Notational Conventions

1.4.1. Variable-Length Integers

MoQT requires a variable-length integer encoding with the following properties:

1. The encoded length can be determined from the first encoded byte.

2. The range of 1 byte values is as large as possible.

3. All 64 bit numbers can be encoded.

The variable-length integer encoding uses the number of leading 1 bits of the first byte to indicate the length of the encoding in bytes. The remaining bits after the first 0 and subsequent bytes, if any, represent the integer value, encoded in network byte order.

Integers are encoded in 1, 2, 3, 4, 5, 6, 8, or 9 bytes and can encode up to 64 bit unsigned integers. The following table summarizes the encoding properties.

The following table contains some example encodings:

11111100 is an invalid code point. An endpoint that receives this value MUST close the session with a PROTOCOL_VIOLATION.

To reduce unnecessary use of bandwidth, variable length integers SHOULD be encoded using the least number of bytes possible to represent the required value.

x (vi64):

Indicates that x holds an integer value using the variable-length encoding as described above.

1.4.2. Location Structure

Location identifies a particular Object in a Group within a Track.

Location {
  Group (vi64),
  Object (vi64)
}

In this document, a Location can be expressed in the form of {GroupID, ObjectID}, where GroupID and ObjectID indicate the Group ID and Object ID of the Location, respectively. The constituent parts of any Location A can be referred to using A.Group or A.Object.

Location A < Location B if:

A.Group < B.Group || (A.Group == B.Group && A.Object < B.Object)

1.4.3. Key-Value-Pair Structure

Key-Value-Pair is a flexible structure designed to carry key/value pairs in which the key is a variable length integer and the value is either a variable length integer or a byte field of arbitrary length.

Key-Value-Pairs encode a Type value as a delta from the previous Type value, or from 0 if there is no previous Type value. This is efficient on the wire and makes it easy to ensure there is only one instance of a type when needed. The previous Type value plus the Delta Type MUST NOT be greater than 2^64 - 1. If a Delta Type is received that would be too large, the Session MUST be closed with a PROTOCOL_VIOLATION.

Key-Value-Pair is used in both the data plane and control plane, but is optimized for use in the data plane.

Key-Value-Pair {
  Delta Type (vi64),
  [Length (vi64),]
  Value (..)
}

• Delta Type: an unsigned integer, encoded as a varint, identifying the Type as a delta encoded value from the previous Type, if any. The Type identifies the type of value and also the subsequent serialization.

• Length: Only present when Type is odd. Specifies the length of the Value field in bytes. The maximum length of a value is 2^16-1 bytes. If an endpoint receives a length larger than the maximum, it MUST close the session with a PROTOCOL_VIOLATION.

• Value: A single varint encoded value when Type is even, otherwise a sequence of Length bytes.

If a receiver understands a Type, and the following Value or Length/Value does not match the serialization defined by that Type, the receiver MUST close the session with error code KEY_VALUE_FORMATTING_ERROR.

1.4.4. Reason Phrase Structure

Reason Phrase provides a way for the sender to encode additional diagnostic information about the error condition, where appropriate.

Reason Phrase {
  Reason Phrase Length (vi64),
  Reason Phrase Value (..)
}

• Reason Phrase Length: A variable-length integer specifying the length of the reason phrase in bytes. The reason phrase length has a maximum value of 1024 bytes. If an endpoint receives a length exceeding the maximum, it MUST close the session with a PROTOCOL_VIOLATION

• Reason Phrase Value: Additional diagnostic information about the error condition. The reason phrase value is encoded as UTF-8 string and does not carry information, such as language tags, that would aid comprehension by any entity other than the one that created the text.