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This is a guest post by Donatas Abraitis, System Engineer at Vinted, with an unusual approach for saving a little bandwidth.
Looking at https://tools.ietf.org/html/rfc1323 there is a nice title: 'TCP Extensions for High Performance'. It's worth to take a look at date May 1992. Timestamps option may appear in any data or ACK segment, adding 12 bytes to the 20-byte TCP header.
Using TCP options, the sender places a timestamp in each data segment, and the receiver reflects these timestamps back in ACK segments. Then a single subtract gives the sender an accurate RTT measurement for every ACK segment.
To prove this let's dig into kernel source:
./include/net/tcp.h:#define TCPOLEN_TSTAMP_ALIGNED 12
./net/ipv4/tcp_output.c:static void tcp_connect_init(struct sock *sk)
...
tp->tcp_header_len = sizeof(struct tcphdr) +
(sysctl_tcp_timestamps ? TCPOLEN_TSTAMP_ALIGNED : 0);
Some visualizations:
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