Chapter 3Transport Layer Computer

Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

Reliable Data Transfer: Getting Started

sendside

receiveside

rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer

udt_send(): called by rdt protocol, to transfer packet over unreliable channel to receiver

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

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

Rdt2.0: FSM Specification

Wait for call from above

snkpkt = make_pkt(data, checksum) udt_send(sndpkt)

extract(rcvpkt,data) deliver_data(data) udt_send(ACK)

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt)

rdt_rcv(rcvpkt) && isACK(rcvpkt)

udt_send(sndpkt)

rdt_rcv(rcvpkt) && isNAK(rcvpkt)

udt_send(NAK)

rdt_rcv(rcvpkt) && corrupt(rcvpkt)

Wait for ACK or NAK

Wait for call from below

sender

receiver

rdt_send(data)

Rdt2.0• rdt2.0 is a stop and wait protocol

– Sender sends one packet, then waits for receiver response

• rdt2.0 has a fatal flaw• What happens if ACK/NAK get

corrupted?– Add checksum bits to

ACK/NAK• How the protocol should recover from

errors in ACK/NAK?– Retransmission on receipt of a

corrupt ACK/NAK– Retransmission causes duplicates– Receiver does not know whether

ACK or NAK it sent was received correctly

– Receiver does not know a priori whether an arriving packet contains new data or is a retransmission

Handling Duplicates: • Sender retransmits current

packet if ACK/NAK garbled• Sender adds sequence number

to each packet• Receiver discards (doesn’t

deliver up) duplicate packet

Rdt2.1: Sender, handles garbled ACK/NAKs

Wait for call 0 from above

sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt)

rdt_send(data)

Wait for ACK or

NAK 0udt_send(sndpkt)

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isNAK(rcvpkt) )

sndpkt = make_pkt(1, data, checksum)udt_send(sndpkt)

rdt_send(data)

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt)

udt_send(sndpkt)

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isNAK(rcvpkt) )

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt)

Wait for call 1 from above

Wait for ACK or NAK 1

Rdt2.1: Receiver, handles garbled ACK/NAKs

Wait for 0 from below

sndpkt = make_pkt(NAK, chksum)udt_send(sndpkt)

rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt)

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt)

extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)

Wait for 1 from below

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt)

extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)

rdt_rcv(rcvpkt) && (corrupt(rcvpkt))

sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)

rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt)

rdt_rcv(rcvpkt) && (corrupt(rcvpkt))

sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)

sndpkt = make_pkt(NAK, chksum)udt_send(sndpkt)

Rdt2.2: a NAK-free protocol

• Same functionality as rdt2.1, using ACKs only• Instead of NAK, receiver sends ACK for last packet

received OK– Receiver must explicitly include seq # of the

packet being ACKed • Duplicate ACK at sender results in same action as

NAK: retransmit current packet

Rdt2.2: Sender, Receiver Fragments

Wait for call 0 from

above

sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)

rdt_send(data)

udt_send(sndpkt)

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) )

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0)

Wait for ACK

0

Sender FSMFragment

Wait for 0 from below

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK,1, chksum)udt_send(sndpkt)

rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt))

udt_send(sndpkt)Receiver FSMFragment

Rdt3.0: Channels with Errors and LossNew Assumption: Underlying channel can also lose packets

(data or ACKs) What to do when packet loss occurs?

Retransmissions How to detect loss?

Sender waits “reasonable” amount of time for ACK Must wait at least as long as RTT plus processing

delay. Retransmits if no ACK received in this time

If packet (or ACK) just delayed (not lost): Retransmission will be duplicate, but use of sequence

numbers can handles this.

Rdt3.0 Sender

sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)start_timer

rdt_send(data)

Wait for ACK 0

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,1) )

Wait for call 1 from above

sndpkt = make_pkt(1, data, checksum)udt_send(sndpkt)start_timer

rdt_send(data)

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0)

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,0) )

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1)

stop_timerstop_timer

udt_send(sndpkt)start_timer

timeout

udt_send(sndpkt)start_timer

timeout

rdt_rcv(rcvpkt)

Wait for call 0 from above

Wait for ACK1

rdt_rcv(rcvpkt)

Pipelined Reliable Data Transfer Protocols

Rdt3.0 works, but performance is poor Poor Performance is due to the fact

that it is a stop and wait protocol Poor utilization of the resource

Solution Sender is allowed to send multiple packets

without waiting for ACKs. Pipelining

Since many sender to receiver packets can be visualized as filling a pipeline

Increase utilization

Pipelined Reliable Data Transfer Protocols Pipelining has the following consequences for

reliable data transfer Range of sequence numbers must be

increased Sender and receiver sides may have to buffer

more than one packet.

Two basic approaches towards pipeline error recovery:

Go-Back-N, Selective Repeat

Go-Back-N (GBN)

Sender: Sender is allowed to transmit multiple packets without

waiting for an acknowledgement Constrained to a certain maximum number N.

Base or send_base Sequence number of oldest unacknowledged packet

Nextseqnum Sequence number of next packet to be sent

The range of sequence numbers for transmitted but not acknowledged packets can be viewed as a window of size N.

This window slides forward as the protocol operates

Go-Back-N

GBN sender must respond to three types of events

Invocation from above (rdt_send() is called): If window is full, returns data to upper layer Maintain synchronization mechanism

Receipt of an ACK: ACK for packet with seq # n is taken as“Cumulative ACK” More shortly in receiver

Time out event: Sender has timer for oldest unacknowledged packet

• If timer expires, retransmit all unacknowledged packets

Go-Back-N

Receiver: If a packet with seq # n is received correctly and

is in order ACK is sent and data is delivered to upper layers

For all other cases Receiver discards the packet and resends ACK for most

recently received in order packet

Packets are delivered one at a time to upper layers If a packet k has been received and delivered, then all

packets with seq # lower than k have also been delivered.

Receiver discards out of order packets No receiver buffering Need only remember expectedseqnum

GBN inaction

http://www.eecis.udel.edu/~amer/450/TransportApplets/GBN/GBNindex.html