• 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.