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Re: on HTTP/QUIC and HTTPBis

From: Adrien de Croy <adrien@qbik.com>
Date: Sun, 12 Mar 2017 21:28:02 +0000
To: "Patrick McManus" <mcmanus@ducksong.com>
Cc: "Mike Bishop" <Michael.Bishop@microsoft.com>, "HTTP working group mailing list" <ietf-http-wg@w3.org>
Message-Id: <em2484435a-146a-42a7-b427-bf699c00f2ae@bodybag>

Hi Patrick

thanks for taking the time to write all that up.

I can see the strong desire to reduce latency / RTTs in setup phase pre 
application data, but it makes me worry about a few things.  SYN flood 
attacks were rendered ineffective by SYN cookies, which meant the server 
didn't need to maintain state about a connection until after the ACK to 
the SYN ACK was received, thereby precluding spoofing source IP.

However if the initial packet has crypto setup as well, the server will 
need to record and retain client state from the initial packet which 
will open it up to a SYN flood style of resource consumption attack?

I also wonder why we need to keep redesigning various mux layers on top 
of IP.  IP is already multiplexing all the TCP connections.  We decided 
for whatever reason that too many TCP connections is bad, and so we went 
to sub-streams (h2) and other mechanisms to provide TCP style behaviour 
without somehow re-introducing the same things that made us decide too 
many TCP connections is bad.  I just wonder whether it won't come to 
pass that we decide too many streams on QUIC is bad, and we need another 
different mux layer.

the obvious question is can't we just fix TCP so that lots of 
connections isn't so bad?

We keep adding more layers and overhead as well, maybe it's time to make 
the internet do bigger packets :)

Adrien


------ Original Message ------
From: "Patrick McManus" <mcmanus@ducksong.com>
To: "Adrien de Croy" <adrien@qbik.com>
Cc: "Mike Bishop" <Michael.Bishop@microsoft.com>; "HTTP working group 
mailing list" <ietf-http-wg@w3.org>
Sent: 11/03/2017 8:38:36 AM
Subject: on HTTP/QUIC and HTTPBis

>
>On Thu, Mar 9, 2017 at 9:59 PM, Adrien de Croy <adrien@qbik.com> wrote:
>>
>>for the benefit of those of us who maybe aren't so familiar with QUIC, 
>>are there any good resources you can refer us to which deal with the 
>>bigger picture such as why even have QUIC?
>>
>
>Thanks Adrien.
>
>[minus quicwg, who doesn't need to hear a summary of their own work :) 
>- and changing the subject line to intentionally break threading]
>
>I'm going to include an opinion here on the value of the quic work from 
>the perspective of the http ecosystem. I think its useful to have 
>something like this that isn't flavored quite as strongly by the Google 
>experience as most of the material out there is. (though we can only 
>thank them for their work and experience - its simply that diversity is 
>valuable).  An important element of the quic-wg charter is that they 
>work closely with httpbis - let's consider this part of that 
>cooperation.
>
>tl;dr; There are motivators both large and small - but for me the big 3 
>are: better perf under high packet loss, lower startup latency, better 
>streaming. This is largely the transport work leftover from http/2 that 
>couldn't be done in the TCP/TLS context this wg chose for h2.
>
>A little bit of history: h2 made two really fundamental design choices. 
>One was to preserve HTTP/1 semantics (i.e. focus on transport issues), 
>and the other was to constrain that work by what could be done in a 
>traditional TLS/TCP context. This latter choice wasn't a forgone 
>conclusion - there was lots of side chatter during the pre-h2 bakeoff 
>phase about whether anyone would propose something more along the lines 
>of sctp/udp/[d]tls (where RTC Datachannels was headed 
>contemporaneously). In the end folks felt that experience and running 
>code were important properties for h2 - building it on tcp/tls lowered 
>the risk of boiling the ocean, improved the latency of getting 
>something deployed, and captured meaningful low hanging fruit. For the 
>issues that it couldn't address there could always be another protocol 
>and we took seriously the need to create an effective means of 
>versioning to facilitate that. Rough consensus for a tcp/tls scope was 
>obvious when all the proposals turned out to be tcp/tls based :)
>
>I think our operational experience has proved that to be a good 
>decision. h2 is a big win for a lot of cases, it has good uptake, its 
>non-standard predecessor has receded, and I don't think we could have 
>done too much better within a TCP/TLS context. Additionally, it can 
>serve as a bootstrap for 'another protocol' and the means for 
>negotiating that via Alt-Svc and ALPN which were both defined by the 
>same effort if not the same rfc. That 'other protocol' turns out to be 
>quic and its a good sign for the vibrancy of http/2 that the quic 
>charter explicitly seeks to map that h2 into its ecosystem. QUIC seems 
>to be taking on exactly the items that were foreseen by the scoping of 
>h2.
>
>The chief performance frustration with h2 happens under higher than 
>normal packet loss. The in-order property of TCP spans different http 
>transactions in h2 - so a single packet loss in one message prevents 
>subsequent independent transactions from being delivered until the loss 
>is repaired (i.e. 2 images in 2 different packets with 1 packet loss 
>can delay them both.l). At the tail of high packet loss this makes h2 
>performance regress from h1. Our community has seen some good data on 
>this recently from Akamai and Fastly. QUIC fixes this - it multiplexes 
>multiple streams onto one connection in a way very familiar to those 
>used by 7540, but gives each stream its own ordering context as if they 
>were independent tcp streams so they can be delivered independently.
>
>Much of the work of the quic/h2 mapping is disentangling the parts of 
>h2 that utilize the in-order property of the connection. .particularly 
>the relationship between control data and streams. hpack obviously has 
>some issues, and the extension-ack as a checkpoint is a problem too. 
>Also lifecycle counting issues (how do you count the number of open 
>streams if you can open them without permission but they can reordered 
>around closes?) etc.. Mike Bishop is doing a lot of the heavy lifting 
>on this (and will present for you all when we meet in Chicago - so come 
>with opinions and send pull requests to the quic wg! or at least read 
>the document :)!) We owe Mike our thanks and support for lots of 
>careful work.
>
>The other major h2 performance issue addressed by QUIC is startup 
>latency. Essentially it gives you common secure 0RTT connections for 
>previously visited hosts. You can just fire off N requests to a known 
>host without blocking on handshaking at the TCP or TLS layer while 
>maintaining a very small amount of closed state for the host (just a 
>little TLS state, no H2 state at all). Congestion control is the only 
>practical limit. Responses come back in 1RTT. This is also resilient to 
>things like NAT rebinding. A world in which you deployed TCP Fast Open 
>with TLS 1.3 Early Data would get you some of the way to the same goal, 
>but you would still be limited to just a single packet in that first 
>round trip and the h2 data would have to share room in that packet with 
>the TCP SYN and the TLS Client Hello - that 1 ethernet sized packet 
>gets crowded real fast.
>
>Speaking of improved latency, quic addresses a common misalignment 
>between typical streamed http data and TLS. QUIC framing is cognizant 
>of UDP boundaries and the crypto operations are done on a per UDP 
>basis. The implications here are important - when doing TLS in a TCP 
>context the atomic crypto operation is done on a TLS record (of 
>relatively arbitrary size - but 4k, 8k, 16k are common). Due to the 
>packet agnostic byte-stream interface of TCP, unless you worked 
>amazingly super duper hard at it these records commonly span packet 
>boundaries - but the delivery time of the application level data (e.g. 
>progressive images) was dependent on the arrival of the last packet in 
>the record. The application would incur ~a round trip of latency 
>rendering partially received data whenever a congestion window limit 
>was hit even without loss or reordering.. in the presence of loss or 
>reordering it can get really bad. Ilya has done some great writing on 
>this topic in the past - but implementations still commonly do it 
>poorly. QUIC automatically optimally aligns the crypto with the UDP 
>boundary (and has some PMTU magic that will generally ensure UDP is not 
>fragmented).
>
>So those are the big 3: more stable under high packet loss, lower 
>startup latency, better streaming. They're really big :) But there's 
>more -
>
>Blending the application and the transport has some advantages - I 
>think the chief one here is that the application is going to have a 
>much better idea of how to manage its buffers as it will have closer 
>access to ack feedback loops. We've seen this several times in H2 
>implementations where code doesn't know how to gauge a BDP worth of 
>data and ends up creating huge amounts of buffering which impact its 
>own ability to multiplex, cancel, and honor priority (which are 
>afterall the major features of h2). The result can be horrible 
>responsiveness. We will have a presenter in Chicago talk about some 
>related academic measurements around this topic as part of our effort 
>to share feedback on the working group's past outputs. This is a subtle 
>thing as I think you can do approximately-the-right-thing in both h2 
>and quic, but the quic architecture should make it easier to get 
>quality implementations.
>
>I also expect that a lot of the weird corner cases that result in data 
>loss in h1/h2 around TCP RST will be eliminated. (RSTs are often 
>routinely generated by application errors and sometimes result in 
>silent kernel discard of data that has already been received and acked 
>but not yet delivered to the application - this combined with large TLS 
>records can sometimes result in the loss of a lot of data. Its a pretty 
>weak part of TCP.)
>
>Further, TCP Tuning for HTTP (1 or 2) is operationally complicated. One 
>of the complications is the administrative separation between transport 
>and application. How do you set IW? Are you doing TLP? RACK? Is Fast 
>Open an option for you? QUIC, because it can be bundled with an 
>application in userspace, provides a deployment option for getting best 
>of breed transport algorithms deployed to hosts that may be running 
>legacy TCP stacks. You can expect a more consistent "best practices" 
>experience across platforms and a pivot for iterating on new designs 
>and ideas for evolution. Speaking of evolution, quic is almost entirely 
>encrypted and basically fully authenticated. This intentionally exposes 
>very few bits to the network with the intention of preventing 
>ossification around patterns introduced in v1 the way things. 
>Ossification has been a huge problem for TCP and http and we'd like to 
>do better.
>
>I'll wrap it up by noting that blending transport and crypto has some 
>advantages beyond packet alignment too - perhaps my favorite is that it 
>seems likely quic will be resistant to RST injections and the 
>modification of basic congestion control by things like peps. (you may 
>or may not consider than an advantage - I'm aware.) Though it will be 
>vulnerable to downgrade attacks to TCP in the same way h2 is vulnerable 
>to downgrade attacks to h1 :(
>
>I think the question for our working group is how we want to engage 
>with all this change - as part of that you can expect regular updates 
>about the quic/http mapping to be part of our process for a while and 
>Mike is doing a great job of making clear choices like how quic 
>interacts with the legacy registries more visible in this audience. I'd 
>like to make it possible for those with only an interest in the HTTP 
>portions of this (to whatever extent that is possible) to be able to 
>share their experience without joining the full QUIC-WG firehose - let 
>Mark and I know if you have any thoughts on how to further facilitate 
>that beyond what we're doing.
>
>>So far our experience with QUIC (as a proxy) has been to simply block 
>>it, since it provides a bypass mechanism for proxy control.
>>
>
>Obviously this hasn't been a proxy centric review, but there is no 
>fundamental reason a client can't speak QUIC to a proxy in the same way 
>some of them support h2 to the proxy now. These are not changes in 
>semantics. I would hope to see it for any client with forward proxy 
>support but it might not be the first thing rolled out.
>
>-Patrick
>
Received on Sunday, 12 March 2017 21:28:41 UTC

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