draft-ietf-tsvwg-rlc-fec-scheme-02.txt   draft-ietf-tsvwg-rlc-fec-scheme-03.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: September 5, 2018 March 4, 2018 Expires: November 7, 2018 May 6, 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-02 draft-ietf-tsvwg-rlc-fec-scheme-03
Abstract Abstract
This document describes two fully-specified FEC Schemes for Sliding This document describes two fully-specified Forward Erasure
Window Random Linear Codes (RLC), one for RLC over GF(2) (binary Correction (FEC) Schemes for Sliding Window Random Linear Codes
case), a second one for RLC over GF(2^^8), both of them with the (RLC), one for RLC over GF(2) (binary case), a second one for RLC
possibility of controlling the code density. They are meant to over GF(2^^8), both of them with the possibility of controlling the
protect arbitrary media streams along the lines defined by FECFRAME code density. They can protect arbitrary media streams along the
extended to sliding window FEC codes. These sliding window FEC codes lines defined by FECFRAME extended to sliding window FEC codes.
rely on an encoding window that slides over the source symbols, These sliding window FEC codes rely on an encoding window that slides
generating new repair symbols whenever needed. Compared to block FEC over the source symbols, generating new repair symbols whenever
codes, these sliding window FEC codes offer key advantages with real- needed. Compared to block FEC codes, these sliding window FEC codes
time flows in terms of reduced FEC-related latency while often offer key advantages with real-time flows in terms of reduced FEC-
providing improved erasure recovery capabilities. related latency while often providing improved packet erasure
recovery capabilities.
Status of This Memo Status of This Memo
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Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
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 . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . 6
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 6 2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 6
3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Parameters Derivation . . . . . . . . . . . . . . . . . . 7 3.1. Possible Parameter Derivation . . . . . . . . . . . . . . 7
3.2. ADU, ADUI and Source Symbols Mappings . . . . . . . . . . 9 3.1.1. Detailed Parameter Derivation for CBR Real-Time Flows 8
3.3. Encoding Window Management . . . . . . . . . . . . . . . 10 3.1.2. Parameter Derivation for Other Real-Time Flows . . . 10
3.4. Pseudo-Random Number Generator . . . . . . . . . . . . . 11 3.1.3. Parameter Derivation for Non Real-Time Flows . . . . 10
3.5. Coding Coefficients Generation Function . . . . . . . . . 12 3.2. ADU, ADUI and Source Symbols Mappings . . . . . . . . . . 11
3.3. Encoding Window Management . . . . . . . . . . . . . . . 12
3.4. Pseudo-Random Number Generator . . . . . . . . . . . . . 13
3.5. Coding Coefficients Generation Function . . . . . . . . . 14
3.6. Linear Combination of Source Symbols Computation . . . . 16
4. Sliding Window RLC FEC Scheme over GF(2^^8) for Arbitrary ADU 4. Sliding Window RLC FEC Scheme over GF(2^^8) for Arbitrary ADU
Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 14 4.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 17
4.1.1. FEC Framework Configuration Information . . . . . . . 14 4.1.1. FEC Framework Configuration Information . . . . . . . 17
4.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 15 4.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 18
4.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 16 4.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 19
4.1.4. Additional Procedures . . . . . . . . . . . . . . . . 17 4.1.4. Additional Procedures . . . . . . . . . . . . . . . . 20
5. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary ADU 5. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary ADU
Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 18 5.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 21
5.1.1. FEC Framework Configuration Information . . . . . . . 18 5.1.1. FEC Framework Configuration Information . . . . . . . 21
5.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 18 5.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 21
5.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 18 5.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 21
5.1.4. Additional Procedures . . . . . . . . . . . . . . . . 18 5.1.4. Additional Procedures . . . . . . . . . . . . . . . . 21
6. FEC Code Specification . . . . . . . . . . . . . . . . . . . 18 6. FEC Code Specification . . . . . . . . . . . . . . . . . . . 21
6.1. Encoding Side . . . . . . . . . . . . . . . . . . . . . . 18 6.1. Encoding Side . . . . . . . . . . . . . . . . . . . . . . 21
6.2. Decoding Side . . . . . . . . . . . . . . . . . . . . . . 19 6.2. Decoding Side . . . . . . . . . . . . . . . . . . . . . . 22
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 20 7. Implementation Status . . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20 8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8.1. Attacks Against the Data Flow . . . . . . . . . . . . . . 20 8.1. Attacks Against the Data Flow . . . . . . . . . . . . . . 24
8.1.1. Access to Confidential Content . . . . . . . . . . . 20 8.1.1. Access to Confidential Content . . . . . . . . . . . 24
8.1.2. Content Corruption . . . . . . . . . . . . . . . . . 21 8.1.2. Content Corruption . . . . . . . . . . . . . . . . . 24
8.2. Attacks Against the FEC Parameters . . . . . . . . . . . 21 8.2. Attacks Against the FEC Parameters . . . . . . . . . . . 24
8.3. When Several Source Flows are to be Protected Together . 21 8.3. When Several Source Flows are to be Protected Together . 25
8.4. Baseline Secure FEC Framework Operation . . . . . . . . . 21 8.4. Baseline Secure FEC Framework Operation . . . . . . . . . 25
9. Operations and Management Considerations . . . . . . . . . . 22 9. Operations and Management Considerations . . . . . . . . . . 25
9.1. Operational Recommendations: Finite Field GF(2) Versus 9.1. Operational Recommendations: Finite Field GF(2) Versus
GF(2^^8) . . . . . . . . . . . . . . . . . . . . . . . . 22 GF(2^^8) . . . . . . . . . . . . . . . . . . . . . . . . 25
9.2. Operational Recommendations: Coding Coefficients Density 9.2. Operational Recommendations: Coding Coefficients Density
Threshold . . . . . . . . . . . . . . . . . . . . . . . . 22 Threshold . . . . . . . . . . . . . . . . . . . . . . . . 25
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
12.1. Normative References . . . . . . . . . . . . . . . . . . 23 12.1. Normative References . . . . . . . . . . . . . . . . . . 26
12.2. Informative References . . . . . . . . . . . . . . . . . 24 12.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Decoding Beyond Maximum Latency Optimization . . . . 26 Appendix A. Decoding Beyond Maximum Latency Optimization . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction 1. Introduction
Application-Level Forward Erasure Correction (AL-FEC) codes, or Application-Level Forward Erasure Correction (AL-FEC) codes, or
simply FEC codes, are a key element of communication systems. They simply FEC codes, are a key element of communication systems. They
are used to recover from packet losses (or erasures) during content are used to recover from packet losses (or erasures) during content
delivery sessions to a large number of receivers (multicast/broadcast delivery sessions to a large number of receivers (multicast/broadcast
transmissions). This is the case with the FLUTE/ALC protocol transmissions). This is the case with the FLUTE/ALC protocol
[RFC6726] in case of reliable file transfers over lossy networks, and [RFC6726] when used for reliable file transfers over lossy networks,
the FECFRAME protocol for reliable continuous media transfers over and the FECFRAME protocol when used for reliable continuous media
lossy networks. 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. blocks of a predefined maximum size.
Defining this block size requires to find an appropriate balance To define this block size, it is required to find an appropriate
between robustness and decoding latency: the larger the block size, balance between robustness and decoding latency: the larger the block
the higher the robustness (e.g., in front of long packet erasure size, the higher the robustness (e.g., in front of long packet
bursts), but also the higher the maximum decoding latency (i.e., the erasure bursts), but also the higher the maximum decoding latency
maximum time required to recover an lost (erased) packet thanks to (i.e., the maximum time required to recover a lost (erased) packet
FEC protection). Therefore, with a multicast/broadcast session where thanks to FEC protection). Therefore, with a multicast/broadcast
different receivers experience different packet loss rates, the block session where different receivers experience different packet loss
size should be chosen by considering the worst communication rates, the block size should be chosen by considering the worst
conditions one wants to support, but without exceeding the desired communication conditions one wants to support, but without exceeding
maximum decoding latency. This choice will impact all receivers. the desired maximum decoding latency. This choice then impacts the
FEC-related latency of all receivers, even those experiencing a good
communication quality, since no FEC encoding can happen until all the
source data of the block is available at the sender, which directly
depends on the block size.
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, and more generally Sliding Window FEC codes, are
feature real-time constraints sent within a multicast/broadcast recommended for instance with media that feature real-time
session. constraints sent within a multicast/broadcast session [Roca17].
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 on-the-fly, computing a random linear
linear combination of the source symbols present in the current combination of the source symbols present in the current encoding
encoding window. window, and passed to the transport layer.
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 optimization equations they are involved in (Appendix A introduces an optimization
that extends the time a variable is considered in the system). Lost that extends the time a variable is considered in the system). Lost
source symbols are then recovered thanks to 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 conditions one dimensioned by considering the worst communication conditions one
wants to support. However the receivers experiencing a good to wants to support. However the receivers experiencing a good to
medium communication quality will observe a FEC-related latency close medium communication quality will observe a reduced FEC-related
to zero [Roca17] since an isolated lost source packet is quickly latency compared to block codes [Roca17] since an isolated lost
recovered with the following repair packet. On the opposite, with a source packet is quickly recovered with the following repair packet.
block code, recovering an isolated lost source packet always requires On the opposite, with a block code, recovering an isolated lost
waiting the end of the block for the first repair packet to arrive. source packet always requires waiting for the first repair packet to
Additionally, under certain situations (e.g., with a limited FEC- arrive after the end of the block. Additionally, under certain
related latency budget and with constant bit rate transmissions after situations (e.g., with a limited FEC-related latency budget and with
FECFRAME encoding), sliding window codes achieve more easily a target constant bitrate transmissions after FECFRAME encoding), sliding
transmission quality (e.g., measured by the residual loss after FEC window codes can more efficiently achieve a target transmission
decoding) by sending fewer repair packets (i.e., higher code rate) quality (e.g., measured by the residual loss after FEC decoding) by
than block codes. sending fewer repair packets (i.e., higher code rate) 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 to limit the packet
transmission overhead. The main requirement is that each repair header overhead. The main requirement is that each repair packet
packet header must enable a receiver to reconstruct the set of source header must enable a receiver to reconstruct the set of source
symbols plus the associated coefficients used during the encoding symbols plus the associated coefficients used during the encoding
process. In order to minimize packet overhead, the set of source 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 reconstruct the set
the set of source symbols considered during encoding, the only of source symbols considered during encoding, the only constraint
constraint being that there cannot be any gap; 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).
skipping to change at page 6, line 32 skipping to change at page 6, line 46
padding fields in addition to the ADU) padding fields in addition to the ADU)
E: size of an encoding symbol (i.e., source or repair symbol), E: size of an encoding symbol (i.e., source or repair symbol),
assumed fixed (in bytes) assumed fixed (in bytes)
br_in: transmission bitrate at the input of the FECFRAME sender, br_in: transmission bitrate at the input of the FECFRAME sender,
assumed fixed (in bits/s) 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: RLC coding rate, ratio between the total number of source cr: RLC coding rate, ratio between the total number of source
symbols and the total number of source plus repair symbols 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_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)
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 DT: coding coefficients density threshold, an integer between 0 and
15 (inclusive) the controls the fraction of coefficients that are 15 (inclusive) the controls the fraction of coefficients that are
non zero 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 these FEC
Scheme. Schemes.
3.1. Parameters Derivation 3.1. Possible Parameter Derivation
The Sliding Window RLC FEC Scheme relies on several key parameters: The Sliding Window RLC FEC Scheme relies on several 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). The maximum FEC-related several flows are globally protected). The maximum FEC-related
latency budget, max_lat, accounts for all sources of latency added latency budget, max_lat, accounts for all sources of latency added
by FEC encoding (at a sender) and FEC decoding (at a receiver). by FEC encoding (at a sender) and FEC decoding (at a receiver).
skipping to change at page 7, line 29 skipping to change at page 7, line 39
are out of scope and must be considered separately (said are out of scope and must be considered separately (said
differently, they have already been deducted from max_lat). differently, they have already been deducted from max_lat).
max_lat can be regarded as the latency budget permitted for all max_lat can be regarded as the latency budget permitted for all
FEC-related operations. This is an input parameter that enables FEC-related operations. This is an input parameter that enables
to derive other internal parameters as explained below; 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: encoding took place. At session start, the encoding window will
probably be small and then progressively increase until it reaches
its maximum value. At any time:
ew_size <= ew_max_size ew_size <= ew_max_size
Decoding window maximum size, dw_max_size (in symbols): at a Decoding window maximum size, dw_max_size (in symbols): at a
receiver, this parameter determines the maximum size of the receiver, this parameter denotes the maximum number of received or
decoding window. Said differently, this is the maximum number of lost source symbols in the linear system (i.e., the variables)
received or lost source symbols in the linear system (i.e., the that are still within their latency budget;
variables) that are still within their latency budget. In Linear system maximum size, ls_max_size (in symbols): The linear
situations where packets are sent with a fixed period, the system maximum size managed by a receiver SHOULD NOT be smaller
dw_max_size parameter directly determines the maximum decoding than this decoding window maximum size, since it would mean that,
latency experienced by the receiver, which necessarily needs to be after receiving a sufficient number of FEC Repair Packets, an ADU
in line with the maximum FEC-related latency budget. Note also may not be recovered just because it has been removed from the
that the optimization detailed in Appendix A can extend the linear linear system, and not because it has timed-out. This would be
system with additional old source symbols (that timed-out) beyond counter-productive. On the opposite, the linear system MAY grow
dw_max_size; beyond this value with old source symbols kept in the linear
system whereas their associated ADUs timed-out (Appendix A);
Symbol size, E (in bytes) and RLC code rate (cr): the E parameter 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 source and repair symbol sizes (necessarily equal).
determines the code rate, i.e., the amount of redundancy added to The cr parameter determines the code rate, i.e., the amount of
the flow (it is the ratio between the total number of source redundancy added to the flow (i.e., cr is the ratio between the
symbols and the total number of source plus repair symbols). total number of source symbols and the total number of source plus
These two parameters are input parameters that enable to derive repair symbols). These two parameters are input parameters that
other internal parameters as explained below. In practice they enable to derive other internal parameters as explained below. An
will usually be fixed, especially with multicast/broadcast implementation at a sender SHOULD fix the E parameter and
transmissions. In specific use-cases, in particular with unicast communicate it as part of the FEC Scheme-Specific Information
transmissions in presence of a feedback mechanism that estimates (Section 4.1.1.2). However there is no need to communicate the cr
the communication quality (out-of-scope of FECFRAME), the code parameter per see (it's not required to process a repair packet at
rate may be adjusted dynamically. a receiver). This code rate parameter can be fixed. However, in
specific use-cases (e.g., 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 The FEC Schemes specified in this document can be used in various
rate (cr) are constant. Let us also assume a constant transmission manners. They can protect one or more source ADU flows having real-
bitrate (br_out, in bits/s) at the output of the FECFRAME sender (as time constraints, or they can protect non-realtime source ADU flows.
in [Roca17]). It means that the source flow bitrate needs to be The source ADU flows may be Constant Bitrate (CBR) flows, while other
adjusted according to the added repair flow overhead in order to keep may be of Variable Bitrate (VBR). The FEC Schemes can be used in
the total transmission bitrate fixed and equal to br_out. In order various environments like the Internet or over a CBR channel. It
to comply with the maximum FEC-related latency budget we need: follows that the FEC Scheme parameters can be derived in different
ways, as described in the following sections.
dw_max_size = (max_lat * br_out * cr) / (8 * E) 3.1.1. Detailed Parameter Derivation for CBR Real-Time Flows
Sometimes the opposite can happen: the source flow bitrate at the In the following, we consider a real-time flow with max_lat latency
input of the FECFRAME sender is fixed (br_in, in bits/s). It means budget. The encoding symbol size (E, in bytes) is constant. The
that the transmission bitrate at the output of the FECFRAME sender code rate (cr) is also constant, in line with the expected
will be higher, depending on the added repair flow overhead. In communication loss model. However the choice of this cr value is out
order to comply with the maximum FEC-related latency budget we need: of scope for this document.
In a first configuration, the source ADU flow bitrate at the input of
the FECFRAME sender is fixed (br_in, in bits/s). It means 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 have:
dw_max_size = (max_lat * br_in) / (8 * E) dw_max_size = (max_lat * br_in) / (8 * E)
Finally, there are situations where no such assumption can be made In a second configuration, the FECFRAME sender generates a fixed
(e.g., with a variable bit rate input flow). In that case the bitrate flow, equal to the CBR channel bitrate (br_out, in bits/s),
encoding and decoding window maximum sizes may be initialized, based as in [Roca17]. The maximum source flow bitrate needs to be such
on the input flow features (e.g., the peak bitrate if it is known) that, with the added repair flow overhead, the total transmission
and great care must be taken on timing aspects at a sender (see bitrate remains (inferior or) equal to br_out. Here we have:
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 dw_max_size = (max_lat * br_out * cr) / (8 * E)
be defined. For decoding to be possible, it is required that the
encoding window maximum size be at most equal to the decoding window For decoding to be possible, it is required that the encoding window
maximum size. It is often good practice to choose [Roca17]: maximum size be at most equal to the decoding window maximum size.
So, once the dw_max_size has been determined, the ew_max_size SHOULD
be computed with ([Roca17]):
ew_max_size = dw_max_size * 0.75 ew_max_size = dw_max_size * 0.75
However any value ew_max_size < dw_max_size can be used without The ew_max_size is the main parameter, used by a FECFRAME sender.
impact on the FEC-related latency budget. Finding the optimal value Whenever the FEC protection (i.e., cr value) is sufficient in front
will depend on the use-case details and should be determined after of the packet loss model, the ew_max_size guaranties that the
simulations or field trials. This is of course out of scope of this recovery of lost ADUs will happen at a FECFRAME receiver on time.
document.
Note that the decoding beyond maximum latency optimization The dw_max_size is computed by a FECFRAME sender but not explicitly
(Appendix A) enables an old source symbol to be kept in the linear communicated to a FECFRAME receiver. However a FECFRAME receiver can
system beyond the FEC-related latency budget, but not delivered to easily evaluate the ew_max_size by observing the maximum Number of
the receiving application. In any case, the linear system maximum Source Symbols (NSS) value contained in the Repair FEC Payload ID of
size is greater than (with the decoding optimization) or equal to received FEC Repair Packets (Section 4.1.3). A receiver can then
(without) the decoding window maximum size: easily compute dw_max_size:
dw_max_size = max_NSS_observed / 0.75
and chose an appropriate maximum linear system size. Having a
limited linear system size is a practical requirement that enables to
forget old source symbols, no longer needed. We have:
ls_max_size >= dw_max_size ls_max_size >= dw_max_size
Using the same maximum size is the minimum. But it is good practice
to use a larger value for ls_max_size as explained in Appendix A,
without impacting maximum latency nor interoperability.
The particular case of session start needs to be managed
appropriately. Here the ew_size progressively increases, upon
receiving new source ADUs at the FECFRAME sender, until it reaches
the ew_max_size value, A FECFRAME receiver SHOULD continuously
observe the received FEC Repair Packets, since the NSS value carried
in the Repair FEC Payload ID will increase too, and adjust the
ls_max_size accordingly.
3.1.2. Parameter Derivation for Other Real-Time Flows
There are situations where the real-time source ADU flow is of
variable bitrate (VBR). A first possibility is to consider the peak
bitrate of the source ADU flow, when this parameter is known, and to
reuse the derivation of Section 3.1.1.
There are also situations where the peak bitrate is not know. In
that case the previous parameter derivation cannot be directly
applied. An approach in that case consists in using ADU timing
information when present (e.g., using the timestamp field of an RTP
packet header) to manage the encoding window accordingly, in
particular removing old symbols whose associated ADUs timed-out.
No matter the choice of the FECFRAME sender, a FECFRAME receiver can
still easily evaluate the ew_max_size by observing the maximum Number
of Source Symbols (NSS) value contained in the Repair FEC Payload ID
of received FEC Repair Packets. A receiver can then compute
dw_max_size and derive an appropriate maximum linear system size,
ls_max_size.
When the observed NSS fluctuates significantly and perhaps slowly, a
FECFRAME receiver may want to adapt its ls_max_size accordingly in
order to avoid managing linear systems that would be significantly
too large. It is worth noticing however that it is preferable to use
an ls_max_size too large than the opposite.
Beyond these general guidelines, the details of how to manage these
situations at a FECFRAME sender and receiver remain out of scope of
this document.
3.1.3. Parameter Derivation for Non Real-Time Flows
Finally there are situations where there is no known real-time
constraints. FECFRAME and the FEC Schemes defined in this document
can still be used. The choice of appropriate parameter values can be
directed by practical considerations. It can be an estimation of the
maximum memory amount that could be dedicated to the linear system at
a FECFRAME receiver, or CPU computation requirements at a FECFRAME
receiver, both of them depending on the ls_max_size. The same
considerations can also apply to the FECFRAME sender, where maximum
memory and CPU computation requirements depend on the ew_max_size.
Here also, the NSS value contained in FEC Repair Packets is used to
inform a FECFRAME receiver of the current coding window size (and
ew_max_size by observing its maximum value over the time).
Beyond these general guidelines, the details of how to manage these
situations at a FECFRAME sender and receiver remain out of scope of
this document.
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 At a sender, an ADU coming from the application cannot directly be
symbols directly. Indeed, a lost ADU recovered at a receiver must mapped to source symbols. When multiple source flows (e.g., media
contain enough information to be assigned to the right application streams) are mapped onto the same FECFRAME instance, each flow is
flow (UDP port numbers and IP addresses cannot be used to that assigned its own Flow ID value (see below). At a sender, this
purpose as they are not protected by FEC encoding). This requires identifier is prepended to each ADU before FEC encoding. This way,
adding the flow identifier to each ADU before doing FEC encoding. FEC decoding at a receiver also recovers this Flow ID and a recovered
ADU can be assigned to the right source flow (note that transport
port numbers and IP addresses cannot be used to that purpose as they
are not recovered during FEC decoding).
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 MUST created as follows. First of
3 bytes are prepended (Figure 1): all, 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 session 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).
skipping to change at page 10, line 17 skipping to change at page 12, line 17
+-+--+---------------------------------------------+-------------+ +-+--+---------------------------------------------+-------------+
|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, and a receiver who lost a certain FEC Source Packet (e.g., encoding, and a receiver who lost a certain FEC Source Packet (e.g.,
the UDP datagram containing this FEC Source Packet) will be able to the UDP datagram containing this FEC Source Packet when UDP is used
recover the ADUI if FEC decoding succeeds. Thanks to the initial 3 as the transport protocol) will be able to recover the ADUI if FEC
bytes, this receiver will get rid of the padding (if any) and decoding succeeds. Thanks to the initial 3 bytes, this receiver will
identify the corresponding ADU flow. get rid of the padding (if any) and 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
skipping to change at page 11, line 23 skipping to change at page 13, line 23
(modulo M), with the following choices: A = 7^^5 = 16807 and M = (modulo M), with the following choices: A = 7^^5 = 16807 and M =
2^^31 - 1 = 2147483647. A validation criteria of such a PRNG is the 2^^31 - 1 = 2147483647. A validation criteria of such a PRNG is the
following: if seed = 1, then the 10,000th value returned MUST be following: if seed = 1, then the 10,000th value returned MUST be
equal to 1043618065. equal to 1043618065.
Several implementations of this PRNG are known and discussed in the Several implementations of this PRNG are known and discussed in the
literature. An optimized implementation of this algorithm, using literature. An optimized implementation of this algorithm, using
only 32-bit mathematics, and which does not require any division, can only 32-bit mathematics, and which does not require any division, can
be found in [rand31pmc]. It uses the Park and Miller algorithm be found in [rand31pmc]. It uses the Park and Miller algorithm
[PM88] with the optimization suggested by D. Carta in [CA90]. The [PM88] with the optimization suggested by D. Carta in [CA90]. The
history behind this algorithm is detailed in [WI08]. Yet, any other history behind this algorithm is detailed in [WI08].
implementation of the PRNG algorithm that matches the above
validation criteria, like the ones detailed in [PM88], is
appropriate.
This PRNG produces, natively, a 31-bit value between 1 and 0x7FFFFFFE This PRNG produces, natively, a 31-bit value between 1 and 0x7FFFFFFE
(2^^31-2) inclusive. Since it is desired to scale the pseudo-random (2^^31-2) inclusive. Since it is desired to scale the pseudo-random
number between 0 and maxv-1 inclusive, one must keep the most number between 0 and maxv-1 inclusive, one must keep the most
significant bits of the value returned by the PRNG (the least significant bits of the value returned by the PRNG (the least
significant bits are known to be less random, and modulo-based significant bits are known to be less random, and modulo-based
solutions should be avoided [PTVF92]). The following algorithm MUST solutions should be avoided [PTVF92]). The following algorithm MUST
be used: be used:
Input: Input:
skipping to change at page 13, line 32 skipping to change at page 15, line 31
return SOMETHING_WENT_WRONG; /* bad repair_key parameter */ return SOMETHING_WENT_WRONG; /* bad repair_key parameter */
} }
switch (m) { switch (m) {
case 1: case 1:
if (dt == 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 */ /* here coefficients are either 0 or 1 */
pmms_srand(repair_key); pmms_srand(repair_key);
pmms_rand(16); /* skip the first PRNG value */
for (i = 0 ; i < cc_nb ; i++) { for (i = 0 ; i < cc_nb ; i++) {
if (pmms_rand(16) <= dt) { 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:
pmms_srand(repair_key); pmms_srand(repair_key);
pmms_rand(256); /* skip the first PRNG value */
if (dt == 15) { 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 {
skipping to change at page 14, line 29 skipping to change at page 16, line 29
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
One can note in the above function that each call to pmms_srand()
(PRNG initialisation) is immediately followed by a call to
pmms_rand() whose return value is ignored. This extra call is
motivated by a possible bias in the first value generated depending
on the way the repair key is managed by a FECFRAME implementation.
Indeed, the PRNG sequences produced by two seeds in sequence have a
high probability of starting with the same value since I1 = A * seed
(modulo M) which is further scaled to a small range (either {0, ...
15} or {0, ... 255}). Producing several times the same first coding
coefficient could reduce the protection of the first source symbol if
multiple repair symbols are produced with the same coding window's
left edge. The extra call avoids such side effects.
3.6. Linear Combination of Source Symbols Computation
The two RLC FEC Schemes require the computation of a linear
combination of source symbols, using the coding coefficients produced
by the generate_coding_coefficients() function and stored in the
cc_tab[] array.
With the RLC over GF(2^^8) FEC Scheme, a linear combination of the
ew_size source symbol present in the encoding window, say src_0 to
src_ew_size_1, in order to generate a repair symbol, is computed as
follows. For each byte of position i in each source and the repair
symbol, where i belongs to {0; E-1}, compute:
repair[i] = cc_tab[0] * src_0[i] + cc_tab[1] * src_1[i] + ... +
cc_tab[ew_size - 1] * src_ew_size_1[i]
where * is the multiplication over GF(2^^8) and + is an XOR
operation. In practice various optimizations need to be used in
order to make this computation efficient (see in particular [PGM13]).
With the RLC over GF(2) FEC Scheme (binary case), a linear
combination is computed as follows. The repair symbol is the XOR sum
of all the source symbols corresponding to a coding coefficient
cc_tab[j] equal to 1 (i.e., the source symbols corresponding to zero
coding coefficients are ignored). The XOR sum of the byte of
position i in each source is computed and stored in the corresponding
byte of the repair symbol, where i belongs to {0; E-1}. In practice,
the XOR sums will be computed several bytes at a time (e.g., on 64
bit words, or on arrays of 16 or more bytes when using SIMD CPU
extensions).
With both FEC Schemes, the details of how to optimize the computation
of these linear combinations are of high practical importance but out
of scope of this document.
4. Sliding Window RLC FEC Scheme over GF(2^^8) 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^^8). 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
The FEC Framework Configuration Information (or FFCI) includes Following the guidelines of [RFC6363], section 5.6, this section
information that MUST be communicated between the sender and provides the FEC Framework Configuration Information (or FFCI). This
receiver(s). More specifically, it enables the synchronization of FCCI needs to be shared (e.g., using SDP) between the FECFRAME sender
the FECFRAME sender and receiver instances. It includes both and receiver instances in order to synchronize them. It includes a
mandatory elements and scheme-specific elements, as detailed below. FEC Encoding ID, mandatory for any FEC Scheme specification, plus
scheme-specific elements.
4.1.1.1. Mandatory Information 4.1.1.1. FEC Encoding ID
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.
4.1.1.2. 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
skipping to change at page 16, line 24 skipping to change at page 19, line 24
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 4.1.3. Repair FEC Payload ID
A FEC Repair Packet can contain one or more repair symbols. When A FEC Repair Packet MAY contain one or more repair symbols. When
there are several repair symbols, all of them MUST have been there are several repair symbols, all of them MUST have been
generated from the same encoding window, using Repair_Key values that generated from the same encoding window, using Repair_Key values that
are managed as explained below. A receiver can easily deduce the are managed as explained below. A receiver can easily deduce the
number of repair symbols within a FEC Repair Packet by comparing the number of repair symbols within a FEC Repair Packet by comparing the
received FEC Repair Packet size (equal to the UDP payload size when received FEC Repair Packet size (equal to the UDP payload size when
UDP is the underlying transport protocol) and the symbol size, E, UDP is the underlying transport protocol) and the symbol size, E,
communicated in the FFCI. communicated in the FFCI.
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. prepended to the repair symbol as illustrated in Figure 6.
skipping to change at page 17, line 14 skipping to change at page 20, line 14
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).
Density Threshold for the coding coefficients, DT (4-bit field): Density Threshold for the coding coefficients, DT (4-bit field):
this unsigned integer carried the Density Threshold (DT) used by this unsigned integer carries the Density Threshold (DT) used by
the 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
skipping to change at page 18, line 14 skipping to change at page 21, line 14
5. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary ADU Flows 5. Sliding Window RLC FEC Scheme over GF(2) 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) (binary case).
5.1. Formats and Codes 5.1. Formats and Codes
5.1.1. FEC Framework Configuration Information 5.1.1. FEC Framework Configuration Information
5.1.1.1. Mandatory Information 5.1.1.1. FEC Encoding ID
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 YYYY, as assigned by IANA (Section 10). Scheme MUST be YYYY, 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 5.1.1.2. FEC Scheme-Specific Information
All the considerations of Section 4.1.1.2 apply here. All the considerations of Section 4.1.1.2 apply here.
skipping to change at page 19, line 14 skipping to change at page 22, line 14
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. In that case the repair key for each of
1 the repair key for each of them, keeping the same ew_size source them MUST be incremented by 1, 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 passed to
source versus repair FEC packet transmission order is out of scope of the transport layer for transmission. The source versus repair FEC
this document and several approaches exist that are implementation packet transmission order is out of scope of this document and
specific. several approaches exist that are implementation specific.
Other solutions are possible to select a repair key value when a new Other solutions are possible to select a repair key value when a new
FEC Repair Packet is needed, for instance by choosing a random FEC Repair Packet is needed, for instance by choosing a random
integer between 1 and 65535. However, selecting the same repair key integer between 1 and 65535. However, selecting the same repair key
as before (which may happen in case of a random process) is only as before (which may happen in case of a random process) is only
meaningful if the encoding window has changed, otherwise the same FEC meaningful if the encoding window has changed, otherwise the same FEC
Repair Packet will be generated. Repair Packet will be generated.
6.2. Decoding Side 6.2. Decoding Side
skipping to change at page 20, line 4 skipping to change at page 23, line 4
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, padding is removed (thanks to the Length field, L, of the recovered, padding is removed (thanks to the Length field, L, of the
ADUI) and the ADU is assigned to the corresponding application flow ADUI) and the ADU is assigned to the corresponding application flow
(thanks to the Flow ID field, F, of the ADUI). This ADU is finally (thanks to the Flow ID field, F, of the ADUI). This ADU is finally
passed to the corresponding upper application. Received FEC Source passed to the corresponding upper application. Received FEC Source
Packets, containing an ADU, can be passed to the application either Packets, containing an ADU, MAY be passed to the application either
immediately or after some time to guaranty an ordered delivery to the immediately or after some time to guaranty an ordered delivery to the
application. This document does not mandate any approach as this is application. This document does not mandate any approach 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 after this delay should not be passed to latency or an ADU received after this delay has no value to the
the application. Instead the associated source symbols should be application. This raises the question of deciding whether or not an
removed from the linear system maintained by the receiver(s). ADU is late. This decision MAY be taken within the FECFRAME receiver
Appendix A discusses a backward compatible optimization whereby those (e.g., using the decoding window, see Section 3.1) or within the
late source symbols may still be used in order to improve the global application (e.g., using RTP timestamps within the ADU). Deciding
robustness. which option to follow and whether or not to pass all ADUs, including
those assumed late, to the application are operational decisions that
depend on the application and are therefore out of scope of this
document. Additionally, Appendix A discusses a backward compatible
optimization whereby late source symbols MAY still be used within the
FECFRAME receiver in order to improve the global 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
skipping to change at page 24, line 11 skipping to change at page 27, line 21
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,
<https://www.rfc-editor.org/info/rfc6364>. <https://www.rfc-editor.org/info/rfc6364>.
12.2. 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.
[PGM13] Plank, J., Greenan, K., and E. Miller, "A Complete
Treatment of Software Implementations of Finite Field
Arithmetic for Erasure Coding Applications", University of
Tennessee Technical Report UT-CS-13-717,
http://web.eecs.utk.edu/~plank/plank/papers/
UT-CS-13-717.html, October 2013,
<http://web.eecs.utk.edu/~plank/plank/papers/
UT-CS-13-717.html>.
[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,
"Numerical Recipies in C; Second Edition", Cambridge "Numerical Recipies in C; Second Edition", Cambridge
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",
skipping to change at page 26, line 15 skipping to change at page 29, line 15
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, by keeping a certain number linear system beyond the decoding window, by keeping a certain number
of old source symbols: 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
When the dw_max_size is very small, it may be preferable to keep a
minimum ls_max_size value (e.g., LS_MIN_SIZE_DEFAULT = 40 symbols).
Going below this threshold will not save a significant amount of
memory nor CPU cycles. Therefore:
ls_max_size = max(2 * dw_max_size, LS_MIN_SIZE_DEFAULT)
Finally, it is worth noting that a good receiver, i.e., a receiver
that benefits from a protection that is significantly sufficient to
recover from the packet losses, can choose to reduce its ls_max_size
significantly. In that case lost ADUs will be recovered rapidly,
without relying on this optimization.
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 the added latency exceeds the maximum value permitted by the if the added latency exceeds the maximum value permitted by the
application. It follows that the corresponding ADUs SHOULD NOT be application. It follows that the corresponding ADUs will not be
delivered to the application and SHOULD be dropped once they are no useful to the application. 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 communication conditions [Roca16]. In any case whether or not to
conditions [Roca16]. In any case whether or not to use this use this optimization and what exact value to use for the ls_max_size
optimization and what exact value to use for the ls_max_size
parameter are decisions made by each receiver independently, without parameter are decisions made by each receiver independently, without
any impact on the other receivers nor on 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
INRIA INRIA
Grenoble Grenoble
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