Adding an Extensible Object Header (moq-wg#502)

This PR adds an extensible header to Objects, as well as updating the
SETUP parameters to allow negotiation of required extension header
parameters. Extensible headers have types which are IANA registered. A
new Extensible header is defined and reserved with type 0x0, allowing
name/value data to be passed in a manner compatible with HTTP headers. A
new example is added showing serialization of an Object with an
extensible header.

Fixes: moq-wg#433

Author: ianswett

draft-ietf-moq-transport.md

@@ -1236,8 +1236,8 @@ the `query` portion of the URI to the parameter.
 #### MAX_SUBSCRIBE_ID {#max-subscribe-id}
 
 The MAX_SUBSCRIBE_ID parameter (Parameter Type 0x02) communicates an initial
-value for the Maximum Subscribe ID to the receiving subscriber. The default value
-is 0, so if not specified, the peer MUST NOT create subscriptions.
+value for the Maximum Subscribe ID to the receiving subscriber. The default
+value is 0, so if not specified, the peer MUST NOT create subscriptions.
 
 ## GOAWAY {#message-goaway}
 
@@ -2291,6 +2291,12 @@ Track or Datagram.
 * Object Status: As enumeration used to indicate missing
 objects or mark the end of a group or track. See {{object-status}} below.
 
+* Object Extension Count: The number of Object Extensions present. A value of 0
+  indicates that no Object Extension Headers are present.
+
+* Object Extensions : A sequence of Object Extension Headers. See
+  {{object-extensions}} below.
+
 * Object Payload: An opaque payload intended for an End Subscriber and SHOULD
 NOT be processed by a relay. Only present when 'Object Status' is Normal (0x0).
 
@@ -2335,6 +2341,44 @@ Any object with a status code other than zero MUST have an empty payload.
 Though some status information could be inferred from QUIC stream state,
 that information is not reliable and cacheable.
 
+#### Object Extension Header {#object-extensions}
+Object Extension Headers are visible to relays and allow the transmission of
+future metadata relevant to MOQT Object distribution. Any Object metadata never
+accessed by the transport or relays SHOULD be serialized as part of the Object
+payload and not as an extension header.
+
+Extension Headers are defined in external specifications and registered in an
+IANA table {{iana}}. These specifications define the type and value of the
+header, along with any rules concerning processing, modification, caching and
+forwarding. A relay which is coded to implement these rules is said to
+"support" the extension.
+
+If unsupported by the relay, Extension Headers MUST NOT be modified, MUST be
+cached as part of the Object and MUST be forwarded by relays.
+
+If supported by the relay and subject to the processing rules specified in the
+definition of the extension, Extension Headers MAY be modified, added, removed,
+and/or cached by relays.
+
+Object Extension Headers are serialized as defined below:
+
+~~~
+Extension Header {
+  Header Type (i),
+  [Header Value (i)]
+  [Header Length (i),
+   Header Value (..)]
+}
+~~~
+{: #object-extension-format title="Object Extension Header Format"}
+
+* Header type: an unsigned integer, encoded as a varint, identifying the type
+  of the extension and also the subsequent serialization.
+* Header values: even types are followed by a single varint encoded value. Odd
+  types are followed by a varint encoded length and then the header value.
+  Header types are registered in the IANA table 'MOQ Extension Headers'.
+  See {{iana}}.
+
 ## Object Datagram {#object-datagram}
 
 An `OBJECT_DATAGRAM` carries a single object in a datagram.
@@ -2352,6 +2396,10 @@ OBJECT_DATAGRAM {
   Group ID (i),
   Object ID (i),
   Publisher Priority (8),
+  Extension Count (i),
+  [Extension headers (...)],
+  Object Payload Length (i),
+  [Object Status (i)],
   Object Payload (..),
 }
 ~~~
@@ -2416,6 +2464,8 @@ The Object Status field is only sent if the Object Payload Length is zero.
 ~~~
 {
   Object ID (i),
+  Extension Count (i),
+  [Extension headers (...)],
   Object Payload Length (i),
   [Object Status (i)],
   Object Payload (..),
@@ -2518,6 +2568,8 @@ Each object sent on a fetch stream after the FETCH_HEADER has the following form
   Subgroup ID (i),
   Object ID (i),
   Publisher Priority (8),
+  Extension Count (i),
+  [Extension headers (...)],
   Object Payload Length (i),
   [Object Status (i)],
   Object Payload (..),
@@ -2555,6 +2607,38 @@ SUBGROUP_HEADER {
 }
 ~~~
 
+Sending a group on one stream, with the first object containing two
+Extension Headers.
+
+~~~
+Stream = 2
+
+STREAM_HEADER_GROUP {
+  Subscribe ID = 2
+  Track Alias = 2
+  Group ID = 0
+  Publisher Priority = 0
+}
+{
+  Object ID = 0
+  Extension Count  = 2
+    { Type = 4
+      Value = 2186796243
+    },
+    { Type = 77
+      Length = 21
+      Value = "traceID:123456"
+    }
+  Object Payload Length = 4
+  Payload = "abcd"
+}
+{
+  Object ID = 1
+  Object Payload Length = 4
+  Payload = "efgh"
+}
+
+~~~
 
 
 # Security Considerations {#security}
@@ -2598,8 +2682,12 @@ TODO: fill out currently missing registries:
 * Announce Error codes
 * Announce Cancel Reason codes
 * Message types
+* MOQ Extension headers - we wish to reserve extension types 0-63 for
+  standards utilization where space is a premium, 64 - 16383 for
+  standards utilization where space is less of a concern, and 16384 and
+  above for first-come-first-served non-standardization usage.
 
-TODO: register the URI scheme and the ALPN
+TODO: register the URI scheme and the ALPN and grease the Extension types
 
 # Contributors
 {:numbered="false"}