Protocols

Basics

Defining Interactions VERTICAL Application Presentation Session Transport Network Data Link Physical Please do this for me OK It’s Done or Sorry I couldn’t

Horizontal Interactions Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical http request http response Route to ? Send to

Problems Solved by Link Layer Protocols Error CORRECTION SenderReceiver Flow Control Speed it Up!

Transmitting large files rather than short messages What kinds of problems ensue?

Sending a big file SenderReceiver

Some pieces are lost SenderReceiver

Some pieces are routed differently SenderReceiver

Some pieces are damaged SenderReceiver

Receiver is busy and misses 2 or misses data because arriving too fast SenderReceiver

Basic Interaction Simplest strategy SenderReceiver Message 1 SenderReceiver ACK SenderReceiver Message 2 etc

But things go wrong As we mentioned previously

SenderReceiver abcdefghijklmnopBill Where is the error? SenderReceiver abcdefghijklmnopBill data corrupted SenderReceiver abcdefghijklmnopBill address corrupted

Whether Lost or Damaged It all looks the same to the sender! Damaged messages are discarded because you can’t be sure the message is even intended for the receiver

General Problems Message is lost Acknowledgement is lost

Message is lost SenderReceiver

Using timers for the messages SenderReceiver time Sender

Problems now created Multiple copies of the message exist How do I set the timers?

Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Network layer passes message To Data Link for transmission

Application Presentation Session Transport Network Data Link Physical 21 Application Presentation Session Transport Network Data Link Physical 1 Data Link layer passes First part of message To Physical layer for transmission

Application Presentation Session Transport Network Data Link Physical 21 Application Presentation Session Transport Network Data Link Physical 1 1 Physical Layer sends across the network

Application Presentation Session Transport Network Data Link Physical 21 Application Presentation Session Transport Network Data Link Physical Ack Receiver accepts, passes up to data link, data link sends ACK ACK is lost

Application Presentation Session Transport Network Data Link Physical 21 Application Presentation Session Transport Network Data Link Physical Ack Sender resends message 1

Application Presentation Session Transport Network Data Link Physical 21 Application Presentation Session Transport Network Data Link Physical Ack Receiver gets message 1 Recognizes it already has 1 Re ACKS Ack

If ACK lost Message only delivered once ACK was retransmitted and eventually sender get ACK Sender moves on to part 2 of the message

Review Message Lost Sender Receiver 1 1 Ack ACK Lost Sender Receiver

Application Presentation Session Transport Network Data Link Physical 2 Application Presentation Session Transport Network Data Link Physical 2 Sender passes Message 2 to Physical Layer to xmit 1

Application Presentation Session Transport Network Data Link Physical 2 Application Presentation Session Transport Network Data Link Physical Receiver gets message 2 ACKS and passes message 2 Up to data link at receiver Ack 1

Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical 2 Upon receiving ACK Sender discard at link layer Assuming xmit complete 1

Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical 2 Receiver passes message 2 up to network layer 1 XMIT of network PDU complete!

Please do this for me Sorry I couldn’t Sends message “n” times but always fails. How many times is it repeated? Horizontal and Vertical Interaction Network Data Link SenderReceiver

Timers How are they set?

Timer Tradeoff Set the alarm time too large –Sender sits around waiting needlessly –Increased delay Set the alarm time too small –Sender repeats transmission unnecessarily –Additional unnecessary network traffic is generated –Unnecessary traffic -> Increased delay

Timer will never be perfect This is a queueing system The time it should take for the receiver to respond is defined by a probability distribution Unpredictable loads –Network traffic –Other receiver cpu loads –Local NIC card

Are there other approaches to retransmission/error correction? Yes!

Correction 3 basic ARQ (Automatic Repeat Request) approaches Idle RQ Go-back-N Selective Repeat