Internet Engineering Task Force (IETF) V. Roca

Internet Engineering Task Force (IETF) V. Roca
Request for Comments: 8680 INRIA
Updates: 6363 A. Begen
Category: Standards Track Networked Media
ISSN: 2070-1721 January 2020

Forward Error Correction (FEC) Framework Extension to Sliding Window
Codes

Abstract

RFC 6363 describes a framework for using Forward Error Correction
(FEC) codes to provide protection against packet loss. The framework
supports applying FEC to arbitrary packet flows over unreliable
transport and is primarily intended for real-time, or streaming,
media. However, FECFRAME as per RFC 6363 is restricted to block FEC
codes. This document updates RFC 6363 to support FEC codes based on
a sliding encoding window, in addition to block FEC codes, in a
backward-compatible way. During multicast/broadcast real-time
content delivery, the use of sliding window codes significantly
improves robustness in harsh environments, with less repair traffic
and lower FEC-related added latency.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8680.

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
2. Terminology
2.1. Definitions and Abbreviations
2.2. Requirements Language
3. Summary of Architecture Overview
4. Procedural Overview
4.1. General
4.2. Sender Operation with Sliding Window FEC Codes
4.3. Receiver Operation with Sliding Window FEC Codes
5. Protocol Specification
5.1. General
5.2. FEC Framework Configuration Information
5.3. FEC Scheme Requirements
6. Feedback
7. Transport Protocols
8. Congestion Control
9. Security Considerations
10. Operations and Management Considerations
11. IANA Considerations
12. References
12.1. Normative References
12.2. Informative References
Appendix A. About Sliding Encoding Window Management
(Informational)
Acknowledgments
Authors' Addresses

1. Introduction

Many applications need to transport a continuous stream of packetized
data from a source (sender) to one or more destinations (receivers)
over networks that do not provide guaranteed packet delivery. In
particular, packets may be lost, which is strictly the focus of this
document: we assume that transmitted packets are either lost (e.g.,
because of a congested router, a poor signal-to-noise ratio in a
wireless network, or because the number of bit errors exceeds the
correction capabilities of the physical-layer error-correcting code)
or were received by the transport protocol without any corruption
(i.e., the bit errors, if any, have been fixed by the physical-layer
error-correcting code and therefore are hidden to the upper layers).

For these use cases, Forward Error Correction (FEC) applied within
the transport or application layer is an efficient technique to
improve packet transmission robustness in the presence of packet
losses (or "erasures") without going through packet retransmissions
that create a delay often incompatible with real-time constraints.
The FEC Building Block defined in [RFC5052] provides a framework for
the definition of Content Delivery Protocols (CDPs) that make use of
separately defined FEC schemes. Any CDP defined according to the
requirements of the FEC Building Block can then easily be used with
any FEC scheme that is also defined according to the requirements of
the FEC Building Block.

Then, FECFRAME [RFC6363] provides a framework to define Content
Delivery Protocols (CDPs) that provide FEC protection for arbitrary
packet flows over an unreliable datagram service transport, such as
UDP. It is primarily intended for real-time or streaming media
applications that are using broadcast, multicast, or on-demand
delivery. A subset of FECFRAME is currently part of the 3GPP Evolved
Multimedia Broadcast/Multicast Service (eMBMS) standard [MBMSTS].

However, [RFC6363] only considers block FEC schemes defined in
accordance with the FEC Building Block [RFC5052] (e.g., [RFC6681],
[RFC6816], or [RFC6865]). These codes require the input flow(s) to
be segmented into a sequence of blocks. Then, FEC encoding (at a
sender or an encoding middlebox) and decoding (at a receiver or a
decoding middlebox) are both performed on a per-block basis. For
instance, if the current block encompasses the 100's to 119's source
symbols (i.e., a block of size 20 symbols) of an input flow, encoding
(and decoding) will be performed on this block independently of other
blocks. This approach has major impacts on FEC encoding and decoding
delays. The data packets of continuous media flow(s) may be passed
to the transport layer immediately, without delay. But the block
creation time, which depends on the number of source symbols in this
block, impacts both the FEC encoding delay (since encoding requires
that all source symbols be known) and, mechanically, the packet loss
recovery delay at a receiver (since no repair symbol for the current
block can be generated and therefore received before that time).
Therefore, a good value for the block size is necessarily a balance
between the maximum FEC decoding latency at the receivers (which must
be in line with the most stringent real-time requirement of the
protected flow(s), hence an incentive to reduce the block size) and
the desired robustness against long loss bursts (which increases with
the block size, hence an incentive to increase this size).

This document updates [RFC6363] in order to also support FEC codes
based on a sliding encoding window (a.k.a., convolutional codes)
[RFC8406]. This encoding window, either fixed or variable size,
slides over the set of source symbols. FEC encoding is launched
whenever needed from the set of source symbols present in the sliding
encoding window at that time. This approach significantly reduces
FEC-related latency, since repair symbols can be generated and passed
to the transport layer on the fly at any time and can be regularly
received by receivers to quickly recover packet losses. Using
sliding window FEC codes is therefore highly beneficial to real-time
flows, one of the primary targets of FECFRAME. [RFC8681] provides an
example of such a FEC scheme for FECFRAME, which is built upon the
simple sliding window Random Linear Code (RLC).

This document is fully backward compatible with [RFC6363]. Indeed:

* This FECFRAME update does not prevent or compromise in any way the
support of block FEC codes. Both types of codes can nicely
coexist, just like different block FEC schemes can coexist.

* Each sliding window FEC scheme is associated with a specific FEC
Encoding ID subject to IANA registration, just like block FEC
schemes.

* Any receiver -- for instance, a legacy receiver that only supports
block FEC schemes -- can easily identify the FEC scheme used in a
FECFRAME session. Indeed, the FEC Encoding ID that identifies the
FEC scheme is carried in FEC Framework Configuration Information
(see Section 5.5 of [RFC6363]). For instance, when the Session
Description Protocol (SDP) is used to carry the FEC Framework
Configuration Information, the FEC Encoding ID can be communicated
in the "encoding-id=" parameter of a "fec-repair-flow" attribute
[RFC6364]. This mechanism is the basic approach for a FECFRAME
receiver to determine whether or not it supports the FEC scheme
used in a given FECFRAME session.

This document leverages on [RFC6363] and reuses its structure. It
proposes new sections specific to sliding window FEC codes whenever
required. The only exception is Section 3, which provides a quick
summary of FECFRAME in order to facilitate the understanding of this
document to readers not familiar with the concepts and terminology.

2. Terminology

2.1. Definitions and Abbreviations

The following list of definitions and abbreviations is copied from
[RFC6363], adding only the Block FEC Code, Sliding Window FEC Code,
and Encoding/Decoding Window definitions (tagged with "ADDED"):

Application Data Unit (ADU):
The unit of source data provided as a payload to the transport
layer. For instance, it can be a payload containing the result of
the RTP packetization of a compressed video frame.

ADU Flow:
A sequence of ADUs associated with a transport-layer flow
identifier (such as the standard 5-tuple {source IP address,
source port, destination IP address, destination port, transport
protocol}).

AL-FEC:
Application-Layer Forward Error Correction.

Application Protocol:
Control protocol used to establish and control the source flow
being protected, e.g., the Real-Time Streaming Protocol (RTSP).

Content Delivery Protocol (CDP):
A complete application protocol specification that, through the
use of the framework defined in this document, is able to make use
of FEC schemes to provide FEC capabilities.

FEC Code:
An algorithm for encoding data such that the encoded data flow is
resilient to data loss. Note that, in general, FEC codes may also
be used to make a data flow resilient to corruption, but that is
not considered in this document.

Block FEC Code: (ADDED)
A FEC code that operates on blocks, i.e., for which the input flow
MUST be segmented into a sequence of blocks, with FEC encoding and
decoding being performed independently on a per-block basis.

Sliding Window FEC Code: (ADDED)
A FEC code that can generate repair symbols on the fly, at any
time, from the set of source symbols present in the sliding
encoding window at that time. These codes are also known as
convolutional codes.

FEC Framework:
A protocol framework for the definition of Content Delivery
Protocols using FEC, such as the framework defined in this
document.

FEC Framework Configuration Information:
Information that controls the operation of the FEC Framework.

FEC Payload ID:
Information that identifies the contents and provides positional
information of a packet with respect to the FEC scheme.

FEC Repair Packet:
At a sender (respectively, at a receiver), a payload submitted to
(respectively, received from) the transport protocol containing
one or more repair symbols along with a Repair FEC Payload ID and
possibly an RTP header.

FEC Scheme:
A specification that defines the additional protocol aspects
required to use a particular FEC code with the FEC Framework.

FEC Source Packet:
At a sender (respectively, at a receiver), a payload submitted to
(respectively, received from) the transport protocol containing an
ADU along with an optional Explicit Source FEC Payload ID.

Repair Flow:
The packet flow carrying FEC data.

Repair FEC Payload ID:
A FEC Payload ID specifically for use with repair packets.

Source Flow:
The packet flow to which FEC protection is to be applied. A
source flow consists of ADUs.

Source FEC Payload ID:
A FEC Payload ID specifically for use with source packets.

Source Protocol:
A protocol used for the source flow being protected, e.g., RTP.

Transport Protocol:
The protocol used for the transport of the source and repair
flows. This protocol needs to provide an unreliable datagram
service, as UDP does ([RFC6363], Section 7).

Encoding Window: (ADDED)
Set of source symbols available at the sender/coding node that are
used (with a Sliding Window FEC code) to generate a repair symbol.

Decoding Window: (ADDED)
Set of received or decoded source and repair symbols available at
a receiver that are used (with a Sliding Window FEC code) to
decode lost source symbols.

Code Rate:
The ratio between the number of source symbols and the number of
encoding symbols. By definition, the code rate is such that 0 <
code rate <= 1. A code rate close to 1 indicates that a small
number of repair symbols have been produced during the encoding
process.

Encoding Symbol:
Unit of data generated by the encoding process. With systematic
codes, source symbols are part of the encoding symbols.

Packet Erasure Channel:
A communication path where packets are either lost (e.g., in our
case, by a congested router, or because the number of transmission
errors exceeds the correction capabilities of the physical-layer
code) or received. When a packet is received, it is assumed that
this packet is not corrupted (i.e., in our case, the bit errors,
if any, are fixed by the physical-layer code and are therefore
hidden to the upper layers).

Repair Symbol:
Encoding symbol that is not a source symbol.

Source Block:
Group of ADUs that are to be FEC protected as a single block.
This notion is restricted to Block FEC codes.

Source Symbol:
Unit of data used during the encoding process.

Systematic Code:
FEC code in which the source symbols are part of the encoding
symbols.

2.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Summary of Architecture Overview

The architecture of Section 3 of [RFC6363] equally applies to this
FECFRAME extension and is not repeated here. However, this section
includes a quick summary to facilitate the understanding of this
document to readers not familiar with the concepts and terminology.

+----------------------+
| Application |
+----------------------+
|
| (1) Application Data Units (ADUs)
|
v
+----------------------+ +----------------+
| FEC Framework | | |
| |-------------------------->| FEC Scheme |
|(2) Construct source |(3) Source Block | |
| blocks | |(4) FEC Encoding|
|(6) Construct FEC |<--------------------------| |
| Source and Repair | | |
| Packets |(5) Explicit Source FEC | |
+----------------------+ Payload IDs +----------------+
| Repair FEC Payload IDs
| Repair symbols
|
|(7) FEC Source and Repair Packets
v
+----------------------+
| Transport Protocol |
+----------------------+

Figure 1: FECFRAME Architecture at a Sender

The FECFRAME architecture is illustrated in Figure 1 for a block FEC
scheme from the sender's point of view. It shows an application
generating an ADU flow (other flows from other applications may
coexist). These ADUs of variable size must be somehow mapped to
source symbols of a fixed size (this fixed size is a requirement of
all FEC schemes, which comes from the way mathematical operations are
applied to the symbols' content). This is the goal of an ADU-to-
symbols mapping process that is FEC scheme specific (see below).
Once the source block is built, taking into account both the FEC
scheme constraints (e.g., in terms of maximum source block size) and
the application's flow constraints (e.g., in terms of real-time
constraints), the associated source symbols are handed to the FEC
scheme in order to produce an appropriate number of repair symbols.
FEC Source Packets (containing ADUs) and FEC Repair Packets
(containing one or more repair symbols each) are then generated and
sent using an appropriate transport protocol (more precisely,
Section 7 of [RFC6363] requires a transport protocol providing an
unreliable datagram service, such as UDP). In practice, FEC Source
Packets may be passed to the transport layer as soon as available
without having to wait for FEC encoding to take place. In that case,
a copy of the associated source symbols needs to be kept within
FECFRAME for future FEC encoding purposes.

At a receiver (not shown), FECFRAME processing operates in a similar
way, taking as input the incoming FEC Source and Repair Packets
received. In case of FEC Source Packet losses, the FEC decoding of
the associated block may recover all (in case of successful decoding)
or a subset that is potentially empty (if decoding fails) of the
missing source symbols. After source-symbol-to-ADU mapping, when
lost ADUs are recovered, they are then assigned to their respective
flow (see below). ADUs are returned to the application(s), either in
their initial transmission order (in which case all ADUs received
after a lost ADU will be delayed until FEC decoding has taken place)
or not (in which case each ADU is returned as soon as it is received
or recovered), depending on the application requirements.

FECFRAME features two subtle mechanisms whose details are FEC scheme
dependent:

* ADUs-to-source-symbols mapping: in order to manage variable size
ADUs, FECFRAME and FEC schemes can use small, fixed-size symbols
and create a mapping between ADUs and symbols. The mapping
details are FEC scheme dependent and must be defined in the
associated document. For instance, with certain FEC schemes, to
each ADU, this mechanism prepends a length field (plus a flow
identifier; see below) and pads the result to a multiple of the
symbol size. A small ADU may be mapped to a single source symbol,
while a large one may be mapped to multiple symbols.

* Assignment of decoded ADUs to flows in multi-flow configurations:
when multiple flows are multiplexed over the same FECFRAME
instance, a problem is to assign a decoded ADU to the right flow
(UDP port numbers and IP addresses traditionally used to map
incoming ADUs to flows are not recovered during FEC decoding).
The mapping details are FEC scheme dependent and must be defined
in the associated document. For instance, with certain FEC
schemes, to make it possible, at the FECFRAME sending instance,
each ADU is prepended with a flow identifier (1 byte) during the
ADU-to-source-symbols mapping (see above). The flow identifiers
are also shared between all FECFRAME instances as part of the FEC
Framework Configuration Information. The ADU Information (ADUI),
which includes the flow identifier, length, application payload,
and padding, is then FEC protected. Therefore, a decoded ADUI
contains enough information to assign the ADU to the right flow.
Note that a FEC scheme may also be restricted to the particular
case of a single flow over a FECFRAME instance; that would make
the above mechanism pointless.

A few aspects are not covered by FECFRAME, namely:

* Section 8 of [RFC6363] does not detail any congestion control
mechanisms and only provides high-level normative requirements.

* The possibility of having feedback from receiver(s) is considered
out of scope, although such a mechanism may exist within the
application (e.g., through RTP Control Protocol (RTCP) messages).

* Flow adaptation at a FECFRAME sender (e.g., how to set the FEC
code rate based on transmission conditions) is not detailed, but
it needs to comply with the congestion control normative
requirements (see above).

4. Procedural Overview

4.1. General

The general considerations of Section 4.1 of [RFC6363] that are
specific to block FEC codes are not repeated here.

With a Sliding Window FEC code, the FEC Source Packet MUST contain
information to identify the position occupied by the ADU within the
source flow in terms specific to the FEC scheme. This information is
known as the Source FEC Payload ID, and the FEC scheme is responsible
for defining and interpreting it.

With a Sliding Window FEC code, the FEC Repair Packets MUST contain
information that identifies the relationship between the contained
repair payloads and the original source symbols used during encoding.
This information is known as the Repair FEC Payload ID, and the FEC
scheme is responsible for defining and interpreting it.

The sender operation ([RFC6363], Section 4.2) and receiver operation
([RFC6363], Section 4.3) are both specific to block FEC codes and are
therefore omitted below. The following two sections detail similar
operations for Sliding Window FEC codes.

4.2. Sender Operation with Sliding Window FEC Codes

With a Sliding Window FEC scheme, the following operations,
illustrated in Figure 2 for the generic case (non-RTP repair flows)
and in Figure 3 for the case of RTP repair flows, describe a possible
way to generate compliant source and repair flows:

1. A new ADU is provided by the application.

2. The FEC Framework communicates this ADU to the FEC scheme.

3. The sliding encoding window is updated by the FEC scheme. The
ADU-to-source-symbol mapping as well as the encoding window
management details are both the responsibility of the FEC scheme
and MUST be detailed there. Appendix A provides non-normative
hints about what FEC scheme designers need to consider.

4. The Source FEC Payload ID information of the source packet is
determined by the FEC scheme. If required by the FEC scheme,
the Source FEC Payload ID is encoded into the Explicit Source
FEC Payload ID field and returned to the FEC Framework.

5. The FEC Framework constructs the FEC Source Packet according to
Figure 6 in [RFC6363], using the Explicit Source FEC Payload ID
provided by the FEC scheme if applicable.

6. The FEC Source Packet is sent using normal transport-layer
procedures. This packet is sent using the same ADU flow
identification information as would have been used for the
original source packet if the FEC Framework were not present
(e.g., the source and destination addresses and UDP port numbers
on the IP datagram carrying the source packet will be the same
whether or not the FEC Framework is applied).

7. When the FEC Framework needs to send one or several FEC Repair
Packets (e.g., according to the target code rate), it asks the
FEC scheme to create one or several repair packet payloads from
the current sliding encoding window along with their Repair FEC
Payload ID.

8. The Repair FEC Payload IDs and repair packet payloads are
provided back by the FEC scheme to the FEC Framework.

9. The FEC Framework constructs FEC Repair Packets according to
Figure 7 in [RFC6363], using the FEC Payload IDs and repair
packet payloads provided by the FEC scheme.

10. The FEC Repair Packets are sent using normal transport-layer
procedures. The port(s) and multicast group(s) to be used for
FEC Repair Packets are defined in the FEC Framework
Configuration Information.

+----------------------+
| Application |
+----------------------+
|
| (1) New Application Data Unit (ADU)
v
+---------------------+ +----------------+
| FEC Framework | | FEC Scheme |
| |-------------------------->| |
| | (2) New ADU |(3) Update of |
| | | encoding |
| |<--------------------------| window |
|(5) Construct FEC | (4) Explicit Source | |
| Source Packet | FEC Payload ID(s) |(7) FEC |
| |<--------------------------| encoding |
|(9) Construct FEC | (8) Repair FEC Payload ID | |
| Repair Packet(s) | + Repair symbol(s) +----------------+
+---------------------+
|
| (6) FEC Source Packet
| (10) FEC Repair Packets
v
+----------------------+
| Transport Protocol |
+----------------------+

Figure 2: Sender Operation with Sliding Window FEC Codes

+----------------------+
| Application |
+----------------------+
|
| (1) New Application Data Unit (ADU)
v
+---------------------+ +----------------+
| FEC Framework | | FEC Scheme |
| |-------------------------->| |
| | (2) New ADU |(3) Update of |
| | | encoding |
| |<--------------------------| window |
|(5) Construct FEC | (4) Explicit Source | |
| Source Packet | FEC Payload ID(s) |(7) FEC |
| |<--------------------------| encoding |
|(9) Construct FEC | (8) Repair FEC Payload ID | |
| Repair Packet(s) | + Repair symbol(s) +----------------+
+---------------------+
| |
|(6) Source |(10) Repair payloads
| packets |
| + -- -- -- -- -+
| | RTP |
| +-- -- -- -- --+
v v
+----------------------+
| Transport Protocol |
+----------------------+

Figure 3: Sender Operation with Sliding Window FEC Codes and RTP
Repair Flows

4.3. Receiver Operation with Sliding Window FEC Codes

With a Sliding Window FEC scheme, the following operations are
illustrated in Figure 4 for the generic case (non-RTP repair flows)
and in Figure 5 for the case of RTP repair flows. The only
differences with respect to block FEC codes lie in steps (4) and (5).
Therefore, this section does not repeat the other steps of
Section 4.3 of [RFC6363] ("Receiver Operation"). The new steps (4)
and (5) are:

4. The FEC scheme uses the received FEC Payload IDs (and derived FEC
Source Payload IDs when the Explicit Source FEC Payload ID field
is not used) to insert source and repair packets into the
decoding window in the right way. If at least one source packet
is missing and at least one repair packet has been received, then
FEC decoding is attempted to recover the missing source payloads.
The FEC scheme determines whether source packets have been lost
and whether enough repair packets have been received to decode
any or all of the missing source payloads.

5. The FEC scheme returns the received and decoded ADUs to the FEC
Framework, along with indications of any ADUs that were missing
and could not be decoded.

+----------------------+
| Application |
+----------------------+
^
|(6) ADUs
|
+----------------------+ +----------------+
| FEC Framework | | FEC Scheme |
| |<--------------------------| |
|(2)Extract FEC Payload|(5) ADUs |(4) FEC Decoding|
| IDs and pass IDs & |-------------------------->| |
| payloads to FEC |(3) Explicit Source FEC +----------------+
| scheme | Payload IDs
+----------------------+ Repair FEC Payload IDs
^ Source payloads
| Repair payloads
|(1) FEC Source
| and Repair Packets
+----------------------+
| Transport Protocol |
+----------------------+

Figure 4: Receiver Operation with Sliding Window FEC Codes

+----------------------+
| Application |
+----------------------+
^
|(6) ADUs
|
+----------------------+ +----------------+
| FEC Framework | | FEC Scheme |
| |<--------------------------| |
|(2)Extract FEC Payload|(5) ADUs |(4) FEC Decoding|
| IDs and pass IDs & |-------------------------->| |
| payloads to FEC |(3) Explicit Source FEC +----------------+
| scheme | Payload IDs
+----------------------+ Repair FEC Payload IDs
^ ^ Source payloads
| | Repair payloads
|Source pkts |Repair payloads
| |
+-- |- -- -- -- -- -- -+
|RTP| | RTP Processing |
| | +-- -- -- --|-- -+
| +-- -- -- -- -- |--+ |
| | RTP Demux | |
+-- -- -- -- -- -- -- -+
^
|(1) FEC Source and Repair Packets
|
+----------------------+
| Transport Protocol |
+----------------------+

Figure 5: Receiver Operation with Sliding Window FEC Codes and
RTP Repair Flows

5. Protocol Specification

5.1. General

This section discusses the protocol elements for the FEC Framework
specific to Sliding Window FEC schemes. The global formats of source
data packets (i.e., [RFC6363], Figure 6) and repair data packets
(i.e., [RFC6363], Figures 7 and 8) remain the same with Sliding
Window FEC codes. They are not repeated here.

5.2. FEC Framework Configuration Information

The FEC Framework Configuration Information considerations of
Section 5.5 of [RFC6363] equally apply to this FECFRAME extension and
are not repeated here.

5.3. FEC Scheme Requirements

The FEC scheme requirements of Section 5.6 of [RFC6363] mostly apply
to this FECFRAME extension and are not repeated here. An exception,
though, is the "full specification of the FEC code", item (4), which
is specific to block FEC codes. In case of a Sliding Window FEC
scheme, then the following item (4-bis) applies:

4-bis.
A full specification of the Sliding Window FEC code.

This specification MUST precisely define the valid FEC-Scheme-
Specific Information values, the valid FEC Payload ID values, and
the valid packet payload sizes (where "packet payload" refers to
the space within a packet dedicated to carrying encoding
symbols).

Furthermore, given valid values of the FEC-Scheme-Specific
Information, a valid Repair FEC Payload ID value, a valid packet
payload size, and a valid encoding window (i.e., a set of source
symbols), the specification MUST uniquely define the values of
the encoding symbol (or symbols) to be included in the repair
packet payload with the given Repair FEC Payload ID value.

Additionally, the FEC scheme associated with a Sliding Window FEC
code:

* MUST define the relationships between ADUs and the associated
source symbols (mapping).

* MUST define the management of the encoding window that slides over
the set of ADUs. Appendix A provides non-normative hints about
what FEC scheme designers need to consider.

* MUST define the management of the decoding window. This usually
consists of managing a system of linear equations (for a linear
FEC code).

6. Feedback

The discussion in Section 6 of [RFC6363] equally applies to this
FECFRAME extension and is not repeated here.

7. Transport Protocols

The discussion in Section 7 of [RFC6363] equally applies to this
FECFRAME extension and is not repeated here.

8. Congestion Control

The discussion in Section 8 of [RFC6363] equally applies to this
FECFRAME extension and is not repeated here.

9. Security Considerations

This FECFRAME extension does not add any new security considerations.
All the considerations of Section 9 of [RFC6363] apply to this
document as well. However, for the sake of completeness, the
following goal can be added to the list provided in Section 9.1 of
[RFC6363] ("Problem Statement"):

* Attacks can try to corrupt source flows in order to modify the
receiver application's behavior (as opposed to just denying
service).

10. Operations and Management Considerations

This FECFRAME extension does not add any new Operations and
Management Considerations. All the considerations of Section 10 of
[RFC6363] apply to this document as well.

11. IANA Considerations

This document has no IANA actions.

A FEC scheme for use with this FEC Framework is identified via its
FEC Encoding ID. It is subject to IANA registration in the "FEC
Framework (FECFRAME) FEC Encoding IDs" registry. All the rules of
Section 11 of [RFC6363] apply and are not repeated here.

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011,
<https://www.rfc-editor.org/info/rfc6363>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2. Informative References

[MBMSTS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
Protocols and codecs", 3GPP TS 26.346, March 2009,
<http://ftp.3gpp.org/specs/html-info/26346.htm>.

[RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052,
DOI 10.17487/RFC5052, August 2007,
<https://www.rfc-editor.org/info/rfc5052>.

[RFC6364] Begen, A., "Session Description Protocol Elements for the
Forward Error Correction (FEC) Framework", RFC 6364,
DOI 10.17487/RFC6364, October 2011,
<https://www.rfc-editor.org/info/rfc6364>.

[RFC6681] Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward
Error Correction (FEC) Schemes for FECFRAME", RFC 6681,
DOI 10.17487/RFC6681, August 2012,
<https://www.rfc-editor.org/info/rfc6681>.

[RFC6816] Roca, V., Cunche, M., and J. Lacan, "Simple Low-Density
Parity Check (LDPC) Staircase Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6816,
DOI 10.17487/RFC6816, December 2012,
<https://www.rfc-editor.org/info/rfc6816>.

[RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K.
Matsuzono, "Simple Reed-Solomon Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6865,
DOI 10.17487/RFC6865, February 2013,
<https://www.rfc-editor.org/info/rfc6865>.

[RFC8406] Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek,
F., Ghanem, S., Lochin, E., Masucci, A., Montpetit, M-J.,
Pedersen, M., Peralta, G., Roca, V., Ed., Saxena, P., and
S. Sivakumar, "Taxonomy of Coding Techniques for Efficient
Network Communications", RFC 8406, DOI 10.17487/RFC8406,
June 2018, <https://www.rfc-editor.org/info/rfc8406>.

[RFC8681] Roca, V. and B. Teibi, "Sliding Window Random Linear Code
(RLC) Forward Erasure Correction (FEC) Schemes for
FECFRAME", RFC 8681, DOI 10.17487/RFC8681, January 2020,
<https://www.rfc-editor.org/info/rfc8681>.

Appendix A. About Sliding Encoding Window Management (Informational)

The FEC Framework does not specify the management of the sliding
encoding window, which is the responsibility of the FEC scheme. This
annex only provides a few informational hints.

Source symbols are added to the sliding encoding window each time a
new ADU is available at the sender after the ADU-to-source-symbol
mapping specific to the FEC scheme has been done.

Source symbols are removed from the sliding encoding window. For
instance:

* After a certain delay, when an "old" ADU of a real-time flow times
out. The source symbol retention delay in the sliding encoding
window should therefore be initialized according to the real-time
features of incoming flow(s) when applicable.

* Once the sliding encoding window has reached its maximum size
(there is usually an upper limit to the sliding encoding window
size). In that case, the oldest symbol is removed each time a new
source symbol is added.

Several considerations can impact the management of this sliding
encoding window:

* At the source flows level: real-time constraints can limit the
total time during which source symbols can remain in the encoding
window.

* At the FEC code level: theoretical or practical limitations (e.g.,
because of computational complexity) can limit the number of
source symbols in the encoding window.

* At the FEC scheme level: signaling and window management are
intrinsically related. For instance, an encoding window composed
of a nonsequential set of source symbols requires appropriate
signaling to inform a receiver of the composition of the encoding
window, and the associated transmission overhead can limit the
maximum encoding window size. On the contrary, an encoding window
always composed of a sequential set of source symbols simplifies
signaling: providing the identity of the first source symbol plus
its number is sufficient, which creates a fixed and relatively
small transmission overhead.

Acknowledgments

The authors would like to thank Christer Holmberg, David Black, Gorry
Fairhurst, Emmanuel Lochin, Spencer Dawkins, Ben Campbell, Benjamin
Kaduk, Eric Rescorla, Adam Roach, and Greg Skinner for their valuable
feedback on this document. This document being an extension of
[RFC6363], the authors would also like to thank Mark Watson as the
main author of that RFC.

Authors' Addresses

Vincent Roca
INRIA
Univ. Grenoble Alpes
France

Email: [email protected]

Ali Begen
Networked Media
Konya/
Turkey

Email: [email protected]