COMP361 Tutorial 6

1
COMP361 Tutorial 6
2
Chapter 3 Review

• Transport services
• rdt protocols
• GBN
• SR

3
Transport Layer Services

• Provide logical communication between application
  processes.
• UDP vs. TCP
• UDP unreliable, unordered delivery
• Multiplexing/de-multiplexing function and light
  error checking, nothing more to IP
• multiplexing and demultiplexing is provided by
  the 4-tuples, ltsource IP address, source port
  number, destination IP address, destination port
  numbergt. This uniquely identifies a communication
  between a pair of processes on two end hosts.
• TCP reliable, in-order delivery
• Flow control and congestion control

4
More questions

• Problem Consider a TCP connection between host A
  and host B. Suppose that the TCP segments
  traveling from host A to host B have source port
  number x and destination port number y. What are
  the source and destination port number for
  segment traveling from host B to host A?
• Solution
• Source port number y and destination port number
  x.

5
More

• Problem Suppose Client A initiates a Telnet
  session with server S. At about the same time,
  client B also initiates a Telnet session with
  server S. Provide possible source and destination
  port numbers for
• the segment sent from A to S.
• the segment sent from B to S.
• the segment sent from S to A.
• the segment sent from S to B.
• (Note that telnet uses port 23)
• If A and B are different hosts, is it possible
  that the source port number in the segments from
  A to S is the same as that from B to S? Yes
• How about if they are the same host? No

6
Example Checksum

• Problem UDP and TCP use 1's complement for their
  checksums. suppose you have the following three
  8-bit words 01010101, 01110000, 11001100. What
  is the 1's complement of the sum of these words?
  Show all work. Solution
• One's complement 0 1 1 0 1 1 0 1.

7
Example Checksum (Cond.)

• Problem With the 1's complement scheme, how does
  the receiver detect errors? Is it possible that a
  1-bit error will go undetected? How about a 2-bit
  error?
• Answer To detect errors, the receiver adds the
  four words (the three original words and the
  checksum). If the sum contains a zero, the
  receiver knows there has been an error. All
  one-bit errors will be detected, but two-bit
  errors can be undetected (e.g., if the last digit
  of the first word is converted to a 0 and the
  last digit of the second word is converted to a
  1).

8
Summary of the protocols

• rdt1.0 all packets arrive correctly
• rdt2.0 all packets arrive, with possible errors
  only in data packets, and introducing ACK and NAK
  (no error)
• rdt2.1 corrupted ACKS/NAKS
• rdt2.2 similar to rdt2.1 remove NAKs
• Rdt3.0 Allows packets to be lost and errors

9
Rtd2.1 revised
SHOW that this receiver, when operating with the
sender shown in above figure, can lead the sender
and receiver to enter into a deadlock state,
where each is waiting for an event that will
never occur.
10
rdt2.1 sender, handles garbled ACK/NAKs
rdt_send(data)
sndpkt make_pkt(0, data, checksum) udt_send(sndp
kt)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isNAK(rcvpkt) )
Wait for call 0 from above
udt_send(sndpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt)
L
L
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isNAK(rcvpkt) )
rdt_send(data)
sndpkt make_pkt(1, data, checksum) udt_send(sndp
kt)
udt_send(sndpkt)
11
Rdt2.1 revised (Cond.)

• Answer
• Suppose the sender is in state "Wait for call 1
  from above" and the receiver is in state "Wait
  for 1 from below". The sender sends a packet with
  sequence number 1, and transitions to "Wait for
  ACK or NAK 1," waiting for an ACK or NAK. Suppose
  now the receiver receives the packet with
  sequence number 1 correctly, sends an ACK, and
  transitions to state "Wait for 0 from below,"
  waiting for a data packet with sequence number 0.
  However, the ACK is corrupted. When the rdt2.1
  sender gets the corrupted ACK, it resends the
  packet with sequence number 1. However, the
  receiver is waiting for a packet with sequence
  number 0 and always sends a NAK when it doesn't
  get a packet with sequence number 0. Hence the
  sender will always be sending a packet with
  sequence number 1, and the receiver will always
  be NAKing that packet. Neither will progress
  forward from that state.

12
rdt2.1 receiver, handles garbled ACK/NAKs
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)
rdt_rcv(rcvpkt) (corrupt(rcvpkt)
sndpkt make_pkt(NAK, chksum) udt_send(sndpkt)
sndpkt make_pkt(NAK, chksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) not corrupt(rcvpkt)
has_seq1(rcvpkt)
rdt_rcv(rcvpkt) not corrupt(rcvpkt)
has_seq0(rcvpkt)
sndpkt make_pkt(ACK, chksum) udt_send(sndpkt)
sndpkt make_pkt(ACK, chksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
has_seq1(rcvpkt)
extract(rcvpkt,data) deliver_data(data) sndpkt
make_pkt(ACK, chksum) udt_send(sndpkt)
13
Rdt2.1 modification

• Consider a channel that can lose packets but has
  a maximum delay that is known. Modify protocol
  rdt2.1 to include sender timeout and retransmit.
  Informally argue why your protocol can
  communicate correctly over this channel.
• Answer Here, we add a timer, whose value is
  greater than the known round-trip propagation
  delay. We add a timeout event to the "Wait for
  ACK or NAK0" and "Wait for ACK or NAK1" states.
  If the timeout event occurs, the most recently
  transmitted packet is retransmitted. Let us see
  why this protocol will still work with the rdt2.1
  receiver.
• Suppose the timeout is caused by a lost data
  packet, i.e., a packet on the sender-to-receiver
  channel. In this case, the receiver never
  received the previous transmission and, from the
  receiver's viewpoint, if the timeout
  retransmission is received, it look exactly the
  same as if the original transmission is being
  received.
• Suppose now that an ACK is lost. The receiver
  will eventually retransmit the packet on a
  timeout. But a retransmission is exactly the same
  action that is take if an ACK is garbled. Thus
  the sender's reaction is the same with a loss, as
  with a garbled ACK. The rdt2.1 receiver can
  already handle the case of a garbled ACK. 

14
rdt2.1 sender, handles garbled ACK/NAKs
rdt_send(data)
sndpkt make_pkt(0, data, checksum) udt_send(sndp
kt)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isNAK(rcvpkt) )
Wait for call 0 from above
udt_send(sndpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt)
L
L
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isNAK(rcvpkt) )
rdt_send(data)
sndpkt make_pkt(1, data, checksum) udt_send(sndp
kt)
udt_send(sndpkt)
15
rdt3.0 sender
udt_send(sndpkt) start_timer
16
Example rdt3.0

• Problem
• Draw a trace of the operation of protocol rdt3.0
  when a data packet is garbled. 
• Draw a trace when an acknowledgement packet is
  garbled.

17
Pipelined protocols

• rdt3.0 stop-and-wait operation
• Low utilization fraction of time sender busy
  sending
• Two generic forms of pipelined protocols
  go-Back-N, selective repeat

18
Go-Back-N (GBN)

• Window size (N) is constant, which is the maximum
  number of packets that a sender can send out
  without receiving any acknowledgment.
• This determines the link utilization
• Applet

19
Receiver in GBN

• The receiver only delivers the packet that it is
  expecting, i.e., the sequence number of the
  packet is expectedseqnum
• It will drop all out of order packets
• This is simple in that the receiver has no buffer

20
Sender in GBN

• Sends packets with the sequence number in the
  range of base, baseN, and the parameter
  nextseqnum is updated
• Keeps only one timer for all packets that has
  been sent yet acknowledged
• a timeout will trigger the sender to resend all
  those packets
• ACK(n) is received, which is cumulative
• i.e., indicating all packets up to n has been
  received by the receiver. The parameter base is
  updated, and a new timer is started if there are
  still other packets transmitted but
  unacknowledged.

21
GBN in action
22
Selective Repeat (SR)

• Individual packet acknowledgment as supposed to
  the cumulative acknowledgement in GBN
• Receiver buffers the out of order arrivals

23
Receiver in SR

• Packet with the sequence number n in the range of
  rcv_base, rcv_baseN-1 is correctly received,
  send ACK(n), adjust parameter rcv_base and
  deliver the packet to upper layer protocol if
  necessary
• Packet arrival with sequence number n rcv_base
  N, rcv_base 1, ACK(n)
• Otherwise, ignore

24
Sender in SR

• Packet from upper layer with sequence number n
  send_base, send_base N, send the packet and
  start the timer for this packet
• Timeout, resend the packet
• ACK received, mark the buffer as received and
  update the parameter if necessary.

25
SR in action