ECE 4450:427/527 - Computer Networks
Spring 2017
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 5.3: Reliable Transmission
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
1
Link Layer: Five Common Problems
• Basic problem: you can’t just send IP datagrams over the link!
• We first consider how to encode bits into the signal at the
source and recover bits at the receiving node
• Once it is possible to transmit bits, we need to figure out
how to package these bits into FRAME
• Assume each node is able to recognize the collections of
bits making up a frame, the third problem is to determine if
those bits are in error: Error Detection and Correction
• If frames arriving at destination contain errors, how to
recover from such losses: ARQ
• Final problem related to multiple-access link: how mediate
access to a shared link so that all nodes have a chance to
transmit: We focus on Ethernet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
2
Discussions
• We have learned so far:
• Error Detection to detect errors in Frames
• Error Correction to correct errors in Frames
• But sometimes, errors are too severe to be corrected and
corrupted frames must be discarded.
• Now we consider a link-level protocol to deal with this
issue so that frames can be delivered reliably:
– A combination of two fundamental mechanisms:
Acknowledgement (ACK) and Timeout – Automatic Repeat
reQuest (ARQ).
– Error detection?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
3
Acknowledgement
• An acknowledgement (ACK for short) is a small
control frame that a protocol sends back to its peer
saying that it has received the earlier frame.
– A control frame is a frame with header only (no data).
• The receipt of an acknowledgement indicates to the
sender of the original frame that its frame was
successfully delivered.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
4
Timeout
• If the sender does not receive an acknowledgment
after a reasonable amount of time, then it
retransmits the original frame.
• The action of waiting a reasonable amount of time is
called a timeout.
• The general strategy of using acknowledgements and
timeouts to implement reliable delivery is sometimes
called Automatic Repeat reQuest (ARQ).
• We now study different ARQ algorithms. We shall use
generic language, i.e., do not give detail about
header fields.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
5
Stop-and-Wait
• Idea of stop-and-wait protocol is straightforward
– After transmitting one frame, the sender waits for an
acknowledgement before transmitting the next frame.
– If the acknowledgement does not arrive after a certain
period of time, the sender times out and retransmits the
original frame
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
6
Stop-and-Wait Protocol
• Let assume we transmit a frame. How many
scenarios we can have with Stop-and-Wait?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
7
Stop-and-Wait Protocol: Issue
ACK is lost or arrives after timeout: what happen?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
8
Stop-and-Wait Protocol: Issue
How to deal with this issue: duplicate copies of
frames will be delivered
•Use 1 bit sequence number (0 or 1) in the header
•When the sender retransmits frame 0, the receiver can
determine that it is seeing a second copy of frame 0
rather than the first copy of frame 1 and therefore can
ignore it (the receiver still acknowledges it, in case the
first acknowledgement was lost)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
9
Stop-and-Wait Protocol: Inefficiency
• How many outstanding frames on the link at a time?
– This may be far below the link’s capacity
• Consider a 1.5 Mbps link with a 45 ms RTT
– The link has a delay  bandwidth product of 67.5 Kb or approximately 8
KB
– Since the sender can send only one frame per RTT and assuming a
frame size of 1 KB
– Maximum Sending rate
• Bits per frame  Time per frame = 1024  8  0.045 = 182 Kbps
Or about one-eighth of the link’s capacity
– How can we use the link fully?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
10
Sliding-Window Protocol
Main idea: Leave up to N frames unacknowledged at any given time:
Sender ready to send (N+1)th frame at the same moment ACK for
the first frame arrives
sender
receiver
first packet bit transmitted
last packet bit transmitted
first packet bit arrives
last packet bit arrives, send
ACK
RTT
ACK arrives, send next
Packet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
11
Sliding-Window Protocol
Main idea: Leave up to N frames/packets unacknowledged at any
given time: Sender ready to send (N+1)th frame at the same
moment ACK for the first frame arrives
sender
receiver
first packet bit transmitted
last bit transmitted
first packet bit arrives
last packet bit arrives, send ACK
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
RTT
ACK arrives, send next
packet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
12
Sliding-Window Protocol
• Perhaps the best known algorithm in computer
networking: Need to understanding it thoroughly!!!
• It can be used to serve different roles in different
layers:
• Reliable delivery at Link Layer
• Flow control: managing the rate of data
transmission between two nodes to prevent a fast
sender from outrunning a slow receiver
• TCP: Also reliable deliver
• We shall consider those later on
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
13
Sliding-Window Protocol: Algorithm
• Sender assigns a sequence number denoted as SeqNum to
each frame
• Sender maintains three variables
– Sending Window Size (SWS)
• Upper bound on the number of outstanding (unacknowledged) frames
that the sender can transmit
– Last Acknowledgement Received (LAR)
• Sequence number of the last acknowledgement received
– Last Frame Sent (LFS)
• Sequence number of the last frame sent
• Sender also maintains the following invariant
LFS – LAR ≤ SWS
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
14
Sliding-Window Protocol
Sliding Window on Sender
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
15
Sliding-Window Protocol
• When an acknowledgement arrives
– the sender moves LAR to right, thereby allowing the sender to transmit
another frame
• Also the sender associates a timer with each frame it
transmits
– It retransmits the frame if the timer expires before the ACK is received
• Note that the sender has to be willing to buffer up to SWS
frames
– WHY?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
16
Sliding-Window Protocol: Example
sender
receiver
first packet bit transmitted
last bit transmitted
first packet bit arrives
last packet bit arrives, send ACK
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
RTT
ACK arrives, send next
packet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
17
Sliding-Window Protocol
• Receiver maintains three variables
– Receiving Window Size (RWS)
• Upper bound on the number of out-of-order
frames that the receiver is willing to accept
– Largest Acceptable Frame (LAF)
• Sequence number of the largest acceptable
frame
– Last Frame Received (LFR)
• Sequence number of the last frame received
(and ACK to the sender)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
18
Sliding-Window Protocol
• Receiver also maintains the following invariant
LAF – LFR ≤ RWS
Sliding Window on Receiver
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
19
Sliding-Window Protocol
• When a frame with sequence number SeqNum
arrives, what does the receiver do?
– If SeqNum ≤ LFR or SeqNum > LAF
• Discard it (the frame is outside the receiver
window)
– If LFR < SeqNum ≤ LAF
• Accept it
• Now the receiver needs to decide whether or
not to send an ACK
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
20
Sliding-Window Protocol
– Let SeqNumToAck
• Denote the largest sequence number not yet
acknowledged, such that all frames with
sequence number less than or equal to
SeqNumToAck have been received
– The receiver acknowledges the receipt of
SeqNumToAck . This acknowledgement is said to
be cumulative.
– The receiver then sets
• LFR = SeqNumToAck and adjusts
• LAF = LFR + RWS
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
21
Sliding-Window Protocol
For example, suppose LFR = 5 and RWS = 4 ->LAF = 9
(i.e. the last ACK that the receiver sent was for seq. no. 5)
If frames 7 and 8 arrive, they will be buffered because they are
within the receiver window
But no ACK will be sent since frame 6 is yet to arrive
Frames 7 and 8 are out of order
Frame 6 arrives (it is late for some reason)
Now Receiver Acknowledges Frame 8
and bumps LFR to 8
and LAF to 12
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
22
Sliding-Window Protocol: Issues
• When timeout occurs, the amount of data in transit decreases
– Since the sender is unable to advance its window
• When the packet loss occurs, this scheme is no longer keeping
the pipe full
– The longer it takes to notice that a packet loss has
occurred, the more severe the problem becomes
• How to improve this
– Negative Acknowledgement (NAK)
– Additional Acknowledgement
– Selective Acknowledgement
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
23
Sliding-Window Protocol: Issues
• Negative Acknowledgement (NAK)
– Receiver sends NAK for frame 6 as soon as frame 7 arrive
(in the previous example)
• Additional Acknowledgement
– Receiver sends additional ACK for frame 5 when frame 7
arrives
• Sender uses duplicate ACK as a clue for frame loss
• Selective Acknowledgement
– Receiver will acknowledge exactly those frames it has
received, rather than the highest number frames
• Receiver will acknowledge frames 7 and 8
• Sender knows frame 6 is lost
• Sender can keep the pipe full, but…..??
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
24
Sliding-Window: Comment Settings
• In practice, how to select SWS?
• Depending on a quite number of factors (which?)
• Now for RWS:
• RWS=1: Go-back-N protocol. But what does this
mean if RWS=1?
• Should we set RWS>SWS?
• RWS=SWS with Selective ACK: Selective Repeat
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
25
Maximum Sequence Number MaxSeqNum
• We already discussed Stop-and-Wait: We need only 1
bit-header: Two sequence numbers 0 and 1, and
reuse them - MaxSeqNum=2;
• Now, for Sliding-Window Protocol: Should sequence
numbers go to infinity?
• We have a finite sequence number:
• Example, 3-bit header: 8 sequences
• Need to reuse or wrap around
• So how we select MaxSeqNum?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
26
Maximum Sequence Number MaxSeqNum
SWS + 1 ≤ MaxSeqNum
– Is this sufficient?
» Depends on RWS
» If RWS = 1, then sufficient
» If RWS = SWS, then not good enough
• For example, we have eight sequence numbers
1, 2, 3, 4, 5, 6, 7,8
RWS = SWS = 7
Sender sends 1, 2, …, 7
Receiver receives 1, 2, … ,7
Receiver acknowledges 1, 2, …, 7
ACK (1, 2, …, 7) are lost
Sender retransmits 1, 2, …, 7
Receiver is expecting 8, 1, …., 6
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
27
Maximum Sequence Number MaxSeqNum
• To avoid this,
If RWS = SWS
SWS < (MaxSeqNum + 1)/2
• Note that this rule is specific to the situation where
RWS=SWS. We might see a more general rule for
arbitrary values of RWS and SWS in assignment
• Also, at this moment, we assume frames are not reordered in transit, which is ony correct for direct
point to point. For different environment, we need
another rule (e.g., in TCP).
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
28
Sliding Window Protocol: Summary
• Sender maintains three variables
– Sending Window Size (SWS)
– Last Acknowledgement Received (LAR)
– Last Frame Sent (LFS)
• Sender also maintains the following invariant
LFS – LAR ≤ SWS
• When ACK arrives
– Move LAR to right, transmit another frame
• Retransmit frame if time expires (time out)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
29
Sliding-Window Protocol: Summary
• Receiver maintains three variables
– Receiving Window Size (RWS)
– Largest Acceptable Frame (LAF)
– Last Frame Received (LFR)
• Receiver also maintains the following invariant
LAF – LFR ≤ RWS
• When a frame with sequence number SeqNum arrives, only accept it if
• If LFR < SeqNum ≤ LAF
• ACK?
• SeqNumToAck : The largest sequence number not yet acknowledged,
such that all frames with sequence number less than or equal to
SeqNumToAck have been received
– The receiver then sets
• LFR = SeqNumToAck and adjusts LAF = LFR + RWS
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
30
Sliding-Window Protocol: Example 1
• Considering the case that SWS=RWS=3. Timeout
interval is about 2RTT. Draw a timeline diagram of
the following
• Frame 4 is lost
• Frames 4-6 are lost
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
31
Sliding-Window Protocol: Issues
• When timeout occurs, the amount of data in transit decreases
– Since the sender is unable to advance its window
• When the packet loss occurs, this scheme is no longer keeping
the pipe full
– The longer it takes to notice that a packet loss has
occurred, the more severe the problem becomes
• How to improve this
– Negative Acknowledgement (NAK)
– Additional Acknowledgement
– Selective Acknowledgement
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
32
Sliding-Window Protocol: Issues
• Negative Acknowledgement (NAK)
– Receiver sends NAK for frame 6 as soon as frame 7 arrive
(in the previous example)
• Additional Acknowledgement
– Receiver sends additional ACK for frame 5 when frame 7
arrives
• Sender uses duplicate ACK as a clue for frame loss
• Selective Acknowledgement
– Receiver will acknowledge exactly those frames it has
received, rather than the highest number frames
• Receiver will acknowledge frames 7 and 8
• Sender knows frame 6 is lost
• Sender can keep the pipe full, but…..??
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
33
Sliding-Window Protocol: Example 2
In the following example, we will see the efficiency of
using NAK or additional ACK
• Considering the case that SWS=RWS=4 and frame 2 is
lost. Timeout interval is about 2RTT.
• First, draw a timeline diagram with retransmission taking place upon time out as usual
• Now, assume NAK[2]/DUPACK[2] is sent after
frame 3 is received. Re-transmission now take
paces either upon time out or upon receipt of
NAK[2]/DUPACK[2]
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
34
Sliding-Window: Comment Settings
• In practice, how to select SWS?
• Depending on a quite number of factors!!
• Now for RWS:
• RWS=1: Go-back-N protocol
• Should we set RWS>SWS?
• RWS=SWS with Selective ACK: Selective Repeat
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
35
Go-Back-N Protocol
• It is Sliding Window Protocol with RWS=1: Receiver will not buffer any
frames that arrive out of order
Demo: http://media.pearsoncmg.com/aw/aw_kurose_network_4/applets/goback-n/index.html
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
36
Selective Repeat
•Window size can be very large for nets with large delay x bandwidth
•Inefficient to retransmit all N frames if one is lost
•Selective repeat allows the re-transmission of only the lost packets
•Accepts out-of-order packets
•Simply increase the RWS up to SWS (does not make sense to allow
for RWS > SWS)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
37
Example with Selective Repeat
Demo: http://media.pearsoncmg.com/aw/aw_kurose_network_4/applets/SR/index.html
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
38
Sliding-Window Protocol
• Perhaps the best known algorithm in computer
networking
• Serves three different roles
•Reliable
•Preserve the order: The receiver makes sure that it
does not pass a frame up to the next higher-level
protocol until it has already passed up all frames with
a smaller sequence number
•Frame control: Receiver is able to throttle senderKeeps sender from overrunning receiver from
transmitting more data than the receiver is able to
process
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
39
Sliding-Window Protocol
• Perhaps the best known algorithm in computer
networking
• It can be used to serve different roles in different
layers:
• Reliable delivery at Link Layer
• TCP: Also reliable deliver
Up to now, we already learned quite a few
techniques/protocols considered in Transport Layer
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
40