[quicwg/base-drafts] Peers that only terminate a single connection on an IP/port cannot migrate (#2441)

[ianswett <[e-mail]>]

Currently the section 5.1(https://tools.ietf.org/html/draft-ietf-quic-transport-18#section-5.1) says:
"An endpoint whose peer has selected a zero-length connection ID MUST continue to use a zero-length connection ID for the lifetime of the connection and MUST NOT send packets from any other local address."

In practice, 0 length connection IDs are only expected to be used when the receiver's IP/port identifies the connection, because otherwise QUIC connections cannot survive NAT rebinding.  Given 0 byte connection IDs are not a privacy concern, the only reason I've heard to prohibit this is that we want to ensure servers never use trial decryption to deal with explicit migration.  I don't believe that's a compelling reason, given there are existing use cases such as Peer-to-peer(ie: WebRTC) where we might want to use 0-byte connection IDs, but supporting migration may also be valuable.  And I don't think any even small-scale server is going to use trial decryption to handle migration due to the excessive CPU cost.

There is already a transport parameter to indicate connection migration is not supported, so the peer should have a clear signal about whether they can migrate with a 0-byte CID.

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