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Keywords: RLC, FEC, FECFRAME, packet loss recovery, reliability

Internet Engineering Task Force (IETF) V. Roca
Request for Comments: 8681 B. Teibi
Category: Standards Track INRIA
ISSN: 2070-1721 January 2020
Sliding Window Random Linear Code (RLC) Forward Erasure Correction (FEC)
Schemes for FECFRAME
Abstract
This document describes two fully specified Forward Erasure
Correction (FEC) Schemes for Sliding Window Random Linear Codes
(RLC), one for RLC over the Galois Field (a.k.a., Finite Field)
GF(2), a second one for RLC over the Galois Field GF(2^(8)), each
time with the possibility of controlling the code density. They can
protect arbitrary media streams along the lines defined by FECFRAME
extended to Sliding Window FEC Codes. These Sliding Window FEC Codes
rely on an encoding window that slides over the source symbols,
generating new repair symbols whenever needed. Compared to block FEC
codes, these Sliding Window FEC Codes offer key advantages with real-
time flows in terms of reduced FEC-related latency while often
providing improved packet erasure recovery capabilities.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8681.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Limits of Block Codes with Real-Time Flows
1.2. Lower Latency and Better Protection of Real-Time Flows with
the Sliding Window RLC Codes
1.3. Small Transmission Overheads with the Sliding Window RLC
FEC Scheme
1.4. Document Organization
2. Definitions and Abbreviations
3. Common Procedures
3.1. Codec Parameters
3.2. ADU, ADUI, and Source Symbols Mappings
3.3. Encoding Window Management
3.4. Source Symbol Identification
3.5. Pseudorandom Number Generator (PRNG)
3.6. Coding Coefficients Generation Function
3.7. Finite Field Operations
3.7.1. Finite Field Definitions
3.7.2. Linear Combination of Source Symbol Computation
4. Sliding Window RLC FEC Scheme over GF(2^(8)) for Arbitrary
Packet Flows
4.1. Formats and Codes
4.1.1. FEC Framework Configuration Information
4.1.2. Explicit Source FEC Payload ID
4.1.3. Repair FEC Payload ID
4.2. Procedures
5. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary Packet
Flows
5.1. Formats and Codes
5.1.1. FEC Framework Configuration Information
5.1.2. Explicit Source FEC Payload ID
5.1.3. Repair FEC Payload ID
5.2. Procedures
6. FEC Code Specification
6.1. Encoding Side
6.2. Decoding Side
7. Security Considerations
7.1. Attacks Against the Data Flow
7.1.1. Access to Confidential Content
7.1.2. Content Corruption
7.2. Attacks Against the FEC Parameters
7.3. When Several Source Flows are to be Protected Together
7.4. Baseline Secure FEC Framework Operation
7.5. Additional Security Considerations for Numerical
Computations
8. Operations and Management Considerations
8.1. Operational Recommendations: Finite Field GF(2) Versus
GF(2^(8))
8.2. Operational Recommendations: Coding Coefficients Density
Threshold
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. TinyMT32 Validation Criteria (Normative)
Appendix B. Assessing the PRNG Adequacy (Informational)
Appendix C. Possible Parameter Derivation (Informational)
C.1. Case of a CBR Real-Time Flow
C.2. Other Types of Real-Time Flow
C.3. Case of a Non-Real-Time Flow
Appendix D. Decoding Beyond Maximum Latency Optimization
(Informational)
Acknowledgments
Authors' Addresses
1. Introduction
Application-Level Forward Erasure Correction (AL-FEC) codes, or
simply FEC codes, are a key element of communication systems. They
are used to recover from packet losses (or erasures) during content
delivery sessions to a potentially large number of receivers
(multicast/broadcast transmissions). This is the case with the File
Delivery over Unidirectional Transport (FLUTE)/Asynchronous Layered
Coding (ALC) protocol [RFC6726] when used for reliable file transfers
over lossy networks, and the FECFRAME protocol [RFC6363] when used
for reliable continuous media transfers over lossy networks.
The present document only focuses on the FECFRAME protocol, which is
used in multicast/broadcast delivery mode, particularly for content
that features stringent real-time constraints: each source packet has
a maximum validity period after which it will not be considered by
the destination application.
1.1. Limits of Block Codes with Real-Time Flows
With FECFRAME, there is a single FEC encoding point (either an end
host/server (source) or a middlebox) and a single FEC decoding point
per receiver (either an end host (receiver) or middlebox). In this
context, currently standardized AL-FEC codes for FECFRAME like Reed-
Solomon [RFC6865], LDPC-Staircase [RFC6816], or Raptor/RaptorQ
[RFC6681], are all linear block codes: they require the data flow to
be segmented into blocks of a predefined maximum size.
To define this block size, it is required to find an appropriate
balance between robustness and decoding latency: the larger the block
size, the higher the robustness (e.g., in case of long packet erasure
bursts), but also the higher the maximum decoding latency (i.e., the
maximum time required to recover a lost (erased) packet thanks to 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 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
Sliding Window RLC Codes
This document introduces two fully specified FEC schemes that do not
follow the block code approach: the Sliding Window Random Linear
Codes (RLC) over either Galois Fields (a.k.a., Finite Fields) GF(2)
(the "binary case") or GF(2^(8)), each time with the possibility of
controlling the code density. These FEC schemes are used to protect
arbitrary media streams along the lines defined by FECFRAME extended
to Sliding Window FEC Codes [RFC8680]. These FEC schemes and, more
generally, Sliding Window FEC Codes are recommended, for instance,
with media that feature real-time constraints sent within a
multicast/broadcast session [Roca17].
The RLC codes belong to the broad class of Sliding Window AL-FEC
Codes (a.k.a., convolutional codes) [RFC8406]. The encoding process
is based on an encoding window that slides over the set of source
packets (in fact source symbols as we will see in Section 3.2), this
window being either of fixed size or variable size (a.k.a., an
elastic window). Repair symbols are generated on-the-fly, by
computing a random linear combination of the source symbols present
in the current encoding window, and passed to the transport layer.
At the receiver, a linear system is managed from the set of received
source and repair packets. New variables (representing source
symbols) and equations (representing the linear combination carried
by each repair symbol received) are added upon receiving new packets.
Variables and the equations they are involved in are removed when
they are too old with respect to their validity period (real-time
constraints). Lost source symbols are then recovered thanks to this
linear system whenever its rank permits to solve it (at least
partially).
The protection of any multicast/broadcast session needs to be
dimensioned by considering the worst communication conditions one
wants to support. This is also true with RLC (more generally, any
sliding window) code. However, the receivers experiencing a good to
medium communication quality will observe a reduced FEC-related
latency compared to block codes [Roca17] since an isolated lost
source packet is quickly recovered with the following repair packet.
On the opposite, with a block code, recovering an isolated lost
source packet always requires waiting for the first repair packet to
arrive after the end of the block. Additionally, under certain
situations (e.g., with a limited FEC-related latency budget and with
constant bitrate transmissions after FECFRAME encoding), Sliding
Window Codes can more efficiently achieve a target transmission
quality (e.g., measured by the residual loss after FEC decoding) by
sending fewer repair packets (i.e., higher code rate) than block
codes.
1.3. Small Transmission Overheads with the Sliding Window RLC FEC
Scheme
The Sliding Window RLC FEC scheme is designed to limit the packet
header overhead. The main requirement is that each repair packet
header must enable a receiver to reconstruct the set of source
symbols plus the associated coefficients used during the encoding
process. In order to minimize packet overhead, the set of source
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
in the linear combination are not individually listed in the repair
packet header. Instead, each FEC Repair Packet header contains:
* the Encoding Symbol Identifier (ESI) of the first source symbol in
the encoding window as well as the number of symbols (since this
number may vary with a variable size, elastic window). These two
pieces of information enable each receiver to reconstruct the set
of source symbols considered during encoding, the only constraint
being that there cannot be any gap;
* the seed and density threshold parameters used by a coding
coefficients generation function (Section 3.6). These two pieces
of information enable each receiver to generate the same set of
coding coefficients over GF(2^(m)) as the sender;
Therefore, no matter the number of source symbols present in the
encoding window, each FEC Repair Packet features a fixed 64-bit long
header, called Repair FEC Payload ID (Figure 8). Similarly, each FEC
Source Packet features a fixed 32-bit long trailer, called Explicit
Source FEC Payload ID (Figure 6), that contains the ESI of the first
source symbol (Section 3.2).
1.4. Document Organization
This fully-specified FEC scheme follows the structure required by
[RFC6363], Section 5.6 ("FEC Scheme Requirements"), namely:
3. Procedures: This section describes procedures specific to this
FEC scheme, namely: RLC parameters derivation, ADUI and source
symbols mapping, pseudorandom number generator, and coding
coefficients generation function;
4. Formats and Codes: This section defines the Source FEC Payload ID
and Repair FEC Payload ID formats, carrying the signaling
information associated to each source or repair symbol. It also
defines the FEC Framework Configuration Information (FFCI)
carrying signaling information for the session;
5. FEC Code Specification: Finally this section provides the code
specification.
2. Definitions and Abbreviations
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the following definitions and abbreviations:
a^(b) a to the power of b
GF(q) denotes a finite field (also known as the Galois Field) with q
elements. We assume that q = 2^(m) in this document
m defines the length of the elements in the finite field, in bits.
In this document, m is equal to 1 or 8
ADU: Application Data Unit
ADUI: Application Data Unit Information (includes the F, L and
padding fields in addition to the ADU)
E: size of an encoding symbol (i.e., source or repair symbol),
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,
assumed fixed (in bits/s)
max_lat: maximum FEC-related latency within FECFRAME (a decimal
number expressed in seconds)
cr: RLC coding rate, ratio between the total number of source
symbols and the total number of source plus repair symbols
ew_size: encoding window current 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)
ls_max_size: linear system maximum size (or width) at a receiver (in
symbols)
WSR: window size ratio parameter used to derive ew_max_size
(encoder) and ls_max_size (decoder).
PRNG: pseudorandom number generator
TinyMT32: PRNG used in this specification.
DT: coding coefficients density threshold, an integer between 0 and
15 (inclusive) the controls the fraction of coefficients that are
nonzero
3. Common Procedures
This section introduces the procedures that are used by these FEC
schemes.
3.1. Codec Parameters
A codec implementing the Sliding Window RLC FEC scheme relies on
several parameters:
Maximum FEC-related latency budget, max_lat (a decimal number
expressed in seconds) with real-time flows:
a source ADU flow can have real-time constraints, and therefore
any FECFRAME related operation should take place within the
validity period of each ADU (Appendix D describes an exception to
this rule). When there are multiple flows with different real-
time constraints, we consider the most stringent constraints (see
item 6 in Section 10.2 of [RFC6363], for recommendations when
several flows are globally protected). The maximum FEC-related
latency budget, max_lat, accounts for all sources of latency added
by FEC encoding (at a sender) and FEC decoding (at a receiver).
Other sources of latency (e.g., added by network communications)
are out of scope and must be considered separately (said
differently, they have already been deducted from max_lat).
max_lat can be regarded as the latency budget permitted for all
FEC-related operations. This is an input parameter that enables a
FECFRAME sender to derive other internal parameters (see
Appendix C);
Encoding window current (resp. maximum) size, ew_size (resp.
ew_max_size) (in symbols):
at a FECFRAME sender, during FEC encoding, a repair symbol is
computed as a linear combination of the ew_size source symbols
present in the encoding window. The ew_max_size is the maximum
size of this window, while ew_size is the current size. For
example, in the common case at session start, upon receiving new
source ADUs, the ew_size progressively increases until it reaches
its maximum value, ew_max_size. We have:
0 < ew_size <= ew_max_size
Decoding window maximum size, dw_max_size (in symbols):
at a FECFRAME receiver, dw_max_size is the maximum number of
received or lost source symbols that are still within their
latency budget;
Linear system maximum size, ls_max_size (in symbols):
at a FECFRAME receiver, the linear system maximum size,
ls_max_size, is the maximum number of received or lost source
symbols in the linear system (i.e., the variables). It SHOULD NOT
be smaller than dw_max_size since it would mean that, even after
receiving a sufficient number of FEC Repair Packets, a lost ADU
may not be recovered just because the associated source symbols
have been prematurely removed from the linear system, which is
usually counter-productive. On the opposite, the linear system
MAY grow beyond the dw_max_size (Appendix D);
Symbol size, E (in bytes):
the E parameter determines the source and repair symbol sizes
(necessarily equal). This is an input parameter that enables a
FECFRAME sender to derive other internal parameters, as explained
below. An implementation at a sender MUST fix the E parameter and
MUST communicate it as part of the FEC Scheme-Specific Information
(Section 4.1.1.2).
Code rate, cr:
The code rate parameter determines the amount of redundancy added
to the flow. More precisely the cr is the ratio between the total
number of source symbols and the total number of source plus
repair symbols and by definition: 0 < cr <= 1. This is an input
parameter that enables a FECFRAME sender to derive other internal
parameters, as explained below. However, there is no need to
communicate the cr parameter per see (it's not required to process
a repair symbol at a receiver). This code rate parameter can be
static. 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.
Appendix C proposes non-normative techniques to derive those
parameters, depending on the use-case specificities.
3.2. ADU, ADUI, and Source Symbols Mappings
At a sender, an ADU coming from the application is not directly
mapped to source symbols. When multiple source flows (e.g., media
streams) are mapped onto the same FECFRAME instance, each flow is
assigned its own Flow ID value (see below). This Flow ID is then
prepended to each ADU before FEC encoding. This way, FEC decoding at
a receiver also recovers this Flow ID and the recovered ADU can be
assigned to the right source flow (note that the 5-tuple used to
identify the right source flow of a received ADU is absent with a
recovered ADU since it is not FEC protected).
Additionally, since ADUs are of variable size, padding is needed so
that each ADU (with its flow identifier) contribute to an integral
number of source symbols. This requires adding the original ADU
length to each ADU before doing FEC encoding. Because of these
requirements, an intermediate format, the ADUI, or ADU Information,
is considered [RFC6363].
For each incoming ADU, an ADUI MUST be created as follows. First of
all, 3 bytes are prepended (Figure 1):
Flow ID (F) (8-bit field): this unsigned byte contains the integer
identifier associated to the source ADU flow to which this ADU
belongs. It is assumed that a single byte is sufficient, which
implies that no more than 256 flows will be protected by a single
FECFRAME session instance.
Length (L) (16-bit field): this unsigned integer contains the length
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
fields.
Then, zero padding is added to the ADU if needed:
Padding (Pad) (variable size field): this field contains zero
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).
The 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
case for ADUIs. However, an ADUI always contributes to an integral
number of source symbols.
symbol length, E E E
< ------------------ >< ------------------ >< ------------------ >
+-+--+---------------------------------------------+-------------+
|F| L| ADU | Pad |
+-+--+---------------------------------------------+-------------+
Figure 1: ADUI Creation Example, Resulting in Three Source Symbols
Note that neither the initial 3 bytes nor the optional padding are
sent over the network. However, they are considered during FEC
encoding, and a receiver that lost a certain FEC Source Packet (e.g.,
the UDP datagram containing this FEC Source Packet when UDP is used
as the transport protocol) will be able to recover the ADUI if FEC
decoding succeeds. Thanks to the initial 3 bytes, this receiver will
get rid of the padding (if any) and identify the corresponding ADU
flow.
3.3. Encoding Window Management
Source symbols and the corresponding ADUs are removed from the
encoding window:
* when the sliding encoding window has reached its maximum size,
ew_max_size. In that case the oldest symbol MUST be removed
before adding a new symbol, so that the current encoding window
size always remains inferior or equal to the maximum size: ew_size
<= ew_max_size;
* when an ADU has reached its maximum validity duration in case of a
real-time flow. When this happens, all source symbols
corresponding to the ADUI that expired SHOULD be removed from the
encoding window;
Source symbols are added to the sliding encoding window each time a
new ADU arrives, once the ADU-to-source symbols mapping has been
performed (Section 3.2). The current size of the encoding window,
ew_size, is updated after adding new source symbols. This process
may require to remove old source symbols so that: ew_size <=
ew_max_size.
Note that a FEC codec may feature practical limits in the number of
source symbols in the encoding window (e.g., for computational
complexity reasons). This factor may further limit the ew_max_size
value, in addition to the maximum FEC-related latency budget
(Section 3.1).
3.4. Source Symbol Identification
Each source symbol is identified by an Encoding Symbol ID (ESI), an
unsigned integer. The ESI of source symbols MUST start with value 0
for the first source symbol and MUST be managed sequentially.
Wrapping to zero happens after reaching the maximum value made
possible by the ESI field size (this maximum value is FEC scheme
dependent, for instance, 2^(32)-1 with FEC schemes 9 and 10).
No such consideration applies to repair symbols.
3.5. Pseudorandom Number Generator (PRNG)
In order to compute coding coefficients (see Section 3.6), the RLC
FEC schemes rely on the TinyMT32 PRNG defined in [RFC8682] with two
additional functions defined in this section.
This PRNG MUST first be initialized with a 32-bit unsigned integer,
used as a seed, with:
void tinymt32_init (tinymt32_t * s, uint32_t seed);
With the FEC schemes defined in this document, the seed is in
practice restricted to a value between 0 and 0xFFFF inclusive (note
that this PRNG accepts a seed value equal to 0), since this is the
Repair_Key 16-bit field value of the Repair FEC Payload ID
(Section 4.1.3). In practice, how to manage the seed and Repair_Key
values (both are equal) is left to the implementer, using a
monotonically increasing counter being one possibility (Section 6.1).
In addition to the seed, this function takes as parameter a pointer
to an instance of a tinymt32_t structure that is used to keep the
internal state of the PRNG.
Then, each time a new pseudorandom integer between 0 and 15 inclusive
(4-bit pseudorandom integer) is needed, the following function is
used:
uint32_t tinymt32_rand16 (tinymt32_t * s);
This function takes as parameter a pointer to the same tinymt32_t
structure (that is left unchanged between successive calls to the
function).
Similarly, each time a new pseudorandom integer between 0 and 255
inclusive (8-bit pseudorandom integer) is needed, the following
function is used:
uint32_t tinymt32_rand256 (tinymt32_t * s);
These two functions keep respectively the 4 or 8 less significant
bits of the 32-bit pseudorandom number generated by the
tinymt32_generate_uint32() function of [RFC8682]. This is done by
computing the result of a binary AND between the
tinymt32_generate_uint32() output and respectively the 0xF or 0xFF
constants, using 32-bit unsigned integer operations. Figure 2 shows
a possible implementation. This is a C language implementation,
written for C99 [C99]. Test results discussed in Appendix B show
that this simple technique, applied to this PRNG, is in line with the
RLC FEC schemes needs.
<CODE BEGINS>
/**
* This function outputs a pseudorandom integer in [0 .. 15] range.
*
* @param s pointer to tinymt internal state.
* @return unsigned integer between 0 and 15 inclusive.
*/
uint32_t tinymt32_rand16(tinymt32_t *s)
{
return (tinymt32_generate_uint32(s) & 0xF);
}
/**
* This function outputs a pseudorandom integer in [0 .. 255] range.
*
* @param s pointer to tinymt internal state.
* @return unsigned integer between 0 and 255 inclusive.
*/
uint32_t tinymt32_rand256(tinymt32_t *s)
{
return (tinymt32_generate_uint32(s) & 0xFF);
}
<CODE ENDS>
Figure 2: 4-bit and 8-bit Mapping Functions for TinyMT32
Any implementation of this PRNG MUST have the same output as that
provided by the reference implementation of [RFC8682]. In order to
increase the compliance confidence, three criteria are proposed: the
one described in [RFC8682] (for the TinyMT32 32-bit unsigned integer
generator), and the two others detailed in Appendix A (for the
mapping to 4-bit and 8-bit intervals). Because of the way the
mapping functions work, it is unlikely that an implementation that
fulfills the first criterion fails to fulfill the two others.
3.6. Coding Coefficients Generation Function
The coding coefficients used during the encoding process are
generated at the RLC encoder by the generate_coding_coefficients()
function each time a new repair symbol needs to be produced. The
fraction of coefficients that are nonzero (i.e., the density) is
controlled by the DT (Density Threshold) parameter. DT has values
between 0 (the minimum value) and 15 (the maximum value), and the
average probability of having a nonzero coefficient equals (DT + 1) /
16. In particular, when DT equals 15 the function guaranties that
all coefficients are nonzero (i.e., maximum density).
These considerations apply to both the RLC over GF(2) and RLC over
GF(2^(8)), the only difference being the value of the m parameter.
With the RLC over GF(2) FEC scheme (Section 5), m is equal to 1.
With RLC over GF(2^(8)) FEC scheme (Section 4), m is equal to 8.
Figure 3 shows the reference generate_coding_coefficients()
implementation. This is a C language implementation, written for C99
[C99].
<CODE BEGINS>
#include <string.h>
/*
* Fills in the table of coding coefficients (of the right size)
* provided with the appropriate number of coding coefficients to
* use for the repair symbol key provided.
*
* (in) repair_key key associated to this repair symbol. This
* parameter is ignored (useless) if m=1 and dt=15
* (in/out) cc_tab pointer to a table of the right size to store
* coding coefficients. All coefficients are
* stored as bytes, regardless of the m parameter,
* upon return of this function.
* (in) cc_nb number of entries in the cc_tab table. This
* value is equal to the current encoding window
* size.
* (in) dt integer between 0 and 15 (inclusive) that
* controls the density. With value 15, all
* coefficients are guaranteed to be nonzero
* (i.e., equal to 1 with GF(2) and equal to a
* value in {1,... 255} with GF(2^^8)), otherwise
* a fraction of them will be 0.
* (in) m Finite Field GF(2^^m) parameter. In this
* document only values 1 and 8 are considered.
* (out) returns 0 in case of success, an error code
* different than 0 otherwise.
*/
int generate_coding_coefficients (uint16_t repair_key,
uint8_t* cc_tab,
uint16_t cc_nb,
uint8_t dt,
uint8_t m)
{
uint32_t i;
tinymt32_t s; /* PRNG internal state */
if (dt > 15) {
return -1; /* error, bad dt parameter */
}
switch (m) {
case 1:
if (dt == 15) {
/* all coefficients are 1 */
memset(cc_tab, 1, cc_nb);
} else {
/* here coefficients are either 0 or 1 */
tinymt32_init(&s, repair_key);
for (i = 0 ; i < cc_nb ; i++) {
cc_tab[i] = (tinymt32_rand16(&s) <= dt) ? 1 : 0;
}
}
break;
case 8:
tinymt32_init(&s, repair_key);
if (dt == 15) {
/* coefficient 0 is avoided here in order to include
* all the source symbols */
for (i = 0 ; i < cc_nb ; i++) {
do {
cc_tab[i] = (uint8_t) tinymt32_rand256(&s);
} while (cc_tab[i] == 0);
}
} else {
/* here a certain number of coefficients should be 0 */
for (i = 0 ; i < cc_nb ; i++) {
if (tinymt32_rand16(&s) <= dt) {
do {
cc_tab[i] = (uint8_t) tinymt32_rand256(&s);
} while (cc_tab[i] == 0);
} else {
cc_tab[i] = 0;
}
}
}
break;
default:
return -2; /* error, bad parameter m */
}
return 0; /* success */
}
<CODE ENDS>
Figure 3: Reference Implementation of the Coding Coefficients
Generation Function
3.7. Finite Field Operations
3.7.1. Finite Field Definitions
The two RLC FEC schemes specified in this document reuse the Finite
Fields defined in [RFC5510], Section 8.1. More specifically, the
elements of the field GF(2^(m)) are represented by polynomials with
binary coefficients (i.e., over GF(2)) and degree lower or equal to
m-1. The addition between two elements is defined as the addition of
binary polynomials in GF(2), which is equivalent to a bitwise XOR
operation on the binary representation of these elements.
With GF(2^(8)), multiplication between two elements is the
multiplication modulo a given irreducible polynomial of degree 8.
The following irreducible polynomial is used for GF(2^(8)):
x^(8) + x^(4) + x^(3) + x^(2) + 1
With GF(2), multiplication corresponds to a logical AND operation.
3.7.2. Linear Combination of Source Symbol 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] XOR cc_tab[1] * src_1[i] XOR ...
XOR cc_tab[ew_size - 1] * src_ew_size_1[i]
where * is the multiplication over GF(2^(8)). 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 Packet
Flows
This fully-specified FEC scheme defines the Sliding Window Random
Linear Codes (RLC) over GF(2^(8)).
4.1. Formats and Codes
4.1.1. FEC Framework Configuration Information
Following the guidelines of Section 5.6 of [RFC6363], this section
provides the FEC Framework Configuration Information (or FFCI). This
FCCI needs to be shared (e.g., using SDP) between the FECFRAME sender
and receiver instances in order to synchronize them. It includes a
FEC Encoding ID, mandatory for any FEC scheme specification, plus
scheme-specific elements.
4.1.1.1. FEC Encoding ID
FEC Encoding ID: the value assigned to this fully specified FEC
scheme MUST be 10, as assigned by IANA (Section 9).
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
The FEC Scheme-Specific Information (FSSI) includes elements that are
specific to the present FEC scheme. More precisely:
Encoding symbol size (E): a non-negative integer that indicates the
size of each encoding symbol in bytes;
Window Size Ratio (WSR) parameter: a non-negative integer between 0
and 255 (both inclusive) used to initialize window sizes. A value
of 0 indicates this parameter is not considered (e.g., a fixed
encoding window size may be chosen). A value between 1 and 255
inclusive is required by certain of the parameter derivation
techniques described in Appendix C;
This element is required both by the sender (RLC encoder) and the
receiver(s) (RLC decoder).
When SDP is used to communicate the FFCI, this FEC Scheme-Specific
Information is carried in the 'fssi' parameter in textual
representation as specified in [RFC6364]. For instance:
fssi=E:1400,WSR:191
In that case the name values "E" and "WSR" are used to convey the E
and WSR parameters respectively.
If another mechanism requires the FSSI to be carried as an opaque
octet string, the encoding format consists of the following three
octets, where the E field is carried in "big-endian" or "network
order" format, that is, most significant byte first:
Encoding symbol length (E): 16-bit field;
Window Size Ratio Parameter (WSR): 8-bit field.
These three octets can be communicated as such, or for instance, be
subject to an additional Base64 encoding.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol Length (E) | WSR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: FSSI Encoding Format
4.1.2. 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 5.
+--------------------------------+
| IP Header |
+--------------------------------+
| Transport Header |
+--------------------------------+
| ADU |
+--------------------------------+
| Explicit Source FEC Payload ID |
+--------------------------------+
Figure 5: Structure of an FEC Source Packet with the Explicit
Source FEC Payload ID
More precisely, the Explicit Source FEC Payload ID is composed of the
following field, carried in "big-endian" or "network order" format,
that is, most significant byte first (Figure 6):
Encoding Symbol ID (ESI) (32-bit field): this unsigned integer
identifies the first source symbol of the ADUI corresponding to
this FEC Source Packet. The ESI is incremented for each new
source symbol, and after reaching the maximum value (2^(32)-1),
wrapping to zero occurs.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol ID (ESI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Source FEC Payload ID Encoding Format
4.1.3. Repair FEC Payload ID
A FEC Repair Packet MAY contain one or more repair symbols. When
there are several repair symbols, all of them MUST have been
generated from the same encoding window, using Repair_Key values that
are managed as explained below. A receiver can easily deduce 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
UDP is the underlying transport protocol) and the symbol size, E,
communicated in the FFCI.
A FEC Repair Packet MUST contain a Repair FEC Payload ID that is
prepended to the repair symbol as illustrated in Figure 7.
+--------------------------------+
| IP Header |
+--------------------------------+
| Transport Header |
+--------------------------------+
| Repair FEC Payload ID |
+--------------------------------+
| Repair Symbol |
+--------------------------------+
Figure 7: Structure of an FEC Repair Packet with the Repair FEC
Payload ID
More precisely, the Repair FEC Payload ID is composed of the
following fields where all integer fields are carried in "big-endian"
or "network order" format, that is, most significant byte first
(Figure 8):
Repair_Key (16-bit field): this unsigned integer is used as a seed
by the coefficient generation function (Section 3.6) in order to
generate the desired number of coding coefficients. This repair
key may be a monotonically increasing integer value that loops
back to 0 after reaching 65535 (see Section 6.1). When a FEC
Repair Packet contains several repair symbols, this repair key
value is that of the first repair symbol. 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 0.
Density Threshold for the coding coefficients, DT (4-bit field):
this unsigned integer carries the Density Threshold (DT) used by
the coding coefficient generation function Section 3.6. More
precisely, it controls the probability of having a nonzero coding
coefficient, which equals (DT+1) / 16. When a FEC Repair Packet
contains several repair symbols, the DT value applies to all of
them;
Number of Source Symbols in the encoding window, NSS (12-bit
field):
this unsigned integer indicates the number of source symbols in
the encoding window when this repair symbol was generated. When a
FEC Repair Packet contains several repair symbols, this NSS value
applies to all of them;
ESI of First Source Symbol in the encoding window, FSS_ESI (32-bit
field):
this unsigned integer indicates the ESI of the first source symbol
in the encoding window when this repair symbol was generated.
When a FEC Repair Packet contains several repair symbols, this
FSS_ESI value applies to all of them;
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Repair_Key | DT |NSS (# src symb in ew) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FSS_ESI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Repair FEC Payload ID Encoding Format
4.2. Procedures
All the procedures of Section 3 apply to this FEC scheme.
5. Sliding Window RLC FEC Scheme over GF(2) for Arbitrary Packet 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. FEC Encoding ID
FEC Encoding ID: the value assigned to this fully specified FEC
scheme MUST be 9, as assigned by IANA (Section 9).
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 4.1.2 apply here.
5.1.3. Repair FEC Payload ID
All the considerations of Section 4.1.3 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
the FECFRAME sender MUST set the Repair_Key field to zero on
transmission and a receiver MUST ignore it on receipt.
5.2. Procedures
All the procedures of Section 3 apply to this FEC scheme.
6. FEC Code Specification
6.1. Encoding Side
This section provides a high level description of a Sliding Window
RLC encoder.
Whenever a new FEC Repair Packet is needed, the RLC encoder instance
first gathers the ew_size source symbols currently in the sliding
encoding window. Then it chooses a repair key, which can be a
monotonically increasing integer value, incremented for each repair
symbol up to a maximum value of 65535 (as it is carried within a
16-bit field) after which it loops back to 0. This repair key is
communicated to the coefficient generation function (Section 3.6) in
order to generate ew_size coding coefficients. Finally, the FECFRAME
sender computes the repair symbol as a linear combination of the
ew_size source symbols using the ew_size coding coefficients
(Section 3.7). When E is small and when there is an incentive to
pack several repair symbols within the same FEC Repair Packet, the
appropriate number of repair symbols are computed. In that case the
repair key for each of them MUST be incremented by 1, keeping the
same ew_size source symbols, since only the first repair key will be
carried in the Repair FEC Payload ID. The FEC Repair Packet can then
be passed to the transport layer for transmission. The source versus
repair FEC packet transmission order is out of scope of this document
and several approaches exist that are implementation-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 0 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. In any case, choosing the repair
key is entirely at the discretion of the sender, since it is
communicated to the receiver(s) in each Repair FEC Payload ID. A
receiver should not make any assumption on the way the repair key is
managed.
6.2. Decoding Side
This section provides a high level description of a Sliding Window
RLC decoder.
A FECFRAME receiver needs to maintain a linear system whose variables
are the received and lost source symbols. Upon receiving a FEC
Repair Packet, a receiver first extracts all the repair symbols it
contains (in case several repair symbols are packed together). For
each repair symbol, when at least one of the corresponding source
symbols it protects has been lost, the receiver adds an equation to
the linear system (or no equation if this repair packet does not
change the linear system rank). This equation of course re-uses the
ew_size coding coefficients that are computed by the same coefficient
generation function (Section 3.6), using the repair key and encoding
window descriptions carried in the Repair FEC Payload ID. 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 recover
lost source symbols. Gaussian elimination is one possible algorithm
to solve this linear system. Each time an ADUI can be totally
recovered, padding is removed (thanks to the Length field, L, of the
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
passed to the corresponding upper application. Received FEC Source
Packets, containing an ADU, MAY be passed to the application either
immediately or after some time to guaranty an ordered delivery to the
application. This document does not mandate any approach as this is
an operational and management decision.
With real-time flows, a lost ADU that is decoded after the maximum
latency or an ADU received after this delay has no value to the
application. This raises the question of deciding whether or not an
ADU is late. This decision MAY be taken within the FECFRAME receiver
(e.g., using the decoding window, see Section 3.1) or within the
application (e.g., using RTP timestamps within the ADU). Deciding
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 D discusses a backward compatible
optimization whereby late source symbols MAY still be used within the
FECFRAME receiver in order to improve transmission robustness.
7. Security Considerations
The FEC Framework document [RFC6363] provides a fairly comprehensive
analysis of security considerations applicable to FEC schemes.
Therefore, the present section follows the security considerations
section of [RFC6363] and only discusses specific topics.
7.1. Attacks Against the Data Flow
7.1.1. Access to Confidential Content
The Sliding Window RLC FEC scheme specified in this document does not
change the recommendations of [RFC6363]. To summarize, if
confidentiality is a concern, it is RECOMMENDED that one of the
solutions mentioned in [RFC6363] is used with special considerations
to the way this solution is applied (e.g., is encryption applied
before or after FEC protection, within the end system or in a
middlebox), to the operational constraints (e.g., performing FEC
decoding in a protected environment may be complicated or even
impossible) and to the threat model.
7.1.2. Content Corruption
The Sliding Window RLC FEC scheme specified in this document does not
change the recommendations of [RFC6363]. To summarize, it is
RECOMMENDED that one of the solutions mentioned in [RFC6363] is used
on both the FEC Source and Repair Packets.
7.2. Attacks Against the FEC Parameters
The FEC scheme specified in this document defines parameters that can
be the basis of attacks. More specifically, the following parameters
of the FFCI may be modified by an attacker who targets receivers
(Section 4.1.1.2):
FEC Encoding ID: changing this parameter leads a receiver to
consider a different FEC scheme. The consequences are severe, the
format of the Explicit Source FEC Payload ID and Repair FEC
Payload ID of received packets will probably differ, leading to
various malfunctions. Even if the original and modified FEC
schemes share the same format, FEC decoding will either fail or
lead to corrupted decoded symbols. This will happen if an
attacker turns value 9 (i.e., RLC over GF(2)) to value 10 (RLC
over GF(2^(8))), an additional consequence being a higher
processing overhead at the receiver. In any case, the attack
results in a form of Denial of Service (DoS) or corrupted content.
Encoding symbol length (E): setting this E parameter to a different
value will confuse a receiver. If the size of a received FEC
Repair Packet is no longer multiple of the modified E value, a
receiver quickly detects a problem and SHOULD reject the packet.
If the new E value is a sub-multiple of the original E value
(e.g., half the original value), then receivers may not detect the
problem immediately. For instance, a receiver may think that a
received FEC Repair Packet contains more repair symbols (e.g.,
twice as many if E is reduced by half), leading to malfunctions
whose nature depends on implementation details. Here also, the
attack always results in a form of DoS or corrupted content.
It is therefore RECOMMENDED that security measures be taken to
guarantee the FFCI integrity, as specified in [RFC6363]. How to
achieve this depends on the way the FFCI is communicated from the
sender to the receiver, which is not specified in this document.
Similarly, attacks are possible against the Explicit Source FEC
Payload ID and Repair FEC Payload ID. More specifically, in case of
a FEC Source Packet, the following value can be modified by an
attacker who targets receivers:
Encoding Symbol ID (ESI): changing the ESI leads a receiver to
consider a wrong ADU, resulting in severe consequences, including
corrupted content passed to the receiving application;
And in case of a FEC Repair Packet:
Repair Key: changing this value leads a receiver to generate a wrong
coding coefficient sequence, and therefore any source symbol
decoded using the repair symbols contained in this packet will be
corrupted;
DT: changing this value also leads a receiver to generate a wrong
coding coefficient sequence, and therefore any source symbol
decoded using the repair symbols contained in this packet will be
corrupted. In addition, if the DT value is significantly
increased, it will generate a higher processing overhead at a
receiver. In case of very large encoding windows, this may impact
the terminal performance;
NSS: changing this value leads a receiver to consider a different
set of source symbols, and therefore any source symbol decoded
using the repair symbols contained in this packet will be
corrupted. In addition, if the NSS value is significantly
increased, it will generate a higher processing overhead at a
receiver, which may impact the terminal performance;
FSS_ESI: changing this value also leads a receiver to consider a
different set of source symbols and therefore any source symbol
decoded using the repair symbols contained in this packet will be
corrupted.
It is therefore RECOMMENDED that security measures are taken to
guarantee the FEC Source and Repair Packets as stated in [RFC6363].
7.3. When Several Source Flows are to be Protected Together
The Sliding Window RLC FEC scheme specified in this document does not
change the recommendations of [RFC6363].
7.4. Baseline Secure FEC Framework Operation
The Sliding Window RLC FEC scheme specified in this document does not
change the recommendations of [RFC6363] concerning the use of the
IPsec/Encapsulating Security Payload (ESP) security protocol as a
mandatory-to-implement (but not mandatory-to-use) security scheme.
This is well suited to situations where the only insecure domain is
the one over which the FEC Framework operates.
7.5. Additional Security Considerations for Numerical Computations
In addition to the above security considerations, inherited from
[RFC6363], the present document introduces several formulae, in
particular in Appendix C.1. It is RECOMMENDED to check that the
computed values stay within reasonable bounds since numerical
overflows, caused by an erroneous implementation or an erroneous
input value, may lead to hazardous behaviors. However, what
"reasonable bounds" means is use-case and implementation dependent
and is not detailed in this document.
Appendix C.2 also mentions the possibility of "using the timestamp
field of an RTP packet header" when applicable. A malicious attacker
may deliberately corrupt this header field in order to trigger
hazardous behaviors at a FECFRAME receiver. Protection against this
type of content corruption can be addressed with the above
recommendations on a baseline secure operation. In addition, it is
also RECOMMENDED to check that the timestamp value be within
reasonable bounds.
8. Operations and Management Considerations
The FEC Framework document [RFC6363] provides a fairly comprehensive
analysis of operations and management considerations applicable to
FEC schemes. Therefore, the present section only discusses specific
topics.
8.1. Operational Recommendations: Finite Field GF(2) Versus GF(2^(8))
The present document specifies two FEC schemes that differ on the
Finite Field used for the coding coefficients. It is expected that
the RLC over GF(2^(8)) FEC scheme will be mostly used since it
warrants a higher packet loss protection. In case of small encoding
windows, the associated processing overhead is not an issue (e.g., we
measured decoding speeds between 745 Mbps and 2.8 Gbps on an ARM
Cortex-A15 embedded board in [Roca17] depending on the code rate and
the channel conditions, using an encoding window of size 18 or 23
symbols; see the above article for the details). 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.
The RLC over GF(2) FEC scheme offers an alternative. In that case
operations symbols can be directly XOR-ed together which warrants
high bitrate encoding and decoding operations, and can be an
advantage with large encoding windows. However, packet loss
protection is significantly reduced by using this FEC scheme.
8.2. Operational Recommendations: Coding Coefficients Density Threshold
In addition to the choice of the Finite Field, the two FEC schemes
define a coding coefficient density threshold (DT) parameter. This
parameter enables a sender to control the code density, i.e., the
proportion of coefficients that are nonzero on average. With RLC
over GF(2^(8)), it is usually appropriate that small encoding windows
be associated to a density threshold equal to 15, the maximum value,
in order to warrant a high loss protection.
On the opposite, with larger encoding windows, it is usually
appropriate that the density threshold be reduced. With large
encoding windows, an alternative can be to use RLC over GF(2) and a
density threshold equal to 7 (i.e., an average density equal to 1/2)
or smaller.
Note that using a density threshold equal to 15 with RLC over GF(2)
is equivalent to using an XOR code that computes the XOR sum of all
the source symbols in the encoding window. In that case: (1) only a
single repair symbol can be produced for any encoding window, and (2)
the repair_key parameter becomes useless (the coding coefficients
generation function does not rely on the PRNG).
9. IANA Considerations
This document registers two values in the "FEC Framework (FECFRAME)
FEC Encoding IDs" registry [RFC6363] as follows:
* 9 refers to the Sliding Window Random Linear Codes (RLC) over
GF(2) FEC Scheme for Arbitrary Packet Flows, as defined in
Section 5 of this document.
* 10 refers to the Sliding Window Random Linear Codes (RLC) over
GF(2^(8)) FEC Scheme for Arbitrary Packet Flows, as defined in
Section 4 of this document.
10. References
10.1. Normative References
[C99] International Organization for Standardization,
"Programming languages - C: C99, correction 3:2007", ISO/
IEC 9899:1999/Cor 3:2007, November 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011,
<https://www.rfc-editor.org/info/rfc6363>.
[RFC6364] Begen, A., "Session Description Protocol Elements for the
Forward Error Correction (FEC) Framework", RFC 6364,
DOI 10.17487/RFC6364, October 2011,
<https://www.rfc-editor.org/info/rfc6364>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8680] Roca, V. and A. Begen, "Forward Error Correction (FEC)
Framework Extension to Sliding Window Codes", RFC 8680,
DOI 10.17487/RFC8680, January 2020,
<https://www.rfc-editor.org/info/rfc8680>.
[RFC8682] Saito, M., Matsumoto, M., Roca, V., Ed., and E. Baccelli,
"TinyMT32 Pseudorandom Number Generator (PRNG)", RFC 8682,
DOI 10.17487/RFC8682, January 2020,
<https://www.rfc-editor.org/info/rfc8682>.
10.2. Informative References
[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, October 2013,
<http://web.eecs.utk.edu/~plank/plank/papers/UT-CS-
13-717.html>.
[RFC5170] Roca, V., Neumann, C., and D. Furodet, "Low Density Parity
Check (LDPC) Staircase and Triangle Forward Error
Correction (FEC) Schemes", RFC 5170, DOI 10.17487/RFC5170,
June 2008, <https://www.rfc-editor.org/info/rfc5170>.
[RFC5510] Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo,
"Reed-Solomon Forward Error Correction (FEC) Schemes",
RFC 5510, DOI 10.17487/RFC5510, April 2009,
<https://www.rfc-editor.org/info/rfc5510>.
[RFC6681] Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward
Error Correction (FEC) Schemes for FECFRAME", RFC 6681,
DOI 10.17487/RFC6681, August 2012,
<https://www.rfc-editor.org/info/rfc6681>.
[RFC6726] Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen,
"FLUTE - File Delivery over Unidirectional Transport",
RFC 6726, DOI 10.17487/RFC6726, November 2012,
<https://www.rfc-editor.org/info/rfc6726>.
[RFC6816] Roca, V., Cunche, M., and J. Lacan, "Simple Low-Density
Parity Check (LDPC) Staircase Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6816,
DOI 10.17487/RFC6816, December 2012,
<https://www.rfc-editor.org/info/rfc6816>.
[RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K.
Matsuzono, "Simple Reed-Solomon Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6865,
DOI 10.17487/RFC6865, February 2013,
<https://www.rfc-editor.org/info/rfc6865>.
[RFC8406] Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek,
F., Ghanem, S., Lochin, E., Masucci, A., Montpetit, M-J.,
Pedersen, M., Peralta, G., Roca, V., Ed., Saxena, P., and
S. Sivakumar, "Taxonomy of Coding Techniques for Efficient
Network Communications", RFC 8406, DOI 10.17487/RFC8406,
June 2018, <https://www.rfc-editor.org/info/rfc8406>.
[Roca16] Roca, V., Teibi, B., Burdinat, C., Tran-Thai, T., and C.
Thienot, "Block or Convolutional AL-FEC Codes? A
Performance Comparison for Robust Low-Latency
Communications", HAL ID hal-01395937v2, February 2017,
<https://hal.inria.fr/hal-01395937/en/>.
[Roca17] Roca, V., Teibi, B., Burdinat, C., Tran, T., and C.
Thienot, "Less Latency and Better Protection with AL-FEC
Sliding Window Codes: a Robust Multimedia CBR Broadcast
Case Study", 13th IEEE International Conference on
Wireless and Mobile Computing, Networking and
Communications (WiMob17), HAL ID hal-01571609, October
2017, <https://hal.inria.fr/hal-01571609v1/en/>.
Appendix A. TinyMT32 Validation Criteria (Normative)
PRNG determinism, for a given seed, is a requirement. Consequently,
in order to validate an implementation of the TinyMT32 PRNG, the
following criteria MUST be met.
The first criterion focuses on the tinymt32_rand256(), where the
32-bit integer of the core TinyMT32 PRNG is scaled down to an 8-bit
integer. Using a seed value of 1, the first 50 values returned by:
tinymt32_rand256() as 8-bit unsigned integers MUST be equal to values
provided in Figure 9, to be read line by line.
37 225 177 176 21
246 54 139 168 237
211 187 62 190 104
135 210 99 176 11
207 35 40 113 179
214 254 101 212 211
226 41 234 232 203
29 194 211 112 107
217 104 197 135 23
89 210 252 109 166
Figure 9: First 50 decimal values (to be read per line) returned by
tinymt32_rand256() as 8-bit unsigned integers, with a seed value of 1
The second criterion focuses on the tinymt32_rand16(), where the
32-bit integer of the core TinyMT32 PRNG is scaled down to a 4-bit
integer. Using a seed value of 1, the first 50 values returned by:
tinymt32_rand16() as 4-bit unsigned integers MUST be equal to values
provided in Figure 10, to be read line by line.
5 1 1 0 5
6 6 11 8 13
3 11 14 14 8
7 2 3 0 11
15 3 8 1 3
6 14 5 4 3
2 9 10 8 11
13 2 3 0 11
9 8 5 7 7
9 2 12 13 6
Figure 10: First 50 decimal values (to be read per line) returned by
tinymt32_rand16() as 4-bit unsigned integers, with a seed value of 1
Appendix B. Assessing the PRNG Adequacy (Informational)
This annex discusses the adequacy of the TinyMT32 PRNG and the
tinymt32_rand16() and tinymt32_rand256() functions, to the RLC FEC
schemes. The goal is to assess the adequacy of these two functions
in producing coding coefficients that are sufficiently different from
one another, across various repair symbols with repair key values in
sequence (we can expect this approach to be commonly used by
implementers, see Section 6.1). This section is purely informational
and does not claim to be a solid evaluation.
The two RLC FEC schemes use the PRNG to produce pseudorandom coding
coefficients (Section 3.6), each time a new repair symbol is needed.
A different repair key is used for each repair symbol, usually by
incrementing the repair key value (Section 6.1). For each repair
symbol, a limited number of pseudorandom numbers is needed, depending
on the DT and encoding window size (Section 3.6), using either
tinymt32_rand16() or tinymt32_rand256(). Therefore, we are more
interested in the randomness of small sequences of random numbers
mapped to 4-bit or 8-bit integers, than in the randomness of a very
large sequence of random numbers which is not representative of the
usage of the PRNG.
Evaluation of tinymt32_rand16(): We first generate a huge number
(1,000,000,000) of small sequences (20 pseudorandom numbers per
sequence), increasing the seed value for each sequence, and perform
statistics on the number of occurrences of each of the 16 possible
values across all sequences. In this first test we consider 32-bit
seed values in order to assess the PRNG quality after output
truncation to 4 bits.
+-------+-------------+----------------+
| Value | Occurrences | Percentage (%) |
+=======+=============+================+
| 0 | 1250036799 | 6.2502 |
+-------+-------------+----------------+
| 1 | 1249995831 | 6.2500 |
+-------+-------------+----------------+
| 2 | 1250038674 | 6.2502 |
+-------+-------------+----------------+
| 3 | 1250000881 | 6.2500 |
+-------+-------------+----------------+
| 4 | 1250023929 | 6.2501 |
+-------+-------------+----------------+
| 5 | 1249986320 | 6.2499 |
+-------+-------------+----------------+
| 6 | 1249995587 | 6.2500 |
+-------+-------------+----------------+
| 7 | 1250020363 | 6.2501 |
+-------+-------------+----------------+
| 8 | 1249995276 | 6.2500 |
+-------+-------------+----------------+
| 9 | 1249982856 | 6.2499 |
+-------+-------------+----------------+
| 10 | 1249984111 | 6.2499 |
+-------+-------------+----------------+
| 11 | 1250009551 | 6.2500 |
+-------+-------------+----------------+
| 12 | 1249955768 | 6.2498 |
+-------+-------------+----------------+
| 13 | 1249994654 | 6.2500 |
+-------+-------------+----------------+
| 14 | 1250000569 | 6.2500 |
+-------+-------------+----------------+
| 15 | 1249978831 | 6.2499 |
+-------+-------------+----------------+
Table 1: tinymt32_rand16()
Occurrence Statistics
Evaluation of tinymt32_rand16(): We first generate a huge number
(1,000,000,000) of small sequences (20 pseudorandom numbers per
sequence), increasing the seed value for each sequence, and perform
statistics on the number of occurrences of each of the 16 possible
values across the 20,000,000,000 numbers of all sequences. In this
first test, we consider 32-bit seed values in order to assess the
PRNG quality after output truncation to 4 bits.
The results (Table 1) show that all possible values are almost
equally represented, or said differently, that the tinymt32_rand16()
output converges to a uniform distribution where each of the 16
possible values would appear exactly 1 / 16 * 100 = 6.25% of times.
Since the RLC FEC schemes use of this PRNG will be limited to 16-bit
seed values, we carried out the same test for the first 2^(16) seed
values only. The distribution (not shown) is of course less uniform,
with value occurrences ranging between 6.2121% (i.e., 81,423
occurrences out of a total of 65536*20=1,310,720) and 6.2948% (i.e.,
82,507 occurrences). However, we do not believe it significantly
impacts the RLC FEC scheme behavior.
Other types of biases may exist that may be visible with smaller
tests, for instance to evaluate the convergence speed to a uniform
distribution. We therefore perform 200 tests, each of them producing
200 sequences, keeping only the first value of each sequence. We use
non-overlapping repair keys for each sequence, starting with value 0
and increasing it after each use.
+-------+-----------------+-----------------+---------------------+
| Value | Min Occurrences | Max Occurrences | Average Occurrences |
+=======+=================+=================+=====================+
| 0 | 4 | 21 | 6.3675 |
+-------+-----------------+-----------------+---------------------+
| 1 | 4 | 22 | 6.0200 |
+-------+-----------------+-----------------+---------------------+
| 2 | 4 | 20 | 6.3125 |
+-------+-----------------+-----------------+---------------------+
| 3 | 5 | 23 | 6.1775 |
+-------+-----------------+-----------------+---------------------+
| 4 | 5 | 24 | 6.1000 |
+-------+-----------------+-----------------+---------------------+
| 5 | 4 | 21 | 6.5925 |
+-------+-----------------+-----------------+---------------------+
| 6 | 5 | 30 | 6.3075 |
+-------+-----------------+-----------------+---------------------+
| 7 | 6 | 22 | 6.2225 |
+-------+-----------------+-----------------+---------------------+
| 8 | 5 | 26 | 6.1750 |
+-------+-----------------+-----------------+---------------------+
| 9 | 3 | 21 | 5.9425 |
+-------+-----------------+-----------------+---------------------+
| 10 | 5 | 24 | 6.3175 |
+-------+-----------------+-----------------+---------------------+
| 11 | 4 | 22 | 6.4300 |
+-------+-----------------+-----------------+---------------------+
| 12 | 5 | 21 | 6.1600 |
+-------+-----------------+-----------------+---------------------+
| 13 | 5 | 22 | 6.3100 |
+-------+-----------------+-----------------+---------------------+
| 14 | 4 | 26 | 6.3950 |
+-------+-----------------+-----------------+---------------------+
| 15 | 4 | 21 | 6.1700 |
+-------+-----------------+-----------------+---------------------+
Table 2: tinymt32_rand16() Occurrence Statistics
Table 2 shows across all 200 tests, for each of the 16 possible
pseudorandom number values, the minimum (resp. maximum) number of
times it appeared in a test, as well as the average number of
occurrences across the 200 tests. Although the distribution is not
perfect, there is no major bias. On the contrary, in the same
conditions, the Park-Miller linear congruential PRNG of [RFC5170]
with a result scaled down to 4-bit values, using seeds in sequence
starting from 1, systematically returns 0 as the first value during
some time. Then, after a certain repair key value threshold, it
systematically returns 1, etc.
Evaluation of tinymt32_rand256(): The same approach is used here.
Results (not shown) are similar: occurrences vary between 7,810,3368
(i.e., 0.3905%) and 7,814,7952 (i.e., 0.3907%). Here also we see a
convergence to the theoretical uniform distribution where each of the
256 possible values would appear exactly 1 / 256 * 100 = 0.390625% of
times.
Appendix C. Possible Parameter Derivation (Informational)
Section 3.1 defines several parameters to control the encoder or
decoder. This annex proposes techniques to derive these parameters
according to the target use-case. This annex is informational, in
the sense that using a different derivation technique will not
prevent the encoder and decoder to interoperate: a decoder can still
recover an erased source symbol without any error. However, in case
of a real-time flow, an inappropriate parameter derivation may lead
to the decoding of erased source packets after their validity period,
making them useless to the target application. This annex proposes
an approach to reduce this risk, among other things.
The FEC schemes defined in this document can be used in various
manners, depending on the target use-case:
* the source ADU flow they protect may or may not have real-time
constraints;
* the source ADU flow may be a Constant Bitrate (CBR) or Variable
Bitrate (VBR) flow;
* with a VBR source ADU flow, the flow's minimum and maximum
bitrates may or may not be known;
* and the communication path between encoder and decoder may be a
CBR communication path (e.g., as with certain LTE-based broadcast
channels) or not (general case, e.g., with Internet).
The parameter derivation technique should be suited to the use-case,
as described in the following sections.
C.1. Case of a CBR Real-Time Flow
In the following, we consider a real-time flow with max_lat latency
budget. The encoding symbol size, E, is constant. The code rate,
cr, is also constant, its value depending on the expected
communication loss model (this choice is out of scope of this
document).
In a first configuration, the source ADU flow bitrate at the input of
the FECFRAME sender is fixed and equal to br_in (in bits/s), and this
value is known by the FECFRAME sender. It follows 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)
assuming that the encoding and decoding times are negligible with
respect to the target max_lat. This is a reasonable assumption in
many situations (e.g., see Section 8.1 in case of small window
sizes). Otherwise the max_lat parameter should be adjusted in order
to avoid the problem. In any case, interoperability will never be
compromised by choosing a too large value.
In a second configuration, the FECFRAME sender generates a fixed
bitrate flow, equal to the CBR communication path bitrate equal to
br_out (in bits/s), and this value is known by the FECFRAME sender,
as in [Roca17]. The maximum source flow bitrate needs to be such
that, with the added repair flow overhead, the total transmission
bitrate remains inferior or equal to br_out. We have:
dw_max_size = (max_lat * br_out * cr) / (8 * E)
assuming here also that the encoding and decoding times are
negligible with respect to the target max_lat.
For decoding to be possible within the latency budget, it is required
that the encoding window maximum size be smaller than or at most
equal to the decoding window maximum size. The ew_max_size is the
main parameter at a FECFRAME sender, but its exact value has no
impact on the FEC-related latency budget. The ew_max_size parameter
is computed as follows:
ew_max_size = dw_max_size * WSR / 255
In line with [Roca17], WSR = 191 is considered as a reasonable value
(the resulting encoding to decoding window size ratio is then close
to 0.75), but other values between 1 and 255 inclusive are possible,
depending on the use-case.
The dw_max_size is computed by a FECFRAME sender but not explicitly
communicated to a FECFRAME receiver. However, a FECFRAME receiver
can 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 (Section 4.1.3). A receiver can then
easily compute dw_max_size:
dw_max_size = max_NSS_observed * 255 / WSR
A receiver can then choose an appropriate linear system maximum size:
ls_max_size >= dw_max_size
It is good practice to use a larger value for ls_max_size as
explained in Appendix D, which does not impact maximum latency nor
interoperability.
In any case, for a given use-case (i.e., for target encoding and
decoding devices and desired protection levels in front of
communication impairments) and for the computed ew_max_size,
dw_max_size and ls_max_size values, it is RECOMMENDED to check that
the maximum encoding time and maximum memory requirements at a
FECFRAME sender, and maximum decoding time and maximum memory
requirements at a FECFRAME receiver, stay within reasonable bounds.
When assuming that the encoding and decoding times are negligible
with respect to the target max_lat, this should be verified as well,
otherwise the max_lat SHOULD be adjusted accordingly.
The particular case of session start needs to be managed
appropriately since the ew_size, starting at zero, increases each
time a new source ADU is received by the FECFRAME sender, until it
reaches the ew_max_size value. Therefore, 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 its ls_max_size accordingly if need be. With a CBR flow,
session start is expected to be the only moment when the encoding
window size will increase. Similarly, with a CBR real-time flow, the
session end is expected to be the only moment when the encoding
window size will progressively decrease. No adjustment of the
ls_max_size is required at the FECFRAME receiver in that case.
C.2. Other Types of Real-Time Flow
In the following, we consider a real-time source ADU flow with a
max_lat latency budget and a variable bitrate (VBR) measured at the
entry of the FECFRAME sender. A first approach consists in
considering the smallest instantaneous bitrate of the source ADU
flow, when this parameter is known, and to reuse the derivation of
Appendix C.1. Considering the smallest bitrate means that the
encoding and decoding window maximum size estimations are
pessimistic: these windows have the smallest size required to enable
on-time decoding at a FECFRAME receiver. If the instantaneous
bitrate is higher than this smallest bitrate, this approach leads to
an encoding window that is unnecessarily small, which reduces
robustness in front of long erasure bursts.
Another approach consists in using ADU timing information (e.g.,
using the timestamp field of an RTP packet header, or registering the
time upon receiving a new ADU). From the global FEC-related latency
budget, the FECFRAME sender can derive a practical maximum latency
budget for encoding operations, max_lat_for_encoding. For the FEC
schemes specified in this document, this latency budget SHOULD be
computed with:
max_lat_for_encoding = max_lat * WSR / 255
It follows that any source symbols associated to an ADU that has
timed-out with respect to max_lat_for_encoding SHOULD be removed from
the encoding window. With this approach there is no pre-determined
ew_size value: this value fluctuates over the time according to the
instantaneous source ADU flow bitrate. For practical reasons, a
FECFRAME sender may still require that ew_size does not increase
beyond a maximum value (Appendix C.3).
With both approaches, and 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 ls_max_size as explained in Appendix C.1.
When the observed NSS fluctuates significantly, a FECFRAME receiver
may want to adapt its ls_max_size accordingly. In particular when
the NSS is significantly reduced, a FECFRAME receiver may want to
reduce the ls_max_size too in order to limit computation complexity.
A balance must be found between using an ls_max_size "too large"
(which increases computation complexity and memory requirements) and
the opposite (which reduces recovery performance).
C.3. Case of a Non-Real-Time Flow
Finally there are configurations where a source ADU flow has no 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. For instance, it
can derive from an estimation of the maximum memory amount that could
be dedicated to the linear system at a FECFRAME receiver, or the
maximum computation complexity at a FECFRAME receiver, both of them
depending on the ls_max_size parameter. The same considerations also
apply to the FECFRAME sender, where the maximum memory amount and
computation complexity depend on the ew_max_size parameter.
Here also, the NSS value contained in FEC Repair Packets is used by a
FECFRAME receiver to determine the current coding window size and
ew_max_size by observing its maximum value over the time.
Appendix D. Decoding Beyond Maximum Latency Optimization
(Informational)
This annex introduces non-normative considerations. It is provided
as suggestions, without any impact on interoperability. For more
information see [Roca16].
With a real-time source ADU flow, it is possible to improve the
decoding performance of Sliding Window Codes without impacting
maximum latency, at the cost of extra memory and CPU overhead. The
optimization consists, for a FECFRAME receiver, to extend the linear
system beyond the decoding window maximum size, by keeping a certain
number of old source symbols whereas their associated ADUs timed-out:
ls_max_size > dw_max_size
Usually the following choice is a good trade-off between decoding
performance and extra CPU overhead:
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 receiver that benefits from an FEC
protection significantly higher than what is required to recover from
packet losses, can choose to reduce the ls_max_size. In that case
lost ADUs will be recovered without relying on this optimization.
ls_max_size
/---------------------------------^-------------------------------\
late source symbols
(pot. decoded but not delivered) dw_max_size
/--------------^-----------------\ /--------------^---------------\
src0 src1 src2 src3 src4 src5 src6 src7 src8 src9 src10 src11 src12
Figure 11: Relationship between Parameters to Decode beyond
Maximum Latency
It means that source symbols, and therefore ADUs, may be decoded even
if the added latency exceeds the maximum value permitted by the
application (the "late source symbols" of Figure 11). It follows
that the corresponding ADUs will not be useful to the application.
However, decoding these "late symbols" significantly improves the
global robustness in bad reception conditions and is therefore
recommended for receivers experiencing bad communication conditions
[Roca16]. In any case whether or not to use this optimization and
what exact value to use for the ls_max_size parameter are local
decisions made by each receiver independently, without any impact on
the other receivers nor on the source.
Acknowledgments
The authors would like to thank the three TSVWG chairs, Wesley Eddy
(our shepherd), David Black, and Gorry Fairhurst; as well as Spencer
Dawkins, our responsible AD; and all those who provided comments --
namely (in alphabetical order), Alan DeKok, Jonathan Detchart, Russ
Housley, Emmanuel Lochin, Marie-Jose Montpetit, and Greg Skinner.
Last but not least, the authors are really grateful to the IESG
members, in particular Benjamin Kaduk, Mirja Kuehlewind, Eric
Rescorla, Adam Roach, and Roman Danyliw for their highly valuable
feedback that greatly contributed to improving this specification.
Authors' Addresses
Vincent Roca
INRIA
Univ. Grenoble Alpes
France
Email: vincent.roca@inria.fr
Belkacem Teibi
INRIA
Univ. Grenoble Alpes
France
Email: belkacem.teibi@gmail.com