NWCRG J. Detchart
Internet-Draft E. Lochin
Intended status: Experimental J. Lacan
Expires: September 10, 2015 ISAE
V. Roca
INRIA
March 9, 2015
Tetrys, an On-the-Fly Network Coding protocol
draft-detchart-nwcrg-tetrys-01
Abstract
This document describes Tetrys, an On-The-Fly Network Coding (NC)
protocol that can be used to transport delay and loss sensitive data
over a lossy network. Tetrys can recover from erasures within a RTT-
independent delay, thanks to the transmission of coded packets
(redundancy). It can be used for both unicast, multicast and anycast
communications.
Status of This Memo
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This Internet-Draft will expire on September 10, 2015.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Definitions, Notations and Abbreviations . . . . . . . . . . 3
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Common Header Format . . . . . . . . . . . . . . . . . . 6
4.1.1. Header Extensions . . . . . . . . . . . . . . . . . . 7
4.2. Source Packet Format . . . . . . . . . . . . . . . . . . 9
4.3. Coded Packet Format . . . . . . . . . . . . . . . . . . . 9
4.4. Acknowledgement Packet Format . . . . . . . . . . . . . . 10
5. The Coding Coefficient Generator Identifiers . . . . . . . . 12
5.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Table of Identifiers . . . . . . . . . . . . . . . . . . 12
6. Tetrys Basic Functions . . . . . . . . . . . . . . . . . . . 12
6.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1.1. Encoding Vector Formats . . . . . . . . . . . . . . . 13
6.1.2. The Elastic Encoding Window . . . . . . . . . . . . . 15
6.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
This document describes Tetrys, a novel network coding protocol.
Network codes were introduced in the early 2000s [AHL-00] to address
the limitations of transmission over the Internet (delay, capacity
and packet loss). While the use of network codes is fairly recent in
the Internet community, the use of application layer erasure codes in
the IETF has already been standardized in the RMT [RMT] and the
FECFRAME [FECFRAME] working groups. The protocol presented here can
be seen as a network coding extension to standards solutions. The
current proposal can be considered as a combination of network
erasure coding and feedback mechanisms [Tetrys].
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The main innovation of the Tetrys protocol is in the generation of
coded packets from an elastic encoding window, the size of which is
periodically updated with the receiver's feedback. This update is
done in such a way that any source packets coming from an input flow
is included in the encoding window as long as it is not acknowledged
or the encoding window did not reach a limit. This mechanism allows
for losses on both the forward and return paths and in particular is
resilient to acknowledgement losses.
With Tetrys, a coded packet is a linear combination of the data
source packets belonging to the coding window over a finite field.
The choice of the finite field of the coefficients is a trade-off
between the best performance (with non-binary coefficients) and the
system constraints (binary codes in an energy constrained
environment) and is driven by the application.
Thanks to the elastic encoding window, the coded packets are built
on-the-fly, by using an algorithm or a function to choose the
coefficients. The redundancy ratio can be dynamically adjusted, and
the coefficients can be generated in different ways along a
transmission. This allows to reduce the bandwidth used, compared to
FEC block codes, and the decoding delay.
1.1. Requirements Notation
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].
2. Definitions, Notations and Abbreviations
The terminology used in this document is presented below. It is
aligned with the FECFRAME terminology as well as with recent
activities in the Network Coding Research Group.
Source symbol: a symbol that has to be transmitted between the
ingress and egress of the network.
Coded symbol: a linear combination over a finite field of a set of
source symbols.
Source symbol ID: a sequence number to identify the source
symbols.
Coded symbol ID: a sequence number to identify the coded symbols.
Encoding vector: a set of the encoding coefficients and input
symbol IDs. One or both sets can be null.
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Source packet: a source packet contains a source symbol with its
associated IDs.
Coded packet: a coded packet contains a coded symbol, the coded
symbol's ID and encoding vector.
Input symbol: a symbol at the input of the Tetrys Encoding
Building Block.
Output symbol: a symbol generated by the Tetrys Encoding Building
Block. For a non systematic mode, all output symbols are coded
symbols. For a systematic mode, output symbols can be the input
symbols and a number of coded symbols that are linear combinations
of the input symbols + the encoding vectors.
Feedback packet: a feedback packet is a packet containing
information about the decoded or received source symbols. It can
also bring additional information about the Packet Error Rate or
the number of various packets in the receiver decoding window
Elastic Encoding Window: an encoder-side buffer that stores all
the non-acknowledged source packets of the input flow that are
part of the coding process.
Coding Coefficient Generator Identifier: a unique identifier to
define a function or an algorithm allowing to generate the
coefficients used to compute the coded packets.
Code rate: Define the rate of generating and sending the
redundancy.
3. Architecture
-- Editor's note: The architecture used in this document should be
aligned with the future NC Architecture document [NWCRG-ARCH]. --
3.1. Use Cases
Tetrys is well suited, but not limited to the use case where there is
a single flow originated by a single source, with intra stream coding
that takes place at a single encoding node. Transmission can be over
a single path or multiple paths. In addition, the flow can be sent
in unicast, multicast, or anycast mode. This is the simplest use-
case, that is very much inline with currently proposed scenarios for
end-to-end streaming.
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3.2. Overview
+-----------+ +-----------+
| Tetrys | output packets | Tetrys |
source | Sender |----------------->| Receiver | source
---------->| | feedback packets | |---------->
symbols | |<-----------------| | symbols
+-----------+ +-----------+
Figure 1: Tetrys Architecture.
The Tetrys protocol features several key functionalities:
o On-the-fly encoding;
o Decoding;
o Signaling, to carry in particular the symbol identifiers in the
encoding window and the associated coding coefficients when
meaningful, in a manner that was previously used in FEC;
o Feedback management;
o Elastic window management;
o Channel estimation;
o Dynamic adjustment of the code rate and flow control;
o Congestion control management (if appropriate);
-- Editor's note: must be discussed --
o Tetrys packet header creation and processing;
o -- Editor's note: something else? --
These functionalities are provided by several building blocks:
o The Tetrys Building Block: this BB is used during encoding and
decoding processes. It must be noted that Tetrys does not mandate
a specific building block. Instead any building block compatible
with the elastic encoding window feature of Tetrys can be used.
o The Window Management Building Block: this building block is in
charge of managing the encoding encoding window at a Tetrys
sender.
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-- Editor's note: Is it worth moving it in a dedicated BB? To
be discussed --
o Other ?
In order to enable future components and services to be added
dynamically, Tetrys adds a header extension mechanism, compatible
with that of LCT, NORM, FECFRAME [REFS].
4. Packet Format
4.1. Common Header Format
All types of Tetrys packets share the same common header format (see
Figure 2).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V | Reserved | C |S| HDR_LEN | Packet Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Congestion Control Information (CCI, length = 32*C bits) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Session Identifier (TSI, length = 32*S bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header Extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Common Header Format
-- Editor's note: this format inherits from the LCT header format
(RFC 5651) with slight modifications. --
o Tetrys version number (V): 4 bits. Indicates the Tetrys version
number. The Tetrys version number for this specification is 1.
o Reserved (Resv): 9 bits. These bits are reserved. In this
version of the specification, they MUST be set to zero by senders
and MUST be ignored by receivers.
o Congestion control flag (C): 2 bits. C=0 indicates the Congestion
Control Information (CCI) field is 0 bits in length. C=1
indicates the CCI field is 32 bits in length. C=2 indicates the
CCI field is 64 bits in length. C=3 indicates the CCI field is 96
bits in length.
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-- Editor's note: version number and congestion control to be
discussed --
o Transport Session Identifier flag (S): 1 bit. This is the number
of full 32-bit words in the TSI field. The TSI field is 32*S bits
in length, i.e., the length is either 0 bits or 32 bits.
o Header length (HDR_LEN): 8 bits. Total length of the Tetrys
header in units of 32-bit words. The length of the Tetrys header
MUST be a multiple of 32 bits. This field can be used to directly
access the portion of the packet beyond the Tetrys header, i.e.,
to the first other header if it exists, or to the packet payload
if it exists and there is no other header, or to the end of the
packet if there are no other headers or packet payload.
o Packet Type: 8 bits. Type of packet.
o Congestion Control Information (CCI): 0, 32, 64, or 96 128 bits
Used to carry congestion control information. For example, the
congestion control information could include layer numbers,
logical channel numbers, and sequence numbers. This field is
opaque for the purpose of this specification. This field MUST be
0 bits (absent) if C=0. This field MUST be 32 bits if C=1. This
field MUST be 64 bits if C=2. This field MUST be 96 bits if C=3.
o Transport Session Identifier (TSI): 0, 16, 32, or 48 bits The TSI
uniquely identifies a session among all sessions from a particular
sender. The TSI is scoped by the IP address of the sender, and
thus the IP address of the sender and the TSI together uniquely
identify the session. Although a TSI in conjunction with the IP
address of the sender always uniquely identifies a session,
whether or not the TSI is included in the Tetrys header depends on
what is used as the TSI value. If the underlying transport is
UDP, then the 16-bit UDP source port number MAY serve as the TSI
for the session. If the TSI value appears multiple times in a
packet, then all occurrences MUST be the same value. If there is
no underlying TSI provided by the network, transport or any other
layer, then the TSI MUST be included in the Tetrys header.
4.1.1. Header Extensions
Header Extensions are used in Tetrys to accommodate optional header
fields that are not always used or have variable size. The presence
of Header Extensions can be inferred by the Tetrys header length
(HDR_LEN). If HDR_LEN is larger than the length of the standard
header, then the remaining header space is taken by Header Extension
fields.
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If present, Header Extensions MUST be processed to ensure that they
are recognized before performing any congestion control procedure or
otherwise accepting a packet. The default action for unrecognized
Header Extensions is to ignore them. This allows the future
introduction of backward-compatible enhancements to Tetrys without
changing the Tetrys version number. Non-backward-compatible Header
Extensions CANNOT be introduced without changing the Tetrys version
number.
There are two formats for Header Extension fields, as depicted in
Figure Figure 3. The first format is used for variable-length
extensions, with Header Extension Type (HET) values between 0 and
127. The second format is used for fixed-length (one 32-bit word)
extensions, using HET values from 127 to 255.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET (<=127) | HEL | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
. .
. Header Extension Content (HEC) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET (>=128) | Header Extension Content (HEC) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Header Extension Format
o Header Extension Type (HET): 8 bits The type of the Header
Extension. This document defines a number of possible types.
Additional types may be defined in future versions of this
specification. HET values from 0 to 127 are used for variable-
length Header Extensions. HET values from 128 to 255 are used for
fixed-length 32-bit Header Extensions.
o Header Extension Length (HEL): 8 bits The length of the whole
Header Extension field, expressed in multiples of 32-bit words.
This field MUST be present for variable-length extensions (HETs
between 0 and 127) and MUST NOT be present for fixed-length
extensions (HETs between 128 and 255).
o Header Extension Content (HEC): variable length The content of the
Header Extension. The format of this sub-field depends on the
Header Extension Type. For fixed-length Header Extensions, the
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HEC is 24 bits. For variable-length Header Extensions, the HEC
field has variable size, as specified by the HEL field. Note that
the length of each Header Extension field MUST be a multiple of 32
bits. Also note that the total size of the Tetrys header,
including all Header Extensions and all optional header fields,
cannot exceed 255 32-bit words.
4.2. Source Packet Format
A source packet is the encapsulation of a source symbol, a source
symbol ID and a Common Packet Header. The source symbols can have
variable sizes.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Common Packet Header /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Payload /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Source Packet Format
Common Packet Header: a common packet header where Packet Type=0.
Source Symbol ID: the sequence number to identify a source symbol.
Payload: the payload (source symbol)
4.3. Coded Packet Format
A coded packet is the encapsulation of a coded symbol, a coded symbol
ID, the associated encoding vector and the Common Packet Header. As
the source symbols can have variable sizes, each source symbol size
need to be encoded, and the result must be stored in the coded
packet. The Encoded Payload Size is 16 bits.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Common Packet Header /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Coded Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Encoding Vector /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded Payload Size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
/ Payload /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Coded Packet Format
Common Packet Header: a common packet header where Packet Type=1.
Coded Symbol ID: the sequence number to identify a coded symbol.
Encoding Vector: an encoding vector to define the linear combination
used (coefficients, and source symbols).
Encoded Payload Size: the coded payload size used if the source
symbols are of variable size.
Payload: the coded symbol.
4.4. Acknowledgement Packet Format
A Tetrys Decoding Building Block MAY send back to a Tetrys Encoding
Building Block some Acknowledgement packets. They contain
information about what it is received and/or decoded, and other
information such as a packet loss rate or the size of the decoding
buffers. The acknowledgement packets are OPTIONAL hence they could
be omitted or lost in transmission without impacting the basic
protocol performance.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Common Packet Header /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nb of missing source symbols |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nb of not already used coded symbols |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Session Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First Source Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SACK size | |
+-+-+-+-+-+-+-+-+ +
| |
/ SACK Vector /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Acknowledgement Packet Format
Common Packet Header: a common packet header where Packet Type=2.
Nb missing source symbols: the number of missing source symbols in
the receiver.
Nb of not already used coded symnbols: the number of not already used
coded symbols in the receiver that have not already been used for
decoding. Meaning the number of linear combinations containing at
least 2 unknown source symbols.
Transport Session Identifier (TSI): the unique identifier for the
session (CAN be 0bit, depending of the Common Packet Header's field
S)
First Source Symbol ID: ID of the first source symbol to acknowledge.
SACK size: the size of the SACK vector in 32-bit words. For
instance, with value 2, the SACK vector is 64 bits long.
SACK vector: bit vector indicating the acknowledged symbols following
the first source symbol ID. The "First Source Symbol" is not
included in this bit vector. A bit equal to 1 at position i means
that the source symbol of ID equal to "First Source Symbol ID" + i +
1 is acknowledged by this acknowledgment packet.
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5. The Coding Coefficient Generator Identifiers
5.1. Definition
The Coding Coefficient Generator Identifiers define a function or an
algorithm to build the coding coefficients used to generate the coded
symbols. They MUST be known by all the Building Blocks.
5.2. Table of Identifiers
0000: GF256 Vandermonde based coefficients. Each coefficient is
build as alpha^( (source_id*repair_id) % 255).
0001: GF16 Vandermonde based coefficients. Each coefficient is build
as alpha^( (source_id*repair_id) % 15).
0010: SRLC.
Others: To be discussed.
6. Tetrys Basic Functions
6.1. Encoding
At the beginning of a transmission, a Tetrys Encoding Building Block
MUST choose an initial code rate (added redundancy) as it doesn't
know the packet loss rate of the channel. In steady state, the
Tetrys Encoding Building Block generates coded symbols when it
receives some information from the decoding blocks.
When a Tetrys Encoding Building Block needs to generate a coded
symbol, it considers the set of source symbols stored in the Elastic
Encoding Window. These source symbols are the set of source symbols
which are not yet acknowledged by the receiver.
A Tetrys Encoding Building Block SHOULD set a limit of the Elastic
Encoding Window size. This allows to reduce the complexity by
considering less source symbols. It also provides a coping mechanism
if all the acknowledgment packets are lost.
At the generation of a coded symbol, the Tetrys Encoding Building
Block generates an encoding vector containing the IDs of the source
symbols stored in the Elastic Encoding Window. For each source
symbol, a finite field coefficient is determined using a Coding
Coefficient Generator. This generator CAN take as input the source
symbol ID and the coded symbol ID and CAN determine a coefficient in
a deterministic way. A classical example of such deterministic
function is a generator matrix where the rows are indexed by the
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source symbol IDs and the columns by the coded symbol IDs. For
example, the entries of this matrix can be built from a Vandermonde
structure, like Reed-Solomon codes, or from a sparse binary matrix,
like Low-Density Generator Matrix codes. Finally, the coded symbol
is the sum of the source symbols multiplied by their corresponding
coefficients.
6.1.1. Encoding Vector Formats
The encoding vectors are sent in each coded symbols. They contain
the source symbol IDs and/or the coefficients.
To avoid the overhead of transmitting all the source symbol IDs, the
following algorithm is used to compress them.
6.1.1.1. Transmitting the source symbol IDs
The source symbol IDs are organized as a sorted list of 32-bit
integers. Instead of sending the full list, a differential transform
to reduce the number of bits needed to represent an ID is used.
6.1.1.1.1. Compressing the Source symbol IDs
Assume the symbol IDs used in the combination are:
[1..3],[5..6],[8..10].
1. Keep the first element in the packet as the first_source_id: 1.
2. Apply a differential transform to the others elements
([3,5,6,8,10]) which removes the element i-1 to the element i,
starting with the first_source_id as i0, and get the list L =>
[2,2,1,2,2]
3. Compute b, the number of bits needed to store all the elements,
which is ceil(log2(max(L))): here, 2 bits.
4. Write b in the corresponding field, and write all the b * [(2 *
NB blocks) - 1] elements in a bit vector, here: 10 10 01 10 10.
6.1.1.1.2. Decompressing the Source symbol IDs
When a Tetrys Decoding Building Block wants to reverse the
operations, this algorithm is used:
1. Rebuild the list of the transmitted elements by reading the bit
vector and b: [10 10 01 10 10] => [2,2,1,2,2]
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2. Apply the reverse transform by adding successively the elements,
starting with first_source_id: [1,1+2,(1+2)+2,(1+2+2)+1,...] =>
[1,3,5,6,8,10]
3. Rebuild the blocks using the list and first_source_id:
[1..3],[5..6],[8..10].
6.1.1.2. Encoding Vector Format
The encoding vector CAN be used to store the source symbol IDs
included in the associated coded symbol, the coefficients used in the
combination, or both. It CAN be used to send only the number of
source symbols included in the coded symbol.
If the source IDs are stored, the nb of blocks MUST be different from
0.
The encoding vector format uses a 4-bit Coding Coefficient Generator
Identifier to identity the algorithm to generate the coefficients,
and contains a set of blocks for the source symbol IDs used in the
combination. In this format, the number of blocks is stored as a
8-bit unsigned integer. To reduce the overhead, a compressed way to
store the symbol IDs is used: the IDs are not stored as themselves,
but stored as the difference between the previous.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EV_LEN | CCGI |I|C| | NB_BLOCKS | NB_COEFS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FIRST_SOURCE_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| b_id | |
+-+-+-+-+-+-+-+ id_bit_vector +-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| b_coef | |
+-+-+-+-+-+-+-+ coef_bit_vector +-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Encoding Vector Format
o Encoding Vector Length (EV_LEN): size in units of 32-bit words.
o Coding Coefficient Generator Identifier (CCGI): 8-bit ID to
identify the algorithm or the function used to generate the
coefficients (see Section 5). As a CCGI is included in each
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encoded vector, it can dynamically change between the generation
of 2 coded symbols.
o Store the IDs flag (I): 1 bit to know if an encoding vector
contains the list of the IDs used. MUST be 1 if the Encoding
Vector stores the source symbol IDs.
o Number of blocks used to store the source symbol IDs (NB_BLOCKS):
the number of blocks used to store all the source symbol IDs.
o Number of coefficients (NB_COEFS): The number of the coefficients
used to generate the associated coded symbol.
o The first source Identifier (FIRST_SOURCE_ID): the first source
symbol ID used in the combination.
o Number of bits for each edge block (b_id): the number of bits
needed to store the edge (see Section 6.1.1.1).
o The compressed edge blocks (id_bit_vector): equal to b_id *
(NB_BLOCKS * 2 - 1).
o Number of bits needed to store each coefficient (b_coef): the
number of bits used to store the coefficients.
o The coefficients (coef_bit_vector): The coefficients stored (as a
vector of b_coef * NB_COEFS).
o Padding: padding to have an Encoding Vector size multiple of
32-bit (for the id and coefficient part).
6.1.2. The Elastic Encoding Window
When an input source symbol is passed to a Tetrys Encoding Building
Block, it is added to the Elastic Encoding Window. This window MUST
have a limit set by the encoding building Block (depending of the use
case: unicast, multicast, file transfer, real-time transfer, ...).
If the Elastic Encoding Window reached its limit, the window slides
over the symbols: the first (oldest) symbols are removed. Then, a
packet containing this symbol can be sent onto the network. As an
element of the coding window, this symbol is included in the next
linear combinations created to generate the coded symbols.
As explained below, the receiver sends periodic feedback indicating
the received or decoded source symbols. In the case of a unicast
transmission, when the sender receives the information that a source
symbol was received and/or decoded by the receiver, it removes this
symbol from the coding window. In a multicast transmission, a source
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symbol is removed from the coding window only when all the receivers
have received or decoded it.
6.2. Decoding
A classical matrix inversion is sufficient to recover the source
symbols.
7. Security Considerations
N/A
8. Privacy Considerations
N/A
9. IANA Considerations
N/A
10. Acknowledgments
N/A
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[AHL-00] Ahlswede, R., Ning Cai, , Li, S., and R. Yeung, "Network
information flow", IEEE Transactions on Information Theory
vol.46, no.4, pp.1204,1216, July 2000.
[FECFRAME]
Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", Request for Comments 6363,
October 2011.
[NWCRG-ARCH]
NWCRG, , "Network Coding Architecture", TBD TBD.
[RMT] Vicisano, L., Gemmel, J., Rizzo, L., Handley, M., and J.
Crowcroft, "Forward Error Correction (FEC) Building
Block", Request for Comments 3452, December 2002.
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[Tetrys] Lacan, J. and E. Lochin, "Rethinking reliability for long-
delay networks", International Workshop on Satellite and
Space Communications 2008 (IWSSC08), October 2008.
Authors' Addresses
Jonathan Detchart
ISAE
10, avenue Edouard-Belin
BP 54032
Toulouse CEDEX 4 31055
France
Email: jonathan.detchart@isae.fr
Emmanuel Lochin
ISAE
10, avenue Edouard-Belin
BP 54032
Toulouse CEDEX 4 31055
France
Email: emmanuel.lochin@isae.fr
Jerome Lacan
ISAE
10, avenue Edouard-Belin
BP 54032
Toulouse CEDEX 4 31055
France
Email: jerome.lacan@isae.fr
Vincent Roca
INRIA
655, av. de l'Europe
Inovallee; Montbonnot
ST ISMIER cedex 38334
France
Email: vincent.roca@inria.fr
URI: http://privatics.inrialpes.fr/people/roca/
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