Tested in chrome # 56.0.2924.87 and canary #58.0.3012.0 on Ubuntu 14.04 & 16.10 and getting the IP address of TCP-FastOpen-enabled servers.Due to security reasons we are unable to share the IP address screen shot/cast.

Steps Followed:
---------------
1.Enabled the flag.
2.Run the provided command in terminal.

@zzq1015:Could you please let me know if i have missed anything and if possible, please create a new profile without extensions & apps.Re-check once and let us know the observation of the issue which would help us to triage the issue further.

Thanks in Advance.

The complete steps are below:
1. Open Chrome
2. Enable TCP Fast Open in chrome://flags
3. Close Chrome
4. execute in the terminal
---START HERE---
sudo sysctl -w net.ipv4.tcp_fastopen=3
sudo ip tcp_metrics flush
---END---
5. Re-open Chrome
6. Go to a TCP-Fastopen-enabled website such as Google or Facebook
7. execute 
ip tcp_metrics show | grep "fo_cookie"
and nothing is produced

Jana can you take a look?

It's hard to tell what may be going on -- can you send a packet trace using tcpdump? Specifically, start the trace after step 5 and stop after 7.

+ycheng, since this seems like a Linux issue and not a Chrome one (as far as I can tell.)

When net.ipv4.tcp_fastopen is set to 3, 
curl --tcp-fastopen www.facebook.com
(execute this multiple times)
uses TCP Fast Open properly
Execute
ip tcp_metrics show | grep "fo_cookie"
and got the Facebook IP address displayed

Thanks for clarifying. A packet capture will be really helpful if you are able to provide it.

TCP Fast Open is activated as long as the LSB of net.ipv4.tcp_fastopen is 1. See
http://lxr.free-electrons.com/source/net/ipv4/tcp.c?v=4.9#L1077  

so Chrome browser either didn't parse the setting correctly or has other bugs.

btw it was nice to learn facebook has enabled Fast Open!

I don't have tcpdump but I installed wireshark on Ubuntu.
tcp_fastopen_1.pcapng is for net.ipv4.tcp_fastopen=1, TCP Fast Open is enabled as normal.
tcp_fastopen_3.pcapng is for net.ipv4.tcp_fastopen=3, TCP Fast Open is disabled even if I haven't touched any chrome://flags setting.
Also, I tried the two kernel settings for curl, and both of them works correctly (TCP Fast Open is enabled for both settings).

Deleted: tcp_fastopen_1.pcapng 1.0 KB

tcp_fastopen_1.pcapng 1.0 KB Download

Deleted: tcp_fastopen_3.pcapng 1.1 KB

tcp_fastopen_3.pcapng 1.1 KB Download

apparently Chrome parses the flag incompletely. 

https://cs.chromium.org/chromium/src/net/socket/tcp_socket_posix.cc?type=cs&l=97

CL in progress: https://codereview.chromium.org/2705323002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6d9ba1044c884aca683404f05509f18d4b63c818

commit 6d9ba1044c884aca683404f05509f18d4b63c818
Author: jri <jri@chromium.org>
Date: Fri Feb 24 21:25:36 2017

Fix incorrect behavior in checking for TCP FastOpen platform support.

BUG= 691881 

Review-Url: https://codereview.chromium.org/2705323002
Cr-Commit-Position: refs/heads/master@{#452926}

[modify] https://crrev.com/6d9ba1044c884aca683404f05509f18d4b63c818/net/socket/tcp_socket_posix.cc

We're removing FastOpen.