Internet DRAFT - draft-roca-tsvwg-fecframev2
draft-roca-tsvwg-fecframev2
TSVWG V. Roca
Internet-Draft INRIA
Intended status: Standards Track A. Begen
Expires: December 29, 2017 Networked Media
June 27, 2017
Forward Error Correction (FEC) Framework Extension to Sliding Window
Codes
draft-roca-tsvwg-fecframev2-04
Abstract
RFC 6363 describes a framework for using Forward Error Correction
(FEC) codes with applications in public and private IP networks 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. The
present document extends FECFRAME 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 29, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 4
3. Architecture Overview . . . . . . . . . . . . . . . . . . . . 7
4. Procedural Overview . . . . . . . . . . . . . . . . . . . . . 9
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Sender Operation with Sliding Window FEC Codes . . . . . 9
4.3. Receiver Operation with Sliding Window FEC Codes . . . . 12
5. Protocol Specification . . . . . . . . . . . . . . . . . . . 14
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. FEC Framework Configuration Information . . . . . . . . . 15
5.3. FEC Scheme Requirements . . . . . . . . . . . . . . . . . 15
6. Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Transport Protocols . . . . . . . . . . . . . . . . . . . . . 16
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . 16
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. Operations and Management Considerations . . . . . . . . . . 17
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
14.1. Normative References . . . . . . . . . . . . . . . . . . 17
14.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. About Sliding Encoding Window Management (non
Normative) . . . . . . . . . . . . . . . . . . . . . 19
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 received
without any corruption or totally lost (e.g., because of a congested
router, of a poor signal-to-noise ratio in a wireless network, or
because the number of bit errors exceeds the correction capabilities
of a low-layer error correcting code).
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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 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 unreliable transports such as UDP. It is primarily
intended for real-time or streaming media applications, using
broadcast, multicast, or on-demand delivery.
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. This approach
has major impacts on FEC encoding and decoding delays. The data
packets of continuous media flow(s) can be sent immediately, without
delay. But the block creation time, that depends on the number k of
source symbols in this block, impacts the FEC encoding delay since
encoding requires that all source symbols be known. This block
creation time also impacts the decoding delay a receiver will
experience in case of erasures, since no repair symbol for the
current block can be received before. Therefore a good value for the
block size is necessarily a balance between the maximum 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 extends [RFC6363] in order to also support FEC codes
based on a sliding encoding window (A.K.A. convolutional codes).
This encoding window, either of 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 sent 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
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highly beneficial to real-time flows, one of the primary targets of
FECFRAME. [RLC-ID] provides an example of such FEC Scheme for
FECFRAME, built upon the simple sliding window Random Linear Codes
(RLC).
This document is fully backward compatible with [RFC6363] that it
extends but does not replace. Indeed:
o this extension does not prevent nor compromize in any way the
support of block FEC codes. Both types of codes can nicely co-
exist, just like different block FEC schemes can co-exist;
o any receiver, for instance a legacy receiver that only supports
block FEC schemes, can easily identify the FEC scheme used in a
FECFRAME session thanks to the associated SDP file and its FEC
Encoding ID information (i.e., the "encoding-id=" parameter of a
"fec-repair-flow" attribute, [RFC6364]). This mechanism is not
specific to this extension but 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 re-uses its structure. It
proposes new sections specific to sliding window FEC codes whenever
required. The only exception is Section Section 3 that 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. Definitions and Abbreviations
The following list of definitions and abbreviations is copied from
[RFC6363], adding only the Block/sliding window FEC Code and
Encoding/Decoding Window definitions:
Application Data Unit (ADU): The unit of source data provided as
payload to the transport layer.
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).
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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: An FEC Code that operates in a block manner, i.e.,
for which the input flow MUST be segmented into a sequence of
blocks, FEC encoding and decoding being performed independently
on a per-block basis.
Sliding Window (or Convolutional) FEC Code: An 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.
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 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.
Protection Amount: The relative increase in data sent due to the use
of FEC.
Repair Flow: The packet flow carrying FEC data.
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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, e.g., UDP and the Datagram Congestion
Control Protocol (DCCP).
Encoding Window: Set of Source Symbols available at the sender/
coding node that are used to generate a repair symbol, with a
Sliding Window FEC Code.
Decoding Window: Set of received or decoded source and repair
symbols available at a receiver that are used to decode erased
source symbols, with a Sliding Window FEC Code.
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., by a congested router, or because the number
of transmission errors exceeds the correction capabilities of the
physical-layer codes) or received. When a packet is received, it
is assumed that this packet is not corrupted.
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.
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Systematic Code: FEC code in which the source symbols are part of
the encoding symbols.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Architecture Overview
The architecture of [RFC6363], Section 3, equally applies to this
FECFRAME extension and is not repeated here. However we provide
hereafter 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 Layer |
| (e.g., UDP) |
+----------------------+
Figure 1: FECFRAME architecture at a sender.
The FECFRAME architecture is illustrated in Figure 1 from the
sender's point of view, in case of a block FEC Scheme. It shows an
application generating an ADU flow (other flows, from other
applications, may co-exist). These ADUs, of variable size, must be
somehow mapped to source symbols of fixed size. This is the goal of
an ADU to symbols mapping process that is FEC Scheme specific (see
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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., 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 UDP (more
precisely [RFC6363], Section 7, requires a transport protocol
providing an unreliable datagram service, like UDP or DCCP). In
practice FEC Source Packets can be sent as soon as available, without
having to wait for FEC encoding to take place. In that case a copy
of the associated source symbols need 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, when the FEC decoding
of the associated block recovers all the missing source symbols, the
lost ADUs are recovered and assigned to their respective flow (see
below). ADUs are then returned to the application(s), either in
order or not depending on the application requirements.
FECFRAME features two subtle mechanisms:
o 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. 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. The mapping details are
FEC Scheme dependant and must be defined there.
o 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/IP addresses traditionally used to map incoming
ADUs to flows are not recovered during FEC decoding). To make it
possible, at the FECFRAME sending instance, each ADU is prepended
with a flow identifier (1 byte) before doing the mapping to source
symbols (see above). This (flow ID + length + application payload
+ padding), called ADUI, is then FEC protected. Therefore a
decoded ADUI contains enough information to assign the ADU to the
right flow.
A few aspects are not considered by FECFRAME, namely:
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o congestion control (see [RFC6363], section 8 for a more detailed
discussion);
o feedbacks from receiver(s) (although they may exist within the
application, e.g., through RCTP control messages);
o flow adaptation at a FECFRAME sender (e.g., by adjusting the FEC
code rate based on channel conditions, since there is no feedback
mechanism within FECFRAME);
4. Procedural Overview
4.1. General
The general considerations of [RFC6363], Section 4.1, 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
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 case of UDP 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 symbols mapping as well as the encoding window
management details are both the responsibility of the FEC scheme
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and MUST be detailed there. Appendix A provides some hints on
the way it might be performed.
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
[RFC6363] Figure 6, 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
(for example, in the UDP case, the UDP source and destination
addresses and ports 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
[RFC6363] Figure 7, 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.
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+----------------------+
| 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 Layer |
| (e.g., UDP) |
+----------------------+
Figure 2: Sender Operation with Convolutional FEC Codes
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+----------------------+
| 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 Layer |
| (e.g., UDP) |
+----------------------+
Figure 3: Sender Operation with RTP Repair Flows
4.3. Receiver Operation with Sliding Window FEC Codes
With a Sliding Window FEC scheme, the following operations,
illustrated in Figure 4 for the case of UDP 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 [RFC6363], Section 4.3,
"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 and
the rank of the associated linear system permits it, then FEC
decoding can be performed in order to recover missing source
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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 Layer |
| (e.g., UDP) |
+----------------------+
Figure 4: Receiver Operation with Sliding Window FEC Codes
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+----------------------+
| 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 Layer |
| (e.g., UDP) |
+----------------------+
Figure 5: Receiver Operation with 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.
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5.2. FEC Framework Configuration Information
The FEC Framework Configuration Information considerations of
[RFC6363], Section 5.5, equally applies to this FECFRAME extension
and is not repeated here.
5.3. FEC Scheme Requirements
The FEC scheme requirements of [RFC6363], Section 5.6, mostly apply
to this FECFRAME extension and are not repeated here. An exception
though is the "full specification of the FEC code", item (4), that is
specific to block FEC codes. The following item (4) applies instead:
4. 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 symbols to be included in the repair packet payload
with the given Repair FEC Payload ID value.
Additionally, the FEC scheme associated to a Sliding Window FEC Code:
o MUST define the relationships between ADUs and the associated
source symbols (mapping);
o MUST define the management of the encoding window that slides over
the set of ADUs. Appendix A provides a non normative example;
o MUST define the management of the decoding window, consisting of a
system of linear equations (in case of a linear FEC code);
6. Feedback
The discussion of [RFC6363], Section 6, equally applies to this
FECFRAME extension and is not repeated here.
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7. Transport Protocols
The discussion of [RFC6363], Section 7, equally applies to this
FECFRAME extension and is not repeated here.
8. Congestion Control
The discussion of [RFC6363], Section 8, equally applies to this
FECFRAME extension and is not repeated here.
9. Implementation Status
Editor's notes: RFC Editor, please remove this section motivated by
RFC 7942 before publishing the RFC. Thanks!
An implementation of FECFRAME extended to Sliding Window codes
exists:
o Organisation: Inria
o Description: This is an implementation of FECFRAME extended to
Sliding Window codes and supporting the RLC FEC Scheme [RLC-ID].
It is based on: (1) a proprietary implementation of FECFRAME, made
by Inria and Expway for which interoperability tests have been
conducted; and (2) a proprietary implementation of RLC Sliding
Window FEC Codes.
o Maturity: the basic FECFRAME maturity is "production", the
FECFRAME extension maturity is "under progress".
o Coverage: the software implements a subset of [RFC6363], as
specialized by the 3GPP eMBMS standard [MBMSTS]. This software
also covers the additional features of FECFRAME extended to
Sliding Window codes, in particular the RLC FEC Scheme.
o Lincensing: proprietary.
o Implementation experience: maximum.
o Information update date: March 2017.
o Contact: vincent.roca@inria.fr
10. Security Considerations
This FECFRAME extension does not add any new security consideration.
All the considerations of [RFC6363], Section 9, apply to this
document as well.
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11. Operations and Management Considerations
This FECFRAME extension does not add any new Operations and
Management Consideration. All the considerations of [RFC6363],
Section 10, apply to this document as well.
12. IANA Considerations
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
[RFC6363], Section 11, apply and are not repeated here.
13. Acknowledgments
TBD
14. References
14.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363, DOI 10.17487/
RFC6363, October 2011,
<http://www.rfc-editor.org/info/rfc6363>.
14.2. 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,
<http://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,
<http://www.rfc-editor.org/info/rfc6364>.
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[RFC6681] Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward
Error Correction (FEC) Schemes for FECFRAME", RFC 6681,
DOI 10.17487/RFC6681, August 2012,
<http://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,
<http://www.rfc-editor.org/info/rfc6816>.
[RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K.
Matsuzono, "Simple Reed-Solomon Forward Error Correction
(FEC) Scheme for FECFRAME", RFC 6865, DOI 10.17487/
RFC6865, February 2013,
<http://www.rfc-editor.org/info/rfc6865>.
[RLC-ID] Roca, V., "The Sliding Window Random Linear Code (RLC)
Forward Erasure Correction (FEC) Scheme for FECFRAME",
Work in Progress, Transport Area Working Group (TSVWG)
draft-roca-tsvwg-rlc-fec-scheme (Work in Progress), June
2017, <https://tools.ietf.org/html/draft-roca-tsvwg-rlc-
fec-scheme>.
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Appendix A. About Sliding Encoding Window Management (non Normative)
The FEC Framework does not specify the management of the sliding
encoding window which is the responsibility of the FEC Scheme. This
annex provides a few hints with respect to the management of this
encoding window.
Source symbols are added to the sliding encoding window each time a
new ADU arrives, where the following information is provided for this
ADU by the FEC Framework: a description of the source flow with which
the ADU is associated, the ADU itself, and the length of the ADU.
This information is sufficient for the FEC scheme to map the ADU to
the corresponding source symbols.
Source symbols and the corresponding ADUs are removed from the
sliding encoding window, for instance:
o 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).
o 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 aspects exist that can impact the sliding encoding window
management:
o at the source flows level: real-time constraints can limit the
total time source symbols can remain in the encoding window;
o at the FEC code level: there may be theoretical or practical
limitations (e.g., because of computational complexity) that limit
the number of source symbols in the encoding window.
o at the FEC scheme level: signaling and window management are
intrinsically related. For instance, an encoding window composed
of a non sequential set of source symbols requires an appropriate
signaling to inform a receiver of the composition of the encoding
window. On the opposite, an encoding window always composed of a
sequential set of source symbols simplifies signaling: providing
the identity of the first source symbol plus their number is
sufficient.
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Authors' Addresses
Vincent Roca
INRIA
Grenoble
France
EMail: vincent.roca@inria.fr
Ali Begen
Networked Media
Konya
Turkey
EMail: ali.begen@networked.media
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