Add a mechanism for communicating the size of a Group ID gap (#922)

Fixes: #731

Author: ianswett

draft-ietf-moq-transport.md

@@ -3071,6 +3071,29 @@ SUBGROUP_HEADER {
 
 ~~~
 
+# Extension Headers
+
+The following Object Extension Headers are defined in MoQT.
+
+## Prior Group ID Gap
+
+Prior Group ID Gap (Extension Header Type 0x40) is a variable length integer
+containing the number of Groups prior to the current Group that do not and will
+never exist. For example, if the Original Publisher published an Object in Group
+7 and knows it will never publish any Objects in Group 8 or Group 9, it can
+include Prior Group ID Gap = 2 in any number of Objects in Group 10, as it sees
+fit.  A track with a Group that contains more than one Object with different
+values for Prior Group ID Gap or has a Prior Group ID Gap larger than the Group
+ID is considered malformed.  If an endpoint receives an Object with a Group ID
+within a previously communicated gap it also treats the track as malformed.
+
+This extension is optional, as publishers might not know the prior gap gize, or
+there may not be a gap. If Prior Group ID Gap is not present, the receiver
+cannot infer any information about the existence of prior groups (see
+{{group-ordering}}).
+
+This extension can be added by the Original Publisher, but MUST NOT be added by
+relays. This extension MUST NOT be modified or removed.
 
 # Security Considerations {#security}