TODAY ON TCP MYTHBUSTERS

HTTP and TLS run over TCP. Every time a new TCP connection is created:

1. TCP protocol handshakes
2. TLS subsequently handshakes, if applicable
3. TCP slowstart

This can be expensive, especially when TLS is involved (which should be all servers, in a perfect world), so care should be taken to re-use existing TCP connections whenever possible. Unfortunately, Ruby doesn't make this as easy as it should be.

I'm doing all of my tests on Ruby 2.3.0

HOW TO DETERMINE WHEN TCP CONNECTIONS ARE MADE

We can tell when TCP connections are made by running the following command:

sudo tcpdump -i <interface_goes_here> -A -s 0 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and dst port <port_goes_here>'

The interface can be determined by running tcpdump -D; on my machine, my loopback interface is lo0.

This command will print out each TCP packet that's either a SYN (connection request) or a FIN (connection termination request).

SHIT, THAT IS COMPLICATED

Yeah, I'm welcome to simpler suggestions. You can see a simpler example by running

curl -v localhost:3005 localhost:3005

cURL will tell you when it's re-using a connection or opening a new one.

BUSTING MYTHS

Run the tcpdump monitoring, the mock webserver, and then run tests individually. Ex:

1. sudo tcpdump -i lo0 -A -s 0 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and dst port 3005'
2. in a separate window, rackup base_server.ru -p 3005
3. in a separate window, ruby <test_file

INTERPRETING RESULTS

By looking at the output of the tcpdump command above, we can determine if the connection is being re-used or not. In situations where it is, we see only a single SYN/FIN pair:

13:56:14.858605 IP localhost.59101 > localhost.geniuslm: Flags [S], seq
1240341411,
....
13:56:14.863307 IP localhost.59101 > localhost.geniuslm: Flags [F.],
....

But, if a new connection is being made per-request, you'll see multiple SYN/FIN pairs:

13:54:11.233058 IP localhost.59095 > localhost.geniuslm: Flags [S], seq
907144824,
...
13:54:11.234127 IP localhost.59095 > localhost.geniuslm: Flags [F.], seq
907144954,
...
13:54:11.234728 IP localhost.59096 > localhost.geniuslm: Flags [S], seq
3385473565,
...
13:54:11.235496 IP localhost.59096 > localhost.geniuslm: Flags [F.],
...

RECOMMENDATIONS

1. Servers should always include a Content-Length header, to ensure clients are able to take advantage of persistent connections.
2. Ruby clients concerned about performance should use net-http-persistent gem instead of Net::HTTP, unless the entire transmission can be completed within a single Net::HTTP.start block. Net::Http.new does not perform as-described.

MORE READING

• TCP