Internet Engineering Task Force RMT WG INTERNET-DRAFT M.Luby/Digital Fountain draft-ietf-rmt-info-fec-00.txt L.Vicisano/Cisco J.Gemmell/Microsoft L.Rizzo/ACIRI and Univ. Pisa M.Handley/ACIRI J. Crowcroft/UCL 17 November 2000 Expires: May 2001 The use of Forward Error Correction in Reliable Multicast This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as a "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. This document is a product of the IETF RMT WG. Comments should be addressed to the authors, or the WG's mailing list at rmt@lbl.gov. Abstract This memo describes the use of Forward Error Correction (FEC) codes within the context of reliable IP multicast transport and provides an introduction to some commonly-used FEC codes. Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft [Page 1]

^L INTERNET-DRAFT Expires: May 2001 November 2000 1. Rationale and Overview There are many ways to provide reliability for transmission protocols. A common method is to use ARQ, automatic request for retransmission. With ARQ, receivers use a back channel to the sender to send requests for retransmission of lost packets. ARQ works well for one-to-one reliable protocols, as evidenced by the pervasive success of TCP/IP. ARQ has also been an effective reliability tool for one-to-many reliability protocols, and in particular for some reliable IP multicast protocols. However, for one-to-very many reliability protocols, ARQ has limitations, including the feedback implosion problem because many receivers are transmitting back to the sender, and the need for a back channel to send these requests from the receiver. Another limitation is that receivers may experience different loss patterns of packets, and thus receivers may be delayed by retransmission of packets that other receivers have lost that but they have already received. This may also cause wasteful use of bandwidth used to retransmit packets that have already been received by many of the receivers. In environments where ARQ is either costly or impossible because there is either a very limited capacity back channel or no back channel at all, such as satellite transmission, a Data Carousel approach to reliability is sometimes used [1]. With a Data Carousel, the sender partitions the object into equal length pieces of data, which we hereafter call source symbols, places them into packets, and then continually cycles through and sends these packets. Receivers continually receive packets until they have received a copy of each packet. Data Carousel has the advantage that it requires no back channel because there is no data that flows from receivers to the sender. However, Data Carousel also has limitations. For example, if a receiver loses a packet in one round of transmission it must wait an entire round before it has a chance to receive that packet again. This may also cause wasteful use of bandwidth, as the sender continually cycles through and transmits the packets until no receiver is missing a packet. FEC codes provide a reliability method that can be used to augment or replace other reliability methods, especially for one-to-many reliability protocols such as reliable IP multicast. We first briefly review some of the basic properties and types of FEC codes before reviewing their uses in the context of reliable IP multicast. Later, we provide a more detailed description of some of FEC codes. In the general literature, FEC refers to the ability to overcome both erasures (losses) and bit-level corruption. However, in the case of IP multicast, lower network layers will detect corrupted packets and discard them. Therefore, an IP multicast protocol need not be concerned with corruption; the focus is solely on erasure codes. The payloads are Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 1. [Page 2]

^L INTERNET-DRAFT Expires: May 2001 November 2000 generated and processed using an FEC erasure encoder and objects are reassembled from reception of packets using the corresponding FEC erasure decoder. The input to an FEC encoder is some number k of equal length source symbols. The FEC encoder generates some number of encoding symbols that are of the same length as the source symbols. The chosen length of the symbols can vary upon each application of the FEC encoder, or it can be fixed. These encoding symbols are placed into packets for transmission. The number of encoding symbols placed into each packet can vary on a per packet basis, or a fixed number of symbols (often one) can be placed into each packet. Also, in each packet is placed enough information to identify the particular encoding symbols carried in that packet. Upon receipt of packets containing encoding symbols, the receiver feeds these encoding symbols into the corresponding FEC decoder to recreate an exact copy of the k source symbols. Ideally, the FEC decoder can recreate an exact copy from any k of the encoding symbols. In a later section, we describe a technique for using FEC codes as described above to handle blocks with variable length source symbols. Block FEC codes work as follows. The input to a block FEC encoder is k source symbols and a number n. The encoder generates n-k redundant symbols yielding an encoding block of n encoding symbols in total composed of the k source symbols and the n-k redundant symbols. A block FEC decoder has the property that any k of the n encoding symbols in the encoding block is sufficient to reconstruct the original k source symbols. Expandable FEC codes work as follows. An expandable FEC encoder takes as input k source symbols and generates as many unique encoding symbols as requested on demand, where the amount of time for generating each encoding symbol is the same independent of how many encoding symbols are generated. Unlike block FEC codes, the source symbols are not considered part of the encoding symbols for an expandable FEC code. An expandable FEC decoder has the property that any k of the unique encoding symbols is sufficient to reconstruct the original k source symbols. Along a different dimension, we classify FEC codes loosely as being either small or large. A small FEC code is efficient in terms of processing time requirements for encoding and decoding for small values of k, and a large FEC code is efficient for encoding and decoding for much large values of k. There are implementations of standard block FEC codes that have encoding times proportional to n times the length of the k source symbols, and decoding times proportional l times the length of the k source symbols, where l is the number of missing source symbols among the k received encoding symbols. Because of the growth rate of Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 1. [Page 3]

^L INTERNET-DRAFT Expires: May 2001 November 2000 the encoding and decoding times as a function of k and n, these are typically considered to be small block FEC codes. There are close approximations to block FEC codes which for all practical purposes can generate n encoding symbols and can decode the k source symbols in time proportional to the length of the n encoding symbols. These codes are considered to be large block FEC codes. There are close approximations to expandable FEC codes which for all practical purposes can generate each encoding symbol in time proportional to its length, and can decode all k source symbols in time proportional to their length. These are considered to be large expandable FEC codes. Ideally, FEC codes in the context of IP multicast can be used to generate encoding symbols that are transmitted in packets in such a way that each received packet is fully useful to a receiver to reassemble the object regardless of previous packet reception patterns. Thus, if some packets are lost in transit between the sender and the receiver, instead of asking for specific retransmission of the lost packets or waiting till the packets are resent using Data Carousel, the receiver can use any other subsequent equal amount of data contained in packets that arrive to reassemble the object. These packets can either be proactively transmitted or they can be explicitly requested by receivers. This implies that the data contained in the same packet is fully useful to all receivers to reassemble the object, even though the receivers may have previously experienced different packet loss patterns. This property can reduce or even eliminate the problems mentioned above associated with ARQ and Data Carousel and thereby dramatically increase the scalability of the protocol to orders of magnitude more receivers. 1.1. Application of FEC codecs For some reliable IP multicast protocols, FEC codes are used in conjunction with ARQ to provide reliability. For example, a large object could be partitioned into a number of source blocks consisting of a small number of source symbols each, and in a first round of transmission all of the source symbols for all the source blocks could be transmitted. Then, receivers could report back to the sender the number of source symbols they are missing from each source block. The sender could then compute the maximum number of missing source symbols from each source block among all receivers. Based on this, a small block FEC encoder could be used to generate for each source block a number of redundant symbols equal to the computed maximum number of missing source symbols from the block among all receivers. In a second round of transmission, the server would then send all of these redundant symbols for all blocks. In this example, if there are no losses in the second round of transmission then all receivers will be able to recreate an exact copy of each original block. In this case, even if different Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 1.1. [Page 4]

^L INTERNET-DRAFT Expires: May 2001 November 2000 receivers are missing different symbols in different blocks, transmitted redundant symbols for a given block are useful to all receivers missing symbols from that block in the second round. For other reliable IP multicast protocols, FEC codes are used in a Data Carousel fashion to provide reliability, which we call an FEC Data Carousel. For example, an FEC Data Carousel using a large block FEC code could work as follows. The large block FEC encoder produces n encoding symbols considering all the k source symbols of an object as one block. The sender cycles through and transmits the n encoding symbols in packets in the same order in each round. An FEC Data Carousel can have much better protection against packet loss than a Data Carousel. For example, a receiver can join the transmission at any point in time, And, as long as the receiver receives at least k encoding symbols during the transmission of the next n encoding symbols, the receiver can completely recover the object. This is true even if the receiver starts receiving packets in the middle of a pass through the encoding symbols. This method can also be used when the object is partitioned into blocks and a short block FEC code is applied to each block separately. In this case, as we explain in more detail below, it is useful to interleave the symbols from the different blocks when they are transmitted. Since any number of encoding symbols can be generated using an expandable FEC encoder, reliable IP multicast protocols that use expandable FEC codes generally rely solely on these codes for reliability. For example, when an expandable FEC code is used in a FEC Data Carousel application, the encoding packets never repeat, and thus any k of the encoding symbols in the potentially unbounded number of encoding symbols are sufficient to recover the original k source symbols. For yet other reliable IP multicast protocols the method to obtain reliability is to generate enough encoding symbols so that each encoding symbol is transmitted at most once. For example, the sender can decide a priori how many encoding symbols it will transmit, use an FEC code to generate that number of encoding symbols from the object, and then transmit the encoding symbols to all receivers. This method is for example applicable to streaming protocols, where the stream is partitioned into objects, the source symbols for each object are encoded into encoding symbols using an FEC code, and then the sets of encoding symbols for each object are transmitted one after the other using IP multicast. Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 1.1. [Page 5]

^L INTERNET-DRAFT Expires: May 2001 November 2000 2. FEC Codes 2.1. Simple codes There are some very simple codes that are effective for repairing packet loss under very low loss conditions. For example, one simple way to provide protection from a single loss is to partition the object into fixed size source symbols and then add a redundant symbol that is the parity (XOR) of all the source symbols. The size of a source symbol is chosen so that it fits perfectly into the payload of a packet, i.e. if the packet payload is 512 bytes then each source symbol is 512 bytes. The header of each packet contains enough information to identify the payload. In this case, this includes a symbol ID. The symbol IDs are numbered consecutively starting from zero independently for the source symbols and for the redundant symbol. Thus, the packet header also contains an encoding flag that indicates whether the symbol in the payload is a source symbol or a redundant symbol, where 1 indicates source symbol and 0 indicates redundant symbol. For example, if the object consists of four source symbols that have values a, b, c and d, then the value of the redundant symbol is e = a XOR b XOR c XOR d. Then, the packets carrying these symbols look like (0, 1: a), (1, 1: b), (2, 1: c), (3, 1: d), (0, 0: e). In this example, the first two fields are in the header of the packet, where the first field is the symbol ID and the second field is the encoding flag. The portion of the packet after the colon is the payload. Any single symbol of the object can be recovered as the parity of all the other symbols. For example, if packets (0, 1: a), (1, 1: b), (3, 1: d), (0, 0: e) are received then the symbol value for the missing source symbol with ID 2 can be recovered by computing a XOR b XOR d XOR e = c. When the number of source symbols in the object is large, a simple block code variant of the above can be used. In this case, the source symbols are grouped together into source blocks of some number k of consecutive symbols each, where k may be different for different blocks. If a block consists of k source symbols then a redundant symbol is added to form an encoding block consisting of k+1 encoding symbols. Then, a source block consisting of k source symbols can be recovered from any k of the k+1 encoding symbols from the associated encoding block. Slightly more sophisticated ways of adding redundant symbols using parity can also be used. For example, one can group a block consisting of k source symbols in an object into a p x p square matrix, where p = sqrt(k). Then, for each row a redundant symbol is added that is the parity of all the source symbols in the row. Similarly, for each column Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 2.1. [Page 6]

^L INTERNET-DRAFT Expires: May 2001 November 2000 a redundant symbol is added that is the parity of all the source symbols in the column. Then, any row of the matrix can be recovered from any p of the p+1 symbols in the row, and similarly for any column. Higher dimensional product codes using this technique can also be used. However, one must be wary of using these constructions without some thought towards the possible loss patterns of symbols. Ideally, the property that one would like to obtain is that if k source symbols are encoded into n encoding symbols (the encoding symbols consist of the source symbols and the redundant symbols) then the k source symbols can be recovered from any k of the n encoding symbols. Using the simple constructions described above does not yield codes that come close to obtaining this ideal behavior. 2.2. Small block FEC codes Reliable IP multicast protocols may use a block (n, k) FEC code [2]. A popular example of these types of codes is a class of Reed-Solomon codes. For such codes, k source symbols are encoded into n > k encoding symbols, such that any k of the encoding symbols can be used to reassemble the original k source symbols. Thus, these codes have 0% reception overhead when used to encode the entire object directly. Block codes are usually systematic, which means that the n encoding symbols consist of the k source symbols and n-k redundant symbols generated from these k source symbols, where the size of a redundant symbol is the same as that for a source symbol. For example, the first simple code (XOR) described in the previous subsection is a (k+1, k) code. In general, the freedom to choose n larger than k+1 is desirable, as this can provide much better protection against losses. Codes of this sort are often based on algebraic methods using finite fields. Some of the most popular such codes are based on linear block codes. Implementations of (n, k) FEC erasure codes are efficient enough to be used by personal computers [16]. For example, [15] describes an implementation where the encoding and decoding speeds decay as C/j, where the constant C is on the order of 10 to 80 Mbytes/second for Pentium class machines of various vintages and j is upper bounded by min(k, n-k). In practice, the values of k and n must be small (below 256) for such FEC codes as large values make encoding and decoding prohibitively expensive. As many objects are longer than k symbols for reasonable values of k and the symbol length (e.g. larger than 16 kilobyte for k = 16 using 1 kilobyte symbols), they can be divided into a number of source blocks. Each source block consists of some number k of source symbols, where k may vary between different source blocks. The FEC encoder is used to encode a k source symbol source block into a n encoding symbol encoding block, where the length n of the encoding block may vary for each source block. For a receiver to completely recover Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 2.2. [Page 7]

^L INTERNET-DRAFT Expires: May 2001 November 2000 the object, for each source block consisting of k source symbols, k distinct encoding symbols (i.e., with different symbol IDs) must be received from the corresponding encoding block. For some encoding blocks, more encoding symbols may be received than there are source symbols in the corresponding source block, in which case any additional encoding symbols are discarded. An example encoding structure is shown in Figure 1. | source symbols | source symbols | | of source block 0 | of source block 1 | | | v v +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |0 |1 |2 |3 |4 |5 |6 |7 |0 |1 |2 |3 | 4|5 |6 |7 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | FEC encoder | v +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |0 |1 |2 |3 | 4| 5| 6| 7| 8| 9| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ ^ ^ | | | encoding symbols | encoding symbols | | of encoding block 0 | of encoding block 1 | Figure 1. Encoding structure for object divided into two source blocks consisting of 8 source symbols each, and the FEC encoder is used to generate 2 additional redundant symbols (10 encoding symbols in total) for each of the two source blocks. In many cases, an object is partitioned into equal length source blocks each consisting of k contiguous source symbols of the object, i.e., block c consists of the range of source symbols [ck, (c+1)k-1]. This ensure that the FEC encoder can be optimized to handle a particular number k of source symbols. This also ensures that memory references are local when the sender reads source symbols to encode, and when the receiver reads encoding symbols to decode. Locality of reference is particularly important when the object is stored on disk, as it reduces the disk seeks required. The block number and the source symbol ID within that block can be used to uniquely specify a source symbol within the object. If the size of the object is not a multiple of k source symbols, then the last source block will contain less than k symbols. Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 2.2. [Page 8]

^L INTERNET-DRAFT Expires: May 2001 November 2000 Encoding symbols can be uniquely identified by block number and encoding symbol ID. The block numbers can be numbered consecutively starting from zero. One way of identifying encoding symbols within a block are to use symbol IDs and an encoding flag that is used to specify whether an encoding symbol is a source symbol or a redundant symbol, where for example 1 indicates source symbol and 0 indicate redundant symbol. The symbol IDs can be numbered consecutively starting from zero for each block independently for the source symbols and for the redundant symbols. Thus, an encoding symbol can be identified by its block number, the encoding flag, and the symbol ID. For example, if the object consists 10 source symbols with values a, b, c, d, e, f, g, h, i, and j, and k = 5 and n = 8, then there are two source blocks consisting of 5 symbols each, and there are two encoding blocks consisting of 8 symbols each. Let p, q and r be the values of the redundant symbols for the first encoding block, and let x, y and z be the values of the redundant symbols for the second encoding block. Then the encoding symbols together with their identifiers are (0, 0, 1: a), (0, 1, 1: b), (0, 2, 1: c), (0, 3, 1: d), (0, 4, 1: e), (0, 0, 0: p), (0, 1, 0: q), (0, 2, 0: r), (1, 0, 1: f), (1, 1, 1: g), (1, 2, 1: h), (1, 3, 1: i), (1, 4, 1: j), (1, 0, 0: x), (1, 1, 0: y), (1, 2, 0: z). In this example, the first three fields identify the encoding symbol, where the first field is the block number, the second field is the symbol ID and the third field is the encoding flag. The value of the encoding symbol is written after the colon. Each block can be recovered from any 5 of the 8 encoding symbols associated with that block. For example, reception of (0, 1, 1: b), (0, 2, 1: c), (0, 3, 1: d), (0, 0, 0: p), (0, 1, 0: q) is sufficient to recover the first source block, and reception of (1, 0, 1: f), (1, 1, 1: g), (1, 0, 0: x), (1, 1, 0: y), (1, 2, 0: z) is sufficient to recover the second source block. 2.3. Large block FEC codes Tornado codes [9] are block FEC codes that provide an alternative to small block FEC codes. An (n, k) Tornado code requires slightly more than k out of n encoding symbols to reassemble k source symbols. However, the advantage is that the value of k may be on the order of tens of thousands and still run efficiently. Because of memory Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 2.3. [Page 9]

^L INTERNET-DRAFT Expires: May 2001 November 2000 considerations, in practice the value of n is restricted to be a small multiple of k, e.g., n = 2k. For example, [3] describes an implementation of Tornado codes where the encoding and decoding speeds are tens of megabytes per second range for Pentium class machines of various vintages when k is in the tens of thousands and n = 2k. The reception overhead for such values of k and n is in the 5-10% range. Tornado codes require a large amount of out of band information to be communicated to all senders and receivers for each different object length, and require an amount of memory on the encoder and decoder which is proportional to the object length times 2n/k. Tornado codes are designed to have low reception overhead on average with respect to reception of a random portion of the encoding packets. Thus, to ensure that a receiver can reassemble the object with low reception overhead, the packets are permuted into a random order before transmission. 2.4. Expandable FEC codes All of the FEC codes described up to this point are block codes. There is a different type of FEC codes that we call expandable FEC codes. Like block codes, an expandable FEC encoder operates on an object of known size that is partitioned into equal length source symbols. Unlike block codes, ideally there is no predetermined number of encoding symbols that can be generated for expandable FEC codes. Instead, an expandable FEC encoder can generate as few or as many unique encoding symbols as required on demand. Also unlike block codes, optimal expandable FEC codes have the additional attractive property that encoding symbols for the same object can be generated and transmitted from multiple servers and concurrently received by a receiver and yet the receiver incurs a 0% reception overhead. LT codes [11] are an example of large expandable FEC codes. An LT encoder uses randomization to generate each encoding symbol randomly and independently of all other encoding symbols. Like Tornado codes, the number of source symbols k may be very large for LT codes, i.e., on the order of tens to hundreds of thousands, and the encoder and decoder run efficiently in software. For example the encoding and decoding speeds for LT codes are in the range 3-20 megabytes per second for Pentium class machines of various vintages when k is in the high tens of thousands. An LT encoder closely approximates the properties of an ideal expandable FEC encoder, as it can generate as few or as many encoding symbols as required on demand. When a new encoding symbol is to be generated by an LT encoder, it is based on a randomly chosen 32-bit encoding symbol ID that uniquely describes how the encoding symbol is to be generated from the input symbols. In general, each encoding symbol ID value corresponds to a unique encoding symbol, and Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 2.4. [Page 10]

^L INTERNET-DRAFT Expires: May 2001 November 2000 thus the space of possible encoding symbols is approximately four billion. Thus, the chance that a particular encoding symbol is the same as any other particular encoding symbol is tiny. An LT decoder has the property that with very high probability the receipt of any set of slightly more than k randomly and independently generated encoding symbols is sufficient to reassemble the k source symbols. For example, when k is on the order of tens to hundreds of thousands the reception overhead is less than 5% with no failures in tens of millions of trials under a variety of loss conditions. Because encoding symbols are randomly and independently generated by choosing random encoding symbol IDs, LT codes have the property that encoding symbols for the same k source symbols can be generated and transmitted from multiple senders ad than if all the encoding symbols were generated by a single sender. The only requirement is that the senders choose their encoding symbol IDs randomly and independently of one another. There is a weak tradeoff between the number of source symbols and the reception overhead for LT codes, and the larger the number of source symbols the smaller the reception overhead. Thus, for shorter objects, it is sometimes advantageous to include multiple symbols in each packet. Normally, and in the discussion below, there is only one symbol per packet. There are a couple of factors for choosing the appropriate symbol length/ number of input symbols tradeoff. The primary consideration is that there is a fixed overhead per symbol component in the overall processing requirements of the encoding and decoding, independent of the number of input symbols. Thus, using shorter symbols means that this fixed overhead processing per symbol will be a larger component of the overall processing requirements, leading to larger overall processing requirements. Because of this, it is advisable to use a reasonably sized fixed symbol length independent of the length of the object, and thus shorter objects will be partitioned into fewer symbols than larger objects. A second much less important consideration is that there is a component of the processing per symbol that depends logarithmically on the number of input symbols, and thus for this reason there is a slight preference towards less input symbols. Like small block codes, there is a point when the object is large enough that it makes sense to partition it into blocks when using LT codes. Generally the object is partitioned into blocks whenever the number of source symbols times the packet payload length is less than the size of the object. Thus, if the packet payload is 1024 bytes and the number of source symbols is 64,000 then any object over 64 megabytes will be partitioned into more than one block. One can choose the number of source symbols to partition the object into, depending on the desired Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 2.4. [Page 11]

^L INTERNET-DRAFT Expires: May 2001 November 2000 encoding and decoding speed versus reception overhead tradeoff desired. Encoding symbols can be uniquely identified by a block number (when the object is large enough to be partitioned into more than one block) and an encoding symbol ID. The block numbers, if they are used, are generally numbered consecutively starting from zero within the object. The block number and the encoding symbol ID are both chosen uniformly and randomly from their range when an encoding symbol is to be generated and transmitted. For example, suppose the number of source symbols is 64,000 and the number of blocks is 2. Then, each packet generated by the LT encoder could be of the form (b, x: y). In this example, the first two fields identify the encoding symbol, where the first field is the block number b = 0 or 1 and the second field is the randomly chosen encoding symbol ID x. The value y after the colon is the value of the encoding symbol. 2.5. Source blocks with variable length source symbols For all the FEC codes described above, all the source symbols in the same source block are all of the same length. In this section, we describe a general technique to handle the case when it is desirable to use source symbols of varying lengths in a single source block. This technique is applicable to block FEC codes. Let l_1, l_2, ... , l_k be the lengths of k varying length source symbols to be considered part of the same source block. Let lmax be the maximum over i = 1, ... , k of l_i. To prepare the source block for the FEC encoder, pad each source symbol i out to length lmax with a suffix of lmax-i zeroes, and then prepend to the beginning of this the value l_i. Thus, each padded source symbol is of length x+lmax, assuming that the length of an original symbol takes x bytes to store. These padded source symbols, each of length x+lmax, are the input to the encoder, together with the value n. The encoder then generates n-k redundant symbols, each of length x+lmax. The encoding symbols that are placed into packets consist of the original k varying length source symbols and n-k redundant symbols, each of length x+lmax. From any k of the received encoding symbols, the FEC decoder recreates the k original source symbols as follows. If all k original source symbols are received, then no decoding is necessary. Otherwise, at least one redundant symbol is received, from which the receiver can easily whether the block was composed of variable-length source symbols: if the redundant simbol(s) has a size different (larger) from the source symbols then the source symbols are variable-length. Note that in a variable-length block the redundant symbols are always larger than the largest source symbol, due to the presence of the encoded symbol-length. The receiver can determine the value of lmax by subtracting x from the length of a received redundant symbol. Note that Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 2.5. [Page 12]

^L INTERNET-DRAFT Expires: May 2001 November 2000 x MUST be a protocol constant. For each of the received original source symbols, the receiver can generate the corresponding padded source symbol as described above. Then, the input to the FEC decoder is the set of received redundant symbols, together with the set of padded source symbols constructed from the received original symbols. The FEC decoder then produces the set of k padded source symbols. Once the k padded source symbols have been recovered, the length l_i of original source symbol i can be recovered from the first x bytes of the ith padded source symbol, and then original source symbol i is obtained by taking the next l_i bytes following the x bytes of the length field. 3. Security Considerations The use of FEC, in and of itself, imposes no additional security considerations versus sending the same information without FEC. However, just like for any transmission system, a malicious sender may intentionally transmit bad symbols. If a receiver accepts one or more bad symbols in place of authentic ones then such a receiver will have its entire object down-load corrupted by the bad symbol. Application- level transmission object authentication can detect the corrupted transfer, but the receiver must then discard the transferred object. Thus, transmitting false symbols is at least an effective denial of service attack. At worst, a malicious sender could add, delete, or replace arbitrary data within the transmitted object. In light of this possibility, FEC receivers may screen the source address of a received symbol against a list of authentic transmitter addresses. Since source addresses may be spoofed, FEC transport protocols may provide mechanisms for robust source authentication of each encoded symbol. Multicast routers along the path of a FEC transfer may provide the capability of discarding multicast packets that originated on that subnet, and whose source IP address does not correspond with that subnet. 4. Intellectual Property Disclosure Tornado codes [9] have both patents issued and patents pending. LT codes [11] have patents pending. 5. Acknowledgments Thanks to Vincent Roca and Hayder Radha for their detailed comments on this draft. Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 5. [Page 13]

^L INTERNET-DRAFT Expires: May 2001 November 2000 6. References [1] Acharya, S., Franklin, M., and Zdonik, S., ``Dissemination- Based Data Delivery Using Broadcast Disks'', IEEE Personal Communications, pp.50-60, Dec 1995. [2] Blahut, R.E., ``Theory and Practice of Error Control Codes'', Addison Wesley, MA 1984. [3] Byers, J.W., Luby, M., Mitzenmacher, M., and Rege, A., ``A Digital Fountain Approach to Reliable Distribution of Bulk Data'', Proceedings ACM SIGCOMM '98, Vancouver, Canada, Sept 1998. [4] Deering, S., ``Host Extensions for IP Multicasting'', RFC 1058, Stanford University, Stanford, CA, 1988. [5] Luby, M., Vicisano, Gemmell, J., L., Rizzo, L., Handley, M., Crowcroft, J., "RMT BB Forward Error Correction Codes", draft-ietf-rmt- bb-fec-01 submited to the IETF RMT working group, November 2000. [6] Gemmell, J., Schooler, E., and Gray, J., ``ALC Scalable Multicast File Distribution: Caching and Parameters Optimizations'' Technical Report MSR-TR-99-14, Microsoft Research, Redmond, WA, April, 1999. [7] Handley, M., and Jacobson, V., ``SDP: Session Description Protocol'', RFC 2327, April 1998. [8] Handley, M., ``SAP: Session Announcement Protocol'', Internet Draft, IETF MMUSIC Working Group, Nov 1996. [9] Luby, M., Mitzenmacher, M., Shokrollahi, A., Spielman, D., Stemann, V., ``Practical Loss-Resilient Codes'' 29th STOC'97. [10] Luby, M., Vicisano, L., Speakman, T. ``Heterogeneous multicast congestion control based on router packet filtering'', presented at RMT meeting in Pisa, March 1999. [11] Digital Fountain Web Site, www.digitalfountain.com [12] Fielding, R., Gettys, J., Mogul, J. Frystyk, H., Berners-Lee, T., Hypertext Transfer Protocol HTTP/1.1 (IETF RFC2068) http://www.rfc- editor.org/rfc/rfc2068.txt [13] Bradner, S., Key words for use in RFCs to Indicate Requirement Levels (IETF RFC 2119) http://www.rfc-editor.org/rfc/rfc2119.txt [14] Rizzo, L, and Vicisano, L., ``Reliable Multicast Data Distribution protocol based on software FEC techniques'', Proceedings of the Fourth Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 6. [Page 14]

^L INTERNET-DRAFT Expires: May 2001 November 2000 IEEE Workshop on the Architecture and Implementation of High Performance Communication Systems, HPCS-97, Chalkidiki, Greece, June 1997. [15] Rizzo, L., ``Effective Erasure Codes for Reliable Computer Communication Protocols'', ACM SIGCOMM Computer Communication Review, Vol.27, No.2, pp.24-36, Apr 1997. [16] Rizzo, L., ``On the Feasibility of Software FEC'', DEIT Tech Report, http://www.iet.unipi.it/~luigi/softfec.ps, Jan 1997. [17] Rubenstein, Dan, Kurose, Jim and Towsley, Don, ``The Impact of Multicast Layering on Network Fairness'', Proceedings of ACM SIGCOMM'99. [18] L.Vicisano, L.Rizzo, J.Crowcroft, ``TCP-like Congestion Control for Layered Multicast Data Transfer'', IEEE Infocom '98, San Francisco, CA, Mar.28-Apr.1 1998. [19] Vicisano, L., ``Notes On a Cumulative Layered Organization of Data Packets Across Multiple Streams with Different Rates'', University College London Computer Science Research Note RN/98/25, Work in Progress (May 1998). 7. Authors' Addresses Michael Luby luby@digitalfountain.com Digital Fountain 600 Alabama Street San Francisco, CA, USA, 94110 Lorenzo Vicisano lorenzo@cisco.com cisco Systems, Inc. 170 West Tasman Dr., San Jose, CA, USA, 95134 Jim Gemmell jgemmell@microsoft.com Microsoft Research 301 Howard St., #830 San Francisco, CA, USA, 94105 Luigi Rizzo luigi@iet.unipi.it ACIRI, 1947 Center St., Berkeley CA 94704 and Dip. di Ing. dell'Informazione Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 7. [Page 15]

^L INTERNET-DRAFT Expires: May 2001 November 2000 Universita` di Pisa via Diotisalvi 2, 56126 Pisa, Italy Mark Handley mjh@aciri.org ACIRI 1947 Center St. Berkeley CA, USA, 94704 Jon Crowcroft J.Crowcroft@cs.ucl.ac.uk Department of Computer Science University College London Gower Street, London WC1E 6BT, UK Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 7. [Page 16]

^L INTERNET-DRAFT Expires: May 2001 November 2000 8. Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 8. [Page 17]

^L INTERNET-DRAFT Expires: May 2001 November 2000 Luby/Vicisano/Gemmell/Rizzo/Handley/Crowcroft Section 8. [Page 18]