Re: [Spud] States in draft-trammell-plus-statefulness-00

"Christian Huitema" <huitema@huitema.net> Mon, 14 November 2016 17:27 UTC

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From: "Christian Huitema" <huitema@huitema.net>
To: "'Brian Trammell'" <ietf@trammell.ch>, "'Dave Dolson'" <ddolson@sandvine.com>
References: <E8355113905631478EFF04F5AA706E9831159645@wtl-exchp-2.sandvine.com> <835E355C-0AF1-4660-B0FF-8BEE0C54788D@trammell.ch>
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Cc: hildjj@cursive.net, mirja.kuehlewind@tik.ee.ethz.ch, spud@ietf.org
Subject: Re: [Spud] States in draft-trammell-plus-statefulness-00
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On Sunday, November 13, 2016 5:56 PM, Brian Trammell wrote:
>...
> Not yet defined; I'll be talking about this in QUIC tomorrow; see also my slides for that talk at 
> https://www.ietf.org/proceedings/97/slides/slides-97-quic-flow-state-signaling-and-quic-00.pdf

The basic connection state machine can be informed by arrival of packets in either direction and timeouts. This is indeed how most NAT handle UDP today, and it is independent of the protocol on top of UDP. It is also fairly robust to route changes, as the new traffic will naturally open the flows in the new route. The main drawbacks come from the use of timers. Too long and they use too much resource in the middle, too short and they require inefficient keep-alive traffic. On the other hand, since endpoints may well just drop off a route without notice, we know that timers are needed in any case. So, we could frame the problem simply as "can we find an alternative to timers for state management." 

My preference would be for standardizing some well-known magic packets with meanings like "keep me alive for another N minutes" or "drop me now", and to rely on bidirectionality to avoid vulnerability to spoofing attacks. Then, the onus would be on end to end protocol to incorporate or work around these magic packets.

The natural temptation is to go analyze the end-to-end protocol, but it can lead to complex code and ossification. I really wonder whether PLUS should do specific work for QUIC, by opposition to "generic work applicable to any UDP based protocol."  I am concerned that implementing protocol-specific logic in the middle of the network leads to ossification of that protocol, and in fact we saw hints of that with boxes that tried to recognize the version number field in QUIC. When the version number changes, these boxes detected an error and started blocking packets. Of course, this was incorrect behavior, but ossification precisely starts there, incorrect behavior distributed all around the network. 

The other concern is that if all the logic is protocol specific, then you need different logic for QUIC, DTLS, RTP, COAP and what have you. This makes the boxes more complex and more error prone, not to mention potential attack surface if these boxes attempt to parse complex protocols. Also, it creates another form of ossification, in which it becomes very hard for our grandchildren to develop the successor to QUIC.

- -- Christian Huitema



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