Reliable_data_transfer

RDT

bit-errors, lossy, duplicate delivery , out-of-order delivery

checksums: bit error
receiver feedback: bit error, ACK/NACK， Cumulative acknowledgments(I have received all packets with sequence numbers up to but not including sn.) allow acknowledgment of numerous packets at a time. They can be useful in pipelined protocols. 可能lost
retransmission : Sender sends another copy of segment, detect loss and allows for duplicate seg
sequence numbers : Distinguish between old and new, Gaps let receiver detect lost segment, find duplicate packets，restoring the transmitted order. have to be a bounded # bits
timer expiration: Segment or receiver feedback is lost (sender: Resends a packet after a timer fires. Sends a new packet after an acknowledgment (positive) arrives.)
window:Control sending of multiple segments, allow for reuse of sequence numbers, also allow for pipeling segments

Simplest Protocol that will handle bit errors and segment loss
- use: checksum, ack, sn, timers
- 1 bit for sequence number
- 为了解决checksum返回的时候有问题。可能ack会lost。带来的问题就是可能重复发几次。所以需要seq num
- 计时发送，看指定时间能否达到response, 没seq num. 接收者不能知道是否是重发。LOST ACK和lost segment对于发送方式一样的

sliding window: A mechanism to control multiple, in-flight segments without overwhelming receiver, Sender is allowed to transmit N segments without waiting for an ACK
“window” of up to N, consecutive unACKed segments allowed
Sets a timer for each in-flight segment
timeout(n): retransmit segment n and all higher seq# segments in window
sender:
- Sender places a k-bit seq# in header
- “window” of up to N, consecutive unACKed segments allowed
- Sets a timer for each in-flight segment
- timeout(n): retransmit segment n and all higher seq# segments in window
- ACK(n): ACKs all segments up to, including seq# n (Cumulative ACK)
receiver: ACK-only: always send ACK for correctly-received segment with highest in-order seq, Receipt of out-of-order segment just discard and send with highest in-order seq
- May generate duplicate ACKs
- Why discard segs received out-of-order： Don’t want to buffer them, going to be re-sent anyway
A single segment error can cause many segments to be retransmitted

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