局域网络可靠传输技术 －－ 滑动窗口技术

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Introduction

Reliable Transmission:

• Acknowledgements
  • 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).
• Timeouts
  • 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).

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Stop-and-Wait protocol

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

Four different scenarios(情节) for the stop-and-wait algorithm:

1. The ACK is received before the timer expires;
2. The original frame is lost;
3. The ACK is lost;
4. The timeout fires too soon (or the ACK is delayed).

If the acknowledgment is lost or delayed in arriving:

• The sender times out and retransmits the original frame.
• As a result, duplicate copies of frames will be delivered.
• To solve this:
  • Use 1 bit sequence number (0 or 1)
  • When the sender retransmits frame 0, the receiver can determine that this is a second copy of frame 0 rather than the first copy of frame 1 and therefore can ignore it.

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Sliding Window Protocol

• Sender assigns a sequence number denoted(为……名称) 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
• 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
• Receiver also maintains the following invariant: LAF–LFR ≤ RWS

Cumulative Acknowledgement

• Let SeqNumToAck denote the largest sequence number not yet acknowledged, such that all frames with sequence number less than SeqNumToAck have been received
• The receiver acknowledges the receipt of SeqNumToAck even if high-numbered packets have been received: This acknowledgement is said to be cumulative.

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Issues with Sliding Window Protocol

When timeout occurs, the amount of data in transit decreases, When the packet loss occurs, this scheme is no longer keeping the pipe full.

• Negative Acknowledgement (NAK)
• Additional Acknowledgement
• Selective Acknowledgement

Select the window size:

• SWS: Delay ✘ Bandwidth
• RWS: = SWS

Finite(有限) Sequence Number:

• Frame sequence number is specified in the header field, So has hinite size and It is necessary to wrap around.
• MaxSeqNum: ≥ SWS+1, fault when ack lost.
• MaxSeqNum: To avoid above case, if RWS = SWS, SWS < (MaxSeqNum+1)/2

Examples:

Sliding Window Protocol provides three features:

• Reliable Transmission
• Preserve the order
  • Each frame has a sequence number.
  • Out of order frames will be buffered.
• Flow control
  • Receiver is able to throttle(压制) the sender by setting the value of RWS.
  • Keeps the sender from overrunning the receiver.

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