CS 352 Internet Technology Transport Protocols

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CS 352Internet TechnologyTransport Protocols

• Dept. of Computer Science
• Rutgers University

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Reliable Data Transfer

• Problem 1 Flow Control
• Receiver can only handle a fixed amount of data
• Problem 2 Reliability
• Want an abstraction of a reliable link even
  though packets can be lost
• Solution to both problems is to keep track of the
  packets
• Not as simple as one would expect

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Data Transfer

• Both problems can be solved using a family of
  related reliable data transfer algorithms
• Alternating Bit Protocol (ABP)
• Go-back-N
• Selective Repeat
• Algorithms range in complexity and performance

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Flow Control

• What happens if the sender tries to transmit
  faster than the receiver can accept?
• Data will be lost unless flow control is
  implemented

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Controlling the Flow of Data
Busy Client
Slow Server
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Some Flow Control Algorithms

• Flow control for the ideal network
• Stop and Wait for noiseless channels
• Stop and Wait for noisy channels
• Sliding window protocols
• Sliding window with error control
• Go Back N
• Selective Repeat

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Flow control in the ideal network

• Assumptions
• Error free transmission link,
• Infinite buffer at the receiver

No acknowledgement of frames necessary Since the
data link is error-free and the receiver can
buffer as many frames as it likes, no frame will
ever be lost
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Flow control in the ideal network (contd)
Busy Client
Slow Server
Infinite Box
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Stop and Wait with Noiseless Channels

• Assumptions
• Error free transmission link,
• Finite buffer at the receiver

• Problem of Buffer overflow at the receiver
• Buffer overflow may happen at the receiver when
  the sender sends frames at a rate faster than the
  receiver can accept

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Stop and Wait with Noiseless Channels (contd)
Busy Client
Slow Server
Finite Box (once full, Letter is lost)
Bit Bucket
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Stop and Wait with Noiseless Channels (contd)

• Solution Stop-and-Wait
• The receiver sends an acknowledgement frame
  telling the sender to transmit the next data
  frame.
• The sender waits for the ACK, and if the ACK
  comes, it transmits the next data frame.

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Stop and Wait with Noiseless Channels (contd)
Data
Data
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Stop and Wait (contd)

• Note that we assume an error-free transmission
  link and therefore ACK frames will not be lost
• In this flow control protocol, there are two
  types of frames data frames and ACK frames. The
  ACK frames dont contain any particular
  information, since only the arrival of the ACK
  frame at the sender is important.

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Stop and Wait with Error

• Assumptions
• Transmission link may cause errors in frames,
• Finite buffer at the receiver

Data and ACK frames may be lost
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Problems introduced by Errors

• Error model
• Packet can be dropped in the network
• Packets can be delayed for a bounded amount of
  time
• maximum packet lifetime
• Problem 1 Loss of a data or ACK frame
• Since the transmission path through the network
  is not error-free, a data or ACK frame may be
  lost, causing the sender to wait indefinitely for
  an ACK

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Loss of an ACK frame
Data
?
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Problems introduced by errors

• Can we solve problem 1 by introducing a timeout
  period for the sender? Yes, but that introduces
  ...
• Problem 2 Duplicated frames
• If the ACK frame for a certain data frame is
  lost, the sender will retransmit the same frame
  after a time-out period, and the receiver will
  then have two copies of the same frame
• Note Setting the timeout longer than the maximum
  packet lifetime does not help

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Duplicated Data Frames
Sell 10 Shares!
Wheres the ACK?
Sell 10 Shares!
Sold 20!
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Alternating Bit Protocol (ABP)

• Solution
• The sender uses a timer to retransmit data frames
  when an ACK has not arrived
• The sender includes a sequence number in each
  frame to distinguish one frame from another.
  This way, the receiver knows when it has received
  duplicate frames.
• The Receiver uses the sequence number in the
  acknowledgement
• The sender alternates the sequence number between
  0 and 1 on each ACKed frame

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Alternating Bit Protocol Normal
Data
Data
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Alternating Bit ProtocolPacket Loss
Data
Wheres the ACK?
Data
Duplicate
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ABP Robustness

• ABP is robust to
• Packet Loss
• ACK loss
• Not Robust to
• Active attacks (spoofing)
• Arbitrary Delay
• Must set timeout gt Maximum packet lifetime in
  the network

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Is ABP the best we can do?

• Alternating Bit protocol is an effective
  reliability protocol, but
• Its not very efficient.
• 1. Only one data frame can be in transit at a
  time
• 2. When waiting for an acknowledgement, the
  sender cannot transmit any frames
• What happens to packet switching in this case?
• Goal Keep multiple packets in the network to
  improve performance
• Solution Sliding window protocols
• Recall packet vs. message switching analysis

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Pipelining
packet 5
packet 4
packet 6
Ack 0
Ack 2
Ack 1
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Pipelining
data stream
99
51
50
A
B
49
1
0
ACK stream

• By allowing several frames onto the link before
  receiving an acknowledgement, pipelining keeps
  the communications path from being idle
• Sliding windows generalize ABPs single bit into
  an N-bit sequence number
• Must keep track of multiple outstanding packets

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Pipelining (cont)
Data
Host A
Switch 1
Switch 2
Host B
Packet 1
Packet 2
Packet 3
Time
Packet 4
Ack1
Ack1
Ack1
Ack2
Ack2
Ack2
Ack3
Ack3
Ack3
Ack4
Ack4
Acknowledgements
Ack4
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Sliding Window ProtocolsDefinitions

• Sequence Number Each frame is assigned a
  sequence number that is incremented as each frame
  is transmitted
• Sender Window Keeps track of sequence numbers
  for frames that have been sent but not yet
  acknowledged
• Receiver Base Last sequence number receiver has
  correctly received
• Receiver Window Keeps track of sequence numbers
  for frames the receiver is allowed to accept
• Maximum Sender Window size The maximum number
  of frames the sender may transmit without
  receiving any acknowledgements
• Maximum Receiver Window size The maximum number
  of frames the receiver may receive before
  returning an acknowledgement to the sender

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Sliding Window Definition
Key
Already Acked
Usable, not yet sent
Sent, not Yet Acked
Not usable
rcv_next
Already Acked
Packets
Not usable
next
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Simple Sliding Window with Window Size of 1

• A sliding window with a maximum window size of 1
  frame
• Modulo-arithmetic results in a circular view
  this 1-packet window using a 3-bit sequence
  number
• E.g. 314 440 (44)8

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Sliding Window example
Sender window
Receiver window
(a)
(b)
(c)
(d)
(a) Initial state, no frames transmitted (b)
Sender transmits frame 0 (c) Receiver receives
frame 0 and ACKs (d) Sender receives ACK
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Simple Sliding Window withWindow size 1 (contd)

• This protocol behaves identically to ABP
• In General
• With window size W
• How do the sender an receiver manipulate base,
  next and max in response to packet arrival and
  timeout events?
• What packets are sent in response to these
  events?

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Sliding Window ProtocolsGeneral Remarks

• The sending and receiving windows do not
  necessarily have the same maximum size
• Any frame whose sequence number falls outside the
  receiver window is discarded at the receiver
• The sender windows size grows and shrinks as
  frames are transmitted and acknowledged
• Unlike the sender window, the receiver window
  always remains at its maximum size

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Sliding Window ProtocolsPiggybacking
Acknowledgements

• Since we have full duplex transmission, we can
  piggyback an ACK onto the header of an outgoing
  data frame to make better use of the channel

When a data frame arrives at a router, instead of
immediately sending a separate ACK frame, the
router waits until it is passed the next data
frame to send. The acknowledgement is attached
to the outgoing data frame.
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Sliding Window with Maximum Sender Window Size WS

• With a maximum window size of 1, the sender waits
  for an ACK before sending another frame
• With a maximum window size of WS, the sender can
  transmit up to WS frames before being blocked
• This allows the sender to transmit several frames
  before waiting for an acknowledgement

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Sender-Side Window with WS2
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(a) Initial window state (b) Send frame 0 (c)
Send frame 1 (d) ACK for frame 0 arrives
(e) Send frame 2 (f ) ACK for frame 1 arrives (g)
ACK for frame 2 arrives, send frame 3 (h) ACK for
frame 3 arrives
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Sliding Window with Maximum Receiver Window Size
WR

• With a maximum window size of 1, the receiver
  must receive and process every frame in sequence
• With a maximum window size of WR, the receiver
  can receive and process up to WR frames before
  acknowledging them
• This is useful when frames are lost the receiver
  can still accept and buffer frames after the
  missing frame

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Receiver-Side Window with WR2
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(a) Initial window state (b) Nothing happens (c)
Frame 0 arrives, ACK frame 0 (d) Nothing happens
(e) Frame 1 arrives, ACK frame 1 (f) Frame 2
arrives, ACK frame 2 (g) Nothing happens (h)
Frame 3 arrives, ACK frame 3
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What about Errors?

• What if a data or acknowledgement frame is lost
  when using a sliding window protocol?
• Two Solutions
• Go Back N
• Selective Repeat

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Sliding Window with Go Back N

• When the receiver notices a missing or erroneous
  frame, it simply discards all frames with greater
  sequence numbers and sends no ACK
• The sender will eventually time out and
  retransmit all the frames in its sending window

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Go Back N
Timeout interval
Sender
0
1
2
3
4
2
3
4
5
6
Maximum window size 8
ACK 6
ACK 0
ACK 1
ACK 2
ACK 3
ACK 4
ACK 5
Receiver
0
1
E
D
D
2
3
4
5
6
Maximum window size 8
Discarded by receiver
Frame with error
Time
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Go Back N (contd)

• Go Back N can recover from erroneous or missing
  frames
• But
• It is wasteful. If there are errors, the sender
  will spend time retransmitting frames the
  receiver has already seen

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Sliding Window with Selective Repeat

• The sender retransmits only the frame with errors
• The receiver stores all the correct frames that
  arrive following the bad one. (Note that the
  receiver requires a frame buffer for each
  sequence number in its receiver window.)
• When the receiver notices a skipped sequence
  number, it keeps acknowledging the last good
  sequence number
• When the sender times out waiting for an
  acknowledgement, it just retransmits the one
  unacknowledged frame, not all its successors.

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Selective Repeat
Key
base
next
Already Acked
Usable, not yet sent
max
Packets
Sent, not Yet Acked
Not usable
Window size N
rcv_base
max
Acked, buffered
Packets
Acceptable
next
Not usable
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Selective Repeat
Timeout interval
Sender
0
1
2
3
4
2
5
6
Maximum window size 8
ACK 6
ACK 0
ACK 1
ACK 4
ACK 5
ACK 1
ACK 1
Receiver
0
1
E
2
5
6
3
4
Maximum window size 8
Buffered by receiver
Frame with error
Time