Internet Draft A. Li draft-ietf-avt-ulp-04.txt F. Liu February 15, 2002 J. Villasenor Expires: August 15, 2002 Univ. of Calif., LA J.H. Park D.S. Park Y.L. Lee Samsung Electronics An RTP Payload Format for Generic FEC with Uneven Level Protection STATUS OF THIS MEMO This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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 anytime. 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. ABSTRACT This document specifies a payload format for generic forward error correction to achieve uneven level protection (ULP) for media data encapsulated in RTP. It is an extension of the forward error correction scheme specified in RFC 2733 [1], and it is based on the same exclusive-or (parity) operation. This payload format allows end systems to apply protection using arbitrary protection lengths and levels, in addition to using arbitrary protection group sizes. It also enables both complete recovery or partial recovery of the critical payload and RTP header fields depending on the packet loss situation. This scheme is completely backward compatible with non-FEC capable hosts and with hosts that are only capable of handling the FEC schemes specified in RFC 2733 [1]. Those receivers that do not know about ULP forward error correction can simply ignore the extensions. Adam H. Li, et al. [Page 1] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 Table of Contents 1. Introduction ................................................... 3 1.1. General Overview ............................................. 4 1.2. Application Statement ........................................ 4 2. Terminology .................................................... 6 3. Basic Operation ................................................ 6 4. RTP Media Packet Structure ..................................... 7 5. ULP FEC Packet Structure ....................................... 7 5.1. RTP Header of ULP FEC Packets ................................ 8 5.2. FEC Header ................................................... 9 5.3. ULP Level Header ............................................. 9 6. Protection Operation .......................................... 10 6.1. Protection Level 0 .......................................... 10 6.2. Protection Level 1 and Higher ............................... 12 7. Recovery Procedure ............................................ 12 7.1. Reconstruction of Level 0 ................................... 12 7.2. Reconstruction of Level 1 and Higher ........................ 13 8. Examples ...................................................... 14 8.1. An Example With Only Protection Level 0 ..................... 14 8.2. An Example That Generates Idential Protection as in RFC 2733 16 8.3. An Example With Two Protection Levels (0 and 1) ............. 17 9. Security and Congestion Considerations ........................ 20 10. Indication ULP FEC Usage in SDP .............................. 21 10.1. ULP FEC as a Separate Stream ............................... 21 10.2. Use with Redundant Encoding ................................ 22 10.3. Usage with RTSP ............................................ 23 11. MIME Registrations ........................................... 24 11.1. Registration of audio/ulpfec ............................... 24 11.2. Registration of video/ulpfec ............................... 25 11.3. Registration of text/ulpfec ................................ 26 11.4. Registration of application/ulpfec ......................... 27 12. Acknowledgements ............................................. 28 13. Bibliography ................................................. 28 14. Authors' Address ............................................. 29 Adam H. Li, et al. [Page 2] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 1. Introduction 1.1. General Overview Because of the real-time nature of many applications, they have more stringent delay requirements than normal data transmissions. As a result, retransmission of the lost packets is generally not a valid option for such applications. In these cases, a better method to attempt recovery of information from packet loss is through Forward Error Correction (FEC). FEC has been one of the main methods used to protect against packet loss over packet switched networks [2]. In many cases, the bandwidth of the network connections is a very limited resource. However, most of traditional FEC schemes are not designed for optimal utilization of the limited bandwidth resource. A more efficient way to utilize the limited bandwidth would be to use unequal error protection to provide different levels of protection for different parts of the data stream which vary in importance. These unequal error protection schemes can make more efficient use of the bandwidth to provide better overall protection of the data stream against the loss. Proper protocol support is essential for realizing these unequal error protection mechanisms. However, the application of most of the unequal error protection schemes requires the knowledge of the importance for different parts of the data stream. Most of such schemes are designed for a particular type of media according to the structure of the media protected, and as a result, are not generic. In many multimedia streams, the more important parts of the data are always at the beginning of the data packet. This is the common practice for most codecs since the beginning of the packet is closer to the re-synchronization marker at the header and thus is more likely to be correctly decoded. Also, almost all media formats have the frame headers at the beginning of the packet, which is the most vital part of the packet. For video streams, most modern formats have optional data partitioning modes to improve error resilience in which the video macroblock header data, the motion vector data, and DCT coefficient data are separated into their individual partitions. In ITU-T H.263 version 3, there is the optional data partitioned syntax of Annex V. In MPEG-4 Visual Simple Profile, there is the optional data partitioning mode. When these modes are enabled, the video macroblock (MB) header and motion vector partitions (which are much more important to the quality of the video reconstruction) are transmitted in the partition(s) at the beginning of the video packet while residue DCT coefficient partitions (which are less important) are transmitted in the partition close to the end of the packet. Because the data is arranged in descending order of importance, it would be beneficial to provide more protection to the beginning part of the packet in transmission. Adam H. Li, et al. [Page 3] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 For audio streams, the bitstreams generated by many of the new audio codecs also contain data with different classes of importance. These different classes are then transmitted in order of descending importance. Thus, applying more protection to the beginning of the packet would also be beneficial in these cases. Even for uniform- significance audio streams, special stretching techniques can be applied to the partially recovered audio data packets. In cases where audio redundancy coding is used, more protection should be applied to the original data located in the first half of the packet. The rest of the packet containing the redundant copies of the data, does not need the same level of protection. It is clear that audio/video applications would generally benefit from an unequal error protection scheme that gives more protection to the beginning part of each packet. This document defines a payload format for RTP [3] that allows for generic forward error correction with unequal error protection for real-time media. The payload data are protected by one or more protection levels. Lower protection levels provide greater protection by using smaller group sizes (compared to higher protection levels) for generating the FEC packet. The data that are closer to the beginning of the packet are protected by lower protection levels because these data are in general more important, and they tend to carry more information than the data further behind in the packet. This document specifies an RTP payload format that extends the generic forward error correction schemes as specified in RFC 2733 [1]. This extension enables different levels of protection to be applied to different parts of the packet. While the whole packet can always be treated as a single level (same as in RFC 2733), this multiple Uneven Level Protection (ULP) can potentially achieve more efficient protection of the media payload. 1.2. Application Statement The ULP algorithm specified in this document is designed to deal with any type of packet loss occurring in transmission, just as does RFC 2733, which it extends. The ULP algorithm is designed to be fully interoperable between the hosts that are ULP-capable and those that are not. Since the media payload is not altered and the protection is sent as additional information, the receivers that are unaware of ULP can simply ignore the additional ULP information and process the main media payload. This interoperability is particularly important for backward compatibility with existing hosts, and also in the scenario where many different hosts need to communicate with each other at the same time, such as during multicast. The ULP algorithm is also a generic protection algorithm with the following features: (1) it is independent of the nature of the media being protected, whether that media is audio, video, or otherwise, (2) it is flexible enough to support a wide variety of FEC mechanisms, (3) it is designed for adaptivity, so that the FEC technique can be modified easily without resorting to out of band Adam H. Li, et al. [Page 4] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 signaling, and (4) it is supportive of a number of different mechanisms for transporting the FEC packets. An Unequal Erasure Protection (UXP) scheme has also been proposed in the AVT Working Group in "An RTP Payload Format for Erasure-Resilient Transmission of Progressive Multimedia Streams". The UXP scheme applies unequal error protection to the media payloads by interleaving the payload stream to be protected with the additional redundancy information obtained using Reed-Solomon operations. By altering the structure of the protected media payload, the UXP scheme sacrifices the backward compatibility with terminals that do not support UXP. This makes it more difficult to apply UXP when backward compatibility is desired. In the case of ULP, however, the media payload remains un-altered and can always be used by the terminals. The extra protection can simply be ignored if the receiving terminals do not support ULP. At the same time, also because the structure of the media payload is altered in UXP, UXP offers the unique ability to change packet size independent of the original media payload structure and protection applied, and is only subject to the protocol overhead constraint. This property is useful in scenarios when altering the packet size of the media at transport level is desired. Because of the interleaving used in UXP, delays will be introduced at both the encoding and decoding sides. For UXP, all data within a transmission block need to arrive before encoding can begin, and a reasonable number of packets must be received before a transmission block can be decoded. The ULP scheme introduces little delay at the encoding side. On the decoding side, correctly received packets can be delivered immediately. Delay is only introduced in ULP when packet losses occur. Because UXP is an interleaved scheme, the un-recoverable errors occurring in data protected by UXP usually result in a number of corrupted holes in the payload stream. In ULP, on the other hand, any errors in the bitstream usually occur in contiguous pieces at the end of the packets. Depending on the encoding of the media payload stream, many applications may find it easier to decode more data from a stream with a contiguous piece missing at the end than from a stream with multiple corrupted holes. The exclusive-or (XOR) parity check operation used by ULP is simpler and faster than the more complex operations required by Reed-Solomon codes. This makes ULP more suitable for applications where computational cost is a constraint. As discussed above, both the ULP and the UXP schemes apply unequal error protection to the RTP media stream, but each uses a different technique. Both schemes have their own unique characteristics, and each can be applied to scenarios with different requirements. Adam H. Li, et al. [Page 5] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 2. Terminology The following terms are used throughout this document: Media Payload: The raw, un-protected user data that are transmitted from the sender. The media payload is placed inside of an RTP packet. Media Header: The RTP header for the packet containing the media payload. Media Packet: The combination of a media payload and media header is called a media packet. ULP FEC Packet: The uneven level protection FEC algorithms at the transmitter take the media packets as an input. They output both the media packets that they are passed, and newly generated packets called ULP FEC packets, which contain redundant media data used for error correction. The ULP FEC packets are formatted according to the rules specified in this document. FEC Header: The header information contained in an FEC packet. FEC Payload: The payload of an FEC packet. Associated: A ULP FEC packet is said to be "associated" with one or more media packets when those media packets are used to generate the ULP FEC packet (by use of the exclusive-or operation). 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 [4]. 3. Basic Operation The payload format described here is used whenever the sender in an RTP session would like to protect the media stream it is sending with uneven level protection (ULP) FEC. The ULP FEC supported by the format is based on the same simple exclusive-or (XOR) parities used in RFC 2733 [1]. The sender takes the packets from the media stream requiring protection and determines the protection levels for these packets and the protection length for each level. The data of each level are grouped as described below in Section 6 to provide each level with a different degree of error resilience. An XOR operation is applied across the payload to generate the ULP FEC information for each level. The lower protection levels (which provide higher protection, or greater error resilience) are applied to the data that are closer to the beginning of the packet to ensure more protection. The result based on the procedures defined here is an RTP packet containing ULP FEC information. This packet can be used at the receiver to recover the packets or parts of the packets used to generate the ULP FEC packet. By using uneven error protection, this Adam H. Li, et al. [Page 6] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 scheme can make more efficient use of the channel bandwidth, and provide more efficient error resilience for transmission over error prone channels. The payload format contains information that allows the sender to tell the receiver exactly which media packets are protected by the ULP FEC packet, and the protection levels and lengths for each of the levels. Specifically, each ULP FEC packet contains a protection length L(k) and a bitmask m(k), called the offset mask, for each protection level k. If the bit i in the mask m(k) is set to 1, then data of length L(k) in media packet number N + i is protected by this ULP FEC packet at level k. N is called the sequence number base, and is sent in the ULP FEC packet as well. The protection length, offset mask and payload type are sufficient to signal ULP forward error correction schemes based on arbitrarily defined parity protection with little overhead. A set of rules is described in Section 5.3 that defines how the mask should be set for different protection levels. This document also describes procedures on transmitting all the protection operation parameters in-band. This allows the sender great flexibility; the sender can adapt the code to current network conditions and be certain the receivers can still make use of the ULP FEC for recovery. At the receiver, the ULP FEC and original media are received. If no media packets are lost, the ULP FEC can be ignored. In the event of a loss, the ULP FEC packets can be combined with other received media and ULP FEC packets to recover all or part of the missing media packets. RTP packets that contain data formatted according to this specification (i.e., ULP FEC packets) use dynamic RTP payload types. 4. RTP Media Packet Structure The formatting of the media packets is unaffected by ULP FEC. If the ULP FEC is sent as a separate stream, the media packets are sent as if there was no FEC. This scheme leads to a very efficient encoding. When little or no ULP FEC is used, the transmitted stream contains mostly media packets. The overhead for using the ULP FEC scheme is only present in ULP FEC packets, and can be easily monitored and adjusted by tracking the amount of FEC in use. 5. ULP FEC Packet Structure A ULP FEC packet is constructed by placing an FEC header and the ULP FEC payload into the RTP payload, as shown in Figure 1: Adam H. Li, et al. [Page 7] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FEC Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ULP Level 0 Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ULP Level 0 Payload | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ULP Level 1 Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ULP Level 1 Payload | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cont. | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: ULP FEC Packet Structure 5.1. RTP Header of ULP FEC Packets The version field is set to 2. The padding bit is computed via the protection operation, defined below. The extension bit is also computed via the protection operation. The SSRC value will generally be the same as the SSRC value of the media stream it protects, though it MAY be different if the FEC stream is being demultiplexed via the SSRC value. The CC value is computed from the protection operation. The CSRC list is never present, independent of the value of the CC field. The extension is never present, independent of the value of the X bit. The marker bit is computed via the protection operation. The sequence number has the standard definition: it MUST be one higher than the sequence number in the previously transmitted FEC packet. The timestamp MUST be set to the value of the media RTP clock at the instant the ULP FEC packet is transmitted. Thus, the TS value in FEC packets is always monotonically increasing. The payload type for the ULP FEC packet is determined through dynamic, out of band means. According to RFC 1889 [3], RTP participants that cannot recognize a payload type must discard it. This provides backwards compatibility. The ULP FEC mechanisms can then be used in a multicast group with mixed ULP-FEC-capable and ULP- FEC-incapable receivers. In such a case, the ULP stream will have a payload type which is not recognized by the ULP-FEC-incapable receivers, and will thus be disregarded. 5.2. FEC Header Adam H. Li, et al. [Page 8] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 This header is 12 bytes. The format of the header is shown in Figure 2 and consists of an SN base field, length recovery field, E field, PT recovery field, mask field and TS recovery field. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SN base | length recovery | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| PT recovery | mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TS recovery | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: FEC Header Format This is exactly the same as the FEC header used in RFC 2733 [1]. The usage will also be exactly the same as specified as in RFC 2733, except that the E bit MUST set to one for this version. 5.3. ULP Level Header The ULP Level Header is 2 bytes for ULP level 0, and 5 bytes for ULP level 1 and higher. The formats of the headers are shown in Figure 3 and Figure 4, and consist of a Protection Length field and a mask field (for level 1 and higher headers). 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protection Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: ULP Level Header Format (Level 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protection Length | mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | mask (cont.) | +-+-+-+-+-+-+-+-+ Figure 4: ULP Level Header Format (Level 1 and higher) The Protection Length field is 16 bits. It indicates the protection length provided by the ULP FEC for the current protection level Adam H. Li, et al. [Page 9] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 (i.e., the payload length for the current protection level after the header). The mask field is 24 bits. If bit i in the mask is set to 1, then the media packet with sequence number N + i is associated with this ULP FEC packet of current protection level, where N is the SN Base field in the ULP FEC packet header. The least significant bit corresponds to i=0, and the most significant to i=23. The SN base field in the FEC header MUST be set to the minimum sequence number of those media packets protected by ULP FEC. This allows for the ULP FEC operation to extend over any string of at most 24 packets. The setting of mask field shall follow the following rules: a. A media packet can only be protected at each protection level once. b. For a media packet to be protected at level p, it must also be protect at level p-1. c. If an ULP FEC packet contains protection at level p, it must also contain protection at level p-1. The payload of the ULP FEC packet of each level is the protection operation applied to the concatenation of the CSRC list, RTP extension, media payload, and padding of the media packets associated with the ULP FEC packet. Details are described in the next section on the protection operation. 6. Protection Operation The protection operation involves copying the payload, padding it with zeroes, and computing the parity (XOR) across the resulting bit strings. In addition, for protection of level 0, it also involves concatenating specific fields from the RTP header of the media packet before the payload data. The resulting bit string is used to generate the ULP FEC packet. The following procedure MAY be followed for the protection operation. Other procedures MAY be used, but the end result MUST be identical to the one described here. 6.1. Protection Level 0 For each media packet to be protected, a bit string is generated by concatenating the following fields together in the order specifed: o Padding Bit (1 bit) Adam H. Li, et al. [Page 10] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 o Extension Bit (1 bit) o CC bits (4 bits) o Marker bit (1 bit) o Payload Type (7 bits) o Timestamp (32 bits) o Unsigned network-ordered 16 bit representation of the sum of the lengths of the CSRC List, length of the padding, length of the extension, and length of the media packet (16 bits) o if CC is nonzero, the CSRC List (variable length) o if X is 1, the Header Extension (variable length) o the payload (variable length) o Padding, if present (variable length) Note that the Padding Bit (first entry above) forms the most significant bit of the bit string. If the lengths of the bit strings are not equal, each bit string that is shorter than the Protection Length 0 plus 96 bits, MUST be padded to that length. Any value may be used for padding. The pad MUST be added at the end of the bit string. The parity operation is then applied across the bit strings. The result is the bit string used to build the ULP FEC packet. We will call this the ULP FEC bit string (level 0). The first (most significant) bit in the ULP FEC bit string is written into the Padding Bit of the ULP FEC packet. The second bit in the ULP FEC bit string is written into the Extension bit of the ULP FEC packet. The next four bits of the ULP FEC bit string are written into the CC field of the ULP FEC packet. The next bit of the ULP FEC bit string is written into the marker bit of the ULP FEC packet. The next 7 bits of the ULP FEC bit string are written into the PT recovery field in the ULP FEC packet header. The next 32 bits of the ULP FEC bit string are written into the TS recovery field in the packet header. The next 16 bits are written into the length recovery field in the ULP FEC packet header. This is exactly the same as in RFC 2733 [1]. The remaining bits (of length Protection Length 0) are set to be the payload of the ULP FEC packet. 6.2. Protection Level 1 and Higher Adam H. Li, et al. [Page 11] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 The protected data of the corresponding packets are copied into the bit strings. If the packet ends before the Protection Length of the current level is reached, the string is padded to that length. Any value may be used for the padding. The padding MUST be added at the end of the bit string. The parity operation is applied across the protected data of the corresponding packets. The generated ULP FEC bit of that level is then appended to the payload of the ULP FEC packet. 7. Recovery Procedures The ULP FEC packets allow end systems to recover from the loss of media packets. All of the header fields of the missing packets, including CSRC lists, extensions, padding bits, marker and payload type, are recoverable. This section describes the procedure for performing this recovery. Recovery requires two distinct operations. The first determines which packets (media and FEC) must be combined in order to recover a missing packet. Once this is done, the second step is to actually reconstruct the data. The second step MUST be performed as described below. The first step MAY be based on any algorithm chosen by the implementer. Different algorithms result in a tradeoff between complexity and the ability to recover missing packets, if possible. 7.1. Reconstruction of Level 0 Let T be the list of packets (ULP FEC and media) which can be combined to recover some media packet xi. The procedure is as follows: 1. For the media packets in T, compute the bit string as described in the protection operation of the previous section. 2. For the ULP FEC packet in T, compute the bit string in the same fashion, except always set the CSRC list, extension, and padding to null. Read the Protection Length 0. Read string of that length from that ULP FEC packet. 3. If any of the bit strings generated from the media packets are shorter than the bit string generated from the ULP FEC packet, pad them to be the same length as the bit string generated from the ULP FEC. The padding MUST be added at the end of the bit string, and MAY be of any value. 4. Perform the exclusive-or (parity) operation across the bit strings, resulting in a recovery bit string. 5. Create a new packet with the standard 12 byte RTP header and no payload. Adam H. Li, et al. [Page 12] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 6. Set the version of the new packet to 2. 7. Set the Padding bit in the new packet to the first bit in the recovery bit string. 8. Set the Extension bit in the new packet to the second bit in the recovery bit string. 9. Set the CC field to the next four bits in the recovery bit string. 10. Set the marker bit in the new packet to the next bit in the recovery bit string. 11. Set the payload type in the new packet to the next 7 bits in the recovery bit string. 12. Set the SN field in the new packet to xi. 13. Set the TS field in the new packet to the next 32 bits in the recovery bit string. 14. Take the next 16 bits of the recovery bit string. Whatever unsigned integer this represents (assuming network-order), take that many bytes from the recovery bit string and append them to the new packet. This represents the CSRC list, extension, payload, and padding. 15. Set the SSRC of the new packet to the SSRC of the media stream it's protecting. This procedure will recover both the header and payload of an RTP packet up to the Protection Length of level 0. 7.2. Reconstruction of Level 1 and Higher Let T be the list of packets (ULP FEC and media) which can be combined to recover some media packet xi. The procedure is as follows: 1. For the media packet in T, get the protection length of that level. Copy the data of the that protection level (data of the length read following the level header) to the bit strings. 2. If any of the bit strings generated from the media packets are shorter than the Protection Length of the current level, pad them to that length. The padding MUST be added at the end of the bit string, and MUST be of the same value as used in the process of generating the ULP FEC packets. 3. Perform the exclusive-or (parity) operation across the bit strings, resulting in a recovery bit string. Adam H. Li, et al. [Page 13] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 The data protected at lower protection level is almost always recoverable if the higher level protected data is recoverable. This procedure (together with the procedure for the lower protection levels) will usually recover both the header and payload of an RTP packet up to the Protection Length of the current level. 8. Examples Consider 4 media packets to be sent, A, B, C and D, from SSRC 2. Their sequence numbers are 8, 9, 10 and 11, respectively, and have timestamps of 3, 5, 7 and 9, respectively. Packet A and C uses payload type 11, and packet B and D uses payload type 18. Packet A is has 200 bytes of payload, packet B 140, packet C 100 and packet D 340. Packet A and C have their marker bit set. 8.1. An Example With Only Protection Level 0 Suppose we want to protect the data of length L0 = 70 bytes of them at the beginning of these packets, as illustrated in Figure 5 below. +------:------------+ Packet A | : | +------:------+-----+ Packet B | : | +------:--+---+ Packet C | : | +------:--+-----------------------+ Packet D | : | +------:--------------------------+ : +------+ Packet FEC | | +------+ : : :<-L0->: Figure 5 ULP FEC scheme with only protection level 0 An ULP FEC packet is generated from these four packets. We assume that payload type 127 is used to indicate an FEC packet. The resulting RTP header is shown in Figure 6. The FEC header in the ULP FEC packet is shown in Figure 7. The ULP header for level 0 in the ULP FEC packet is shown in Figure 8. 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 Adam H. Li, et al. [Page 14] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 0|0|0|0 0 0 0|0|1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version: 2 Padding: 0 Extension: 0 Marker: 0 PT: 127 SN: 1 TS: 9 SSRC: 2 Figure 6: RTP Header of ULP FEC for Packets A, B, C and D (one level) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|0 0 0 0 0 0 0 1 0 1 1 1 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ SN base: 8 [min(8,9,10,11)] len. rec.: 372 [200 XOR 140 XOR 100 XOR 340] E: 1 [ULP FEC] PT rec.: 0 [11 XOR 18 XOR 11 XOR 18] mask: 15 TS rec.: 8 [3 XOR 5 XOR 7 XOR 9] Figure 7: FEC Header of ULP Packet (one level) 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L0: 70 The payload length for level 0 is 70 bytes. Figure 8: ULP Level Header (Level 0) 8.2. An Example That Generates Identical Protection as in RFC 2733 Adam H. Li, et al. [Page 15] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 We can choose to extend the level 0 protection to cover the whole length of the packets (as shown in Figure 9). This gives almost identical protection as provided in RFC 2733. Please note that when using ULP this way, each ULP FEC packet will use two more bytes (for the level 0 payload length field) than that of RFC 2733 - a small price to pay for the added flexbility. +-------------------+ : Packet A | | : +-------------+-----+ : Packet B | | : +---------+---+ : Packet C | | : +---------+-----------------------+ Packet D | | +---------------------------------+ : +---------------------------------+ Packet FEC | | +---------------------------------+ : : :<------------- L0 -------------->: Figure 9 ULP FEC scheme with only protection level 0 The resulting ULP FEC packet will have the RTP header same as shown in Figure 6 and FEC header same as shown in Figure 7. The ULP level header is shown in Figure 10. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L0: 340 [max(200,140,100,340)] The payload length for level 0 is 340 bytes. Figure 10: ULP Level Header (Level 0) 8.3. An Example With Two Protection Levels (0 and 1) Adam H. Li, et al. [Page 16] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 A more complete example is to use ULP at two levels. The level 0 ULP will provide greater protection to the beginning part of the payload packets. The level 1 ULP will apply additional protection to the rest of the packets. This is illustrated in Figure 11. In this example, we take L0 = 70 and L1 = 90. +------:--------:---+ Packet A | : : | +------:------+-:---+ Packet B | : | : +------:--+---+ : : : +------+ : ULP #1 | | : +------+ : : : +------:--+ : Packet C | : | : +------:--+-----:-----------------+ Packet D | : : | +------:--------:-----------------+ : : +------:--------+ ULP #2 | : | +------:--------+ : : : :<-L0->:<--L1-->: Figure 11 ULP FEC scheme with protection level 0 and level 1 This will result in two ULP FEC packets - #1 and #2. The resulting ULP FEC packet #1 will have the RTP header as shown in Figure 12. The FEC header for ULP FEC packet #1 will be as shown in Figure 13. The level 0 ULP header for #1 will be shown in Figure 14. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Adam H. Li, et al. [Page 17] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 |1 0|0|0|0 0 0 0|1|1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version: 2 Padding: 0 Extension: 0 Marker: 1 PT: 127 SN: 1 TS: 5 SSRC: 2 Figure 12: RTP Header of ULP FEC #1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0 0 1 1 0 0 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ SN base: 8 [min(8,9)] len. rec.: 68 [200 XOR 140] E: 1 [ULP FEC] PT rec.: 25 [11 XOR 18] mask: 3 TS rec.: 6 [3 XOR 5] Figure 13: FEC Header of ULP FEC Packet #1 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L0: 70 The payload length for level 0 is 70 bytes. Figure 14: ULP Level Header (Level 0) for ULP FEC Packet #1 The resulting ULP FEC packet #2 will have the RTP header as shown in Figure 15. The FEC header for ULP FEC packet #2 will be as shown in Adam H. Li, et al. [Page 18] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 Figure 16. The level 0 ULP header for #2 will be shown in Figure 17. The level 1 ULP header for #2 will be shown in Figure 18. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 0|0|0|0 0 0 0|1|1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version: 2 Padding: 0 Extension: 0 Marker: 1 PT: 127 SN: 2 TS: 9 SSRC: 2 Figure 15: RTP Header of ULP FEC Packet #2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|0 0 0 0 0 0 0 1 0 0 1 1 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0 0 1 1 0 0 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ SN base: 8 [min(8,9,10,11)] len. rec.: 308 [100 XOR 340] E: 1 [ULP FEC] PT rec.: 25 [11 XOR 18] mask: 12 TS rec.: 6 [7 XOR 9] Figure 16: FEC Header of ULP Packet #2 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 Adam H. Li, et al. [Page 19] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L0: 70 The payload length for level 0 is 70 bytes. Figure 17: ULP Level Header (Level 0) for ULP Packet #2 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 1 0 1 1 0 1 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 1 1 1 1| +-+-+-+-+-+-+-+-+ L1: 90 mask: 15 The payload length for level 1 is 90 bytes. Figure 18: ULP Level Header (Level 1) for ULP Packet #2 9. Security and Congestion Considerations The use of ULP FEC has implications on the usage and changing of keys for encryption. As the ULP FEC packets do consist of a separate stream, there are a number of combinations on the usage of encryption. These include: o The ULP FEC stream may be encrypted, while the media stream is not. o The media stream may be encrypted, while the ULP FEC stream is not. o The media stream and ULP FEC stream are both encrypted, but using different keys. o The media stream and ULP FEC stream are both encrypted, but using the same key. The first three of these would require all application level signaling protocols used to be aware of the usage of ULP FEC, and to thus exchange keys and negotiate encryption usage on the media and ULP FEC streams separately. In the final case, no such additional mechanisms are needed. The first two cases present a layering violation, as ULP FEC packets should be treated no differently than other RTP packets. Encrypting just one stream may also make certain Adam H. Li, et al. [Page 20] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 known-plaintext attacks possible. For these reasons, applications utilizing encryption SHOULD encrypt both streams. The changing of encryption keys is another crucial issue needs to be addressed. Consider the case where two packets a and b are sent along with the ULP FEC packet that protects them. The keys used to encrypt a and b are different, so which key should be used to decode the ULP FEC packet? In general, old keys need to be cached, so that when the keys change for the media stream, the old key can be used until it is determined that the key has changed for the ULP FEC packets as well. Another issue with the use of ULP FEC is its impact on network congestion. In many situations, the packet loss in the network is induced by congestions. In such scenarios, adding FEC when encountering increasing network losses should be avoided. If it is used on a widespread basis, this can result in increased congestion and eventual congestion collapse. The applications may include stronger protections while at the same time reduce the bandwidth for the payload packets. In any event, implementations MUST NOT substantially increase the total amount of bandwidth in use (including the payload and the ULP FEC) as network losses increase. 10. Indicating ULP FEC Usage in SDP FEC packets contain RTP packets with dynamic payload type values. In addition, the FEC packets can be sent on separate multicast groups or separate ports from the media. The ULP FEC can even be carried in packets containing media using the redundant encoding payload format [5]. These configuration options MUST be indicated out of band. This section describes how this can be accomplished using the Session Description Protocol (SDP), specified in RFC 2327 [6]. 10.1. ULP FEC as a Separate Stream In the first case, the ULP FEC packets are sent as a separate stream. This means that they can be sent on a different port and/or multicast group from the media. When this is done, several pieces of information must be conveyed: o The address and port where the ULP FEC is being sent to o The payload type number for the ULP FEC o Which media stream the ULP FEC is protecting The payload type number for the ULP FEC is conveyed in the m line of the media it is protecting, listed as if it were another valid encoding for the stream. There is no static payload type assignment for ULP FEC, so dynamic payload type numbers MUST be used. The binding to the number is indicated by an rtpmap attribute. The name used in this binding is "ulpfec". Adam H. Li, et al. [Page 21] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 The presence of the payload type number in the m line of the media it is protecting does not mean the ULP FEC is sent to the same address and port as the media. Instead, this information is conveyed through an fmtp attribute line. The presence of the ULP FEC payload type on the m line of the media serves only to indicate which stream the ULP FEC is protecting. The format for the fmtp line for ULP FEC is: a=fmtp: where 'number' is the payload type number present in the m line. Port is the port number where the ULP FEC is sent to. The remaining three items - network type, address type, and connection address - have the same syntax and semantics as the c line from SDP. This allows the fmtp line to be partially parsed by the same parser used on the c lines. Note that since ULP FEC cannot be hierarchically encoded, the parameter MUST NOT appear in the connection address. The following is an example SDP for ULP FEC: v=0 o=hamming 2890844526 2890842807 IN IP4 128.97.90.168 s=ULP FEC Seminar c=IN IP4 224.2.17.12/127 t=0 0 m=audio 49170 RTP/AVP 0 78 a=rtpmap:78 ulpfec/8000 a=fmtp:78 49172 IN IP4 224.2.17.12/127 m=video 51372 RTP/AVP 31 79 a=rtpmap:79 ulpfec/8000 a=fmtp:79 51372 IN IP4 224.2.17.13/127 The presence of two m lines in this SDP indicates that there are two media streams - one audio and one video. The media format of 0 indicates that the audio uses PCM, and is protected by ULP FEC with payload type number 78. The ULP FEC is sent to the same multicast group and TTL as the audio, but on a port number two higher (49172). The video is protected by ULP FEC with payload type number 79. The ULP FEC appears on the same port as the video (51372), but on a different multicast address. 10.2. Use with Redundant Encoding When the ULP FEC stream is being sent as a secondary codec in the redundant encoding format, this must be signaled through SDP. To do this, the procedures defined in RFC 2198 [5] are used to signal the use of redundant encoding. The ULP FEC payload type is indicated in the same fashion as any other secondary codec. An rtpmap attribute MUST be used to indicate a dynamic payload type number for the ULP Adam H. Li, et al. [Page 22] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 FEC packets. The ULP FEC MUST protect only the main codec. In this case, the fmtp attribute for the ULP FEC MUST NOT be present. For example: m=audio 12345 RTP/AVP 121 0 5 100 a=rtpmap:121 red/8000/1 a=rtpmap:100 ulpfec/8000 a=fmtp:121 0/5/100 This SDP indicates that there is a single audio stream, which can consist of PCM (media format 0) , DVI (media format 5), the redundant encodings (indicated by media format 121, which is bound to read through the rtpmap attribute), or ULP FEC (media format 100, which is bound to ulpfec through the rtpmap attribute). Although the ULP FEC format is specified as a possible coding for this stream, the ULP FEC MUST NOT be sent by itself for this stream. Its presence in the m line is required only because non-primary codecs must be listed here according to RFC 2198. The fmtp attribute indicates that the redundant encodings format can be used, with DVI as a secondary coding and ULP FEC as a tertiary encoding. 10.3. Usage with RTSP RTSP [7] can be used to request ULP FEC packets to be sent as a separate stream. When SDP is used with RTSP, the Session Description does not include a connection address and port number for each stream. Instead, RTSP uses the concept of a "Control URL". Control URLs are used in SDP in two distinct ways. 1. There is a single control URL for all streams. This is referred to as "aggregate control". In this case, the fmtp line for the ULP FEC stream is omitted. 2. There is a Control URL assigned to each stream. This is referred to as "non-aggregate control". In this case, the fmtp line specifies the Control URL for the stream of ULP FEC packets. The URL may be used in a SETUP command by an RTSP client. The format for the fmtp line for ULP FEC with RTSP and non-aggregate control is: a=fmtp: where 'number' is the payload type number present in the m line. Control URL is the URL used to control the stream of ULP FEC packets. Note that the Control URL does not need to be an absolute URL. The rules for converting a relative Control URL to an absolute URL are given in RFC 2326, Section C.1.1. 11. MIME Registrations Adam H. Li, et al. [Page 23] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 Four new MIME sub-type as described in this section is to be registered. 11.1. Registration of audio/ulpfec To: ietf-types@iana.org Subject: Registration of MIME media type audio/ulpfec MIME media type name: audio MIME subtype name: ulpfec Required parameters: none Note that it is mandated that RTP payload formats without a defined rate must define a rate parameter as part of their MIME registration. The payload format for ULP FEC does not specify a rate parameter. However, the rate for ULP FEC data is equal to the rate of the media data it protects. Optional parameters: none Typical optional parameters [8], such as the number of channels, and the duration of audio per packet, do not apply to ULP FEC data. The number of channels is effectively the same as the media data it protects; the same is true for the duration of audio per packet. Encoding considerations: This format is only defined for transport within the Real Time Transport protocol (RTP) [3]. Its transport within RTP is fully specified with RFC xxxx. Security considerations: the same security considerations apply to these MIME registrations as to the payloads for them, as detailed in RFC xxxx. Interoperability considerations: none Published specification: RFC xxxx. Applications which use this media type: Audio and video streaming tools which seek to improve resiliency to loss by sending additional data with the media stream. Additional information: none Person & email address to contact for further information: Adam Li Department of Electrical Engineering University of California Los Angeles, CA 90095 adamli@icsl.ucla.edu Adam H. Li, et al. [Page 24] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 Intended usage: COMMON Author/Change controller: This registration is part of the IETF registration tree. RTP and SDP Issues: Usage of this format within RTP and the Session Description Protocol (SDP) [6] are fully specified within Section 10 of RFC xxxx. 11.2. Registration of video/ulpfec To: ietf-types@iana.org Subject: Registration of MIME media type video/ulpfec MIME media type name: video MIME subtype name: ulpfec Required parameters: none Note that it is mandated that RTP payload formats without a defined rate must define a rate parameter as part of their MIME registration. The payload format for ULP FEC does not specify a rate parameter. However, the rate for ULP FEC data is equal to the rate of the media data it protects. Optional parameters: none Typical optional parameters [8], such as the number of channels, and the duration of audio per packet, do not apply to ULP FEC data. The number of channels is effectively the same as the media data it protects; the same is true for the duration of video per packet. Encoding considerations: This format is only defined for transport within the Real Time Transport protocol (RTP) [3]. Its transport within RTP is fully specified with RFC xxxx. Security considerations: the same security considerations apply to these MIME registrations as to the payloads for them, as detailed in RFC xxxx. Interoperability considerations: none Published specification: RFC xxxx. Applications which use this media type: Audio and video streaming tools which seek to improve resiliency to loss by sending additional data with the media stream. Additional information: none Adam H. Li, et al. [Page 25] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 Person & email address to contact for further information: Adam Li Department of Electrical Engineering University of California Los Angeles, CA 90095 adamli@icsl.ucla.edu Intended usage: COMMON Author/Change controller: This registration is part of the IETF registration tree. RTP and SDP Issues: Usage of this format within RTP and the Session Description Protocol (SDP) [6] are fully specified within Section 10 of RFC xxxx. 11.3. Registration of text/ulpfec To: ietf-types@iana.org Subject: Registration of MIME media type text/ulpfec MIME media type name: text MIME subtype name: ulpfec Required parameters: none Note that it is mandated that RTP payload formats without a defined rate must define a rate parameter as part of their MIME registration. The payload format for ULP FEC does not specify a rate parameter. However, the rate for ULP FEC data is equal to the rate of the media data it protects. Optional parameters: none Typical optional parameters [8], such as the number of channels, and the duration of audio per packet, do not apply to ULP FEC data. The number of channels is effectively the same as the media data it protects; the same is true for the duration of video per packet. Encoding considerations: This format is only defined for transport within the Real Time Transport protocol (RTP) [3]. Its transport within RTP is fully specified with RFC xxxx. Security considerations: the same security considerations apply to these MIME registrations as to the payloads for them, as detailed in RFC xxxx. Interoperability considerations: none Published specification: RFC xxxx. Adam H. Li, et al. [Page 26] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 Applications which use this media type: Audio, video and text streaming tools which seek to improve resiliency to loss by sending additional data with the media stream. Additional information: none Person & email address to contact for further information: Adam Li Department of Electrical Engineering University of California Los Angeles, CA 90095 adamli@icsl.ucla.edu Intended usage: COMMON Author/Change controller: This registration is part of the IETF registration tree. RTP and SDP Issues: Usage of this format within RTP and the Session Description Protocol (SDP) [6] are fully specified within Section 10 of RFC xxxx. 11.4. Registration of application/ulpfec To: ietf-types@iana.org Subject: Registration of MIME media type application/ulpfec MIME media type name: application MIME subtype name: ulpfec Required parameters: none Note that it is mandated that RTP payload formats without a defined rate must define a rate parameter as part of their MIME registration. The payload format for ULP FEC does not specify a rate parameter. However, the rate for ULP FEC data is equal to the rate of the media data it protects. Optional parameters: none Typical optional parameters [8], such as the number of channels, and the duration of audio per packet, do not apply to ULP FEC data. The number of channels is effectively the same as the media data it protects; the same is true for the duration of video per packet. Encoding considerations: This format is only defined for transport within the Real Time Transport protocol (RTP) [3]. Its transport within RTP is fully specified with RFC xxxx. Adam H. Li, et al. [Page 27] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 Security considerations: the same security considerations apply to these MIME registrations as to the payloads for them, as detailed in RFC xxxx. Interoperability considerations: none Published specification: RFC xxxx. Applications which use this media type: Audio and video streaming tools which seek to improve resiliency to loss by sending additional data with the media stream. Additional information: none Person & email address to contact for further information: Adam Li Department of Electrical Engineering University of California Los Angeles, CA 90095 adamli@icsl.ucla.edu Intended usage: COMMON Author/Change controller: This registration is part of the IETF registration tree. RTP and SDP Issues: Usage of this format within RTP and the Session Description Protocol (SDP) [6] are fully specified within Section 10 of RFC xxxx. 12. Acknowledgments This text is partially based on an RFC 2733 [1] and RFC 3009 [9] on generic RTP FEC payload format by H. Schulzrinne and J. Rosenburg. The authors would also like to acknowledge the suggestions from many people, particularly Tao Tian, Matthieu Tisserand, Stephen Wenger, Jay Fahlen, and Jeffery Tseng. 13. Bibliography [1] J. Rosenberg and H. Schulzrine, "An RTP Payload Format for Generic Forward Error Correction," Request for Comments (Proposed Standard) 2733, Internet Engineering Task Force, December 1999. [2] C. Perkins and O. Hodson, "Options for repair of streaming media, "Request for Comments (Informational) 2354, Internet Engineering Task Force, June 1998. [3] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a transport protocol for real-time applications," Request for Comments Adam H. Li, et al. [Page 28] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 (Proposed Standard) 1889, Internet Engineering Task Force, January 1996. [4] S. Bradner, "Key words for use in RFCs to indicate requirement levels," Request for Comments (Best Current Practice) 2119, Internet Engineering Task Force, March 1997. [5] C. Perkins, I. Kouvelas, O. Hodson, V. Hardman, M. Handley, J.C. Bolot, A. Vega-Garcia, and S. Fosse-Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, September 1997. [6] M. Handley, and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [7] H. Schulzrinne, A. Rao, and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998. [8] S. Casner, and P. Hoschka, "MIME type registration of RTP payload formats", Work in Progress. [9] J. Rosenberg and H. Schulzrine, "Registration of parityfec MIME types", Request for Comments (Proposed Standard) 3009, Internet Engineering Task Force, November 2000. 14. Authors' Addresses Adam H. Li Electrical Engineering Department University of California, Los Angeles Los Angeles, CA 90095 USA Phone: +1-310-825-5178 Fax : +1-310-825-7928 EMail: adamli@icsl.ucla.edu Fang Liu Electrical Engineering Department University of California, Los Angeles Los Angeles, CA 90095 USA Phone: +1-310-825-5178 Fax : +1-310-825-7928 EMail: fanliu@icsl.ucla.edu John D. Villasenor Electrical Engineering Department University of California, Los Angeles Los Angeles, CA 90095 USA Phone: +1-310-825-0228 Adam H. Li, et al. [Page 29] I-Draft An RTP Payload Format for Generic FEC with ULP Feb 2002 Fax : +1-310-825-7928 EMail: villa@icsl.ucla.edu Jeong-Hoon Park Samsung Electronics Suwon City, Kyungki-Do Korea 442-742 Phone: +82-31-200-3747 Fax : +82-31-200-3147 Email: jeonghoon@samsung.com Dong-Seek Park Samsung Electronics Suwon City, Kyungki-Do Korea 442-742 Phone: +82-31-200-3674 Fax : +82-31-200-3147 Email: dspark@samsung.com Yung-Lyul Lee Samsung Electronics Suwon City, Kyungki-Do Korea 442-742 Phone: +82-31-200-3719 Fax : +82-31-200-3147 Email: yllee@samsung.com Adam H. Li, et al. [Page 30]