AVT J. Ott Internet-Draft Univ. Bremen Expires: October 28, 2005 G. Sullivan Microsoft S. Wenger TU Berlin R. Even Polycom April 26, 2005 RTP Payload Format for the 1998 Version of ITU-T Rec. H.263 Video (H.263+) draft-ietf-avt-rfc2429-bis-05.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 28, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document describes a scheme to packetize an H.263 video stream for transport using the Real-time Transport Protocol, RTP, with any Ott, et al. Expires October 28, 2005 [Page 1] Internet-Draft H.263 RTP payload format April 2005 of the underlying protocols that carry RTP. The document also describe the syntax and semantics of the SDP parameters needed to support the H.263 video codec. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. New H.263 features . . . . . . . . . . . . . . . . . . . . . . 4 3. Usage of RTP . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 RTP Header Usage . . . . . . . . . . . . . . . . . . . . . 6 3.2 Video Packet Structure . . . . . . . . . . . . . . . . . . 7 4. Design Considerations . . . . . . . . . . . . . . . . . . . . 9 5. H.263+ Payload Header . . . . . . . . . . . . . . . . . . . . 11 5.1 General H.263+ payload header . . . . . . . . . . . . . . 11 5.2 Video Redundancy Coding Header Extension . . . . . . . . . 12 6. Packetization schemes . . . . . . . . . . . . . . . . . . . . 15 6.1 Picture Segment Packets and Sequence Ending Packets (P=1) . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1.1 Packets that begin with a Picture Start Code . . . . . 15 6.1.2 Packets that begin with GBSC or SSC . . . . . . . . . 16 6.1.3 Packets that Begin with an EOS or EOSBS Code . . . . . 17 6.2 Encapsulating Follow-On Packet (P=0) . . . . . . . . . . . 17 7. Use of this payload specification . . . . . . . . . . . . . . 19 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 8.1 Media Type Registrations . . . . . . . . . . . . . . . . . 21 8.1.1 Registration of MIME media type video/H263-1998 . . . 21 8.1.2 Registration of MIME media type video/H263-2000 . . . 24 8.2 SDP Parameters . . . . . . . . . . . . . . . . . . . . . . 25 8.2.1 Usage with the SDP Offer Answer Model . . . . . . . . 26 9. Backward Compatibility to RFC2429 . . . . . . . . . . . . . . 28 9.1 New SDP optional parameters . . . . . . . . . . . . . . . 28 10. Security Considerations . . . . . . . . . . . . . . . . . . 29 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 12. Changes from previous versions of the document . . . . . . . 31 12.1 changes from RFC 2429 . . . . . . . . . . . . . . . . . . 31 12.2 Changes from rfc2429-bis-00 . . . . . . . . . . . . . . . 31 12.3 Changes from rfc2429-bis-03 . . . . . . . . . . . . . . . 31 12.4 Changes from rfc2429-bis-03 . . . . . . . . . . . . . . . 31 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 13.1 Normative References . . . . . . . . . . . . . . . . . . . 32 13.2 Informative References . . . . . . . . . . . . . . . . . . 33 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 33 Intellectual Property and Copyright Statements . . . . . . . . 34 Ott, et al. Expires October 28, 2005 [Page 2] Internet-Draft H.263 RTP payload format April 2005 1. Introduction This document specifies an RTP payload header format applicable to the transmission of video streams generated based on the 1998 and 2000 versions of ITU-T Recommendation H.263 [H263P]. Because the 1998 and 2000 versions of H.263 are a superset of the 1996 syntax, this format can also be used with the 1996 version of H.263 [H263], and is recommended for this use by new implementations. This format replaces the payload format in RFC 2190[RFC2190], which continues to be used by some existing implementations, and can be useful for backward compatibility. New implementations supporting H.263 will use the payload format described in this document. This document updates RFC2429. 1.1 Terminology 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 RFC2119[RFC2119] and indicate requirement levels for compliant RTP implementations. Ott, et al. Expires October 28, 2005 [Page 3] Internet-Draft H.263 RTP payload format April 2005 2. New H.263 features The 1998 version of ITU-T Recommendation H.263 added numerous coding options to improve codec performance over the 1996 version. The 1998 version is referred to as H.263+ in this document. The 2000 version is referred to as H.263++ in this document. Among the new options, the ones with the biggest impact on the RTP payload specification and the error resilience of the video content are the slice structured mode, the independent segment decoding mode, the reference picture selection mode, and the scalability mode. This section summarizes the impact of these new coding options on packetization. Refer to [H263P] for more information on coding options. The slice structured mode was added to H.263+ for three purposes: to provide enhanced error resilience capability, to make the bitstream more amenable to use with an underlying packet transport such as RTP, and to minimize video delay. The slice structured mode supports fragmentation at macroblock boundaries. With the independent segment decoding (ISD) option, a video picture frame is broken into segments and encoded in such a way that each segment is independently decodable. Utilizing ISD in a lossy network environment helps to prevent the propagation of errors from one segment of the picture to others. The reference picture selection mode allows the use of an older reference picture rather than the one immediately preceding the current picture. Usually, the last transmitted frame is implicitly used as the reference picture for inter-frame prediction. If the reference picture selection mode is used, the data stream carries information on what reference frame should be used, indicated by the temporal reference as an ID for that reference frame. The reference picture selection mode may be used with or without a back channel, which provides information to the encoder about the internal status of the decoder. However, no special provision is made herein for carrying back channel information. The Extended RTP Profile for RTCP-based Feedback [AVPF] MAY be used as a back channel mechanism. H.263+ also includes bitstream scalability as an optional coding mode. Three kinds of scalability are defined: temporal, signal-to- noise ratio (SNR), and spatial scalability. Temporal scalability is achieved via the disposable nature of bi-directionally predicted frames, or B-frames. (A low-delay form of temporal scalability known as P-picture temporal scalability can also be achieved by using the reference picture selection mode described in the previous paragraph.) SNR scalability permits refinement of encoded video Ott, et al. Expires October 28, 2005 [Page 4] Internet-Draft H.263 RTP payload format April 2005 frames, thereby improving the quality (or SNR). Spatial scalability is similar to SNR scalability except the refinement layer is twice the size of the base layer in the horizontal dimension, vertical dimension, or both. Ott, et al. Expires October 28, 2005 [Page 5] Internet-Draft H.263 RTP payload format April 2005 3. Usage of RTP When transmitting H.263+ video streams over the Internet, the output of the encoder can be packetized directly. All the bits resulting from the bitstream including the fixed length codes and variable length codes will be included in the packet, with the only exception being that when the payload of a packet begins with a Picture, GOB, Slice, EOS, or EOSBS start code, the first two (all-zero) bytes of the start code shall be removed and replaced by setting an indicator bit in the payload header. For H.263+ bitstreams coded with temporal, spatial, or SNR scalability, each layer may be transported to a different network address. More specifically, each layer may use a unique IP address and port number combination. The temporal relations between layers shall be expressed using the RTP timestamp so that they can be synchronized at the receiving ends in multicast or unicast applications. The H.263+ video stream will be carried as payload data within RTP packets. A new H.263+ payload header is defined in section 3. This section defines the usage of the RTP fixed header and H.263+ video packet structure. 3.1 RTP Header Usage Each RTP packet starts with a fixed RTP header. The following fields of the RTP fixed header are used for H.263+ video streams: Marker bit (M bit): The Marker bit of the RTP header is set to 1 when the current packet carries the end of current frame, and is 0 otherwise. Payload Type (PT): The RTP profile for a particular class of applications will assign a payload type for this encoding, or if that is not done, then a payload type in the dynamic range shall be chosen by the sender. Timestamp: The RTP Timestamp encodes the sampling instance of the first video frame data contained in the RTP data packet. The RTP timestamp shall be the same on successive packets if a video frame occupies more than one packet. In a multilayer scenario, all pictures corresponding to the same temporal reference should use the same timestamp. If temporal scalability is used (if B-frames are present), the timestamp may not be monotonically increasing in the RTP stream. If B-frames are transmitted on a separate layer and address, they must be synchronized properly with the reference frames. Refer to the 1998 ITU-T Recommendation H.263 [H263P] for Ott, et al. Expires October 28, 2005 [Page 6] Internet-Draft H.263 RTP payload format April 2005 information on required transmission order to a decoder. For an H.263+ video stream, the RTP timestamp is based on a 90 kHz clock, the same as that of the RTP payload for H.261 stream [RFC2032]. Since both the H.263+ data and the RTP header contain time information, those timing information must run synchronously. That is, both the RTP timestamp and the temporal reference (TR in the picture header of H.263) should carry the same relative timing information. Any H.263+ picture clock frequency can be expressed as 1800000/(cd*cf) source pictures per second, in which cd is an integer from 1 to 127 and cf is either 1000 or 1001. Using the 90 kHz clock of the RTP timestamp, the time increment between each coded H.263+ picture should therefore be a integer multiple of (cd*cf)/20. This will always be an integer for any "reasonable" picture clock frequency (for example, it is 3003 for 29.97 Hz NTSC, 3600 for 25 Hz PAL, 3750 for 24 Hz film, and 1500, 1250 and 1200 for the computer display update rates of 60, 72 and 75 Hz, respectively). For RTP packetization of hypothetical H.263+ bitstreams using "unreasonable" custom picture clock frequencies, mathematical rounding could become necessary for generating the RTP timestamps. 3.2 Video Packet Structure A section of an H.263+ compressed bitstream is carried as a payload within each RTP packet. For each RTP packet, the RTP header is followed by an H.263+ payload header, which is followed by a number of bytes of a standard H.263+ compressed bitstream. The size of the H.263+ payload header is variable depending on the payload involved as detailed in the section 4. The layout of the RTP H.263+ video packet is shown as: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | H.263+ Payload Header +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | H.263+ Compressed Data Stream +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Any H.263+ start codes can be byte aligned by an encoder by using the stuffing mechanisms of H.263+. As specified in H.263+, picture, slice, and EOSBS starts codes shall always be byte aligned, and GOB and EOS start codes may be byte aligned. For packetization purposes, GOB start codes should be byte aligned; however, since this is not required in H.263+, there may be some cases where GOB start codes are not aligned, such as when transmitting existing content, or when Ott, et al. Expires October 28, 2005 [Page 7] Internet-Draft H.263 RTP payload format April 2005 using H.263 encoders that do not support GOB start code alignment. In this case, follow-on packets (see section 5.2) should be used for packetization. All H.263+ start codes (Picture, GOB, Slice, EOS, and EOSBS) begin with 16 zero-valued bits. If a start code is byte aligned and it occurs at the beginning of a packet, these two bytes shall be removed from the H.263+ compressed data stream in the packetization process and shall instead be represented by setting a bit (the P bit) in the payload header. Ott, et al. Expires October 28, 2005 [Page 8] Internet-Draft H.263 RTP payload format April 2005 4. Design Considerations The goals of this payload format are to specify an efficient way of encapsulating an H.263+ standard compliant bitstream and to enhance the resiliency towards packet losses. Due to the large number of different possible coding schemes in H.263+, a copy of the picture header with configuration information is inserted into the payload header when appropriate. The use of that copy of the picture header along with the payload data can allow decoding of a received packet even in such cases in which another packet containing the original picture header becomes lost. There are a few assumptions and constraints associated with this H.263+ payload header design. The purpose of this section is to point out various design issues and also to discuss several coding options provided by H.263+ that may impact the performance of network-based H.263+ video. o The optional slice structured mode described in Annex K of H.263+ [H263P] enables more flexibility for packetization. Similar to a picture segment that begins with a GOB header, the motion vector predictors in a slice are restricted to reside within its boundaries. However, slices provide much greater freedom in the selection of the size and shape of the area which is represented as a distinct decodable region. In particular, slices can have a size which is dynamically selected to allow the data for each slice to fit into a chosen packet size. Slices can also be chosen to have a rectangular shape which is conducive for minimizing the impact of errors and packet losses on motion compensated prediction. For these reasons, the use of the slice structured mode is strongly recommended for any applications used in environments where significant packet loss occurs. o In non-rectangular slice structured mode, only complete slices should be included in a packet. In other words, slices should not be fragmented across packet boundaries. The only reasonable need for a slice to be fragmented across packet boundaries is when the encoder which generated the H.263+ data stream could not be influenced by an awareness of the packetization process (such as when sending H.263+ data through a network other than the one to which the encoder is attached, as in network gateway implementations). Optimally, each packet will contain only one slice. o The independent segment decoding (ISD) described in Annex R of [H263P] prevents any data dependency across slice or GOB boundaries in the reference picture. It can be utilized to further improve resiliency in high loss conditions. Ott, et al. Expires October 28, 2005 [Page 9] Internet-Draft H.263 RTP payload format April 2005 o If ISD is used in conjunction with the slice structure, the rectangular slice submode shall be enabled and the dimensions and quantity of the slices present in a frame shall remain the same between each two intra-coded frames (I-frames), as required in H.263+. The individual ISD segments may also be entirely intra coded from time to time to realize quick error recovery without adding the latency time associated with sending complete INTRA- pictures. o When the slice structure is not applied, the insertion of a (preferably byte-aligned) GOB header can be used to provide resync boundaries in the bitstream, as the presence of a GOB header eliminates the dependency of motion vector prediction across GOB boundaries. These resync boundaries provide natural locations for packet payload boundaries. o H.263+ allows picture headers to be sent in an abbreviated form in order to prevent repetition of overhead information that does not change from picture to picture. For resiliency, sending a complete picture header for every frame is often advisable. This means that (especially in cases with high packet loss probability in which picture header contents are not expected to be highly predictable), the sender may find it advisable to always set the subfield UFEP in PLUSPTYPE to '001' in the H.263+ video bitstream. (See [H263P] for the definition of the UFEP and PLUSPTYPE fields). o In a multi-layer scenario, each layer may be transmitted to a different network address. The configuration of each layer such as the enhancement layer number (ELNUM), reference layer number (RLNUM), and scalability type should be determined at the start of the session and should not change during the course of the session. o All start codes can be byte aligned, and picture, slice, and EOSBS start codes are always byte aligned. The boundaries of these syntactical elements provide ideal locations for placing packet boundaries. o We assume that a maximum Picture Header size of 504 bits is sufficient. The syntax of H.263+ does not explicitly prohibit larger picture header sizes, but the use of such extremely large picture headers is not expected. Ott, et al. Expires October 28, 2005 [Page 10] Internet-Draft H.263 RTP payload format April 2005 5. H.263+ Payload Header For H.263+ video streams, each RTP packet shall carry only one H.263+ video packet. The H.263+ payload header shall always be present for each H.263+ video packet. The payload header is of variable length. A 16 bit field of the basic payload header may be followed by an 8 bit field for Video Redundancy Coding (VRC) information, and/or by a variable length extra picture header as indicated by PLEN. These optional fields appear in the order given above when present. If an extra picture header is included in the payload header, the length of the picture header in number of bytes is specified by PLEN. The minimum length of the payload header is 16 bits, corresponding to PLEN equal to 0 and no VRC information present. The remainder of this section defines the various components of the RTP payload header. Section six defines the various packet types that are used to carry different types of H.263+ coded data, and section seven summarizes how to distinguish between the various packet types. 5.1 General H.263+ payload header The H.263+ payload header is structured as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RR |P|V| PLEN |PEBIT| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ RR: 5 bits Reserved bits. Shall be zero. MUST be ignored by receivers P: 1 bit Indicates the picture start or a picture segment (GOB/Slice) start or a video sequence end (EOS or EOSBS). Two bytes of zero bits then have to be prefixed to the payload of such a packet to compose a complete picture/GOB/slice/EOS/EOSBS start code. This bit allows the omission of the two first bytes of the start codes, thus improving the compression ratio. V: 1 bit Ott, et al. Expires October 28, 2005 [Page 11] Internet-Draft H.263 RTP payload format April 2005 Indicates the presence of an 8 bit field containing information for Video Redundancy Coding (VRC), which follows immediately after the initial 16 bits of the payload header if present. For syntax and semantics of that 8 bit VRC field see section 5.2. PLEN: 6 bits Length in bytes of the extra picture header. If no extra picture header is attached, PLEN is 0. If PLEN>0, the extra picture header is attached immediately following the rest of the payload header. Note the length reflects the omission of the first two bytes of the picture start code (PSC). See section 6.1. PEBIT: 3 bits Indicates the number of bits that shall be ignored in the last byte of the picture header. If PLEN is not zero, the ignored bits shall be the least significant bits of the byte. If PLEN is zero, then PEBIT shall also be zero. 5.2 Video Redundancy Coding Header Extension Video Redundancy Coding (VRC) is an optional mechanism intended to improve error resilience over packet networks. Implementing VRC in H.263+ will require the Reference Picture Selection option described in Annex N of [H263P]. By having multiple "threads" of independently inter-frame predicted pictures, damage of individual frame will cause distortions only within its own thread but leave the other threads unaffected. From time to time, all threads converge to a so-called sync frame (an INTRA picture or a non-INTRA picture which is redundantly represented within multiple threads); from this sync frame, the independent threads are started again. For more information on codec support for VRC see [Vredun]. P-picture temporal scalability is another use of the reference picture selection mode and can be considered a special case of VRC in which only one copy of each sync frame may be sent. It offers a thread-based method of temporal scalability without the increased delay caused by the use of B pictures. In this use, sync frames sent in the first thread of pictures are also used for the prediction of a second thread of pictures which fall temporally between the sync frames to increase the resulting frame rate. In this use, the pictures in the second thread can be discarded in order to obtain a reduction of bit rate or decoding complexity without harming the ability to decode later pictures. A third or more threads can also be added as well, but each thread is predicted only from the sync frames (which are sent at least in thread 0) or from frames within Ott, et al. Expires October 28, 2005 [Page 12] Internet-Draft H.263 RTP payload format April 2005 the same thread. While a VRC data stream is - like all H.263+ data - totally self- contained, it may be useful for the transport hierarchy implementation to have knowledge about the current damage status of each thread. On the Internet, this status can easily be determined by observing the marker bit, the sequence number of the RTP header, and the thread-id and a circling "packet per thread" number. The latter two numbers are coded in the VRC header extension. The format of the VRC header extension is as follows: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | TID | Trun |S| +-+-+-+-+-+-+-+-+ TID: 3 bits Thread ID. Up to 7 threads are allowed. Each frame of H.263+ VRC data will use as reference information only sync frames or frames within the same thread. By convention, thread 0 is expected to be the "canonical" thread, which is the thread from which the sync frame should ideally be used. In the case of corruption or loss of the thread 0 representation, a representation of the sync frame with a higher thread number can be used by the decoder. Lower thread numbers are expected to contain equal or better representations of the sync frames than higher thread numbers in the absence of data corruption or loss. See [Vredun] for a detailed discussion of VRC. Trun: 4 bits Monotonically increasing (modulo 16) 4 bit number counting the packet number within each thread. S: 1 bit A bit that indicates that the packet content is for a sync frame. An encoder using VRC may send several representations of the same "sync" picture, in order to ensure that regardless of which thread of pictures is corrupted by errors or packet losses, the reception of at least one representation of a particular picture is ensured (within at least one thread). The sync picture can then be used for the prediction of any thread. If packet losses have not occurred, then the sync frame contents of thread 0 can be used and those of other threads can be discarded (and similarly for other threads). Thread 0 is considered the "canonical" thread, the use of which is preferable to all others. The contents of packets having lower thread numbers Ott, et al. Expires October 28, 2005 [Page 13] Internet-Draft H.263 RTP payload format April 2005 shall be considered as having a higher processing and delivery priority than those with higher thread numbers. Thus packets having lower thread numbers for a given sync frame shall be delivered first to the decoder under loss-free and low-time-jitter conditions, which will result in the discarding of the sync contents of the higher- numbered threads as specified in Annex N of [H263P]. Ott, et al. Expires October 28, 2005 [Page 14] Internet-Draft H.263 RTP payload format April 2005 6. Packetization schemes 6.1 Picture Segment Packets and Sequence Ending Packets (P=1) A picture segment packet is defined as a packet that starts at the location of a Picture, GOB, or slice start code in the H.263+ data stream. This corresponds to the definition of the start of a video picture segment as defined in H.263+. For such packets, P=1 always. An extra picture header can sometimes be attached in the payload header of such packets. Whenever an extra picture header is attached as signified by PLEN>0, only the last six bits of its picture start code, '100000', are included in the payload header. A complete H.263+ picture header with byte aligned picture start code can be conveniently assembled on the receiving end by prepending the sixteen leading '0' bits. When PLEN>0, the end bit position corresponding to the last byte of the picture header data is indicated by PEBIT. The actual bitstream data shall begin on an 8-bit byte boundary following the payload header. A sequence ending packet is defined as a packet that starts at the location of an EOS or EOSBS code in the H.263+ data stream. This delineates the end of a sequence of H.263+ video data (more H.263+ video data may still follow later, however, as specified in ITU-T Recommendation H.263). For such packets, P=1 and PLEN=0 always. The optional header extension for VRC may or may not be present as indicated by the V bit flag. 6.1.1 Packets that begin with a Picture Start Code Any packet that contains the whole or the start of a coded picture shall start at the location of the picture start code (PSC), and should normally be encapsulated with no extra copy of the picture header. In other words, normally PLEN=0 in such a case. However, if the coded picture contains an incomplete picture header (UFEP = "000"), then a representation of the complete (UFEP = "001") picture header may be attached during packetization in order to provide greater error resilience. Thus, for packets that start at the location of a picture start code, PLEN shall be zero unless both of the following conditions apply: 1) The picture header in the H.263+ bitstream payload is incomplete (PLUSPTYPE present and UFEP="000"), and 2) The additional picture header which is attached is not incomplete Ott, et al. Expires October 28, 2005 [Page 15] Internet-Draft H.263 RTP payload format April 2005 (UFEP="001"). A packet which begins at the location of a Picture, GOB, slice, EOS, or EOSBS start code shall omit the first two (all zero) bytes from the H.263+ bitstream, and signify their presence by setting P=1 in the payload header. Here is an example of encapsulating the first packet in a frame (without an attached redundant complete picture header): 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RR |1|V|0|0|0|0|0|0|0|0|0| bitstream data without the | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | first two 0 bytes of the PSC +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 6.1.2 Packets that begin with GBSC or SSC For a packet that begins at the location of a GOB or slice start code, PLEN may be zero or may be nonzero, depending on whether a redundant picture header is attached to the packet. In environments with very low packet loss rates, or when picture header contents are very seldom likely to change (except as can be detected from the GFID syntax of H.263+), a redundant copy of the picture header is not required. However, in less ideal circumstances a redundant picture header should be attached for enhanced error resilience, and its presence is indicated by PLEN>0. Assuming a PLEN of 9 and P=1, below is an example of a packet that begins with a byte aligned GBSC or a SSC: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RR |1|V|0 0 1 0 0 1|PEBIT|1 0 0 0 0 0| picture header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | starting with TR, PTYPE ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | bitstream | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data starting with GBSC/SSC without its first two 0 bytes +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Notice that only the last six bits of the picture start code, '100000', are included in the payload header. A complete H.263+ Ott, et al. Expires October 28, 2005 [Page 16] Internet-Draft H.263 RTP payload format April 2005 picture header with byte aligned picture start code can be conveniently assembled if needed on the receiving end by prepending the sixteen leading '0' bits. 6.1.3 Packets that Begin with an EOS or EOSBS Code For a packet that begins with an EOS or EOSBS code, PLEN shall be zero, and no Picture, GOB, or Slice start codes shall be included within the same packet. As with other packets beginning with start codes, the two all-zero bytes that begin the EOS or EOSBS code at the beginning of the packet shall be omitted, and their presence shall be indicated by setting the P bit to 1 in the payload header. System designers should be aware that some decoders may interpret the loss of a packet containing only EOS or EOSBS information as the loss of essential video data and may thus respond by not displaying some subsequent video information. Since EOS and EOSBS codes do not actually affect the decoding of video pictures, they are somewhat unnecessary to send at all. Because of the danger of misinterpretation of the loss of such a packet (which can be detected by the sequence number), encoders are generally to be discouraged from sending EOS and EOSBS. Below is an example of a packet containing an EOS code: 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RR |1|V|0|0|0|0|0|0|0|0|0|1|1|1|1|1|1|0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 6.2 Encapsulating Follow-On Packet (P=0) A Follow-on packet contains a number of bytes of coded H.263+ data which does not start at a synchronization point. That is, a Follow-On packet does not start with a Picture, GOB, Slice, EOS, or EOSBS header, and it may or may not start at a macroblock boundary. Since Follow-on packets do not start at synchronization points, the data at the beginning of a follow-on packet is not independently decodable. For such packets, P=0 always. If the preceding packet of a Follow-on packet got lost, the receiver may discard that Follow-on packet as well as all other following Follow-on packets. Better behavior, of course, would be for the receiver to scan the interior of the packet payload content to determine whether any start codes are found in the interior of the packet which can be used as resync points. The use of an attached copy of a picture header for a follow-on packet is useful only if the interior of the packet or some Ott, et al. Expires October 28, 2005 [Page 17] Internet-Draft H.263 RTP payload format April 2005 subsequent follow-on packet contains a resync code such as a GOB or slice start code. PLEN>0 is allowed, since it may allow resync in the interior of the packet. The decoder may also be resynchronized at the next segment or picture packet. Here is an example of a follow-on packet (with PLEN=0): 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- | RR |0|V|0|0|0|0|0|0|0|0|0| bitstream data +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Ott, et al. Expires October 28, 2005 [Page 18] Internet-Draft H.263 RTP payload format April 2005 7. Use of this payload specification There is no syntactical difference between a picture segment packet and a Follow-on packet, other than the indication P=1 for picture segment or sequence ending packets and P=0 for Follow-on packets. See the following for a summary of the entire packet types and ways to distinguish between them. It is possible to distinguish between the different packet types by checking the P bit and the first 6 bits of the payload along with the header information. The following table shows the packet type for permutations of this information (see also the picture/GOB/Slice header descriptions in H.263+ for details): -------------+--------------+----------------------+---------------- First 6 bits | P-Bit | PLEN | Packet | Remarks of Payload |(payload hdr.)| | -------------+--------------+----------------------+---------------- 100000 | 1 | 0 | Picture | Typical Picture 100000 | 1 | > 0 | Picture | Note UFEP 1xxxxx | 1 | 0 | GOB/Slice/EOS/EOSBS | See possible GNs 1xxxxx | 1 | > 0 | GOB/Slice | See possible GNs Xxxxxx | 0 | 0 | Follow-on | Xxxxxx | 0 | > 0 | Follow-on | Interior Resync -------------+--------------+----------------------+---------------- The details regarding the possible values of the five bit Group Number (GN) field which follows the initial "1" bit when the P-bit is "1" for a GOB, Slice, EOS, or EOSBS packet are found in section 5.2.3 of H.263[H263P]. As defined in this specification, every start of a coded frame (as indicated by the presence of a PSC) has to be encapsulated as a picture segment packet. If the whole coded picture fits into one packet of reasonable size (which is dependent on the connection characteristics), this is the only type of packet that may need to be used. Due to the high compression ratio achieved by H.263+ it is often possible to use this mechanism, especially for small spatial picture formats such as QCIF and typical Internet packet sizes around 1500 bytes. If the complete coded frame does not fit into a single packet, two different ways for the packetization may be chosen. In case of very low or zero packet loss probability, one or more Follow-on packets may be used for coding the rest of the picture. Doing so leads to minimal coding and packetization overhead as well as to an optimal use of the maximal packet size, but does not provide any added error resilience. Ott, et al. Expires October 28, 2005 [Page 19] Internet-Draft H.263 RTP payload format April 2005 The alternative is to break the picture into reasonably small partitions - called Segments - (by using the Slice or GOB mechanism), that do offer synchronization points. By doing so and using the Picture Segment payload with PLEN>0, decoding of the transmitted packets is possible even in such cases in which the Picture packet containing the picture header was lost (provided any necessary reference picture is available). Picture Segment packets can also be used in conjunction with Follow-on packets for large segment sizes. Ott, et al. Expires October 28, 2005 [Page 20] Internet-Draft H.263 RTP payload format April 2005 8. IANA Considerations This section updates the H.263(1998) and H.263 (2000) media types described in RFC3555 [RFC3555]. This section specifies optional parameters that MAY be used to select optional features of the H.263 codec. The parameters are specified here as part of the MIME subtype registration for the ITU-T H.263 codec. A mapping of the parameters into the Session Description Protocol (SDP)[RFC2327] is also provided for those applications that use SDP. Multiple parameters SHOULD be expressed as a MIME media type string, in the form of a semicolon separated list of parameter=value pairs 8.1 Media Type Registrations This section describes the Media types and names associated with this payload format. The section updates the previous registered version in RFC 3555 [RFC3555]. The section registers the Media types, as per RFC2048[RFC2048] 8.1.1 Registration of MIME media type video/H263-1998 MIME media type name: video MIME subtype name: H263-1998 Required parameters: None Optional parameters: SQCIF: Specifies the MPI (Minimum Picture Interval) for SQCIF resolution. Permissible value are integer values from 1 to 32, which correspond to a maximum frame rate of 29.97/ the specified value frames per second. QCIF: Specifies the MPI (Minimum Picture Interval) for QCIF resolution. Permissible value are integer values from 1 to 32, which correspond to a maximum frame rate of 29.97/ the specified value frames per second. CIF: Specifies the MPI (Minimum Picture Interval) for CIF resolution. Permissible value are integer values from 1 to 32, which correspond to a maximum frame rate of 29.97/ the specified value frames per second. CIF4: Specifies the MPI (Minimum Picture Interval) for 4CIF Ott, et al. Expires October 28, 2005 [Page 21] Internet-Draft H.263 RTP payload format April 2005 resolution. Permissible value are integer values from 1 to 32, which correspond to a maximum frame rate of 29.97/ the specified value frames per second. CIF16: Specifies the MPI (Minimum Picture Interval) for 16CIF resolution. Permissible value are integer values from 1 to 32, which correspond to a maximum frame rate of 29.97/ the specified value frames per second. CUSTOM: Specifies the MPI (Minimum Picture Interval) for a custom defined resolution. The custom parameter receives three comma separated values Xmax , Ymax and MPI. The Xmax and Ymax parameters describe the number of pixels in the X and Y axis and must be evenly dividable by 4. The permissible value for MPI, are integer values from 1 to 32, which correspond to a maximum frame rate of 29.97/ the specified value. A system that declares support of a specific MPI for one of the resolutions SHALL also implicitly support a lower resolution with the same MPI. A list of optional annexes specifies which annexes of H.263 are supported. The optional annexes are defined as part of H263-1998, H263-2000. Annex X of H.263-2000 defines profiles which group annexes for specific applications. A system that support a specific annex SHALL specify it support using the optional parameters. The allowed optional parameters for the annexes are "F", "I", "J", "T" which do not get any values and "K", "N" and "P". "K" can receive one of four values 1-4: 1: - Slices In Order-Non Rectangular 2: - Slices In Order-Rectangular 3: - Slices Not Ordered - Non Rectangular 4: - Slices Not Ordered - Rectangular "N" - Reference Picture Selection mode - Four numeric choices (1-4) are available representing the following modes: 1: NEITHER: In which no back-channel data is returned from the decoder to the encoder. 2: ACK: In which the decoder returns only acknowledgment messages. Ott, et al. Expires October 28, 2005 [Page 22] Internet-Draft H.263 RTP payload format April 2005 3: NACK: In which the decoder returns only non-acknowledgment messages 4: ACK+NACK: In which the decoder returns both acknowledgment and non-acknowledgment messages. No special provision is made herein for carrying back channel information. The Extended RTP Profile for RTCP-based Feedback [AVPF] MAY be used as a back channel mechanism. "P" - Reference Picture Resampling with the following submodes represented as a number fro 1 to 4: 1: dynamicPictureResizingByFour 2: dynamicPictureResizingBySixteenthPel 3: dynamicWarpingHalfPel 4: dynamicWarpingSixteenthPel Example: P=1,3 PAR - Arbitrary Pixel Aspect Ratio : defines the width:height ratio by two colon separated integers between 0 and 255. Default ratio is 12:11 if not otherwise specified. CPCF - Arbitrary (Custom) Picture Clock Frequency: CPCF is a floating point value. Default value is 29.97 Hz. BPP - BitsPerPictureMaxKb: Maximum number of bits in units of 1024 bits allowed to represent a single picture. If this parameter is not present, then the default value, based on the maximum supported resolution, is used. BPP is integer value between 0 and 65536. HRD - Hypothetical Reference Decoder: See annex B of H.263 specification[H263P]. Encoding considerations: This type is only defined for transfer via RTP [RFC3550] Security considerations: See Section 10 Interoperability considerations: These are receiver options, current implementations will not send any optional parameters in their SDP. They will ignore the optional parameters and will encode the H.263 stream without any of the annexes. Most decoders support at least Ott, et al. Expires October 28, 2005 [Page 23] Internet-Draft H.263 RTP payload format April 2005 QCIF and CIF fixed resolutions and they are expected to be available almost in every H.263 based video application. Published specification: RFC yyy ( This RFC) Applications which use this media type: Audio and video streaming and conferencing tools. Additional information: none Person and email address to contact for further information : Roni Even: roni.even@polycom.co.il Intended usage: COMMON Author: Roni Even Change controller: IETF Audio/Video Transport working group delegated from the IESG. 8.1.2 Registration of MIME media type video/H263-2000 MIME media type name: video MIME subtype name: H263-2000 Required parameters: None Optional parameters: The optional parameters of the H263-1998 type MAY be used with this MIME subtype. Specific optional parameters that may be used with the H263-2000 type are: PROFILE: H.263 profile number, in the range 0 through 10, specifying the supported H.263 annexes/subparts based on H.263 annex X. LEVEL: Level of bitstream operation, in the range 0 through 100, specifying the level of computational complexity of the decoding process. A system that specifies support of a PROFILE MUST specify the supported LEVEL. Ott, et al. Expires October 28, 2005 [Page 24] Internet-Draft H.263 RTP payload format April 2005 INTERLACE: Interlaced or 60 fields indicates the support for interlace display mode. Encoding considerations: This type is only defined for transfer via RTP [RFC3550] Security considerations: See Section 10 Interoperability considerations: The optional parameters PROFILE and LEVEL SHALL NOT be used with any of the other optional parameters. Published specification: RFC yyy Applications which use this media type: Audio and video streaming and conferencing tools. Additional information: none Person and email address to contact for further information : Roni Even: roni.even@polycom.co.il Intended usage: COMMON Author: Roni Even Change controller: IETF Audio/Video Transport working group delegated from the IESG. 8.2 SDP Parameters The media types video/H263-1998 and video/H263-2000 are mapped to fields in the Session Description Protocol (SDP) as follows: o The media name in the "m=" line of SDP MUST be video. o The encoding name in the "a=rtpmap" line of SDP MUST be H263-1998 or H263-2000 (the MIME subtype). o The clock rate in the "a=rtpmap" line MUST be 90000. o The optional parameters if any, MUST be included in the "a=fmtp" line of SDP. These parameters are expressed as a media type string, Ott, et al. Expires October 28, 2005 [Page 25] Internet-Draft H.263 RTP payload format April 2005 in the form of a semicolon separated list of parameter=value pairs. The optional parameters PROFILE and LEVEL SHALL NOT be used with any of the other optional parameters. 8.2.1 Usage with the SDP Offer Answer Model When offering H.263 over RTP using SDP in an Offer/Answer model[RFC3264] the following considerations are necessary. Codec options: (F,I,J,K,N,P,T) These options MUST NOT appear unless the sender of this SDP message is able to decode those options. These options are receiver's capability even when send in a "sendonly" offer. Picture sizes and MPI: Supported picture sizes and their corresponding minimum picture interval (MPI) information for H.263 can be combined. All picture sizes can be advertised to the other party, or only a subset of it. Terminal announces only those picture sizes (with their MPIs) which it is willing to receive. For example, MPI=2 means that maximum (decodeable) picture rate per second is about 15. If the receiver does not specify the picture size /MPI optional parameter then it SHOULD be ready to receive QCIF resolution with MPI=1. Parameters offered first are the most preferred picture mode to be received. Example of the usage of these parameters: CIF=4;QCIF=3;SQCIF=2;CUSTOM=360,240,2 This means that encoder SHOULD send CIF picture size, which it can decode at MPI=4. If that is not possible, then QCIF with MPI value 3, if neither are possible, then SQCIF with MPI value=2. The receiver is able of (but least preferred) decoding custom picture sizes (max 360x240) with MPI=2. Note that most decoders support at least QCIF and CIF fixed resolutions and they are expected to be available almost in every H.263 based video application. Below is an example of H.263 SDP in an offer. a=fmtp:xx CIF=4;QCIF=2;F;K=1 This means that the sender of this message can decode H.263 bit stream with following options and parameters: Preferred resolution is Ott, et al. Expires October 28, 2005 [Page 26] Internet-Draft H.263 RTP payload format April 2005 CIF (its MPI is 4), but if that is not possible then QCIF size is ok. AP and slicesInOrder-NonRect options MAY be used. Ott, et al. Expires October 28, 2005 [Page 27] Internet-Draft H.263 RTP payload format April 2005 9. Backward Compatibility to RFC2429 The current draft updates RFC2429. This section will address the major backward compatibility issues. 9.1 New SDP optional parameters The draft adds new optional parameters to the H263-1998 and H263-2000 payload type. Since these are optional parameters we expect old implementation to ignore these parameters while new implementations that will receive the H263-1998 and H263-2000 payload type with no parameters will behave as if the other side can accept H.263 at QCIF resolution and 30 frames per second. Ott, et al. Expires October 28, 2005 [Page 28] Internet-Draft H.263 RTP payload format April 2005 10. Security Considerations RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [RFC3550], and any appropriate RTP profile (for example [RFC3551]). 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 encoding 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. The usage of authentication of at least the RTP packet is RECOMMENDED 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 may be implemented in future versions of IGMP [RFC2032] 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. Ott, et al. Expires October 28, 2005 [Page 29] Internet-Draft H.263 RTP payload format April 2005 11. Acknowledgements This is to acknowledge the work done by Carsten Bormann from Universitaet Bremen and Chad Zhu, Linda Cline, Gim Deisher, Tom Gardos, Christian Maciocco, Donald Newell from Intel Corp. who co- authored RFC2429. I would also like to acknowledge the work of Petri Koskelainen from Nokia and Nermeen Ismail from Cisco who helped with drafting the text for the new media types. Ott, et al. Expires October 28, 2005 [Page 30] Internet-Draft H.263 RTP payload format April 2005 12. Changes from previous versions of the document 12.1 changes from RFC 2429 The changes from the RFC 2429 are: 1. The H.263 1998 and 2000 MIME type are now in the payload specification. 2. Added optional parameters to the H.263 MIME types 3. Mandate the usage of RFC2429 for all H.263. RFC2190 payload format should be used only to interact with legacy systems. 4. Editorial changes to be in line with RFC editing procedures 12.2 Changes from rfc2429-bis-00 1. Changed from space separated parameters to semi colon according to the AVT guidelines 2. Took out the MaxBR optional parameter. 12.3 Changes from rfc2429-bis-03 1. Change the section order between section 3 and 4. 2. check for correct usage of keywords. 12.4 Changes from rfc2429-bis-03 1. Change the section order between section 4 and 3 to reduce the difference from RFC2429 2. Editorial changes to reduce diff from RFC2429 Ott, et al. Expires October 28, 2005 [Page 31] Internet-Draft H.263 RTP payload format April 2005 13. References 13.1 Normative References [H263] International Telecommunications Union, "Video coding for low bit rate communication", ITU Recommendation H.263, March 1996. [H263P] International Telecommunications Union, "Video coding for low bit rate communication", ITU Recommendation H.263P, February 1998. [H263X] International Telecommunications Union, "Annex X: Profiles and levels definition", ITU Recommendation H.263AnxX, April 2001. [RFC2032] Turletti, T., "RTP Payload Format for H.261 Video Streams", RFC 2032, October 1996. [RFC2048] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", BCP 13, RFC 2048, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2190] Zhu, C., "RTP Payload Format for H.263 Video Streams", RFC 2190, September 1997. [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003. [RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP Payload Formats", RFC 3555, July 2003. [Vredun] Wenger, S., "Video Redundancy Coding in H.263+", Proc. Ott, et al. Expires October 28, 2005 [Page 32] Internet-Draft H.263 RTP payload format April 2005 Audio-Visual Services over Packet Networks, Aberdeen, U.K. 9/1997, September 1997. 13.2 Informative References [AVPF] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, "Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)", January 2004. Authors' Addresses Joerg Ott Univ. Bremen Email: jo@acm.org Gary Sullivan Microsoft Email: garysull@windows.microsoft.com Stephan Wenger TU Berlin Email: stewe@cs.tu-berlin.de Roni Even Polycom 94 Derech Em Hamoshavot Petach Tikva 49130 Israel Email: roni.even@polycom.co.il Ott, et al. Expires October 28, 2005 [Page 33] Internet-Draft H.263 RTP payload format April 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Ott, et al. Expires October 28, 2005 [Page 34]