Internet DRAFT - draft-swett-nwcrg-coding-for-quic
draft-swett-nwcrg-coding-for-quic
nwcrg I. Swett
Internet-Draft Google
Intended status: Informational M-J. Montpetit
Expires: September 10, 2020 Triangle Video
V. Roca
INRIA
F. Michel
UCLouvain
March 9, 2020
Coding for QUIC
draft-swett-nwcrg-coding-for-quic-04
Abstract
This document focuses on the integration of FEC coding in the QUIC
transport protocol, in order to recover from packet losses. This
document does not specify any FEC code but defines mechanisms to
negotiate and integrate FEC Schemes in QUIC. By using proactive loss
recovery, it is expected to improve QUIC performance in sessions
impacted by packet losses. More precisely it is expected to improve
QUIC performance with real-time sessions (since FEC coding makes
packet loss recovery insensitive to the round trip time), with short
sessions (since FEC coding can help recovering from tail losses more
rapidely than through retransmissions), with multicast sessions
(since the same repair packet can recover several different losses at
several receivers), and with multipath sessions (since repair packets
add diversity and flexibility).
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 10, 2020.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 3
3. General Design Considerations . . . . . . . . . . . . . . . . 4
3.1. FEC Code versus FEC Scheme, Block Codes versus Sliding
Window Codes . . . . . . . . . . . . . . . . . . . . . . 4
3.2. FEC Scheme Negotiation . . . . . . . . . . . . . . . . . 4
3.3. FEC Protection Within an Encrypted Channel . . . . . . . 5
3.4. About Middleboxes . . . . . . . . . . . . . . . . . . . . 5
4. FEC Protection Principles . . . . . . . . . . . . . . . . . . 5
4.1. Cross Packet Frames FEC Encoding . . . . . . . . . . . . 5
4.2. Source Symbol Definition . . . . . . . . . . . . . . . . 6
4.2.1. Packet Payload to Packet Chunk Mapping . . . . . . . 6
4.2.2. Packet Chunk to Source Symbol Mapping . . . . . . . . 7
4.2.2.1. Open questions: Content of Source Symbols
Metadata? Removing certain frames from FEC
protection? . . . . . . . . . . . . . . . . . . . 9
4.2.3. Source Symbol Size (E) Considerations . . . . . . . . 10
4.3. Source Symbol Signaling . . . . . . . . . . . . . . . . . 11
4.4. Repair Symbol Signaling . . . . . . . . . . . . . . . . . 11
4.5. Signaling a Symbol Recovery . . . . . . . . . . . . . . . 11
4.6. About Gaps in the Set of Source Symbols Considered During
Encoding . . . . . . . . . . . . . . . . . . . . . . . . 12
5. FEC Scheme Negotiation in QUIC . . . . . . . . . . . . . . . 12
5.1. FEC Scheme Negotiation . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
QUIC is a new transport that aims at improving network performance by
enabling out of order delivery, partial reliability, and methods of
recovery besides retransmission, while also improving security. This
document specifies a framework to enable FEC codes to be used to
recover from lost packets within a single QUIC stream or across
several QUIC streams.
The ability to add FEC coding in QUIC may be beneficial in several
situations:
o for a robust transmission of latency sensitive traffic, for
instance real-time flows, since it enables to recover packet
losses independently of the round trip time;
o for short sessions, in order to protect the last few packets sent,
since it enables to recover from tail losses more rapidely than
through retransmissions;
o for the transmission of contents to a large set of QUIC reception
endpoints, since the same repair frame may help recovering several
different packet losses at different receivers;
o for multipath communications, since repair traffic adds diversity
and flexibility.
This framework does not mandate the use of any specific FEC code
(i.e., how to encode and decode) nor FEC Scheme (i.e., that specifies
both a FEC code and how to use it, in particular in terms of
signaling). Instead it allows to negotiate the FEC Scheme to use at
session startup, assuming that more than one solution could
potentially be offered concurrently. Without loss of generality, we
assume that the encoding operations compute a linear combination of
QUIC packets, regardless of whether these codes are of block type (as
with Reed-Solomon codes [RFC5510]) or sliding window type (as with
RLC codes [RFC8681]).
2. Definitions and Abbreviations
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].
Terms and definitions that apply to coding are available in
[nc-taxonomy]. More specifically, this document uses the following
definitions:
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Packet versus Symbol: a Packet is the unit of data that is exchanged
over the network while a Symbol is the unit of data that is
manipulated during the encoding and decoding operations
Source Symbol: a unit of data originating from the source that is
used as input to encoding operations
Repair Symbol: a unit of data that is the result of a coding
operation
This document uses the following abbreviations:
E: size of an encoding symbol (i.e., source or repair symbol),
assumed fixed (in bytes)
3. General Design Considerations
This section lists a few general considerations that govern the
framework for FEC coding support in QUIC.
3.1. FEC Code versus FEC Scheme, Block Codes versus Sliding Window
Codes
A FEC code specifies the details of encoding and decoding operations.
In addition to that, a FEC Scheme defines the additional protocol
aspects required to use a particular FEC code [nc-taxonomy]. In
particular the FEC Scheme defines signaling (e.g., information
contained in Source and Repair Packet header or trailers) needed to
synchronize encoders and decoders.
Block coding (e.g., Reed-Solomon [RFC5510]) and sliding window coding
(e.g., RLC [RFC8681]) are two broad classes of FEC codes
[nc-taxonomy]. In the first case, the input flow must be first
segmented into a sequence of blocks, FEC encoding and decoding being
performed independently on a per-block basis. In the second case
rely, a sliding encoding window continuously slides over the input
flow. It is envisioned that the two classes of codes could be used
to bring FEC protection to QUIC, usually with an advantage for
sliding window codes when it comes to low latency communications.
3.2. FEC Scheme Negotiation
There are multiple FEC Scheme candidates. Therefore a negotiation
step is needed to select one or more codes to be used over a QUIC
session. This will be implemented using the one step negotiation of
the new QUIC negotiation mechanism [QUIC-transport], during the QUIC
handshake.
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Editor's notes:
* It is likely that FEC Scheme negotiation requires the use of a
new dedicated Extension Frame Type. To Be Clarified and text
updated.
* It is not clear whether negotiation is meant to select a
**single** FEC Scheme or **multiple** FEC Schemes. In the
second case (multiple FEC) it is required to have a
complementary mechanism to indicate which FEC Scheme is used
in a given REPAIR frame (which could be done through as many
REPAIR frame type values as potential FEC Scheme negotiated).
Is it what we want to achieve? Not sure.
3.3. FEC Protection Within an Encrypted Channel
FEC encoding is applied before any QUIC encryption and authentication
processing. Source symbols, that constitute the data units used by
the FEC codec, contain cleartext data (application and/or QUIC data).
3.4. About Middleboxes
The coding approach described in this document does not allow on path
elements (middleboxes) to take part in FEC protection. The traffic
being encrypted end-to-end, the middleboxes are not in position to
perform FEC decoding, nor to add any redundant traffic.
4. FEC Protection Principles
The present section explains how FEC encoding can be applied to QUIC.
It defines the general ideas for mapping QUIC packet frames to source
symbols, as well as the associated signaling. This section does not
define the FEC Scheme specific details that need to be specified in a
companion document.
4.1. Cross Packet Frames FEC Encoding
A QUIC packet payload consists in a set of QUIC frames. These frames
either carry application data (e.g., in a STREAM or DATAGRAM frame)
or control information (e.g., a MAX_DATA frame). Each packet is
either entirely received or lost, and is uniquely identified by a
monotonically increasing Packet Number.
Through the use of FEC encoding, application data can be protected
proactively against packet losses, without requiring to go through
packet retransmission. In addition to application data, QUIC
transfers might benefit from protecting control frames having a
potential impact on the transmission throughput, such as MAX_DATA or
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MAX_STREAM_DATA frames. Therefore this document introduces an FEC
protection across all -- or a subset of -- the frames of a given QUIC
packet. This design choice impacts the QUIC packet to source symbols
mapping, as well as signaling aspects, both of them being discussed
hereafter.
4.2. Source Symbol Definition
The cross packet frames FEC encoding approach considers the sequence
of frames (or a sub-sequence of them) transmitted within a given QUIC
packet, seen as the QUIC packet payload. From this payload, it
defines a mapping to source symbols (see Section 4.2.1 and
Section 4.2.2). Source symbols are then used for encoding purposes,
producing one or more repair symbols, the details of which depend on
the FEC Scheme considered. However source symbols are never sent per
se on the network. Instead the original QUIC packet, plus a
dedicated signaling header, are sent and therefore implicitely carry
those source symbols. The QUIC packets, containing one or more
repair symbols, are sent on the network.
The only modification to the original QUIC packet is the addition of
a dedicated FEC_SRC_FPI frame type, meant to carry source symbol
signaling (known as Source FEC Payload Information, or FPI). On the
opposite, frames that carry one or more repair symbols use a
dedicated REPAIR frame type. In both cases, in order to facilitate
experiments and enable backward compatibility, the FEC_SRC_FPI and
REPAIR frame types are chosen within the type range dedicated to
"Extension Frames". Thereby, a legacy receiver will automatically
ignore these unknown frame types. As QUIC packets can be of
different lengths, a special care must be taken to ensure having a
fixed Source Symbol size to ease FEC Scheme implementations.
4.2.1. Packet Payload to Packet Chunk Mapping
This section defines a mechanism to segment a QUIC packet payload,
composed of several frames, into fixed-size payload chunks, of size
E-1 bytes or E-1-4 bytes for the first chunk when the QUIC Packet
Number needs to be added ((Section 4.2.2). Depending on the relative
value of E-1 (or E-1-4) and the QUIC packet payload size, a packet
can potentially contain more than one chunks. This is a first step
into producing source symbols. Figure 1 illustrates this process.
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|<E-1-4>|< E-1 >|< E-1 >|< E-1 >|
| | | | |
+------|-------|-------|-------|-------|
QUIC pkt 0 |Header| Packet Payload | chunks 0, 1, 2, 3
+------|-----+-|-------|-------|-------+
QUIC pkt 1 |Header| 0 | Packet Payload | chunks 4, 5, 6
+------|---+-+-|-------|-------|
QUIC pkt 2 |Header| 0 | Packet Payload | chunks 7, 8, 9
+------|---+---|-------|-------|
Figure 1: Example of QUIC packet to chunk mapping, when the E-1 value
is relatively small, with prepended zero padding when needed (here
packets 1 and 2), and assuming the first chunk contains the QUIC
Packet Number in 4 bytes compressed version.
4.2.2. Packet Chunk to Source Symbol Mapping
The second step consists in producing the source symbols. A source
symbols is the concatenation of a single byte of metadata,
potentially followed by the Packet Number of the associated source,
plus a packet chunk. Figure 2 illustrates the situation where a
compressed QUIC packet number is added (in general for the first
chunk of a QUIC packet). Figure 3 illustrates the situation where
there is no QUIC packet number (in general for the following chunk(s)
of a QUIC packet). When the QUIC packet number is present, this
identifier can be recovered by a receiver after successful FEC
decoding. It means that a RECOVERED frame can be generated to the
sender to indicated that this packet (identified by the QUIC packet
number) has been recovered. Each source symbol is of fixed-size E
bytes. These source symbols are only used during encoding and
decoding and are not sent as-is on the network.
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
+-+-+-+-+-+-+-+-+
| meta data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Number (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet chunk (E-1-4 bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Source symbol format with Packet Number information (e.g.,
first packet chunk).
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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
+-+-+-+-+-+-+-+-+
| meta data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet chunk (E-1 bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Source symbol format without Packet Number information
(e.g., packet chunks except the first one).
7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|Resvd (0)|N|S|E|
+-+-+-+-+-+-+-+-+
Figure 4: Source symbol metadata format.
Figure 4 shows the format of the 1 byte metadata. The fields are the
following:
Reserved field (5 bits): for this specification, this field MUST be
equal to zero.
Packet Number (N) field (1 bit): this field indicates that the
following 4 bytes contain the Packet Number (short 32-bit
representation) of the associated QUIC packet ([QUIC-transport]
section 17.1., Packet Number Encoding and Decoding).
Start (S) bit (1 bit): this field, when set to 1, indicates that
this source symbol contains the first chunk of the packet
payload.
End bit (E) (1 bit): this field, when set to 1, indicates that this
source symbol contains the last chunk of the packet payload.
Note that with a QUIC packet containing a single chunk, the
associated metadata will contain S=E=1. On the opposite, a source
symbol containing a intermediate chunk (i.e., neither the first nor
the last chunk of the QUIC packet), the associated metadata will
contain S=E=0.
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QUIC packet
|<E-1-4>|< E-1 >|< E-1 >|< E-1 >|
+------|----+--|-------|-------|-------|
|Header| 0 | Packet Payload | 4 packet chunks
+------|----+--|-------|-------|-------|
/ | | \
v v v v
+-+--+----+ +-+-------+ +-+-------+ +-+-------+
|m|pn|chnk| |m| chunk| |m| chunk| |m| chunk| 4 source symbols
+-+--+----+ +-+-------+ +-+-------+ +-+-------+
| | | | | | | |
|< --E-- >| |< --E-- >| |< --E-- >| |< --E-- >|
Figure 5: Example of packet chunk to source symbol mapping, when the
E value is relatively small, in presence of the QUIC Packet Number
for the first chunk.
Figure 5 shows an example where the 4 source symbols are created from
the payload of a given QUIC packet. The first chunk may contain zero
padding at the beginning in order to align the protected packet
payload size to a multiple of E-1, and the first source symbol may
contain the QUIC Packet Number.
Each source symbol is uniquely identified allowing to determine
unambiguously its position in the source symbol flow. What
information to associate to a source symbol to uniquely identify it
is FEC Scheme dependent. Section 4.3 gives insight on this topic.
4.2.2.1. Open questions: Content of Source Symbols Metadata? Removing
certain frames from FEC protection?
NB: section to remove once fixed.
During the FEC encoding phase, additional data can be added to the
source symbol. These data are only added during the encoding and
MUST NOT be transmitted on the network. The encoder and decoder MUST
agree on the addition of these data to the source symbol in order to
avoid decoding errors. Here are some examples of data that can be
added to a source symbol during encoding and that will be decoded
upon a source symbol recovery:
o The packet number: adding the packet number allows the decoder to
know which packet has been recovered and potentially send a
feedback of which packet has been recovered to the QUIC sender.
o Additional QUIC frames: the FEC encoder can for example add
PADDING frames to a source symbol before proceeding to encoding.
Adding PADDING frames to source symbols before encoding allows
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protecting packets of different sizes. The smaller packet payload
will be added PADDING frames to reach a size that is a multiple of
E-1.
Note: Maybe the decision of adding data such as padding in the
source symbols should be left to the underlying FEC Scheme.
Besides adding data to source symbols before encoding, some frames
can be removed from the source symbol if their protection is not
crucial for the transmission in order to reduce the size of the
source symbol. For example, ACK frames can be systematically
stripped out of the source symbols. Stream frames of non-delay-
sensitive streams could also be removed from the source symbol. The
encoder and decoder MUST agree on which frames must be stripped out
of packet payloads. This information might for example be encoded in
the Source Symbol ID by the FEC encoder.
Note: We might want to propose standard ways/algorithms to add/
remove data before the encoding ?
TODO: Add a mechanism to add QUIC packet identifier to the metadata.
It's useful.
4.2.3. Source Symbol Size (E) Considerations
The source symbol size, E, MUST be strictly greater than zero bytes
and strictly smaller than the minimum PMTU value allowed by QUIC.
The packet header is not part of the FEC-protected data. When the
packet payload size is not a multiple of E-1, zero-padding MUST be
added at the beginning of the first chunk of the packet payload.
This is equivalent to inserting PADDING frames at the beginning of
the payload. This zero-padding, only used for FEC encoding, SHOULD
NOT be sent on the wire.
The choice of an appropriate value for E may depends on the use case
(in particular on the nature of application data). A reasonably
small value reduces the expected value of the added padding needed to
align the payload size with a multiple of E-1, which can be a good
approach when dealing with QUIC packets whose size significantly
vary. However an overly small value also increases processing
complexity (FEC encoding and decoding are performed over a larger
linear system since there are more source symbols), so there is an
incentive to use a larger value. An appropriate balance should be
found, and this choice is considered as out of scope for this
document. Since a repair symbol will transit through a frame, the E
value must take this into account to avoid having REPAIR frames that
do not fit into a single QUIC packet.
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4.3. Source Symbol Signaling
An explicit signaling is needed by a decoder to identify the source
symbols and their position in the block (i.e., for block codes) or
coding window (i.e., for sliding window codes). While the QUIC
packet number increases monotonically, it cannot be used to identify
the position of a packet in the coding window as the packet number is
not needed to increase by 1 for each new packet. There is thus an
ambiguity on the decoder-side between lost packets and packets that
do not exist. Similarly to FECFRAME, we propose to assign a
identifier to source symbols to avoid this ambiguity. This
identifier is opaque to the protocol and will be defined by the
underlying FEC schemes. This is out of the scope of this document.
An example of identifier could be an integer increasing by 1 for each
new source symbol
In order to announce the source symbol identifier to the FEC decoder,
we propose to add a new frame, the FEC_SRC_FPI frame to packets whose
payload will contain one or more source symbols from the FEC decoder
point of view. The FEC_SRC_FPI frame is part of the packet payload
itself. Any packet containing a FEC_SRC_FPI frame MUST see its
payload considered as one or more source symbol(s).
The FEC_SRC_FPI frame format is FEC Scheme specific and MUST be
specified in the associated document.
4.4. Repair Symbol Signaling
An explicit signaling is needed by a decoder for each repair symbol
received through a REPAIR frame. The goals are manyfold: identifying
the repair symbols and their position in the block (i.e., for block
codes) or coding window (i.e., for sliding window codes); carrying
information on the way this repair symbol has been produced (e.g.,
with sliding window codes, it can indicate the encoding window
composition).
One or more repair symbols can be present in a given QUIC packet.
When there are multiple symbols, they SHOULD be concatenated in the
same REPAIR frame. How to achieve this goal is FEC Scheme specific
and therefore must be defined in the document describing this FEC
Scheme.
4.5. Signaling a Symbol Recovery
When all the source symbols of a given QUIC packet have been lost but
are recovered during FEC decoding, a QUIC receiver SHOULD advertise
it to the sender in order to avoid the retransmission of already
available data. However, the QUIC receiver MUST NOT acknowledge this
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recovered packet through a regular acknowledement, as it would
interfere with the behaviour of loss-based congestion controls such
as [Cubic]. Therefore this document introduces a dedicated RECOVERED
frame, that enables a receiver to indicate that a specific QUIC
packet has been recovered through FEC decoding.
The RECOVERED frame works at the packet level. It is therefore
required to be able to identify to which packet the recovered source
symbols belong to. This is made possible by the QUIC packet
identifier field added to the meta data prior to FEC encoding
(Section 4.2.2).
4.6. About Gaps in the Set of Source Symbols Considered During Encoding
A given FEC Scheme MAY support or not the presence of gaps in the set
of source symbols that constitute a block (for Block codes) or an
encoding window (for Sliding Window codes). A potential cause for
non contiguous sets of source symbols is the acknowledgment of one of
them. When this happens, the QUIC sending endpoint may want to
remove this source symbol from further FEC encodings. This is
particularly true with Sliding Window codes because of their
flexibility during FEC encoding (i.e., the encoding window can change
between two consecutive FEC encodings).
Supporting gaps can be motivated by the desire to reduce encoding and
decoding complexity since there are fewer variables. However this
choice has major consequences in terms of signaling. Indeed each
repair symbol transmitted MUST be accompanied by enough information
for the QUIC decoding endpoint to unambiguously identify the exact
composition of the block or encoding window. Without any gap, the
identity of the first source symbol plus the number of symbols in the
block or encoding window is sufficient. With gaps, a more complex
encoding needs to be used, perhaps similar to the encoding used for
selective acknowledgments.
Whether gaps are supported MUST be clarified in each FEC Scheme.
5. FEC Scheme Negotiation in QUIC
FEC Scheme negotiation has two goals:
o Selecting a FEC Scheme (or FEC Schemes) that can be used by the
QUIC transmission and reception endpoints. This process requires
an exchange between them;
o Communicating a certain number of parameters, the "Configuration
Information", that are not expected to change over the session
lifetime. For instance, this is the case of the symbol size
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parameter, E (in bytes), that needs either to be agreed between
the endpoints, or chosen by the sender and communicated to the
receiver(s);
Editor's notes:
* It is likely that FEC Scheme negotiation requires the use of a
new dedicated Extension Frame Type. The details remain TBD.
* The Negotiation Frame Type format remains TBD.
* How to communicate the parameters remains TBD.
* The present document only provides high level principles, the
details are of course the responsibility of the FEC Scheme.
* In case negotiation is different when protecting a single
versus several streams, this section may be moved to the
respective sections.
* How does it work in case of a multicast session?
* Do we negotiate here a FEC Scheme on a per-Stream basis (or
group of Streams to be protected jointly)? Or do we negotiate
a FEC Scheme on a QUIC session basis, therefore to be used for
all the Streams that need FEC protection?
5.1. FEC Scheme Negotiation
Before defining the transport parameters, we define two structures,
encoder_fec_scheme_t and decoder_fec_scheme_t, in Figure 6. The
config field is an opaque field allowing the decoder to define
supported configuration information for the associated FEC Scheme. A
FEC Scheme specification MUST define the set of valid configurations
for the FEC Scheme.
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struct {
varint fec_scheme_id;
} encoder_fec_scheme_t
struct {
varint fec_scheme_id;
uint16_t config_length;
uint8_t config[config_length];
} decoder_fec_scheme_t
Figure 6: encoder_fec_scheme_t and decoder_fec_scheme_t structures.
The following three transport parameters are used by the QUIC
endpoints to negotiate the FEC Scheme used during the connection.
o supported_encoder_fec_schemes: list of supported FEC schemes for
the encoding part listed from the most to the least preferred.
The value of this parameter consists in a list of
encoder_fec_scheme_t. When announcing a FEC Scheme, the encoder
MUST be able handle every FEC Scheme configuration considered
valid.
o supported_decoder_fec_schemes: list of supported FEC schemes for
the decoding part listed from the most to the least preferred.
The value of this parameter consists in a list of
decoder_fec_scheme_t, each one representing the ID of a supported
FEC Scheme.
o receiving_symbol_size: the size in bytes of the symbols the peer
is willing to receive and recover. The value is a 16-bits
integer.
Since communications can be bidirectional, each QUIC endpoint can
provide the three parameters. Conversely, providing an empty list
indicates this endpoint does not support FEC for the associated
communication path (e.g., an empty supported_decoder_fec_schemes list
indicates this endpoint cannot perform FEC decoding).
The decoding FEC Scheme of a QUIC endpoint is set to the first FEC
Scheme listed in its own supported_decoder_fec_schemes that also
appears in the peer's supported_encoder_fec_schemes. The encoding
FEC Scheme of a QUIC endpoint is set to the first FEC Scheme listed
in the peer's supported_decoder_fec_schemes that also appears in its
own supported_encoder_fec_schemes. The encoder-side symbol size (E)
of a QUIC endpoint is set to the value announced by the peer's
receiving_symbol_size transport parameter. The decoder-side symbol
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size of a QUIC endpoint is set to the value announced in its own
receiving_symbol_size transport parameter.
Host 1 Host 2
< - - - - - - - - - - - - - - - - - - - - - - - - - - - -
supported_encoder_fec_schemes{RLC_GF256,REED_SOLOMON,XOR}
supported_decoder_fec_schemes{REED_SOLOMON,XOR}
receiving_symbol_size{500}
- - - - - - - - - - - - - - - - - - - - - - - - - - - - >
supported_encoder_fec_schemes{RLC_GF256,REED_SOLOMON,XOR}
supported_decoder_fec_schemes{RLC_GF256,REED_SOLOMON}
receiving_symbol_size{200}
ENCODER_FEC_SCHEME = REED_SOLOMON
DECODER_FEC_SCHEME = RLC_GF256
ENCODER_SYMBOL_SIZE = 500
DECODER_SYMBOL_SIZE = 200
ENCODER_FEC_SCHEME = RLC_GF256
DECODER_FEC_SCHEME = REED_SOLOMON
ENCODER_SYMBOL_SIZE = 200
DECODER_SYMBOL_SIZE = 500
Figure 7: Example FEC Schemes negotiation during the QUIC handshake.
It is possible that the QUIC endpoint that receives one or more FEC
Scheme proposals from the initiator cannot select any of them. In
that case the negotiation process fails and no FEC protection is
used.
6. Security Considerations
TBD
7. IANA Considerations
TBD
8. Acknowledgments
TBD
9. References
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9.1. Normative References
[Cubic] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>.
[QUIC-transport]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic-
transport (Work in Progress) (work in progress), January
2019, <https://datatracker.ietf.org/doc/draft-ietf-quic-
transport/>.
[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>.
9.2. Informative References
[nc-taxonomy]
Roca (Ed.) et al., V., "Taxonomy of Coding Techniques for
Efficient Network Communications", Request For
Comments RFC 8406, June 2018,
<https://datatracker.ietf.org/doc/draft-irtf-nwcrg-
network-coding-taxonomy/>.
[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>.
[RFC8681] Roca, V. and B. Teibi, "Sliding Window Random Linear Code
(RLC) Forward Erasure Correction (FEC) Schemes for
FECFRAME", RFC 8681, DOI 10.17487/RFC8681, January 2020,
<https://www.rfc-editor.org/info/rfc8681>.
Authors' Addresses
Ian Swett
Google
Cambridge, MA
US
Email: ianswett@google.com
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Marie-Jose Montpetit
Triangle Video
Boston, MA
US
Email: marie@mjmontpetit.com
Vincent Roca
INRIA
Univ. Grenoble Alpes
France
Email: vincent.roca@inria.fr
Francois Michel
UCLouvain
Louvain-la-Neuve
Belgium
Email: francois.michel@uclouvain.be
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