Internet Draft A. Li draft-ietf-avt-ulp-07.txt F. Liu November 4, 2002 J. Villasenor Expires: May 4, 2003 Univ. of Calif., LA J.H. Park D.S. Park Y.L. Lee Samsung Electronics J. Rosenberg DynamicSoft H. Shulzrinne Columbia University 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 based on the same exclusive-or (parity) operation as in RFC 2733 [1], but it generalized and included the algorithms of that RFC. This draft will obsolete RFC 2733 and RFC 3009. The payload format described in this draft 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 Adam H. Li, et al. [Page 1] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 scheme is completely backward compatible with non-FEC capable hosts. Those receivers that do not know about ULP forward error correction can simply ignore the extensions. Table of Contents 1. Introduction ................................................... 3 1.1. General Overview ............................................. 3 1.2. Application Statement ........................................ 4 2. Terminology .................................................... 6 3. Basic Operation ................................................ 6 4. RTP Media Packet Structure ..................................... 7 5. ULP FEC Packet Structure ....................................... 8 5.1. RTP Header of ULP FEC Packets ................................ 8 5.2. FEC Header ................................................... 9 5.3. ULP Level Header ............................................. 9 6. Protection Operation .......................................... 10 7. Recovery Procedure ............................................ 11 8. Examples ...................................................... 12 8.1. An Example With Only Protection Level 0 ..................... 12 8.2. An Example That Has Identical Protection as in RFC 2733 ..... 13 8.3. An Example With Two Protection Levels (0 and 1) ............. 14 9. Security and Congestion Considerations ........................ 18 10. Indication ULP FEC Usage in SDP .............................. 19 10.1. ULP FEC as a Separate Stream ............................... 19 10.2. Use with Redundant Encoding ................................ 20 10.3. Usage with RTSP ............................................ 21 11. MIME Registrations ........................................... 21 11.1. Registration of audio/ulpfec ............................... 21 11.2. Registration of video/ulpfec ............................... 22 11.3. Registration of text/ulpfec ................................ 23 11.4. Registration of application/ulpfec ......................... 25 12. Acknowledgements ............................................. 26 13. Bibliography ................................................. 26 14. Authors' Address ............................................. 27 Adam H. Li, et al. [Page 2] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 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 Nov. 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 and settings, (3) it is designed for adaptivity, so that the FEC parameters 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 Nov. 2002 signaling, and (4) it supports 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, the unrecoverable errors due to packet loss in the bitstream usually appear as contiguous missing pieces at the end of the packets. Depending on the encoding of the media payload stream, many applications may find it easier to parse and extract data from a packet with only a contiguous piece missing at the end than a packet with multiple corrupted holes, especially when the holes are not coincident with the independently decodable fragment boundaries. 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 Adam H. Li, et al. [Page 5] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 technique. Both schemes have their own unique characteristics, and each can be applied to scenarios with different requirements. 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 Adam H. Li, et al. [Page 6] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 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 an offset mask m(k) for each protection level k. If the bit i in the mask m(k) is set to 1, then 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 amount of data that are protected at level k is indicated by L(k). 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, with examples in Section 8. 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. When ULP is used, the decoder is expected to receive and handle partially recovered packets with contiguous pieces missing at the end of the 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. Adam H. Li, et al. [Page 7] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ULP FEC Header (2 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ULP Level 0 Header (5 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ULP Level 0 Payload | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ULP Level 1 Header (5 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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 extension bit is set to 0. The market bit is set to 0. The padding bit, the CC field, and the CSRC list are set according to RFC 1889[3]. 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 ULP FEC stream is being demultiplexed via the SSRC value. The sequence number has the standard definition: it MUST be one higher than the sequence number in the previously transmitted ULP 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 ULP 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. Adam H. Li, et al. [Page 8] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 5.2. FEC Header This header is 2-octet long. The format of the header is shown in Figure 2 and consists of an SN base field and the length 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: FEC Header Format The SN base field MUST be set to the minimum sequence number of those media packets protected by FEC. This allows for the FEC operation to extend over any string of at most 24 packets. The length recovery field is used to determine the length of any recovered packets. It is computed via the protection operation applied to the unsigned network-ordered 16 bit representation of the sums of the lengths (in bytes) of the media payload, CSRC list, extension and padding of media packets associated with this FEC packet (in other words, the CSRC list, extension, and padding, if present, are "counted" as part of the payload). This allows the FEC procedure to be applied even when the lengths of the media packets are not identical. For example, assume an FEC packet is being generated by xor'ing two media packets together. The length of the two media packets are 3 (0b011) and 5 (0b101) bytes, respectively. The length recovery field is then encoded as 0b011 xor 0b101 = 0b110. 5.3. ULP Level Header The ULP Level Header is 5-octet long. The formats of the headers are shown in Figure 3 and consist of a Protection Length field and a mask 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protection Length | mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | mask (cont.) | +-+-+-+-+-+-+-+-+ Figure 3: ULP Level Header Format 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 Nov. 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. 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. Starting from the first octet of the media RTP packet header for the Level 0, the protected data of the corresponding packets of length of Protection Length 0 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. Adam H. Li, et al. [Page 10] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 This operation is repeated for all the assigned protection levels. 7. Recovery Procedures The ULP FEC packets allow end systems to recover from the loss of media packets. 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. 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. 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. Adam H. Li, et al. [Page 11] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 4 below. +------:------------+ Packet A | : | +------:------+-----+ Packet B | : | +------:--+---+ Packet C | : | +------:--+-----------------------+ Packet D | : | +------:--------------------------+ : +------+ Packet FEC | | +------+ : : :<-L0->: Figure 4 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 5. The FEC header in the ULP FEC packet is shown in Figure 6. The ULP header for level 0 in the ULP FEC packet is shown in Figure 7. 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|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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Adam H. Li, et al. [Page 12] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 Version: 2 Padding: 0 Extension: 0 Marker: 0 PT: 127 SN: 1 TS: 9 SSRC: 2 Figure 5: 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 [with Packet 8, 9, 10, and 11 marked] TS rec.: 8 [3 XOR 5 XOR 7 XOR 9] Figure 6: 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 7: ULP Level Header (Level 0) 8.2. An Example That Has Identical Protection as in RFC 2733 We can choose to extend the level 0 protection to cover the whole length of the packets (as shown in Figure 8). 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. Adam H. Li, et al. [Page 13] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 +-------------------+ : Packet A | | : +-------------+-----+ : Packet B | | : +---------+---+ : Packet C | | : +---------+-----------------------+ Packet D | | +---------------------------------+ : +---------------------------------+ Packet FEC | | +---------------------------------+ : : :<------------- L0 -------------->: Figure 8 ULP FEC scheme with only protection level 0 The resulting ULP FEC packet will have the RTP header same as shown in Figure 5 and FEC header same as shown in Figure 6. The ULP level header is shown in Figure 9. 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 9: ULP Level Header (Level 0) 8.3. An Example With Two Protection Levels (0 and 1) 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 10. In this example, we take L0 = 70 and L1 = 90. Adam H. Li, et al. [Page 14] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 +------:--------:---+ Packet A | : : | +------:------+-:---+ Packet B | : | : +------:--+---+ : : : +------+ : ULP #1 | | : +------+ : : : +------:--+ : Packet C | : | : +------:--+-----:-----------------+ Packet D | : : | +------:--------:-----------------+ : : +------:--------+ ULP #2 | : | +------:--------+ : : : :<-L0->:<--L1-->: Figure 10 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 11. The FEC header for ULP FEC packet #1 will be as shown in Figure 12. The level 0 ULP header for #1 will be shown in Figure 13. 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 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 11: RTP Header of ULP FEC #1 Adam H. Li, et al. [Page 15] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |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 [Packet 8 and 9 marked] TS rec.: 6 [3 XOR 5] Figure 12: 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 13: 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 14. The FEC header for ULP FEC packet #2 will be as shown in Figure 15. The level 0 ULP header for #2 will be shown in Figure 16. The level 1 ULP header for #2 will be shown in Figure 17. 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 Adam H. Li, et al. [Page 16] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 SN: 2 TS: 9 SSRC: 2 Figure 14: 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 [Packet 10 and 11 marked] TS rec.: 6 [7 XOR 9] Figure 15: FEC Header of ULP Packet #2 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 16: 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 [Packet 8, 9, 10, and 11 marked] The payload length for level 1 is 90 bytes. Figure 17: ULP Level Header (Level 1) for ULP Packet #2 Adam H. Li, et al. [Page 17] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 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. Adam H. Li, et al. [Page 18] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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". 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: Adam H. Li, et al. [Page 19] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 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. Adam H. Li, et al. [Page 20] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 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 Adam H. Li, et al. [Page 21] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 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 Adam H. Li, et al. [Page 22] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 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 Adam H. Li, et al. [Page 23] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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. 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. Adam H. Li, et al. [Page 24] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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. 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/video streaming tools and other applications 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 Adam H. Li, et al. [Page 25] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 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 (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. Adam H. Li, et al. [Page 26] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 [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 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: +82-31-279-5090 Fax : +82-31-279-5130 Email: dspark@samsung.com Adam H. Li, et al. [Page 27] I-Draft An RTP Payload Format for Generic FEC with ULP Nov. 2002 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 Jonathan D. Rosenberg DynamicSoft Inc. 72 Eagle Rock Avenue East Hanover, NJ 07936 USA Phone: +1-973-952-5000 Fax : +1-983-952-5050 Email: jdrosen@dynamicsoft.com Henning Schulzrinne Department of Computer Science Columbia University New York, NY 10027 USA Phone: +1-212-939-7000 Fax : +1-212-981-4470 Email: hgs@cs.columbia.edu Adam H. Li, et al. [Page 28]