Network Working Group L. Berc Request for Comments: 2435 Digital Equipment Corporation Obsoletes: 2035 W. Fenner Category: Standards Track Xerox PARC R. Frederick Xerox PARC S. McCanne Lawrence Berkeley Laboratory P. Stewart Xerox PARC October 1998
RTP Payload Format for JPEG-compressed Video
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This memo describes the RTP payload format for JPEG video streams. The packet format is optimized for real-time video streams where codec parameters change rarely from frame to frame.
This document is a product of the Audio-Video Transport working group within the Internet Engineering Task Force. Comments are solicited and should be addressed to the working group's mailing list at rem- conf@es.net and/or the author(s).
Most of this memo is identical to RFC 2035. The changes made to the protocol are summarized in Appendix D.
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Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [9].
The Joint Photographic Experts Group (JPEG) standard [1,2,3] defines a family of compression algorithms for continuous-tone, still images. This still image compression standard can be applied to video by compressing each frame of video as an independent still image and transmitting them in series. Video coded in this fashion is often called Motion-JPEG.
We first give an overview of JPEG and then describe the specific subset of JPEG that is supported in RTP and the mechanism by which JPEG frames are carried as RTP payloads.
The JPEG standard defines four modes of operation: the sequential DCT mode, the progressive DCT mode, the lossless mode, and the hierarchical mode. Depending on the mode, the image is represented in one or more passes. Each pass (called a frame in the JPEG standard) is further broken down into one or more scans. Within each scan, there are one to four components, which represent the three components of a color signal (e.g., "red, green, and blue", or a luminance signal and two chrominance signals). These components can be encoded as separate scans or interleaved into a single scan.
Each frame and scan is preceded with a header containing optional definitions for compression parameters like quantization tables and Huffman coding tables. The headers and optional parameters are identified with "markers" and comprise a marker segment; each scan appears as an entropy-coded bit stream within two marker segments. Markers are aligned to byte boundaries and (in general) cannot appear in the entropy-coded segment, allowing scan boundaries to be determined without parsing the bit stream.
Compressed data is represented in one of three formats: the interchange format, the abbreviated format, or the table- specification format. The interchange format contains definitions for all the tables used by the entropy-coded segments, while the abbreviated format might omit some assuming they were defined out- of-band or by a "previous" image.
The JPEG standard does not define the meaning or format of the components that comprise the image. Attributes like the color space and pixel aspect ratio must be specified out-of-band with respect to
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the JPEG bit stream. The JPEG File Interchange Format (JFIF) [4] is a de-facto standard that provides this extra information using an application marker segment (APP0). Note that a JFIF file is simply a JPEG interchange format image along with the APP0 segment. In the case of video, additional parameters must be defined out-of-band (e.g., frame rate, interlaced vs. non-interlaced, etc.).
While the JPEG standard provides a rich set of algorithms for flexible compression, cost-effective hardware implementations of the full standard have not appeared. Instead, most hardware JPEG video codecs implement only a subset of the sequential DCT mode of operation. Typically, marker segments are interpreted in software (which "re-programs" the hardware) and the hardware is presented with a single, interleaved entropy-coded scan represented in the YUV color space.
The scan contains an ordered sequence of Minimum Coded Units, or MCUs, which are the smallest group of image data coded in a JPEG bit stream. Each MCU defines the image data for a small rectangular block of the output image.
Restart markers in the JPEG data denote a point where the decoder should reset its state. As defined by JPEG, restart markers are the only type of marker that may appear embedded in the entropy-coded segment, and they may only appear on an MCU boundary. A "restart interval" is defined to be a block of data containing a restart marker followed by some fixed number of MCUs. An exception is made for the first restart interval in each frame, which omits the initial restart marker and just begins with the MCU data. When these markers are used, each frame is composed of some fixed number of back-to-back restart intervals.
To maximize interoperability among hardware-based codecs, we assume the sequential DCT operating mode [1,Annex F] and restrict the set of predefined RTP/JPEG "type codes" (defined below) to single-scan, interleaved images. While this is more restrictive than even baseline JPEG, many hardware implementation fall short of the baseline specification (e.g., most hardware cannot decode non- interleaved scans).
In practice, most of the table-specification data rarely changes from frame to frame within a single video stream. Therefore RTP/JPEG data is represented in abbreviated format, with all of the tables omitted from the bit stream where possible. Each frame begins immediately with the (single) entropy-coded scan. The information that would otherwise be in both the frame and scan headers is represented
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entirely within the RTP/JPEG header (defined below) that lies between the RTP header and the JPEG payload.
While parameters like Huffman tables and color space are likely to remain fixed for the lifetime of the video stream, other parameters should be allowed to vary, notably the quantization tables and image size (e.g., to implement rate-adaptive transmission or allow a user to adjust the "quality level" or resolution manually). Thus explicit fields in the RTP/JPEG header are allocated to represent this information. Since only a small set of quantization tables are typically used, we encode the entire set of quantization tables in a small integer field. Customized quantization tables are accommodated by using a special range of values in this field, and then placing the table before the beginning of the JPEG payload. The image width and height are encoded explicitly.
Because JPEG frames are typically larger than the underlying network's maximum packet size, frames must often be fragmented into several packets. One approach is to allow the network layer below RTP (e.g., IP) to perform the fragmentation. However, this precludes rate-controlling the resulting packet stream or partial delivery in the presence of loss, and frames may be larger than the maximum network layer reassembly length (see [10] for more information). To avoid these limitations, RTP/JPEG defines a simple fragmentation and reassembly scheme at the RTP level.
The RTP timestamp is in units of 90000Hz. The same timestamp MUST appear in each fragment of a given frame. The RTP marker bit MUST be set in the last packet of a frame.
Each packet contains a special JPEG header which immediately follows the RTP header. The first 8 bytes of this header, called the "main JPEG header", are as follows:
Interpretation depends on the value of the type field. If no interpretation is specified, this field MUST be zeroed on transmission and ignored on reception.
The Fragment Offset is the offset in bytes of the current packet in the JPEG frame data. This value is encoded in network byte order (most significant byte first). The Fragment Offset plus the length of the payload data in the packet MUST NOT exceed 2^24 bytes.
The type field specifies the information that would otherwise be present in a JPEG abbreviated table-specification as well as the additional JFIF-style parameters not defined by JPEG. Types 0-63 are reserved as fixed, well-known mappings to be defined by this document and future revisions of this document. Types 64-127 are the same as types 0-63, except that restart markers are present in the JPEG data and a Restart Marker header appears immediately following the main JPEG header. Types 128-255 are free to be dynamically defined by a session setup protocol (which is beyond the scope of this document).
The Q field defines the quantization tables for this frame. Q values 0-127 indicate the quantization tables are computed using an algorithm determined by the Type field (see below). Q values 128-255 indicate that a Quantization Table header appears after the main JPEG header (and the Restart Marker header, if present) in the first packet of the frame (fragment offset 0). This header can be used to explicitly specify the quantization tables in-band.
This field encodes the width of the image in 8-pixel multiples (e.g., a width of 40 denotes an image 320 pixels wide). The maximum width is 2040 pixels.
This field encodes the height of the image in 8-pixel multiples (e.g., a height of 30 denotes an image 240 pixels tall). When encoding interlaced video, this is the height of a video field, since fields are individually JPEG encoded. The maximum height is 2040 pixels.
This header MUST be present immediately after the main JPEG header when using types 64-127. It provides the additional information required to properly decode a data stream containing restart markers.
The Restart Interval field specifies the number of MCUs that appear between restart markers. It is identical to the 16 bit value that would appear in the DRI marker segment of a JFIF header. This value MUST NOT be zero.
If the restart intervals in a frame are not guaranteed to be aligned with packet boundaries, the F (first) and L (last) bits MUST be set to 1 and the Restart Count MUST be set to 0x3FFF. This indicates that a receiver MUST reassemble the entire frame before decoding it.
To support partial frame decoding, the frame is broken into "chunks" each containing an integral number of restart intervals. The Restart Count field contains the position of the first restart interval in the current "chunk" so that receivers know which part of the frame this data corresponds to. A Restart Interval value SHOULD be chosen to allow a "chunk" to completely fit within a single packet. In this case, both the F and L bits of the packet are set to 1. However, if a chunk needs to be spread across multiple packets, the F bit will be set to 1 in the first packet of the chunk (and only that one) and the L bit will be set to 1 in the last packet of the chunk (and only that one).
This header MUST be present after the main JPEG header (and after the Restart Marker header, if present) when using Q values 128-255. It provides a way to specify the quantization tables associated with this Q value in-band.
The Length field is set to the length in bytes of the quantization table data to follow. The Length field MAY be set to zero to indicate that no quantization table data is included in this frame. See section 4.2 for more information. If the Length field in a received packet is larger than the remaining number of bytes, the packet MUST be discarded.
When table data is included, the number of tables present depends on the JPEG type field. For example, type 0 uses two tables (one for the luminance component and one shared by the chrominance components). Each table is an array of 64 values given in zig-zag order, identical to the format used in a JFIF DQT marker segment.
For each quantization table present, a bit in the Precision field specifies the size of the coefficients in that table. If the bit is zero, the coefficients are 8 bits yielding a table length of 64 bytes. If the bit is one, the coefficients are 16 bits for a table length of 128 bytes. For 16 bit tables, the coefficients are presented in network byte order. The rightmost bit in the Precision field (bit 15 in the diagram above) corresponds to the first table and each additional table uses the next bit to the left. Bits beyond those corresponding to the tables needed by the type in use MUST be ignored.
For Q values from 128 to 254, the Q value to quantization table data mapping MUST be static, i.e., the receivers are guaranteed that they only need to read the table data once in order to correctly decode frames sent with that Q value. A Q value of 255 denotes that the quantization table mapping is dynamic and can change on every frame. Decoders MUST NOT depend on any previous version of the tables, and need to reload these tables on every frame. Packets MUST NOT contain Q = 255 and Length = 0.
The data following the RTP/JPEG headers is an entropy-coded segment consisting of a single scan. The scan header is not present and is inferred from the RTP/JPEG header. The scan is terminated either implicitly (i.e., the point at which the image is fully parsed), or
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explicitly with an EOI marker. The scan may be padded to arbitrary length with undefined bytes. (Some existing hardware codecs generate extra lines at the bottom of a video frame and removal of these lines would require a Huffman-decoding pass over the data.)
The type code determines whether restart markers are present. If a type supports restart markers, the packet MUST contain a non-zero Restart Interval value in a Restart Marker Header and restart markers MUST appear on byte aligned boundaries beginning with an 0xFF between MCUs at that interval. Additional 0xFF bytes MAY appear between restart intervals. This can be used in the packetization process to align data to something like a word boundary for more efficient copying. Restart markers MUST NOT appear anywhere else in the JPEG payload. Types which do not support restart makers MUST NOT contain restart markers anywhere in the JPEG payload. All packets MUST contain a "stuffed" 0x00 byte following any true 0xFF byte generated by the entropy coder [1, Sec. B.1.1.5].
The Type field defines the abbreviated table-specification and additional JFIF-style parameters not defined by JPEG, since they are not present in the body of the transmitted JPEG data.
Three ranges of the type field are currently defined. Types 0-63 are reserved as fixed, well-known mappings to be defined by this document and future revisions of this document. Types 64-127 are the same as types 0-63, except that restart markers are present in the JPEG data and a Restart Marker header appears immediately following the main JPEG header. Types 128-255 are free to be dynamically defined by a session setup protocol (which is beyond the scope of this document).
Of the first group of fixed mappings, types 0 and 1 are currently defined, along with the corresponding types 64 and 65 that indicate the presence of restart markers. They correspond to an abbreviated table-specification indicating the "Baseline DCT sequential" mode, 8-bit samples, square pixels, three components in the YUV color space, standard Huffman tables as defined in [1, Annex K.3], and a single interleaved scan with a scan component selector indicating components 1, 2, and 3 in that order. The Y, U, and V color planes correspond to component numbers 1, 2, and 3, respectively. Component 1 (i.e., the luminance plane) uses Huffman table number 0 and quantization table number 0 (defined below) and components 2 and 3 (i.e., the chrominance planes) use Huffman table number 1 and quantization table number 1 (defined below).
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Type numbers 2-5 are reserved and SHOULD NOT be used. Applications based on previous versions of this document (RFC 2035) should be updated to indicate the presence of restart markers with type 64 or 65 and the Restart Marker header.
The two RTP/JPEG types currently defined are described below:
These sampling factors indicate that the chrominance components of type 0 video is downsampled horizontally by 2 (often called 4:2:2) while the chrominance components of type 1 video are downsampled both horizontally and vertically by 2 (often called 4:2:0).
Types 0 and 1 can be used to carry both progressively scanned and interlaced image data. This is encoded using the Type-specific field in the main JPEG header. The following values are defined:
0 : Image is progressively scanned. On a computer monitor, it can be displayed as-is at the specified width and height.
1 : Image is an odd field of an interlaced video signal. The height specified in the main JPEG header is half of the height of the entire displayed image. This field should be de- interlaced with the even field following it such that lines from each of the images alternate. Corresponding lines from the even field should appear just above those same lines from the odd field.
2 : Image is an even field of an interlaced video signal.
3 : Image is a single field from an interlaced video signal, but it should be displayed full frame as if it were received as both the odd & even fields of the frame. On a computer monitor, each line in the image should be displayed twice, doubling the height of the image.
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Appendix B contains C source code for transforming the RTP/JPEG header parameters into the JPEG frame and scan headers that are absent from the data payload.
For JPEG types 0 and 1 (and their corresponding types 64 and 65), Q values between 1 and 99 inclusive are defined as follows. Other values less than 128 are reserved. Additional types are encouraged to use this definition if applicable.
Both type 0 and type 1 JPEG require two quantization tables. These tables are calculated as follows. For 1 <= Q <= 99, the Independent JPEG Group's formula [5] is used to produce a scale factor S as:
S = 5000 / Q for 1 <= Q <= 50 = 200 - 2 * Q for 51 <= Q <= 99
This value is then used to scale Tables K.1 and K.2 from [1] (saturating each value to 8 bits) to give quantization table numbers 0 and 1, respectively. C source code is provided in Appendix A to compute these tables.
For Q values 128-255, dynamically defined quantization tables are used. These tables may be specified either in-band or out of band by something like a session setup protocol, but the Quantization Table header MUST be present in the first packet of every frame. When the tables are specified out of band, they may be omitted from the packet by setting the Length field in this header to 0.
When the quantization tables are sent in-band, they need not be sent with every frame. Like the out of band case, frames which do not contain tables will have a Quantization Table header with a Length field of 0. While this does decrease the overhead of including the tables, new receivers will be unable to properly decode frames from the time they start up until they receive the tables.
Since JPEG frames can be large, they must often be fragmented. Frames SHOULD be fragmented into packets in a manner avoiding fragmentation at a lower level. If support for partial frame decoding is desired, frames SHOULD be fragmented such that each packet contains an integral number of restart intervals (see below).
Each packet that makes up a single frame MUST have the same timestamp, and the RTP marker bit MUST be set on the last packet in a frame. The fragment offset field of each packet is set to the byte
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offset of its payload data within the original frame. Packets making up a frame SHOULD be sent sequentially and the fragments they contain MUST NOT overlap one another.
An entire frame can be identified as a sequence of packets beginning with a packet having a zero fragment offset and ending with a packet having the RTP marker bit set. Missing packets can be detected either with RTP sequence numbers or with the fragment offset and lengths of each packet. Reassembly could be carried out without the offset field (i.e., using only the RTP marker bit and sequence numbers), but an efficient single-copy implementation would not otherwise be possible in the presence of misordered packets. Moreover, if the last packet of the previous frame (containing the marker bit) were dropped, then a receiver could not always detect that the current frame is entirely intact.
Restart markers indicate a point in the JPEG stream at which the Huffman decoder and DC predictors are reset, allowing partial decoding starting at that point. To fully take advantage of this, however, a decoder must know which MCUs of a frame a particular restart interval encodes. While the original JPEG specification does provide a small sequence number field in the restart markers for this purpose, it is not large enough to properly cope with the loss of an entire packet's worth of data at a typical network MTU size. The RTP/JPEG Restart Marker header contains the additional information needed to accomplish this.
The size of restart intervals SHOULD be chosen to always allow an integral number of restart intervals to fit within a single packet. This will guarantee that packets can be decoded independently from one another. If a restart interval ends up being larger than a packet, the F and L bits in the Restart Marker header can be used to fragment it, but the resulting set of packets must all be received by a decoder for that restart interval to be decoded properly.
Once a decoder has received either a single packet with both the F and L bits set on or a contiguous sequence of packets (based on the RTP sequence number) which begin with an F bit and end with an L bit, it can begin decoding. The position of the MCU at the beginning of the data can be determined by multiplying the Restart Count value by the Restart Interval value. A packet (or group of packets as identified by the F and L bits) may contain any number of consecutive restart intervals.
To accommodate encoders which generate frames with restart markers in them but cannot fragment the data in this manner, the Restart Count
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field may be set to 0x3FFF with the F and L bits both set to 1. This indicates to decoders that the entire frame must be reassembled before decoding it.
RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [6], and any appropriate RTP profile (for example [7]). This implies that confidentiality of the media streams is achieved by encryption. Because the data compression used with this payload format is applied end-to-end, encryption may be performed after compression so there is no conflict between the two operations.
A potential denial-of-service threat exists for data encodings using compression techniques that have non-uniform receiver-end computational load. The attacker can inject pathological datagrams into the stream which are complex to decode and cause the receiver to be overloaded. However, this encoding does not exhibit any significant non-uniformity.
Another potential denial-of-service threat exists around the fragmentation mechanism presented here. Receivers should be prepared to limit the total amount of data associated with assembling received frames so as to avoid resource exhaustion.
As with any IP-based protocol, in some circumstances a receiver may be overloaded simply by the receipt of too many packets, either desired or undesired. Network-layer authentication may be used to discard packets from undesired sources, but the processing cost of the authentication itself may be too high. In a multicast environment, pruning of specific sources will be implemented in a future version of IGMP [8] and in multicast routing protocols to allow a receiver to select which sources are allowed to reach it.
A security review of this payload format found no additional considerations beyond those in the RTP specification.
[6] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996.
[7] Schulzrinne, H., "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 1890, January 1996.
[8] Fenner, W., "Internet Group Management Protocol Version 2", RFC 2236, November 1997.
[9] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[10] Kent C., and J. Mogul, "Fragmentation Considered Harmful", Proceedings of the ACM SIGCOMM '87 Workshop on Frontiers in Computer Communications Technology, August 1987.
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Appendix A
The following code can be used to create a quantization table from a Q factor:
/* * Call MakeTables with the Q factor and two u_char[64] return arrays */ void MakeTables(int q, u_char *lqt, u_char *cqt) { int i; int factor = q;
if (q < 1) factor = 1; if (q > 99) factor = 99; if (q < 50) q = 5000 / factor; else q = 200 - factor*2;
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for (i=0; i < 64; i++) { int lq = (jpeg_luma_quantizer[i] * q + 50) / 100; int cq = (jpeg_chroma_quantizer[i] * q + 50) / 100;
/* Limit the quantizers to 1 <= q <= 255 */ if (lq < 1) lq = 1; else if (lq > 255) lq = 255; lqt[i] = lq;
if (cq < 1) cq = 1; else if (cq > 255) cq = 255; cqt[i] = cq; } }
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Appendix B
The following routines can be used to create the JPEG marker segments corresponding to the table-specification data that is absent from the RTP/JPEG body.
/* * Arguments: * type, width, height: as supplied in RTP/JPEG header * lqt, cqt: quantization tables as either derived from * the Q field using MakeTables() or as specified * in section 4.2. * dri: restart interval in MCUs, or 0 if no restarts. * * p: pointer to return area * * Return value: * The length of the generated headers. * * Generate a frame and scan headers that can be prepended to the * RTP/JPEG data payload to produce a JPEG compressed image in * interchange format (except for possible trailing garbage and * absence of an EOI marker to terminate the scan). */ int MakeHeaders(u_char *p, int type, int w, int h, u_char *lqt, u_char *cqt, u_short dri) { u_char *start = p;
/* convert from blocks to pixels */ w <<= 3; h <<= 3;
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*p++ = 0xff; *p++ = 0xd8; /* SOI */
p = MakeQuantHeader(p, lqt, 0); p = MakeQuantHeader(p, cqt, 1);
The following routine is used to illustrate the RTP/JPEG packet fragmentation and header creation.
For clarity and brevity, the structure definitions are only valid for 32-bit big-endian (most significant octet first) architectures. Bit fields are assumed to be packed tightly in big-endian bit order, with no additional padding. Modifications would be required to construct a portable implementation.
/* * RTP data header from RFC1889 */ typedef struct { unsigned int version:2; /* protocol version */ unsigned int p:1; /* padding flag */ unsigned int x:1; /* header extension flag */ unsigned int cc:4; /* CSRC count */ unsigned int m:1; /* marker bit */ unsigned int pt:7; /* payload type */ u_int16 seq; /* sequence number */ u_int32 ts; /* timestamp */ u_int32 ssrc; /* synchronization source */ u_int32 csrc[1]; /* optional CSRC list */ } rtp_hdr_t;
#define RTP_HDR_SZ 12
/* The following definition is from RFC1890 */ #define RTP_PT_JPEG 26
struct jpeghdr { unsigned int tspec:8; /* type-specific field */ unsigned int off:24; /* fragment byte offset */ u_int8 type; /* id of jpeg decoder params */ u_int8 q; /* quantization factor (or table id) */ u_int8 width; /* frame width in 8 pixel blocks */ u_int8 height; /* frame height in 8 pixel blocks */ };
struct jpeghdr_rst { u_int16 dri; unsigned int f:1; unsigned int l:1; unsigned int count:14; };
/* Procedure SendFrame: * * Arguments: * start_seq: The sequence number for the first packet of the current * frame. * ts: RTP timestamp for the current frame * ssrc: RTP SSRC value * jpeg_data: Huffman encoded JPEG scan data * len: Length of the JPEG scan data * type: The value the RTP/JPEG type field should be set to * typespec: The value the RTP/JPEG type-specific field should be set * to * width: The width in pixels of the JPEG image * height: The height in pixels of the JPEG image * dri: The number of MCUs between restart markers (or 0 if there * are no restart markers in the data * q: The Q factor of the data, to be specified using the Independent * JPEG group's algorithm if 1 <= q <= 99, specified explicitly * with lqt and cqt if q >= 128, or undefined otherwise. * lqt: The quantization table for the luminance channel if q >= 128 * cqt: The quantization table for the chrominance channels if * q >= 128 * * Return value: * the sequence number to be sent for the first packet of the next * frame. * * The following are assumed to be defined: * * PACKET_SIZE - The size of the outgoing packet * send_packet(u_int8 *data, int len) - Sends the packet to the network */
u_int16 SendFrame(u_int16 start_seq, u_int32 ts, u_int32 ssrc, u_int8 *jpeg_data, int len, u_int8 type, u_int8 typespec, int width, int height, int dri, u_int8 q, u_int8 *lqt, u_int8 *cqt) { rtp_hdr_t rtphdr; struct jpeghdr jpghdr; struct jpeghdr_rst rsthdr;
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struct jpeghdr_qtable qtblhdr; u_int8 packet_buf[PACKET_SIZE]; u_int8 *ptr; int bytes_left = len; int seq = start_seq; int pkt_len, data_len;
This section outlines the changes between this document and its precdecessor, RFC 2035. The changes to the protocol were made with an eye towards causing as few interoperability problems between implementations based on the older text and newer implementations, and indeed, many of the obsolete conventions can still be unambiguously decoded by a newer implementation. However, use of the older conventions in newer implementations is strongly discouraged.
o Types 0 and 1 have been augmented to allow for the encoding of interlaced video images, using 2 bits of the type-specific field. See section 4.1 for details.
o There has been discussion in the working group arguing for more flexibility in specifying the JPEG quantization tables. This memo allows table coefficients to be specified explicitly through the use of an optional Quantization Table header, discussed in sections 3.1.8 and 4.2.
o In RFC 2035, the encoding of restart marker information in the Type field made it difficult to add new types. Additionally, the type- specific field was used for the restart count, making it unavailable for other type-specific purposes. This memo moves the restart marker indication to a particular bit in the Type field, and adds an optional header to hold the additional information required, leaving the type-specific field free for its intended purpose. The handling of partial frame decoding was also made more robust against packet loss. See sections 3.1.7 and 4.4 for details.
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