Is the network graph applied to the localhost loopback connections?

[haxxpop]

I think the loopback connections should have no limit on bandwidth and nearly zero latency. Are the bandwidth limit, latency and the packet loss rate specified in the network graph applied to the loopback connection?

[stevenengler]

No they should not be applied to the loopback connection. There is a 1 ns latency for localhost packets, but hopefully we can remove that in the future.

See the is_local check and the surrounding code:

shadow/src/main/host/network_interface.c

Lines 484 to 487 in 89442dc

	// Local packets arrive on our own interface, they do not go through the
	// upstream router and do not consume bandwidth.
	bool is_local =
	address_toNetworkIP(interface->address) == packet_getDestinationIP(peeked_packet);

The packet loss occurs in worker_sendPacket, but this function is not called for localhost packets.

(We might be changing this code soon in #2632.)

[stevenengler]

It looks like there might be a bug in a specific case (a host sends packets to itself using its public IP rather than 127.0.0.1) that can cause the rate limit to be applied to localhost packets if the host is also sending a lot of non-localhost packets. #2677

[stevenengler]

Also, Shadow currently uses delayed ACKs on localhost, which can occasionally make localhost packets have a larger-than-expected 1 ms delay. #2683