Re: Asymmetric CIDs

Eric Rescorla <ekr@rtfm.com> Fri, 16 February 2018 18:39 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 16 Feb 2018 10:38:55 -0800
Message-ID: <CABcZeBOyiOwyd=UFHfAxzXx=Gv_5MdZwvGkk5vHHM7m3BGCqxA@mail.gmail.com>
Subject: Re: Asymmetric CIDs
To: Martin Duke <martin.h.duke@gmail.com>
Cc: Ian Swett <ianswett@google.com>, IETF QUIC WG <quic@ietf.org>
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On Fri, Feb 16, 2018 at 9:52 AM, Martin Duke <martin.h.duke@gmail.com>
wrote:

> Thanks for writing this up, ekr. This incorporates many of the suggestions
> I made in objection to this project (perhaps coincidentally) and I like it
> a lot.
>
> Advantages of this:
> - In the common case where most of the data is server->client, clients can
> get away with shorter Conn IDs to reduce overhead.
> - Omit-conn-id just becomes a case where length = 0. Does this sidestep
> some of Google's transition issues?
> - If a client sends NEW_CONNECTION_ID, that indicates an intention to
> migrate, and is a good cue for the server to send the same. We should add a
> SHOULD to specify this behavior.
>
> I must be missing something, however, regarding implicit CIDs.
>
> If there's a NAT rebinding, how is the server supposed to extract the CID?
>

The assumption is that the server either:

(a) always uses the same CID length
(b) has a structured CID which starts with the length.

But that we don't need to mandate which one in the protoocl


> Furthermore, this obviates the entire concept of using Connection ID for
> routing; it's not obviously a savings to store CID length in a table vs.
> just storing the destination server.
>

I don't think you hae to do this, but maybe I am confused. See above and
tell me if you think I'm wrong

-Ekr


> Lastly, if we encode the length somewhere that seems to solve the
> Stateless Reset issue.
>
>
> On Fri, Feb 16, 2018 at 9:25 AM, Ian Swett <ianswett@google.com> wrote:
>
>> Thanks for the excellent summary EKR.  I like this design and think the
>> breakage of stateless reset in certain cases is acceptable, since it only
>> applies if both sides must have their preferred connection ID present in
>> order to route correctly, which is a use case that's impossible in the
>> status quo.  I have not come up with any other downsides.
>>
>> On Fri, Feb 16, 2018 at 12:01 PM, Eric Rescorla <ekr@rtfm.com> wrote:
>>
>>> Hi folks,
>>>
>>> After a bunch of discussion, the CID task force came down to rough
>>> consensus that asymmetric conn IDs were probably the right
>>> direction (CID task force members, please feel free to voice dissent
>>> here). Here's a complete writeup of what I think would be needed
>>> for asymmetric connection IDs. It's not a PR, because I think
>>> something self-contained is cleaner.
>>>
>>> Note that if we adopt this direction, we would be sacrificing
>>> public reset under some conditions (see previous emails to the
>>> list) and we would need to decide if it was worth keeping at all.
>>>
>>>
>>> OVERVIEW
>>> The basic idea is that each side gets to dictate the connection IDs
>>> that are used to send to it. During the handshake, you establish those
>>> CIDs and then each side can issue new CIDs during the connection.  The
>>> main advantage of this is that it allows for symmetric topologies in
>>> which
>>> the client is also behind some kind of stateless LB/router rather than
>>> just the server. See Issue #1091 for more info on this.
>>>
>>>
>>> The overall handshake looks something like this:
>>>
>>> Client                                      Server
>>>
>>> Initial [CID=XXX] {recv-CID=YYY} ---------------->
>>> <-------------- Cleartext [CID=YYY] {recv-CID=ZZZ}
>>> Cleartext [CID=ZZZ], {recv-CID=YYY} ------------->
>>> <-------------------------- Short header [CID=YYY]
>>> Short header [CID=ZZZ] -------------------------->
>>>
>>>
>>> The client's initial CID (XXX) is special, and either consists of
>>>
>>>     (a) a randomly chosen dummy CID. Proposal: require this to be
>>>         8 bytes or at least a minimum. This should be the same
>>>         for all Initial packets in a connection (unless a stateless
>>>         reject is received, as below).
>>>     (b) a CID which it received from the server in a stateless reject
>>>
>>> All the server's packets are sent with the client's receive CID (YYY)
>>> and all subsequent client packets are sent with the server's receive
>>> CID (ZZZ). The general rule is that you should send with the
>>> connection ID that you most recently received (where recently
>>> is defined as highest PN).
>>>
>>> Note: I believe it's safe to just use the sending CID as the mixin
>>> for the KDF, but I haven't thought this entirely through yet.
>>>
>>> Finally, you can send NEW_CONNECTION_ID in either direction to provide
>>> a new connection ID for the other side to use. The general assumption
>>> is that you can do this at any time, just as with current QUIC, and
>>> that any time you send to a new remote 3-tuple you should change CIDs
>>> if you can. Note that this means that endpoints should try to make
>>> sure that the other side has spare CIDs in case they need to migrate.
>>>
>>>
>>> WIRE ENCODING
>>> As we discussed in the meeting the short header should just have
>>> an implicit length CID. This gives us the following short header:
>>>
>>>    +-+-+-+-+-+-+-+-+
>>>    |0|C|K| Type (5)|
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |                                                               |
>>>    +                     [Connection ID (*)]                       +  <-
>>> change from 64
>>>    |                                                               |
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |                      Packet Number (8/16/32)                ...
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |                     Protected Payload (*)                   ...
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>
>>> Note that we may also be able to dispense with the C bit, if each
>>> side just gets to say "send me this CID exactly", why do we want
>>> to say "here is my CID but you can omit it".
>>>
>>>
>>> We have several options about the long header. The first question
>>> is where recv-CIDs go. In previous versions I suggested putting
>>> them in transport parameters, or elsewhere in the TLS handshake,
>>> and that might still be viable, though it has some drawbacks [0],
>>> so the other alternative is to put both CIDs in in the long header.
>>> This would look something like:
>>>
>>>    +-+-+-+-+-+-+-+-+
>>>    |1|   Type (7)  |
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |  DCID-Length  |                                               |
>>>    +-+-+-+-+-+-+-+-+   Dst Connection ID (*)                       +
>>>    |                                                               |
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |  SCID-Length  |                                               |
>>>    +-+-+-+-+-+-+-+-+   Src Connection ID (*)                       +
>>>    |                                                               |
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |                         Version (32)                          |
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |                       Packet Number (32)                      |
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |                          Payload (*)                        ...
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>
>>> The semantics here are that the first value is the CID you want to
>>> send to and the second one is the value you want used to send to you
>>> (I've inverted these to keep the order the same as short header).
>>>
>>> Two notes about this encoding:
>>>
>>> 1. I think we agreed that we didn't want arbitrary length CIDs up to
>>> 255 bytes, and yet we have room in this length byte. I propose we
>>> limit it to 31 bytes and then grease the remaining 3 bits [1].
>>>
>>> 2. Because the client sends its CID first, there's no way to get the
>>> current QUIC semantics of the server just dictates the CID.  I propose
>>> we fix that by defining a special sentinel CID (all 1s, all 0s,
>>> whatever) of whatever our maximum length is that means "just use your
>>> own CID".
>>>
>>> We can endlessly bikeshed on this structure.
>>>
>>>
>>> Finally, we will need to update NEW_CONNECTION_ID to allow a variable
>>> length CID. This would look like this:
>>>
>>>     0                   1                   2                   3
>>>     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |                          Sequence (i)                       ...
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |  CID-Length   |                                               |
>>>    +-+-+-+-+-+-+-+-+       Connection ID (*)                       +
>>>    |                                                               |
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>    |                                                               |
>>>    +                                                               +
>>>    |                                                               |
>>>    +                   Stateless Reset Token (128)                 +
>>>    |                                                               |
>>>    +                                                               +
>>>    |                                                               |
>>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>
>>>
>>>
>>> [0] However, in the transport parameters design, if the server's
>>> handshake gets reordered, the client might need to send some ACKs with
>>> the initial CID. However, we've agreed that the client's IP address
>>> has to be stable, so this isn't a problem. Alternately, you could
>>> change C->S CIDs in the short header if that was easier.
>>>
>>> [1] An alternative would be to have a sparse range (e.g., you can
>>> express 0-7 and then 8-22 by 2s, assuming I have counted correctly)
>>> and then we could pack both lengths into a single byte. As I said,
>>> lots of opportunities for bikeshedding here.
>>>
>>>
>>
>