draft-ietf-tsvwg-rlc-fec-scheme-00.txt   draft-ietf-tsvwg-rlc-fec-scheme-01.txt 
TSVWG V. Roca TSVWG V. Roca
Internet-Draft INRIA Internet-Draft B. Teibi
Intended status: Standards Track July 17, 2017 Intended status: Standards Track INRIA
Expires: January 18, 2018 Expires: April 29, 2018 October 26, 2017
Sliding Window Random Linear Code (RLC) Forward Erasure Correction (FEC) Sliding Window Random Linear Code (RLC) Forward Erasure Correction (FEC)
Scheme for FECFRAME Schemes for FECFRAME
draft-ietf-tsvwg-rlc-fec-scheme-00 draft-ietf-tsvwg-rlc-fec-scheme-01
Abstract Abstract
This document describes a fully-specified FEC scheme for the Sliding This document describes two fully-specified FEC Schemes for Sliding
Window Random Linear Codes (RLC) over GF(2^^m), where m equals 1 Window Random Linear Codes (RLC), one for RLC over GF(2) (binary
(binary case), 4 or 8, that can be used to protect arbitrary media case), a second one for RLC over GF(2^^8), both of them with the
streams along the lines defined by FECFRAME extended to sliding possibility of controlling the code density. They are meant to
window codes. These sliding window FEC codes rely on an encoding protect arbitrary media streams along the lines defined by FECFRAME
window that slides over the source symbols, generating new repair extended to sliding window FEC codes. These sliding window FEC codes
symbols whenever needed. Compared to block FEC codes, these sliding rely on an encoding window that slides over the source symbols,
window FEC codes offer key advantages with real-time flows in terms generating new repair symbols whenever needed. Compared to block FEC
of reduced FEC-related latency while often providing improved erasure codes, these sliding window FEC codes offer key advantages with real-
recovery capabilities. time flows in terms of reduced FEC-related latency while often
providing improved erasure recovery capabilities.
Status of This Memo Status of This Memo
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Limits of Block Codes with Real-Time Flows . . . . . . . 3 1.1. Limits of Block Codes with Real-Time Flows . . . . . . . 3
1.2. Lower Latency and Better Protection of Real-Time Flows 1.2. Lower Latency and Better Protection of Real-Time Flows
with the Sliding Window RLC Codes . . . . . . . . . . . . 3 with the Sliding Window RLC Codes . . . . . . . . . . . . 4
1.3. Small Transmission Overheads with the Sliding Window RLC 1.3. Small Transmission Overheads with the Sliding Window RLC
FEC Scheme . . . . . . . . . . . . . . . . . . . . . . . 4 FEC Scheme . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Document Organization . . . . . . . . . . . . . . . . . . 5 1.4. Document Organization . . . . . . . . . . . . . . . . . . 5
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 5 2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 6
3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Parameters Derivation . . . . . . . . . . . . . . . . . . 6 3.1. Parameters Derivation . . . . . . . . . . . . . . . . . . 6
3.2. ADU, ADUI and Source Symbols Mappings . . . . . . . . . . 7 3.2. ADU, ADUI and Source Symbols Mappings . . . . . . . . . . 8
3.3. Encoding Window Management . . . . . . . . . . . . . . . 9 3.3. Encoding Window Management . . . . . . . . . . . . . . . 9
3.4. Pseudo-Random Number Generator . . . . . . . . . . . . . 9 3.4. Pseudo-Random Number Generator . . . . . . . . . . . . . 10
3.5. Coding Coefficients Generation Function . . . . . . . . . 10 3.5. Coding Coefficients Generation Function . . . . . . . . . 11
4. Sliding Window RLC FEC Scheme for Arbitrary ADU Flows . . . . 12 4. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary ADU
4.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 12 Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1.1. FEC Framework Configuration Information . . . . . . . 12 4.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 13
4.1.1. FEC Framework Configuration Information . . . . . . . 13
4.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 13 4.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 13
4.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 13 4.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 14
4.1.4. Additional Procedures . . . . . . . . . . . . . . . . 15 4.1.4. Additional Procedures . . . . . . . . . . . . . . . . 14
5. FEC Code Specification . . . . . . . . . . . . . . . . . . . 15 5. Sliding Window RLC FEC Scheme over GF(2^^8) for Arbitrary ADU
5.1. Encoding Side . . . . . . . . . . . . . . . . . . . . . . 15 Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. Decoding Side . . . . . . . . . . . . . . . . . . . . . . 16 5.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 14
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 16 5.1.1. FEC Framework Configuration Information . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 5.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 15
7.1. Attacks Against the Data Flow . . . . . . . . . . . . . . 17 5.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 16
7.1.1. Access to Confidential Content . . . . . . . . . . . 17 5.1.4. Additional Procedures . . . . . . . . . . . . . . . . 17
7.1.2. Content Corruption . . . . . . . . . . . . . . . . . 17 6. FEC Code Specification . . . . . . . . . . . . . . . . . . . 17
7.2. Attacks Against the FEC Parameters . . . . . . . . . . . 17 6.1. Encoding Side . . . . . . . . . . . . . . . . . . . . . . 17
7.3. When Several Source Flows are to be Protected Together . 18 6.2. Decoding Side . . . . . . . . . . . . . . . . . . . . . . 18
7.4. Baseline Secure FEC Framework Operation . . . . . . . . . 18 7. Implementation Status . . . . . . . . . . . . . . . . . . . . 18
8. Operations and Management Considerations . . . . . . . . . . 18 8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8.1. Operational Recommendations: Finite Field Element Size (m 8.1. Attacks Against the Data Flow . . . . . . . . . . . . . . 19
Parameter) . . . . . . . . . . . . . . . . . . . . . . . 19 8.1.1. Access to Confidential Content . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 8.1.2. Content Corruption . . . . . . . . . . . . . . . . . 19
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19 8.2. Attacks Against the FEC Parameters . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 8.3. When Several Source Flows are to be Protected Together . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . 19 8.4. Baseline Secure FEC Framework Operation . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . 20 9. Operations and Management Considerations . . . . . . . . . . 20
9.1. Operational Recommendations: Finite Field GF(2) Versus
Appendix A. Decoding Beyond Maximum Latency Optimization . . . . 22 GF(2^^8) . . . . . . . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22 9.2. Operational Recommendations: Coding Coefficients Density
Threshold . . . . . . . . . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Decoding Beyond Maximum Latency Optimization . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction 1. Introduction
Application-Level Forward Erasure Correction (AL-FEC) codes are a key Application-Level Forward Erasure Correction (AL-FEC) codes are a key
element of communication systems. They are used to recover from element of communication systems. They are used to recover from
packet losses (or erasures) during content delivery sessions to a packet losses (or erasures) during content delivery sessions to a
large number of receivers (multicast/broadcast transmissions). This large number of receivers (multicast/broadcast transmissions). This
is the case with the FLUTE/ALC protocol [RFC6726] in case of reliable is the case with the FLUTE/ALC protocol [RFC6726] in case of reliable
file transfers over lossy networks, and the FECFRAME protocol for file transfers over lossy networks, and the FECFRAME protocol for
reliable continuous media transfers over lossy networks. reliable continuous media transfers over lossy networks.
skipping to change at page 3, line 44 skipping to change at page 4, line 8
which there is an incentive to increase the block size) and maximum which there is an incentive to increase the block size) and maximum
decoding latency (for which there is an incentive to decrease the decoding latency (for which there is an incentive to decrease the
block size). Therefore, with a multicast/broadcast session, the block size). Therefore, with a multicast/broadcast session, the
block code is dimensioned by considering the worst communication block code is dimensioned by considering the worst communication
channel one wants to support, and this choice impacts all receivers, channel one wants to support, and this choice impacts all receivers,
no matter their individual channel quality. no matter their individual channel quality.
1.2. Lower Latency and Better Protection of Real-Time Flows with the 1.2. Lower Latency and Better Protection of Real-Time Flows with the
Sliding Window RLC Codes Sliding Window RLC Codes
This document introduces a fully-specified FEC scheme that follows a This document introduces two fully-specified FEC Schemes that follow
totally different approach: the Sliding Window Random Linear Codes a totally different approach: the Sliding Window Random Linear Codes
(RLC) over GF(2^^m), where m equals 1, 4 or 8. This FEC scheme is (RLC) over either Finite Field GF(2) or GF(8). These FEC Schemes are
used to protect arbitrary media streams along the lines defined by used to protect arbitrary media streams along the lines defined by
FECFRAME extended to sliding window codes [fecframe-ext]. This FEC FECFRAME extended to sliding window FEC codes [fecframe-ext]. These
scheme is extremely efficient for instance with media that feature FEC Schemes are extremely efficient for instance with media that
real-time constraints sent within a multicast/broadcast session. feature real-time constraints sent within a multicast/broadcast
session.
The RLC codes belong to the broad class of sliding window AL-FEC The RLC codes belong to the broad class of sliding window AL-FEC
codes (A.K.A. convolutional codes). The encoding process is based on codes (A.K.A. convolutional codes). The encoding process is based on
an encoding window that slides over the set of source packets (in an encoding window that slides over the set of source packets (in
fact source symbols as we will see in Section 3.2), and which is fact source symbols as we will see in Section 3.2), and which is
either of fixed or variable size (elastic window). Repair packets either of fixed or variable size (elastic window). Repair packets
(symbols) are generated and sent on-the-fly, after computing a random (symbols) are generated and sent on-the-fly, after computing a random
linear combination of the source symbols present in the current linear combination of the source symbols present in the current
encoding window. encoding window.
skipping to change at page 4, line 29 skipping to change at page 4, line 41
validity period (real-time constraints), as well as the associated validity period (real-time constraints), as well as the associated
equations they are involved in (Appendix A introduces an optimisation equations they are involved in (Appendix A introduces an optimisation
that extends the time a variable is considered in the system). that extends the time a variable is considered in the system).
Erased source symbols are then recovered thanks this linear system Erased source symbols are then recovered thanks this linear system
whenever its rank permits it. whenever its rank permits it.
With RLC codes (more generally with sliding window codes), the With RLC codes (more generally with sliding window codes), the
protection of a multicast/broadcast session also needs to be protection of a multicast/broadcast session also needs to be
dimensioned by considering the worst communication channel one wants dimensioned by considering the worst communication channel one wants
to support. However the receivers experiencing a good to medium to support. However the receivers experiencing a good to medium
channel quality observe a FEC-related latency close to zero [Roca16] channel quality observe a FEC-related latency close to zero [Roca17]
since an isolated erased source packet is quickly recovered by the since an isolated erased source packet is quickly recovered by the
following repair packet. On the opposite, with a block code, following repair packet. On the opposite, with a block code,
recovering an isolated erased source packet always requires waiting recovering an isolated erased source packet always requires waiting
the end of the block for the first repair packet to arrive. the end of the block for the first repair packet to arrive.
Additionally, under certain situations (e.g., with a limited FEC- Additionally, under certain situations (e.g., with a limited FEC-
related latency budget and with constant bit rate transmissions after related latency budget and with constant bit rate transmissions after
FECFRAME encoding), sliding window codes achieve more easily a target FECFRAME encoding), sliding window codes achieve more easily a target
transmission quality (e.g., measured by the residual loss after FEC transmission quality (e.g., measured by the residual loss after FEC
decoding) by sending fewer repair packets (i.e., higher code rate) decoding) by sending fewer repair packets (i.e., higher code rate)
than block codes. than block codes.
1.3. Small Transmission Overheads with the Sliding Window RLC FEC 1.3. Small Transmission Overheads with the Sliding Window RLC FEC
Scheme Scheme
The Sliding Window RLC FEC scheme is designed so as to reduce the The Sliding Window RLC FEC Scheme is designed so as to reduce the
transmission overhead. The main requirement is that each repair transmission overhead. The main requirement is that each repair
packet header must enable a receiver to reconstruct the list of packet header must enable a receiver to reconstruct the list of
source symbols and the associated random coefficients used during the source symbols and the associated random coefficients used during the
encoding process. In order to minimize packet overhead, the set of encoding process. In order to minimize packet overhead, the set of
symbols in the encoding window as well as the set of coefficients symbols in the encoding window as well as the set of coefficients
over GF(2^^m) used in the linear combination are not individually over GF(2^^m) (where m is 1 or 8, depending on the FEC Scheme) used
listed in the repair packet header. Instead, each FEC repair packet in the linear combination are not individually listed in the repair
header contains: packet header. Instead, each FEC repair packet header contains:
o the Encoding Symbol Identifier (ESI) of the first source symbol in o the Encoding Symbol Identifier (ESI) of the first source symbol in
the encoding window as well as the number of symbols (since this the encoding window as well as the number of symbols (since this
number may vary with a variable size, elastic window). These two number may vary with a variable size, elastic window). These two
pieces of information enable each receiver to easily reconstruct pieces of information enable each receiver to easily reconstruct
the set of source symbols considered during encoding, the only the set of source symbols considered during encoding, the only
constraint being that there cannot be any gap; constraint being that there cannot be any gap;
o the seed used by a coding coefficients generation function o the seed used by a coding coefficients generation function
(Section 3.5). This information enables each receiver to generate (Section 3.5). This information enables each receiver to generate
the same set of coding coefficients over GF(2^^m) as the sender; the same set of coding coefficients over GF(2^^m) as the sender;
Therefore, no matter the number of source symbols present in the Therefore, no matter the number of source symbols present in the
encoding window, each FEC repair packet features a fixed 64-bit long encoding window, each FEC repair packet features a fixed 64-bit long
header, called Repair FEC Payload ID (Figure 7). Similarly, each FEC header, called Repair FEC Payload ID (Figure 7). Similarly, each FEC
source packet features a fixed 32-bit long trailer, called Explicit source packet features a fixed 32-bit long trailer, called Explicit
Source FEC Payload ID (Figure 5), that contains the ESI of the first Source FEC Payload ID (Figure 5), that contains the ESI of the first
source symbol (see the ADUI and source symbol mapping, Section 3.2). source symbol (see the ADUI and source symbol mapping, Section 3.2).
1.4. Document Organization 1.4. Document Organization
This fully-specified FEC scheme follows the structure required by This fully-specified FEC Scheme follows the structure required by
[RFC6363], section 5.6. "FEC Scheme Requirements", namely: [RFC6363], section 5.6. "FEC Scheme Requirements", namely:
3. Procedures: This section describes procedures specific to this 3. Procedures: This section describes procedures specific to this
FEC scheme, namely: RLC parameters derivation, ADUI and source FEC Scheme, namely: RLC parameters derivation, ADUI and source
symbols mapping, pseudo-random number generator, and coding symbols mapping, pseudo-random number generator, and coding
coefficients generation function; coefficients generation function;
4. Formats and Codes: This section defines the Source FEC Payload 4. Formats and Codes: This section defines the Source FEC Payload
ID and Repair FEC Payload ID formats, carrying the signalling ID and Repair FEC Payload ID formats, carrying the signalling
information associated to each source or repair symbol. It also information associated to each source or repair symbol. It also
defines the FEC Framework Configuration Information (FFCI) defines the FEC Framework Configuration Information (FFCI)
carrying signalling information for the session; carrying signalling information for the session;
5. FEC Code Specification: Finally this section provides the code 5. FEC Code Specification: Finally this section provides the code
specification. specification.
skipping to change at page 5, line 50 skipping to change at page 6, line 16
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
This document uses the following definitions and abbreviations: This document uses the following definitions and abbreviations:
GF(q) denotes a finite field (also known as the Galois Field) with q GF(q) denotes a finite field (also known as the Galois Field) with q
elements. We assume that q = 2^^m in this document elements. We assume that q = 2^^m in this document
m defines the length of the elements in the finite field, in bits. m defines the length of the elements in the finite field, in bits.
In this document, m is equal to 1, 4 or 8 In this document, m is equal to 1 or 8
ADU: Application Data Unit ADU: Application Data Unit
ADUI: Application Data Unit Information (includes the F, L and ADUI: Application Data Unit Information (includes the F, L and
padding fields in addition to the ADU) padding fields in addition to the ADU)
E: encoding symbol size (i.e., source or repair symbol), assumed E: encoding symbol size (i.e., source or repair symbol), assumed
fixed (in bytes) fixed (in bytes)
br_out: transmission bitrate at the output of the FECFRAME sender, br_out: transmission bitrate at the output of the FECFRAME sender,
assumed fixed (in bits/s) assumed fixed (in bits/s)
max_lat: maximum FEC-related latency within FECFRAME (in seconds) max_lat: maximum FEC-related latency within FECFRAME (in seconds)
cr: AL-FEC coding rate cr: AL-FEC coding rate
plr: packet loss rate on the erasure channel plr: packet loss rate on the erasure channel
skipping to change at page 6, line 28 skipping to change at page 6, line 42
symbols) symbols)
ls_size: linear system current size (or width) at a receiver (in ls_size: linear system current size (or width) at a receiver (in
symbols) symbols)
PRNG: pseudo-random number generator PRNG: pseudo-random number generator
pmms_rand(maxv): PRNG defined in Section 3.4 and used in this pmms_rand(maxv): PRNG defined in Section 3.4 and used in this
specification, that returns a new random integer in [0; maxv-1] specification, that returns a new random integer in [0; maxv-1]
3. Procedures 3. Procedures
This section introduces the procedures that are used by this FEC This section introduces the procedures that are used by this FEC
scheme. Scheme.
3.1. Parameters Derivation 3.1. Parameters Derivation
The Sliding Window RLC FEC Scheme relies on several key internal The Sliding Window RLC FEC Scheme relies on several key internal
parameters: parameters:
Maximum FEC-related latency budget, max_lat (in seconds) A source Maximum FEC-related latency budget, max_lat (in seconds) A source
ADU flow can have real-time constraints, and therefore any ADU flow can have real-time constraints, and therefore any
FECFRAME related operation must take place within the validity FECFRAME related operation must take place within the validity
period of each ADU. When there are multiple flows with different period of each ADU. When there are multiple flows with different
skipping to change at page 10, line 43 skipping to change at page 11, line 16
implements the Park-Miller "minimal standard" algorithm, defined implements the Park-Miller "minimal standard" algorithm, defined
above, and that scales the raw value between 0 and maxv-1 inclusive, above, and that scales the raw value between 0 and maxv-1 inclusive,
using the above scaling algorithm. using the above scaling algorithm.
Additionally, the pmms_srand(seed) function must be provided to Additionally, the pmms_srand(seed) function must be provided to
enable the initialization of the PRNG with a seed before calling enable the initialization of the PRNG with a seed before calling
pmms_rand(maxv) the first time. The seed is a 31-bit integer between pmms_rand(maxv) the first time. The seed is a 31-bit integer between
1 and 0x7FFFFFFE inclusive. In this specification, the seed is 1 and 0x7FFFFFFE inclusive. In this specification, the seed is
restricted to a value between 1 and 0xFFFF inclusive, as this is the restricted to a value between 1 and 0xFFFF inclusive, as this is the
Repair_Key 16-bit field value of the Repair FEC Payload ID Repair_Key 16-bit field value of the Repair FEC Payload ID
(Section 4.1.3). (Section 5.1.3).
3.5. Coding Coefficients Generation Function 3.5. Coding Coefficients Generation Function
The coding coefficients, used during the encoding process, are The coding coefficients, used during the encoding process, are
generated at the RLC encoder by the following function each time a generated at the RLC encoder by the generate_coding_coefficients()
new repair symbol needs to be produced: function each time a new repair symbol needs to be produced. Note
that the fraction of coefficients that are non zero (density) is
controlled by a dedicated parameter, DT (Density Threshold). When
this parameter equals 15, the maximum value, the function guaranties
that all coefficients are non zero (i.e., maximum density). When the
parameter is between 0 (minimum value) and strictly inferior to 15,
the average probability of having a non zero coefficients equals (DT
+1) / 16. The density is reduced in a controlled manner.
These considerations apply both the RLC over GF(2) and RLC over
GF(2^^8), the only difference being the value of the m parameter.
With the RLC over GF(2) FEC Scheme (Section 4), m MUST be equal to 1.
With RLC over GF(2^^8) FEC Scheme (Section 5), m MUST be equal to 8.
<CODE BEGINS> <CODE BEGINS>
/* /*
* Fills in the table of coding coefficients (of the right size) * Fills in the table of coding coefficients (of the right size)
* provided with the appropriate number of coding coefficients to * provided with the appropriate number of coding coefficients to
* use for the repair symbol key provided. * use for the repair symbol key provided.
* *
* (in) repair_key key associated to this repair symbol * (in) repair_key key associated to this repair symbol
* (in) cc_tab[] pointer to a table of the right size to store * (in) cc_tab[] pointer to a table of the right size to store
* coding coefficients. All coefficients are * coding coefficients. All coefficients are
* stored as bytes, regardless of the m parameter, * stored as bytes, regardless of the m parameter,
* upon return of this function. * upon return of this function.
* (in) cc_nb[] number of entries in the table. This value is * (in) cc_nb[] number of entries in the table. This value is
* equal to the current encoding window size. * equal to the current encoding window size.
* (in) m Finite Field GF(2^^m) parameter. * (in) density_threshold value between 0 and 15 (inclusive) that
* controls the density. With value 15, all
* coefficients are guaranteed to be non zero
* (i.e. equal to 1 with GF(2) and equal to a
* value in {1,... 255} with GF(2^^8)), otherwise
* a fraction of them will be 0.
* (in) m Finite Field GF(2^^m) parameter. In this
* version only 1 and 8 are considered.
* (out) returns an error code * (out) returns an error code
*/ */
int generate_coding_coefficients (UINT16 repair_key, int generate_coding_coefficients (UINT16 repair_key,
UINT8 cc_tab[], UINT8 cc_tab[],
UINT16 cc_nb, UINT16 cc_nb,
UINT8 density_threshold,
UINT8 m) UINT8 m)
{ {
UINT32 i; UINT32 i;
if (repair_key == 0) { if (repair_key == 0 || density_threshold > 15) {
/* bad parameters */
return SOMETHING_WENT_WRONG; return SOMETHING_WENT_WRONG;
} }
pmms_srand(repair_key); pmms_srand(repair_key);
if (m == 1) { switch (m) {
/* 0 is a valid coefficient value in binary GF */ case 1:
for (i = 0 ; i < cc_nb ; i ++) { if (density_threshold == 15) {
cc_tab[i] = (UINT8) pmms_rand(2); /* all coefficients are 1 */
memset(cc_tab, 1, cc_nb);
} else {
for (i = 0 ; i < cc_nb ; i++) {
if (pmms_rand(16) <= density_threshold) {
cc_tab[i] = (UINT8) 1;
} else {
cc_tab[i] = (UINT8) 0;
}
}
} }
} else { break;
/* coefficient 0 is avoided in non-binary GF to consider each
* source symbol */ case 8:
UINT32 maxv; if (density_threshold == 15) {
maxv = get_gf_size(); /* i.e., 16 if m=4 or 256 if m=8 */ /* coefficient 0 is avoided here in order to include
for (i = 0 ; i < cc_nb ; i ++) { * all the source symbols */
do { for (i = 0 ; i < cc_nb ; i++) {
cc_tab[i] = (UINT8) pmms_rand(maxv); do {
} while (cc_tab[i] == 0) cc_tab[i] = (UINT8) pmms_rand(256);
} while (cc_tab[i] == 0);
}
} else {
/* here a certain fraction of coefficients should be 0 */
for (i = 0 ; i < cc_nb ; i++) {
if (pmms_rand(16) <= density_threshold) {
do {
cc_tab[i] = (UINT8) pmms_rand(256);
} while (cc_tab[i] == 0);
} else {
cc_tab[i] = 0;
}
}
} }
break;
default:
/* bad parameter m */
return SOMETHING_WENT_WRONG;
} }
return EVERYTHING_IS_OKAY; return EVERYTHING_IS_OKAY;
} }
<CODE ENDS> <CODE ENDS>
Figure 2: Coding Coefficients Generation Function pseudo-code Figure 2: Coding Coefficients Generation Function pseudo-code
4. Sliding Window RLC FEC Scheme for Arbitrary ADU Flows 4. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary ADU Flows
This fully-specified FEC Scheme defines the Sliding Window Random
Linear Codes (RLC) over GF(2) (binary case).
4.1. Formats and Codes 4.1. Formats and Codes
4.1.1. FEC Framework Configuration Information 4.1.1. FEC Framework Configuration Information
4.1.1.1. Mandatory Information
o FEC Encoding ID: the value assigned to this fully specified FEC
Scheme MUST be YYYY, as assigned by IANA (Section 10).
When SDP is used to communicate the FFCI, this FEC Encoding ID is
carried in the 'encoding-id' parameter.
4.1.1.2. FEC Scheme-Specific Information
All the considerations of Section 5.1.1.2 apply equally here.
4.1.2. Explicit Source FEC Payload ID
All the considerations of Section 5.1.1.2 apply equally here.
4.1.3. Repair FEC Payload ID
All the considerations of Section 5.1.1.2 apply equally here.
4.1.4. Additional Procedures
All the considerations of Section 5.1.1.2 apply equally here.
5. Sliding Window RLC FEC Scheme over GF(2^^8) for Arbitrary ADU Flows
This fully-specified FEC Scheme defines the Sliding Window Random
Linear Codes (RLC) over GF(2^^8).
5.1. Formats and Codes
5.1.1. FEC Framework Configuration Information
The FEC Framework Configuration Information (or FFCI) includes The FEC Framework Configuration Information (or FFCI) includes
information that MUST be communicated between the sender and information that MUST be communicated between the sender and
receiver(s). More specifically, it enables the synchronization of receiver(s). More specifically, it enables the synchronization of
the FECFRAME sender and receiver instances. It includes both the FECFRAME sender and receiver instances. It includes both
mandatory elements and scheme-specific elements, as detailed below. mandatory elements and scheme-specific elements, as detailed below.
4.1.1.1. Mandatory Information 5.1.1.1. Mandatory Information
o FEC Encoding ID: the value assigned to this fully specified FEC o FEC Encoding ID: the value assigned to this fully specified FEC
scheme MUST be XXXX, as assigned by IANA (Section 9). Scheme MUST be XXXX, as assigned by IANA (Section 10).
When SDP is used to communicate the FFCI, this FEC Encoding ID is When SDP is used to communicate the FFCI, this FEC Encoding ID is
carried in the 'encoding-id' parameter. carried in the 'encoding-id' parameter.
4.1.1.2. FEC Scheme-Specific Information 5.1.1.2. FEC Scheme-Specific Information
The FEC Scheme-Specific Information (FSSI) includes elements that are The FEC Scheme-Specific Information (FSSI) includes elements that are
specific to the present FEC scheme. More precisely: specific to the present FEC Scheme. More precisely:
Encoding symbol size (E): a non-negative integer that indicates the Encoding symbol size (E): a non-negative integer that indicates the
size of each encoding symbol in bytes; size of each encoding symbol in bytes;
m parameter (m): the length of the elements in the finite field, in
bits, where m is equal to 1, 4 or 8;
These elements are required both by the sender (RLC encoder) and the This element is required both by the sender (RLC encoder) and the
receiver(s) (RLC decoder). receiver(s) (RLC decoder).
When SDP is used to communicate the FFCI, this FEC scheme-specific When SDP is used to communicate the FFCI, this FEC Scheme-specific
information is carried in the 'fssi' parameter in textual information is carried in the 'fssi' parameter in textual
representation as specified in [RFC6364]. For instance: representation as specified in [RFC6364]. For instance:
fssi=E:1400,m:8 fssi=E:1400
If another mechanism requires the FSSI to be carried as an opaque If another mechanism requires the FSSI to be carried as an opaque
octet string (for instance, after a Base64 encoding), the encoding octet string (for instance, after a Base64 encoding), the encoding
format consists of the following 2 octets: format consists of the following 2 octets:
Encoding symbol length (E): 16-bit field. Encoding symbol length (E): 16-bit field.
m parameter (m): 8-bit field.
0 1 2 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol Length (E) | m | | Encoding Symbol Length (E) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: FSSI Encoding Format Figure 3: FSSI Encoding Format
4.1.2. Explicit Source FEC Payload ID 5.1.2. Explicit Source FEC Payload ID
A FEC source packet MUST contain an Explicit Source FEC Payload ID A FEC source packet MUST contain an Explicit Source FEC Payload ID
that is appended to the end of the packet as illustrated in Figure 4. that is appended to the end of the packet as illustrated in Figure 4.
+--------------------------------+ +--------------------------------+
| IP Header | | IP Header |
+--------------------------------+ +--------------------------------+
| Transport Header | | Transport Header |
+--------------------------------+ +--------------------------------+
| ADU | | ADU |
skipping to change at page 13, line 48 skipping to change at page 16, line 5
wrapping to zero occurs. wrapping to zero occurs.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol ID (ESI) | | Encoding Symbol ID (ESI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Source FEC Payload ID Encoding Format Figure 5: Source FEC Payload ID Encoding Format
4.1.3. Repair FEC Payload ID 5.1.3. Repair FEC Payload ID
A FEC repair packet MUST contain a Repair FEC Payload ID that is A FEC repair packet MUST contain a Repair FEC Payload ID that is
prepended to the repair symbol as illustrated in Figure 6. There can prepended to the repair symbol as illustrated in Figure 6. There can
be one or more repair symbols per FEC repair packet. When this is be one or more repair symbols per FEC repair packet. When this is
the case, the number of repair symbols within this FEC repair packet the case, the number of repair symbols within this FEC repair packet
is easily deduced by comparing the known received FEC repair packet is easily deduced by comparing the known received FEC repair packet
size (equal to the UDP payload size when UDP is the underlying size (equal to the UDP payload size when UDP is the underlying
transport protocol) and the symbol size, E, communicated in the FFCI. transport protocol) and the symbol size, E, communicated in the FFCI.
When this is the case, all the repair symbols MUST have been When this is the case, all the repair symbols MUST have been
generated from the same encoding window. generated from the same encoding window.
skipping to change at page 14, line 35 skipping to change at page 16, line 41
following fields (Figure 7): following fields (Figure 7):
Repair_Key (16-bit field): this unsigned integer is used as a seed Repair_Key (16-bit field): this unsigned integer is used as a seed
by the coefficient generation function (Section 3.5) in order to by the coefficient generation function (Section 3.5) in order to
generate the desired number of coding coefficients. Value 0 MUST generate the desired number of coding coefficients. Value 0 MUST
NOT be used. When a FEC repair packet contains several repair NOT be used. When a FEC repair packet contains several repair
symbols, this repair key value is that of the first repair symbol. symbols, this repair key value is that of the first repair symbol.
The remaining repair keys can be deduced by incrementing by 1 this The remaining repair keys can be deduced by incrementing by 1 this
value, up to a maximum value of 65535 after which it loops back to value, up to a maximum value of 65535 after which it loops back to
1 (note that 0 is not a valid value). 1 (note that 0 is not a valid value).
Number of Source Symbols in the Encoding Window, NSS (16-bit field): Coding coefficients Density Threshold, DT (4-bit field): this
unsigned integer carried the Density Threshold (DT) used by the
coding coefficient generation function Section 3.5. More
precisely, it controls the probability of having a non zero coding
coefficient, which equals (DT+1) / 16. When a FEC repair packet
contains several repair symbols, the DT value applies to all of
them;
Number of Source Symbols in the Encoding Window, NSS (12-bit field):
this unsigned integer indicates the number of source symbols in this unsigned integer indicates the number of source symbols in
the encoding window when this repair symbol was generated. When a the encoding window when this repair symbol was generated. When a
FEC repair packet contains several repair symbols, this NSS value FEC repair packet contains several repair symbols, this NSS value
applies to all of them; applies to all of them;
ESI of first source symbol in encoding window, FSS_ESI (32-bit ESI of first source symbol in encoding window, FSS_ESI (32-bit
field): field):
this unsigned integer indicates the ESI of the first source symbol this unsigned integer indicates the ESI of the first source symbol
in the encoding window when this repair symbol was generated. in the encoding window when this repair symbol was generated.
When a FEC repair packet contains several repair symbols, this When a FEC repair packet contains several repair symbols, this
FSS_ESI value applies to all of them; FSS_ESI value applies to all of them;
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Repair_Key | NSS (# source symbols in ew) | | Repair_Key | DT |NSS (# src symb in ew) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FSS_ESI | | FSS_ESI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Repair FEC Payload ID Encoding Format Figure 7: Repair FEC Payload ID Encoding Format
4.1.4. Additional Procedures 5.1.4. Additional Procedures
The following procedure applies: The following procedure applies:
o The ESI of source symbols MUST start with value 0 for the first o The ESI of source symbols MUST start with value 0 for the first
source symbol and MUST be managed sequentially. Wrapping to zero source symbol and MUST be managed sequentially. Wrapping to zero
will happen after reaching the maximum 32-bit value. will happen after reaching the maximum 32-bit value.
5. FEC Code Specification 6. FEC Code Specification
5.1. Encoding Side 6.1. Encoding Side
This section provides a high level description of a Sliding Window This section provides a high level description of a Sliding Window
RLC encoder. RLC encoder.
Whenever a new FEC repair packet is needed, the RLC encoder instance Whenever a new FEC repair packet is needed, the RLC encoder instance
first gathers the ew_size source symbols currently in the sliding first gathers the ew_size source symbols currently in the sliding
encoding window. Then it chooses a repair key, which can be a non encoding window. Then it chooses a repair key, which can be a non
zero monotonically increasing integer value, incremented for each zero monotonically increasing integer value, incremented for each
repair symbol up to a maximum value of 65535 (as it is carried within repair symbol up to a maximum value of 65535 (as it is carried within
a 16-bit field) after which it loops back to 1 (indeed, being used as a 16-bit field) after which it loops back to 1 (indeed, being used as
skipping to change at page 16, line 5 skipping to change at page 18, line 11
is small and when there is an incentive to pack several repair is small and when there is an incentive to pack several repair
symbols within the same FEC Repair Packet, the appropriate number of symbols within the same FEC Repair Packet, the appropriate number of
repair symbols are computed. The only constraint is to increment by repair symbols are computed. The only constraint is to increment by
1 the repair key for each of them, keeping the same ew_size source 1 the repair key for each of them, keeping the same ew_size source
symbols, since only the first repair key will be carried in the symbols, since only the first repair key will be carried in the
Repair FEC Payload ID. The FEC repair packet can then be sent. The Repair FEC Payload ID. The FEC repair packet can then be sent. The
source versus repair FEC packet transmission order is out of scope of source versus repair FEC packet transmission order is out of scope of
this document and several approaches exist that are implementation this document and several approaches exist that are implementation
specific. specific.
5.2. Decoding Side 6.2. Decoding Side
This section provides a high level description of a Sliding Window This section provides a high level description of a Sliding Window
RLC decoder. RLC decoder.
A FECFRAME receiver needs to maintain a linear system whose variables A FECFRAME receiver needs to maintain a linear system whose variables
are the received and lost source symbols. Upon receiving a FEC are the received and lost source symbols. Upon receiving a FEC
repair packet, a receiver first extracts all the repair symbols it repair packet, a receiver first extracts all the repair symbols it
contains (in case several repair symbols are packed together). For contains (in case several repair symbols are packed together). For
each repair symbol, when at least one of the corresponding source each repair symbol, when at least one of the corresponding source
symbols it protects has been lost, the receiver adds an equation to symbols it protects has been lost, the receiver adds an equation to
skipping to change at page 16, line 41 skipping to change at page 18, line 47
as this is an operational and management decision. as this is an operational and management decision.
With real-time flows, a lost ADU that is decoded after the maximum With real-time flows, a lost ADU that is decoded after the maximum
latency (or an ADU received far too late) should not be considered by latency (or an ADU received far too late) should not be considered by
the application. Instead the associated source symbols should be the application. Instead the associated source symbols should be
removed from the linear system maintained by the receiver(s). removed from the linear system maintained by the receiver(s).
Appendix A discusses a backward compatible optimization whereby those Appendix A discusses a backward compatible optimization whereby those
late source symbols may still be useful to improve the global loss late source symbols may still be useful to improve the global loss
recovery performance. recovery performance.
6. Implementation Status 7. Implementation Status
Editor's notes: RFC Editor, please remove this section motivated by Editor's notes: RFC Editor, please remove this section motivated by
RFC 6982 before publishing the RFC. Thanks. RFC 6982 before publishing the RFC. Thanks.
An implementation of the Sliding Window RLC FEC Scheme for FECFRAME An implementation of the Sliding Window RLC FEC Scheme for FECFRAME
exists: exists:
o Organisation: Inria o Organisation: Inria
o Description: This is an implementation of the Sliding Window RLC o Description: This is an implementation of the Sliding Window RLC
FEC Scheme. It relies on a modified version of our OpenFEC FEC Scheme. It relies on a modified version of our OpenFEC
(http://openfec.org) FEC code library. It is integrated in our (http://openfec.org) FEC code library. It is integrated in our
FECFRAME software (see [fecframe-ext]). FECFRAME software (see [fecframe-ext]).
o Maturity: prototype. o Maturity: prototype.
o Coverage: this software complies with the Sliding Window RLC FEC o Coverage: this software complies with the Sliding Window RLC FEC
Scheme (limited to m=8 as of June, 2017). Scheme (limited to m=8 as of June, 2017).
o Lincensing: proprietary. o Lincensing: proprietary.
o Contact: vincent.roca@inria.fr o Contact: vincent.roca@inria.fr
7. Security Considerations 8. Security Considerations
The FEC Framework document [RFC6363] provides a comprehensive The FEC Framework document [RFC6363] provides a comprehensive
analysis of security considerations applicable to FEC schemes. analysis of security considerations applicable to FEC Schemes.
Therefore, the present section follows the security considerations Therefore, the present section follows the security considerations
section of [RFC6363] and only discusses specific topics. section of [RFC6363] and only discusses specific topics.
7.1. Attacks Against the Data Flow 8.1. Attacks Against the Data Flow
7.1.1. Access to Confidential Content 8.1.1. Access to Confidential Content
The Sliding Window RLC FEC Scheme specified in this document does not The Sliding Window RLC FEC Scheme specified in this document does not
change the recommendations of [RFC6363]. To summarize, if change the recommendations of [RFC6363]. To summarize, if
confidentiality is a concern, it is RECOMMENDED that one of the confidentiality is a concern, it is RECOMMENDED that one of the
solutions mentioned in [RFC6363] is used with special considerations solutions mentioned in [RFC6363] is used with special considerations
to the way this solution is applied (e.g., is encryption applied to the way this solution is applied (e.g., is encryption applied
before or after FEC protection, within the end-system or in a before or after FEC protection, within the end-system or in a
middlebox) to the operational constraints (e.g., performing FEC middlebox) to the operational constraints (e.g., performing FEC
decoding in a protected environment may be complicated or even decoding in a protected environment may be complicated or even
impossible) and to the threat model. impossible) and to the threat model.
7.1.2. Content Corruption 8.1.2. Content Corruption
The Sliding Window RLC FEC Scheme specified in this document does not The Sliding Window RLC FEC Scheme specified in this document does not
change the recommendations of [RFC6363]. To summarize, it is change the recommendations of [RFC6363]. To summarize, it is
RECOMMENDED that one of the solutions mentioned in [RFC6363] is used RECOMMENDED that one of the solutions mentioned in [RFC6363] is used
on both the FEC Source and Repair Packets. on both the FEC Source and Repair Packets.
7.2. Attacks Against the FEC Parameters 8.2. Attacks Against the FEC Parameters
The FEC Scheme specified in this document defines parameters that can The FEC Scheme specified in this document defines parameters that can
be the basis of attacks. More specifically, the following parameters be the basis of attacks. More specifically, the following parameters
of the FFCI may be modified by an attacker who only targets receivers of the FFCI may be modified by an attacker who only targets receivers
(Section 4.1.1.2): (Section 5.1.1.2):
o FEC Encoding ID: changing this parameter leads the receivers to o FEC Encoding ID: changing this parameter leads the receivers to
consider a different FEC Scheme, which enables an attacker to consider a different FEC Scheme, which enables an attacker to
create a Denial of Service (DoS); create a Denial of Service (DoS);
o Encoding symbol length (E): setting this E parameter to a o Encoding symbol length (E): setting this E parameter to a
different value will confuse the receivers and create a DoS. More different value will confuse the receivers and create a DoS. More
precisely, the FEC Repair Packets received will probably no longer precisely, the FEC Repair Packets received will probably no longer
be multiple of E, leading receivers to reject them; be multiple of E, leading receivers to reject them;
o m parameter: changing this parameter triggers a DoS since the
receivers will generate a different set of coding coefficients.
The recovered source symbols (and thereafter ADUs) will be
corrupted.
An attacker who only targets a sender will achieve the same results. An attacker who only targets a sender will achieve the same results.
However if the attacker targets both sender and receivers at the same However if the attacker targets both sender and receivers at the same
time (the same wrong piece of information is communicated to time (the same wrong piece of information is communicated to
everybody), the results will be suboptimal but less severe. everybody), the results will be suboptimal but less severe.
It is therefore RECOMMENDED that security measures are taken to It is therefore RECOMMENDED that security measures are taken to
guarantee the FFCI integrity, as specified in [RFC6363]. How to guarantee the FFCI integrity, as specified in [RFC6363]. How to
achieve this depends on the way the FFCI is communicated from the achieve this depends on the way the FFCI is communicated from the
sender to the receiver, which is not specified in this document. sender to the receiver, which is not specified in this document.
Similarly, attacks are possible against the Explicit Source FEC Similarly, attacks are possible against the Explicit Source FEC
Payload ID and Repair FEC Payload ID: by modifying the Encoding Payload ID and Repair FEC Payload ID: by modifying the Encoding
Symbol ID (ESI), or the repair key, NSS or FSS_ESI. It is therefore Symbol ID (ESI), or the repair key, NSS or FSS_ESI. It is therefore
RECOMMENDED that security measures are taken to guarantee the FEC RECOMMENDED that security measures are taken to guarantee the FEC
Source and Repair Packets as stated in [RFC6363]. Source and Repair Packets as stated in [RFC6363].
7.3. When Several Source Flows are to be Protected Together 8.3. When Several Source Flows are to be Protected Together
The Sliding Window RLC FEC Scheme specified in this document does not The Sliding Window RLC FEC Scheme specified in this document does not
change the recommendations of [RFC6363]. change the recommendations of [RFC6363].
7.4. Baseline Secure FEC Framework Operation 8.4. Baseline Secure FEC Framework Operation
The Sliding Window RLC FEC Scheme specified in this document does not The Sliding Window RLC FEC Scheme specified in this document does not
change the recommendations of [RFC6363] concerning the use of the change the recommendations of [RFC6363] concerning the use of the
IPsec/ESP security protocol as a mandatory to implement (but not IPsec/ESP security protocol as a mandatory to implement (but not
mandatory to use) security scheme. This is well suited to situations mandatory to use) security scheme. This is well suited to situations
where the only insecure domain is the one over which the FEC where the only insecure domain is the one over which the FEC
Framework operates. Framework operates.
8. Operations and Management Considerations 9. Operations and Management Considerations
The FEC Framework document [RFC6363] provides a comprehensive The FEC Framework document [RFC6363] provides a comprehensive
analysis of operations and management considerations applicable to analysis of operations and management considerations applicable to
FEC schemes. Therefore, the present section only discusses specific FEC Schemes. Therefore, the present section only discusses specific
topics. topics.
8.1. Operational Recommendations: Finite Field Element Size (m 9.1. Operational Recommendations: Finite Field GF(2) Versus GF(2^^8)
Parameter)
The present document requires that m equals 1 (binary case), 4 or 8.
It is expected that m = 8 will be mostly used since it warrants a
high loss protection. Additionally, elements in the finite field are
8 bits long, which makes read/write memory operations aligned on
bytes during encoding and decoding.
An alternative when one can accommodate a lower loss protection is The present document specifies two FEC Schemes that differ on the
m = 4. Elements in the finite field are 4 bits long, so if 2 associated Finite Field used for the coding coefficients. It is
elements are accessed at a time, read/write memory operations are expected that the RLC over GF(2^^8) FEC Scheme will be mostly used
aligned on bytes during encoding and decoding. since it warrants a high loss protection. Additionally, elements in
the finite field are 8 bits long, which makes read/write memory
operations aligned on bytes during encoding and decoding.
Finally, in particular when dealing with large encoding windows, an Finally, in particular when dealing with large encoding windows, an
alternative is m = 1. In that case operations symbols can be alternative is the RLC over GF(2) FEC Scheme. In that case
directly XORed together which warrants high bitrate encoding and operations symbols can be directly XORed together which warrants high
decoding operations. bitrate encoding and decoding operations.
Since several values for the m parameter are possible, the use case 9.2. Operational Recommendations: Coding Coefficients Density Threshold
SHOULD define which value or values need to be supported. In any
case, any compliant implementation MUST support at least the default
m = 8 value.
9. IANA Considerations In addition to the choice of the Finite Field, the two FEC Schemes
define a coding coefficient density threshold parameter. This
parameter enables a sender to control the code density, i.e., the
proportion of coefficients that are non zero on average. With RLC
over GF(2^^8), it is recommended that small encoding windows be
associated to a density threshold equal to 15, the maximum value, in
order to warrant a high loss protection.
This document registers one value in the "FEC Framework (FECFRAME) On the opposite, with large encoding windows, it it recommened that
the density threshold be reduced. With large encoding windows, an
alternative can be to use RLC over GF(2) and a density threshold
equal to 8 (i.e., an average density equal to 1/2) or smaller.
Note also that using a density threshold equal to 15 with RLC over
GF(2) is equivalent to using code that XOR's all the source symbols
of the encoding window. In that case it follows that: (1) a single
repair symbol can be produced for a given encoding window, and (2)
the repair_key parameter is useless (the coding coefficients
generation function does not rely on the PRNG).
10. IANA Considerations
This document registers two values in the "FEC Framework (FECFRAME)
FEC Encoding IDs" registry [RFC6363] as follows: FEC Encoding IDs" registry [RFC6363] as follows:
o XXX refers to the Sliding Window Random Linear Codes (RLC) FEC o YYYY refers to the Sliding Window Random Linear Codes (RLC) over
Scheme for Arbitrary Packet Flows, as defined in Section XXX of GF(2) FEC Scheme for Arbitrary Packet Flows, as defined in
this document. Section 4 of this document.
o XXXX refers to the Sliding Window Random Linear Codes (RLC) over
GF(2^^8) FEC Scheme for Arbitrary Packet Flows, as defined in
Section 5 of this document.
10. Acknowledgments 11. Acknowledgments
The authors would like to thank Belkacem Teibi (Inria) who in The authors would like to thank Marie-Jose Montpetit for her valuable
particular implemented the RLC codec. The author would also like to feedbacks on this document.
thank Marie-Jose Montpetit for her valuable feedbacks on this
document.
11. References 12. References
11.1. Normative References 12.1. Normative References
[fecframe-ext] [fecframe-ext]
Roca, V. and A. Begen, "Forward Error Correction (FEC) Roca, V. and A. Begen, "Forward Error Correction (FEC)
Framework Extension to Sliding Window Codes", Transport Framework Extension to Sliding Window Codes", Transport
Area Working Group (TSVWG) draft-roca-tsvwg-fecframev2 Area Working Group (TSVWG) draft-roca-tsvwg-fecframev2
(Work in Progress), June 2017, (Work in Progress), June 2017,
<https://tools.ietf.org/html/draft-roca-tsvwg-fecframev2>. <https://tools.ietf.org/html/draft-roca-tsvwg-fecframev2>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363, Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011, DOI 10.17487/RFC6363, October 2011,
<http://www.rfc-editor.org/info/rfc6363>. <https://www.rfc-editor.org/info/rfc6363>.
[RFC6364] Begen, A., "Session Description Protocol Elements for the [RFC6364] Begen, A., "Session Description Protocol Elements for the
Forward Error Correction (FEC) Framework", RFC 6364, Forward Error Correction (FEC) Framework", RFC 6364,
DOI 10.17487/RFC6364, October 2011, DOI 10.17487/RFC6364, October 2011,
<http://www.rfc-editor.org/info/rfc6364>. <https://www.rfc-editor.org/info/rfc6364>.
11.2. Informative References 12.2. Informative References
[CA90] Carta, D., "Two Fast Implementations of the Minimal [CA90] Carta, D., "Two Fast Implementations of the Minimal
Standard Random Number Generator", Communications of the Standard Random Number Generator", Communications of the
ACM, Vol. 33, No. 1, pp.87-88, January 1990. ACM, Vol. 33, No. 1, pp.87-88, January 1990.
[PM88] Park, S. and K. Miller, "Random Number Generators: Good [PM88] Park, S. and K. Miller, "Random Number Generators: Good
Ones are Hard to Find", Communications of the ACM, Vol. Ones are Hard to Find", Communications of the ACM, Vol.
31, No. 10, pp.1192-1201, 1988. 31, No. 10, pp.1192-1201, 1988.
[PTVF92] Press, W., Teukolsky, S., Vetterling, W., and B. Flannery, [PTVF92] Press, W., Teukolsky, S., Vetterling, W., and B. Flannery,
skipping to change at page 20, line 49 skipping to change at page 23, line 13
University Press, ISBN: 0-521-43108-5, 1992. University Press, ISBN: 0-521-43108-5, 1992.
[rand31pmc] [rand31pmc]
Whittle, R., "31 bit pseudo-random number generator", Whittle, R., "31 bit pseudo-random number generator",
September 2005, <http://www.firstpr.com.au/dsp/rand31/ September 2005, <http://www.firstpr.com.au/dsp/rand31/
rand31-park-miller-carta.cc.txt>. rand31-park-miller-carta.cc.txt>.
[RFC5170] Roca, V., Neumann, C., and D. Furodet, "Low Density Parity [RFC5170] Roca, V., Neumann, C., and D. Furodet, "Low Density Parity
Check (LDPC) Staircase and Triangle Forward Error Check (LDPC) Staircase and Triangle Forward Error
Correction (FEC) Schemes", RFC 5170, DOI 10.17487/RFC5170, Correction (FEC) Schemes", RFC 5170, DOI 10.17487/RFC5170,
June 2008, <http://www.rfc-editor.org/info/rfc5170>. June 2008, <https://www.rfc-editor.org/info/rfc5170>.
[RFC6726] Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen, [RFC6726] Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen,
"FLUTE - File Delivery over Unidirectional Transport", "FLUTE - File Delivery over Unidirectional Transport",
RFC 6726, DOI 10.17487/RFC6726, November 2012, RFC 6726, DOI 10.17487/RFC6726, November 2012,
<http://www.rfc-editor.org/info/rfc6726>. <https://www.rfc-editor.org/info/rfc6726>.
[RFC6816] Roca, V., Cunche, M., and J. Lacan, "Simple Low-Density [RFC6816] Roca, V., Cunche, M., and J. Lacan, "Simple Low-Density
Parity Check (LDPC) Staircase Forward Error Correction Parity Check (LDPC) Staircase Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6816, (FEC) Scheme for FECFRAME", RFC 6816,
DOI 10.17487/RFC6816, December 2012, DOI 10.17487/RFC6816, December 2012,
<http://www.rfc-editor.org/info/rfc6816>. <https://www.rfc-editor.org/info/rfc6816>.
[RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K. [RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K.
Matsuzono, "Simple Reed-Solomon Forward Error Correction Matsuzono, "Simple Reed-Solomon Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6865, (FEC) Scheme for FECFRAME", RFC 6865,
DOI 10.17487/RFC6865, February 2013, DOI 10.17487/RFC6865, February 2013,
<http://www.rfc-editor.org/info/rfc6865>. <https://www.rfc-editor.org/info/rfc6865>.
[Roca16] Roca, V., Teibi, B., Burdinat, C., Tran, T., and C. [Roca16] Roca, V., Teibi, B., Burdinat, C., Tran, T., and C.
Thienot, "Block or Convolutional AL-FEC Codes? A Thienot, "Block or Convolutional AL-FEC Codes? A
Performance Comparison for Robust Low-Latency Performance Comparison for Robust Low-Latency
Communications", Submitted for publication Communications", HAL open-archive document,hal-01395937
https://hal.inria.fr/hal-01395937/en/, November 2016, < https://hal.inria.fr/hal-01395937/en/, November 2016, <
https://hal.inria.fr/hal-01395937/en/>. https://hal.inria.fr/hal-01395937/en/>.
[Roca17] Roca, V., Teibi, B., Burdinat, C., Tran, T., and C.
Thienot, "Less Latency and Better Protection with AL-FEC
Sliding Window Codes: a Robust Multimedia CBR Broadcast
Case Study", 13th IEEE International Conference on
Wireless and Mobile Computing, Networking and
Communications (WiMob17), October
2017 https://hal.inria.fr/hal-01571609v1/en/, October
2017, < https://hal.inria.fr/hal-01395937/en/>.
[WI08] Whittle, R., "Park-Miller-Carta Pseudo-Random Number [WI08] Whittle, R., "Park-Miller-Carta Pseudo-Random Number
Generator", http://www.firstpr.com.au/dsp/rand31/, Generator", http://www.firstpr.com.au/dsp/rand31/,
January 2008, <http://www.firstpr.com.au/dsp/rand31/>. January 2008, <http://www.firstpr.com.au/dsp/rand31/>.
Appendix A. Decoding Beyond Maximum Latency Optimization Appendix A. Decoding Beyond Maximum Latency Optimization
This annex introduces non normative considerations. They are This annex introduces non normative considerations. They are
provided as suggestions, without any impact on interoperability. For provided as suggestions, without any impact on interoperability. For
more information see [Roca16]. more information see [Roca16].
skipping to change at page 22, line 46 skipping to change at page 24, line 46
permitted by the application. It follows that these source symbols permitted by the application. It follows that these source symbols
SHOULD NOT be delivered to the application and SHOULD be dropped once SHOULD NOT be delivered to the application and SHOULD be dropped once
they are no longer needed. However, decoding these late symbols they are no longer needed. However, decoding these late symbols
significantly improves the global robustness in bad reception significantly improves the global robustness in bad reception
conditions and is therefore recommended for receivers experiencing conditions and is therefore recommended for receivers experiencing
bad channels[Roca16]. In any case whether or not to use this bad channels[Roca16]. In any case whether or not to use this
facility and what exact value to use for the ls_max_size parameter facility and what exact value to use for the ls_max_size parameter
are decisions made by each receiver independently, without any impact are decisions made by each receiver independently, without any impact
on others, neither the other receivers nor the source. on others, neither the other receivers nor the source.
Author's Address Authors' Addresses
Vincent Roca Vincent Roca
INRIA INRIA
Grenoble Grenoble
France France
EMail: vincent.roca@inria.fr EMail: vincent.roca@inria.fr
Belkacem Teibi
INRIA
Grenoble
France
EMail: belkacem.teibi@inria.fr
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