Active Stream Format for Holding Multiple Media Streams

The present invention relates generally to data processing systems and more particularly to an active stream format for holding multiple media streams.

Conventional file and/or stream formats for transmitting multiple data streams of varying media are limited in several respects. First, these formats are generally limited in the packet sizes that are available for encapsulating data. Such formats, if they specify packets, specify the packets as a given fixed size. Another limitation of such formats is that they do not facilitate the use of error correction codes. A further weakness of these conventional formats is that they do not provide flexibility in timing models for rendering the data encapsulated within the format. An additional limitation with such formats is that they are not well adapted for different transport mediums that have different levels of reliability and different transmission capabilities.

In accordance with a first aspect of the present invention, a computer system has a logical structure for encapsulating multiple streams of data that are partitioned into packets for holding samples of data from the multiple data streams. A method of incorporating error correction into the logical structure is performed on the computer system. In accordance with this method, a portion of at least one packet is designated for holding error correcting data. The error correcting data is then stored in the designated portion of the packet.

In accordance with another aspect of the present invention, multiple streams of data are stored in packets and error correcting data is stored in at least some of the packets. The packets are encapsulated into a larger stream and information regarding what error correcting methods are employed for the packets is also stored in the packets.

In accordance with yet another aspect of the present invention, samples of data from multiple data streams are stored in packets, and replicas of information are stored in at least some of the packets. A flag is set in each of the packets that holds replicas to indicate that the packets hold the replicas. The packets are encapsulated into a larger logical structure and transmitted to a destination.

In accordance with a further aspect of the present invention, a logical structure is provided for encapsulating multiple streams of data where the streams of data are stored in packets. Clock licenses that dictate advancement of a clock are stored in multiple ones of the packets. The logical structure is transmitted from a source computer to a destination computer. The clock is advanced at the destination computer as dictated by the clock license for each packet that holds a clock license in response to the receipt or processing of the packet at the destination computer.

In accordance with an additional aspect of the present invention, a stream format is provided for encapsulating multiple streams of data. The stream format includes a field for specifying a packet size for holding samples of the multiple streams of data. In a logical structure that adopts the stream format, a value is stored in the field that corresponds to the desired packet size. Packets of the desired size are stored within the logical structure and the logical structure is transmitted over a transport medium to the destination.

In accordance with a further aspect of the present invention, a stream format is provided for encapsulating multiple streams of data. A field is included in a logical structure that adopts the stream format for holding a value that specifies a maximum bit rate at which the multiple streams may be rendered at the destination. A value is stored in the field and the logical structure is transmitted over a transport medium to a destination.

In accordance with another aspect of the present invention, a stream format is provided for encapsulating multiple data streams and a new media type is dynamically defined. An identifier of the media type is stored in a logical structure that adopts the stream format and packets of the new media type are stored in the logical structure.

A preferred embodiment of the present invention will be described below relative to the following drawings.

FIG. 1 is a block diagram illustrating a computer system that is suitable for practicing the preferred embodiment of the present invention.

FIG. 2 is a flowchart illustrating use of the ASF stream in accordance with a preferred embodiment of the present invention.

FIG. 3 is a block diagram illustrating the components of the ASF stream.

FIG. 4 is a block diagram illustrating the format of the header.sub.-- object.

FIG. 5 is a block diagram illustrating the format of the properties.sub.-- object.

FIG. 6A is a flowchart illustrating the steps that are performed to fill in packet size fields within the ASF stream.

FIG. 6B is a diagram illustrating different packet sizes and respective ASF streams.

FIG. 7 is a block diagram illustrating the format of the stream.sub.-- properties.sub.-- object.

FIG. 8 is a diagram that illustrates the partitioning of a sample for storage in multiple packets.

FIG. 9 is a diagram that illustrates the format of the content.sub.-- description.sub.-- object.

FIG. 10A is a diagram illustrating the format of the marker.sub.-- object.

FIG. 10B is a diagram illustrating the format of a marker entry.

FIG. 11 is a diagram illustrating the format of the error.sub.-- correction.sub.-- object.

FIG. 12 is flowchart illustrating the steps that are performed to utilize error correcting information in accordance with a preferred embodiment of the present invention.

FIG. 13 is a diagram illustrating format of the clock.sub.-- object.

FIG. 14A is a diagram illustrating the format of the script.sub.-- command.sub.-- object.

FIG. 14B is a diagram illustrating the format of a type.sub.-- names.sub.-- struc.

FIG. 14C is a diagram illustrating the format of a command.sub.-- entry.

FIG. 15A is a diagram illustrating the format of the codec.sub.-- object.

FIG. 15B is a diagram of a CodecEntry.

FIG. 16 is a diagram illustrating the format of the data.sub.-- object.

FIG. 17 illustrates the format of a packet.

FIG. 18A illustrates a first format that the initial.sub.-- structure may assume.

FIG. 18B illustrates a second format that the initial.sub.-- structure may assume.

FIG. 19 illustrates the format of a payload.sub.-- struc.

FIG. 20 is a diagram illustrating the format of the index.sub.-- object.

The preferred embodiment of the present invention employs an active stream format (ASF) for holding multiple media streams. ASF is well suited for storage of multimedia streams as well as transmission of multiple media streams over a transport medium. ASF is constructed to encapsulate diverse multimedia streams and facilitates optimal interleaving of respective media streams. ASF specifies the packetization of data and provides flexibility in choosing packet sizes. In addition, ASF enables the specification of a maximum data transmission rate. As such, the packetization and transmission of media streams may be tailored to facilitate the bandwidth limitations of the system on which media streams are stored or transmitted.

ASF facilitates the use of error correction and error concealment techniques on the media streams. In unreliable transport mediums, such error correction and error concealment is highly beneficial. ASF is independent of media types and is extensible to handle newly defined media types. ASF supports flexible timing approaches and allows an author of an ASF stream to specify the synchronization of events. ASF supports synchronized rendering using a variety of synchronization clock types and provides index information which can be used as markers for lookup to provide playback features such as fast forward and fast reverse.

FIG. 1 is a block diagram of an illustrative system for practicing the preferred embodiment of the present invention. FIG. 2 is a flowchart that illustrates the steps that are performed in the illustrative embodiment of FIG. 1. An ASF stream 16 is built by an author (step 20 in FIG. 2) and stored on a storage 14 on a source computer 10. As will be described in more detail below, ASF allows the author to design the stream for a most efficient storage based on the type of source computer 10 on which it is stored. Sometime later, the ASF stream 16 is transferred over a transport media 17, such as a network connection, to a destination computer 12 (step 24 in FIG. 2). The destination computer 12 includes a number of renderers 18 for rendering the media types that are present within the ASF stream 16. For example, the ASF stream 16 may include audio-type data and video-type data. The renderers 18 at the destination 12 include an audio renderer and a video renderer. The renderers may begin rendering data as soon as they receive data prior to the complete transmission of the entire ASF stream 16 (see step 26 in FIG. 2). The renderers need not immediately render the data, but rather may render the data at a later point in time.

FIG. 3 depicts the basic logical organization of an ASF stream 16. It is up to the author to fill in the contents of the ASF stream in accordance with this format. The ASF stream 16 is divisible into a header section 28, a data section 30 and an index section 49. In general, the header section is first transmitted from the source computer 10 to the destination computer 12 so that the destination computer may process the information within the header section. Subsequently, the data section 30 is transmitted from the source computer 10 to the destination computer 12 on a packet-by-packet basis and the index section 49 is transmitted. The header section 28 includes a number of objects that describe the ASF stream 16 in aggregate. The header section 28 includes a header.sub.-- object 32 that identifies the beginning of the ASF header section 28 and specifies the number of objects contained within the header section. FIG. 4 depicts the format of the header.sub.-- object 32 in more detail. The header.sub.-- object 32 includes an object.sub.-- id field 50 that holds a UUID for the header.sub.-- object. The UUID is an identifier. The header.sub.-- object 32 also includes a size field 52 that specifies a 64-bit quantity that describes the size of the header section 28 in bytes. The header.sub.-- object 32 additionally includes a number.sub.-- headers field 54 that holds a 32-bit number that specifies a count of the objects contained within the header section that follow the header.sub.-- object 32. An alignment field 55 specifies packing alignment of objects within the header (e.g., byte alignment or word alignment). The architecture field 57 identifies the computer architecture type of the data section 30 at the index section 49. The architecture field 57 specifies the architecture of these sections as little endian or big endian.

The header.sub.-- object 32 is followed in the header section 28 by a properties.sub.-- object 34, such as depicted in FIG. 5. The properties.sub.-- object 34 describes properties about the ASF stream 16. As can be seen in FIG. 5, the properties.sub.-- object 34 includes an object.sub.-- id field 56 that holds a UUID and a size field 58 that specifies the size of the properties.sub.-- object 34. The properties.sub.-- object 34 also includes a multimedia.sub.-- stream.sub.-- id field 60 that contains a UUID that identifies a multimedia ASF stream. A total.sub.-- size field 62 is included in the properties.sub.-- object 34 to hold a 64-bit value that expresses the size of the entire ASF multimedia stream.

The properties.sub.-- object 34 also holds a created field 64 that holds a timestamp that specifies when the ASF stream was created. A num.sub.-- packet field 65 holds a 64-bit value that defines the number of packets in the data section 30. A play.sub.-- duration field 66 holds a 32-bit number that specifies the play duration of the entire ASF stream in 100-nanosecond units. For example, if the ASF stream 16 holds a movie, the duration field 66 may hold the duration of the movie. The play.sub.-- duration field 66 is followed by a send.sub.-- duration field 67 that corresponds to send the ASF stream in 100-nanosecond units. A preroll field 68 specifies the amount of time to buffer data before starting to play, and the flags field 70 holds 32-bits of bit flags.

The properties.sub.-- object 34 includes a min.sub.-- packet.sub.-- size field 72 and a max.sub.-- packet.sub.-- size field 74. These fields 72 and 74 specify the size of the smallest and largest packets 48 in the data section 30, respectively. These fields help to determine if the ASF stream 16 is playable from servers that are constrained by packet size. For constant bit rate streams, these values are set to have the same values. A maximum.sub.-- bit.sub.-- rate field 76 holds a value that specifies the maximum instantaneous bit rate (in bits per second) of the ASF stream.

FIG. 6A is a flowchart illustrating how these values are identified and assigned during authoring of the ASF stream 16. First, the size of the smallest packet in the data section 30 is identified (step 78 in FIG. 6A). The size of the smallest packet is stored in the min.sub.-- packet.sub.-- size field 72 (step 80 in FIG. 6A). The size of the largest packet in the data section 30 is identified (step 82 in FIG. 6A), and the size is assigned to the max.sub.-- packet.sub.-- size field 74 (step 84 in FIG. 6A).

One of the beneficial features of ASF is its ability for facilitating different packet sizes for data of multiple media streams. FIG. 6B shows one example of two different streams 83 and 85. In stream 83, each of the packets is chosen to have a size of 512 bytes, whereas in stream 85 each of the packets 48 holds 256 bytes. The decision as to the size of the packets may be influenced by the speed of the transport mechanism over which the ASF stream is to be transmitted, the protocol adopted by the transport medium, and the reliability of the transport medium.

As mentioned above, the properties.sub.-- object 34 holds a value in the maximum.sub.-- bit.sub.-- rate field 76 that specifies an instantaneous maximum bit rate in bits per second that is required to play the ASF stream 16. The inclusion of this field 76 helps to identify the requirements necessary to play the ASF stream 16.

The header section 28 (FIG. 3) must also include at least one stream.sub.-- properties.sub.-- object 36. The stream.sub.-- properties.sub.-- object 36 is associated with a particular type of media stream that is encapsulated within the ASF stream 16. For example, one of the stream.sub.-- properties.sub.-- objects 36 in the header section 28 may be associated with an audio stream, while another such object is associated with a video stream. FIG. 7 depicts a format for such stream.sub.-- properties.sub.-- objects 36. Each stream.sub.-- properties.sub.-- object 36 includes an object.sub.-- id field 86 for holding a UUID for the object and a size field 88 for holding a value that specifies the size of the object in bytes. A stream.sub.-- type field 90 holds a value that identifies the media type of the associated stream.

The stream.sub.-- properties.sub.-- object 36 holds at least three fields 92, 98 and 104 for holding information relating to error concealment strategies. In general, ASF facilitates the use of error concealment strategies that seek to reduce the effect of losing information regarding a given sample of media data. An example of an error concealment strategy is depicted in FIG. 8. A sample 106 is divided into four sections S.sub.1, S.sub.2, S.sub.3 and S.sub.4. When the sample is incorporated into packets in the ASF stream, the samples are distributed into separate packets P.sub.1, P.sub.2, P.sub.3 and P.sub.4 so that if any of the packets are lost, the amount of data that is lost relative to the sample is not as great, and techniques, such as interpolation, may be applied to conceal the error. Each sample has a number of associated properties that describe how big the sample is, how the sample should be presented to a viewer, and what the sample holds. Since the loss of the property information could prevent the reconstruction of the sample, the properties information for the entire sample is incorporated with the portions of the sample in the packets.

The error.sub.-- concealment.sub.-- strategy field 92 holds a UUID that identifies the error concealment strategy that is employed by the associated stream. The error.sub.-- concealment.sub.-- len field 98 describes the number of bytes in an error concealment data block that is held in the error.sub.-- concealment.sub.-- data entries 104. The properties associated with the error concealment strategy are placed in the error.sub.-- concealment.sub.-- data entries 104. The number of entries will vary depending upon the error concealment strategy that is adopted.

The stream.sub.-- properties.sub.-- object 36 includes a stream.sub.-- number field 100 that holds an alias to a stream instance. The stream.sub.-- properties.sub.-- object 36 also includes an offset field 94 that holds an offset value to the stream in milliseconds. This value is added to all of the timestamps of the samples in the associated stream to account for the offset of the stream with respect to the timeline of the program that renders the stream. Lastly, the stream.sub.-- properties.sub.-- object 36 holds a type.sub.-- specific.sub.-- len field 96 that holds a value that describes the number of bytes in the type.sub.-- specific.sub.-- data entries 102. The type.sub.-- specific.sub.-- data entries 102 hold properties values that are associated with the stream type.

The header section 28 (FIG. 3) may also include a number of optional objects 38, 40, 42, 44, 45 and 46. These optional objects include a content.sub.-- description.sub.-- object 38 that holds information such as the title, author, copyright information, and ratings information regarding the ASF stream. This information may be useful and necessary in instances wherein the ASF stream 16 is a movie or other artistic work. The content.sub.-- description.sub.-- object 38 includes an object.sub.-- id field 110 and a size field 112 like the other objects in the header section 28. A title.sub.-- len field 114 specifies the size in bytes of the title entries 119 that hold character data for the title of the ASF stream 16. An author.sub.-- len field 115 specifies the size in bytes of the author entries 120 which hold the characters that specify the author of the ASF stream 16. The copyright.sub.-- len field 116 holds the value that specifies the length in bytes of the copyright entries 121 that hold copyright information regarding the ASF stream 16. The description.sub.-- len field 117 holds a value that specifies the length in bytes of the description entries 122. The description entries 122 hold a narrative description of the ASF stream 16. Lastly, the rating.sub.-- len field 118 specifies a size in bytes of the rating entries 123 that hold rating information (e.g., X, R, PG-13) for the ASF stream content.

The header section 28 may include a marker.sub.-- object 40. The marker.sub.-- object 40 holds a pointer to a specific time within the data section 30. The marker.sub.-- object enables a user to quickly jump forward or backward to specific data points (e.g., audio tracks) that are designated by markers held within the marker.sub.-- object 40.

FIG. 10A shows the marker.sub.-- object 40 in more detail. The marker.sub.-- object 40 includes an object.sub.-- id field 126 that holds a UUID, and a size field 128 specifies the size of the marker.sub.-- object in bytes. A marker.sub.-- id field 130 contains a UUID that identifies the marker data strategy, and a num.sub.-- entries field 132 specifies the number of marker entries in the marker.sub.-- object 40. An entry.sub.-- alignment field 134 identifies the byte alignment of the marker data, and a name.sub.-- len field 136 specifies how many Unicode characters are held in the name field 138, which holds the name of the marker.sub.-- object 40. Lastly, the marker.sub.-- data field 140 holds the markers in a table. Each marker has an associated entry in the table.

FIG. 10B shows the format of a marker entry 141 such as found in the marker.sub.-- data field 140. An offset field 142 holds an offset in bytes from the start of packets in the data.sub.-- object 47 indicating the position of the marker entry 141. A time field 144 specifies a time stamp for the marker entry 141. An entry.sub.-- len field 146 specifies the size of an entry.sub.-- data field 148, which is an array holding the data for the marker entry.

The header section 28 may also include an error.sub.-- correction.sub.-- object 42 for an error correction method that is employed in the ASF stream. Up to four error correction methods may be defined for the ASF stream 16 and, thus, up to four error.sub.-- correction.sub.-- objects 42 may be stored within the header section 28 of the ASF stream 16. FIG. 11 depicts the format of the error.sub.-- correction.sub.-- object 42.

The error.sub.-- correction.sub.-- object 42 includes an object.sub.-- id field 150 and a size field 152, like those described above for the other objects in the header section 28. The error.sub.-- correction.sub.-- object 42 also includes an error.sub.-- correction.sub.-- id 154 that holds UUID that identifies the error correcting methodology associated with the object 42. The error.sub.-- correction.sub.-- data.sub.-- len field 156 specifies the length in bytes of the error.sub.-- correction.sub.-- data entries 158 that hold octets for error correction. The error.sub.-- correction.sub.-- object 42 is used by the destination computer 12 (FIG. 1) in playing the ASF stream 16.

FIG. 12 depicts a flowchart of how error correcting may be applied in the preferred embodiment of the present invention. In particular, an error correction methodology such as an N+1 parity scheme, is applied to one or more streams within the ASF stream 16 (step 160 in FIG. 12). Information regarding the error correcting methodology is then stored in the error.sub.-- correction.sub.-- object 42 within the header section 28 (step 162 in FIG. 12). The source computer then accesses the error correcting methodology information stored in the error.sub.-- correction.sub.-- object 42 in playing back the ASF stream 16 (step 164 in FIG. 12). Error correcting data is stored in the interleave.sub.-- packets 48.

The header section 28 of the ASF stream 16 may also hold a clock.sub.-- object 44 that defines properties for the timeline for which events are synchronized and against which multimedia objects are presented. FIG. 13 depicts the format of the clock.sub.-- object 44. An object.sub.-- ID field 166 holds a UUID to identify the object, and a size field 168 identifies the size of the clock.sub.-- object 44 in bytes. A packet.sub.-- clock.sub.-- type field 170 identifies the UUID of the clock.sub.-- type that is used by the object. A packet.sub.-- clock.sub.-- size field 172 identifies the clock size. A clock.sub.-- specific.sub.-- len field 174 identifies the size and bytes of the clock.sub.-- specific.sub.-- data field 176 which contains clock-specific data. The clock type alternatives include a clock that has a 32-bit source value and a 16-bit duration value, a clock type that has a 64-bit source value and a 32-bit duration value and a clock type that has a 64-bit source value and a 64-bit duration value.

The ASF stream 16 enables script commands to be embedded as a table in the script.sub.-- command.sub.-- object 45. This object 45 may be found in the header section 28 of the ASF stream 16. The script commands ride the ASF stream 16 to the client where they are grabbed by event handlers and executed. FIG. 14A illustrates the format of the script.sub.-- command.sub.-- object 45. Like many of the other objects in the header section 28, this object 45 may include an object.sub.-- ID field 178 for holding a UUID for the object and a size field 180 for holding the size in bytes of the object. A command.sub.-- ID field 182 identifies the structure of the command entry that is held within the object.

The num.sub.-- commands field 184 specifies the total number of script commands that are to be executed. The num.sub.-- types field 186 specifies the total number of different types of script.sub.-- command types that have been specified. The type.sub.-- names field 188 is an array of type.sub.-- names.sub.-- struc data structures. FIG. 14B depicts the format of this data structure 192. The type.sub.-- name.sub.-- len field 194 specifies the number of Unicode characters in the type.sub.-- names field 196, which is a Unicode string array holding names that specify script command types.

The command.sub.-- entry field 190 identifies what commands should be executed at which point in the timeline. The command.sub.-- entry field 190 is implemented as a table of script commands. Each command has an associated command.sub.-- entry element 198 as shown in FIG. 14C. Each such element 198 has a time field 200 that specifies when the script command is to be executed and a type field 202 that is an index into the type.sub.-- names array 196 that identifies the start of a Unicode string for the command type. A parameter field 204 holds a parameter value for the script command type.

The script commands may be of a URL type that causes a client browser to be executed to display an indicated URL. The script command may also be of a file name type that launches another ASF file to facilitate "continuous play" audio or video presentations. Those skilled in the art will appreciate that other types of script commands may also be used.

The header section 28 of the ASF stream 16 may also include a codec.sub.-- object 46. The codec.sub.-- object 46 provides a mechanism to embed information about a codec dependency that is needed to render the data stream by that codec. The codec object includes a list of codec types (e.g., ACM or ICM) and a descriptive name which enables the construction of a codec property page on the client. FIG. 15A depicts the format of a codec.sub.-- object 46. The object.sub.-- id field 206 holds a UUID for the codec.sub.-- object 46 and the size field 208 specifies the size of the object 46 in bytes. The codec.sub.-- ID field 210 holds a UUID that specifies the codec.sub.-- type used by the object. The codec.sub.-- entry.sub.-- len field 212 specifies the number of CodecEntry entries that are in the codec.sub.-- entry field 214. The codec.sub.-- entry field 214 contains codec-specific data and is an array of CodecEntry elements.

FIG. 15B depicts the format of a single CodecEntry element 216 as found in the codec.sub.-- entry field 214. A type field 218 specifies the type of codec. A name field 222 holds an array of Unicode characters that specifies the name of the codec and a name.sub.-- len field 220 specifies the number of Unicode characters in the name field. The description field 226 holds a description of the codec in Unicode characters and the description.sub.-- len field 224 specifies the number of Unicode characters held within the description field. The cbinfo field 230 holds an array of octets that identify the type of the codec and the cbinfo.sub.-- len field 228 holds the number of bytes in the cbinfo field 230.

As mentioned above, the data section 30 follows the header section 28 in the ASF stream 16. The data section includes a data.sub.-- object 47 and interleave.sub.-- packets 48. A data.sub.-- object 47 marks the beginning of the data section 30 and correlates the header section 28 with the data section 30. The packets 48 hold the data payloads for the media stream stored within the ASF stream 16.

FIG. 16 depicts the format of the data.sub.-- object 46. Like other objects in the ASF stream 16, data.sub.-- object 46 includes an object.sub.-- id field 232 and a size field 234. The data.sub.-- object 46 also includes a multimedia.sub.-- stream.sub.-- id field 236 that holds a UUID for the ASF stream 16. This value must match the value held in the multimedia.sub.-- stream.sub.-- id field 60 in the properties.sub.-- object 34 in the header section 28. The data.sub.-- object 46 also includes a num.sub.-- packets field 238 that specifies the number of interleave.sub.-- packets 48 in the data section 30. An alignment field 240 specifies the packing alignment within packets (e.g., byte alignment or word alignment), and the packet.sub.-- alignment field 242 specifies the packet packing alignment.

Each packet 48 has a format like that depicted in FIG. 17. Each packet 48 begins with an initial.sub.-- structure 244. The format of the initial.sub.-- structures 244 depends upon whether the first bit held within the structure is set or not. FIG. 18A depicts a first format of the initial.sub.-- structure 244 when the most significant bit is cleared (i.e., has a value of zero). The most significant bit is the error.sub.-- correction.sub.-- present flag 270 that specifies whether error correction information is present within the initial.sub.-- structure 244 or not. In this case, because the bit 270 is cleared, there is no error correction information contained within the initial.sub.-- structure 244. This bit indicates whether or not error correction is used within the packet. The two bits that constitute the packet.sub.-- len.sub.-- type field 272 specify the size of the packet.sub.-- len field 256, which will be described in more detail below. The next two bits constitute the padding.sub.-- len.sub.-- type field 274 and specify the length of the padding.sub.-- len field 260, which will also be discussed in more detail below. The next two bits constitute the sequence.sub.-- type field 276 and specify the size of the sequence field 258. The final bit is the multiple.sub.-- payloads.sub.-- present flag 278 which specifies whether or not multiple payloads are present within the packet. A value of 1 indicates that multiple media stream samples (i.e., multiple payloads) are present within the packet.

FIG. 18B depicts the format of the initial.sub.-- structure 244 when the error.sub.-- correction.sub.-- present bit is set (i.e., has a value of 1). In this instance, the first byte of the initial.sub.-- structure 244 constitutes the ec.sub.-- flag field 280. The first bit within the ec.sub.-- flag field is the error.sub.-- correction.sub.-- present bit 270, which has been described above. The two bits that follow the error.sub.-- correction.sub.-- present bit 270 constitute the error.sub.-- correction.sub.-- len.sub.-- type field 284 and specify the size of the error.sub.-- correction.sub.-- data.sub.-- len field 290. The next bit constitutes the opaque.sub.-- data flag 286 which specifies whether opaque data exists or not. The final four bits constitute the error.sub.-- correction.sub.-- data length field 288. If the error.sub.-- correction.sub.-- len.sub.-- type field 284 has a value of "00" then the error.sub.-- correction.sub.-- data.sub.-- length field 288 holds the error.sub.-- correction.sub.-- data.sub.-- len value and the error.sub.-- correction.sub.-- data.sub.-- len field 290 does not exist. Otherwise this field 288 has a value of "0000." When the error.sub.-- correction.sub.-- data.sub.-- len field 290 is present, it specifies the number of bytes in the error.sub.-- correction.sub.-- data array 292. The error.sub.-- correction.sub.-- data array 292 holds an array of bytes that contain the actual per-packet data required to implement the selected error correction method.

The initial.sub.-- structure 244 may also include opaque data 300 if the opaque.sub.-- data bit 286 is set. The initial structure includes a byte of flags 302. The most significant bit is a reserved bit 304 that is set to a value of "0." The next two bits constitute the packet.sub.-- len.sub.-- type field 306 that indicate the size of the packet.sub.-- len field 256. The next subsequent two bits constitute the padding.sub.-- len.sub.-- type field 272 that indicate the size of the padding.sub.-- len field 274. These two bits are followed by another 2-bit field that constitutes the sequence.sub.-- type of field 276 that specifies the size of the sequence field 258. The last bit is the multiple.sub.-- payloads.sub.-- present bit 278 that specifies whether are not multiple payloads are present.

The initial.sub.-- structure 244 is followed by a stream.sub.-- flag field 246 that holds a byte consisting of four 2-bit fields. The first two bits constitute a stream.sub.-- id.sub.-- type field 248 that specifies the size of the stream.sub.-- id field 314 within the payload.sub.-- struc 266. The second most significant bits constitute the object.sub.-- id.sub.-- type field 250 and indicate the number of bits in the object.sub.-- id field 316 of the payload.sub.-- struc 266 as either 0-bits, 8-bits, 16-bits or 32-bits. The third most significant two bits constitute the offset.sub.-- type field 252, which specifies the length of the offset field 318 within the payload.sub.-- struc 266 as either 0-bits, 8-bits, 16-bits or 32-bits. The least two significant bits constitute the replicated.sub.-- data.sub.-- type field 254 and these bits indicate the number of bits that are present for the replicated.sub.-- data.sub.-- len field 320 of the payload.sub.-- struc 266.

The packet 48 also includes a packet.sub.-- len field 256 that specifies the packet length size. The sequence field 258 specifies the sequence number for the packet. The padding.sub.-- len field 260 contains a number that specifies the number of padding bytes that are present at the end of the packet to pad out the packet to a desirable size.

The packet 48 also contains a clock.sub.-- data field 262 that contains data representing time information. This data may include a clock license that contains a system clock reference that drives the progression of the time line under the timing model and a duration that specifies the effective duration of the clock license. The duration field limits the validity of the license to a time specified in milliseconds. Under the model adopted by the preferred embodiment of the present invention, the source computer 10 issues a clock license to the destination computer 12 that allows the clock of the destination computer 12 to progress forward for a period of time. The progression of time is gated by the arrival of a new piece of data that contains a clock value with a valid clock license that is not expired.

The packet 48 also includes a payload.sub.-- flag field 264 that specifies a payload length type and a designation of the number of payloads present in the packet. The payload.sub.-- flag field 264 is followed by one or more payload.sub.-- strucs 266. These structures contain payload information which will be described in more detail below. The final bits within the packet 48 may constitute padding 268.

FIG. 19 depicts the payload.sub.-- struc 266 in more detail. The stream.sub.-- id field 314 is an optional field that identifies the stream type of the payload. The object.sub.-- id field 316 may be included to hold an object identifier. An offset field 318 may be included to specify an offset of the payload within the ASF stream. The offset represents the starting address within a zero-address-based media stream sample where the packet payload should be copied.

The payload.sub.-- struc 266 may also include a replicated.sub.-- data.sub.-- len field 320 that specifies the number of bytes of replicated data present in the replicated.sub.-- data field 322. As was discussed above, for protection against possible errors, the packet 48 may include replicated data. This replicated data is stored within the replicated.sub.-- data field 322.

The payload.sub.-- len field 323 specifies the number of payload bytes present in the payload held within the payload.sub.-- data field 325. The payload.sub.-- data field 326 holds an array of payloads (i.e., the data).

The ASF stream may also include an index.sub.-- object 49 that holds index information regarding the ASF stream 16. FIG. 20 depicts the format of the index.sub.-- object 49. The index.sub.-- object includes a number of index entries. The index.sub.-- object 49 includes an object.sub.-- id field 324 and a size field 326. In addition, the index.sub.-- object 49 includes an index.sub.-- id field 328 that holds a UUID for the index type. Multiple index.sub.-- name.sub.-- entries may be stored depending on the number of entries required to hold the characters of the name. For example, each entry may hold 16 characters in an illustrative embodiment.

The index.sub.-- object includes a time.sub.-- delta field 330 that specifies a time interval between index entries. The time represents a point on the timeline for the ASF stream 16. A max.sub.-- packets field 332 specifies a maximum value for packet.sub.-- count fields, which will be described in more detail below. A num.sub.-- entries field 334 is a 32-bit unsigned integer that describes the maximum number of index entries that are defined within the index.sub.-- info array 336. This array 336 is an array of index.sub.-- information structures. Each index.sub.-- info structure holds a packet field that holds a packet number associated with the index entry and a packet.sub.-- count field specifies the number of the packet to send with the index entry so as to associate the index entries with the packets. In FIG. 21, the index.sub.-- info array structure 336 holds N index.sub.-- information structures and each index.sub.-- information structure has a packet field 338A-338N and a packet.sub.-- count field 340A-340N.

While the present invention has been described with reference to a preferred embodiment thereof, those skilled in the art will appreciate that various changes in form and detail may be made without departing from the intended scope of the invention as defined in the appended claims. For example, the present invention may be practiced with a stream format that differs from the format described above. The particulars described above are intended merely to be illustrative. The present invention may be practiced with stream formats that include only a subset of the above-described fields or include additional fields that differ from those described above. Moreover, the length of the values held within the fields and the organization of the structures described above are not intended to limit the scope of the present invention.