Hacker News new | past | comments | ask | show | jobs | submit login

That still irks me. The real problem is not tinygram prevention. It's ACK delays, and that stupid fixed timer. They both went into TCP around the same time, but independently. I did tinygram prevention (the Nagle algorithm) and Berkeley did delayed ACKs, both in the early 1980s. The combination of the two is awful. Unfortunately by the time I found about delayed ACKs, I had changed jobs, was out of networking, and doing a product for Autodesk on non-networked PCs.

Delayed ACKs are a win only in certain circumstances - mostly character echo for Telnet. (When Berkeley installed delayed ACKs, they were doing a lot of Telnet from terminal concentrators in student terminal rooms to host VAX machines doing the work. For that particular situation, it made sense.) The delayed ACK timer is scaled to expected human response time. A delayed ACK is a bet that the other end will reply to what you just sent almost immediately. Except for some RPC protocols, this is unlikely. So the ACK delay mechanism loses the bet, over and over, delaying the ACK, waiting for a packet on which the ACK can be piggybacked, not getting it, and then sending the ACK, delayed. There's nothing in TCP to automatically turn this off. However, Linux (and I think Windows) now have a TCP_QUICKACK socket option. Turn that on unless you have a very unusual application.

Turning on TCP_NODELAY has similar effects, but can make throughput worse for small writes. If you write a loop which sends just a few bytes (worst case, one byte) to a socket with "write()", and the Nagle algorithm is disabled with TCP_NODELAY, each write becomes one IP packet. This increases traffic by a factor of 40, with IP and TCP headers for each payload. Tinygram prevention won't let you send a second packet if you have one in flight, unless you have enough data to fill the maximum sized packet. It accumulates bytes for one round trip time, then sends everything in the queue. That's almost always what you want. If you have TCP_NODELAY set, you need to be much more aware of buffering and flushing issues.

None of this matters for bulk one-way transfers, which is most HTTP today. (I've never looked at the impact of this on the SSL handshake, where it might matter.)

Short version: set TCP_QUICKACK. If you find a case where that makes things worse, let me know.

John Nagle

How did you end up switching from networking to animating falling bodies? What are you doing these days?

I wish you hadn't signed your comment, so we could have had the "I am John Nagle" moment when someone inevitably tried to pedantically correct you. :)

"How did you end up switching from networking to animating falling bodies? What are you doing these days?"

Ford Aerospace got out of networking, then computer science, then closed the Palo Alto facility. I was out long before then.

What am I doing now? Robotics, again. Most recent GitHub commit: [1]

[1] https://github.com/John-Nagle/uarm_util

> The combination of the two is awful.

Apparently Greg Minshall proposed tinygram prevention alternations 15 years ago to fix the problematic interaction: https://tools.ietf.org/html/draft-minshall-nagle-01

OSX seems to have implemented this in 2007 and be less/not sensitive to the issue e.g. http://neophob.com/2013/09/rpc-calls-and-mysterious-40ms-del... notes that there was no delay on OSX

> it took around 40ms until my application get’s the data. I tested the application on a regular Linux system (Ubuntu) with the same result, so it’s not a RPi limitation. On my OSX MacBook Air however the RPC call needed only 3ms!

alterations not alternations, of course

One thing that confuses me is -- are ACK delays part of the default TCP implementation on Linux? I originally assumed this was some kind of edge case / unusual behavior.

So it would appear, according to the man pages: http://linux.die.net/man/7/tcp

In quickack mode, acks are sent immediately, rather than delayed if needed in accordance to normal TCP operation.

So "normal TCP operation" is to delay ACKs "if needed". Not sure if "needed" is the right word to use, but whatever.

Looks like RHEL has a system-wide fix: https://access.redhat.com/documentation/en-US/Red_Hat_Enterp...

There is also "ip route change ROUTE quickack 1"

It still seems wasteful to wait an entire round trip before sending, rather than 1/4 round trip or so.

Guidelines | FAQ | Support | API | Security | Lists | Bookmarklet | Legal | Apply to YC | Contact