Coding for QUIC
draft-swett-nwcrg-coding-for-quic-02
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draft-swett-nwcrg-coding-for-quic-02
nwcrg I. Swett
Internet-Draft Google
Intended status: Informational M-J. Montpetit
Expires: August 8, 2019 Triangle Video
V. Roca
INRIA
February 4, 2019
Coding for QUIC
draft-swett-nwcrg-coding-for-quic-02
Abstract
This document focusses 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
multicast sessions (since the same repair packet can recover several
different losses at several receivers), and with multipath sessions
(since repair packets add diversity).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 8, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
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
3.5. FEC Protection at the Stream Level . . . . . . . . . . . 5
3.6. About Gaps in the Set of Source Symbols Considered During
Encoding . . . . . . . . . . . . . . . . . . . . . . . . 5
4. FEC Scheme Negotiation in QUIC . . . . . . . . . . . . . . . 6
4.1. FEC Scheme Selection Process . . . . . . . . . . . . . . 7
4.2. FEC Scheme Configuration Information . . . . . . . . . . 8
5. Procedures when Protecting a Single QUIC Stream . . . . . . . 8
5.1. Application data, STREAM Frame data and Source Symbols . 8
5.2. Signaling Considerations within STREAM and REPAIR Frames 9
5.3. Management of Silent Periods and End of Stream . . . . . 10
6. Procedures when Protecting Several QUIC Streams . . . . . . . 11
6.1. Application data, STREAM Frame data and Source Symbols . 12
6.2. Block or Encoding Window Management . . . . . . . . . . . 12
6.3. Signaling Considerations within STREAM and REPAIR Frames 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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
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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 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.
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 [RLC]).
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:
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:
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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 [RLC]) 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.
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
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REPAIR frame type values as potential FEC Scheme negotiated).
Is it what we want to achieve? Not sure.
* It is not clear whether negotiation is carried out at QUIC
level (and therefore for multiple streams) or at a stream
level (and therefore multiple streams may use multiple FEC
Schemes). The terminology used above should be updated to
reflect the choice.
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 application 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.
3.5. FEC Protection at the Stream Level
Streams in QUIC provide a lightweight, ordered byte-stream
abstraction. FEC encoding is applied at the stream level, within a
single stream or across two or more streams of the same QUIC session.
This is motivated by the fact that FEC protection is not necessarily
beneficial to all data streams, but only to a subset of them. For
instance FEC protection can be highly beneficial to live video
streams to which the proactive erasure correction feature of FEC,
independent of the RTT, should be highly beneficial. On the
opposite, FEC protection is probably less attractive for latency
insensitive bulk unicast flows.
In order to facilitate experiments, and in order to enable backward
compatibility, the STREAM frames that carry application data are kept
unmodified. On the opposite, frames that carry one or more repair
symbols use a dedicated REPAIR frame type, chosen within the type
range dedicated to "Extension Frames".
3.6. 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
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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 with 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 or not gaps are supported MUST be clarified in each FEC
Scheme.
4. 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
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.
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* 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?
4.1. FEC Scheme Selection Process
Let us consider the FEC Scheme selection process between the QUIC
endpoints. Figure 1 illustrates the principle when a QUIC reception
endpoint initiates the exchange.
QUIC sender QUIC receiver
< - - - - - - - - - - - - - - - - - - - - - -
supported_fec_scheme_32b{FEC_Encoding_ID1 | other}
supported_fec_scheme_64b{FEC_Encoding_ID2 | other}
choose FEC Scheme 1
- - - - - - - - - - - - - - - - - - - - - - >
supported_fec_scheme_32b{FEC_Encoding_ID1 | other}
Figure 1: Example FEC Scheme selection process, during the initial
negotiation.
The supported_fec_scheme_16b and supported_fec_scheme_32b are two new
TransportParameterId to be added to the "Table 7: Initial QUIC
Transport Parameters Entries" Section 13.1, of [QUIC-transport]. The
supported_fec_scheme_32b contains a 32-bit data field to carry opaque
32-bit value, while the supported_fec_scheme_64b contains a 64-bit
data field to carry opaque 64-bit value (see Section 4.2).
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...
Editor's notes:
* So what? How does it finishes? Consequences?
* Can the second QUIC endpoint change the proposed static
parameters? In that case can the initator refuse?
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4.2. FEC Scheme Configuration Information
Let us now focus on the communication of configuration information
specific to the selected FEC Scheme. In Figure 1, the
supported_fec_scheme_32b{FS1_Encoding_ID} contains a field meant to
carry the FEC Encoding ID of the FEC Scheme selected plus addditional
configuration information if any. If a 32 bit opaque field is not
sufficient, the supported_fec_scheme_64b can be used instead and
proposes a 64 bit opaque field.
5. Procedures when Protecting a Single QUIC Stream
This section focusses on the simple case where FEC protection is
applied to a single QUIC stream. We consider a unidirectional data
flow between a QUIC sending endpoint and one (or more) QUIC reception
endpoints.
5.1. Application data, STREAM Frame data and Source Symbols
Application data is kept in a transmission buffer at a QUIC sending
endpoint, and sent within STREAM frames. Each STREAM frame that
carries data contains an Offset field that indicates the offset
within the stream of the first byte of the Stream Data field, as well
as a Length field that indicates the number of bytes contained in the
Stream Data field. Upon receiving a STREAM frame, using the Offset
and Length fields, a QUIC reception endpoint can easily store data in
its reception buffer. But since a QUIC Packet may be lost during
transmission, the reception buffer may have gaps.
Figure 2 illustrates how source symbols are mapped to the QUIC
transmission or reception buffers (same principle on either side).
Since any source (and repair) symbol is of fixed size (E bytes) for a
given stream, since QUIC guaranties that the first byte in the stream
has an offset of 0, the position of each source symbol is known by
both ends.
< -E- > < -E- > < -E- > < -E- >
+-------+-------+-------+-------+
|< -- Frame 1 -- >< ----- Frame | source symbols 0, 1, 2, 3
+-------+-------+-------+-------+
| 2 ----- >< --- Frame 3 -- >< -| source symbols 4, 5, 6, 7
+-------+-------+----+--+-------+
| Frame 4 - >< -F5- >| source symbols 8, 9 and 10
+-------+-------+----+ (incomplete)
Figure 2: Example of source symbol mapping, when the E value is
relatively small.
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Any value for E is possible, from a single byte to several hundreds
or thousands of bytes, as long as a frame containing a repair symbol
(E bytes long) can fit into a QUIC packet. In general, the source
symbols are not aligned with data chunks sent in the STREAM frames.
A given STREAM frame may carry all the bytes of a given source
symbol. But when a source symbol straddles two or more (e.g., if E
is large compared to usual frame size) STREAM frames, a proper
reception of these two (or more) STREAM frames is needed for a QUIC
reception endpoint to consider that the source symbol is available
for FEC decoding operations. The choice of an appropriate value for
E may depend on the use case (in particular on the nature of
application data). A reasonably small value reduces the probability
that a source symbol straddles two or more STREAM frames, a situation
that is considered as potentially harmful (the unit of control, the
source symbol, and unit of transmission, the frame, are not aligned).
However an overly small value also increases processing complexity
(FEC encoding and decoding are performed over a larger linear system)
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.
5.2. Signaling Considerations within STREAM and REPAIR Frames
Once the initial negotiation succeeded and an appropriate FEC Scheme
has been chosen between the QUIC endpoints, data is exchanged as
follows. Source data is transmitted within STREAM frames, as would
happen without any FEC based loss recovery mechanism (in particular
without considering source symbols boundaries). Repair data,
computed during FEC encoding, on the opposite, is sent within a
dedicated REPAIR frame type, chosen within the type range dedicated
to "Extension Frames". In both cases, the same Stream ID is used
since both flows relate to the same stream.
The REPAIR frame format is FEC Scheme dependent. The document
specifying a FEC Scheme to be used with QUIC MUST define the REPAIR
frame format, among other things. The REPAIR frame MUST carry enough
information for a QUIC reception endpoint to understand exactly how
this repair symbol(s) has(ve) been generated. It implies that each
REPAIR symbol MUST communicate the block (with block codes) or
encoding window (with Sliding Window codes) composition. When there
is no gap in the source symbol set, this MAY be achieved by
communicating:
o the offset of the first source symbol of the block or encoding
window;
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o the number of source symbols in the block or encoding window,
which can be either a number of symbols or a number of bytes since
symbols are of fixed size, E.
Note that unlike FEC Schemes for FLUTE/ALC, NORM, and FECFRAME, here
there is no notion of Encoding Symbol Id (ESI). The use of an offset
within the stream, with the guaranty that no wrapping to zero can
occur, provides an alternative mechanism to identify any source
symbol.
As explained above, source data is transmitted without any
modification, which provides backward compatibility. This is an
advantage in situations where the same QUIC stream is simultaneously
delivered to several QUIC reception endpoints (multicast): it enables
a given FEC Scheme to be used even if a subset of the QUIC reception
endpoints do not support it.
Editor's notes:
* This I-D proposes to define a single generic REPAIR frame
type, but an alternative could be to have a one-to-one mapping
between a REPAIR frame type and a specific FEC Scheme.
* The use of frame type within the Extension Frames range for
REPAIR frames is meant to facilitate experimentations. If the
use of coding in QUIC is recognized as having benefits, a
dedicated (or more, see above) frame type could be selected
later on.
5.3. Management of Silent Periods and End of Stream
If an application does not submit fresh data for some time, the last
source symbol may not be totally filled. It follows that this last
source symbol cannot be considered during FEC encoding and therefore
the associated bytes of the application stream are not protected. A
similar problem arrives when a stream is finished, the application
having no more data to submit to QUIC. Here also, the bytes of the
last incomplete source symbol are not protected by FEC encoding.
In order to solve this problem, it is RECOMMENDED that a QUIC sending
endpoint:
o Identifies when such a situation is likely to occur, for instance
by waiting no more than a certain time during an application
silent period;
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o Upon time-out, the application falls back to the alternative re-
transmission based loss recovery mechanism for the bytes of the
last incomplete source symbol;
Editor's notes: Clearly, the above mechanism requires more thoughts
as well as experimental work. The "end of stream" situation may
be addressed through zero padding perhaps easily. However the
use of zero padding for transitory silent periods may add a lot
of specification and implementation complexity...
6. Procedures when Protecting Several QUIC Streams
This section focusses on the general case where FEC protection is
globally applied across two or more QUIC streams.
Editor's notes: It is not clear whether this use-case is needed. It
adds specification and implementation complexity that need to be
balanced with the expected benefits.
* Receiver: A first complexity comes from the requirement to
identify to which stream a decoded source symbol belongs to.
This is also one of the main difficulty for FECFRAME (both
with block and sliding window codes) which required to
distinguish an ADU (submitted by the application) from an ADUI
(the same ADU plus an additional FlowID among other things).
Do we want this level of complexity?
* Sender: Another complexity comes from the encoding window
management at a sender. With multiple streams, shifting the
encoding window to the right needs to be done based on
timestamps associated to source symbols of the various
streams: the oldest source symbol across all the streams will
be removed.
* When two largely different streams are protected togethers
(e.g., a high definition 4K video flow plus the associated
relatively low-rate audio stream), is this extra complexity
balanced by significant performance improvements compared to
an independent protection on each stream (intuition is yes,
the low bitrate flow is better protected iff the encoding
window is large enough)? And when the various streams have a
comparable bitrate? More work (incl. experimental work) is
needed to answer this question.
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6.1. Application data, STREAM Frame data and Source Symbols
Within each stream, the source symbols MUST be defined as in the
simple case of a single stream. Figure 2 remains valid.
6.2. Block or Encoding Window Management
The details of how to create the block or encoding window are
specific to the FEC Scheme. A possible approach is the following.
When creating the block (block FEC code) or encoding window (sliding
window FEC code), the source symbols to consider of each stream are
appended. All the relevant source symbols of the first stream are
appended, followed by all the source symbols of the second stream,
etc. These sequences do not follow any timing consideration in order
to simplify signaling.
Figure 3 illustrates, in case of a Sliding Window FEC Scheme, an
encoding window with source symbols belonging to two streams, of
Stream ID 120 and 51 respectively.
< ----------- Stream ID 120 ---------- > < --- Stream ID 51 --- >
+-------+-------+-------+-------+-------+-------+-------+-------+
| | | | | | | | |
+-------+-------+-------+-------+-------+-------+-------+-------+
^ < -E- > ^
| |
offset = 0x42f0, length = 5*E offset = 0x0f24, length = 3*E
Figure 3: Example of encoding window of a Sliding Window FEC Scheme
and FEC protection across two streams.
6.3. Signaling Considerations within STREAM and REPAIR Frames
Source data on each stream is transmitted within STREAM frames, as
would happen without any FEC based loss recovery mechanism.
Repair symbols, generated during FEC encoding as a linear combination
of source symbols that belong to one or more of the streams, are
transmitted within REPAIR frames. Each REPAIR frame can be
associated to any of the input streams it protects, and therefore
associated to any of the associated Stream IDs.
Editor's notes: Check that indeed, with FEC protection across
several streams, assigning a REPAIR frame to any of the streams
it protects is meaningful. Should an approach for selecting one
stream (and Stream ID) be preferred?
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The REPAIR frame format is FEC Scheme dependent and MUST be defined
by document specifying a FEC Scheme. One of the key information of
this REPAIR frame is the composition of the block (with block codes)
or encoding window (with sliding window codes) used to perform FEC
encoding. Indeed, this is the only manner to convey this information
since an application flow is not predictable (e.g., if an application
flow is momentarily suspended, the composition of the block or
encoding window will be affected). One possibility is to list, in
each REPAIR frame header:
o the actual number of streams considered (the maximum number is
known after the negotiation step, but if one of the streams
remains silent for some time, it may not contribute during
encoding and therefore be absent from the block or encoding
window);
o for each stream concerned, its Stream ID, the offset of the first
source symbol considered as well as the length, i.e., the number
of bytes considered.
This approach does not enable to keep track of the source symbol
ordering across streams, but enables a non ambiguous description of
the encoding window.
The FEC Scheme specification MUST also detail how to manage the block
or encoding window. For instance, should the oldest source symbol of
any stream be removed from the encoding window when this latter is
shifted to the right? This would mean that a timestamp is attached
to each source symbol in order to identify the oldest one across all
streams.
7. Security Considerations
TBD
8. IANA Considerations
TBD
9. Acknowledgments
TBD
10. References
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10.1. Normative References
[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>.
10.2. 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>.
[RLC] Roca, V. and B. Teibi, "Sliding Window Random Linear Code
(RLC) Forward Erasure Correction (FEC) Scheme for
FECFRAME", Work in Progress, Transport Area Working Group
(TSVWG) draft-ietf-tsvwg-rlc-fec-scheme (Work in
Progress), February 2019, <https://tools.ietf.org/html/
draft-ietf-tsvwg-rlc-fec-scheme>.
Authors' Addresses
Ian Swett
Google
Cambridge, MA
US
Email: ianswett@google.com
Swett, et al. Expires August 8, 2019 [Page 14]
Internet-Draft Coding for QUIC February 2019
Marie-Jose Montpetit
Triangle Video
Boston, MA
US
Email: marie@mjmontpetit.com
Vincent Roca
INRIA
Univ. Grenoble Alpes
France
Email: vincent.roca@inria.fr
Swett, et al. Expires August 8, 2019 [Page 15]