| RMT | V. Roca |
| Internet-Draft | A. Roumy |
| Intended status: Experimental Protocol | INRIA |
| Expires: April 26, 2012 | B. Sayadi |
| Alcatel-Lucent, Bell Labs | |
| October 24, 2011 |
The Generalized Object Encoding (GOE) LDPC-Staircase FEC Scheme
This document describes a Generalized Object Encoding (GOE) FEC Scheme for the protection of one or multiple objects, in the context of a Content Delivery Protocol (CDP) like FLUTE/ALC, FCAST/ALC or FCAST/NORM. Unlike [RFC5052], the GOE approach [GOE] decouples the definition of Generalized Objects over which FEC encoding takes place homogeneously, from the natural source object boundaries. This separation enables either an Unequal Erasure Protection (UEP) of different portions of a given source object, or an efficient and global protection of a set of potentially small files, depending on the way the Generalized Objects are defined.
The present document defines the GOE LDPC-Staircase FEC Scheme, i.e., the GOE version of the FEC Encoding ID 3 (LDPC-Staircase) defined in [RFC5170] with the further restriction that the number of encoding symbols per group (i.e., the number of symbols sent in the same packet) MUST be equal to 1 (G=1). This document does not change the LDPC-Staircase code definition, and therefore it inherits most of [RFC5170]. It only modifies the FEC Payload ID and FEC OTI, i.e., it addresses the problem of UEP and efficient file bundle protection by means of pure signaling approach.
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The use of Forward Error Correction (FEC) codes is a classic solution to improve the reliability of unicast, multicast and broadcast Content Delivery Protocols (CDP) and applications [RFC3453]. The [RFC5052] document describes a generic framework to use FEC schemes with objects (e.g., files) delivery applications based on the ALC [RFC5775] and NORM [RFC5740] reliable multicast transport protocols.
More specifically, the [RFC5053] (Raptor) and [RFC5170] (LDPC-Staircase) FEC schemes introduce erasure codes based on sparse parity check matrices for object delivery protocols like ALC and NORM. Similarly, the [RFC5510] document introduces Reed-Solomon codes based on Vandermonde matrices for the same object delivery protocols.
The way these FEC schemes is used leads to two limitations. First of all, [RFC5052] defines an approach where the same FEC encoding is applied to all the blocks of a given object, i.e., the whole object is encoded using the same FEC scheme, with the same target code rate, resulting in an equivalent protection. This approach may not suit situations where some subsets of an object deserve a higher erasure protection than the others.
A second limitation is associated to the protection of a large set of small objects. [RFC5052] defines an approach where each object is protected individually. This feature limits the robustness of their delivery: since there is a small number of source and repair packets for a given small object, a significant number of these packets may be erased thereby preventing this object to be decoded at a receiver. For instance, if the source and repair packets of a given object are transmitted in sequence (which may not be the best strategy), a packet erasure burst will significantly impact transmission robustness. Other transmission ordering strategies (e.g., with long packet interleavings or random ordering strategies) can reduce the impacts of packet erasure bursts, but they do not solve the fundamental problem of the protection of small objects. On the opposite a global FEC protection of all the objects of this set, using a single FEC encoding (when possible), provides optimal transmission robustness, since all the objects can be decoded as long as the erasure rate remains lower than the protection brought by the FEC code rate.
In order to mitigate the limitations of the traditional FEC Schemes, a better approach consists in decoupling FEC protection from the natural object boundaries. This is the goal of the Generalized Object Encoding (GOE) approach [GOE]. The set of source objects is first encoded using the No-Code FEC Scheme [RFC5445]. Each source symbol of each source object is therefore individually identified by its {TOI (i.e., ALC or NORM object identifier); SBN (source block identifier); ESI (symbol identifier)} tupple. Each Generalized Object is then defined as a sequence of consecutive No-Code encoding symbols, that starts at a given symbol, identified by its {TOI, SBN, ESI} tuple, and that is composed of a given number of such symbols. Each Generalized Object is then FEC encoded using an appropriate FEC code, with an appropriate code rate. Of course a Generalized Object may be a subset of a given source object or at the opposite may encompass several source objects. The key point when defining Generalized Objects is that all the corresponding source symbols require an equal erasure protection.
The GOE approach is independent of the nature of the FEC code, in the sense that the general mechanisms it defines is not restricted to a single type of FEC code. On the opposite, the GOE approach can be associated to any of the existing FEC schemes, re-using their code definition. However a new FEC Encoding ID value, a new FEC Object Transmission Information (FEC OTI) and a new FEC Payload ID (FPI) must be defined in order to accommodate the GOE specifics. This means that a dedicated FEC Scheme must be defined. For instance, [GOE] defines the GOE Reed-Solomon FEC Scheme for the particular case of Reed-Solomon codes over GF(2^^8) and no encoding symbol group, the GOE equivalent to FEC Encoding ID 5 defined in [RFC5510].
The present document defines the GOE LDPC-Staircase FEC Scheme, i.e., the GOE version of the FEC Encoding ID 3 (LDPC-Staircase) defined in [RFC5170], with the further restriction that the number of encoding symbols per group (i.e., the number of symbols sent in the same packet) MUST be equal to 1 (G=1).
Please refer to [GOE] for the details on the GOE procedures at a sender and at a receiver. An evaluation of GOE can also be found in [GOE.RR7699]. Finally [GOEatIETF81] provides a high level overview of GOE.
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 [RFC2119].
This document uses the following terms and definitions. Some of them are FEC scheme specific and are in line with [RFC5052]:
This document uses the following notations:
This document uses the following abbreviations:
This section introduces the formats and codes associated with the Fully-Specified FEC Scheme with FEC Encoding ID XXX, which focuses on LDPC-Staircase Codes. This GOE FEC Scheme is the GOE equivalent to FEC Encoding ID 3 defined in [RFC5170], with the further restriction that the number of encoding symbols per group (i.e., the number of symbols sent in the same packet) MUST be equal to 1 (G=1).
The FEC Payload ID, to be used only with repair packets, i.e., packets containing a repair symbol each, is composed of the Source Block Number (SBN) and the Encoding Symbol ID (ESI). There is no change in terms of format with respect to [RFC5170] but a restriction in terms of valid ESI as explained below:
There MUST be exactly one FEC Payload ID per repair packet (since G=1). This FEC Payload ID refers to the one and only symbol of the packet.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Block Number | Encoding Symbol ID (20 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Common elements are the same as those specified in [RFC5170] for FEC Encoding ID 3, namely: the Transfer-Length (L), the Encoding-Symbol-Length (E), the Maximum-Source-Block-Length (B), and the Max-Number-of-Encoding-Symbols (max_n). These common elements refer to the Generalized Object for which LDPC-Staircase encoding is needed.
The following element MUST be defined with the present FEC scheme. It defines the composition of a generalized object:
This section shows the two possible encoding formats of the above FEC OTI. The present document does not specify when one encoding format or the other should be used.
The FEC OTI binary format is the following, when the EXT_FTI mechanism is used (e.g., within the ALC [RFC5775] or NORM [RFC5740] protocols).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET = 64 | HEL | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Transfer-Length (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol Length (E) | N1m3| G = 1 | B (MSB) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| B (LSB) | Max Nb of Enc. Symbols (max_n) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PRNG seed |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|*_O| |
+-+-+ ISS_TOI (length = 32*ISS_O + 30 bits) +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ISS Source Block Number | ISS Encoding Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generalized Object Size (GOS) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
When it is desired that the FEC OTI be carried in the FDT Instance of a FLUTE session [FLUTE], the following XML attributes must be described for the associated object:
The FEC-OTI-Scheme-Specific-Info contains the string resulting from the Base64 encoding (in the XML Schema xs:base64Binary sense) of the following value:
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PRNG seed | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |*_O| | +-+-+ ISS_TOI (length = 32*ISS_O + 30 bits) + | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ISS Source Block Number | ISS Encoding Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Generalized Object Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | N1m3| G = 1 | +-+-+-+-+-+-+-+-+
During Base64 encoding, the FEC OTI Scheme-Specific Information (of variable length) is transformed into a string of printable characters (in the 64-character alphabet) that is added to the FEC-OTI-Scheme-Specific-Info attribute.
This section defines procedures that MUST be applied to FEC Encoding ID XXX. The block partitioning algorithm that is defined in Section 9.1 of [RFC5052] MUST be used. The procedure called "Determining the Maximum Source Block Length (B)" in [RFC5170] MUST be used. The procedure called "Determining the Maximum Number of Encoding Symbols Generated for Any Source Block (max_n)" in [RFC5170] MUST be used. The procedure called "Determining the Number of Encoding Symbols of a Block" in [RFC5170] MUST be used. The procedure called "Identifying the G Symbols of an Encoding Symbol Group" in [RFC5170] MUST NOT be used, since this specification requires that the number of encoding symbols per group MUST be equal to 1 (G=1). The procedure called "Pseudo-Random Number Generator" in [RFC5170] MUST be used.
The E parameter usually depends on the maximum transmission unit on the path Maximum Transmission Unit (PMTU) from the source to each receiver. This PMTU may be known, may be discovered, or may be estimated, depending on the target use case. In order to minimize the protocol header overhead (e.g., the Layered Coding Transport (LCT), UDP, IPv4, or IPv6 headers in the case of ALC), E MAY be chosen to be as large as possible. In that case, E is chosen so that the size of a packet composed of a single encoding symbol remains below but close to the PMTU (or by the minimum PMTU to each possible destinations in case of one-to-many sessions). This value E is also the source symbol size (i.e., the source symbols, before FEC encoding, and the encoding symbols, after FEC encoding, are of equal size).
This size MUST be used to segment all of the NO source objects considered by the GOE FEC schemes for this CDP into source symbols. By doing so, a Generalized Object that straddles several objects (among the NO possibles) benefits from the same source symbol size across source object boundaries.
TBD
LDPC-Staircase codes have excellent erasure recovery capabilities with large source blocks, close to ideal MDS codes. For instance, with a medium source block size k=1024, CR=2/3, N1=5, G=1, with a hybrid ITerative/Maximum Likelihood (IT/ML) decoding approach (see below) and when all symbols are sent in a random order (see below), the average overhead amounts to 0.64% (corresponding to 6.5 symbols in addition to k) and receiving 1043 symbols (corresponding to a 1.9% overhead) is sufficient to reduce the decoding failure probability to 5.1*10^^-5.
LDPC-Staircase codes are also a good solution whenever processing requirements at a software encoder or decoder must be kept to a minimum. This is true when the decoder uses an IT decoding algorithm, or an ML algorithm (we use a Gaussian Elimination as the ML algorithm) when this latter is carefully implemented and the source block size kept reasonable, or a mixture of both techniques which is the recommended solution. For instance an average decoding speed between 1.3 Gbps (corresponding to a very bad channel, close to the theoretical decoding limit and requiring an ML decoding) and 4.3 Gbps (corresponding to a medium quality channel where IT decoding is sufficient) are easily achieved with a source block size composed of k=1024 source symbols, a code rate CR=2/3 (i.e., 512 repair symbols), 1024 byte long symbols, G=1, and N1=5, on an Intel Xeon 5120/1.86GHz workstation running Linux/64 bits. Additionally, with a hybrid IT/ML approach, a receiver can decide if and when ML decoding is used, depending on local criteria (e.g., battery or CPU capabilities), independently from other receivers.
As the source block size decreases, the erasure recovery capabilities of LDPC codes in general also decrease. In the case of LDPC-Staircase codes, in order to compensate this phenomenon, it is recommended to increase the N1 parameter and to use a hybrid IT/ML decoding approach. For instance, with a small source block size k=256 symbols, CR=2/3, N1=7, and G=1, the average overhead amounts to 0.67% (corresponding to 1.7 symbols in addition to k), and receiving 267 symbols (corresponding to a 4.3% overhead) is sufficient to reduce the decoding failure probability to 1.4*10^^-5. Using N1=9 further improves these results if need be, which also enables to use LDPC-Staircase codes with k=100 symbols for instance.
With very small source blocks (e.g., a few tens symbols), using for instance Reed-Solomon codes [RFC5510] or 2D parity check codes MAY be more appropriate.
The way the FEC Repair Packets are transmitted is of high importance. A good strategy, that works well for any kind of channel loss model, consists in sending FEC Repair Packets in random order (rather than in sequence) while FEC Source Packets are sent first and in sequence. Sending all packets in a random order is another possibility, but it requires that all repair symbols for a source block be produced first, which adds some extra delay at a sender.
For further information, the interested reader can refer for instance to [Cunche08][CunchePHD10].
Values of FEC Encoding IDs and FEC Instance IDs are subject to IANA registration. For general guidelines on IANA considerations as they apply to this document, see [RFC5052].
This document assigns the Fully-Specified FEC Encoding ID XXX under the "ietf:rmt:fec:encoding" name-space to "Generalized Object Encoding for LDPC-Staircase codes".
TBD
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, . |
| [GOE] | Roca, V., Roumy, A. and S. Bessem, "The Generalized Object Encoding (GOE) Approach for the Forward Erasure Correction (FEC) Protection of Objects and its Application to Reed-Solomon Codes over GF(2^^8)", Work in progress draft-roca-rmt-goe-fec, July 2011. |
| [RFC5170] | Roca, V., Neumann, C. and D. Furodet, "Low Density Parity Check (LDPC) Forward Error Correction", RFC 5170, June 2008. |