我如何在centOS6上获取`tcp option timestamp`

Question

on centOS7,I can use getsockopt(x,IPPROTO_TCP, TCP_TIMESTAMP, x, x) to get tcp timestamp, and TCP_TIMESTAMP was defined in netinet/tcp.h, but I can't find it when I use centOS6.

So did anyone do it successfully on centOS6? Thanks a lot!

Answer 1

rfc-1323 , which is where tcp timestamp is introduced, in page 4, states:

(3) Round-Trip Measurement

       TCP implements reliable data delivery by retransmitting
       segments that are not acknowledged within some retransmission
       timeout (RTO) interval.  Accurate dynamic determination of an
       appropriate RTO is essential to TCP performance.  RTO is
       determined by estimating the mean and variance of the
       measured round-trip time (RTT), i.e., the time interval
       between sending a segment and receiving an acknowledgment for
       it [Jacobson88a].

       Section 4 introduces a new TCP option, "Timestamps", and then
       defines a mechanism using this option that allows nearly
       every segment, including retransmissions, to be timed at
       negligible computational cost.  We use the mnemonic RTTM
       (Round Trip Time Measurement) for this mechanism, to
       distinguish it from other uses of the Timestamps option.

So, the timestamp introduced there is for TCP's at both sides of a connection could precisely determine the Retransmission TimeOut interval, by attaching precise timestamps to the segments and determine the round trip time from one side to the other.

As such, it is normal that you see no socket api access to get these timestamps (or to insert them), as they have nothing to do with user space, you cannot determine when to put them and when they have been attached to a segment. These are normally seen in the SYN init segment, to determine the initial round trip time, and then periodically, as the connection goes, to determine variations or network congestion.

Also, as an option introduced after the TCP protocol definition, it is considered optional, and you can encounter implementations that don't support it.