introduction to computer networks...sliding window protocol using selective repeat • if the...
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Introduction to Computer Networks
Tian Song (嵩天), Ph.D., Assoc. [email protected]
Data Link Protocol
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Outline
• Elementary Data Link Protocol
• Sliding Window Protocols
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Elementary Data Link Protocol
1: An Unrestricted Simplex Protocol
2: A Simplex Stop‐and‐Wait Protocol
3: A Simplex Protocol for a Noisy Channel
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Elementary Data Link Protocol
Some basic assumptions:–We have a machine A that wants to send data to machine B
– There is always enough data for A to send– There is a well‐defined interface to the network layer, and to the physical layer
– The receiver generally waits for an event to happen by calling wait_for_event
boolean, seq_nr, packet, frame_kind, frame
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Elementary Data Link Protocol1: An Unrestricted Simplex Protocol
sender receiver
packet = from_network()
packet ‐> frame
to_physical(frame)
wait_for_event()
frame = from_physical()
frame ‐> packet
to_network(packet)
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Elementary Data Link Protocol
Issue of Protocol 1:
How to prevent the sender from flooding?
Solutions:
Control the pace of sending and receiving
Stop‐and‐Wait
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Elementary Data Link Protocol2: A Simple Stop‐and‐Wait Protocol
sender receiver
packet = from_network()
packet ‐> frame
to_physical(frame)
wait_for_event()
wait_for_event()
frame = from_physical()
frame ‐> packet
to_network(packet)
receiver sender
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Elementary Data Link Protocol
Issue of Protocol 2:
How about we lost some frames?
Solutions:
(1) Lost a data frame ‐> Timer
(2) Lost a feedback frame ‐> Sequence Number
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Elementary Data Link Protocol3: A Simplex Protocol for a Noisy Channel
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Elementary Data Link Protocol3: A Simplex Protocol for a Noisy Channel
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Improvement
Motivation:
from Simplex (单工) to Duplex (双工)
Observation:
Feedback frame is very small compared to data.
Solution:
Piggybacking (捎带确认)
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Sliding Window Protocols
Motivation:
Send more packets at one time
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a: Initially. b: After sending frame #1.
c: After receiving frame #1. d: After receiving ack for frame #1.
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How to Understand?
Page 196, Q 23
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Sliding Window Protocols4: One‐Bit Sliding Window Protocol
maximum window size: 1
stop and wait
piggybacking
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(a) Normal case. (b) Abnormal case.
(序号, 确认, 包号)
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Sliding Window ProtocolsProblem:
What should the receiver do if a frame is damaged?
Solution:(1) Request retransmission starting from frame #N
5: Sliding Window Protocol Using Go Back N
(2) Request just retransmission of damaged frame
6: Sliding Window Protocol Using Selective Repeat
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Sliding Window Protocols
否定确认
pipeline: 流水线
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Sliding Window Protocol Using Selective Repeat
• If the receiver delivers all frames in its window just after sending an ACK for the entire window, we may have a serious problem:
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Sliding Window Protocol Using Selective Repeat
• Solution
we must avoid overlapping send and receive windows, the
highest sequence number must be at least twice the
window size.
If window size: 0 – MAX_SEQ
Then, protocol 6:
window size: (MAX_SEQ+1)/2.
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Link Utilization
2. Stop‐and‐Wait
5. Sliding Window, Go back N
6. Sliding Window, selective repeat
Q 32, page 197
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Conclusion
• Elementary Data Link Protocol
• Sliding Window Protocols
• Six Protocols
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End
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