Mandate the use of short header for stateless reset

Closes #2599.

draft-ietf-quic-transport.md

@@ -2343,14 +2343,18 @@ the packet it receives.  Endpoints MUST discard packets that are too small to be
 valid QUIC packets.  With the set of AEAD functions defined in {{QUIC-TLS}},
 packets that are smaller than 21 bytes are never valid.
 
+Endpoints MUST send stateless reset packets formatted as a packet with a short
+header.  However, endpoints MUST treat any packet ending in a valid stateless
+reset token as a stateless reset, as other QUIC versions might allow the use of
+a long header.
+
 An endpoint MAY send a stateless reset in response to a packet with a long
-header and MAY send a stateless reset that is formatted to appear as a packet
-with a long header.  Sending a stateless reset is not effective prior to the
-stateless reset token being available to a peer.  In this QUIC version, packets
-with a long header are only used during connection establishment.   Because the
-stateless reset token is not available until connection establishment is
-complete or near completion, ignoring an unknown packet with a long header might
-be more effective than sending a stateless reset.
+header.  Sending a stateless reset is not effective prior to the stateless reset
+token being available to a peer.  In this QUIC version, packets with a long
+header are only used during connection establishment.   Because the stateless
+reset token is not available until connection establishment is complete or near
+completion, ignoring an unknown packet with a long header might be more
+effective than sending a stateless reset.
 
 An endpoint cannot determine the Source Connection ID from a packet with a short
 header, therefore it cannot set the Destination Connection ID in the stateless