Anti-amplification limits

[MikeBishop]

RFC 9000 requires that:

• Datagrams carrying Initial packets be expanded to 1200+ bytes
• Servers verify the client address on handshake and when the address changes through migration / rebinding
• The server can send no more than 3x the received bytes until the client's address is validated, both during the handshake and following client migration / rebinding

This draft states in the Security Considerations that "the anti-amplification limits as specified in Section 8 of [QUIC-TRANSPORT] need to be followed to limit the amplification risk." However, there's no text that describes how this maps into multipath behavior.

My proposal is that:

• The packet on a new path SHOULD be expanded to 1200+ bytes (PADDING, etc.) for the first 1 RTT or until validation completes
• When a packet is received from an unverified address, an endpoint MUST NOT send more than 3x the received bytes until address validation completes