Comment by dpc_pw | Hacker News Reader

Comment by dpc_pw | Hacker News Reader

I did not do any real tests and I might be completely wrong etc. but it seems to me that http2 is going to perform poorly over wireless links like 3g.

With http1 one had N tcp connections, and with the way tcp slowly increases the bandwidth used, and rapidly decreases it when packet is lost, even if any packet were dropped (which will happen quite a lot on 3g) other tcp streams were not delayed, or blocked, and can even utilize the leftover bandwidth, yielded by the stream that lost the packet.

With http2 however there's one tcp connection, so dropped packets will cause under-utilization of the bandwidth. On top of that dropped packets will cause all frames after them, to be delayed in kernel receiving buffer until the dropped packet is retransmitted, while in http1 case they would be available at the app level right away.

HTTP2 being implemented on top of TCP always seemed like a weird choice. It should have been UDP, IMO. That's why network accelerators like PacketZoom make so much sense. Note: I work in PacketZoom, I did not do any in-depth research on HTTP2, and this is my opinion, not necessarily of the company.

This is a real worry, but as with all things the actual behaviour of H2 on lossy networks is more complex than that.

TCP's congestion control algorithms don't work that well when you have many TCP streams competing for the same bandwidth. This is because while packet loss is a property of the link, not an individual TCP stream, each packet loss event necessarily only affects one TCP stream. This means the others don't get the true feedback about the lossiness of the connection. This behaviour can lead to a situation where all of your TCP streams try to over-ramp.

A single stream generally behaves better on such a link: it's getting a much more complete picture of the world.

However, your HOL blocking concern is real. This is why QUIC is being worked on. In QUIC, each HTTP request/response is streamed independently over UDP, which gets the behaviour you're talking about here, while also maintaining an overall view of packet loss for rate limiting purposes.

Agreed about possibility of over-ramp on good links. But on wireless packets drops can be quite frequent and unrelated to reaching bandwidth limit.

> can lead to a situation where all of your TCP streams try to over-ramp. ... A single stream generally behaves better on such a link: it's getting a much more complete picture of the world

Multiple HTTP connections work better for exactly this reason, because they are 'stealing' bandwidth from streaming video by 'not playing fair'. For example, 6 connections ramping back up bandwidth at 6x the rate of a single connection or sometimes only scaling back on 1/6th of the streams at once.

...which is fine because multiple parallel HTTP connections is usually a browser doing so for short-lived data transfers for active users and it is not the bulk of the data on the network.

I worry about TCP window scaling (and full TCP windows) when only using one TCP connection. There is a good reason download managers use multiple connections to download one file, because depending on the latency the maximum transfer rate is capped because only so many TCP packets can be in flight simultaneously. I wonder if nobody ever thought about that... HTTP1/x solved that (more by chance) with multiple connections...

Has anybody ever used this for denial of service? Make a large request so the TCP window scales up then just stop sending ACKs. All that data has to remain in the server's memory for retransmit. Even with a really short timeout before connections are dropped you could probably tie up a lot of server memory.

Have you looked at QUIC? It seems like it addresses the problems with HTTP2 over TCP as well as providing some additional benefits like like speeding up the secure connection establishment.

Yes, I'm aware of it, though not as in-depth as I'd like to. AFAIK, it was a bit unfinished yet eg. not having any congestion control story, which is very important. But yeah... it's what what HTTP2 probably should have been in the first place.

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