08 transport layer
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08 - Transport Layer. deliver_data(): called by rdt to deliver data to upper. rdt_send(): called from above, (e.g., by app.). udt_send(): called by rdt, to transfer packet over unreliable channel to receiver. rdt_rcv(): called when packet arrives on rcv-side of channel.

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08 - Transport Layer

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08 transport layer

08 - Transport Layer

Transport Layer


Reliable data transfer getting started

deliver_data():called by rdt to deliver data to upper

rdt_send():called from above, (e.g., by app.).

udt_send():called by rdt,

to transfer packet over

unreliable channel to receiver

rdt_rcv():called when packet arrives on rcv-side of channel

Reliable data transfer: getting started

send

side

receive

side

Transport Layer


Reliable data transfer getting started1

We’ll:

incrementally develop sender, receiver sides of reliable data transfer protocol (rdt)

consider only unidirectional data transfer

but control info will flow on both directions!

use finite state machines (FSM) to specify sender, receiver

event

state

1

state

2

actions

Reliable data transfer: getting started

event causing state transition

actions taken on state transition

state: when in this “state” next state uniquely determined by next event

Transport Layer


R eliable transfer over a reliable channel

underlying channel perfectly reliable

no bit errors

no loss of packets

separate FSMs for sender, receiver:

sender sends data into underlying channel

receiver read data from underlying channel

Reliable transfer over a reliable channel

rdt_send(data)

rdt_rcv(packet)

Wait for call from below

Wait for call from above

extract (packet,data)

deliver_data(data)

packet = make_pkt(data)

udt_send(packet)

sender

receiver

Transport Layer


Channel with bit errors

underlying channel may flip bits in packet

checksum to detect bit errors

the question: how to recover from errors:

acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK

negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors

sender retransmits pkt on receipt of NAK

new mechanisms in rdt2.0 (beyond rdt1.0):

error detection

receiver feedback: control msgs (ACK,NAK) rcvr->sender

Imagine a telephone conversation.

How do humans recover from “errors”

during conversation?

Channel with bit errors

Transport Layer


Channel with bit errors1

underlying channel may flip bits in packet

checksum to detect bit errors

the question: how to recover from errors:

acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK

negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors

sender retransmits pkt on receipt of NAK

Channel with bit errors

Transport Layer


Group assignment 1

Group Assignment #1

Divide into groups of 4 and modify our original FSM to include ACKsand NACKs. Note that an ACK is sent when the packet arrives at the receiver with no errors and a NACK is sent when the packet is corrupted in transmission.

rdt_send(data)

rdt_rcv(packet)

Wait for call from below

Wait for call from above

extract (packet,data)

deliver_data(data)

packet = make_pkt(data)

udt_send(packet)

sender

receiver

Transport Layer


On the board

On the Board

Let’s discuss your solution (Please make any changes to your quiz notes if necessary).

Transport Layer


Group assignment 2

Group Assignment #2

What happens if ACK/NAK corrupted?

  • sender doesn’t know what happened at receiver!

  • can’t just retransmit: possible duplicates

Transport Layer


On the board1

On the Board

Let’s discuss your solution (Please make any changes to your quiz notes if necessary).

Key: Introduce sequence numbers.

Stop and Wait!

Transport Layer


Group assignment 3

Group Assignment #3

How would you replace NACKs in your FSM?

Transport Layer


On the board2

On the Board

Let’s discuss your solution (Please make any changes to your quiz notes if necessary).

Key: instead of NAK, receiver sends ACK for last pkt received OK

  • receiver must explicitly include seq # of pkt being ACKed

  • duplicate ACK at sender results in same action as NAK: retransmit current pkt

Transport Layer


Group assignment 4

Group Assignment #4

What if the underlying channels can also lose packets (data or ACKs)?

Transport Layer


On the board3

On the Board

Let’s discuss your solution (Please make any changes to your quiz notes if necessary).

Key: sender waits “reasonable” amount of time for ACK

  • retransmits if no ACK received in this time

  • if pkt (or ACK) just delayed (not lost):

    • retransmission will be duplicate, but use of seq. #’s already handles this

    • receiver must specify seq # of pkt being ACKed

  • requires countdown timer

Transport Layer


Rdt3 0 in action

rdt3.0 in action

Transport Layer


Rdt3 0 in action1

rdt3.0 in action

Transport Layer


For next time

For Next Time…

Pipelined Protocols

Take a look at the Go-Back-N and Selective Repeat protocol applets on the textbook website. How do these two protocols work?

Transport Layer


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