Communication Networks (9)

Reliable Data Transfer: Intuition

rdt

reliable data transfer protocol

udt

unreliable data transfer protocol

Reliable Channel

channel is perfectly reliable:

• no bit errors
• no loss of packets

Channel with Bit Errors

• underlying channel may flip bits in packet

Detect error: checksum

Recover from errors: receiver tells sender with:

ACK

acknowledgements

NAKs
:negative acknowledgements

if ACK/NAK corrupted

• sender doesn’t know what happened at receiver
• can’t just retransmit: possible duplicate
• sender adds sequence number to each pkt to deal with duplicating

FSM

Notice that the 2 sequence numbers (0 and 1) suffies
{: .prompt-tip }

NAK-free protocol

• using ACKs only
• instead of NAK, receiver sends ACK for last pkt received OK
• in sender, duplicate ACK means NAK

Channels with Errors and Loss

• Channel can also lose packets (data, ACKs)
• sender waits “reasonable” amount of time for ACK
• retransmits if no ACK received in this time

Performance

utilization $U_{\text {sender }}$ (1 Gbps link, 15 ms prop. delay, 8000 bit packet)

$$U_{\text {sender }}=\frac{L / R}{R T+L / R}=\frac{.008}{30.008}=0.00027$$

Pipelining

e.g. $3\times U_{\text {sender }}$

Go-back-N

• sender can have up to N unacked packets in pipeline
• receiver only sends cumulative ack, Doesn’t ack packet if there’s a gap
• sender has timer for oldest unacked packet
• when timer expires, retransmit all unacked packets

Selective Repeat

• sender can have up to N unacked packets in pipeline
• rcvr sends individual ack for each packet
• sender maintains timer for each unacked packet
• when timer expires, retransmit only that unacked packet