[+] Explicit Path ID: Clarify that endpoints use the same Path ID in both directions.

draft-ietf-quic-multipath.md

@@ -312,6 +312,19 @@ Each endpoint associates a Receiver Packet Number space to each Path Identifier
 that it provides to the peer. Each endpoint associates a Sender Packet Number space
 to each Path Identifier received from the peer.
 
+
+Endpoints use the same Path ID for one specific path in both directions.
+For a client-initiated path, the client decides which Path ID is used for the new path
+by picking one of the server allocated CID with the specified Path ID.
+Then, the client sends a PATH_CHALLENGE with the chosen CID. If the server receives the PATH_CHALLENGE,
+it picks a Connection ID with the same path ID for sending the PATH_RESPONSE.
+
+The client MUST choose a previously unused Path ID for which both endpoints have already issued at least one connection ID.
+If the server receives a PATH_CHALLENGE before receiving a MP_NEW_CONNECTION_ID
+for the specific path, it MAY choose to ignore the PATH_CHALLENGE, or it can
+choose to send the PATH_RESPONSE frame upon reception of a
+MP_NEW_CONNECTION_ID frame containing the corresponding Path ID.
+
 The Path Identifier associated with the Destination Connection ID is used to
 construct the packet protection nonce defined in {#multipath-aead}.
 
@@ -474,6 +487,12 @@ However, implicit signals such as idle time or packet losses might be
 the only way for an endhost to detect path closure (see
 {{idle-time-close}}).
 
+PATH_ABANDON frame causes all CID allocated by both of the endpoints for the specified Path ID to be retired.
+
+When path validation of a new path fails, the used Path ID is anyway consumed, 
+and the endpoint MUST abandon the path by sending a PATH_ABANDON frame
+on another path to inform the peer that the Path ID cannot be used anymore.
+
 Note that other explicit closing mechanisms of {{QUIC-TRANSPORT}} still
 apply on the whole connection. In particular, the reception of either a
 CONNECTION_CLOSE ({{Section 10.2 of QUIC-TRANSPORT}}) or a Stateless
@@ -1210,8 +1229,7 @@ PATH_ABANDON frames are formatted as shown in {{fig-path-abandon-format}}.
 PATH_ABANDON frames contain the following fields:
 
 Path Identifier:
-: The Path Identifier of the Destination Connection ID used by the
-  receiver of the frame to send packets over the path to abandon.
+: The Path Identifier to abandon.
 
 Error Code:
 : A variable-length integer that indicates the reason for abandoning
@@ -1253,9 +1271,8 @@ PATH_STANDBY frames are formatted as shown in {{fig-path-standby-format}}.
 PATH_STANDBY Frames contain the following fields:
 
 Path Identifier:
-: The Path Identifier of the Destination Connection ID used by the
-  receiver of this frame to send packets over the path the status update
-  corresponds to. All Destination Connection IDs that have been issued
+: The Path Identifier the status update corresponds to.
+  All Destination Connection IDs that have been issued
   MAY be specified, even if they are not yet in use over a path.
 
 Path Status sequence number:
@@ -1303,9 +1320,7 @@ PATH_AVAILABLE frames are formatted as shown in {{fig-path-available-format}}.
 PATH_AVAILABLE frames contain the following fields:
 
 Path Identifier:
-: The Path Identifier of the Destination Connection ID used by the
-  receiver of this frame to send packets over the path the status update
-  corresponds to.
+: The Path Identifier the status update corresponds to.
 
 Path Status sequence number:
 : A variable-length integer specifying