Internet Research Task Force (IRTF) J. Detchart
Request for Comments: 9407 ISAE-SUPAERO
Category: Experimental E. Lochin
ISSN: 2070-1721 ENAC
J. Lacan
ISAE-SUPAERO
V. Roca
INRIA
May
June 2023
Tetrys: An On-the-Fly Network Coding Protocol
Abstract
This document describes Tetrys, which is an on-the-fly network coding
protocol that can be used to transport delay-sensitive and loss-
sensitive data over a lossy network. Tetrys may recover from
erasures within an RTT-independent delay thanks to the transmission
of coded packets. This document is a record of the experience gained
by the authors while developing and testing the Tetrys protocol in
real conditions.
This document is a product of the Coding for Efficient NetWork
Communications Research Group (NWCRG). It conforms to the NWCRG
taxonomy described in RFC 8406.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Research Task
Force (IRTF). The IRTF publishes the results of Internet-related
research and development activities. These results might not be
suitable for deployment. This RFC represents the consensus of the
Coding for Efficient NetWork Communications Research Group of the
Internet Research Task Force (IRTF). Documents approved for
publication by the IRSG are not candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9407.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction
1.1. Requirements Notation
2. Definitions, Notations, and Abbreviations
3. Architecture
3.1. Use Cases
3.2. Overview
4. Tetrys Basic Functions
4.1. Encoding
4.2. The Elastic Encoding Window
4.3. Decoding
5. Packet Format
5.1. Common Header Format
5.1.1. Header Extensions
5.2. Source Packet Format
5.3. Coded Packet Format
5.3.1. The Encoding Vector
5.4. Window Update Packet Format
6. Research Issues
6.1. Interaction with Congestion Control
6.2. Adaptive Coding Rate
6.3. Using Tetrys below the IP Layer for Tunneling
7. Security Considerations
7.1. Problem Statement
7.2. Attacks against the Data Flow
7.3. Attacks against Signaling
7.4. Attacks against the Network
7.5. Baseline Security Operation
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
This document is a product of and represents the collaborative work
and consensus of the Coding for Efficient NetWork Communications
Research Group (NWCRG). It is not an IETF product or an IETF
standard.
This document describes Tetrys, which is an on-the-fly network coding
protocol that can be used to transport delay-sensitive and and loss-
sensitive data over a lossy network. 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 network codes have seen some deployment fairly recently in the
Internet community, the use of application-layer erasure codes in the
IETF has already been standardized in the RMT [RFC5052] [RFC5445] and
FECFRAME [RFC8680] Working Groups. The protocol presented here may
be seen as a network-coding extension to standard unicast transport
protocols (or even multicast or anycast with a few modifications).
The current proposal may be considered a combination of network
erasure coding and feedback mechanisms [Tetrys] [Tetrys-RT].
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 feedback. These feedback messages provide
to the sender information about the highest sequence number received
or rebuilt, which can enable the flushing the corresponding source
packets stored in the encoding window. The size of this window may
be fixed or dynamically updated. If the window is full, incoming
source packets replace older source packets that are dropped. As a
matter of fact, its limit should be correctly sized. Finally, Tetrys
allows dealing with losses on both the forward and return paths and
is particularly resilient to acknowledgment losses. All these
operations are further detailed in Section 4.
With Tetrys, a coded packet is a linear combination over a finite
field of the data source packets belonging to the coding window. The
choice of coefficients, as finite fields elements, is a trade-off
between the best erasure recovery performance (finite fields of 256
elements) and the system constraints (finite fields of 16 elements
are preferred) and is driven by the application.
Thanks to the elastic encoding window, the coded packets are built
on-the-fly by using a predefined method to choose the coefficients.
The redundancy ratio may be dynamically adjusted and the coefficients
may be generated in different ways during the transmission. Compared
to Forward Error Correction (FEC) block codes, this reduces the
bandwidth use and the decoding delay.
The design description of the Tetrys protocol in this document is
complemented by a record of the experience gained by the authors
while developing and testing the Tetrys protocol in realistic
conditions. In particular, several research issues are discussed in
Section 6 following our own experience and observations.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Definitions, Notations, and Abbreviations
The notation used in this document is based on the NWCRG taxonomy
[RFC8406].
Source Symbol: A symbol that is 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 encoder.
Output Symbol: A symbol generated by the Tetrys encoder. For a non-
systematic mode, all output symbols are coded symbols. For a
systematic mode, output symbols MAY be the input symbols and a
number of coded symbols that are linear combinations of the input
symbols plus the encoding vectors.
Feedback Packet: A feedback packet is a packet containing
information about the decoded or received source symbols. It MAY
also contain 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
unacknowledged source packets of the input flow involved in the
coding process.
Coding Coefficient Generator Identifier (CCGI): A unique identifier
that defines a function or an algorithm allowing the generation of
the encoding vector.
Code Rate: Defines the rate between the number of input symbols and
the number of output symbols.
3. Architecture
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 MAY be
a multiplex of several upper-layer streams. Transmission MAY be over
a single path or multiple paths. This is the simplest use case that
is quite 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 include:
* on-the-fly encoding;
* decoding;
* signaling, to carry in particular the symbol IDs in the encoding
window and the associated coding coefficients when meaningful;
* feedback management;
* elastic window management; and
* Tetrys packet header creation and processing.
The optional features include:
* channel estimation;
* dynamic adjustment of the code rate and flow control; and
* congestion control management (if appropriate). See Section 6.1
for further details.
Several building blocks provide the following functionalities:
The Tetrys Building Block: This building block embeds both the
Tetrys decoder and Tetrys encoder; thus, it 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 may be used.
The Window Management Building Block: This building block is in
charge of managing the encoding window at a Tetrys sender.
To ease the addition of future components and services, Tetrys adds a
header extension mechanism that is compatible with that of Layered
Coding Transport (LCT) [RFC5651], NACK-Oriented Reliable Multicast
(NORM) [RFC5740], and FEC Framework (FECFRAME) [RFC8680].
4. Tetrys Basic Functions
4.1. Encoding
At the beginning of a transmission, a Tetrys encoder MUST choose an
initial code rate that adds redundancy as it doesn't know the packet
loss rate of the channel. In the steady state, the Tetrys encoder
MAY generate coded symbols when it receives a source symbol from the
application or some feedback from the decoding blocks depending on
the code rate.
When a Tetrys encoder needs to generate a coded symbol, it considers
the set of source symbols stored in the elastic encoding window and
generates an encoding vector with the coded symbol. These source
symbols are the set of source symbols that are not yet acknowledged
by the receiver. For each source symbol, a finite field coefficient
is determined using a Coding Coefficient Generator. This generator
MAY take the source symbol IDs and the coded symbol ID as an input
and MAY determine a coefficient in a deterministic way as presented
in Section 5.3. Finally, the coded symbol is the sum of the source
symbols multiplied by their corresponding coefficients.
A Tetrys encoder MUST 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 all window update packets are lost
or absent.
4.2. The Elastic Encoding Window
When an input source symbol is passed to a Tetrys encoder, it is
added to the elastic encoding window. This window MUST have a limit
set by the encoding building block. If the elastic encoding window
has reached its limit, the window slides over the symbols. The first
(oldest) symbol is removed, and the newest symbol is added. 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 Tetrys decoder sends periodic feedback
indicating the received or decoded source symbols. When the sender
receives the information that a source symbol was received or decoded
by the receiver, it removes this symbol from the coding window.
4.3. Decoding
A standard Gaussian elimination is sufficient to recover the erased
source symbols when the matrix rank enables it.
5. Packet Format
5.1. 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 | C |S| Reserved | HDR_LEN | PKT_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 noted above, this format is inspired by, and inherits from, the
LCT header format [RFC5651] with slight modifications.
Tetrys version number (V): 4 bits. Indicates the Tetrys version
number. The Tetrys version number for this specification is 1.
Congestion control flag (C): 2 bits. C set to 0b00 indicates the
Congestion Control Information (CCI) field is 0 bits in length. C
set to 0b01 indicates the CCI field is 32 bits in length. C set
to 0b10 indicates the CCI field is 64 bits in length. C set to
0b11 indicates the CCI field is 96 bits in length.
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.
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.
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 may be used to directly
access the portion of the packet beyond the Tetrys header, i.e.,
to the first next header if it exists, 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.
Tetrys packet type (PKT_TYPE): 8 bits. There are three types of
packets: the PKT_TYPE_SOURCE (0b00) defined in Section 5.2, the
PKT_TYPE_CODED (0b01) defined in Section 5.3 and the
PKT_TYPE_WND_UPT (0b11) for window update packets defined in
Section 5.4.
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 is set to 0b00. This field MUST be 32 bits if C is set to
0b01. This field MUST be 64 bits if C is set to 0b10. This field
MUST be 96 bits if C is set to 0b11.
Transport Session Identifier (TSI): 0 or 32 bits. The TSI uniquely
identifies a session among all sessions from a particular Tetrys
encoder. The TSI is scoped by the IP address of the sender; thus,
the IP address of the sender and the TSI together uniquely
identify the session. Although a TSI always uniquely identifies a
session conjointly with the IP address of the sender, whether 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.
5.1.1. Header Extensions
Header extensions are used in Tetrys to accommodate optional header
fields that are not always used or have variable sizes. The presence
of header extensions MAY 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 for the future
introduction of backward-compatible enhancements to Tetrys without
changing the Tetrys version number. Header extensions that are not
backward-compatible MUST NOT 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.
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
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.
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).
Header Extension Content (HEC): Length of the variable. The content
of the header extension. The format of this subfield 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 a variable size as specified by the HEL field. Note
that the length of each header extension MUST be a multiple of 32
bits. Additionally, the total size of the Tetrys header,
including all header extensions and optional header fields, cannot
exceed 255 32-bit words.
5.2. Source Packet Format
A source packet is a common packet header encapsulation, a source
symbol ID, and a source symbol (payload). The source symbols MAY
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 (as common header
format) where packet type is set to 0b00.
Source Symbol ID: The sequence number to identify a source symbol.
Payload: The payload (source symbol).
5.3. 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 MAY have variable sizes, all the
source symbol sizes need to be encoded. To generate this encoded
payload size as a 16-bit unsigned value, the linear combination uses
the same coefficients as the coded payload. 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 5.3.1).
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 is set to 0b01.
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 5.3.1).
Payload: The coded symbol.
5.3.1. The Encoding Vector
An encoding vector contains all the information about the linear
combination used to generate a coded symbol. The information
includes the source identifiers and the coefficients used for each
source symbol. It MAY be stored in different ways depending on the
situation.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ coef_bit_vector +-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Encoding Vector Format
Encoding Vector Length (EV_LEN): 8 bits. The size in units of
32-bit words.
Coding Coefficient Generator Identifier (CCGI): 4-bit ID to identify
the algorithm or function used to generate the coefficients. As a
CCGI is included in each encoded vector, it MAY dynamically change
between the generation of two coded symbols. The CCGI builds the
coding coefficients used to generate the coded symbols. They MUST
be known by all the Tetrys encoders or decoders. The two RLC FEC
schemes specified in this document reuse the finite fields defined
in [RFC5510], Section 8.1. More specifically, the elements of the
field GF(2^(m)) are represented by polynomials with binary
coefficients (i.e., over GF(2)) and with degree lower or equal to
m-1. The addition between two elements is defined as the addition
of binary polynomials in GF(2), which is equivalent to a bitwise
XOR operation on the binary representation of these elements.
With GF(2^(8)), multiplication between two elements is the
multiplication modulo a given irreducible polynomial of degree 8.
The following irreducible polynomial is used for GF(2^(8)):
x^(8) + x^(4) + x^(3) + x^(2) + 1
With GF(2^(4)), multiplication between two elements is the
multiplication modulo a given irreducible polynomial of degree 4.
The following irreducible polynomial is used for GF(2^(4)):
x^(4) + x + 1
* 0b00: Vandermonde-based coefficients over the finite field
GF(2^(4)) as defined below. Each coefficient is built as
alpha^( (source_symbol_id*coded-symbol_id) % 16), with alpha
the root of the primitive polynomial.
* 0b01: Vandermonde-based coefficients over the finite field
GF(2^(8)) as defined below. Each coefficient is built as
alpha^( (source_symbol_id*coded-symbol_id) % 256), with alpha
the root of the primitive polynomial.
* Suppose we want to generate the coded symbol 2 as a linear
combination of the source symbols 1, 2, and 4 using CCGI set to
0b01. The coefficients will be alpha^( (1 * 1) % 256), alpha^(
(1 * 2) % 256), and alpha^( (1 * 4) % 256).
Store the Source Symbol ID Format (I) (2 bits):
* 0b00 means there is no source symbol ID information.
* 0b01 means the encoding vector contains the edge blocks of the
source symbol IDs without compression.
* 0b10 means the encoding vector contains the compressed list of
the source symbol IDs.
* 0b11 means the encoding vector contains the compressed edge
blocks of the source symbol IDs.
Store the Encoding Coefficients (C): 1 bit to indicate if an
encoding vector contains information about the coefficients used.
Having Source Symbols with Variable Size Encoding (V): Set V to 0b01
if the combination that 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.
NB_IDS: The number of source IDs stored in the encoding vector
(depending on I).
Number of Coefficients (NB_COEFS): The number of the coefficients
used to generate the associated coded symbol.
The First Source Identifier (FIRST_SOURCE_ID): The first source
symbol ID used in the combination.
Number of Bits for Each Edge Block (b_id): The number of bits needed
to store the edge.
Information about the Source Symbol IDs (id_bit_vector): If I is set
to 0b01, store the edge blocks as b_id * (NB_IDS * 2 - 1). If I
is set to 0b10, store the edge blocks in a compressed way.
The Coefficients (coef_bit_vector): The coefficients stored
depending on the CCGI (4 or 8 bits for each coefficient).
Padding: Padding to have an encoding vector size that is a multiple
of 32 bits (for the ID and coefficient part).
The source symbol IDs are organized as a sorted list of 32-bit
unsigned integers. Depending on the feedback, the source symbol IDs
in the list MAY be successive or not. If they are successive, the
boundaries are stored in the encoding vector; it just needs 2*32 bits
of information. If not, the full list or the edge blocks MAY be
stored and a differential transform to reduce the number of bits
needed to represent an identifier MAY be used.
For the following subsections, let's take as an example the
generation of an encoding vector for a coded symbol that is a linear
combination of the source symbols with IDs 1, 2, 3, 5, 6, 8, 9, and
10 (or as edge blocks: [1..3], [5..6], [8..10]).
There are several ways to store the source symbol IDs into the
encoding vector:
* If no information about the source symbol IDs is needed, the field
I MUST be set to 0b00: no b_id and no id_bit_vector field.
* If the edge blocks are stored without compression, the field I
MUST be set to 0b01. In this case, set b_id to 32 (as a Symbol ID
is 32 bits), and store the list of 32-bit unsigned integers (1, 3,
4, 5, 6, 10) into id_bit_vectors.
* If the source symbol IDs are stored as a list with compression,
the field I MUST be set to 0b10. In this case, see
Section 5.3.1.1, but rather than compressing the edge blocks, we
compress the full list of the source symbol IDs.
* If the edge blocks are stored with compression, the field I MUST
be set to 0b11. In this case, see Section 5.3.1.1.
5.3.1.1. Compressed List of Source Symbol IDs
Let's continue with our coded symbol defined in the previous section.
The source symbol IDs used in the linear combination are: [1..3],
[5..6], [8..10].
If we want to compress and store this list into the encoding vector,
we MUST follow this procedure:
1. Keep the first element in the packet as the first_source_id: 1.
2. Apply a differential transform to the other elements ([3, 5, 6,
8, 10]) that 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))), where max(L) represents the maximum
of the elements of the list L; here, it is 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.
5.3.1.2. Decompressing the Source Symbol IDs
When a Tetrys decoding 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].
5.4. Window Update Packet Format
A Tetrys decoder MAY send window update packets back to another
building block. They contain information about what the packets
received, decoded, or dropped, and other information such as a packet
loss rate or the size of the decoding buffers. They are used to
optimize the content of the encoding window. The window update
packets are OPTIONAL; hence, they could be omitted or lost in
transmission without impacting the protocol behavior.
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_missing_src |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nb_not_used_coded_symb |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| first_src_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| plr | sack_size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
/ SACK Vector /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Window Update Packet Format
Common Packet Header: A common packet header (as common header
format) where packet type is set to 0b10.
nb_missing_src: The number of missing source symbols in the receiver
since the beginning of the session.
nb_not_used_coded_symb: The number of coded symbols at the receiver
that have not already been used for decoding (e.g., the linear
combinations contain at least two unknown source symbols).
first_src_id: ID of the first source symbol to consider in the
selective acknowledgment (SACK) vector.
plr: Packet loss ratio expressed as a percentage normalized to an
8-bit unsigned integer. For example, 2.5% will be stored as
floor(2.5 * 256/100) = 6. Conversely, if 6 is the stored value,
the corresponding packet loss ratio expressed as a percentage is
6*100/256 = 2.34%. This value is used in the case of dynamic code
rate or for a statistical purpose. The choice of calculation is
left to the Tetrys decoder, depending on a window observation, but
should be the PLR seen before decoding.
sack_size: The size of the SACK vector in 32-bit words. For
instance, with a value of 2, the SACK vector is 64 bits long.
SACK vector: Bit vector indicating symbols that must be removed in
the encoding window from the first source symbol ID. In most
cases, these symbols were received by the receiver. The other
cases concern some events with non-recoverable packets (i.e., in
the case of a burst of losses) where it is better to drop and
abandon some packets and remove them from the encoding window to
allow the recovery of the following packets. The "First Source
Symbol" is included in this bit vector. A bit equal to 1 at the
i-th position means that this window update packet removes the
source symbol of the ID equal to "First Source Symbol ID" + i from
the encoding window.
6. Research Issues
The present document describes the baseline protocol, allowing
communications between a Tetrys encoder and Tetrys decoder. In
practice, Tetrys can be used either as a standalone protocol or
embedded inside an existing protocol, and either above, within, or
below the transport layer. There are different research questions
related to each of these scenarios that should be investigated for
future protocol improvements. We summarize them in the following
subsections.
6.1. Interaction with Congestion Control
The Tetrys and congestion control components generate two separate
channels (see [RFC9265], Section 2.1):
* The Tetrys channel carries source and coded packets (from the
sender to the receiver) and information from the receiver to the
sender (e.g., signaling which symbols have been recovered, loss
rate before and/or after decoding, etc.).
* The congestion control channel carries packets from a sender to a
receiver and packets signaling information about the network
(e.g., number of packets received versus lost, Explicit Congestion
Notification (ECN) marks, etc.) from the receiver to the sender.
The following topics, which are identified and discussed by
[RFC9265], are adapted to the particular deployment cases of Tetrys
(i.e., above, within, or below the transport layer):
* Congestion-related losses may be hidden if Tetrys is deployed
below the transport layer without any precaution (i.e., Tetrys
recovering packets lost because of a congested router), which can
severely impact the congestion control efficiency. An approach is
suggested to avoid hiding such signals in [RFC9265], Section 5.
* Tetrys and non-Tetrys flows sharing the same network links can
raise fairness issues between these flows. In particular, the
situation depends on whether some of these flows and not others
are congestion controlled and which type of congestion control is
used. The details are out of scope of this document, but may have
major impacts in practice.
* Coding rate adaptation within Tetrys can have major impacts on
congestion control if done inappropriately. This topic is
discussed more in detail in Section 6.2.
* Tetrys can leverage multipath transmissions, with the Tetrys
packets being sent to the same receiver through multiple paths.
Since paths can largely differ, a per-path flow control and
congestion control adaptation could be needed.
* Protecting several application flows within a single Tetrys flow
raises additional questions. This topic is discussed more in
detail in Section 6.3.
6.2. Adaptive Coding Rate
When the network conditions (e.g., delay and loss rate) strongly vary
over time, an adaptive coding rate can be used to increase or reduce
the amount of coded packets among a transmission dynamically (i.e.,
the added redundancy) with the help of a dedicated algorithm similar
to [A-FEC]. Once again, the strategy differs depending on which
layer Tetrys is deployed (i.e., above, within, or below the transport
layer). Basically, we can split these strategies into two distinct
classes: Tetrys deployment inside the transport layer versus outside
the transport layer (i.e., above or below). A deployment within the
transport layer means that interactions between transport protocol
mechanisms such as error recovery, congestion control, and/or flow
control are envisioned. Otherwise, deploying Tetrys within a
transport protocol that is not congestion controlled, like UDP, would
not bring out any other advantage than deploying it below or above
the transport layer.
The impact deploying a FEC mechanism within the transport layer is
further discussed in Section 4 of [RFC9265], where considerations
concerning the interactions between congestion control and coding
rates, or the impact of fairness, are investigated. This adaptation
may be done jointly with the congestion control mechanism of a
transport layer protocol as proposed by [CTCP]. This allows the use
of monitored congestion control metrics (e.g., RTT, congestion
events, or current congestion window size) to adapt the coding rate
conjointly with the computed transport sending rate. The rationale
is to compute an amount of repair traffic that does not lead to
congestion. This joint optimization is mandatory to prevent flows
from consuming the whole available capacity as discussed in
[RMCAT-ADAPTIVE-FEC], where the authors point out that an increase in
the repair ratio should be done conjointly with a decrease in the
source sending rate.
Finally, adapting a coding rate can also be done outside the
transport layer without considering transport-layer metrics. In
particular, this adaptation may be done jointly with the network as
proposed in [RED-FEC]. In this paper, the authors propose a Random
Early Detection FEC mechanism in the context of video transmission
over wireless networks. Briefly, 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 with Tetrys
has been proposed [THAI]. This approach is interesting as it
illustrates a joint collaboration between the application
requirements and the network conditions and combines both signals
coming from the application needs and the network state (i.e.,
signals below or above the transport layer).
To conclude, there are multiple ways to enable an adaptive coding
rate. However, all of them depend on:
* the signal metrics that can be monitored and used to adapt the
coding rate;
* the transport layer used, whether it is congestion controlled or
not; and
* the objective sought (e.g., to minimize congestion or to fit
application requirements).
6.3. Using Tetrys below the IP Layer for Tunneling
The use of Tetrys to protect an aggregate of flows raises research
questions when Tetrys is used to recover from IP datagram losses
while tunneling. Applying redundancy without flow differentiation
may contradict the service requirements of individual flows: some
flows may be penalized more by high latency and jitter than by
partial reliability, while other flows may be penalized more by
partial reliability. In practice, head-of-line blocking impacts all
flows in a similar manner despite their different needs, which
indicates that more elaborate strategies inside Tetrys are needed.
7. Security Considerations
First of all, it must be clear that the use of FEC protection on a
data stream does not provide any kind of security per se. On the
contrary, the use of FEC protection on a data stream raises security
risks. The situation with Tetrys is mostly similar to that of other
content delivery protocols making use of FEC protection; this is well
described in FECFRAME [RFC6363]. This section builds on this
reference, adding new considerations to comply with Tetrys
specificities when meaningful.
7.1. Problem Statement
An attacker can either target the content, protocol, or network. The
consequences will largely differ reflecting various types of goals,
like gaining access to confidential content, corrupting the content,
compromising the Tetrys encoder and/or Tetrys decoder, or
compromising the network behavior. In particular, several of these
attacks aim at creating a Denial-of-Service (DoS) with consequences
that may be limited to a single node (e.g., the Tetrys decoder), or
that may impact all the nodes attached to the targeted network (e.g.,
by making flows unresponsive to congestion signals).
In the following sections, we discuss these attacks, according to the
component targeted by the attacker.
7.2. Attacks against the Data Flow
An attacker may want to access confidential content by eavesdropping
the traffic between the Tetrys encoder/decoder. Traffic encryption
is the usual approach to mitigate this risk, and this encryption can
be applied to the source flow upstream of the Tetrys encoder or to
the output packets downstream of the Tetrys encoder. The choice on
where to apply encryption depends on various criteria, in particular
the attacker model (e.g., when encryption happens below Tetrys, the
security risk is assumed to be on the interconnection network).
An attacker may also want to corrupt the content (e.g., by injecting
forged or modified source and coded packets to prevent the Tetrys
decoder from recovering the original source flow). Content integrity
and source authentication services at the packet level are then
needed to mitigate this risk. Here, these services need to be
provided below Tetrys in order to enable the receiver to drop
undesired packets and only transfer legitimate packets to the Tetrys
decoder. It should be noted that forging or modifying feedback
packets will not corrupt the content, although it will certainly
compromise Tetrys operation (see Section 7.3).
7.3. Attacks against Signaling
Attacks on signaling information (e.g., by forging or modifying
feedback packets to falsify the good reception or recovery of source
content) can easily prevent the Tetrys decoder from recovering the
source flow, thereby creating a DoS. In order to prevent this type
of attack, content integrity and source authentication services at
the packet level are needed for the feedback flow from the Tetrys
decoder to the Tetrys encoder as well. These services need to be
provided below Tetrys in order to drop undesired packets and only
transfer legitimate feedback packets to the Tetrys encoder.
Conversely, an attacker in position to selectively drop feedback
packets (instead of modifying them) will not severely impact the
function of Tetrys since it is naturally robust when challenged with
such losses. However, it will have side impacts, such as the use of
bigger linear systems (since the Tetrys encoder cannot remove well-
received or decoded source packets from its linear system), which
mechanically increases computational costs on both sides (encoder and
decoder).
7.4. Attacks against the Network
Tetrys can react to congestion signals (Section 6.1) in order to
provide a certain level of fairness with other flows on a shared
network. This ability could be exploited by an attacker to create or
reinforce congestion events (e.g., by forging or modifying feedback
packets) that can potentially impact a significant number of nodes
attached to the network. In order to mitigate the risk, content
integrity and source authentication services at the packet level are
needed to enable the receiver to drop undesired packets and only
transfer legitimate packets to the Tetrys encoder and decoder.
7.5. Baseline Security Operation
Tetrys can benefit from an IPsec / Encapsulating Security Payload
(IPsec/ESP) [RFC4303] that provides confidentiality, origin
authentication, integrity, and anti-replay services in particular.
IPsec/ESP can be used to protect the Tetrys data flows (both
directions) against attackers located within the interconnection
network or attackers in position to eavesdrop traffic, inject forged
traffic, or replay legitimate traffic.
8. IANA Considerations
This document has no IANA actions.
9. References
9.1. 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>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052,
DOI 10.17487/RFC5052, August 2007,
<https://www.rfc-editor.org/info/rfc5052>.
[RFC5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, DOI 10.17487/RFC5445, March 2009,
<https://www.rfc-editor.org/info/rfc5445>.
[RFC5510] Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo,
"Reed-Solomon Forward Error Correction (FEC) Schemes",
RFC 5510, DOI 10.17487/RFC5510, April 2009,
<https://www.rfc-editor.org/info/rfc5510>.
[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>.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011,
<https://www.rfc-editor.org/info/rfc6363>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8406] Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek,
F., Ghanem, S., Lochin, E., Masucci, A., Montpetit, M.,
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>.
[RFC9265] Kuhn, N., Lochin, E., Michel, F., and M. Welzl, "Forward
Erasure Correction (FEC) Coding and Congestion Control in
Transport", RFC 9265, DOI 10.17487/RFC9265, July 2022,
<https://www.rfc-editor.org/info/rfc9265>.
9.2. Informative References
[A-FEC] Bolot, J., Fosse-Parisis, S., and D. Towsley, "Adaptive
FEC-based error control for Internet telephony", IEEE
INFOCOM '99, Conference on Computer Communications, New
York, NY, USA, Vol. 3, pp. 1453-1460,
DOI 10.1109/INFCOM.1999.752166, March 1999,
<https://doi.org/10.1109/INFCOM.1999.752166>.
[AHL-00] Ahlswede, R., Cai, N., Li, S., and R. Yeung, "Network
information flow", IEEE Transactions on Information
Theory, Vol. 46, Issue 4, pp. 1204-1216,
DOI 10.1109/18.850663, July 2000,
<https://doi.org/10.1109/18.850663>.
[CTCP] Kim, M., Cloud, J., ParandehGheibi, A., Urbina, L., Fouli,
K., Leith, D., and M. Medard, "Network Coded TCP (CTCP)",
arXiv 1212.2291v3, April 2013,
<https://arxiv.org/abs/1212.2291>.
[RED-FEC] Lin, C., Shieh, C., Chilamkurti, N., Ke, C., and W. Hwang,
"A RED-FEC Mechanism for Video Transmission Over WLANs",
IEEE Transactions on Broadcasting, Vol. 54, Issue 3, pp.
517-524, DOI 10.1109/TBC.2008.2001713, September 2008,
<https://doi.org/10.1109/TBC.2008.2001713>.
[RMCAT-ADAPTIVE-FEC]
Singh, V., Nagy, M., Ott, J., and L. Eggert, "Congestion
Control Using FEC for Conversational Media", Work in
Progress, Internet-Draft, draft-singh-rmcat-adaptive-fec-
03, 20 March 2016, <https://datatracker.ietf.org/doc/html/
draft-singh-rmcat-adaptive-fec-03>.
[Tetrys] Lacan, J. and E. Lochin, "Rethinking reliability for long-
delay networks", International Workshop on Satellite and
Space Communications, Toulouse, France, pp. 90-94,
DOI 10.1109/IWSSC.2008.4656755, October 2008,
<https://doi.org/10.1109/IWSSC.2008.4656755>.
[Tetrys-RT]
Tournoux, P., Lochin, E., Lacan, J., Bouabdallah, A., and
V. Roca, "On-the-Fly Erasure Coding for Real-Time Video
Applications", IEEE Transactions on Multimedia, Vol. 13,
Issue 4, pp. 797-812, DOI 10.1109/TMM.2011.2126564, August
2011, <http://dx.doi.org/10.1109/TMM.2011.2126564>.
[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
Issue 4, pp. 449-461, DOI 10.1016/j.image.2014.02.003,
April 2014, <https://doi.org/10.1016/j.image.2014.02.003>.
Acknowledgments
First, the authors want sincerely to 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.
Finally, the authors would like to thank Colin Perkins for providing
comments and feedback on the document.
Authors' Addresses
Jonathan Detchart
ISAE-SUPAERO
BP 54032
10, avenue Edouard Belin
31055 Toulouse CEDEX 4
France
Email: jonathan.detchart@isae-supaero.fr
Emmanuel Lochin
ENAC
7, avenue Edouard Belin
31400 Toulouse
France
Email: emmanuel.lochin@enac.fr
Jerome Lacan
ISAE-SUPAERO
BP 54032
10, avenue Edouard Belin
31055 Toulouse CEDEX 4
France
Email: jerome.lacan@isae-supaero.fr
Vincent Roca
INRIA
Inovallee; Montbonnot
655, avenue de l'Europe
38334 St Ismier CEDEX
France
Email: vincent.roca@inria.fr