NWCRG J. Detchart
Internet-Draft ISAE-SUPAERO
Intended status: Experimental E. Lochin
Expires: April 20, 2022 ENAC
J. Lacan
ISAE-SUPAERO
V. Roca
INRIA
October 17, 2021
Tetrys, an On-the-Fly Network Coding protocol
draft-detchart-nwcrg-tetrys-08
Abstract
This document is a product of the Coding for Efficient Network
Communications Research Group (NWCRG). It conforms to the directions
found in the NWCRG taxonomy [RFC8406] .
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 an
RTT-independent delay, thanks to the transmission of coded packets.
It can be used for both unicast, multicast and anycast
communications.
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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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 20, 2022.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Definitions, Notations and Abbreviations . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Common Header Format . . . . . . . . . . . . . . . . . . 6
4.1.1. Header Extensions . . . . . . . . . . . . . . . . . . 8
4.2. Source Packet Format . . . . . . . . . . . . . . . . . . 9
4.3. Coded Packet Format . . . . . . . . . . . . . . . . . . . 10
4.4. Acknowledgement Packet Format . . . . . . . . . . . . . . 11
5. The Coding Coefficient Generator Identifiers . . . . . . . . 13
5.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Table of Identifiers . . . . . . . . . . . . . . . . . . 13
6. Tetrys Basic Functions . . . . . . . . . . . . . . . . . . . 13
6.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1.1. Encoding Vector Formats . . . . . . . . . . . . . . . 14
6.2. The Elastic Encoding Window . . . . . . . . . . . . . . . 17
6.3. Decoding . . . . . . . . . . . . . . . . . . . . . . . . 18
7. Research Issues . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Interaction with Existing Congestion-Controlled Transport
Protocol . . . . . . . . . . . . . . . . . . . . . . . . 18
7.2. Adaptive Coding Rate . . . . . . . . . . . . . . . . . . 18
7.3. Using Tetrys Above The IP Layer For Tunneling . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . 20
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12.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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 [RFC3452] and the
FECFRAME [RFC8680] working groups. The protocol presented here can
be seen as a network coding extension to standards solutions. The
current proposal can be considered a combination of network erasure
coding and feedback mechanisms [Tetrys] .
The main innovation of the Tetrys protocol is in the generation of
coded packets from an elastic encoding window. This window is filled
by any source packets coming from an input flow and is periodically
updated with the receiver's feedbacks. These feedbacks return to the
sender the highest sequence number received or rebuilt, which allows
to flush the corresponding source packets stored in the window. The
size of this window can be fixed or dynamically updated. If the
window is full, incoming source packets are dropped. As a matter of
fact, its limit should be correctly sized. Finally, Tetrys allows to
deal with losses on both the forward and return paths and in
particular, is resilient to acknowledgment losses.
With Tetrys, a coded packet is a linear combination over a finite
field of the data source packets belonging to the coding window. The
coefficients finite field's choice 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 with a
transmission. Compared to FEC block codes, this allows reducing the
bandwidth use 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] .
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2. Definitions, Notations and Abbreviations
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 coefficients: elements of the finite field characterizing
the linear combination used to generate coded symbols.
Encoding vector: a set of the coding coefficients and input source
symbol IDs.
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 involved in
the coding process.
Coding Coefficient Generator Identifier: a unique identifier that
defines a function or an algorithm allowing to generate the
encoding vector.
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Code rate: Define the rate between the number of input symbols and
the number of output symbols.
3. Architecture
The notation used in this document is based on the NWCRG taxonomy
[RFC8406] .
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
at a single encoding node. Note that the input stream can be a
multiplex of several upper layer streams. Transmission can be over a
single path or multiple paths. Besides, the flow can be sent in
unicast, multicast, or anycast mode. This is the simplest use-case,
that is very much aligned with currently proposed scenarios for end-
to-end streaming.
3.2. Overview
+----------+ +----------+
| | | |
| App | | App |
| | | |
+----------+ +----------+
| ^
| source source |
| symbols symbols |
| |
v |
+----------+ +----------+
| | output packets | |
| Tetrys |--------------->| Tetrys |
| Encoder |feedback packets| Decoder |
| |<---------------| |
+----------+ +----------+
Figure 1: Tetrys Architecture
The Tetrys protocol features several key functionalities. The
mandatory features are :
o on-the-fly encoding;
o decoding;
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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 Tetrys packet header creation and processing;
and the optional features are :
o channel estimation;
o dynamic adjustment of the code rate and flow control;
o congestion control management (if appropriate). See
Section Section 7.1 for further details;
Several building blocks provide these functionalities:
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 window at a Tetrys sender.
o Other ?
To ease the addition of future components and services, Tetrys adds a
header extension mechanism, compatible with that of LCT [RFC5651] ,
NORM [RFC5740] , FECFRAME [RFC8680] .
4. Packet Format
4.1. Common Header Format
All types of Tetrys packets share the same common header format (see
Figure 2 ).
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V | C |S| Reserved | 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
As already noted above in the document, this format is compatible
with LCT and inherits from the LCT header format [RFC5651] 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 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.
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 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 Header length (HDR_LEN): 8 bits. The 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 next 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.
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o Congestion Control Information (CCI): 0, 32, 64, or 96 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 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 or 32 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 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
Extensions.
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 Extensions, as depicted in 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 128 to 255.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 several 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
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 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 a Common Packet Header encapsulation, a Source
Symbol ID and a source symbol (payload). The source symbols can have
variable sizes.
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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 /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Payload /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Source Packet Format
Common Packet Header: a common packet header (as common header
format) 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 Common Packet Header, a
Coded Symbol ID, the associated Encoding Vector, and a coded symbol
(payload). As the source symbols CAN have variable sizes, each
source symbol size need to be encoded. The result must be stored in
the coded packet as the Encoded Payload Size (16 bits): as it is an
optional field, the encoding vector MUST signal the use of variable
source symbol sizes with the field V (see Section 6.1.1.2 ).
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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 (as common header
format) 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 have a variable size (optional, Section 6.1.1.2 )).
Payload: the coded symbol.
4.4. Acknowledgement Packet Format
A Tetrys Decoding Building Block MAY send back to another building
block some Acknowledgement packets. They contain information about
what it has received and/or decoded, and other information such as a
packet loss rate or the size of the decoding buffers. The
acknowledgment packets are OPTIONAL hence they could be omitted or
lost in transmission without impacting the protocol behavior.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First Source Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PLR | SACK size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
/ SACK Vector /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Acknowledgement Packet Format
Common Packet Header: a common packet header (as common header
format) where Packet Type=2.
Nb missing source symbols: the number of missing source symbols in
the receiver since the beginning of the session.
Nb of not already used coded symbols: the number of coded symbols at
the receiver that have not already been used for decoding (e.g., the
linear combinations contain at least 2 unknown source symbols).
First Source Symbol ID: ID of the first source symbol to consider for
acknowledgment.
PLR: packet loss ratio expressed as a percentage. This value is used
in the case of dynamic code rate or for statistical purpose. The
choice of calculation is left to the appreciation of the developer
but should be the PLR seen before decoding.
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 from the
first source symbol ID. The "First Source Symbol" is included in
this bit vector. A bit equal to 1 at the i-th position means that
this acknowledgment packet acknowledges the source symbol of ID equal
to "First Source Symbol ID" + i.
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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 Tetrys encoders or decoders.
5.2. Table of Identifiers
0000: GF2 (or GF(2**1)) Vandermonde based coefficients. Each
coefficient is built as alpha^( (source_symbol_id*coded-symbol_id) %
2).
0001: GF16 (or GF(2**4)) Vandermonde based coefficients. Each
coefficient is built as alpha^( (source_symbol_id*coded-symbol_id) %
16).
0010: GF256 (or GF(2**8)) Vandermonde based coefficients. Each
coefficient is built as alpha^( (source_symbol_id*coded_symbol_id) %
256).
0011: SRLC.
Others: To be discussed.
6. Tetrys Basic Functions
6.1. Encoding
At the beginning of a transmission, a Tetrys Encoding Building Block
or MUST choose an initial code rate (added redundancy) as it doesn't
know the packet loss rate of the channel. In the steady state,
depending on the code-rate, the Tetrys Encoding Building Block CAN
generate coded symbols when it receives a source symbol from the
application or some feedback 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
that are not yet acknowledged by the receiver.
A Tetrys Encoding Building Block SHOULD set a limit to the Elastic
Encoding Window maximum size. This controls the algorithmic
complexity at the encoder and decoder by limiting the size of linear
combinations. It is also needed in situations where acknowledgment
packets are all lost or absent.
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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 typical example of such a deterministic
function is a generator matrix where the rows are indexed by the
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 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
Each coded packet contains an encoding vector. The encoding vectors
CAN contain the ID and/or coefficient of each source symbol contained
in the coded symbol.
6.1.1.1. Transmitting the source symbol IDs
The source symbol IDs are organized as a sorted list of 32-bit
unsigned integers. Depending on the feedback, the source symbol IDs
can be successive or not in the list.
If they are successive, the boundaries are stored in the encoding
vector: it just needs 2*32-bit of information.
If not, the edge blocks CAN be stored directly, or a differential
transform to reduce the number of bits needed to represent an ID CAN
be used.
6.1.1.1.1. Compressed list of 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.
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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]
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 number of blocks MUST be different
from 0.
The encoding vector format uses a 4-bit Coding Coefficient Generator
Identifier to identify the algorithm to generate the coefficients.
It 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.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EV_LEN | CCGI | I |C|V| NB_IDS | 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) (8-bits): size in units of 32-bit
words.
o Coding Coefficient Generator Identifier (CCGI): 4-bit ID to
identify the algorithm or the function used to generate the
coefficients (see Section 5 ). As a CCGI is included in each
encoded vector, it can dynamically change between the generation
of 2 coded symbols.
o Store the Source symbol IDs (I) (2 bits):
* 00 means there is no source symbol ID information.
* 01 means the encoding vector contains the edge blocks of the
source symbol IDs without compression.
* 10 means the encoding vector contains the compressed list of
the source symbol IDs.
* 11 means the encoding vector contains the compressed edge
blocks of the source symbol IDs.
o Store the coefficients (C): 1 bit to know if an encoding vector
contains information about the coefficients used.
o Having source symbols with variable size (V): set V to 1 if the
combination which refers to the encoding vector is a combination
of source symbols with variable sizes. In this case, the coded
packets MUST have the 'Encoded Payload Size' field.
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o Number of IDs used to store the source symbol IDs (NB_IDS): the
number of IDs stored (depending on I).
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 Information about the source symbol IDs (id_bit_vector): if I=01,
store the edge blocks as b_id * (NB_IDS * 2 - 1). If I=10, store
in a compressed way the edge blocks.
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).
6.2. 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 on 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 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:
o If the acknowledgment packets are not enabled, the coding window
grows up to a limit. When the limit is reached, the oldest
symbols are removed from the coding window.
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o If the acknowledgment packets are enabled, a source symbol is
removed from the coding window when all the receivers have
received or decoded it or when the coding window reaches its
limit.
6.3. Decoding
A classical matrix inversion is sufficient to recover the source
symbols.
7. Research Issues
The design of Tetrys protocol presented in this document provides the
baseline allowing communication between a Tetrys encoder and a Tetrys
decoder. At this stage, the detailed specifications only focus on
the coding and decoding aspects. The objective of this document is
first to provide guidelines to implement Tetrys as a standalone
protocol or to embed Tetrys inside an existing protocol at the
application layer or the IP layer. However, both cases raise
manifold research efforts to come up with a complete protocol
specification. Despite mandatory communication protocol operations
such as opening/closing procedures and timeout/reset, we identified
the following research issues that would need further discussion.
7.1. Interaction with Existing Congestion-Controlled Transport Protocol
Tetrys coding and congestion control can be seen as two separate
channels. In practice, implementations may mix the signals exchanged
on these channels. This raises several concerns that must be tackled
when considering using Tetrys conjointly with a congestion-controlled
transport protocol. All these numerous research issues are discussed
in a separate document [I-D.irtf-nwcrg-coding-and-congestion] . In
particular, this document investigates end-to-end unicast data
transfer with FEC coding in the application (above the transport),
within the transport, or directly below the transport; the
relationship between transport layer and application requirements;
and the case of transport multipath and multi-streams applications.
7.2. Adaptive Coding Rate
In a particular context, a redundancy adaptation algorithm might be
considered helpful or mandatory when the network condition (e.g.,
delay, loss rate) strongly varies over time. Hence, it requires an
enhanced mechanism for erasure codes to adapt to network dynamics
similarly to [A-FEC] . However, the dynamic adaptation of an on-the-
fly coding rate is slightly more complex than a block code.
Furthermore, this adaptation can be done conjointly with the network
as proposed in [RED-FEC] . In this paper, the authors propose a
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Random Early Detection FEC mechanism in the context of video
transmission over wireless networks. In brief, the idea is to add
more redundancy packets if the queue at the access point is less
occupied and vice versa. A first theoretical attempt for video
delivery has been proposed [THAI] with Tetrys. However, this kind of
algorithms should deserve more research to be deployed in practice.
7.3. Using Tetrys Above The IP Layer For Tunneling
The use of Tetrys to protect from losses an aggregate of flows raise
various issues. This occurs when an encoding mechanism is enabled
below the IP layer and builds redundancy without flows
differentiation. This is typically the case in a tunnel. The main
problem relates to head-of-line blocking when decoding multiple
flows. The number of source packets might vary following their own
loss probability and lead to decoding blocking in waiting for source
data packets to be suppressed from a given repair packet. This kind
of issue could lead to a decrease of the decoding performance and
should be further investigated. Note this research issue joins the
topics discussed in the IRTF LOOPS working group
[I-D.li-tsvwg-loops-problem-opportunities] .
8. Security Considerations
Tetrys inherits a subset of the security issues described as those
described in FECFRAME [RFC8680] and in particular in sections "9.2.2.
Content Corruption" and "9.3. Attacks against the FEC Parameters".
As an application layer end-to-end protocol, security considerations
of Tetrys should also be comparable to those of HTTP/2 with TLS. The
considerations from Section 10 of HTTP2 [RFC7540] also apply in
addition to those listed here.
9. Privacy Considerations
N/A
10. IANA Considerations
N/A
11. Acknowledgments
First, the authors want to sincerely thank Marie-Jose Montpetit for
continuous help and support on Tetrys. Marie-Jo, many thanks!
The authors also wish to thank NWCRG group members for numerous
discussions on on-the-fly coding that helped finalize this document.
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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>.
[RFC3452] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley,
M., and J. Crowcroft, "Forward Error Correction (FEC)
Building Block", RFC 3452, DOI 10.17487/RFC3452, December
2002, <https://www.rfc-editor.org/info/rfc3452>.
[RFC5651] Luby, M., Watson, M., and L. Vicisano, "Layered Coding
Transport (LCT) Building Block", RFC 5651,
DOI 10.17487/RFC5651, October 2009,
<https://www.rfc-editor.org/info/rfc5651>.
[RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker,
"NACK-Oriented Reliable Multicast (NORM) Transport
Protocol", RFC 5740, DOI 10.17487/RFC5740, November 2009,
<https://www.rfc-editor.org/info/rfc5740>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[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>.
[RFC8680] Roca, V. and A. Begen, "Forward Error Correction (FEC)
Framework Extension to Sliding Window Codes", RFC 8680,
DOI 10.17487/RFC8680, January 2020,
<https://www.rfc-editor.org/info/rfc8680>.
12.2. Informative References
[A-FEC] Bolot, J., Fosse-Parisis, S., and D. Towsley, "Adaptive
FEC-based error control for Internet telephony", IEEE
INFOCOM 99, pp. 1453-1460 vol. 3 DOI
10.1109/INFCOM.1999.752166, 1999.
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[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.
[I-D.irtf-nwcrg-coding-and-congestion]
Kuhn, N., Lochin, E., Michel, F., and M. Welzl, "Coding
and congestion control in transport", draft-irtf-nwcrg-
coding-and-congestion-09 (work in progress), June 2021.
[I-D.li-tsvwg-loops-problem-opportunities]
Li, Y., Zhou, X., Boucadair, M., Wang, J., and F. Qin,
"LOOPS (Localized Optimizations on Path Segments) Problem
Statement and Opportunities for Network-Assisted
Performance Enhancement", draft-li-tsvwg-loops-problem-
opportunities-06 (work in progress), July 2020.
[RED-FEC] Lin, C., Shieh, C., Chilamkurti, N., Ke, C., and H. Hwang,
"A RED-FEC Mechanism for Video Transmission Over WLANs",
IEEE Transactions on Broadcasting, vol. 54, no. 3, pp.
517-524 DOI 10.1109/TBC.2008.2001713, September 2008.
[Tetrys] Lacan, J. and E. Lochin, "Rethinking reliability for long-
delay networks", International Workshop on Satellite and
Space Communications 2008 (IWSSC08), October 2008.
[THAI] Tran-Thai, T., Lacan, J., and E. Lochin, "Joint on-the-fly
network coding/video quality adaptation for real-time
delivery", Signal Processing: Image Communication, vol. 29
(no. 4), pp. 449-461 ISSN 0923-5965, 2014.
Authors' Addresses
Jonathan Detchart
ISAE-SUPAERO
10, avenue Edouard Belin
BP 54032
Toulouse CEDEX 4 31055
France
Email: jonathan.detchart@isae-supaero.fr
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Emmanuel Lochin
ENAC
7, avenue Edouard Belin
Toulouse 31400
France
Email: emmanuel.lochin@enac.fr
Jerome Lacan
ISAE-SUPAERO
10, avenue Edouard Belin
BP 54032
Toulouse CEDEX 4 31055
France
Email: jerome.lacan@isae-supaero.fr
Vincent Roca
INRIA
655, avenue de l'Europe
Inovallee; Montbonnot
ST ISMIER cedex 38334
France
Email: vincent.roca@inria.fr
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