rdt3'0: channels with errors and loss

1
rdt3.0 channels with errors and loss

• New assumption underlying channel can also lose
  packets (data or ACKs)

• checksum, seq. , ACKs, retransmissions will be
  of help, but not enough
• Q how to deal with loss?

• sender waits until certain data or ACK lost, then
  retransmits
• yuck drawbacks?

2
rdt3.0 channels with errors and loss

• Approach sender waits reasonable amount of
  time for ACK a timeout event
• 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
• So how long a sender should wait?
• requires countdown timer
• Associate with Each packet
• Associate with a TO event
• Can be stopped if an ACK is received.

3
rdt3.0 sender
rdt_send(data)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isACK(rcvpkt,1) )
sndpkt make_pkt(0, data, checksum) udt_send(sndp
kt) start_timer
L
rdt_rcv(rcvpkt)
L
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt,1)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt,0)
stop_timer
stop_timer
rdt_rcv(rcvpkt)
L
rdt_send(data)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isACK(rcvpkt,0) )
sndpkt make_pkt(1, data, checksum) udt_send(sndp
kt) start_timer
L
4
rdt2.2 and rdt 3.0 receiver
5
Rdt 3.0 in action (p0,p1,p0,p1,p0,p1)
P0 Ack0 P1 Ack0 Retx p1 Ack1
Retx p1 Ack1
p0 Ack0
6
rdt3.0 in action
7
rdt3.0 in action
8
Performance of rdt3.0

• example 1 Gbps link, 15 ms e-e prop. delay, 1KB
  packet

L (packet length in bits)
8kb/pkt
T


8 microsec
transmit
R (transmission rate, bps)
109 b/sec

• U sender utilization fraction of time sender
  busy sending

• 1KB pkt every 30 msec -gt 33kB/sec throughput over
  1 Gbps link
• network protocol limits use of physical resources!

9
rdt3.0 stop-and-wait operation
rdt3.0 works, but performance ?
10
How to improve?
Can we do the following?
11
Pipelined protocols

• Pipelining sender allows multiple, in-flight,
  yet-to-be-acknowledged pkts

12
Outline

• 3.4 Principles of reliable data transfer
• rdt3.0 Channel with bit errors and packet
  losses
• Pipelined protocols
• Go-Back-N
• Sliding window at sender
• ACKs only correctly-received pkt with highest
  in-order seq , cumulative ACK,
• Retransmit the whole window
• Selective Repeat
• 3.5 Connection-oriented transport TCP
• reliable data transfer

13
Pipelined protocols

• Pipelining
• range of sequence numbers must be increased
• buffering at sender and/or receiver

• Two generic forms of pipelined protocols
  go-Back-N, selective repeat

14
Pipelining Protocols

• Go-back-N big picture
• Sender can have up to N unacked packets in
  pipeline
• Rcvr only sends cumulative acks
• Doesnt ack packet if theres a gap
• Sender has timer for oldest unacked packet
• If timer expires, retransmit all unacked packets

15
Go-Back-N

• Sender
• window of up to N, consecutive unacked pkts
  allowed
• Sliding-window protocol

16
Go-Back-N

• Sender
• k-bit seq in pkt header
• Fill window as much as possible
• ACK(n) ACKs all pkts up to, including seq n
  (cumulative ACK)
• Send_base sets to pkt n1, slides window
• timer for each in-flight pkt
• timeout(n) retransmit pkt n and all pkts in
  window
• go-back-N

pkt n
17
GBN sender extended FSM
rdt_send(data)
if (nextseqnum lt baseN) sndpktnextseqnum
make_pkt(nextseqnum,data,chksum)
udt_send(sndpktnextseqnum) if (base
nextseqnum) start_timer nextseqnum
else refuse_data(data)
L
base1 nextseqnum1
timeout
start_timer udt_send(sndpktbase) udt_send(sndpkt
base1) udt_send(sndpktnextseqnum-1)
rdt_rcv(rcvpkt) corrupt(rcvpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
base getacknum(rcvpkt)1 If (base
nextseqnum) stop_timer else start_timer
18
GBN receiver extended FSM
default
udt_send(sndpkt)
rdt_rcv(rcvpkt) notcurrupt(rcvpkt)
hasseqnum(rcvpkt,expectedseqnum)
L
Wait
extract(rcvpkt,data) deliver_data(data) sndpkt
make_pkt(expectedseqnum,ACK,chksum) udt_send(sndpk
t) expectedseqnum
expectedseqnum1 sndpkt
make_pkt(expectedseqnum,ACK,chksum)

• ACK-only always send ACK for correctly-received
  pkt with highest in-order seq
• may generate duplicate ACKs
• need only remember expectedseqnum
• out-of-order pkt
• discard (dont buffer) -gt no receiver buffering!
• Re-ACK pkt with highest in-order seq

19
GBN inaction
20
Pipelining Protocols

• Go-back-N big picture
• Sender can have up to N unacked packets in
  pipeline
• Rcvr only sends cumulative acks
• Doesnt ack packet if theres a gap
• Sender has timer for oldest unacked packet
• If timer expires, retransmit all unacked packets

• Selective Repeat big pic
• Sender can have up to N unacked packets in
  pipeline
• Rcvr acks individual packets
• Sender maintains timer for each unacked packet
• When timer expires, retransmit only unack packet

21
Selective Repeat

• receiver individually acknowledges all correctly
  received pkts
• buffers pkts, as needed, for eventual in-order
  delivery to upper layer
• sender only resends pkts for which ACK not
  received
• sender timer for each unACKed pkt
• sender window
• N consecutive seq s
• again limits of sent, yet unACKed pkts

22
Selective repeat sender, receiver windows
23
Selective repeat

• pkt n in rcvbase, rcvbaseN-1
• send ACK(n)
• out-of-order buffer
• in-order deliver (also deliver buffered,
  in-order pkts), advance window to next
  not-yet-received pkt
• pkt n in rcvbase-N,rcvbase-1
• ACK(n)
• otherwise
• ignore

• data from above
• if next available seq in window, send pkt
• timeout(n)
• Each pkt has a timer
• resend pkt n, restart timer
• ACK(n) in sendbase,sendbaseN
• mark pkt n as received
• if n smallest unACKed pkt(send_base), advance
  window base to next unACKed seq , send new pkts
  if any.

24
Selective repeat in action
Pkt 3,4,5 buffered
25
Selective repeat dilemma

• Example
• seq s 0, 1, 2, 3
• window size3
• receiver sees no difference in two scenarios!
• incorrectly passes duplicate data as new in (a)
• Q what relationship between seq size and
  window size?

26
Outline

• 3.4 Principles of reliable data transfer
• rdt3.0 Channel with bit errors and packet
  losses
• Pipelined protocols
• Go-Back-N
• Selective Repeat
• 3.5 Connection-oriented transport TCP
• reliable data transfer

27
TCP reliable data transfer

• TCP creates rdt service on top of IPs unreliable
  service
• Pipelined segments
• Cumulative acks
• TCP uses single retransmission timer

• Retransmissions are triggered by
• timeout events
• duplicate acks
• Initially consider simplified TCP sender
• ignore duplicate acks
• ignore flow control, congestion control

28
TCP seq. s and ACKs

• Seq. s
• byte stream number of first byte in segments
  data
• segments 0 999, seq 0
• 1000 1999, seq 1000
• ACKs
• seq of next byte expected from other side
• Receive 0 999, ACK seq 1000
• 1000 1999, ACK seq 2000
• cumulative ACK
• Q how receiver handles out-of-order segments
• A TCP spec doesnt say, -- up to implementer
• What we have learned?

29
TCP seq. s and ACKs (A-gtB)
Host B
Host A
User types C
Seq42, ACK79, data C
host ACKs receipt of C, echoes back C
Seq79, ACK43, data C
host ACKs receipt of echoed C
Seq43, ACK80

• Random choose an initial seq
• A-gtB
• Full duplex not emphasized.

simple telnet scenario
30
TCP seq. s and ACKs (bi-direction)
Host B
Host A
User types C
Seq42, ACK79, data C
host ACKs receipt of C, echoes back C
Seq79, ACK43, data C
host ACKs receipt of echoed C
Seq43, ACK80

• Random choose an initial seq
• Full duplex piggybacking ACKs

simple telnet scenario
31
TCP sender events

• data rcvd from app
• Create segment with seq
• seq is byte-stream number of first data byte in
  segment

• Ack rcvd
• If acknowledges previously unacked segments
• update what is known to be acked
• start timer if there are outstanding segments

• start timer if not already running (think of
  timer as for oldest unacked segment)
• expiration interval TimeOutInterval

• timeout
• retransmit segment that caused timeout
• restart timer

32
TCP sender(simplified)
NextSeqNum InitialSeqNum
SendBase InitialSeqNum loop (forever)
switch(event) event
data received from application above
create TCP segment with sequence number
NextSeqNum if (timer currently
not running) start timer
pass segment to IP
NextSeqNum NextSeqNum length(data)
event timer timeout
retransmit not-yet-acknowledged segment with
smallest sequence number
start timer event ACK
received, with ACK field value of y
if (y gt SendBase)
SendBase y if (there are
currently not-yet-acknowledged segments)
start timer
/ end of loop forever /

• Comment
• SendBase-1 last
• cumulatively acked byte
• Example
• SendBase-1 71y 73, so the rcvrwants 73
  y gt SendBase, sothat new data is acked

33
TCP retransmission scenarios
Host A
Host B
Seq92, 8 bytes data
Seq100, 20 bytes data
ACK100
ACK120
Seq92, 8 bytes data
Sendbase 100
SendBase 120
ACK120
Seq92 timeout
SendBase 100
SendBase 120
premature timeout
34
TCP retransmission scenarios (more)
SendBase 120
35
TCP ACK generation RFC 1122, RFC 2581
TCP Receiver action Delayed ACK. Wait up to
500ms for next segment. If no next segment, send
ACK Immediately send single cumulative ACK,
ACKing both in-order segments Immediately send
duplicate ACK, indicating seq. of next
expected byte Immediate send ACK, provided
that segment starts at lower end of gap
Event at Receiver Arrival of in-order segment
with expected seq . All data up to expected seq
already ACKed Arrival of in-order segment
with expected seq . One other segment has ACK
pending Arrival of out-of-order
segment higher-than-expect seq. . Gap
detected Arrival of segment that partially or
completely fills gap