Igor Radovanovic, [e-mail]

1
Computer Networks (2IC10)

• Communication Protocols
• Igor Radovanovic
• Thanks to
• P. van der Stok
• A. B. Forouzan
• A. Tanenbaum

2
Packet transmission in ideal network
N. Olifer, V. Olifer
Computer Networks,
principles, technologies and protocols
for network design, Wiley Sons

• All packets delivered to the destination node
  without loss or distortion
• All packets delivered in the same order
• Delivery with minimum delay (d1d2d3)

3
Communication problems

• corruption of packets
• loss of packets
• replication of packets
• ordering of packets
• independent failures of nodes

4
Packet transmission in a real network
?1
2
3
4
5
6
1
t
1
2
3
4
6
d1
d4
t
d2
?1
d3
N. Olifer, V. Olifer
Computer Networks,
principles, technologies and protocols
for network design, Wiley Sons

• Packets are delivered with variable delays (d1?d2
  ?d3)
• Packets are delivered out of order
• Packets may be lost or corrupted
• Average rate of information flow is different at
  the input and at the output of the network

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Packet corruption

• Error detection
• Error correction
• Types of errors
• single bit only 1 bit in the data unit has
  changed
• burst error 2 or more bits in the data unit have
  changed

Example 0.01 s of burst impulse noise
1200 bps data rate
12 bits of errors
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Error detection

• Redundancy adding extra bits for detecting
  errors at the destination

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Error detection

• All methods use redundancy

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Parity check
even parity

• A parity bit is added to every data unit so that
  the total number of 1s is even (or odd for odd
  parity)

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Parity check-example-

• Suppose the sender wants to send the word world.
  In ASCII, the five characters
  are coded as
• 1110111 1101111 1110010 1101100
  1100100
• The following shows the actual bits sent
• 11101110 11011110 11100100 11011000
  11001001
• Parity check can detect single-bit errors
• What about burst errors?

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Error detection
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CRC check

• based on binary division
• CRC added to the data unit so that the resulting
  data becomes exactly divisible by a predetermined
  number
• CRC
• exactly 1 less bit than the divisor

• A string of n zeros added to the data unit
• New data divided by divisor
• Replace 0s by CRC

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CRC generator

• Modulo-2 division

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CRC generator (cntd)
Name Polynomial Application
CRC-8 x8 x2 x 1 ATM header
CRC-10 x10 x9 x5 x4 x 2 1 ATM AAL
ITU-16 x16 x12 x5 1 HDLC
ITU-32 x32 x26 x23 x22 x16 x12 x11 x10 x8 x7 x5 x4 x2 x 1 LANs
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CRC performance

• CRC can detect
• all burst errors that affect odd number of bits
• all burst errors of length ? degree of polynomial
• with high probability burst errors gt degree of
  polynomial
• Example
• CRC-12 (degree of 12)
• OK
• OK
• 99.97

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Error detection
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Checksum
Example IP header

1. The data unit is divided into k sections, each of
   n bits
2. All sections are added using 1s complement
3. The sum is complemented
4. The checksum is sent with data

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1s complement

• Suppose the following block of 16 bits is to be
  sent using a checksum of 8 bits.
• 10101001 00111001
• The numbers are added using ones complement
• 10101001
• 00111001
  ------------
• Sum 11100010
• Checksum 00011101
• The pattern sent is 10101001 00111001
  00011101

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Checksum (cntd)

• Performance
• Can we use this technique to detect an error if 2
  corresponding opposite bits in 2 data segments
  become corrupted?
• An example
• Transmitted sequence 10101001 00111001
  00011101
• Received sequence 00101001 10111001
  00011101

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Error correction

• By retransmission
• flow and error control protocols
• Forward Error Correction (FEC)
• require more redundancy bits
• should locate the invalid bit or bits
• n-bit code word contains m data bits r
  redundancy bits
• nmr
• mr1 bits discoverable by r bits
• Example
• For m5 data bits can r be 3?

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Hamming code

• Example 7-bit ASCII code 4 bit redundancy
• Each r bit is the parity bit for one combination
  of data bits
• r1 bits 1, 3, 5, 7, 9, 11
• r2 bits 2, 3, 6, 7, 10, 11
• r3 bits 4, 5, 6, 7
• r4 bits 8, 9, 10, 11

20 1 1 1 1 1 1
21 2 2 2 2 2 2
22 4 4 4 4
23 8 8 8 8

? ? ? ? ? ? ? ? ? ? ?
1 2 3 4 5 6 7 8 9 10 11
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Hamming code (cntd)

• Example of redundancy bit calculation (even
  parity)

22
Hamming code (cntd)

• Error detection using Hamming code

once the bit is identified the receiver can
reverse its value
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Communication protocols
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Communication protocols-Wanted behavior-

• Communicating sender and receiver
• Order
• When sender has sent mi before mj and receiver
  has accepted mj, then receiver has also accepted
  mi
• Validity
• When receiver has accepted mi then mi was sent by
  sender
• Progress
• A sent message is eventually accepted

25
Stop-and-Wait Automatic Repeat reQuest

• simplest flow and error control mechanism
• the sending device keeps a copy of the last frame
  transmitted until it receives an acknowledgement
• identification of duplicate transmission (lost or
  delayed ACK)
• a damaged or lost frame is treated in the same
  way
• timers introduced
• positive ACK sent only for frames received safe
  sound

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A Simplex Stop-and-Wait ARQ

1. normal operation
2. the frame is lost
3. the ACK is lost
4. the ACK is delayed

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Stop-and-Wait ARQ -Normal operation-

• The sender will not send the next piece of data
  until it is sure that the current one is
  correctly received
• sequence number necessary to check for duplicated
  packets
• No NACK when packet is corrupted duplicate
  ACKs instead

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Stop-and-Wait ARQ -Lost or damaged frame-
roundtrip delay
processing in the receiver
Should be as short as possible. Why?
29
Stop-and-Wait ARQ -Lost ACK-
Importance of numbering
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Stop-and-Wait ARQ -Delayed ACK-
Importance of ACK numbering
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Duplex Stop-and-Wait ARQ

• Piggybacking
• combine data with ACK (less overhead saves
  bandwidth)

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Drawbacks of Stop-and-Wait ARQ

• after each frame sent the host must wait for an
  ACK
• inefficient use of bandwidth
• to improve efficiency ACK should be sent after
  multiple frames
• Alternatives Sliding Window protocols
• Go-back-N ARQ
• Selective Repeat ARQ

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Pipelining

• One task begins before the other one ends
• increases efficiency in transmission

34
Sliding Window Protocols

• Sequence numbers
• sent frames are numbered sequentially
• number of frames stored in the header
• if the number of bits in the header is m than
  sequence number goes from 0 to
  2m-1
• Sliding window
• to hold the
• unacknowledged
• outstanding frames
• the receiver
• window size
• always 1

sequence number
frame
acknowledged frames
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Question

• What is the consequence of the window size in the
  receiver on the order of the received packets?
• Answer
• Window size 1 means that the packets are received
  in order
• Note this is not the case for the larger
  receiver window size

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Go-back-N-Control variables-

• S- holds the sequence number of the recently sent
  frame
• SF holds sequence number of the first frame in
  the window
• SL holds the sequence number of the last frame
• R sequence number of the frame expected to be
  received

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The name of Go-back-N why?

• Re-sending frame
• when the frame is damaged the sender goes back
  and sends a set of frames starting from the last
  one ACKnd
• the number of retransmitted frames is N
• Example
• The window size is 4.
• A sender has sent frame 6 and the timer expires
  for frame 3 (frame 3 not ACKnd). The sender goes
  back and re-sends the frames 3, 4, 5 and 6.

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Go-back-N -normal operation-

• How many frames can be transmitted without
  acknowledgment?
• ACK1 not necessary if ACK2 is sent
• Cumulative ACK

expected sequence number
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Go-back-N -damaged or lost frame-
damaged frames are discarded! Why are correctly
received packets not buffered? What is the
disadvantage of this?
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Go-back-N -sender window size-
sequence number
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Go-back-N

• Inefficient
• all out of order received packets are discarded
• This is a problem in a noisy link
• many frames must be retransmitted -gt bandwidth
  consuming
• Solution
• re-send only the damaged frames
• Selective Repeat ARQ
• avoid unnecessary retransmissions

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Selective Repeat ARQ

• Processing at the receiver more complex
• The window size is reduced to 2m-1 (at most)
• Both the transmitter and the receiver have the
  same window size
• Receiver expects frames within the range of the
  sequence numbers

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Selective Repeat ARQ-lost frame-
Note retransmission triggered with NACK and not
with expired timer
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Selective Repeat ARQ-sender window size-
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Communication problems

• corruption of packets
• loss of packets
• replication of packets
• ordering of packets
• independent failures of nodes

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Communication protocols

• We have assumed until now that packets cannot be
  reordered
• this when the parties are connected by a single
  physical line
• What if the channel is a network?
• the packets may arrive out of order
• Sequence number might be reused duplications
  possible
• How to solve this problem?
• Introduce the lifetime of the packet

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Connection establishment

• Connection request PDU
• Connection accepted PDU
• Is this enough?
• What if the network can store or duplicate
  packets?
• What if the subnet is so congested that ACK or
  datagrams cannot go through?
• Example bank transaction (delayed duplicates)
• Solutions
• new addresses for each connection?
• incremented sequence numbers?
• PACKET LIFETIME

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Packet lifetime

• Can be restricted using
• restricted subnet design
• prevents looping
• bounding congestion delay over the longest
  possible path
• putting a hop counter in each packet
• decrementing default value dropping when zero
• time stamping each packet
• carry the time when it was created
• discarded after some predefined time
• all the routers synchronized nontrivial
• Lifetime expires
• Not only the packets but all the ACKs must be
  dead

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Connection Establishment (2)
dead time after crash (max packet lifetime)
data rate lt clk rate

• Time-of-day clock (not necessarily sync) 2 PDU
  with the same seq. num. are never outstanding at
  the same time
• solves the delayed duplicate problem for data
  PDU
• (a) PDUs may not enter the forbidden region.
• (b) The resynchronization problem.

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Connection Establishment (2)
Three protocol scenarios for establishing a
connection using a three-way handshake. CR
denotes CONNECTION REQUEST. (a)
Normal operation, (b) Old CONNECTION REQUEST
appearing out of nowhere. (c) Duplicate
CONNECTION REQUEST and duplicate ACK.
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Connection Release

• asymmetric release
• a telephone line
• connection is broken when one party hangs up
• abrupt disconnection with loss of data.
• symmetric release
• each connection released separately
• continue to receive after disconnect PDU is sent
• avoid data loss

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Connection Release (2)
unreliable communication channel

• The two-army problem.

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Connection Release (3)
disconnect request
6-14, a, b

• Four protocol scenarios for releasing a
  connection.
• (a) Normal case of a three-way handshake. (b)
  final ACK lost.

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Connection Release (4)
6-14, c,d

• (c) Response lost. (d) Response lost and
  subsequent DRs lost.