In HTTP/2, What's the relationship between req/resp, frame and TCP packet?

Question

Image is from https://hpbn.co/http2/#streams-messages-and-frames.

I want to make sure I really understand what's going on, so here's my understanding:

a TCP packet may contain several frames, and they can belong to different streams.
a request or response is composed of one or more frames that belong to the same stream.
Physically there's no "stream", it's just a logical concept(Yeah I know each frame contains a stream ID).

Am I correct?

HTTP/2 is an application layer protocol while TCP is a transport layer streaming protocol. There is no specific relation between a TCP packet and specific parts of the application layer, i.e. there might be multiple application layer entities inside a single TCP packet, partial entities etc. — Steffen Ullrich
– Steffen Ullrich, Commented Sep 28, 2016 at 6:19
Frames and streams are abstractions built on TCP. Consider that a single browser request is built on 7 layers of abstraction: en.wikipedia.org/wiki/OSI_model#Description_of_OSI_layers — B Seven
– B Seven, Commented Oct 26, 2016 at 18:31

e.dan · Accepted Answer · 2016-09-28 06:26:43Z

5

I believe everything you said is correct, but I'd clarify:

The main point is that a single TCP connection may contain frames from many different HTTP/2 streams, interleaved. The relationship to TCP packets is not important here - TCP packets are reassembled into TCP streams by your TCP stack, and should have no bearing on your understanding of HTTP/2.
The reason why that first point is important is that it is a huge step forward from HTTP/1, in which the TCP stream is "blocked" by any given request/response pair, since the response to the current request must be sent before any other ones. This is the multiplexing feature that unblocks a huge bottleneck of HTTP/1.
A request or response is called a message and yes, it's composed of one or more frames.
There is no physical HTTP/2 stream just like there is no physical TCP stream - it's a higher level concept/abstraction that is handled by the layer in question, which reassembles individual packets or frames into a stream, which makes it infinitely easier to deal with.

Hope that helps.

answered Sep 28, 2016 at 6:26

e.dan

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2 Comments

laike9m Over a year ago

The reason I'm asking about TCP packet is that I'm using WireShark to debug http2, and each "line" in Wireshark is a TCP packet (I guess?). I see each line contain multiple frames, that's why I'm asking. BTW I don't really understand what you mean by "TCP packets are reassembled into TCP streams by your TCP stack, and should have no bearing".

Steffen Ullrich Over a year ago

@laike9m: Each TCP "packet" contain just some part of the TCP stream. These packets have no relation to any kind of message - because TCP is a single stream and not a sequence of messages. Thus an application layer frame may be packet in multiple TCP packets, a single packet may contain multiple application layer frames etc. There simply is no defined mapping from application layer frames to TCP packets.

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