draft-ietf-tsvwg-rlc-fec-scheme-01.txt   draft-ietf-tsvwg-rlc-fec-scheme-02.txt 
TSVWG V. Roca TSVWG V. Roca
Internet-Draft B. Teibi Internet-Draft B. Teibi
Intended status: Standards Track INRIA Intended status: Standards Track INRIA
Expires: April 29, 2018 October 26, 2017 Expires: September 5, 2018 March 4, 2018
Sliding Window Random Linear Code (RLC) Forward Erasure Correction (FEC) Sliding Window Random Linear Code (RLC) Forward Erasure Correction (FEC)
Schemes for FECFRAME Schemes for FECFRAME
draft-ietf-tsvwg-rlc-fec-scheme-01 draft-ietf-tsvwg-rlc-fec-scheme-02
Abstract Abstract
This document describes two fully-specified FEC Schemes for Sliding This document describes two fully-specified FEC Schemes for Sliding
Window Random Linear Codes (RLC), one for RLC over GF(2) (binary Window Random Linear Codes (RLC), one for RLC over GF(2) (binary
case), a second one for RLC over GF(2^^8), both of them with the case), a second one for RLC over GF(2^^8), both of them with the
possibility of controlling the code density. They are meant to possibility of controlling the code density. They are meant to
protect arbitrary media streams along the lines defined by FECFRAME protect arbitrary media streams along the lines defined by FECFRAME
extended to sliding window FEC codes. These sliding window FEC codes extended to sliding window FEC codes. These sliding window FEC codes
rely on an encoding window that slides over the source symbols, rely on an encoding window that slides over the source symbols,
skipping to change at page 1, line 41 skipping to change at page 1, line 41
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This Internet-Draft will expire on April 29, 2018. This Internet-Draft will expire on September 5, 2018.
Copyright Notice Copyright Notice
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skipping to change at page 2, line 23 skipping to change at page 2, line 23
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 . . . . . . . . . . . . 4 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 . . . . . . . . . . . . . . . . . . . . . . . 5 FEC Scheme . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Document Organization . . . . . . . . . . . . . . . . . . 5 1.4. Document Organization . . . . . . . . . . . . . . . . . . 5
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 6 2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 6
3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Parameters Derivation . . . . . . . . . . . . . . . . . . 6 3.1. Parameters Derivation . . . . . . . . . . . . . . . . . . 7
3.2. ADU, ADUI and Source Symbols Mappings . . . . . . . . . . 8 3.2. ADU, ADUI and Source Symbols Mappings . . . . . . . . . . 9
3.3. Encoding Window Management . . . . . . . . . . . . . . . 9 3.3. Encoding Window Management . . . . . . . . . . . . . . . 10
3.4. Pseudo-Random Number Generator . . . . . . . . . . . . . 10 3.4. Pseudo-Random Number Generator . . . . . . . . . . . . . 11
3.5. Coding Coefficients Generation Function . . . . . . . . . 11 3.5. Coding Coefficients Generation Function . . . . . . . . . 12
4. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary ADU 4. Sliding Window RLC FEC Scheme over GF(2^^8) for Arbitrary ADU
Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 13
4.1.1. FEC Framework Configuration Information . . . . . . . 13
4.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 13
4.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 14
4.1.4. Additional Procedures . . . . . . . . . . . . . . . . 14
5. Sliding Window RLC FEC Scheme over GF(2^^8) for Arbitrary ADU
Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 14 4.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 14
5.1.1. FEC Framework Configuration Information . . . . . . . 14 4.1.1. FEC Framework Configuration Information . . . . . . . 14
5.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 15 4.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 15
5.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 16 4.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 16
5.1.4. Additional Procedures . . . . . . . . . . . . . . . . 17 4.1.4. Additional Procedures . . . . . . . . . . . . . . . . 17
6. FEC Code Specification . . . . . . . . . . . . . . . . . . . 17 5. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary ADU
6.1. Encoding Side . . . . . . . . . . . . . . . . . . . . . . 17 Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Decoding Side . . . . . . . . . . . . . . . . . . . . . . 18 5.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 18
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 18 5.1.1. FEC Framework Configuration Information . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 5.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 18
8.1. Attacks Against the Data Flow . . . . . . . . . . . . . . 19 5.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 18
8.1.1. Access to Confidential Content . . . . . . . . . . . 19 5.1.4. Additional Procedures . . . . . . . . . . . . . . . . 18
8.1.2. Content Corruption . . . . . . . . . . . . . . . . . 19 6. FEC Code Specification . . . . . . . . . . . . . . . . . . . 18
8.2. Attacks Against the FEC Parameters . . . . . . . . . . . 19 6.1. Encoding Side . . . . . . . . . . . . . . . . . . . . . . 18
8.3. When Several Source Flows are to be Protected Together . 20 6.2. Decoding Side . . . . . . . . . . . . . . . . . . . . . . 19
8.4. Baseline Secure FEC Framework Operation . . . . . . . . . 20 7. Implementation Status . . . . . . . . . . . . . . . . . . . . 20
9. Operations and Management Considerations . . . . . . . . . . 20 8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8.1. Attacks Against the Data Flow . . . . . . . . . . . . . . 20
8.1.1. Access to Confidential Content . . . . . . . . . . . 20
8.1.2. Content Corruption . . . . . . . . . . . . . . . . . 21
8.2. Attacks Against the FEC Parameters . . . . . . . . . . . 21
8.3. When Several Source Flows are to be Protected Together . 21
8.4. Baseline Secure FEC Framework Operation . . . . . . . . . 21
9. Operations and Management Considerations . . . . . . . . . . 22
9.1. Operational Recommendations: Finite Field GF(2) Versus 9.1. Operational Recommendations: Finite Field GF(2) Versus
GF(2^^8) . . . . . . . . . . . . . . . . . . . . . . . . 21 GF(2^^8) . . . . . . . . . . . . . . . . . . . . . . . . 22
9.2. Operational Recommendations: Coding Coefficients Density 9.2. Operational Recommendations: Coding Coefficients Density
Threshold . . . . . . . . . . . . . . . . . . . . . . . . 21 Threshold . . . . . . . . . . . . . . . . . . . . . . . . 22
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
12.1. Normative References . . . . . . . . . . . . . . . . . . 22 12.1. Normative References . . . . . . . . . . . . . . . . . . 23
12.2. Informative References . . . . . . . . . . . . . . . . . 22 12.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Decoding Beyond Maximum Latency Optimization . . . . 24 Appendix A. Decoding Beyond Maximum Latency Optimization . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction 1. Introduction
Application-Level Forward Erasure Correction (AL-FEC) codes are a key Application-Level Forward Erasure Correction (AL-FEC) codes, or
element of communication systems. They are used to recover from simply FEC codes, are a key element of communication systems. They
packet losses (or erasures) during content delivery sessions to a are used to recover from packet losses (or erasures) during content
large number of receivers (multicast/broadcast transmissions). This delivery sessions to a large number of receivers (multicast/broadcast
is the case with the FLUTE/ALC protocol [RFC6726] in case of reliable transmissions). This is the case with the FLUTE/ALC protocol
file transfers over lossy networks, and the FECFRAME protocol for [RFC6726] in case of reliable file transfers over lossy networks, and
reliable continuous media transfers over lossy networks. the FECFRAME protocol for reliable continuous media transfers over
lossy networks.
The present document only focusses on the FECFRAME protocol, used in The present document only focusses on the FECFRAME protocol, used in
multicast/broadcast delivery mode, with contents that feature multicast/broadcast delivery mode, with contents that feature
stringent real-time constraints: each source packet has a maximum stringent real-time constraints: each source packet has a maximum
validity period after which it will not be considered by the validity period after which it will not be considered by the
destination application. destination application.
1.1. Limits of Block Codes with Real-Time Flows 1.1. Limits of Block Codes with Real-Time Flows
With FECFRAME, there is a single FEC encoding point (either a end- With FECFRAME, there is a single FEC encoding point (either a end-
host/server (source) or a middlebox) and a single FEC decoding point host/server (source) or a middlebox) and a single FEC decoding point
(either a end-host (receiver) or middlebox). In this context, (either a end-host (receiver) or middlebox). In this context,
currently standardized AL-FEC codes for FECFRAME like Reed-Solomon currently standardized AL-FEC codes for FECFRAME like Reed-Solomon
[RFC6865], LDPC-Staircase [RFC6816], or Raptor/RaptorQ, are all [RFC6865], LDPC-Staircase [RFC6816], or Raptor/RaptorQ, are all
linear block codes: they require the data flow to be segmented into linear block codes: they require the data flow to be segmented into
blocks of a predefined maximum size. The block size is a balance blocks of a predefined maximum size.
between robustness (in particular in front of long erasure bursts for
which there is an incentive to increase the block size) and maximum Defining this block size requires to find an appropriate balance
decoding latency (for which there is an incentive to decrease the between robustness and decoding latency: the larger the block size,
block size). Therefore, with a multicast/broadcast session, the the higher the robustness (e.g., in front of long packet erasure
block code is dimensioned by considering the worst communication bursts), but also the higher the maximum decoding latency (i.e., the
channel one wants to support, and this choice impacts all receivers, maximum time required to recover an lost (erased) packet thanks to
no matter their individual channel quality. FEC protection). Therefore, with a multicast/broadcast session where
different receivers experience different packet loss rates, the block
size should be chosen by considering the worst communication
conditions one wants to support, but without exceeding the desired
maximum decoding latency. This choice will impact all receivers.
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 two fully-specified FEC Schemes that follow This document introduces two fully-specified FEC Schemes that follow
a totally different approach: the Sliding Window Random Linear Codes a totally different approach: the Sliding Window Random Linear Codes
(RLC) over either Finite Field GF(2) or GF(8). These FEC Schemes are (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 FEC codes [fecframe-ext]. These FECFRAME extended to sliding window FEC codes [fecframe-ext]. These
FEC Schemes are extremely efficient for instance with media that FEC Schemes are extremely efficient for instance with media that
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(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.
At the receiver, a linear system is managed from the set of received At the receiver, a linear system is managed from the set of received
source and repair packets. New variables (representing source source and repair packets. New variables (representing source
symbols) and equations (representing the linear combination of each symbols) and equations (representing the linear combination of each
repair symbol received) are added upon receiving new packets. repair symbol received) are added upon receiving new packets.
Variables are removed when they are too old with respect to their Variables are removed when they are too old with respect to their
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 optimization
that extends the time a variable is considered in the system). that extends the time a variable is considered in the system). Lost
Erased source symbols are then recovered thanks this linear system source symbols are then recovered thanks to 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 conditions one
to support. However the receivers experiencing a good to medium wants to support. However the receivers experiencing a good to
channel quality observe a FEC-related latency close to zero [Roca17] medium communication quality will observe a FEC-related latency close
since an isolated erased source packet is quickly recovered by the to zero [Roca17] since an isolated lost source packet is quickly
following repair packet. On the opposite, with a block code, recovered with the following repair packet. On the opposite, with a
recovering an isolated erased source packet always requires waiting block code, recovering an isolated lost source packet always requires
the end of the block for the first repair packet to arrive. waiting 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 set of source
source symbols and the associated random coefficients used during the symbols plus the associated coefficients used during the encoding
encoding process. In order to minimize packet overhead, the set of process. In order to minimize packet overhead, the set of source
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) (where m is 1 or 8, depending on the FEC Scheme) used over GF(2^^m) (where m is 1 or 8, depending on the FEC Scheme) used
in the linear combination are not individually listed in the repair in the linear combination are not individually listed in the repair
packet header. Instead, each FEC repair packet 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
skipping to change at page 6, line 20 skipping to change at page 6, line 23
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 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: size of an encoding symbol (i.e., source or repair symbol),
fixed (in bytes) assumed fixed (in bytes)
br_in: transmission bitrate at the input of the FECFRAME sender,
assumed fixed (in bits/s)
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: RLC coding rate, ratio between the total number of source
plr: packet loss rate on the erasure channel symbols and the total number of source plus repair symbols
plr: packet loss rate on the packet erasure channel
ew_size: encoding window current size at a sender (in symbols) ew_size: encoding window current size at a sender (in symbols)
ew_max_size: encoding window maximum size at a sender (in symbols) ew_max_size: encoding window maximum size at a sender (in symbols)
dw_size: decoding window current size at a receiver (in symbols)
dw_max_size: decoding window maximum size at a receiver (in symbols) dw_max_size: decoding window maximum size at a receiver (in symbols)
ls_max_size: linear system maximum size (or width) at a receiver (in ls_max_size: linear system maximum size (or width) at a receiver (in
symbols) symbols)
ls_size: linear system current size (or width) at a receiver (in
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]
DT: coding coefficients density threshold, an integer between 0 and
15 (inclusive) the controls the fraction of coefficients that are
non zero
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 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
real-time constraints, we consider the most stringent constraints real-time constraints, we consider the most stringent constraints
(see [RFC6363], Section 10.2, item 6, for recommendations when (see [RFC6363], Section 10.2, item 6, for recommendations when
several flows are globally protected). This maximum FEC-related several flows are globally protected). The maximum FEC-related
latency accounts for all sources of latency added by FEC encoding latency budget, max_lat, accounts for all sources of latency added
(sender) and FEC decoding (receiver). Other sources of latency by FEC encoding (at a sender) and FEC decoding (at a receiver).
(e.g., added by network communications) are out of scope and must Other sources of latency (e.g., added by network communications)
be considered separately (e.g., they have already been deducted). are out of scope and must be considered separately (said
It can be regarded as the latency budget permitted for all FEC- differently, they have already been deducted from max_lat).
related operations. This is also an input parameter that enables max_lat can be regarded as the latency budget permitted for all
to derive other internal parameters; FEC-related operations. This is an input parameter that enables
to derive other internal parameters as explained below;
Encoding window current (resp. maximum) size, ew_size (resp. Encoding window current (resp. maximum) size, ew_size (resp.
ew_max_size) (in symbols): ew_max_size) (in symbols):
these parameters are used by a sender during FEC encoding. More these parameters are used by a sender during FEC encoding. More
precisely, each repair symbol is a linear combination of the precisely, each repair symbol is a linear combination of the
ew_size source symbols present in the encoding window when RLC ew_size source symbols present in the encoding window when RLC
encoding took place. In all situations, we MUST have ew_size <= encoding took place. In all situations, we MUST have:
ew_max_size;
Decoding window current (resp. maximum) size, dw_size (resp.
dw_max_size) (in symbols):
these parameters are used by a receiver when managing the linear
system used for decoding. dw_size is the current size of the
decoding window, i.e., the set of received or erased source
symbols that are currently part of the linear system. In all
situations, we MUST have dw_size <= dw_max_size;
In order to comply with the maximum FEC-related latency budget, ew_size <= ew_max_size
assuming a constant transmission bitrate at the output of the Decoding window maximum size, dw_max_size (in symbols): at a
FECFRAME sender (br_out), encoding symbol size (E), and code rate receiver, this parameter determines the maximum size of the
(cr), we have: decoding window. Said differently, this is the maximum number of
received or lost source symbols in the linear system (i.e., the
variables) that are still within their latency budget. In
situations where packets are sent with a fixed period, the
dw_max_size parameter directly determines the maximum decoding
latency experienced by the receiver, which necessarily needs to be
in line with the maximum FEC-related latency budget. Note also
that the optimization detailed in Appendix A can extend the linear
system with additional old source symbols (that timed-out) beyond
dw_max_size;
Symbol size, E (in bytes) and RLC code rate (cr): the E parameter
determines the (source or repair) symbol sizes. The cr parameter
determines the code rate, i.e., the amount of redundancy added to
the flow (it is the ratio between the total number of source
symbols and the total number of source plus repair symbols).
These two parameters are input parameters that enable to derive
other internal parameters as explained below. In practice they
will usually be fixed, especially with multicast/broadcast
transmissions. In specific use-cases, in particular with unicast
transmissions in presence of a feedback mechanism that estimates
the communication quality (out-of-scope of FECFRAME), the code
rate may be adjusted dynamically.
Let us assume that the encoding symbol size (E, in bytes) and code
rate (cr) are constant. Let us also assume a constant transmission
bitrate (br_out, in bits/s) at the output of the FECFRAME sender (as
in [Roca17]). It means that the source flow bitrate needs to be
adjusted according to the added repair flow overhead in order to keep
the total transmission bitrate fixed and equal to br_out. In order
to comply with the maximum FEC-related latency budget we need:
dw_max_size = (max_lat * br_out * cr) / (8 * E) dw_max_size = (max_lat * br_out * cr) / (8 * E)
This dw_max_size defines the maximum delay after which an old source Sometimes the opposite can happen: the source flow bitrate at the
symbol may be recovered: after this delay, this old source symbol input of the FECFRAME sender is fixed (br_in, in bits/s). It means
symbol will be removed from the decoding window. that the transmission bitrate at the output of the FECFRAME sender
will be higher, depending on the added repair flow overhead. In
order to comply with the maximum FEC-related latency budget we need:
It is often good practice to choose: dw_max_size = (max_lat * br_in) / (8 * E)
ew_max_size = dw_max_size / 2 Finally, there are situations where no such assumption can be made
(e.g., with a variable bit rate input flow). In that case the
encoding and decoding window maximum sizes may be initialized, based
on the input flow features (e.g., the peak bitrate if it is known)
and great care must be taken on timing aspects at a sender (see
Section 3.3) and receiver. The details of how to manage these
situations are use-case dependent and out of scope of this document.
Then, once the dw_max_size has been determined, the ew_max_size can
be defined. For decoding to be possible, it is required that the
encoding window maximum size be at most equal to the decoding window
maximum size. It is often good practice to choose [Roca17]:
ew_max_size = dw_max_size * 0.75
However any value ew_max_size < dw_max_size can be used without However any value ew_max_size < dw_max_size can be used without
impact on the FEC-related latency budget. Finding the optimal value impact on the FEC-related latency budget. Finding the optimal value
can depend on the erasure channel one wants to support and should be will depend on the use-case details and should be determined after
determined after simulations or field trials. simulations or field trials. This is of course out of scope of this
document.
Note that the decoding beyond maximum latency optimisation Note that the decoding beyond maximum latency optimization
(Appendix A) enables an old source symbol to be kept in the linear (Appendix A) enables an old source symbol to be kept in the linear
system beyond the FEC-related latency budget, but not delivered to system beyond the FEC-related latency budget, but not delivered to
the receiving application. Here we have: ls_size >= dw_max_size the receiving application. In any case, the linear system maximum
size is greater than (with the decoding optimization) or equal to
(without) the decoding window maximum size:
ls_max_size >= dw_max_size
3.2. ADU, ADUI and Source Symbols Mappings 3.2. ADU, ADUI and Source Symbols Mappings
An ADU, coming from the application, cannot be mapped to source An ADU, coming from the application, cannot be mapped to source
symbols directly. Indeed, an erased ADU recovered at a receiver must symbols directly. Indeed, a lost ADU recovered at a receiver must
contain enough information to be assigned to the right application contain enough information to be assigned to the right application
flow (UDP port numbers and IP addresses cannot be used to that flow (UDP port numbers and IP addresses cannot be used to that
purpose as they are not protected by FEC encoding). This requires purpose as they are not protected by FEC encoding). This requires
adding the flow identifier to each ADU before doing FEC encoding. adding the flow identifier to each ADU before doing FEC encoding.
Additionally, since ADUs are of variable size, padding is needed so Additionally, since ADUs are of variable size, padding is needed so
that each ADU (with its flow identifier) contribute to an integral that each ADU (with its flow identifier) contribute to an integral
number of source symbols. This requires adding the original ADU number of source symbols. This requires adding the original ADU
length to each ADU before doing FEC encoding. Because of these length to each ADU before doing FEC encoding. Because of these
requirements, an intermediate format, the ADUI, or ADU Information, requirements, an intermediate format, the ADUI, or ADU Information,
is considered [RFC6363]. is considered [RFC6363].
For each incoming ADU, an ADUI is created as follows. First of all, For each incoming ADU, an ADUI is created as follows. First of all,
3 bytes are prepended: (Figure 1): 3 bytes are prepended (Figure 1):
Flow ID (F) (8-bit field): this unsigned byte contains the integer Flow ID (F) (8-bit field): this unsigned byte contains the integer
identifier associated to the source ADU flow to which this ADU identifier associated to the source ADU flow to which this ADU
belongs. It is assumed that a single byte is sufficient, which belongs. It is assumed that a single byte is sufficient, which
implies that no more than 256 flows will be protected by a single implies that no more than 256 flows will be protected by a single
FECFRAME instance. FECFRAME instance.
Length (L) (16-bit field): this unsigned integer contains the length Length (L) (16-bit field): this unsigned integer contains the length
of this ADU, in network byte order (i.e., big endian). This of this ADU, in network byte order (i.e., big endian). This
length is for the ADU itself and does not include the F, L, or Pad length is for the ADU itself and does not include the F, L, or Pad
fields. fields.
Then, zero padding is added to the ADU if needed: Then, zero padding is added to the ADU if needed:
Padding (Pad) (variable size field): this field contains zero Padding (Pad) (variable size field): this field contains zero
padding to align the F, L, ADU and padding up to a size that is padding to align the F, L, ADU and padding up to a size that is
multiple of E bytes (i.e., the source and repair symbol length). multiple of E bytes (i.e., the source and repair symbol length).
Each ADUI contributes to an integral number of source symbols. The The data unit resulting from the ADU and the F, L, and Pad fields is
data unit resulting from the ADU and the F, L, and Pad fields is called ADUI. Since ADUs can have different sizes, this is also the
called ADU Information (or ADUI). Since ADUs can be of different case for ADUIs. However an ADUI always contributes to an integral
size, this is also the case for ADUIs. number of source symbols.
symbol length, E E E symbol length, E E E
< ------------------ >< ------------------ >< ------------------ > < ------------------ >< ------------------ >< ------------------ >
+-+--+---------------------------------------------+-------------+ +-+--+---------------------------------------------+-------------+
|F| L| ADU | Pad | |F| L| ADU | Pad |
+-+--+---------------------------------------------+-------------+ +-+--+---------------------------------------------+-------------+
Figure 1: ADUI Creation example (here 3 source symbols are created Figure 1: ADUI Creation example (here 3 source symbols are created
for this ADUI). for this ADUI).
Note that neither the initial 3 bytes nor the optional padding are Note that neither the initial 3 bytes nor the optional padding are
sent over the network. However, they are considered during FEC sent over the network. However, they are considered during FEC
encoding. It means that a receiver who lost a certain FEC source encoding, and a receiver who lost a certain FEC Source Packet (e.g.,
packet (e.g., the UDP datagram containing this FEC source packet) the UDP datagram containing this FEC Source Packet) will be able to
will be able to recover the ADUI if FEC decoding succeeds. Thanks to recover the ADUI if FEC decoding succeeds. Thanks to the initial 3
the initial 3 bytes, this receiver will get rid of the padding (if bytes, this receiver will get rid of the padding (if any) and
any) and identify the corresponding ADU flow. identify the corresponding ADU flow.
3.3. Encoding Window Management 3.3. Encoding Window Management
Source symbols and the corresponding ADUs are removed from the Source symbols and the corresponding ADUs are removed from the
encoding window: encoding window:
o when the sliding encoding window has reached its maximum size, o when the sliding encoding window has reached its maximum size,
ew_max_size. In that case the oldest symbol MUST be removed ew_max_size. In that case the oldest symbol MUST be removed
before adding a new symbol, so that the current encoding window before adding a new symbol, so that the current encoding window
size always remains inferior or equal to the maximum size: ew_size size always remains inferior or equal to the maximum size: ew_size
<= ew_max_size; <= ew_max_size;
o when an ADU has reached its maximum validity duration in case of a o when an ADU has reached its maximum validity duration in case of a
real-time flow. When this happens, all source symbols real-time flow. When this happens, all source symbols
corresponding to the ADUI that expired SHOULD be removed from the corresponding to the ADUI that expired SHOULD be removed from the
encoding window; encoding window;
Source symbols are added to the sliding encoding window each time a Source symbols are added to the sliding encoding window each time a
new ADU arrives, once the ADU to ADUI and then to source symbols new ADU arrives, once the ADU to source symbols mapping has been
mapping has been performed (Section 3.2). The current size of the performed (Section 3.2). The current size of the encoding window,
encoding window, ew_size, is updated after adding new source symbols. ew_size, is updated after adding new source symbols. This process
This process may require to remove old source symbols so that: may require to remove old source symbols so that: ew_size <=
ew_size <= ew_max_size. ew_max_size.
Note that a FEC codec may feature practical limits in the number of Note that a FEC codec may feature practical limits in the number of
source symbols in the encoding window (e.g., for computational source symbols in the encoding window (e.g., for computational
complexity reasons). This factor may further limit the ew_max_lat complexity reasons). This factor may further limit the ew_max_size
value, in addition to the maximum FEC-related latency budget value, in addition to the maximum FEC-related latency budget
(Section 3.1). (Section 3.1).
3.4. Pseudo-Random Number Generator 3.4. Pseudo-Random Number Generator
The RLC codes rely on the following Pseudo-Random Number Generator The RLC codes rely on the following Pseudo-Random Number Generator
(PRNG), identical to the PRNG used with LDPC-Staircase codes (PRNG), identical to the PRNG used with LDPC-Staircase codes
([RFC5170], section 5.7). ([RFC5170], section 5.7).
The Park-Miler "minimal standard" PRNG [PM88] MUST be used. It The Park-Miler "minimal standard" PRNG [PM88] MUST be used. It
skipping to change at page 11, line 16 skipping to change at page 12, 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 5.1.3). (Section 4.1.3).
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 generate_coding_coefficients() generated at the RLC encoder by the generate_coding_coefficients()
function each time a new repair symbol needs to be produced. Note function each time a new repair symbol needs to be produced. The
that the fraction of coefficients that are non zero (density) is fraction of coefficients that are non zero (i.e., the density) is
controlled by a dedicated parameter, DT (Density Threshold). When controlled by the DT (Density Threshold) parameter. When DT equals
this parameter equals 15, the maximum value, the function guaranties 15, the maximum value, the function guaranties that all coefficients
that all coefficients are non zero (i.e., maximum density). When the are non zero (i.e., maximum density). When DT is between 0 (minimum
parameter is between 0 (minimum value) and strictly inferior to 15, value) and strictly inferior to 15, the average probability of having
the average probability of having a non zero coefficients equals (DT a non zero coefficient equals (DT +1) / 16.
+1) / 16. The density is reduced in a controlled manner.
These considerations apply both the RLC over GF(2) and RLC over 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. 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 the RLC over GF(2) FEC Scheme (Section 5), m MUST be equal to 1.
With RLC over GF(2^^8) FEC Scheme (Section 5), m MUST be equal to 8. With RLC over GF(2^^8) FEC Scheme (Section 4), 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. This
* parameter is ignored (useless) if m=2 and dt=15
* (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) density_threshold value between 0 and 15 (inclusive) that * (in) dt integer between 0 and 15 (inclusive) that
* controls the density. With value 15, all * controls the density. With value 15, all
* coefficients are guaranteed to be non zero * coefficients are guaranteed to be non zero
* (i.e. equal to 1 with GF(2) and equal to a * (i.e. equal to 1 with GF(2) and equal to a
* value in {1,... 255} with GF(2^^8)), otherwise * value in {1,... 255} with GF(2^^8)), otherwise
* a fraction of them will be 0. * a fraction of them will be 0.
* (in) m Finite Field GF(2^^m) parameter. In this * (in) m Finite Field GF(2^^m) parameter. In this
* version only 1 and 8 are considered. * document only values 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 dt,
UINT8 m) UINT8 m)
{ {
UINT32 i; UINT32 i;
if (repair_key == 0 || density_threshold > 15) { if (dt > 15) {
/* bad parameters */ return SOMETHING_WENT_WRONG; /* bad dt parameter */
return SOMETHING_WENT_WRONG; }
if (repair_key == 0 && dt != 15 && m != 2) {
return SOMETHING_WENT_WRONG; /* bad repair_key parameter */
} }
pmms_srand(repair_key);
switch (m) { switch (m) {
case 1: case 1:
if (density_threshold == 15) { if (dt == 15) {
/* all coefficients are 1 */ /* all coefficients are 1 */
memset(cc_tab, 1, cc_nb); memset(cc_tab, 1, cc_nb);
} else { } else {
/* here coefficients are either 0 or 1 */
pmms_srand(repair_key);
for (i = 0 ; i < cc_nb ; i++) { for (i = 0 ; i < cc_nb ; i++) {
if (pmms_rand(16) <= density_threshold) { if (pmms_rand(16) <= dt) {
cc_tab[i] = (UINT8) 1; cc_tab[i] = (UINT8) 1;
} else { } else {
cc_tab[i] = (UINT8) 0; cc_tab[i] = (UINT8) 0;
} }
} }
} }
break; break;
case 8: case 8:
if (density_threshold == 15) { pmms_srand(repair_key);
if (dt == 15) {
/* coefficient 0 is avoided here in order to include /* coefficient 0 is avoided here in order to include
* all the source symbols */ * all the source symbols */
for (i = 0 ; i < cc_nb ; i++) { for (i = 0 ; i < cc_nb ; i++) {
do { do {
cc_tab[i] = (UINT8) pmms_rand(256); cc_tab[i] = (UINT8) pmms_rand(256);
} while (cc_tab[i] == 0); } while (cc_tab[i] == 0);
} }
} else { } else {
/* here a certain fraction of coefficients should be 0 */ /* here a certain fraction of coefficients should be 0 */
for (i = 0 ; i < cc_nb ; i++) { for (i = 0 ; i < cc_nb ; i++) {
if (pmms_rand(16) <= density_threshold) { if (pmms_rand(16) <= dt) {
do { do {
cc_tab[i] = (UINT8) pmms_rand(256); cc_tab[i] = (UINT8) pmms_rand(256);
} while (cc_tab[i] == 0); } while (cc_tab[i] == 0);
} else { } else {
cc_tab[i] = 0; cc_tab[i] = 0;
} }
} }
} }
break; break;
default: default:
/* bad parameter m */ /* bad parameter m */
return SOMETHING_WENT_WRONG; 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 over GF(2) for Arbitrary ADU Flows 4. Sliding Window RLC FEC Scheme over GF(2^^8) for Arbitrary ADU Flows
This fully-specified FEC Scheme defines the Sliding Window Random This fully-specified FEC Scheme defines the Sliding Window Random
Linear Codes (RLC) over GF(2) (binary case). Linear Codes (RLC) over GF(2^^8).
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.
5.1.1.1. Mandatory Information 4.1.1.1. 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 10). 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.
5.1.1.2. FEC Scheme-Specific Information 4.1.1.2. 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;
This element is 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).
skipping to change at page 15, line 18 skipping to change at page 15, line 36
Encoding symbol length (E): 16-bit field. Encoding symbol length (E): 16-bit field.
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol Length (E) | | Encoding Symbol Length (E) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: FSSI Encoding Format Figure 3: FSSI Encoding Format
5.1.2. Explicit Source FEC Payload ID 4.1.2. 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 |
+--------------------------------+ +--------------------------------+
| Explicit Source FEC Payload ID | | Explicit Source FEC Payload ID |
+--------------------------------+ +--------------------------------+
Figure 4: Structure of an FEC Source Packet with the Explicit Source Figure 4: Structure of an FEC Source Packet with the Explicit Source
FEC Payload ID FEC Payload ID
More precisely, the Explicit Source FEC Payload ID is composed of the More precisely, the Explicit Source FEC Payload ID is composed of the
following field (Figure 5): following field (Figure 5):
Encoding Symbol ID (ESI) (32-bit field): this unsigned integer Encoding Symbol ID (ESI) (32-bit field): this unsigned integer
identifies the first source symbol of the ADUI corresponding to identifies the first source symbol of the ADUI corresponding to
this FEC source packet. The ESI is incremented for each new this FEC Source Packet. The ESI is incremented for each new
source symbol, and after reaching the maximum value (2^32-1), source symbol, and after reaching the maximum value (2^32-1),
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
5.1.3. Repair FEC Payload ID 4.1.3. Repair FEC Payload ID
A FEC repair packet MUST contain a Repair FEC Payload ID that is A FEC Repair Packet can contain one or more repair symbols. When
prepended to the repair symbol as illustrated in Figure 6. There can there are several repair symbols, all of them MUST have been
be one or more repair symbols per FEC repair packet. When this is generated from the same encoding window, using Repair_Key values that
the case, the number of repair symbols within this FEC repair packet are managed as explained below. A receiver can easily deduce the
is easily deduced by comparing the known received FEC repair packet number of repair symbols within a FEC Repair Packet by comparing the
size (equal to the UDP payload size when UDP is the underlying received FEC Repair Packet size (equal to the UDP payload size when
transport protocol) and the symbol size, E, communicated in the FFCI. UDP is the underlying transport protocol) and the symbol size, E,
When this is the case, all the repair symbols MUST have been communicated in the FFCI.
generated from the same encoding window.
A FEC Repair Packet MUST contain a Repair FEC Payload ID that is
prepended to the repair symbol as illustrated in Figure 6.
+--------------------------------+ +--------------------------------+
| IP Header | | IP Header |
+--------------------------------+ +--------------------------------+
| Transport Header | | Transport Header |
+--------------------------------+ +--------------------------------+
| Repair FEC Payload ID | | Repair FEC Payload ID |
+--------------------------------+ +--------------------------------+
| Repair Symbol | | Repair Symbol |
+--------------------------------+ +--------------------------------+
Figure 6: Structure of an FEC Repair Packet with the Repair FEC Figure 6: Structure of an FEC Repair Packet with the Repair FEC
Payload ID Payload ID
More precisely, the Repair FEC Payload ID is composed of the More precisely, the Repair FEC Payload ID is composed of the
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).
Coding coefficients Density Threshold, DT (4-bit field): this Density Threshold for the coding coefficients, DT (4-bit field):
unsigned integer carried the Density Threshold (DT) used by the this unsigned integer carried the Density Threshold (DT) used by
coding coefficient generation function Section 3.5. More the coding coefficient generation function Section 3.5. More
precisely, it controls the probability of having a non zero coding precisely, it controls the probability of having a non zero coding
coefficient, which equals (DT+1) / 16. When a FEC repair packet coefficient, which equals (DT+1) / 16. When a FEC Repair Packet
contains several repair symbols, the DT value applies to all of contains several repair symbols, the DT value applies to all of
them; them;
Number of Source Symbols in the Encoding Window, NSS (12-bit field): 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 the 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 | DT |NSS (# src symb 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
5.1.4. Additional Procedures 4.1.4. 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. happens after reaching the maximum 32-bit value.
5. 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).
5.1. Formats and Codes
5.1.1. FEC Framework Configuration Information
5.1.1.1. 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.
5.1.1.2. FEC Scheme-Specific Information
All the considerations of Section 4.1.1.2 apply here.
5.1.2. Explicit Source FEC Payload ID
All the considerations of Section 4.1.1.2 apply here.
5.1.3. Repair FEC Payload ID
All the considerations of Section 4.1.1.2 apply here, with the only
exception that the Repair_Key field is useless if DT = 15 (indeed, in
that case all the coefficients are necessarily equal to 1 and the
coefficient generation function does not use any PRNG). When DT = 15
it is RECOMMENDED that the sender use value 0 for the Repair_Key
field, but a receiver SHALL ignore this field.
5.1.4. Additional Procedures
All the considerations of Section 4.1.1.2 apply here.
6. FEC Code Specification 6. FEC Code Specification
6.1. 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
a PRNG seed, value 0 is prohibited). This repair key is communicated a PRNG seed, value 0 is prohibited). This repair key is communicated
to the coefficient generation function (Section Section 3.5) in order to the coefficient generation function (Section Section 3.5) in order
to generate ew_size coding coefficients. Finally, the FECFRAME to generate ew_size coding coefficients. Finally, the FECFRAME
sender computes the repair symbol as a linear combination of the sender computes the repair symbol as a linear combination of the
ew_size source symbols using the ew_size coding coefficients. When E ew_size source symbols using the ew_size coding coefficients. When E
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.
Other solutions are possible to select a repair key value when a new
FEC Repair Packet is needed, for instance by choosing a random
integer between 1 and 65535. However, selecting the same repair key
as before (which may happen in case of a random process) is only
meaningful if the encoding window has changed, otherwise the same FEC
Repair Packet will be generated.
6.2. 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
the linear system (or no equation if this repair packet does not the linear system (or no equation if this repair packet does not
change the linear system rank). This equation of course re-uses the change the linear system rank). This equation of course re-uses the
ew_size coding coefficients that are computed by the same coefficient ew_size coding coefficients that are computed by the same coefficient
generation function (Section Section 3.5), using the repair key and generation function (Section Section 3.5), using the repair key and
encoding window descriptions carried in the Repair FEC Payload ID. encoding window descriptions carried in the Repair FEC Payload ID.
Whenever possible (i.e., when a sub-system covering one or more lost Whenever possible (i.e., when a sub-system covering one or more lost
source symbols is of full rank), decoding is performed in order to source symbols is of full rank), decoding is performed in order to
recover lost source symbols. Each time an ADUI can be totally recover lost source symbols. Each time an ADUI can be totally
recovered, it is assigned to the corresponding application flow recovered, padding is removed (thanks to the Length field, L, of the
(thanks to the Flow ID (F) field of the ADUI) and padding (if any) ADUI) and the ADU is assigned to the corresponding application flow
removed (thanks to the Length (L) field of the ADUI). This ADU is (thanks to the Flow ID field, F, of the ADUI). This ADU is finally
finally passed to the corresponding upper application. Received FEC passed to the corresponding upper application. Received FEC Source
source packets, containing an ADU, can be passed to the application Packets, containing an ADU, can be passed to the application either
either immediately or after some time to guaranty an ordered delivery immediately or after some time to guaranty an ordered delivery to the
to the application(s). This document does not mandate any approach application. This document does not mandate any approach as this is
as this is an operational and management decision. 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 after this delay should not be passed to
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 used in order to improve the global
recovery performance. robustness.
7. 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 limited to GF(2^^8). It relies on a modified version
(http://openfec.org) FEC code library. It is integrated in our of our OpenFEC (http://openfec.org) FEC code library. It is
FECFRAME software (see [fecframe-ext]). integrated in our 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.
o Lincensing: proprietary. o Lincensing: proprietary.
o Contact: vincent.roca@inria.fr o Contact: vincent.roca@inria.fr
8. 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.
skipping to change at page 19, line 51 skipping to change at page 21, line 20
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.
8.2. 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 targets receivers
(Section 5.1.1.2): (Section 4.1.1.2):
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;
An attacker who only targets a sender will achieve the same results.
However if the attacker targets both sender and receivers at the same
time (the same wrong piece of information is communicated to
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].
skipping to change at page 21, line 8 skipping to change at page 22, line 18
9. 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.
9.1. Operational Recommendations: Finite Field GF(2) Versus GF(2^^8) 9.1. Operational Recommendations: Finite Field GF(2) Versus GF(2^^8)
The present document specifies two FEC Schemes that differ on the The present document specifies two FEC Schemes that differ on the
associated Finite Field used for the coding coefficients. It is Finite Field used for the coding coefficients. It is expected that
expected that the RLC over GF(2^^8) FEC Scheme will be mostly used the RLC over GF(2^^8) FEC Scheme will be mostly used since it
since it warrants a high loss protection. Additionally, elements in warrants a higher packet loss protection. In case of small encoding
the finite field are 8 bits long, which makes read/write memory windows, the associated processing overhead is not an issue (e.g., we
operations aligned on bytes during encoding and decoding. measured decoding speeds between 745 Mbps and 2.8 Gbps on an ARM
Cortex-A15 embedded board in [Roca17]). Of course the CPU overhead
will increase with the encoding window size, because more operations
in the GF(2^^8) finite field will be needed.
Finally, in particular when dealing with large encoding windows, an The RLC over GF(2) FEC Scheme offers an alternative. In that case
alternative is the RLC over GF(2) FEC Scheme. In that case operations symbols can be directly XOR-ed together which warrants
operations symbols can be directly XORed together which warrants high high bitrate encoding and decoding operations, and can be an
bitrate encoding and decoding operations. advantage with large encoding windows. However packet loss
protection is significantly reduced by using this FEC Scheme.
9.2. Operational Recommendations: Coding Coefficients Density Threshold 9.2. Operational Recommendations: Coding Coefficients Density Threshold
In addition to the choice of the Finite Field, the two FEC Schemes In addition to the choice of the Finite Field, the two FEC Schemes
define a coding coefficient density threshold parameter. This define a coding coefficient density threshold (DT) parameter. This
parameter enables a sender to control the code density, i.e., the parameter enables a sender to control the code density, i.e., the
proportion of coefficients that are non zero on average. With RLC proportion of coefficients that are non zero on average. With RLC
over GF(2^^8), it is recommended that small encoding windows be over GF(2^^8), it is usually appropriate that small encoding windows
associated to a density threshold equal to 15, the maximum value, in be associated to a density threshold equal to 15, the maximum value,
order to warrant a high loss protection. in order to warrant a high loss protection.
On the opposite, with large encoding windows, it it recommened that On the opposite, with larger encoding windows, it is usually
the density threshold be reduced. With large encoding windows, an appropriate that the density threshold be reduced. With large
alternative can be to use RLC over GF(2) and a density threshold encoding windows, an alternative can be to use RLC over GF(2) and a
equal to 8 (i.e., an average density equal to 1/2) or smaller. density threshold equal to 7 (i.e., an average density equal to 1/2)
or smaller.
Note also that using a density threshold equal to 15 with RLC over Note that using a density threshold equal to 15 with RLC over GF(2)
GF(2) is equivalent to using code that XOR's all the source symbols is equivalent to using an XOR code that compute the XOR sum of all
of the encoding window. In that case it follows that: (1) a single the source symbols in the encoding window. In that case: (1) a
repair symbol can be produced for a given encoding window, and (2) single repair symbol can be produced for any encoding window, and (2)
the repair_key parameter is useless (the coding coefficients the repair_key parameter becomes useless (the coding coefficients
generation function does not rely on the PRNG). generation function does not rely on the PRNG).
10. IANA Considerations 10. IANA Considerations
This document registers two values in the "FEC Framework (FECFRAME) 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 YYYY refers to the Sliding Window Random Linear Codes (RLC) over o YYYY refers to the Sliding Window Random Linear Codes (RLC) over
GF(2) FEC Scheme for Arbitrary Packet Flows, as defined in GF(2) FEC Scheme for Arbitrary Packet Flows, as defined in
Section 4 of this document. Section 5 of this document.
o XXXX refers to the Sliding Window Random Linear Codes (RLC) over o XXXX refers to the Sliding Window Random Linear Codes (RLC) over
GF(2^^8) FEC Scheme for Arbitrary Packet Flows, as defined in GF(2^^8) FEC Scheme for Arbitrary Packet Flows, as defined in
Section 5 of this document. Section 4 of this document.
11. Acknowledgments 11. Acknowledgments
The authors would like to thank Marie-Jose Montpetit for her valuable The authors would like to thank Marie-Jose Montpetit for her valuable
feedbacks on this document. feedbacks on this document.
12. References 12. References
12.1. 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-ietf-tsvwg-fecframe-ext
(Work in Progress), June 2017, (Work in Progress), March 2018,
<https://tools.ietf.org/html/draft-roca-tsvwg-fecframev2>. <https://tools.ietf.org/html/
draft-ietf-tsvwg-fecframe-ext>.
[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,
<https://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,
<https://www.rfc-editor.org/info/rfc6363>. <https://www.rfc-editor.org/info/rfc6363>.
skipping to change at page 23, line 36 skipping to change at page 24, line 50
[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,
<https://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", HAL open-archive document,hal-01395937 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. [Roca17] Roca, V., Teibi, B., Burdinat, C., Tran, T., and C.
Thienot, "Less Latency and Better Protection with AL-FEC Thienot, "Less Latency and Better Protection with AL-FEC
Sliding Window Codes: a Robust Multimedia CBR Broadcast Sliding Window Codes: a Robust Multimedia CBR Broadcast
Case Study", 13th IEEE International Conference on Case Study", 13th IEEE International Conference on
Wireless and Mobile Computing, Networking and Wireless and Mobile Computing, Networking and
Communications (WiMob17), October Communications (WiMob17), October
2017 https://hal.inria.fr/hal-01571609v1/en/, October 2017 https://hal.inria.fr/hal-01571609v1/en/, October
2017, < https://hal.inria.fr/hal-01395937/en/>. 2017, <https://hal.inria.fr/hal-01571609v1/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].
It is possible to improve the decoding performance of sliding window It is possible to improve the decoding performance of sliding window
codes without impacting maximum latency, at the cost of extra CPU codes without impacting maximum latency, at the cost of extra CPU
overhead. The optimization consists, for a receiver, to extend the overhead. The optimization consists, for a receiver, to extend the
linear system beyond the decoding window: linear system beyond the decoding window, by keeping a certain number
of old source symbols:
ls_max_size > dw_max_size ls_max_size > dw_max_size
Usually the following choice is a good trade-off between decoding Usually the following choice is a good trade-off between decoding
performance and extra CPU overhead: performance and extra CPU overhead:
ls_max_size = 2 * dw_max_size ls_max_size = 2 * dw_max_size
ls_max_size ls_max_size
/---------------------------------^-------------------------------\ /---------------------------------^-------------------------------\
late source symbols late source symbols
(pot. decoded but not delivered) dw_max_size (pot. decoded but not delivered) dw_max_size
/--------------^-----------------\ /--------------^---------------\ /--------------^-----------------\ /--------------^---------------\
src0 src1 src2 src3 src4 src5 src6 src7 src8 src9 src10 src11 src12 src0 src1 src2 src3 src4 src5 src6 src7 src8 src9 src10 src11 src12
Figure 8: Relationship between parameters to decode beyond maximum Figure 8: Relationship between parameters to decode beyond maximum
latency. latency.
It means that source symbols (and therefore ADUs) may be decoded even It means that source symbols, and therefore ADUs, may be decoded even
if their transport protocol added latency exceeds the maximum value if the added latency exceeds the maximum value permitted by the
permitted by the application. It follows that these source symbols application. It follows that the corresponding ADUs SHOULD NOT be
SHOULD NOT be delivered to the application and SHOULD be dropped once delivered to the application and SHOULD be dropped once they are no
they are no longer needed. However, decoding these late symbols longer needed. However, decoding these "late symbols" significantly
significantly improves the global robustness in bad reception improves the global robustness in bad reception conditions and is
conditions and is therefore recommended for receivers experiencing therefore recommended for receivers experiencing bad communication
bad channels[Roca16]. In any case whether or not to use this conditions [Roca16]. In any case whether or not to use this
facility and what exact value to use for the ls_max_size parameter optimization and what exact value to use for the ls_max_size
are decisions made by each receiver independently, without any impact parameter are decisions made by each receiver independently, without
on others, neither the other receivers nor the source. any impact on the other receivers nor on the source.
Authors' Addresses Authors' Addresses
Vincent Roca Vincent Roca
INRIA INRIA
Grenoble Grenoble
France France
EMail: vincent.roca@inria.fr EMail: vincent.roca@inria.fr
Belkacem Teibi Belkacem Teibi
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