Editing Transmission Control Protocol (section)

===TCP timestamps===
TCP timestamps, defined in {{harvtxt|RFC 1323}} in 1992, can help TCP determine in which order packets were sent. TCP timestamps are not normally aligned to the system clock and start at some random value. Many operating systems will increment the timestamp for every elapsed millisecond; however, the RFC only states that the ticks should be proportional.

There are two timestamp fields:
* a 4-byte sender timestamp value (my timestamp)
* a 4-byte echo reply timestamp value (the most recent timestamp received from you).

TCP timestamps are used in an algorithm known as ''Protection Against Wrapped Sequence'' numbers, or ''PAWS''. PAWS is used when the receive window crosses the sequence number wraparound boundary. In the case where a packet was potentially retransmitted, it answers the question: "Is this sequence number in the first 4&nbsp;GB or the second?" And the timestamp is used to break the tie.

Also, the Eifel detection algorithm uses TCP timestamps to determine if retransmissions are occurring because packets are lost or simply out of order.{{sfn|RFC 3522}}

TCP timestamps are enabled by default in Linux,<ref>{{cite web |title=IP sysctl |url=https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt |website=Linux Kernel Documentation |access-date=15 December 2018 |archive-date=5 March 2016 |archive-url=https://web.archive.org/web/20160305080444/https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt |url-status=live }}</ref> and disabled by default in Windows Server 2008, 2012 and 2016.<ref>{{cite web |last1=Wang |first1=Eve |title=TCP timestamp is disabled |url=https://social.technet.microsoft.com/Forums/office/en-US/6b1e4653-320f-4dbf-8b1a-64d27d8464fc/tcp-timestamp-is-disabled |website=Technet – Windows Server 2012 Essentials |publisher=Microsoft |access-date=2018-12-15 |archive-url=https://web.archive.org/web/20181215225201/https://social.technet.microsoft.com/Forums/office/en-US/6b1e4653-320f-4dbf-8b1a-64d27d8464fc/tcp-timestamp-is-disabled |archive-date=2018-12-15 |url-status=dead }}</ref>

Recent Statistics show that the level of TCP timestamp adoption has stagnated, at ~40%, owing to Windows Server dropping support since Windows Server 2008.<ref name="2017stats">{{cite web |url=http://profiles.murdoch.edu.au/myprofile/david-murray/files/2012/06/An_Analysis_of_Changing_Enterprise_Network_Traffic_Characteristics-22.pdf |title=An Analysis of Changing Enterprise Network Traffic Characteristics |author1=David Murray |author2=Terry Koziniec |author3=Sebastian Zander |author4=Michael Dixon |author5=Polychronis Koutsakis |publisher=The 23rd Asia-Pacific Conference on Communications (APCC 2017) |date=2017 |access-date=3 October 2017 |archive-date=3 October 2017 |archive-url=https://web.archive.org/web/20171003124654/http://profiles.murdoch.edu.au/myprofile/david-murray/files/2012/06/An_Analysis_of_Changing_Enterprise_Network_Traffic_Characteristics-22.pdf |url-status=live }}</ref>