Clarify default for max_receive_timestamps_per_ack is 0 and prohibits frames

draft-ietf-quic-receive-ts.md

@@ -267,10 +267,18 @@ max_receive_timestamps_per_ack (0x4ac07 temporary value for draft use):
 : A variable-length integer indicating that the maximum number of receive
   timestamps the sending endpoint would like to receive in an
   ACK_RECEIVE_TIMESTAMPS or PATH_ACK_RECEIVE_TIMESTAMPS frame.
+  If this transport parameter is absent, this extension is not supported, which
+  is equivalent to a value of 0.
 
   Each ACK_RECEIVE_TIMESTAMPS or PATH_ACK_RECEIVE_TIMESTAMPS frame sent MUST
   NOT contain more than the peer's maximum number of receive timestamps.
 
+  An endpoint MUST NOT send ACK_RECEIVE_TIMESTAMPS or
+  PATH_ACK_RECEIVE_TIMESTAMPS frames to a peer whose
+  max_receive_timestamps_per_ack is not present or 0.
+  A value of 0 does not permit sending these
+  frames with zero timestamps; it prohibits sending them entirely.
+
 receive_timestamps_exponent (0x4ac26 temporary value for draft use):
 
 : A variable-length integer indicating the exponent to be used when encoding and
@@ -279,8 +287,8 @@ receive_timestamps_exponent (0x4ac26 temporary value for draft use):
   peer (see {{ts-ranges}}). If this value is absent, a default value of 0 is
   assumed (indicating microsecond precision). Values above 20 are invalid.
   If the receive_timestamps_exponent transport parameter is present and
-  max_receive_timestamps_per_ack is not, receive timestamps are not supported
-  and receive_timestamps_exponent MUST be ignored.
+  max_receive_timestamps_per_ack is absent or has a value of 0, receive
+  timestamps are not supported and receive_timestamps_exponent MUST be ignored.
 
 ## Receive Timestamp Basis {#ts-basis}