Allow the Track Alias to be selected by the publisher (#258)

Currently SUBSCRIBE_OK and OBJECT will race. If OBJECT wins (>50% chance
for multi-threaded QUIC libraries), then the subscriber will receive an
unknown OBJECT that needs to be buffered. Moving Track ID to SUBSCRIBE
fixes this race, removes the need to buffer OBJECTs in the MoqT library,
and simplifies the flow. See #145

Author: ianswett

draft-ietf-moq-transport.md

@@ -400,19 +400,21 @@ Section 5}}).
 The application MAY use any error message and SHOULD use a relevant
 code, as defined below:
 
-|------|--------------------|
-| Code | Reason             |
-|-----:|:-------------------|
-| 0x0  | No Error           |
-|------|--------------------|
-| 0x1  | Generic Error      |
-|------|--------------------|
-| 0x2  | Unauthorized       |
-|------|--------------------|
-| 0x3  | Protocol Violation |
-|------|--------------------|
-| 0x10 | GOAWAY Timeout     |
-|------|--------------------|
+|------|-----------------------|
+| Code | Reason                |
+|-----:|:----------------------|
+| 0x0  | No Error              |
+|------|-----------------------|
+| 0x1  | Generic Error         |
+|------|-----------------------|
+| 0x2  | Unauthorized          |
+|------|-----------------------|
+| 0x3  | Protocol Violation    |
+|------|-----------------------|
+| 0x4  | Duplicate Track Alias |
+|------|-----------------------|
+| 0x10 | GOAWAY Timeout        |
+|------|-----------------------|
 
 * No Error: The session is being terminated without an error.
 
@@ -424,6 +426,9 @@ code, as defined below:
 * Protocol Violation: The remote endpoint performed an action that was
   disallowed by the specification.
 
+* Duplicate Track Alias: The endpoint attempted to use a Track Alias
+  that was already in use.
+
 * GOAWAY Timeout: The session was closed because the client took too long to
   close the session in response to a GOAWAY ({{message-goaway}}) message.
   See session migration ({{session-migration}}).
@@ -862,8 +867,8 @@ OBJECT Message {
 ~~~
 {: #moq-transport-object-format title="MOQT OBJECT Message"}
 
-* Track Alias : The compressed full track name obtained as part of
-subscription and/or publish control message exchanges.
+* Track Alias :The track identifier as specified in the
+  SUBSCRIBE message {{message-subscribe-req}}.
 
 * Group ID : The object is a member of the indicated group ID
 {{model-group}} within the track.
@@ -931,6 +936,7 @@ The format of SUBSCRIBE is as follows:
 ~~~
 SUBSCRIBE Message {
   Subscribe ID (i),
+  Track Alias (i),
   Track Namespace (b),
   Track Name (b),
   StartGroup (Location),
@@ -942,18 +948,24 @@ SUBSCRIBE Message {
 ~~~
 {: #moq-transport-subscribe-format title="MOQT SUBSCRIBE Message"}
 
-* Track Namespace: Identifies the namespace of the track as defined in
-({{track-name}}).
-
-* Track Name: Identifies the track name as defined in ({{track-name}}).
-
 * Subscribe ID: The subscription identifier that is unique within the session.
 `Subscribe ID` is a monotonically increasing variable length integer which
 MUST not be reused within a session. `Subscribe ID` is used by subscribers and
 the publishers to identify a given subscription. Subscribers specify the
 `Subscribe ID` and it is included in the corresponding SUBSCRIBE_OK or
 SUBSCRIBE_ERROR messages.
 
+* Track Alias: A session specific identifier for the track.
+Messages that reference a track, such as OBJECT ({{message-object}}),
+reference this Track Alias instead of the Track Name and Track Namespace to
+reduce overhead. If the Track Alias is already in use, the publisher MUST
+close the session with a Duplicate Track Alias error ({{session-termination}}).
+
+* Track Namespace: Identifies the namespace of the track as defined in
+({{track-name}}).
+
+* Track Name: Identifies the track name as defined in ({{track-name}}).
+
 * StartGroup: The Location of the requested group.  StartGroup's Mode MUST NOT be
 None.
 
@@ -1042,9 +1054,6 @@ A SUBSCRIBE_OK control message is sent for successful subscriptions.
 SUBSCRIBE_OK
 {
   Subscribe ID (i),
-  Track Namespace (b),
-  Track Name (b),
-  Track Alias (i),
   Expires (i)
 }
 ~~~
@@ -1057,11 +1066,6 @@ SUBSCRIBE_OK
 
 * Track Name: Identifies the track name as defined in ({{track-name}}).
 
-* Track Alias: Session specific identifier that is used as an alias for the
-Full Track Name in the Track Alias field of the OBJECT ({{message-object}})
-message headers of the requested track. Track Aliases are shorter
-than Full Track Names and thus reduce the overhead in OBJECT messages.
-
 * Expires: Time in milliseconds after which the subscription is no
 longer valid. A value of 0 indicates that the subscription stays active
 until it is explicitly unsubscribed.
@@ -1078,6 +1082,7 @@ SUBSCRIBE_ERROR
   Subscribe ID (i),
   Error Code (i),
   Reason Phrase (b),
+  Track Alias (i),
 }
 ~~~
 {: #moq-transport-subscribe-error format title="MOQT SUBSCRIBE_ERROR Message"}
@@ -1086,10 +1091,14 @@ SUBSCRIBE_ERROR
 
 * Error Code: Identifies an integer error code for subscription failure.
 
-* Reason Phrase Length: The length in bytes of the reason phrase.
-
 * Reason Phrase: Provides the reason for subscription error.
 
+* Track Alias: When Error Code is TBD, the subscriber SHOULD re-issue the
+  SUBSCRIBE with this Track Alias instead. If this Track Alias is already in use,
+  the receiver MUST close the connection with a Duplicate Track Alias error
+  ({{session-termination}}).
+  TODO: Add a registry for subscribe error codes and make this field conditional.
+
 
 ## UNSUBSCRIBE {#message-unsubscribe}