Re: [quicwg/base-drafts] PMTUD details (#614)

[Magnus Westerlund <[e-mail]>]

@martinduke I have also re-read RFC 4821. And I first of all I think being explicit on the things you consider redundant is the better approach. This from the aspect, that if we can spend time to write a sentence or two, and save time and get more consistent behavior from implementations it is well spent time in my view. 

Additional issues worth discussing in the text are:

- ICMP verification for any received Message to too Big (Not only relevant for PMTUD). This is likely rather straightforward, but do require the QUIC implementation to be able to match the truncated part of the encrypted QUIC packet against itself. This likely implies storing X number of bytes of the encrypted packet for this verification. Or will we actually use any encryption that will be able to perform partial decryption and use that to verify that these initial bytes matches valid QUIC headers?
  
- Issues with setting DF bit from user land (as SHOULD requirement) in 4821. It is somewhat discussed in RFC4821, but clearly an issue for many QUIC implementations. What advancements since 4821 is it worth bringing up here? 

- Storage and reuse of PMTUD information in stack instance releated to differnet paths. Applies for multi-path as well as implementations opening a number of connections. At a minimum a clear definition of what QUIC semantics corresponds to a path, and thus can have different MTU values.
 
- For user-land QUIC implementation it can actually be a bit of a challenge to correctly know how much headers would be added. A challenging example would be QUIC over ICE in a WebRTC context. Where it is actually the ICE stack that controlls which path and what headers and NAT/FW traversal mechanism are in place, such as TURN that changes the header overhead between candidate pairs. This is in relation to RFC 4821 Section 5.1. 
 

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