Subject: [quicwg/base-drafts] Recovery: Size of MSS and relation to MTU (#592)

From: Magnus Westerlund <[email-masked]>
Date: Tue, 06 Jun 2017 04:42:32 -0700

In draft-ietf-quic-recovery-03, section 4.3: 

   kDefaultMss (default 1460 bytes):  The default max packet size used
      for calculating default and minimum congestion windows.

   kInitialWindow (default 10 * kDefaultMss):  Default limit on the
      amount of outstanding data in bytes.

   kMinimumWindow (default 2 * kDefaultMss):  Default minimum congestion
      window.

The document doesn't discuss MSS more, however it clearly have significant impact on the protocol behavior. Thus, I really like to get some clarifications on how MSS are handled in QUIC. Some questions I have are:

1. What is the relation between MSS and IP layer MTU?
2. Is the constants above intended to be related to actual packets possible to send, where MSS = MTU is important, or only to indicate a certain number of bytes? 
3. What manipulation is allowed of MSS during a connections life time. In QUIC change of MTU has to be expected for several reasons. Thus, should MSS be updated due to MTU changes, and how are they performed?

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