TSVWG V. Roca
Internet-Draft INRIA
Intended status: Standards Track February 7, 2017
Expires: August 11, 2017
Random Linear Codes (RLC) Forward Error Correction (FEC) Scheme for
FECFRAME
draft-roca-tsvwg-rlc-fec-scheme-00
Abstract
This document describes a fully-specified FEC scheme for the
convolutional Random Linear Codes (RLC) over GF(2^^m), where m equals
1 (binary case), 4 or 8, that can be used to protect arbitrary media
streams along the lines defined by FECFRAME extended to convolutional
codes. These convolutional 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 convolutional
FEC codes offer key advantages in terms of reduced FEC-related
latency while often providing improved erasure recovery capabilities.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Limits of Block Codes with Real-Time Flows . . . . . . . 3
1.2. Lower Latency and Better Protection with RLC
Convolutional Codes . . . . . . . . . . . . . . . . . . . 3
1.3. Small Transmission Overheads with the RLC FEC Scheme . . 4
1.4. Document Organization . . . . . . . . . . . . . . . . . . 5
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 5
3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. RLC parameters derivation . . . . . . . . . . . . . . . . 6
3.2. ADU, ADUI and Source Symbols Mappings . . . . . . . . . . 7
3.3. Encoding Window Management . . . . . . . . . . . . . . . 8
3.4. Pseudo-Random Number Generator . . . . . . . . . . . . . 9
3.5. Coding Coefficients Generation Function . . . . . . . . . 10
4. RLC FEC Scheme for Arbitrary ADU Flows . . . . . . . . . . . 12
4.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 12
4.1.1. FEC Framework Configuration Information . . . . . . . 12
4.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . 13
4.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 13
4.1.4. Additional Procedures . . . . . . . . . . . . . . . . 14
5. FEC Code Specification . . . . . . . . . . . . . . . . . . . 15
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. Operations and Management Considerations . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Decoding Beyond Maximum Latency Optimization . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
Application-Level Forward Erasure Correction (AL-FEC) codes are a key
element of telecommunication systems. They are used to recover from
packet erasures during content delivery sessions to a large number of
receivers (multicast/broadcast transmissions). This is the case with
the FLUTE/ALC protocol [RFC6726] in case of reliable file transfers
over lossy networks, and the FECFRAME protocol for reliable
continuous media transfers over lossy networks.
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The present document focusses only on the FECFRAME protocol, used in
multicast/broadcast delivery mode, with contents that feature
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 a end-
host/server (source) or a middlebox) and a single FEC decoding point
(either a 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, are all
linear block codes: they require the data flow to be segmented into
blocks of a predefined maximum size. The block size is a balance
between robustness (in particular in front of long erasure bursts for
which there is an incentive to increase the block size) and maximum
decoding latency (for which there is an incentive to decrease the
block size). Therefore, with a multicast/broadcast session, the
block code is dimensioned by considering the worst communication
channel one wants to support, and this choice impacts all receivers,
no matter their individual channel quality.
1.2. Lower Latency and Better Protection with RLC Convolutional Codes
This document introduces a fully-specified FEC scheme that follows a
totally different approach: the Random Linear Codes (RLC) over
GF(2^^m), where m equals 1, 4 or 8. This FEC scheme is used to
protect arbitrary media streams along the lines defined by FECFRAME
extended to convolutional codes [fecframe-ext]. This FEC scheme is
extremmely efficient in case of media with real-time constraints,
sent within a multicast/broadcast session.
The RLC codes belong to the broad class of convolutional AL-FEC
codes. 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), and which is either of fixed or variable
size (elastic window). Repair packets (symbols) are generated and
sent on-the-fly, after computing a random linear combination of the
source symbols present in the current encoding window.
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 of each
repair symbol received) are added upon receiving new packets.
Variables are removed when they are too old with respect to their
validity period (real-time constraints), as well as the associated
equations they are involved in (Appendix A introduces an optimisation
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that extends the time a variable is considered in the system).
Erased source symbols are then recovered thanks this linear system
whenever its rank permits it.
With RLC (more generally with convolutional codes), the protection of
a multicast/broadcast session also needs to be dimensioned by
considering the worst communication channel one wants to support.
However the receivers experiencing a good to medium channel quality
observe a FEC-related latency close to zero [Roca16] since an
isolated erased source packet is quickly recovered by the following
repair packet. On the opposite, with a block code, recovering an
isolated erased source packet always requires waiting the end of the
block for the first repair packet to arrive. Additionally, under
certain situations (e.g., with a limited FEC-related latency budget
and with constant bit rate transmissions after FECFRAME encoding),
convolutional codes achieve more easily 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 RLC FEC Scheme
The RLC FEC scheme is designed so as to reduce the transmission
overhead. The main requirement is that each repair packet header
must enable a receiver to reconstruct the list of source symbols and
the associated random coefficients used during the encoding process.
In order to minimize packet overhead, the set of symbols in the
encoding window as well as the set of coefficients over GF(2^^m) used
in the linear combination are not individually listed in the repair
packet header. Instead, each FEC repair packet header contains:
o 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 easily reconstruct
the set of source symbols considered during encoding, the only
constraint being that there cannot be any gap;
o the seed used by a coding coefficients generation function
(Section 3.5). This information enables 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, also called Repair FEC Payload ID (Figure 7). Similarly,
each FEC source packet features a fixed 32-bit lon trailer, also
called Explicit 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).
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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, pseudo-random 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses the following definitions and abbreviations:
ADU: Application Data Unit
E: encoding symbol size (i.e., source or repair symbol), assumed
fixed (in bytes)
bw_out: transmission bandwidth at the output of the FECFRAME sender,
assumed fixed (in bits/s)
max_lat: maximum FEC-related latency within FECFRAME (in seconds)
cr: AL-FEC coding rate
plr: packet loss rate on the erasure channel
ew_size: encoding window current size at a sender (in symbols)
ew_max_size: encoding window maximum size at a sender (in symbols)
dw_size: decoding window current size at a receiver (in symbols).
dw_max_size: decoding window maximum size at a receiver (in symbols)
ls_max_size: linear system maximum size (or width) at a receiver (in
symbols)
ls_size: linear system current size (or width) at a receiver (in
symbols)
PRNG: pseudo-random number generator
pmms_rand(maxv): PRNG defined in Section 3.4 and used in this
specification, that returns a new random integer in [0; maxv-1]
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3. Procedures
This section introduces the procedures that are used by this FEC
scheme.
3.1. RLC parameters derivation
The RLC FEC Scheme relies on several key internal parameters:
Maximum FEC-related latency budget, max_lat (in seconds) A source
ADU flow can have real-time constraints, and therefore any
FECFRAME related operation must take place within the validity
period of each ADU. When there are multiple flows with different
real-time constraints, we consider the most stringent constraints
(see [RFC6363], Section 10.2, item 6, for recommendations when
several flows are globally protected). This maximum FEC-related
latency accounts for all sources of latency added by FEC encoding
(sender) and FEC decoding (receiver). Any other source of latency
(e.g., added by network communications) is not considered in this
latency budget; It can be regarded as the latency budget permitted
for all FEC-related operations. This is also an input parameter
that enables to derive other internal parameters;
Encoding window current (resp. maximum) size, ew_size (resp.
ew_max_size) (in symbols):
these parameters are used by a sender during FEC encoding. More
precisely, each repair symbol is a linear combination of the
ew_size source symbols present in the encoding window when RLC
encoding took place. In all situations, we MUST have ew_size <=
ew_max_size;
Decoding window current (resp. maximum) size, dw_size (resp.
dw_max_size) (in symbols):
these parameters are used by a receiver when managing the linear
system used for decoding. dw_size is the current size of the
decoding window, i.e., the set of received or erased source
symbols that are currently part of the linear system. In all
situations, we MUST have dw_size <= dw_max_size;
In order to comply with the maximum FEC-related latency budget,
assuming a constant transmission bandwidth at the output of the
FECFRAME sender (bw_out), encoding symbol size (E), and code rate
(cr), we have:
dw_max_size = (max_lat * bw_out * cr) / (8 * E)
This dw_max_size defines the maximum delay after which an old source
symbol may be recovered: after this delay, this old source symbol
symbol will be removed from the decoding window.
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It is often good practice to choose:
ew_max_size = dw_max_size / 2
However any value ew_max_size < dw_max_size can be used without
impact on the FEC-related latency budget. Finding the optimal value
can depend on the erasure channel one wants to support and should be
determined after simulations or field trials.
Note that the decoding beyond maximum latency optimisation
(Appendix A) enables an old source symbol to be kept in the linear
system beyond the FEC-related latency budget, but not delivered to
the receiving application. Here we have: ls_size >= dw_max_size
3.2. ADU, ADUI and Source Symbols Mappings
An ADU, coming from the application, cannot be mapped to source
symbols directly. Indeed, an erased ADU recovered at a receiver must
contain enough information to be assigned to the right application
flow (UDP port numbers and IP addresses cannot be used to that
purpose as they are not protected by FEC encoding). This requires
adding the flow identifier to each ADU before doing FEC encoding.
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 is 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 instance.
Length (L) (16-bit field): this unsigned integer contain 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 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).
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Each ADUI contributes to an integral number of source symbols. The
data unit resulting from the ADU and the F, L, and Pad fields is
called ADU Information (or ADUI). Since ADUs can be of different
size, this is also the case for ADUIs.
symbol length, E E E
< ------------------ >< ------------------ >< ------------------ >
+-+--+---------------------------------------------+-------------+
|F| L| ADU | Pad |
+-+--+---------------------------------------------+-------------+
Figure 1: ADUI Creation example (here 3 source symbols are created
for this ADUI).
Note that neither the initial 3 bytes nor the optional padding are
sent over the network. However, they are considered during FEC
encoding. It means that a receiver who lost a certain FEC source
packet (e.g., the UDP datagram containing this FEC source packet)
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:
o 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;
o when an ADU has reached its maximum validity duration in case of a
realtime 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 ADUI and then 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_lat
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value, in addition to the maximum FEC-related latency budget
(Section 3.1).
3.4. Pseudo-Random Number Generator
The RLC codes rely on the following Pseudo-Random Number Generator
(PRNG), identical to the PRNG used with LDPC-Staircase codes
([RFC5170], section 5.7).
The Park-Miler "minimal standard" PRNG [PM88] MUST be used. It
defines a simple multiplicative congruential algorithm: Ij+1 = A * Ij
(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
following: if seed = 1, then the 10,000th value returned MUST be
equal to 1043618065.
Several implementations of this PRNG are known and discussed in the
literature. An optimized implementation of this algorithm, using
only 32-bit mathematics, and which does not require any division, can
be found in [rand31pmc]. It uses the Park and Miller algorithm
[PM88] with the optimization suggested by D. Carta in [CA90]. The
history behind this algorithm is detailed in [WI08]. Yet, any other
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
(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
significant bits of the value returned by the PRNG (the least
significant bits are known to be less random, and modulo-based
solutions should be avoided [PTVF92]). The following algorithm MUST
be used:
Input:
raw_value: random integer generated by the inner PRNG algorithm,
between 1 and 0x7FFFFFFE (2^^31-2) inclusive.
maxv: upper bound used during the scaling operation.
Output:
scaled_value: random integer between 0 and maxv-1 inclusive.
Algorithm:
scaled_value = (unsigned long) ((double)maxv * (double)raw_value /
(double)0x7FFFFFFF);
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(NB: the above C type casting to unsigned long is equivalent to
using floor() with positive floating point values.)
In this document, pmms_rand(maxv) denotes the PRNG function that
implements the Park-Miller "minimal standard" algorithm, defined
above, and that scales the raw value between 0 and maxv-1 inclusive,
using the above scaling algorithm.
Additionally, the pmms_srand(seed) function must be provided to
enable the initialization of the PRNG with a seed before calling
pmms_rand(maxv) the first time. The seed is a 31-bit integer between
1 and 0x7FFFFFFE inclusive. In this specification, the seed is
restricted to a value between 1 and 0xFFFF inclusive, as this is the
Repair_Key 16-bit field value of the Repair FEC Payload ID
(Section 4.1.3).
3.5. Coding Coefficients Generation Function
The coding coefficients, used during the encoding process, are
generated at the RLC encoder by the following function each time a
new repair symbol needs to be produced:
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<CODE BEGINS>
/*
* 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
* (in) 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 table. This value is
* equal to the current encoding window size.
* (in) m Finite Field GF(2^^m) parameter.
* (out) returns an error code
*/
int generate_coding_coefficients (UINT16 repair_key,
UINT8 cc_tab[],
UINT16 cc_nb,
UINT8 m)
{
UINT32 i;
if (repair_key == 0) {
return SOMETHING_WENT_WRONG;
}
pmms_srand(repair_key);
if (m == 1) {
/* 0 is a valid coefficient value in binary GF */
for (i = 0 ; i < cc_nb ; i ++) {
cc_tab[i] = (UINT8) pmms_rand(2);
}
} else {
/* coefficient 0 is avoided in non-binary GF to consider each
* source symbol */
UINT32 maxv;
maxv = get_gf_size(); /* i.e., 16 if m=4 or 256 if m=8 */
for (i = 0 ; i < cc_nb ; i ++) {
do {
cc_tab[i] = (UINT8) pmms_rand(maxv);
} while (cc_tab[i] == 0)
}
}
return EVERYTHING_IS_OKAY;
}
<CODE ENDS>
Figure 2: Coding Coefficients Generation Function pseudo-code
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4. RLC FEC Scheme for Arbitrary ADU Flows
4.1. Formats and Codes
4.1.1. FEC Framework Configuration Information
The FEC Framework Configuration Information (or FFCI) includes
information that MUST be communicated between the sender and
receiver(s). More specifically, it enables the synchronization of
the FECFRAME sender and receiver instances. It includes both
mandatory elements and scheme-specific elements, as detailed below.
4.1.1.1. Mandatory Information
o FEC Encoding ID: the value assigned to this fully specified FEC
scheme MUST be XXXX, 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 length (E): a non-negative integer that indicates
the length of each encoding symbol in bytes;
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
If another mechanism requires the FSSI to be carried as an opaque
octet string (for instance, after a Base64 encoding), the encoding
format consists of the following 2 octets:
Encoding symbol length (E) field (16-bits): Length, in number of
bytes, of the source and repair symbols.
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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 Length (E) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: 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 4.
+--------------------------------+
| IP Header |
+--------------------------------+
| Transport Header |
+--------------------------------+
| ADU |
+--------------------------------+
| Explicit Source FEC Payload ID |
+--------------------------------+
Figure 4: 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 (Figure 5):
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 5: Source FEC Payload ID Encoding Format
4.1.3. Repair FEC Payload ID
A FEC repair packet MUST contain a Repair FEC Payload ID that is
prepended to the repair symbol as illustrated in Figure 6. There
MUST be a single repair symbol per FEC repair packet.
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+--------------------------------+
| IP Header |
+--------------------------------+
| Transport Header |
+--------------------------------+
| Repair FEC Payload ID |
+--------------------------------+
| Repair Symbol |
+--------------------------------+
Figure 6: 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 (Figure 7):
Repair_Key (16-bit field): this unsigned integer is used as a seed
by the coefficient generation function Section 3.5, in order to
generate the desired number of coding coefficients. Value 0 MUST
NOT be used.
Number of Source Symbols in the Encoding Window, NSS (16-bit field):
this unsigned integer indicates the number of source symbols in
the encoding window when this repair symbol was generated.
ESI of first source symbol in 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.
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 | NSS (# source symbols in ew) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FSS_ESI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Repair FEC Payload ID Encoding Format
4.1.4. Additional Procedures
The following procedure applies:
o The ESI of source symbols MUST start with value 0 for the first
source symbol and MUST be managed sequentially. Wrapping to zero
will happen after reaching the maximum 32-bit value.
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5. FEC Code Specification
TBD... Describe a typical sender's operation, when using the RLC FEC
scheme. Describe a typical receiver operation, when using the RLC
FEC scheme.
(summary, to be detailed): The FECFRAME sender generates a linear
combination of source symbols, using the coding coefficients
generation function and sends it within an FEC repair packet. This
linear combination encompasses all the source symbols currently in
the encoding window. FEC repair packets are sent immediately after
having been created, inter-mixed with FEC source packets.
(summary, to be detailed): A FECFRAME receiver, upon receiving a FEC
repair packet, adds an equation to the linear system it maintains (or
no equation if this repair packet does not change the linear system
rank). Whenever possible, decoding is performed in order to recover
erased source symbols if any.
6. Implementation Status
Editor's notes:
o RFC Editor, please remove this section motivated by RFC 6982
before publishing the RFC. Thanks.
An implementation of the RLC convolutional FEC Scheme for FECFRAME
exists:
o Organisation: Inria
o Description: This is an implementation of the RLC Convolutional
FEC Scheme. It relies on a modified version of our OpenFEC
(http://openfec.org) FEC code library. It is integrated in our
FECFRAME software (see [fecframe-ext]).
o Maturity: prototype.
o Coverage: this software complies with the RLC FEC Scheme (limited
to m=8 as of end of January, 2017).
o Lincensing: proprietary.
o Contact: vincent.roca@inria.fr
7. Security Considerations
TBD
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8. Operations and Management Considerations
9. IANA Considerations
This document registers one value in the "FEC Framework (FECFRAME)
FEC Encoding IDs" registry [RFC6363] as follows:
o XXX refers to the convolutional Random Linear Codes (RLC) FEC
Scheme for Arbitrary Packet Flows, as defined in Section XXX of
this document.
10. Acknowledgments
11. References
11.1. Normative References
[fecframe-ext]
Roca, V. and A. Begen, "Forward Error Correction (FEC)
Framework version 2", Transport Area Working Group
(TSVWG) draft-roca-tsvwg-fecframev2 (Work in Progress),
October 2016, <https://tools.ietf.org/html/draft-roca-
tsvwg-fecframev2-02>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc6364>.
11.2. Informative References
[CA90] Carta, D., "Two Fast Implementations of the Minimal
Standard Random Number Generator", Communications of the
ACM, Vol. 33, No. 1, pp.87-88, January 1990.
[PM88] Park, S. and K. Miller, "Random Number Generators: Good
Ones are Hard to Find", Communications of the ACM, Vol.
31, No. 10, pp.1192-1201, 1988.
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[PTVF92] Press, W., Teukolsky, S., Vetterling, W., and B. Flannery,
"Numerical Recipies in C; Second Edition", Cambridge
University Press, ISBN: 0-521-43108-5, 1992.
[rand31pmc]
Whittle, R., "31 bit pseudo-random number generator",
September 2005, <http://www.firstpr.com.au/dsp/rand31/
rand31-park-miller-carta.cc.txt>.
[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, <http://www.rfc-editor.org/info/rfc5170>.
[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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc6865>.
[Roca16] Roca, V., Teibi, B., Burdinat, C., Tran, T., and C.
Thienot, "Block or Convolutional AL-FEC Codes? A
Performance Comparison for Robust Low-Latency
Communications", Submitted for publication
https://hal.inria.fr/hal-01395937/en/, November 2016, <
https://hal.inria.fr/hal-01395937/en/>.
[WI08] Whittle, R., "Park-Miller-Carta Pseudo-Random Number
Generator", http://www.firstpr.com.au/dsp/rand31/,
January 2008, <http://www.firstpr.com.au/dsp/rand31/>.
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Appendix A. Decoding Beyond Maximum Latency Optimization
This annex introduces non normative considerations. They are
provided as suggestions, without any impact on interoperability. For
more information see [Roca16].
It is possible to improve the decoding performance of convolutional
codes without impacting maximum latency, at the cost of extra CPU
overhead. The optimization consists, for a receiver, to extend the
linear system beyond the decoding window:
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
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 8: Relationship between parameters to decode beyond maximum
latency.
It means that source symbols (and therefore ADUs) may be decoded even
if their transport protocol added latency exceeds the maximum value
permitted by the application. It follows that these source symbols
SHOULD NOT be delivered to the application and SHOULD be dropped once
they are no longer needed. However, decoding these late symbols
significantly improves the global robustness in bad reception
conditions and is therefore recommended for receivers experiencing
bad channels[Roca16]. In any case whether or not to use this
facility and what exact value to use for the ls_max_size parameter
are decisions made by each receiver independantly, without any impact
on others, neither the other receivers nor the source.
Author's Address
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Vincent Roca
INRIA
655, av. de l'Europe
Inovallee; Montbonnot
ST ISMIER cedex 38334
France
EMail: vincent.roca@inria.fr
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