmits if timeout Receiver only accepts the correct packet (a sequence number is required to do this). –Sliding window protocol PAR + pipelined communication. Senders allows multiple outstanding packets Receivers buffer out of order packets

Error detection/correction –CRC checksum Sender/receiver agrees on the generator Sender makes sure that all messages sent can be divisible by the generator –Add n 0-bits to the message –Perform binary division –Add the remainder to the message Receiver verifies a message is valid by dividing the message by the generator CRC checksum used in Ethernet frame –Internet checksum 1’s complement of the sums of 16 bit numbers Used in UDP, TCP, etc.

Error detection/correction –Error correction. Is typically more expensive than detecting errors and retransmission Used only when retransmission is expensive or when retransmission is inefficient –Ad hoc mobile networks –Satellite link –Real time applications (Video streaming)

Medium access control: –Use when multiple stations can access a shared channel –There are three types of medium access control protocols. Protocols that allow collisions: try out your luck and fix the problem if collision occurs. Collision free protocols: make sure that no collision can possibly happen Limited collision protocols: have the combined advantages of the previous two types of protocols.

Protocols that allow collisions –ALOHA, slotted ALOHA, CSMA, CSMA/CD Collision free protocols –Bitmap method, binary countdown, token based. Limited collision protocols –Adaptive tree walk

Routing –Flooding –Shortest path routing (Dijkstra algorithm) Centralized algorithm –Distributed routing algorithms have two components: Maintaining the topology information Compute the routes –Distance vector algorithm Each router distributes its routing table to its neighbors Each router updates its routing table based on the tables received from its neighbors.

Routing –Link state algorithm Each router obtain the link information for links directly connected to the router Each router flood the network with the link state information (all routers in the system receive link state packets). Shortest path algorithm is used to compute routes based on the link state database.

Congestion control: –Congestion is a global event. –Congestion happens when the network cannot handle the number of data injected into the network. –Congestion control techniques: Open loop or close loop Can be done either in the network layer or transport layer Traffic shaping –Leaky bucket algorithm and token bucket algorithm Admission control Chock packet Hop-by hop chock packet Load sheding

Congestion control mechanism in the Internet: –Done in TCP –Slow start, AIMD –Early random drop.