Client TCP segments arrive in batches

tonso

11/10/23, 6:29 PM

The scenario is the following: a WebSocket server and clients exchange messages at some pace (like 40-50 times in a second). However, for one client, once in a few minutes I'm observing that there is a 5-second (always the same span) period, when TCP segments (containing client's WebSocket messages, as well as ACKs to server messages) from the client arrive in batches (one batch is accumulated for a few hundred ms, so there are kind of successive bursts of client messages), so the tcpdump output schematically looks like this:

...
t0      S > C <server-message-1>
t0 + 20 S > C <server-message-2>
...
t0 + 100 S > C <server-message-n>
t0 + 105 C > S <client-message-1>
t0 + 105 C > S <client-message-2>
t0 + 105 C > S <server-message-1-ack>
...
t0 + 105 C > S <client-message-m>
t0 + 105 C > S <server-message-n-ack>
...

Among other things, the Nagle's algorithm is disabled for the sockets, as well as net.ipv4.tcp_autocorking = 0. Tried various TCP congestion algorithms, doesn't seem to change the situation. How can I detect if this is problem on the server, or something is wrong on the client's side?

Any suggestions why this may happen and how to resolve or at least pinpoint the issue?

1 + 0

networking

linux

tcp

tcpdump

sysctl

Score:1

Server

Brennen Smith

11/11/23, 12:21 AM

Take a look at RSO and GSO as that will combine segments into larger frames that are then delivered to the kernel. That shouldn't have a 5 second delay, but that may be mis-configured.

+ 2

tonso

11/11/23, 10:47 AM

Thanks, will try. It's not actually a solid 5-second interval, but a series of bursts, each containing client segments, generated for up to a few hundred ms.

Brennen Smith

11/13/23, 8:54 PM

^ that definitely sounds like GSO/RSO, try disabling it with `ethtool` and see if it improves the situation. It could either be on the receiver, or on the sender side. If you can access the network devices inline, you could take a `pcap` of the segments and see if the segments are combined in a single ethernet frame at that point.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Client TCP segments arrive in batches

Post an answer

Most people don’t grasp that asking a lot of questions unlocks learning and improves interpersonal bonding. In Alison’s studies, for example, though people could accurately recall how many questions had been asked in their conversations, they didn’t intuit the link between questions and liking. Across four studies, in which participants were engaged in conversations themselves or read transcripts of others’ conversations, people tended not to realize that question asking would influence—or had influenced—the level of amity between the conversationalists.