TCP

Work in Progress

Summary

Protocol	Timeout	Retransmit	Handle out-of-order
Go-Back-N	oldest unACKed packet	all unACKed, `>=n`	discard
Selective repeat	individual unACKed packets	one, `n`	buffer, discard only if too far
TCP	oldest unACKed packet	one, oldest unACKed packet	unspecified

Timeout

ideally, timeout > RTT

the exponential weighted moving average(EWMA) encodes all the sampled RTTs into a single value

Concept

Transmission Control Protocol(TCP)

connection-oriented, have to use source information to determine which socket to direct to
reliable
byte-stream, TCP divides the data into segments
flow control
congestion control
full duplex - bi-directional data flow

0                   1                   2                   3   
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data |           |U|A|P|R|S|F|                               |
| Offset| Reserved  |R|C|S|S|Y|I|            Window             |
|       |           |G|K|H|T|N|N|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Checksum            |         Urgent Pointer        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Options (if offset > 5)                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             data                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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Sequence and acknowledgement numbers

sequence: first byte in this segment’s data
acknowledgement: next byte of data expected, receiver sends this as the cumulative ACK

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fast retransmission - if 3 duplicates ACKs are received, resend that segment immediately, without waiting for timeout

3-way handshake

establishing a connection

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SYN flooding - send SYN, but not ACK
SYN/ACK - send SYN-ACK packets that don’t belong to any session, server has to lookup everytime

Closing connection

send signal to close
wait for leftover data

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