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<?xml version='1.0' encoding='utf-8'?><?xml version='1.0' encoding='utf-8'?>
<!-- [rfced] updated by Chris /07/18/19 --><!-- [rfced] updated by Chris /07/18/19 -->
<!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent"><!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent">
<rfc xmlns:xi="http://www.w3.org/2001/XInclude" submissionType="IETF"category="<rfc xmlns:xi="http://www.w3.org/2001/XInclude" submissionType="IETF"
std" consensus="yes"number="XXXX" ipr="trust200902" obsoletes="" updates=""xmlcategory="std" consensus="yes"number="0000" ipr="trust200902"
:lang="en" tocInclude="true" symRefs="true" sortRefs="true"version="3"> obsoletes="" updates=""xml:lang="en" tocInclude="true" symRefs="true"
sortRefs="true"version="3" docName="draft-ietf-tsvwg-rlc-fec-scheme-16">
<!-- xml2rfc v2v3 conversion 2.34.0 --> <!-- xml2rfc v2v3 conversion 2.34.0 -->
<front> <front>
<title abbrev="RLC FEC Scheme">Sliding Window Random Linear Code (RLC) Forward Erasure Correction (FEC) Schemes for FECFRAME</title> <title abbrev="RLC FEC Scheme">Sliding Window Random Linear Code (RLC) Forward Erasure Correction (FEC) Schemes for FECFRAME</title>
<seriesInfo name="RFC" value="XXXX"/> <seriesInfo name="RFC" value="0000"/>
<author fullname="Vincent Roca" initials="V" surname="Roca"> <author fullname="Vincent Roca" initials="V" surname="Roca">
<organization>INRIA</organization> <organization>INRIA</organization>
<address> <address>
<postal> <postal>
<street/> <street/>
<city>Univ. Grenoble Alpes</city><city/>
<code/> <code/>
<extaddr>Univ. Grenoble Alpes</extaddr>
<country>France</country> <country>France</country>
</postal> </postal>
<email>vincent.roca@inria.fr</email> <email>vincent.roca@inria.fr</email>
</address> </address>
</author> </author>
<author fullname="Belkacem Teibi" initials="B" surname="Teibi"> <author fullname="Belkacem Teibi" initials="B" surname="Teibi">
<organization>INRIA</organization> <organization>INRIA</organization>
<address> <address>
<postal> <postal>
<street/> <street/>
<city>Univ. Grenoble Alpes</city><city/>
<code/> <code/>
<extaddr>Univ. Grenoble Alpes</extaddr>
<country>France</country> <country>France</country>
</postal> </postal>
<email>belkacem.teibi@gmail.com</email> <email>belkacem.teibi@gmail.com</email>
</address> </address>
</author> </author>
<date year="2019" month="September"/> <date year="2019" month="September"/>
<workgroup>TSVWG</workgroup> <workgroup>TSVWG</workgroup>
<!-- [rfced] Please insert any keywords (beyond those that appear in the title) for use on https://www.rfc-editor.org/search. --> <!-- [rfced] Please insert any keywords (beyond those that appear in the title) for use on https://www.rfc-editor.org/search. -->
<keyword>example</keyword> <keyword>example</keyword>
<abstract> <abstract>
skipping to change at line 126skipping to change at line 131
</ul> </ul>
<t> <t>
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 (<xref format="default"/>).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 (<xref format="default"/>).
Similarly, each FEC Source Packet features a fixed 32-bit long trailer, called Explicit Source FEC Payload ID (<xref format="default"/>), that contains the ESI of the first source symbol (<xref format="default"/>).Similarly, each FEC Source Packet features a fixed 32-bit long trailer, called Explicit Source FEC Payload ID (<xref format="default"/>), that contains the ESI of the first source symbol (<xref format="default"/>).
</t></t>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Document Organization</name> <name>Document Organization</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
This fully-specified FEC Scheme follows the structure required by <xreftarget="This fully-specified FEC Scheme follows the structure required by <xref
RFC6363" format="default"/>, section 5.6. "FEC Scheme Requirements", namely:target="RFC6363" format="default" sectionFormat="comma" section="5.6"/> ("FEC Sc
heme Requirements"), namely:
</t></t>
<dl newline="false" spacing="normal">
<dt>3. Procedures:</dt><ol type="1" start="3">
<dd> <li>Procedures:
This section describes procedures specific to this FEC Scheme, namely: RL This section describes procedures specific to this FEC Scheme, namely: RL
C parameters derivation, ADUI and source symbols mapping, pseudorandom number geC parameters derivation, ADUI and source symbols mapping, pseudorandom number ge
nerator, and coding coefficients generationfunction;</dd>nerator, and coding coefficients generationfunction;</li>
<dt>4. Formats andCodes:</dt> <li>Formats andCodes:
<dd>
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. 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) carryi It also defines the FEC Framework Configuration Information (FFCI) carryi
ng signaling information for thesession;</dd>ng signaling information for thesession;</li>
<dt>5. FEC CodeSpecification:</dt> <li>FEC CodeSpecification:
<dd>
Finally this section provides the codespecification.</dd> Finally this section provides the codespecification.</li>
</dl> </ol>
</section> </section>
</section> </section>
<section anchor="definitionsAndAbbreviations" numbered="true" toc="default"> <section anchor="definitionsAndAbbreviations" numbered="true" toc="default">
<name>Definitions and Abbreviations</name> <name>Definitions and Abbreviations</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
The key words"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLThe key words"<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
"MAY", and"OPTIONAL" in this document are to beinterpreted asNOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECO
MMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and"<bcp14>OPTIONAL</bcp14>" in this document are to bei
nterpreted as
described in BCP 14 <xref format="default"/> <xref format="default"/>described in BCP 14 <xref format="default"/> <xref format="default"/>
when, and only when, they appear in all capitals, as shown here.when, and only when, they appear in all capitals, as shown here.
</t></t>
<t>This document uses the following definitions and abbreviations: </t> <t>This document uses the following definitions and abbreviations: </t>
<dl newline="false" spacing="normal"> <dl newline="false" spacing="normal">
<dt>a^^b</dt> <dt>a^^b</dt>
<dd> a to the power of b</dd> <dd> a to the power of b</dd>
<dt>GF(q)</dt> <dt>GF(q)</dt>
<dd> denotes a finite field (also known as the Galois Field) with q elements. <dd> denotes a finite field (also known as the Galois Field) with q elements.
We assume that q = 2^^m in this document</dd> We assume that q = 2^^m in this document</dd>
skipping to change at line 209skipping to change at line 216
<!-- ================ --> <!-- ================ -->
<t> <t>
This section introduces the procedures that are used by these FEC Schemes.This section introduces the procedures that are used by these FEC Schemes.
</t></t>
<section anchor="CommonProc_rlcParameters" numbered="true" toc="default"> <section anchor="CommonProc_rlcParameters" numbered="true" toc="default">
<name>Codec Parameters</name> <name>Codec Parameters</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
A codec implementing the Sliding Window RLC FEC Scheme relies on several parameters:A codec implementing the Sliding Window RLC FEC Scheme relies on several parameters:
</t></t>
<dl newline="false" spacing="normal"> <dl newline="true" spacing="normal">
<dt>Maximum FEC-related latency budget, max_lat (a decimal number expressed in seconds) with real-time flows:</dt> <dt>Maximum FEC-related latency budget, max_lat (a decimal number expressed in seconds) with real-time flows:</dt>
<dd> <dd>
a source ADU flow can have real-time constraints, and therefore any FECFRAME related operation should take place within the validity 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 (<xref format="default"/> describes an exception to this rule). period of each ADU (<xref format="default"/> describes an exception to this rule).
When there are multiple flows with different real-timeconstraint When there are multiple flows with different real-time
s, we consider the most stringent constraints (see <xrefformatconstraints, we consider the most stringent constraints (see
="default"/>,item 6 in <xrefformat="default"
Section 10.2, item 6, for recommendations when several flows are sectionFormat="of" section="10.2"/>,
globally protected). 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). 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). 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. 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 <xref format="default"/>); This is an input parameter that enables a FECFRAME sender to derive other internal parameters (see <xref format="default"/>);
</dd> </dd>
<dt>Encoding window current (resp. maximum) size, ew_size (resp. ew_max_size) (in symbols):</dt> <dt>Encoding window current (resp. maximum) size, ew_size (resp. ew_max_size) (in symbols):</dt>
<dd> <dd>
<t> <t>
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. 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. 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. 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: We have:
</t> </t>
<ul spacing="normal"> <ul spacing="normal" empty="true">
<li> 0 &lt; ew_size &lt;= ew_max_size </li> <li> 0 &lt; ew_size &lt;= ew_max_size </li>
</ul> </ul>
</dd> </dd>
<dt>Decoding window maximum size, dw_max_size (in symbols):</dt> <dt>Decoding window maximum size, dw_max_size (in symbols):</dt>
<dd> <dd>
at a FECFRAME receiver, dw_max_size is the maximum number of received or lost source symbols that are still within their latency budget; at a FECFRAME receiver, dw_max_size is the maximum number of received or lost source symbols that are still within their latency budget;
</dd> </dd>
<dt>Linear system maximum size, ls_max_size (in symbols):</dt> <dt>Linear system maximum size, ls_max_size (in symbols):</dt>
<dd> <dd>
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). 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).
ItSHOULD NOT be smaller than dw_max_size since it would meantha It<bcp14>SHOULD NOT</bcp14> be smaller than dw_max_size since it
t, even after receiving a sufficient number of FEC RepairPackets, a lost ADUma would meanthat, even after receiving a sufficient number of FEC RepairPackets
y not be recovered just because the associated source symbolshave beenprematur, a lost ADUmay not be recovered just because the associated source symbolshav
ely removed from the linear system, which is usuallycounter-productive.e beenprematurely removed from the linear system, which is usuallycounter-prod
On the opposite, the linear systemMAY grow beyond thedw_max_sizuctive.
e (<xref format="default"/>); On the opposite, the linear system<bcp14>MAY</bcp14> grow beyond
thedw_max_size (<xref format="default"/>);
<!-- with old source symbols kept in the linear system whereas their associated ADUs timed-out --> <!-- with old source symbols kept in the linear system whereas their associated ADUs timed-out -->
</dd> </dd>
<dt>Symbol size, E (in bytes):</dt> <dt>Symbol size, E (in bytes):</dt>
<dd> <dd>
the E parameter determines the source and repair symbol sizes (necessarily equal). 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. This is an input parameter that enables a FECFRAME sender to derive other internal parameters, as explained below.
An implementation at a senderMUST fix the E parameter and MUST communicate it as part of the FEC Scheme-Specific Information (<xref format="default"/>). An implementation at a sender<bcp14>MUST</bcp14> fix the E parameter and <bcp14>MUST</bcp14> communicate it as part of the FEC Scheme-Specific Information (<xref format="default"/>).
</dd> </dd>
<dt>Code rate, cr:</dt> <dt>Code rate, cr:</dt>
<dd> <dd>
The code rate parameter determines the amount of redundancy added to the flow. 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 &lt; cr &lt;= 1. 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 &lt; cr &lt;= 1.
This is an input parameter that enables a FECFRAME sender to derive other internal parameters, as explained below. 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). 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. 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. 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.
</dd> </dd>
skipping to change at line 286skipping to change at line 296
This requires adding the flow identifier to each ADU before doing FEC encoding.This requires adding the flow identifier to each ADU before doing FEC encoding.
-->-->
</t></t>
<t> <t>
Additionally, since ADUs are of variable size, padding is needed so that each ADU (with its flow identifier) contributeAdditionally, 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.to an integral number of source symbols.
This requires adding the original ADU length to each ADU before doing FEC encoding.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 <xref format="default"/>.Because of these requirements, an intermediate format, the ADUI, or ADU Information, is considered <xref format="default"/>.
</t></t>
<t> <t>
For each incoming ADU, an ADUIMUST be created as follows.For each incoming ADU, an ADUI<bcp14>MUST</bcp14> be created as follows.
First of all, 3 bytes are prepended (<xref format="default"/>):First of all, 3 bytes are prepended (<xref format="default"/>):
</t></t>
<dl newline="false" spacing="normal"> <dl newline="false" spacing="normal">
<dt>Flow ID (F) (8-bit field):</dt> <dt>Flow ID (F) (8-bit field):</dt>
<dd> <dd>
this unsigned byte contains the integer identifier associated to the source ADU flow to which this ADU belongs. 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 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.</dd> a single FECFRAME session instance.</dd>
<dt>Length (L) (16-bit field):</dt> <dt>Length (L) (16-bit field):</dt>
<dd> <dd>
skipping to change at line 335skipping to change at line 345
Figure 1: ADUI Creation Example Figure 1: ADUI Creation Example
-->-->
<figure anchor="fig_adui_creation"> <figure anchor="fig_adui_creation">
<name>ADUI Creation Example</name> <name>ADUI Creation Example</name>
<artwork name="" type="" align="left" alt=""><![CDATA[ <artwork name="" type="" align="left" alt=""><![CDATA[
symbol length, E E E symbol length, E E E
< ------------------ >< ------------------ >< ------------------ >< ------------------ >< ------------------ >< ------------------ >
+-+--+---------------------------------------------+-------------++-+--+---------------------------------------------+-------------+
|F| L| ADU | Pad ||F| L| ADU | Pad |
+-+--+---------------------------------------------+-------------++-+--+---------------------------------------------+-------------+
]]></artwork></figure>
<t>Here, three source symbols are created for this ADUI.</t>
Here, three source symbols are created for this ADUI. ]]></artwork>
</figure>
<t> <t>
Note that neither the initial 3 bytes nor the optional padding are sent over the network.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 who lost a certain FEC Source Packet (e.g., the UDP datagramHowever, they are considered during FEC encoding, and a receiver who 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.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.Thanks to the initial 3 bytes, this receiver will get rid of the padding (if any) and identify the corresponding ADU flow.
</t></t>
</section> </section>
<section anchor="encodingWindowManagement" numbered="true" toc="default"> <section anchor="encodingWindowManagement" numbered="true" toc="default">
<name>Encoding Window Management</name> <name>Encoding Window Management</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
Source symbols and the corresponding ADUs are removed from the encoding window:Source symbols and the corresponding ADUs are removed from the encoding window:
</t></t>
<ul spacing="normal"> <ul spacing="normal">
<li> when the sliding encoding window has reached its maximum size, ew_max_size. <li> when the sliding encoding window has reached its maximum size, ew_max_size.
In that case the oldest symbolMUST be removed before adding a new symbol, so that the current encoding window size always In that case the oldest symbol<bcp14>MUST</bcp14> 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 &lt;= ew_max_size;</li> remains inferior or equal to the maximum size: ew_size &lt;= ew_max_size;</li>
<li> when an ADU has reached its maximum validity duration in case of a real-time flow. <li> 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 expiredSHOULD be removed from the encoding window; </li> When this happens, all source symbols corresponding to the ADUI that expired<bcp14>SHOULD</bcp14> be removed from the encoding window; </li>
</ul> </ul>
<t> <t>
Source symbols are added to the sliding encoding window each time a new ADU arrives, once the ADU-to-source symbols mapping has been performedSource symbols are added to the sliding encoding window each time a new ADU arrives, once the ADU-to-source symbols mapping has been performed
(<xref format="default"/>).(<xref format="default"/>).
The current size of the encoding window, ew_size, is updated after adding new source symbols.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 &lt;= ew_max_size.This process may require to remove old source symbols so that: ew_size &lt;= ew_max_size.
</t></t>
<t> <t>
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).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 (<xref format="default"/>).This factor may further limit the ew_max_size value, in addition to the maximum FEC-related latency budget (<xref format="default"/>).
</t></t>
<!-- <!--
<t><t>
LimitationsMAY exist that impact the encoding window management. For instance:Limitations<bcp14>MAY</bcp14> exist that impact the encoding window management. For instance:
<list><list>
<t> at the FEC Framework level: the source flows can have real-time constraints that limit the number of ADUs in the encoding window;</t><t> at the FEC Framework level: the source flows can have real-time constraints that limit the number of ADUs in the encoding window;</t>
<t> at the FEC Scheme level: there may be theoretical or practical limitations (e.g., because of computational complexity aspect or field size limits in the signaling headers) that limit the number of ADUs in the encoding window.</t><t> at the FEC Scheme level: there may be theoretical or practical limitations (e.g., because of computational complexity aspect or field size limits in the signaling headers) that limit the number of ADUs in the encoding window.</t>
</list></list>
The most stringent limitation defines the maximum encoding window size, either in terms of number of source symbols or number of ADUs, whichever applies.The most stringent limitation defines the maximum encoding window size, either in terms of number of source symbols or number of ADUs, whichever applies.
</t></t>
-->-->
</section> </section>
<section anchor="CommonProc_esi" numbered="true" toc="default"> <section anchor="CommonProc_esi" numbered="true" toc="default">
<name>Source Symbol Identification</name> <name>Source Symbol Identification</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
Each source symbol is identified by an Encoding Symbol ID (ESI), an unsigned integer.Each source symbol is identified by an Encoding Symbol ID (ESI), an unsigned integer.
The ESI of source symbolsMUST start with value 0 for the first source symbol and MUST be managed sequentially.The ESI of source symbols<bcp14>MUST</bcp14> start with value 0 for the first source symbol and <bcp14>MUST</bcp14> be managed sequentially.
Wrapping to zero happens after reaching the maximum value made possible by the ESI field sizeWrapping to zero happens after reaching the maximum value made possible by the ESI field size
(this maximum value is FEC Scheme dependant, for instance, 2^32-1 with FEC Schemes XXX and YYY).(this maximum value is FEC Scheme dependant, for instance, 2^32-1 with FEC Schemes XXX and YYY).
</t></t>
<t> <t>
No such consideration applies to repair symbols.No such consideration applies to repair symbols.
</t></t>
</section> </section>
<section anchor="CommonProc_prng" numbered="true" toc="default"> <section anchor="CommonProc_prng" numbered="true" toc="default">
<name>Pseudorandom Number Generator (PRNG)</name> <name>Pseudorandom Number Generator (PRNG)</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
In order to compute coding coefficients (see <xref format="default"/>), the RLC FEC Schemes rely on the TinyMT32 PRNG defined in <xref format="default"/> with two additional functions defined in this section.In order to compute coding coefficients (see <xref format="default"/>), the RLC FEC Schemes rely on the TinyMT32 PRNG defined in <xref format="default"/> with two additional functions defined in this section.
</t></t>
<t> <t>
This PRNGMUST first be initialized with a 32-bit unsigned integer, used as a seed, with:This PRNG<bcp14>MUST</bcp14> first be initialized with a 32-bit unsigned integer, used as a seed, with:
</t></t>
<ul empty="true" spacing="normal"><sourcecode type="c">
<li>void tinymt32_init (tinymt32_t * s, uint32_tseed);</li> void tinymt32_init (tinymt32_t * s, uint32_tseed);
</ul></sourcecode>
<t>
<t>
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),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 (<xref format="default"/>).since this is the Repair_Key 16-bit field value of the Repair FEC Payload ID (<xref format="default"/>).
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 (<xref format="default"/>).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 (<xref format="default"/>).
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.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.
</t></t>
<t> <t>
Then, each time a new pseudorandom integer between 0 and 15 inclusive (4-bit pseudorandom integer) is needed, the following function is used:Then, each time a new pseudorandom integer between 0 and 15 inclusive (4-bit pseudorandom integer) is needed, the following function is used:
</t></t>
<ul empty="true" spacing="normal"><sourcecode type="c">
<li>uint32_t tinymt32_rand16 (tinymt32_t *s);</li> uint32_t tinymt32_rand16 (tinymt32_t *s);
</ul></sourcecode>
<t> <t>
This function takes as parameter a pointer to the same tinymt32_t structure (that is left unchanged between successive calls to the function).This function takes as parameter a pointer to the same tinymt32_t structure (that is left unchanged between successive calls to the function).
</t></t>
<t> <t>
Similarly, each time a new pseudorandom integer between 0 and 255 inclusive (8-bit pseudorandom integer) is needed, the following function is used:Similarly, each time a new pseudorandom integer between 0 and 255 inclusive (8-bit pseudorandom integer) is needed, the following function is used:
</t></t>
<ul empty="true" spacing="normal"><sourcecode type="c">
<li>uint32_t tinymt32_rand256 (tinymt32_t *s);</li> uint32_t tinymt32_rand256 (tinymt32_t *s);
</ul> </sourcecode>
<t> <t>
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 <xref format="default"/>.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 <xref format="default"/>.
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.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.
<xref format="default"/> shows a possible implementation.<xref format="default"/> shows a possible implementation.
This is a C language implementation, written for C99 <xref format="default"/>.This is a C language implementation, written for C99 <xref format="default"/>.
Test results discussed in <xref format="default"/> show that this simple technique, applied to this PRNG, is in line with the RLC FEC Schemes needs.Test results discussed in <xref format="default"/> show that this simple technique, applied to this PRNG, is in line with the RLC FEC Schemes needs.
</t></t>
<figure anchor="fig_tinymt32_mapping"> <figure anchor="fig_tinymt32_mapping">
<name>4-bit and 8-bit Mapping Functions for TinyMT32</name> <name>4-bit and 8-bit Mapping Functions for TinyMT32</name>
<sourcecode name="" type="" markers="true"><![CDATA[ <sourcecode name="" type="c" markers="true"><![CDATA[
/**/**
* This function outputs a pseudorandom integer in [0 .. 15] range. * This function outputs a pseudorandom integer in [0 .. 15] range.
* *
* @param s pointer to tinymt internal state. * @param s pointer to tinymt internal state.
* @return unsigned integer between 0 and 15 inclusive. * @return unsigned integer between 0 and 15 inclusive.
*/ */
uint32_t tinymt32_rand16(tinymt32_t *s)uint32_t tinymt32_rand16(tinymt32_t *s)
{{
return (tinymt32_generate_uint32(s) & 0xF); return (tinymt32_generate_uint32(s) & 0xF);
}}
skipping to change at line 459skipping to change at line 470
* @param s pointer to tinymt internal state. * @param s pointer to tinymt internal state.
* @return unsigned integer between 0 and 255 inclusive. * @return unsigned integer between 0 and 255 inclusive.
*/ */
uint32_t tinymt32_rand256(tinymt32_t *s)uint32_t tinymt32_rand256(tinymt32_t *s)
{{
return (tinymt32_generate_uint32(s) & 0xFF); return (tinymt32_generate_uint32(s) & 0xFF);
}}
]]></sourcecode>]]></sourcecode>
</figure> </figure>
<t> <t>
Any implementation of this PRNGMUST have the same output as that provided by the reference implementation of <xref format="default"/>.Any implementation of this PRNG<bcp14>MUST</bcp14> have the same output as that provided by the reference implementation of <xref format="default"/>.
In order to increase the compliancy confidence, three criteria are proposed: the one described in <xref format="default"/> (for the TinyMT32 32-bit unsigned integer generator), and the two others detailed in <xref format="default"/> (for the mapping to 4-bit and 8-bit intervals).In order to increase the compliancy confidence, three criteria are proposed: the one described in <xref format="default"/> (for the TinyMT32 32-bit unsigned integer generator), and the two others detailed in <xref format="default"/> (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.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.
</t></t>
</section> </section>
<section anchor="CommonProc_coef_generation_func" numbered="true" toc="default"> <section anchor="CommonProc_coef_generation_func" numbered="true" toc="default">
<name>Coding Coefficients Generation Function</name> <name>Coding Coefficients Generation Function</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
The coding coefficients, used during the encoding process, are generated at the RLC encoder by the generate_coding_coefficients()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.function each time a new repair symbol needs to be produced.
skipping to change at line 485skipping to change at line 496
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.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 (<xref format="default"/>), m is equal to 1.With the RLC over GF(2) FEC Scheme (<xref format="default"/>), m is equal to 1.
With RLC over GF(2^^8) FEC Scheme (<xref format="default"/>), m is equal to 8.With RLC over GF(2^^8) FEC Scheme (<xref format="default"/>), m is equal to 8.
</t></t>
<t> <t>
<xref format="default"/> shows the reference generate_coding_coefficients() implementation.<xref format="default"/> shows the reference generate_coding_coefficients() implementation.
This is a C language implementation, written for C99 <xref format="default"/>.This is a C language implementation, written for C99 <xref format="default"/>.
</t></t>
<figure anchor="fig_coef_generation_func"> <figure anchor="fig_coef_generation_func">
<name>Coding Coefficients Generation Function Reference Implementation</name> <name>Coding Coefficients Generation Function Reference Implementation</name>
<sourcecode name="" type="" markers="true"><![CDATA[ <sourcecode name="" type="c" markers="true"><![CDATA[
#include <string.h>#include <string.h>
/*/*
* Fills in the table of coding coefficients (of the right size) * Fills in the table of coding coefficients (of the right size)
* provided with the appropriate number of coding coefficients to * provided with the appropriate number of coding coefficients to
* use for the repair symbol key provided. * use for the repair symbol key provided.
* *
* (in) repair_key key associated to this repair symbol. This * (in) repair_key key associated to this repair symbol. This
* parameter is ignored (useless) if m=1 and dt=15 * parameter is ignored (useless) if m=1 and dt=15
* (in/out) cc_tab pointer to a table of the right size to store * (in/out) cc_tab pointer to a table of the right size to store
skipping to change at line 603skipping to change at line 614
<section anchor="CommonProc_linear_combination_computation" numbered="true" toc="default"> <section anchor="CommonProc_linear_combination_computation" numbered="true" toc="default">
<name>Linear Combination of Source Symbol Computation</name> <name>Linear Combination of Source Symbol Computation</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
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.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.
</t></t>
<t> <t>
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.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:For each byte of position i in each source and the repair symbol, where i belongs to [0; E-1], compute:
</t></t>
<ul spacing="normal">
<li> repair[i] = cc_tab[0] * src_0[i] XOR cc_tab[1] * src_1[i] XOR.<sourcecode type="pseudocode">
.. XOR cc_tab[ew_size - 1] *src_ew_size_1[i]</li> repair[i] = cc_tab[0] * src_0[i] XOR cc_tab[1] * src_1[i] XOR...
</ul> XOR cc_tab[ew_size - 1] *src_ew_size_1[i]
</sourcecode>
<t> <t>
where * is the multiplication over GF(2^^8).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 <xref format="default"/>).In practice various optimizations need to be used in order to make this computation efficient (see in particular <xref format="default"/>).
</t></t>
<t> <t>
With the RLC over GF(2) FEC Scheme (binary case), a linear combination is computed as follows.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 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].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).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).
</t></t>
skipping to change at line 635skipping to change at line 648
<t> <t>
This fully-specified FEC Scheme defines the Sliding Window Random Linear Codes (RLC) over GF(2^^8).This fully-specified FEC Scheme defines the Sliding Window Random Linear Codes (RLC) over GF(2^^8).
</t></t>
<section anchor="ArbitraryFlows_formatsAndCodes" numbered="true" toc="default"> <section anchor="ArbitraryFlows_formatsAndCodes" numbered="true" toc="default">
<name>Formats and Codes</name> <name>Formats and Codes</name>
<!-- ==================================== --> <!-- ==================================== -->
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>FEC Framework Configuration Information</name> <name>FEC Framework Configuration Information</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
Following the guidelines of <xrefformat="default"/>, section 5Following the guidelines of <xrefformat="default"
.6, this section providessectionFormat="of" section="5.6"/>, this section provides
the FEC Framework Configuration Information (or FFCI).the FEC Framework Configuration Information (or FFCI).
This FCCI needs to be shared (e.g., using SDP) between the FECFRAME sender and receiverThis FCCI needs to be shared (e.g., using SDP) between the FECFRAME sender and receiver
instances in order to synchronize them.instances in order to synchronize them.
It includes a FEC Encoding ID, mandatory for any FEC Scheme specification, plus scheme-specific elements.It includes a FEC Encoding ID, mandatory for any FEC Scheme specification, plus scheme-specific elements.
</t></t>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>FEC Encoding ID</name> <name>FEC Encoding ID</name>
<!-- ================ --> <!-- ================ -->
<ul spacing="normal"><dl>
<li>FEC EncodingID: <dt>FEC EncodingID:</dt>
the value assigned to this fully specified FEC SchemeMUST be XXXX, <dd>the value assigned to this fully specified FEC Scheme<bcp14>MUST</bc
as assigned by IANA (<xrefformat="default"/>).</li>p14> be XXXX,
</ul> as assigned by IANA (<xrefformat="default"/>).</dd>
</dl>
<t> <t>
When SDP is used to communicate the FFCI, this FEC Encoding ID is carried inWhen SDP is used to communicate the FFCI, this FEC Encoding ID is carried in
the 'encoding-id' parameter.the 'encoding-id' parameter.
</t></t>
</section> </section>
<section anchor="ArbitraryFlows_fssi" numbered="true" toc="default"> <section anchor="ArbitraryFlows_fssi" numbered="true" toc="default">
<name>FEC Scheme-Specific Information</name> <name>FEC Scheme-Specific Information</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
The FEC Scheme-Specific Information (FSSI) includes elements that are specific to the present FEC Scheme.The FEC Scheme-Specific Information (FSSI) includes elements that are specific to the present FEC Scheme.
skipping to change at line 679skipping to change at line 693
A value between 1 and 255 inclusive is required by certain of the parameter derivation techniques described in <xref format="default"/>;</dd> A value between 1 and 255 inclusive is required by certain of the parameter derivation techniques described in <xref format="default"/>;</dd>
</dl> </dl>
<t> <t>
This element is required both by the sender (RLC encoder) and the receiver(s) (RLC decoder).This element is required both by the sender (RLC encoder) and the receiver(s) (RLC decoder).
</t></t>
<t> <t>
When SDP is used to communicate the FFCI, this FEC Scheme-specific information is carried inWhen SDP is used to communicate the FFCI, this FEC Scheme-specific information is carried in
the 'fssi' parameter in textual representation as specified in <xref format="default"/>.the 'fssi' parameter in textual representation as specified in <xref format="default"/>.
For instance:For instance:
</t></t>
<t><sourcecode type="sdp">
fssi=E:1400,WSR:191 fssi=E:1400,WSR:191
</t></sourcecode>
<t> <t>
In that case the name values "E" and "WSR" are used to convey the E and WSR parameters respectively.In that case the name values "E" and "WSR" are used to convey the E and WSR parameters respectively.
</t></t>
<t> <t>
If another mechanism requires the FSSI to be carried as an opaque octet string, the encoding format consistsIf 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,of the following three octets, where the E field is carried in "big-endian" or "network order" format, that is,
most significant byte first:most significant byte first:
</t></t>
<dl newline="false" spacing="normal"> <dl newline="false" spacing="normal">
skipping to change at line 708skipping to change at line 722
These three octets can be communicated as such, or for instance, be subject to an additional Base64 encoding.These three octets can be communicated as such, or for instance, be subject to an additional Base64 encoding.
</t></t>
<figure anchor="fig_ArbitraryFlows_fssi_binary"> <figure anchor="fig_ArbitraryFlows_fssi_binary">
<name>FSSI Encoding Format</name> <name>FSSI Encoding Format</name>
<artwork name="" type="" align="left" alt=""><![CDATA[ <artwork name="" type="" align="left" alt=""><![CDATA[
0 1 2 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 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 || Encoding Symbol Length (E) | WSR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>]]></artwork>
</figure> </figure>
</section> </section>
</section> </section>
<section anchor="ArbitraryFlows_src_fpi" numbered="true" toc="default"> <section anchor="ArbitraryFlows_src_fpi" numbered="true" toc="default">
<name>Explicit Source FEC Payload ID</name> <name>Explicit Source FEC Payload ID</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
A FEC Source PacketMUST contain an Explicit Source FEC Payload ID that is appended to theA FEC Source Packet<bcp14>MUST</bcp14> contain an Explicit Source FEC Payload ID that is appended to the
end of the packet as illustrated in <xref format="default"/>.end of the packet as illustrated in <xref format="default"/>.
</t></t>
<figure anchor="fig_src_pkt_format"> <figure anchor="fig_src_pkt_format">
<name>Structure of an FEC Source Packet with the Explicit Source FEC Payload ID</name> <name>Structure of an FEC Source Packet with the Explicit Source FEC Payload ID</name>
<artwork name="" type="" align="left" alt=""><![CDATA[ <artwork name="" type="" align="left" alt=""><![CDATA[
+--------------------------------++--------------------------------+
| IP Header || IP Header |
+--------------------------------++--------------------------------+
| Transport Header || Transport Header |
+--------------------------------++--------------------------------+
| ADU || ADU |
+--------------------------------++--------------------------------+
| Explicit Source FEC Payload ID || Explicit Source FEC Payload ID |
+--------------------------------++--------------------------------+
]]></artwork>]]></artwork>
</figure> </figure>
<t> <t>
More precisely, the Explicit Source FEC Payload ID is composed of the following field,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 firstcarried in "big-endian" or "network order" format, that is, most significant byte first
(<xref format="default"/>):(<xref format="default"/>):
</t></t>
<dl newline="false" spacing="normal"> <dl newline="false" spacing="normal">
<dt>Encoding Symbol ID (ESI) (32-bit field):</dt> <dt>Encoding Symbol ID (ESI) (32-bit field):</dt>
<dd> <dd>
this unsigned integer identifies the first source symbol of the ADUI corresponding to this FEC Source Packet. this unsigned integer identifies the first source symbol of the ADUI corresponding to this FEC Source Packet.
skipping to change at line 754skipping to change at line 768
</dd> </dd>
</dl> </dl>
<figure anchor="fig_src_fpi"> <figure anchor="fig_src_fpi">
<name>Source FEC Payload ID Encoding Format</name> <name>Source FEC Payload ID Encoding Format</name>
<artwork name="" type="" align="left" alt=""><![CDATA[ <artwork name="" type="" align="left" alt=""><![CDATA[
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol ID (ESI) || Encoding Symbol ID (ESI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>]]></artwork>
</figure> </figure>
</section> </section>
<section anchor="ArbitraryFlows_repair_fpi" numbered="true" toc="default"> <section anchor="ArbitraryFlows_repair_fpi" numbered="true" toc="default">
<name>Repair FEC Payload ID</name> <name>Repair FEC Payload ID</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
A FEC Repair PacketMAY contain one or more repair symbols.A FEC Repair Packet<bcp14>MAY</bcp14> contain one or more repair symbols.
When there are several repair symbols, all of themMUST have been generated fromWhen there are several repair symbols, all of them<bcp14>MUST</bcp14> have been
the same encoding window, generated from the same encoding window,
using Repair_Key values that are managed as explained below.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 byA 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 underlyingcomparing 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.transport protocol) and the symbol size, E, communicated in the FFCI.
</t></t>
<t> <t>
A FEC Repair PacketMUST contain a Repair FEC Payload ID that is prepended to theA FEC Repair Packet<bcp14>MUST</bcp14> contain a Repair FEC Payload ID that is prepended to the
repair symbol as illustrated in <xref format="default"/>.repair symbol as illustrated in <xref format="default"/>.
</t></t>
<figure anchor="fig_repair_pkt_format"> <figure anchor="fig_repair_pkt_format">
<name>Structure of an FEC Repair Packet with the Repair FEC Payload ID</name> <name>Structure of an FEC Repair Packet with the Repair FEC Payload ID</name>
<artwork name="" type="" align="left" alt=""><![CDATA[ <artwork name="" type="" align="left" alt=""><![CDATA[
+--------------------------------++--------------------------------+
| IP Header || IP Header |
+--------------------------------++--------------------------------+
| Transport Header || Transport Header |
+--------------------------------++--------------------------------+
| Repair FEC Payload ID || Repair FEC Payload ID |
+--------------------------------++--------------------------------+
| Repair Symbol || Repair Symbol |
+--------------------------------++--------------------------------+
]]></artwork>]]></artwork>
</figure> </figure>
<t> <t>
More precisely, the Repair FEC Payload ID is composed of the following fields where all integer fields are carriedMore 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 (<xref format="default"/>):in "big-endian" or "network order" format, that is, most significant byte first (<xref format="default"/>):
</t></t>
<dl newline="false" spacing="normal"> <dl newline="false" spacing="normal">
<dt>Repair_Key (16-bit field):</dt> <dt>Repair_Key (16-bit field):</dt>
<dd> <dd>
this unsigned integer is used as a seed by the coefficient generation function (<xref format="default"/>) this unsigned integer is used as a seed by the coefficient generation function (<xref format="default"/>)
in order to generate the desired number of coding coefficients. in order to generate the desired number of coding coefficients.
skipping to change at line 824skipping to change at line 838
<figure anchor="fig_repair_fpi"> <figure anchor="fig_repair_fpi">
<name>Repair FEC Payload ID Encoding Format</name> <name>Repair FEC Payload ID Encoding Format</name>
<artwork name="" type="" align="left" alt=""><![CDATA[ <artwork name="" type="" align="left" alt=""><![CDATA[
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Repair_Key | DT |NSS (# src symb in ew) || Repair_Key | DT |NSS (# src symb in ew) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FSS_ESI || FSS_ESI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>]]></artwork>
</figure> </figure>
</section> </section>
</section> </section>
<section anchor="ArbitraryFlows_Procedures" numbered="true" toc="default"> <section anchor="ArbitraryFlows_Procedures" numbered="true" toc="default">
<name>Procedures</name> <name>Procedures</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
All the procedures of <xref format="default"/> apply to this FEC Scheme.All the procedures of <xref format="default"/> apply to this FEC Scheme.
</t></t>
</section> </section>
skipping to change at line 852skipping to change at line 866
</t></t>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Formats and Codes</name> <name>Formats and Codes</name>
<!-- ==================================== --> <!-- ==================================== -->
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>FEC Framework Configuration Information</name> <name>FEC Framework Configuration Information</name>
<!-- ================ --> <!-- ================ -->
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>FEC Encoding ID</name> <name>FEC Encoding ID</name>
<!-- ================ --> <!-- ================ -->
<ul spacing="normal"><dl>
<li>FEC EncodingID: <dt>FEC EncodingID:</dt>
the value assigned to this fully specified FEC SchemeMUST be YYYY, <dd>the value assigned to this fully specified FEC Scheme<bcp14>MUST</bc
as assigned by IANA (<xrefformat="default"/>).</li>p14> be YYYY,
</ul> as assigned by IANA (<xrefformat="default"/>).</dd>
</dl>
<t> <t>
When SDP is used to communicate the FFCI, this FEC Encoding ID is carried inWhen SDP is used to communicate the FFCI, this FEC Encoding ID is carried in
the 'encoding-id' parameter.the 'encoding-id' parameter.
</t></t>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>FEC Scheme-Specific Information</name> <name>FEC Scheme-Specific Information</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
All the considerations of <xref format="default"/> apply here.All the considerations of <xref format="default"/> apply here.
skipping to change at line 883skipping to change at line 897
<t> <t>
All the considerations of <xref format="default"/> apply here.All the considerations of <xref format="default"/> apply here.
</t></t>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Repair FEC Payload ID</name> <name>Repair FEC Payload ID</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
All the considerations of <xref format="default"/> apply here, with the only exception that the Repair_Key fieldAll the considerations of <xref format="default"/> 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).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 senderMUST set the Repair_Key field to zero on transmission and a receiver MUST ignore it on receipt.When DT = 15 the FECFRAME sender<bcp14>MUST</bcp14> set the Repair_Key field to zero on transmission and a receiver <bcp14>MUST</bcp14> ignore it on receipt.
</t></t>
</section> </section>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Procedures</name> <name>Procedures</name>
<!-- ================ --> <!-- ================ -->
<t> <t>
All the procedures of <xref format="default"/> apply to this FEC Scheme.All the procedures of <xref format="default"/> apply to this FEC Scheme.
</t></t>
</section> </section>
skipping to change at line 915skipping to change at line 929
<t> <t>
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.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 maximumThen 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.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 (<xref target="CommonProc_coef_generation_func" format="default"/>) in order to generateThis repair key is communicated to the coefficient generation function (<xref target="CommonProc_coef_generation_func" format="default"/>) in order to generate
ew_size coding coefficients.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 coefficientsFinally, the FECFRAME sender computes the repair symbol as a linear combination of the ew_size source symbols using the ew_size coding coefficients
(<xref format="default"/>).(<xref format="default"/>).
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 symbolsWhen 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.are computed.
In that case the repair key for each of themMUST be incremented by 1, keeping the same ew_size source symbols, since only the first repair key willIn that case the repair key for each of them<bcp14>MUST</bcp14> 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.be carried in the Repair FEC Payload ID.
The FEC Repair Packet can then be passed to the transport layer for transmission.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.The source versus repair FEC packet transmission order is out of scope of this document and several approaches exist that are implementation-specific.
</t></t>
<t> <t>
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.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,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.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.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.
</t></t>
skipping to change at line 946skipping to change at line 960
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 systemFor 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).(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 functionThis equation of course re-uses the ew_size coding coefficients that are computed by the same coefficient generation function
(<xref format="default"/>), using the repair key and encoding window descriptions carried in the Repair FEC Payload ID.(<xref format="default"/>), 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 recoverWhenever 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.lost source symbols.
Gaussian elimination is one possible algorithm to solve this linear system.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 correspondingEach 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).application flow (thanks to the Flow ID field, F, of the ADUI).
This ADU is finally passed to the corresponding upper application.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 toReceived FEC Source Packets, containing an ADU,<bcp14>MAY</bcp14> be passed to the application either immediately or after some time to guaranty an ordered delivery to
the application.the application.
This document does not mandate any approach as this is an operational and management decision.This document does not mandate any approach as this is an operational and management decision.
</t></t>
<t> <t>
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.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 raises the question of deciding whether or not an ADU is late.
This decisionMAY be taken within the FECFRAME receiver (e.g., using the decoding window, see <xref format="default"/>)This decision<bcp14>MAY</bcp14> be taken within the FECFRAME receiver (e.g., using the decoding window, see <xref format="default"/>)
or within the application (e.g., using RTP timestamps within the ADU).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 dependDeciding 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.on the application and are therefore out of scope of this document.
Additionally, <xref format="default"/> discusses a backward compatible optimization whereby late source symbolsMAY still be used withinAdditionally, <xref format="default"/> discusses a backward compatible optimization whereby late source symbols<bcp14>MAY</bcp14> still be used within
the FECFRAME receiver in order to improve transmission robustness.the FECFRAME receiver in order to improve transmission robustness.
</t></t>
</section> </section>
</section> </section>
<!-- =========================================================================================== --> <!-- =========================================================================================== -->
<section anchor="SecurityConsiderations" numbered="true" toc="default"> <section anchor="SecurityConsiderations" numbered="true" toc="default">
<name>Security Considerations</name> <name>Security Considerations</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
The FEC Framework document <xref format="default"/> provides a fairly comprehensiveThe FEC Framework document <xref format="default"/> provides a fairly comprehensive
skipping to change at line 980skipping to change at line 994
<xref format="default"/> and only discusses specific topics.<xref format="default"/> and only discusses specific topics.
</t></t>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Attacks Against the Data Flow</name> <name>Attacks Against the Data Flow</name>
<!-- ====================== --> <!-- ====================== -->
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Access to Confidential Content</name> <name>Access to Confidential Content</name>
<!-- ====================== --> <!-- ====================== -->
<t>The Sliding Window RLC FEC Scheme specified in this document does not change the <t>The Sliding Window RLC FEC Scheme specified in this document does not change the
recommendations of <xref format="default"/>.recommendations of <xref format="default"/>.
To summarize, if confidentiality is a concern, it isRECOMMENDEDthat one of theTo summarize, if confidentiality is a concern, it is<bcp14>RECOMMENDED</bcp14>that one of the
solutions mentioned in <xref format="default"/> is used with specialsolutions mentioned in <xref format="default"/> is used with special
considerations to the way this solution is applied (e.g., is encryption appliedconsiderations 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 operationalbefore 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 beconstraints (e.g., performing FEC decoding in a protected environment may be
complicated or even impossible) and to the threat model.complicated or even impossible) and to the threat model.
</t></t>
</section> </section>
<section anchor="sec_content_corruption" numbered="true" toc="default"> <section anchor="sec_content_corruption" numbered="true" toc="default">
<name>Content Corruption</name> <name>Content Corruption</name>
<!-- ====================== --> <!-- ====================== -->
<t>The Sliding Window RLC FEC Scheme specified in this document does not change the <t>The Sliding Window RLC FEC Scheme specified in this document does not change the
recommendations of <xref format="default"/>.recommendations of <xref format="default"/>.
To summarize, it isRECOMMENDED that one of the solutions mentioned inTo summarize, it is<bcp14>RECOMMENDED</bcp14> that one of the solutions mentioned in
<xref format="default"/> is used on both the FEC Source and Repair Packets.<xref format="default"/> is used on both the FEC Source and Repair Packets.
</t></t>
</section> </section>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Attacks Against the FEC Parameters</name> <name>Attacks Against the FEC Parameters</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
The FEC Scheme specified in this document defines parameters thatThe FEC Scheme specified in this document defines parameters that
can be the basis of attacks.can be the basis of attacks.
More specifically, the following parameters of the FFCI may be modifiedMore specifically, the following parameters of the FFCI may be modified
by an attacker who targets receivers (<xref format="default"/>):by an attacker who targets receivers (<xref format="default"/>):
</t></t>
<ul spacing="normal"><dl>
<li>FEC EncodingID: <dt>FEC EncodingID:</dt>
changing this parameter leads a receiver to consider adifferent <dd>changing this parameter leads a receiver to consider adiffer
FEC Scheme.ent FEC Scheme.
The consequences are severe, the format of the Explicit Source FEC Payload ID 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 and Repair FEC Payload ID of received packets will probably differ, leading to
various malfunctions. various malfunctions.
Even if the original and modified FEC Schemes share the same format, FEC decoding Even if the original and modified FEC Schemes share the same format, FEC decoding
will either fail or lead to corrupted decoded symbols. will either fail or lead to corrupted decoded symbols.
This will happen if an attacker turns value YYYY (i.e., RLC over GF(2)) to value XXXX (RLC over GF(2^^8)), This will happen if an attacker turns value YYYY (i.e., RLC over GF(2)) to value XXXX (RLC over GF(2^^8)),
an additional consequence being a higher processing overhead at the receiver. 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. In any case, the attack results in a form of Denial of Service (DoS) or corrupted content.
</li></dd>
<li>Encoding symbol length(E): <dt>Encoding symbol length(E):</dt>
setting this E parameter to a different value will confuse arece <dd>setting this E parameter to a different value will confuse a
iver.receiver.
If the size of a received FEC Repair Packet is no longer multiple of the modified E value, If the size of a received FEC Repair Packet is no longer multiple of the modified E value,
a receiver quickly detects a problem andSHOULD reject the packet. a receiver quickly detects a problem and<bcp14>SHOULD</bcp14> reject the packet.
If the new E value is a sub-multiple of the original E value (e.g., half the original value), 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. then receivers may not detect the problem immediately.
For instance, a receiver may think that a received FEC Repair Packet contains more repair symbols 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 (e.g., twice as many if E is reduced by half), leading to malfunctions whose nature depends on
implementation details. implementation details.
Here also, the attack always results in a form of DoS or corrupted content. Here also, the attack always results in a form of DoS or corrupted content.
</li></dd>
</ul> </dl>
<t> <t>
It is thereforeRECOMMENDED that security measures be taken toIt is therefore<bcp14>RECOMMENDED</bcp14> that security measures be taken to
guarantee the FFCI integrity, as specified in <xref format="default"/>.guarantee the FFCI integrity, as specified in <xref format="default"/>.
How to achieve this depends on the way the FFCI is communicated from the senderHow to achieve this depends on the way the FFCI is communicated from the sender
to the receiver, which is not specified in this document.to the receiver, which is not specified in this document.
</t></t>
<t> <t>
Similarly, attacks are possible against the Explicit Source FEC Payload IDSimilarly, attacks are possible against the Explicit Source FEC Payload ID
and Repair 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:More specifically, in case of a FEC Source Packet, the following value can be modified by an attacker who targets receivers:
</t></t>
<ul spacing="normal"><dl>
<li>Encoding Symbol ID(ESI): <dt>Encoding Symbol ID(ESI):</dt>
changing the ESI leads a receiver to consider a wrong ADU,result <dd>changing the ESI leads a receiver to consider a wrong ADU,re
ing in severe consequences, includingsulting in severe consequences, including
corrupted content passed to the receiving application; corrupted content passed to the receiving application;
</li></dd>
</ul> </dl>
<t> <t>
And in case of a FEC Repair Packet:And in case of a FEC Repair Packet:
</t></t>
<ul spacing="normal"><dl>
<li>Repair Key: <dt>Repair Key:</dt>
changing this value leads a receiver to generate a wrongcoding c <dd>changing this value leads a receiver to generate a wrongcodi
oefficient sequence, and thereforeng coefficient sequence, and therefore
any source symbol decoded using the repair symbols contained in this packet will be corrupted; any source symbol decoded using the repair symbols contained in this packet will be corrupted;
</li></dd>
<li>DT: <dt>DT:</dt>
changing this value also leads a receiver to generate a wrongcod <dd>changing this value also leads a receiver to generate a wrong
ing coefficient sequence, and thereforecoding coefficient sequence, and therefore
any source symbol decoded using the repair symbols contained in this packet will be corrupted. 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 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; In case of very large encoding windows, this may impact the terminal performance;
</li></dd>
<li>NSS: <dt>NSS:</dt>
changing this value leads a receiver to consider a different set <dd>changing this value leads a receiver to consider a different
of source symbols, and thereforeset of source symbols, and therefore
any source symbol decoded using the repair symbols contained in this packet will be corrupted. 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, 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; which may impact the terminal performance;
</li></dd>
<li>FSS_ESI: <dt>FSS_ESI:</dt>
changing this value also leads a receiver to consider adifferent <dd>changing this value also leads a receiver to consider adiffe
set of source symbols and thereforerent set of source symbols and therefore
any source symbol decoded using the repair symbols contained in this packet will be corrupted. any source symbol decoded using the repair symbols contained in this packet will be corrupted.
</li></dd>
</ul> </dl>
<t> <t>
It is thereforeRECOMMENDED that security measures are taken to guarantee theIt is therefore<bcp14>RECOMMENDED</bcp14> that security measures are taken to guarantee the
FEC Source and Repair Packets as stated in <xref format="default"/>.FEC Source and Repair Packets as stated in <xref format="default"/>.
</t></t>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>When Several Source Flows are to be Protected Together</name> <name>When Several Source Flows are to be Protected Together</name>
<!-- ====================== --> <!-- ====================== -->
<t>The Sliding Window RLC FEC Scheme specified in this document does not change the <t>The Sliding Window RLC FEC Scheme specified in this document does not change the
recommendations of <xref format="default"/>.</t>recommendations of <xref format="default"/>.</t>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
skipping to change at line 1100skipping to change at line 1114
This is well suited to situations where the only insecure domain is the oneThis is well suited to situations where the only insecure domain is the one
over which the FEC Framework operates.over which the FEC Framework operates.
</t></t>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Additional Security Considerations for Numerical Computations</name> <name>Additional Security Considerations for Numerical Computations</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
In addition to the above security considerations, inherited from <xref format="default"/>,In addition to the above security considerations, inherited from <xref format="default"/>,
the present document introduces several formulae, in particular in <xref target="param_derivation_cbr_realtime" format="default"/>.the present document introduces several formulae, in particular in <xref target="param_derivation_cbr_realtime" format="default"/>.
It isRECOMMENDED to check that the computed values stay within reasonable bounds since numerical overflows,It is<bcp14>RECOMMENDED</bcp14> 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.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.However, what "reasonable bounds" means is use-case and implementation dependent and is not detailed in this document.
</t></t>
<t> <t>
<xref format="default"/> also mentions the possibility of "using the<xref format="default"/> also mentions the possibility of "using the
timestamp field of an RTP packet header" when applicable.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.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.Protection against this type of content corruption can be addressed with the above recommendations on a baseline secure operation.
In addition, it is alsoRECOMMENDED to check that the timestamp value be withinreasonable bounds.In addition, it is also<bcp14>RECOMMENDED</bcp14> to check that the timestamp value be withinreasonable bounds.
</t></t>
</section> </section>
</section> </section>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Operations and Management Considerations</name> <name>Operations and Management Considerations</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
The FEC Framework document <xref format="default"/> provides a fairly comprehensiveThe FEC Framework document <xref format="default"/> provides a fairly comprehensive
analysis of operations and management considerations applicable to FEC Schemes.analysis of operations and management considerations applicable to FEC Schemes.
Therefore, the present section only discusses specific topics.Therefore, the present section only discusses specific topics.
skipping to change at line 1182skipping to change at line 1196
<references> <references>
<name>References</name> <name>References</name>
<references> <references>
<name>Normative References</name> <name>Normative References</name>
<!-- ====================== --> <!-- ====================== -->
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/> <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/> <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6363.xml"/> <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6363.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6364.xml"/> <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6364.xml"/>
<!-- <reference anchor="fecframe-ext">; RFC Ed Queue --> <!-- <reference anchor="fecframe-ext">; RFC Ed Queue -->
<reference anchor="RFCYYY1"> <reference anchor="RFCYYY1">
<front> <front>
<title>Forward Error Correction (FEC) Framework Extension to Sliding Window Codes</title> <title>Forward Error Correction (FEC) Framework Extension to Sliding Window Codes</title>
<seriesInfo name="DOI" value="10.17487/RFCYYY1"/> <seriesInfo name="DOI" value="10.17487/RFCYYY1"/>
<seriesInfo name="RFC" value="YYY1"/> <seriesInfo name="RFC" value="YYY1"/>
<author initials="V" surname="Roca" fullname="Vincent Roca"> <author initials="V" surname="Roca" fullname="Vincent Roca">
<organization/> <organization/>
</author> </author>
<author initials="A" surname="Begen" fullname="Ali Begen"> <author initials="A" surname="Begen" fullname="Ali Begen">
<organization/> <organization/>
</author> </author>
<date month="September" year="2019"/> <date month="September" year="2019"/>
</front> </front>
</reference> </reference>
<!-- <reference anchor="tinymt32">; Replaced by draft-ietf-tsvwg-tinymt32; RFC Ed Queue --> <!-- <reference anchor="tinymt32">; Replaced by draft-ietf-tsvwg-tinymt32; RFC Ed Queue -->
<reference anchor="RFCYYY2"> <reference anchor="RFCYYY2">
<front> <front>
<title>TinyMT32 Pseudo Random Number Generator (PRNG)</title> <title>TinyMT32 Pseudo Random Number Generator (PRNG)</title>
<seriesInfo name="DOI" value="10.17487/RFCYYY2"/> <seriesInfo name="DOI" value="10.17487/RFCYYY2"/>
<seriesInfo name="RFC" value="YYY2"/> <seriesInfo name="RFC" value="YYY2"/>
<author initials="M" surname="Saito" fullname="Mutsuo Saito"> <author initials="M" surname="Saito" fullname="Mutsuo Saito">
<organization/> <organization/>
</author> </author>
<author initials="M" surname="Matsumoto" fullname="Makoto Matsumoto"> <author initials="M" surname="Matsumoto" fullname="Makoto Matsumoto">
<organization/> <organization/>
</author> </author>
skipping to change at line 1329skipping to change at line 1343
</front> </front>
</reference> </reference>
</references> </references>
</references> </references>
<!-- ====================== --> <!-- ====================== -->
<section anchor="annex_tinymt32_validation" numbered="true" toc="default"> <section anchor="annex_tinymt32_validation" numbered="true" toc="default">
<name>TinyMT32 Validation Criteria (Normative)</name> <name>TinyMT32 Validation Criteria (Normative)</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
PRNG determinism, for a given seed, is a requirement.PRNG determinism, for a given seed, is a requirement.
Consequently, in order to validate an implementation of the TinyMT32 PRNG, the following criteriaMUST be met.Consequently, in order to validate an implementation of the TinyMT32 PRNG, the following criteria<bcp14>MUST</bcp14> be met.
</t></t>
<t> <t>
The first criterion focusses on the tinymt32_rand256(), where the 32-bit integer of the core TinyMT32 PRNG is scaled down to an 8-bit integer.The first criterion focusses 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 integersUsing 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 <xref format="default"/>, to be read line by line.<bcp14>MUST</bcp14> be equal to values provided in <xref format="default"/>, to be read line by line.
</t></t>
<!-- [rfced] Would more concise figure titles be helpful for readers, <!-- [rfced] Would more concise figure titles be helpful for readers,
especially since the same information is in the text preceding the figures?especially since the same information is in the text preceding the figures?
OriginalOriginal
Figure 9: First 50 decimal values (to be read per line) returned by 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. tinymt32_rand256() as 8-bit unsigned integers, with a seed value of 1.
Figure 10: First 50 decimal values (to be read per line) returned by 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. tinymt32_rand16() as 4-bit unsigned integers, with a seed value of 1.
skipping to change at line 1384skipping to change at line 1398
214 254 101 212 211 214 254 101 212 211
226 41 234 232 203 226 41 234 232 203
29 194 211 112 107 29 194 211 112 107
217 104 197 135 23 217 104 197 135 23
89 210 252 109 166 89 210 252 109 166
]]></artwork>]]></artwork>
</figure> </figure>
<t> <t>
The second criterion focusses on the tinymt32_rand16(), where the 32-bit integer of the core TinyMT32 PRNG is scaled down to a 4-bit integer.The second criterion focusses 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 integersUsing 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 <xref format="default"/>, to be read line by line.<bcp14>MUST</bcp14> be equal to values provided in <xref format="default"/>, to be read line by line.
</t></t>
<figure anchor="fig_tinymt32_out_truncated_16"> <figure anchor="fig_tinymt32_out_truncated_16">
<name>First 50 decimal values (to be read per line) returned by tinymt32_rand16() as 4-bit unsigned integers, with a seed value of 1.</name> <name>First 50 decimal values (to be read per line) returned by tinymt32_rand16() as 4-bit unsigned integers, with a seed value of 1.</name>
<artwork name="" type="" align="left" alt=""><![CDATA[ <artwork name="" type="" align="left" alt=""><![CDATA[
5 1 1 0 5 5 1 1 0 5
6 6 11 8 13 6 6 11 8 13
3 11 14 14 8 3 11 14 14 8
7 2 3 0 11 7 2 3 0 11
15 3 8 1 3 15 3 8 1 3
6 14 5 4 3 6 14 5 4 3
skipping to change at line 1421skipping to change at line 1435
The two RLC FEC Schemes use the PRNG to produce pseudorandom coding coefficients (<xref format="default"/>), each time a new repair symbol is needed.The two RLC FEC Schemes use the PRNG to produce pseudorandom coding coefficients (<xref format="default"/>), 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 (<xref format="default"/>).A different repair key is used for each repair symbol, usually by incrementing the repair key value (<xref format="default"/>).
For each repair symbol, a limited number of pseudorandom numbers is needed, depending on the DT and encoding window size (<xref format="default"/>), using either tinymt32_rand16() or tinymt32_rand256().For each repair symbol, a limited number of pseudorandom numbers is needed, depending on the DT and encoding window size (<xref format="default"/>), 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.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.
</t></t>
<t> <t>
Evaluation of tinymt32_rand16():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.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.In this first test we consider 32-bit seed values in order to assess the PRNG quality after output truncation to 4 bits.
</t></t>
<figure anchor="fig_tinymt32_out_truncated_16_huge_nb_small_seq">
<name>tinymt32_rand16(): occurrence statistics across a huge number(1,0<!--[rfced] Note: the text that was originally the table's title has
00,000,000) of small sequences (20 pseudorandom numbers per sequence), with 0 asbeen moved to a sentence below the table. Please review and let us
the first PRNGseed.</name>know any updates.
<artwork name="" type="" align="left" alt=""><![CDATA[
Value Occurrences Percentage (%) (total of20000000000)Original:
0 1250036799 6.2502
1 1249995831 6.2500Current:
2 1250038674 6.2502-->
3 1250000881 6.2500
4 1250023929 6.2501<table anchor="table_tinymt32_out_truncated_16_huge_nb_small_seq">
5 1249986320 6.2499<name>tinymt32_rand16() Occurrence Statistics across 1M Tests</name>
6 1249995587 6.2500<thead>
7 1250020363 6.2501<tr><th>Value</th><th> Occurrences</th><th>Percentage (%)</th></tr>
8 1249995276 6.2500</thead>
9 1249982856 6.2499<tbody>
10 1249984111 6.2499<tr><td>0</td> <td>1250036799</td> <td>6.2502</td></tr>
11 1250009551 6.2500<tr><td>1</td> <td>1249995831</td> <td>6.2500</td></tr>
12 1249955768 6.2498<tr><td>2</td> <td>1250038674</td> <td>6.2502</td></tr>
13 1249994654 6.2500<tr><td>3</td> <td>1250000881</td> <td>6.2500</td></tr>
14 1250000569 6.2500<tr><td>4</td> <td>1250023929</td> <td>6.2501</td></tr>
15 1249978831 6.2499<tr><td>5</td> <td>1249986320</td> <td>6.2499</td></tr>
]]></artwork><tr><td>6</td> <td>1249995587</td> <td>6.2500</td></tr>
</figure><tr><td>7</td> <td>1250020363</td> <td>6.2501</td></tr>
<tr><td>8</td> <td>1249995276</td> <td>6.2500</td></tr>
<tr><td>9</td> <td>1249982856</td> <td>6.2499</td></tr>
<tr><td>10</td> <td>1249984111</td> <td>6.2499</td></tr>
<tr><td>11</td> <td>1250009551</td> <td>6.2500</td></tr>
<tr><td>12</td> <td>1249955768</td> <td>6.2498</td></tr>
<tr><td>13</td> <td>1249994654</td> <td>6.2500</td></tr>
<tr><td>14</td> <td>1250000569</td> <td>6.2500</td></tr>
<tr><td>15</td> <td>1249978831</td> <td>6.2499</td></tr>
</tbody></table>
<t>The table above shows tinymt32_rand16() occurrence statistics
across a huge number(1,000,000,000) of small sequences (20
pseudorandom numbers per sequence), with 0 as the first PRNGseed. The
percentage is from a total of20,000,000,000.</t>
<t> <t>
The results (<xref format="default"/>) 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.The results (<xref format="default"/>) 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.
</t></t>
<t> <t>
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.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).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.However, we do not believe it significantly impacts the RLC FEC Scheme behavior.
</t></t>
<t> <t>
Other types of biases may exist that may be visible with smaller tests, for instance to evaluate the convergence speed to a uniform distribution.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 consisting in producing 200 sequences, keeping only the first value of each sequence.We therefore perform 200 tests, each of them consisting in 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.We use non-overlapping repair keys for each sequence, starting with value 0 and increasing it after each use.
skipping to change at line 1480skipping to change at line 1511
10 12 6.000010 12 6.0000
11 11 5.500011 11 5.5000
12 12 6.000012 12 6.0000
13 17 8.500013 17 8.5000
14 13 6.500014 13 6.5000
15 8 4.000015 8 4.0000
]]></artwork>]]></artwork>
</figure></figure>
-->-->
</t></t>
<figure anchor="fig_tinymt32_out_truncated_16_small_nb_small_seq">
<name>tinymt32_rand16(): occurrence statistics across 200 tests, each of<!--[rfced] Note: the text that was originally the table's title has
them consisting in 200 sequences of 1 pseudorandom number each, with nonoverlabeen moved to a sentence below the table. Please review and let us
pping PRNG seeds in sequence starting from0.</name>know any updates.
<artwork name="" type="" align="left" alt=""><![CDATA[
Value Min Occurrences Max Occurrences Average OccurrencesOriginal:
0 4 21 6.3675
1 4 22 6.0200Current:
2 4 20 6.3125-->
3 5 23 6.1775
4 5 24 6.1000<table anchor="table_tinymt32_out_truncated_16_small_nb_small_seq">
5 4 21 6.5925<name>tinymt32_rand16() Occurrence Statistics across 200 Tests</name>
6 5 30 6.3075<thead>
7 6 22 6.2225<tr><th>Value</th><th>Min Occurrences</th><th>Max Occurrences</th><th>Average Oc
8 5 26 6.1750currences</th></tr>
9 3 21 5.9425</thead>
10 5 24 6.3175<tbody>
11 4 22 6.4300<tr><td>0</td> <td>4</td> <td>21</td> <td>6.3675</td></tr>
12 5 21 6.1600<tr><td>1</td> <td>4</td> <td>22</td> <td>6.0200</td></tr>
13 5 22 6.3100<tr><td>2</td> <td>4</td> <td>20</td> <td>6.3125</td></tr>
14 4 26 6.3950<tr><td>3</td> <td>5</td> <td>23</td> <td>6.1775</td></tr>
15 4 21 6.1700<tr><td>4</td> <td>5</td> <td>24</td> <td>6.1000</td></tr>
]]></artwork><tr><td>5</td> <td>4</td> <td>21</td> <td>6.5925</td></tr>
</figure><tr><td>6</td> <td>5</td> <td>30</td> <td>6.3075</td></tr>
<tr><td>7</td> <td>6</td> <td>22</td> <td>6.2225</td></tr>
<tr><td>8</td> <td>5</td> <td>26</td> <td>6.1750</td></tr>
<tr><td>9</td> <td>3</td> <td>21</td> <td>5.9425</td></tr>
<tr><td>10 </td> <td>5</td> <td>24</td> <td>6.3175</td></tr>
<tr><td>11 </td> <td>4</td> <td>22</td> <td>6.4300</td></tr>
<tr><td>12 </td> <td>5</td> <td>21</td> <td>6.1600</td></tr>
<tr><td>13 </td> <td>5</td> <td>22</td> <td>6.3100</td></tr>
<tr><td>14 </td> <td>4</td> <td>26</td> <td>6.3950</td></tr>
<tr><td>15 </td> <td>4</td> <td>21</td> <td>6.1700</td></tr>
</tbody>
</table>
<t>
The table above shows tinymt32_rand16() occurrence statistics across 200 tests,
each of them
consisting in 200 sequences of 1 pseudorandom number each, with non
overlapping PRNG seeds in sequence starting from0.
</t>
<t> <t>
<xref format="default"/> 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.<xref format="default"/> 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.Although the distribution is not perfect, there is no major bias.
On the opposite, in the same conditions, the Park-Miller linear congruential PRNG of <xref format="default"/> with a result scaled down to 4-bit values, using seeds in sequence starting from 1, returns systematically 0 as the first value during some time, then after a certain repair key value threshold, it systematically returns 1, etc.On the opposite, in the same conditions, the Park-Miller linear congruential PRNG of <xref format="default"/> with a result scaled down to 4-bit values, using seeds in sequence starting from 1, returns systematically 0 as the first value during some time, then after a certain repair key value threshold, it systematically returns 1, etc.
</t></t>
<t> <t>
Evaluation of tinymt32_rand256():Evaluation of tinymt32_rand256():
The same approach is used here.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%).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.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.
</t></t>
</section> </section>
skipping to change at line 1549skipping to change at line 1597
<t> <t>
In the following, we consider a real-time flow with max_lat latency budget.In the following, we consider a real-time flow with max_lat latency budget.
The encoding symbol size, E, is constant.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).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).
</t></t>
<t> <t>
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.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.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:In order to comply with the maximum FEC-related latency budget, we have:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> dw_max_size = (max_lat * br_in) / (8 * E) </li> <li> dw_max_size = (max_lat * br_in) / (8 * E) </li>
</ul> </ul>
<t> <t>
assuming that the encoding and decoding times are negligible with respect to the target max_lat.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 <xref format="default"/> in case of small window sizes).This is a reasonable assumption in many situations (e.g., see <xref format="default"/> in case of small window sizes).
Otherwise the max_lat parameter should be adjusted in order to avoid the problem.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 any case, interoperability will never be compromised by choosing a too large value.
</t></t>
<t> <t>
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 <xref format="default"/>.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 <xref format="default"/>.
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.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:We have:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> dw_max_size = (max_lat * br_out * cr) / (8 * E) </li> <li> dw_max_size = (max_lat * br_out * cr) / (8 * E) </li>
</ul> </ul>
<t> <t>
assuming here also that the encoding and decoding times are negligible with respect to the target max_lat.assuming here also that the encoding and decoding times are negligible with respect to the target max_lat.
</t></t>
<t> <t>
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.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 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:The ew_max_size parameter is computed as follows:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> ew_max_size = dw_max_size * WSR / 255</li> <li> ew_max_size = dw_max_size * WSR / 255</li>
<!-- <t> ew_max_size = dw_max_size * 0.75 </t> --> <!-- <t> ew_max_size = dw_max_size * 0.75 </t> -->
</ul> </ul>
<t> <t>
In line with <xref format="default"/>, 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.In line with <xref format="default"/>, 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.
<!-- It is always RECOMMENDED to check that the ew_max_size value stays within reasonable bounds in order to avoid hazardous behaviours. --><!-- It is always RECOMMENDED to check that the ew_max_size value stays within reasonable bounds in order to avoid hazardous behaviours. -->
<!--<!--
However, any value ew_max_size &lt; dw_max_size can be used without impact on the FEC-related latency budget.However, any value ew_max_size &lt; dw_max_size can be used without impact on the FEC-related latency budget.
Another value could be determined depending on the use-case details, which is out of scope of this document.Another value could be determined depending on the use-case details, which is out of scope of this document.
Whenever the FEC protection (i.e., cr value) is sufficient in front of the experienced packet losses, the ew_max_size guaranties that the recovery of lost ADUs will happen at a FECFRAME receiver on time.Whenever the FEC protection (i.e., cr value) is sufficient in front of the experienced packet losses, the ew_max_size guaranties that the recovery of lost ADUs will happen at a FECFRAME receiver on time.
-->-->
</t></t>
<t> <t>
The dw_max_size is computed by a FECFRAME sender but not explicitly communicated to a FECFRAME receiver.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 (<xref format="default"/>).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 (<xref format="default"/>).
A receiver can then easily compute dw_max_size:A receiver can then easily compute dw_max_size:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> dw_max_size = max_NSS_observed * 255 / WSR </li> <li> dw_max_size = max_NSS_observed * 255 / WSR </li>
<!-- <t> dw_max_size = max_NSS_observed / 0.75 </t> --> <!-- <t> dw_max_size = max_NSS_observed / 0.75 </t> -->
</ul> </ul>
<t> <t>
A receiver can then choose an appropriate linear system maximum size:A receiver can then choose an appropriate linear system maximum size:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> ls_max_size &gt;= dw_max_size </li> <li> ls_max_size &gt;= dw_max_size </li>
</ul> </ul>
<t> <t>
It is good practice to use a larger value for ls_max_size as explained in <xref target="decodingBeyondMaxLatency" format="default"/>, which does not impact maximum latency nor interoperability.It is good practice to use a larger value for ls_max_size as explained in <xref target="decodingBeyondMaxLatency" format="default"/>, which does not impact maximum latency nor interoperability.
</t></t>
<t> <t>
In any case, for a given use-case (i.e., for target encoding and decoding deviceIn any case, for a given use-case (i.e., for target encoding and decoding device
s and desired protection levels in front of communication impairments) and for ts and desired protection levels in front of communication impairments) and for t
he computed ew_max_size, dw_max_size and ls_max_size values, it isRECOMMENDED the computed ew_max_size, dw_max_size and ls_max_size values, it is<bcp14>RECOMM
o check that the maximum encoding time and maximum memoryrequirements at aFECFENDED</bcp14> to check that the maximum encoding time and maximum memoryrequire
RAME sender, and maximum decoding time and maximum memoryrequirements at aFECFments at aFECFRAME sender, and maximum decoding time and maximum memoryrequire
RAME receiver, stay within reasonable bounds.ments at aFECFRAME receiver, stay within reasonable bounds.
When assuming that the encoding and decoding times are negligible with respect tWhen assuming that the encoding and decoding times are negligible with respect t
o the target max_lat, this should be verified as well, otherwise the max_latSHOo the target max_lat, this should be verified as well, otherwise the max_lat<bc
ULD be adjusted accordingly.p14>SHOULD</bcp14> be adjusted accordingly.
</t></t>
<t> <t>
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.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 receiverSHOULD 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.Therefore, a FECFRAME receiver<bcp14>SHOULD</bcp14> 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.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.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.No adjustment of the ls_max_size is required at the FECFRAME receiver in that case.
</t></t>
</section> </section>
<section anchor="param_derivation_other_realtime_flows" numbered="true" toc="default"> <section anchor="param_derivation_other_realtime_flows" numbered="true" toc="default">
<name>Other Types of Real-Time Flow</name> <name>Other Types of Real-Time Flow</name>
<!-- ====================== --> <!-- ====================== -->
<t> <t>
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.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 <xref format="default"/>.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 <xref format="default"/>.
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.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.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.
</t></t>
<t> <t>
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).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.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 budgetSHOULD be computed with:For the FEC Schemes specified in this document, this latency budget<bcp14>SHOULD</bcp14> be computed with:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> max_lat_for_encoding = max_lat * WSR / 255 </li> <li> max_lat_for_encoding = max_lat * WSR / 255 </li>
<!-- <t> max_lat_for_encoding = max_lat * 0.75 </t> --> <!-- <t> max_lat_for_encoding = max_lat * 0.75 </t> -->
</ul> </ul>
<t> <t>
It follows that any source symbols associated to an ADU that has timed-out with respect to max_lat_for_encodingSHOULD be removed from the encoding window.It follows that any source symbols associated to an ADU that has timed-out with respect to max_lat_for_encoding<bcp14>SHOULD</bcp14> 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.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 (<xref format="default"/>).For practical reasons, a FECFRAME sender may still require that ew_size does not increase beyond a maximum value (<xref format="default"/>).
</t></t>
<t> <t>
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.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 <xref format="default"/>.A receiver can then compute dw_max_size and derive an appropriate ls_max_size as explained in <xref format="default"/>.
</t></t>
<t> <t>
When the observed NSS fluctuates significantly, a FECFRAME receiver may want to adapt its ls_max_size accordingly.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.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.
skipping to change at line 1687skipping to change at line 1735
<!-- ====================== --> <!-- ====================== -->
<t> <t>
This annex introduces non-normative considerations.This annex introduces non-normative considerations.
It is provided as suggestions, without any impact on interoperability.It is provided as suggestions, without any impact on interoperability.
For more information see <xref format="default"/>.For more information see <xref format="default"/>.
</t></t>
<t> <t>
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.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: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:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> ls_max_size &gt; dw_max_size </li> <li> ls_max_size &gt; dw_max_size </li>
</ul> </ul>
<t> <t>
Usually the following choice is a good trade-off between decoding performance and extra CPU overhead:Usually the following choice is a good trade-off between decoding performance and extra CPU overhead:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> ls_max_size = 2 * dw_max_size </li> <li> ls_max_size = 2 * dw_max_size </li>
</ul> </ul>
<t> <t>
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).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.Going below this threshold will not save a significant amount of memory nor CPU cycles.
Therefore:Therefore:
</t></t>
<ul spacing="normal"> <ulempty="true"spacing="normal">
<li> ls_max_size = max(2 * dw_max_size, LS_MIN_SIZE_DEFAULT) </li> <li> ls_max_size = max(2 * dw_max_size, LS_MIN_SIZE_DEFAULT) </li>
</ul> </ul>
<t> <t>
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.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.In that case lost ADUs will be recovered without relying on this optimization.
</t></t>
<figure anchor="fig_decoding_beyond_max_laetency"> <figure anchor="fig_decoding_beyond_max_laetency">
<name>Relationship between Parameters to Decode beyond Maximum Latency</name> <name>Relationship between Parameters to Decode beyond Maximum Latency</name>
<artwork name="" type="" align="left" alt=""><![CDATA[ <artwork name="" type="" align="left" alt=""><![CDATA[
ls_max_size ls_max_size
/---------------------------------^-------------------------------\/---------------------------------^-------------------------------\
late source symbols late source symbols
(pot. decoded but not delivered) dw_max_size (pot. decoded but not delivered) dw_max_size
/--------------^-----------------\ /--------------^---------------\/--------------^-----------------\ /--------------^---------------\
src0 src1 src2 src3 src4 src5 src6 src7 src8 src9 src10 src11 src12src0 src1 src2 src3 src4 src5 src6 src7 src8 src9 src10 src11 src12
]]></artwork>]]></artwork>
</figure> </figure>
<t> <t>
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 <xref format="default"/>).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 <xref format="default"/>).
It follows that the corresponding ADUs will not be useful to the application.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 <xref format="default"/>.However, decoding these "late symbols" significantly improves the global robustness in bad reception conditions and is therefore recommended for receivers experiencing bad communication conditions <xref format="default"/>.
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.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.
</t></t>
</section> </section>
<section numbered="false" toc="default"> <section numbered="false" toc="default">
<name>Acknowledgments</name> <name>Acknowledgments</name>
 End of changes. 87 change blocks. 
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