What are the retransmission rules for TCP

Question

The Wikipedia article on TCP indicates that the IP packets transporting TCP segments can sometimes go lost  and that TCP  requests retransmission of lost data    What exactly are the rules for requesting retransmission of lost data  At what time frequency are the retransmission requests performed  Is there an upper bound on the number  Is there functionality for the client to indicate to the server to forget about the whole TCP segment for which part went missing when the IP packet went missing

User · Answer

There s no fixed time for retransmission   Simple implementations estimate the RTT  round-trip-time  and if no ACK to send data has been received in 2x that time then they re-send     They then double the wait-time and re-send once more if again there is no reply   Rinse   Repeat   More sophisticated systems make better estimates of how long it should take for the ACK as well as guesses about exactly which data has been lost   The bottom-line is that there is no hard-and-fast rule about exactly when to retransmit   It s up to the implementation   All retransmissions are triggered solely by the sender based on lack of response from the receiver   TCP never drops data so no  there is no way to indicate a server should forget about some segment

User · Answer

What exactly are the rules for requesting retransmission of lost data   The receiver does not request the retransmission  The sender waits for an ACK for the byte-range sent to the client and when not received  resends the packets  after a particular interval  This is ARQ  Automatic Repeat reQuest   There are several ways in which this is implemented    Stop-and-wait ARQ Go-Back-N ARQ Selective Repeat ARQ   are detailed in the RFC 3366   At what time frequency are the retransmission requests performed   The retransmissions-times and the number of attempts isn t enforced by the standard  It is implemented differently by different operating systems  but the methodology is fixed   One of the ways to fingerprint OSs perhaps    The timeouts are measured in terms of the RTT  Round Trip Time  times  But this isn t needed very often due to Fast-retransmit which kicks in when 3 Duplicate ACKs are received   Is there an upper bound on the number   Yes there is  After a certain number of retries  the host is considered to be  down  and the sender gives up and tears down the TCP connection   Is there functionality for the client to indicate to the server to forget about the whole TCP segment for which part went missing when the IP packet went missing   The whole point is reliable communication  If you wanted the client to forget about some part  you wouldn t be using TCP in the first place   UDP perhaps

[networking] What are the retransmission rules for TCP?

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