Internet DRAFT - draft-irtf-nwcrg-coding-and-congestion
draft-irtf-nwcrg-coding-and-congestion
NWCRG N. Kuhn
Internet-Draft CNES
Intended status: Informational E. Lochin
Expires: 26 August 2022 ENAC
F. Michel
UCLouvain
M. Welzl
University of Oslo
22 February 2022
Coding and congestion control in transport
draft-irtf-nwcrg-coding-and-congestion-12
Abstract
Forward Erasure Correction (FEC) is a reliability mechanism that is
distinct and separate from the retransmission logic in reliable
transfer protocols such as TCP. FEC coding can help deal with losses
at the end of transfers or with networks having non-congestion
losses. However, FEC coding mechanisms should not hide congestion
signals. This memo offers a discussion of how FEC coding and
congestion control can coexist. Another objective is to encourage
the research community to also consider congestion control aspects
when proposing and comparing FEC coding solutions in communication
systems.
This document is the product of the Coding for Efficient Network
Communications Research Group (NWCRG). The scope of the document is
end-to-end communications: FEC coding for tunnels is out-of-the scope
of the document.
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 26 August 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Fairness, Quantifying and Limiting Harm, and Policy
Concerns . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Separate channels, separate entities . . . . . . . . . . 5
2.3. Relation between transport layer and application
requirements . . . . . . . . . . . . . . . . . . . . . . 7
2.4. Scope of the document concerning transport multipath and
multi-streams applications . . . . . . . . . . . . . . . 8
2.5. Types of coding . . . . . . . . . . . . . . . . . . . . . 9
3. FEC above the transport . . . . . . . . . . . . . . . . . . . 10
3.1. Fairness and impact on non-coded flows . . . . . . . . . 11
3.2. Congestion control and recovered symbols . . . . . . . . 11
3.3. Interactions between congestion control and coding
rates . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4. On useless repair symbols . . . . . . . . . . . . . . . . 12
3.5. On partial ordering at FEC level . . . . . . . . . . . . 12
3.6. On partial reliability at FEC level . . . . . . . . . . . 12
3.7. On multipath transport and FEC mechanism . . . . . . . . 12
4. FEC within the transport . . . . . . . . . . . . . . . . . . 12
4.1. Fairness and impact on non-coded flows . . . . . . . . . 14
4.2. Interactions between congestion control and coding
rates . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3. On useless repair symbols . . . . . . . . . . . . . . . . 14
4.4. On partial ordering at FEC and/or transport level . . . . 15
4.5. On partial reliability at FEC level . . . . . . . . . . . 15
4.6. On transport multipath and subpath FEC coding rate . . . 15
5. FEC below the transport . . . . . . . . . . . . . . . . . . . 15
5.1. Fairness and impact on non-coded flows . . . . . . . . . 17
5.2. Congestion control and recovered symbols . . . . . . . . 17
5.3. Interactions between congestion control and coding
rates . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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5.4. On useless repair symbols . . . . . . . . . . . . . . . . 17
5.5. On partial ordering at FEC level with in-order delivery
transport . . . . . . . . . . . . . . . . . . . . . . . . 17
5.6. On partial reliability at FEC level . . . . . . . . . . . 18
5.7. FEC not aware of transport multipath . . . . . . . . . . 18
6. Research recommendations and questions . . . . . . . . . . . 18
6.1. Activities related to congestion control and coding . . . 18
6.2. Open research questions . . . . . . . . . . . . . . . . . 18
6.2.1. Parameter derivation . . . . . . . . . . . . . . . . 19
6.2.2. New signaling methods and fairness . . . . . . . . . 19
6.3. Recommendations and advice for evaluating coding
mechanisms . . . . . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. Informative References . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
There are cases where deploying FEC coding improves the performance
of a transmission. As an example, it may take time for a sender to
detect transfer tail losses (losses that occur at the end of a
transfer, where, e.g., TCP obtains no more ACKs that would enable it
to quickly repair the loss via retransmission). Allowing the
receiver to recover such losses instead of having to rely on a
retransmission could improve the experience of applications using
short flows. Another example is a network where non-congestion
losses are persistent and prevent a sender from exploiting the link
capacity.
Coding and the loss detection of congestion controls are two distinct
and separate reliability mechanisms that is distinct and separate
from the loss detection of congestion controls. Since FEC coding
repairs losses, blindly applying FEC may easily lead to an
implementation that also hides a congestion signal from the sender.
It is important to ensure that such information hiding does not
occur, because loss may be the only congestion signal available to
the sender (e.g. TCP [RFC5681]).
FEC coding and congestion control can be seen as two separate
channels. In practice, implementations may mix the signals that are
exchanged on these channels. This memo offers a discussion of how
FEC coding and congestion control coexist. Another objective is to
encourage the research community also to consider congestion control
aspects when proposing and comparing FEC coding solutions in
communication systems. This document does not aim at proposing
guidelines for characterizing FEC coding solutions.
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We consider three architectures for end-to-end unicast data transfer:
* with FEC coding in the application (above the transport)
(Section 3),
* within the transport (Section 4), or
* directly below the transport (Section 5).
A typical scenario for the considerations in this document is a
client browsing the web or watching a live video.
This document represents the collaborative work and consensus of the
Coding for Efficient Network Communications Research Group (NWCRG);
it is not an IETF product and is not a standard. The document
follows the terminology proposed in the taxonomy document [RFC8406].
2. Context
2.1. Fairness, Quantifying and Limiting Harm, and Policy Concerns
Traffic from or to different end users may share various types of
bottlenecks. When such a shared bottleneck does not implement some
form of flow protection, the share of the available capacity between
single flows can help assess when one flow starves the other.
As one example, for residential accesses, the data rate can be
guaranteed for the customer premises equipment, but not necessarily
for the end user. The quality of service that guarantees fairness
between the different clients can be seen as a policy concern
[I-D.briscoe-tsvarea-fair].
While past efforts have focused on achieving fairness, quantifying
and limiting harm caused by new algorithms (or algorithms with
coding) is more practical [BEYONDJAIN]. This document considers
fairness as the impact of the addition of coded flows on non-coded
flows when they share the same bottleneck. It is assumed that the
non-coded flows respond to congestion signals from the network. This
document does not contribute to the definition of fairness at a wider
scale.
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2.2. Separate channels, separate entities
Figure 1 and Figure 2 present the notations that will be used in this
document and introduces the Forward Erasure Correction (FEC) and
Congestion Control (CC) channels. The Forward Erasure Correction
channel carries repair symbols (from the sender to the receiver) and
information from the receiver to the sender (e.g. signaling which
symbols have been recovered, loss rate prior and/or after decoding,
etc.). The Congestion Control channel carries network packets from a
sender to a receiver, and packets signaling information about the
network (number of packets received vs. lost, Explicit Congestion
Notification (ECN) [RFC3168] marks, etc.) from the receiver to the
sender. The network packets that are sent by the Congestion Control
channel may be composed of source packets and/or repair symbols.
SENDER RECEIVER
+------+ +------+
| | ----- network packets ---->| |
| CC | | CC |
| | <--- network information ---| |
+------+ +------+
Figure 1: Congestion Control (CC) channel
SENDER RECEIVER
+------+ +------+
| | source and/or | |
| | ----- repair symbols ---->| |
| FEC | | FEC |
| | signaling | |
| | <--- recovered symbols ----| |
+------+ +------+
Figure 2: Forward Erasure Correction (FEC) channel
Inside a host, the CC and FEC entities can be regarded as
conceptually separate:
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| ^ | ^
| source | coding |packets | sending
| packets | rate |requirements | rate (or
v | v | window)
+---------------+source +-----------------+
| FEC |and/or | CC |
| |repair | |network
| |symbols | |packets
+---------------+==> +-----------------+==>
^ ^
| signaling | network
| recovered symbols | information
Figure 3: Separate entities (sender-side)
| |
| source and/or | network
| repair symbols | packets
v v
+---------------+ +-----------------+
| FEC |signaling | CC |
| |recovered | |network
| |symbols | |information
+---------------+==> +-----------------+==>
Figure 4: Separate entities (receiver-side)
Figure 3 and Figure 4 provide more details than Figure 1 and
Figure 2. Some elements are introduced:
* 'network information' (input control plane for the transport
including CC): refers not only to the network information that is
explicitly signaled from the receiver, but all the information a
congestion control obtains from a network.
* 'requirements' (input control plane for the transport including
CC): refers to application requirements such as upper/lower rate
bounds, periods of quiescence, or a priority.
* 'sending rate (or window)' (output control plane for the transport
including CC): refers to the rate at which a congestion control
decides to transmit packets based on 'network information'.
* 'signaling recovered symbols' (input control plane for the FEC):
refers to the information a FEC sender can obtain from a FEC
receiver about the performance of the FEC solution as seen by the
receiver.
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* 'coding rate' (output control plane for the FEC): refers to the
coding rate that is used by the FEC solution (i.e. proportion of
transmitted symbols that carry useful data).
* 'network packets' (output data plane for the CC): refers to the
data that is transmitted by a CC sender to a CC receiver. The
network packets may contain source and/or repair symbols.
* 'source and/or repair symbols' (data plane for the FEC): refers to
the data that is transmitted by a FEC sender to a FEC receiver.
The sender can decide to send source symbols only (meaning that
the coding rate is 0), repair symbols only (if the solution
decides not to send the original source symbols) or a mix of both.
The inputs to FEC (incoming data packets without repair symbols, and
signaling from the receiver about losses and/or recovered symbols)
are distinct from the inputs to CC. The latter calculates a sending
rate or window from network information, and it takes the packet to
send as input, sometimes along with application requirements such as
upper/lower rate bounds, periods of quiescence, or a priority. It is
not clear that the ACK signals feeding into a congestion control
algorithm are useful to FEC in their raw form, and vice versa -
information about recovered blocks may be quite irrelevant to a CC
algorithm.
2.3. Relation between transport layer and application requirements
The choice of the adequate transport layer may be related to
application requirements and the services offered by a transport
protocol [RFC8095]:
* The transport layer may implement a retransmission mechanism to
guarantee the reliability of a data transfer (e.g. TCP).
Depending on how the FEC and CC functions are scheduled (FEC above
CC (Section 3), FEC in CC (Section 4), FEC below CC (Section 5)),
the impact of reliable transport on the FEC reliability mechanisms
is different.
The transport layer may provide an unreliable transport service (e.g.
UDP or DCCP [RFC4340]) or a partially reliable transport service
(e.g. SCTP with the partial reliability extension [RFC3758] or QUIC
with the unreliable datagram extension [I-D.ietf-quic-datagram]).
Depending on the amount of redundancy and network conditions, there
could be cases where it becomes impossible to carry traffic. This is
further discussed in Section 3 where "FEC above CC" case is assessed
and in Section 4 and in Section 5 where "FEC in CC" and "FEC below
CC" are assessed.
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2.4. Scope of the document concerning transport multipath and multi-
streams applications
The application layer can be composed of several streams above FEC
and transport layers instances. The transport layer can exploit a
multipath mechanism. The different streams could exploit different
paths between the sender and the receiver. Moreover, a single-stream
application could also exploit a multipath transport mechanism. This
section describes what is in the scope of this document in regards
with multi-streams applications and multipath transport protocols.
The different combinations between multi-stream applications and
multipath transport are the following: (1) one application layer
stream as input packets above a combination of FEC and multipath
(Mpath) transport layers (Figure 5), and (2) multiple application
layer streams as input packets above a combination of FEC and
multipath (Mpath) or single path (Spath) transport layers (Figure 6).
This document further details cases I (in Section 3.7), II (in
Section 4.6) and III (in Section 5.7) illustrated in Figure 5. Cases
IV, V and VI of Figure 6 are related to how multiple streams are
managed by a single transport or FEC layer: this does not directly
concerns the interaction between FEC and the transport and is out of
the scope of this document.
CASE I CASE II CASE III
+---------------+ +---------------+ +---------------+
| Stream 1 | | Stream 2 | | Stream 3 |
+---------------+ +---------------+ +---------------+
+---------------+ +---------------+ +---------------+
| FEC | | FEC | |Mpath Transport|
+---------------+ | in | +---------------+
|Mpath Transport|
+---------------+ | | +-----+ +-----+
|Mpath Transport| | | |Flow1|...|FlowM|
+---------------+ +---------------+ +-----+ +-----+
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
|Flow1|...|FlowM| |Flow1|...|FlowM| | FEC |...| FEC |
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
Figure 5: Transport multipath and single stream applications - in
the scope of the document
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CASE IV CASE V CASE VI
+-------+ +-------+ +-------+ +-------+ +-------+ +-------+
|Stream1|...|StreamM| |Stream1|...|StreamM| |Stream1|...|StreamM|
+-------+ +-------+ +-------+ +-------+ +-------+ +-------+
+-------------------+ +-------------------+ +-------------------+
| | | FEC | | Mpath Transport |
| FEC | +-------------------+ +-------------------+
| above/in/below |
| Spath Transport | +-------------------+ +-------------------+
| | | Mpath Transport | | FEC |
+-------------------+ +-------------------+ +-------------------+
+-------------------+ +-----+ +-----+ +-----+ +-----+
| Flow | |Flow1| ... |FlowM| |Flow1| ... |FlowM|
+-------------------+ +-----+ +-----+ +-----+ +-----+
Figure 6: Transport single path, transport multipath and multi-stream
applications - out of the scope of the document
2.5. Types of coding
[RFC8406] summarizes recommended terminology for Network Coding
concepts and constructs. In particular, the document identifies the
following coding types (among many others):
* Block Coding: Coding technique where the input Flow must first be
segmented into a sequence of blocks; FEC encoding and decoding are
performed independently on a per-block basis.
* Sliding Window Coding: general class of coding techniques that
rely on a sliding encoding window.
The decoding scheme may not be able to decode all the symbols. The
chance of decoding the erased packets depends on the size of the
encoding window, the coding rate and the distribution of erasure in
the transmission channel. The FEC channel may let the client
transmit information related to the need of supplementary symbols to
adapt the level of reliability. Partial and full reliability could
be envisioned.
* Full reliability: The receiver may hold symbols until the decoding
of source symbols is possible. In particular, if the codec does
not enable a subset of the system to be inverted, the receiver
would have to wait for a certain minimum amount of repair packets
before it can recover all the source symbols.
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* Partial reliability: The receiver cannot deliver source symbols
that could not have been decoded to the upper layer. For a fixed
size of encoding window (for Sliding Window Coding) or of blocks
(for Block Coding) containing the source symbols, increasing the
amount of repair symbols would increase the chances of recovering
the erased symbols. However, this would impact on memory
requirements, on the cost of encoding and decoding processes and
on the network overhead.
3. FEC above the transport
| source ^ source
| packets | packets
v |
+-------------+ +-------------+
|FEC | signaling|FEC |
| | recovered| |
| | symbols| |
| | <==| |
+-------------+ +-------------+
| source ^ ^ source
| and/or | sending | and/or
| repair | rate | repair
| symbols | (or window) | symbols
v | |
+-------------+ +-------------+
|Transport | network|Transport |
|(incl. CC) | information| |
| |network <==| |
| |packets | |
+-------------+==> +-------------+
SENDER RECEIVER
Figure 7: FEC above the transport
Figure 7 presents an architecture where FEC operates on top of the
transport.
The advantage of this approach is that the FEC overhead does not
contribute to congestion in the network when congestion control is
implemented at the transport layer, because the repair symbols are
sent following the congestion window or rate determined by the CC
mechanism. This can result in improved quality of experience for
latency sensitive applications such as Voice over IP (VoIP) or any
not-fully reliable services.
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This approach requires that the transport protocol does not implement
a fully reliable in-order data transfer service (e.g., like TCP).
QUIC with unreliable datagram extension [I-D.ietf-quic-datagram] is
an example of a protocol for which this is relevant. In cases where
the partially reliable transport is blocked and a fall-back to a
reliable transport is proposed, there is a risk for bad interactions
between reliability at the transport level and coding schemes. For
reliable transfers, coding usage does not guarantee better
performance; instead, it would mainly reduce goodput.
3.1. Fairness and impact on non-coded flows
The addition of coding within the flow does not influence the
interaction between coded and non-coded flows. This interaction
would mainly depend on the congestion controls associated with each
flow.
3.2. Congestion control and recovered symbols
The congestion control mechanism receives network packets and may not
be able to differentiate repair symbols from actual source ones.
This differentiation requires a transport protocol providing more
than the services described in [RFC8095], in particular specifically
indicating what information has been repaired. The relevance of
adding coding at the application layer is related to the needs of the
application. For real-time applications using an unreliable or
partially reliable transport, this approach may reduce the number of
losses perceived by the application.
3.3. Interactions between congestion control and coding rates
The coding rate applied at the application layer mainly depends on
the available rate or congestion window given by the congestion
control underneath. The coding rate could be adapted to avoid adding
overhead when the minimum required data rate of the application is
not provided by the congestion control underneath. When the
congestion control allows sending faster than the application needs,
adding coding can reduce packet losses and improve the quality of
experience (provided that an unreliable or partially reliable
transport is used).
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3.4. On useless repair symbols
The only case where adding useless repair symbols does not obviously
result in reduced goodput is when the application rate is limited
(e.g., VoIP traffic). In this case, useless repair symbols would
only impact the amount of data generated in the network. Extra data
in the network can, however, increase the likelihood of increasing
delay and/or packet loss, which could provoke a congestion control
reaction that would degrade goodput.
3.5. On partial ordering at FEC level
Irrespective of the transport protocol, a FEC mechanism does not
require to implement a reordering mechanism if the application does
not need it. However, if the application needs in-order delivery of
packets, a reordering mechanism at the receiver is required.
3.6. On partial reliability at FEC level
The application may require partial reliability. In this case, the
coding rate of a FEC mechanism could be adapted based on inputs from
the application and the trade-off between latency and packet loss.
Partial reliability impacts the type of FEC and type of codec that
can be used, such as discussed in Section 2.5.
3.7. On multipath transport and FEC mechanism
Whether the transport protocol exploits multiple paths or not does
not have an impact on the FEC mechanism.
4. FEC within the transport
| source ^ source
| packets | packets
v |
+------------+ +------------+
| Transport | | Transport |
| | | |
| +---+ +--+ | signaling| +---+ +--+ |
| |FEC| |CC| | recovered| |FEC| |CC| |
| +---+ +--+ | symbols| +---+ +--+ |
| | <==| |
| |network network| |
| |packets information| |
+------------+ ==> <==+------------+
SENDER RECEIVER
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Figure 8: FEC in the transport
Figure 8 presents an architecture where FEC operates within the
transport. The repair symbols are sent within what the congestion
window or calculated rate allows, such as in [CTCP].
The advantage of this approach is that it allows a joint optimization
of CC and FEC. Moreover, the transmission of repair symbols does not
add congestion in potentially congested networks but helps repair
lost packets (such as tail losses). This joint optimization is the
key to prevent flows to consume the whole available capacity. The
amount of repair traffic injected should not lead to congestion. As
denoted in [I-D.singh-rmcat-adaptive-fec], an increase of the repair
ratio should be done conjointly with a decrease of the source sending
rate.
The drawback of this approach is that it may require specific
signaling and transport services that may not be described in
[RFC8095]. Therefore, development and maintenance may require
specific efforts at both transport and coding level and the design of
the solution may end up being complex to suit different deployment
needs.
For reliable transfers, including redundancy reduces goodput for long
transfers but the amount of repair symbols can be adapted, e.g.
depending on the congestion window size. There is a trade-off
between 1) the capacity that could have been exploited by application
data instead of transmitting source packets, and 2) the benefits
derived from transmitting repair symbols (e.g. unlocking the receive
buffer if it is limiting). The coding ratio needs to be carefully
designed. For small files, sending repair symbols when there is no
more data to transmit could help to reduce the transfer time.
Sending repair symbols can avoid the silence period between the
transmission of the last packet in the send buffer and 1) firing a
retransmission of lost packets, or 2) the transmission of new
packets.
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Examples of the solution could be to add a given percentage of the
congestion window or rate as supplementary symbols, or to send a
fixed amount of repair symbols at a fixed rate. The redundancy flow
can be decorrelated from the congestion control that manages source
packets: a separate congestion control entity could be introduced to
manage the amount of recovered symbols to transmit on the FEC
channel. The separate congestion control instances could be made to
work together while adhering to priorities, as in coupled congestion
control for RTP media [RFC8699] in case all traffic can be assumed to
take the same path, or otherwise with a multipath congestion window
coupling mechanism as in Multipath TCP [RFC6356]. Another
possibility would be to exploit a lower than best-effort congestion
control [RFC6297] for repair symbols.
4.1. Fairness and impact on non-coded flows
Specific interaction between congestion controls and coding schemes
can be proposed (see Section 4.2 and Section 4.3). If no specific
interaction is introduced, the coding scheme may hide congestion
losses from the congestion controller and the description of
Section 5 may apply.
4.2. Interactions between congestion control and coding rates
The receiver can differentiate between source packets and repair
symbols. The receiver may indicate both the number of source packets
received and repair symbols that were actually useful in the recovery
process of packets. The congestion control at the sender can then
exploit this information to tune congestion control behavior.
There is an important flexibility in the trade-off, inherent to the
use of coding, between (1) reducing goodput when useless repair
symbols are transmitted and (2) helping to recover from losses
earlier than with retransmissions. The receiver may indicate to the
sender the number of packets that have been received or recovered.
The sender may use this information to tune the coding ratio. For
example, coupling an increased transmission rate with an increasing
or decreasing coding rate could be envisioned. A server may use a
decreasing coding rate as a probe of the channel capacity and adapt
the congestion control transmission rate.
4.3. On useless repair symbols
The sender may exploit the information given by the receiver to
reduce the number of useless repair symbols, and improve goodput.
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4.4. On partial ordering at FEC and/or transport level
The application may require in-order delivery of packets. In this
case, both FEC and transport layer mechanisms should guarantee that
packets are delivered in order. If partial ordering is requested by
the application, both the FEC and transport could relax the
constraints related to in-order delivery: partial ordering impacts
both the congestion control and the type of FEC and type of codec
that can be used, mostly at the receiver that may need to implement
partial reordering.
4.5. On partial reliability at FEC level
The application may require partial reliability. The reliability
offered by FEC may be sufficient, with no retransmission required.
This depends on application needs and the trade-off between latency
and loss. Partial reliability impacts the type of FEC and type of
codec that can be used, such as discussed in Section 2.5.
4.6. On transport multipath and subpath FEC coding rate
The sender may adapt the coding rate of each of the single subpaths,
whether the congestion control is coupled or not. There is an
important flexibility on how the coding rate is tuned depending on
the characteristics of each subpath.
5. FEC below the transport
| source ^ source
| packets | packets
v |
+--------------+ +--------------+
|Transport | network|Transport |
|(including CC)| information| |
| | <==| |
+--------------+ +--------------+
| network packets ^ network packets
v |
+--------------+ +--------------+
| FEC |source | FEC |
| |and/or signaling| |
| |repair recovered| |
| |symbols symbols| |
| |==> <==| |
+--------------+ +--------------+
SENDER RECEIVER
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Figure 9: FEC below the transport
Figure 9 presents an architecture where FEC is applied end-to-end
below the transport layer, but above the link layer. Note that it is
common to apply FEC at the link layer on one or more of the links
that make up the end-to-end path. The application of FEC at the link
layer contributes to the total capacity that a link exposes to upper
layers, but may not be visible to either the end-to-end sender or
receiver, if the end-to-end sender and receiver are separated by more
than one link, and therefore is out of scope for this document. This
includes the use of FEC on top of a link layer in scenarios where the
link is known by configuration. In the scenario considered here, the
repair symbols are not visible to the end-to-end congestion
controller and may be sent on top of what is allowed by the
congestion control.
Including redundancy adds traffic without reducing goodput but incurs
potential fairness issues. The effective bit-rate is higher than the
CC's computed fair share due to the transmission of repair symbols,
and losses are hidden from the transport. This may cause a problem
for loss-based congestion detection, but it is not a problem for
delay-based congestion detection.
The advantage of this approach is that it can result in performance
gains when there are persistent transmission losses along the path.
The drawback of this approach is that it can induce congestion in
already congested networks. The coding ratio needs to be carefully
designed.
Examples of the solution could be to add a given percentage of the
congestion window or rate as supplementary symbols, or to send a
fixed amount of repair symbols at a fixed rate. The redundancy flow
can be decorrelated from the congestion control that manages source
packets: a separate congestion control entity could be introduced to
manage the amount of recovered symbols to transmit on the FEC
channel. The separate congestion control instances could be made to
work together while adhering to priorities, as in coupled congestion
control for RTP media [RFC8699] in case all traffic can be assumed to
take the same path, or otherwise with a multipath congestion window
coupling mechanism as in Multipath TCP [RFC6356]. Another
possibility would be to exploit a lower than best-effort congestion
control [RFC6297] for repair symbols.
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5.1. Fairness and impact on non-coded flows
The coding scheme may hide congestion losses from the congestion
controller. There are cases where this can drastically reduce the
goodput of non-coded flows. Depending on the congestion control, it
may be possible to signal to the congestion control mechanism that
there was congestion (loss) even when a packet has been recovered,
e.g. using ECN, to reduce the impact on the non-coded flows (see
Section 5.2 and [TENTET]).
5.2. Congestion control and recovered symbols
The congestion control may not be aware of the existence of a coding
scheme underneath it. The congestion control may behave as if no
coding scheme had been introduced. The only way for a coding channel
to indicate that symbols have been lost but recovered is to exploit
existing signaling that is understood by the congestion control
mechanism. An example would be to indicate to a TCP sender that a
packet has been received, yet congestion has occurred, by using ECN
signaling [TENTET].
5.3. Interactions between congestion control and coding rates
The coding rate can be tuned depending on the number of recovered
symbols and the rate at which the sender transmits data. If the
coding scheme is not aware of the congestion control implementation,
it is hard for the coding scheme to apply the relevant coding rate.
5.4. On useless repair symbols
Useless repair symbols only impact the load on the network without
actual gain for the coded flow. Using feedback signaling, FEC
mechanisms can measure the ratio between the number of symbols that
were actually used and the number of symbols that useless, and adjust
the coding rate.
5.5. On partial ordering at FEC level with in-order delivery transport
The transport above the FEC channel may support out-of-order delivery
of packets: reordering mechanisms at the receiver may not be
necessary. In cases where the transport requires in-order delivery,
the FEC channel may need to implement a reordering mechanism.
Otherwise, spurious retransmissions may occur at the transport level.
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5.6. On partial reliability at FEC level
The transport or application layer above the FEC channel may require
partial reliability only. FEC may provide an unnecessary service
unless it is aware of the reliability requirements. Partial
reliability impacts the type of FEC and type of codec that can be
used, such as discussed in Section 2.5.
5.7. FEC not aware of transport multipath
The transport may exploit multiple paths without the FEC channel
being aware of it. If FEC is aware that multiple paths are in use,
FEC can be applied to all subflows as an aggregate, or to each of the
subflows individually. If FEC is not aware that multiple paths are
in use, FEC can only be applied to each subflow individually. When
FEC is applied to all the flows as an aggregate, the varying
characteristics of the individual paths may lead to a risk for the
coding rate to be inadequate for the characteristics of the
individual paths.
6. Research recommendations and questions
This section provides a short state-of-the art overview of activities
related to congestion control and coding. The objective is to
identify open research questions and contribute to advice when
evaluating coding mechanisms.
6.1. Activities related to congestion control and coding
We map activities related to congestion control and coding with the
organization presented in this document:
* For the FEC above transport case: [RFC8680].
* For the FEC within transport case:
[I-D.swett-nwcrg-coding-for-quic], [QUIC-FEC], [RFC5109].
* For the FEC below transport case: [NCTCP],
[I-D.detchart-nwcrg-tetrys].
6.2. Open research questions
There is a general trade-off, inherent to the use of coding, between
(1) reducing goodput when useless repair symbols are transmitted and
(2) helping to recover from transmission and congestion losses.
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6.2.1. Parameter derivation
There is a trade-off related to the amount of redundancy to add, as a
function of the transport layer protocol and application
requirements.
[RFC8095] describes the mechanisms provided by existing IETF
protocols such as TCP, SCTP or RTP. [RFC8406] describes the variety
of coding techniques. The number of combinations makes the
determination of an optimum parameters derivation very complex. This
depends on application requirements and deployment context.
Appendix C of [RFC8681] describes how to tune the parameters for
target use-case. However, this discussion does not integrate
congestion-controlled end points.
Research question 1 : "Is there a way to dynamically adjust the codec
characteristics depending on the transmission channel, the transport
protocol and application requirements ?"
Research question 2 : "Should we apply specific per-stream FEC
mechanisms when multiple streams with different reliability needs are
carried out ?"
6.2.2. New signaling methods and fairness
Recovering lost symbols may hide congestion losses from the
congestion control. Disambiguate acked packets from rebuilt packets
would help the sender adapt its sending rate accordingly. There are
opportunities for introducing interaction between congestion control
and coding schemes to improve the quality of experience while
guaranteeing fairness with other flows.
Some existing solutions already propose to disambiguate acked packets
from rebuilt packets [QUIC-FEC]. New signaling methods and FEC-
recovery-aware congestion controls could be proposed. This would
allow the design of adaptive coding rates.
Research question 3 : "Should we quantify the harm that a coded flow
would induce on a non-coded flow ? How can this be reduced while
still benefiting from advantages brought by FEC ?"
Research question 4 : "If transport and FEC senders are collocated
and close to the client, and FEC is applied only on the last mile,
e.g. to ignore losses on a noisy wireless link, would this raise
fairness issues ?"
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Research question 5 : "Should we propose a generic API to allow
dynamic interactions between a transport protocol and a coding scheme
? This should consider existing APIs between application and
transport layers."
6.3. Recommendations and advice for evaluating coding mechanisms
Research Recommendation 1: "From a congestion control point-of-view,
a recovered packet must be considered as a lost packet. This does
not apply to the usage of FEC on a path that is known to be lossy."
Research Recommendation 2: "New research contributions should be
mapped following the organization of this document (above, below, in
the congestion control) and should consider congestion control
aspects when proposing and comparing FEC coding solutions in
communication systems."
Research Recommendation 3: "When a research work aims at improving
throughput by hiding the packet loss signal from congestion control
(e.g., because the path between the sender and receiver is known to
consist of a noisy wireless link), the authors should 1) discuss the
advantages of using the proposed FEC solution compared to replacing
the congestion control by one that ignores a portion of the
encountered losses, 2) critically discuss the impact of hiding packet
loss from the congestion control mechanism."
7. Acknowledgements
Many thanks to Spencer Dawkins, Dave Oran, Carsten Bormann, Vincent
Roca and Marie-Jose Montpetit for their useful comments that helped
improve the document.
8. IANA Considerations
This memo includes no request to IANA.
9. Security Considerations
FEC and CC schemes can contribute to DoS attacks. Moreover, the
transmission of signaling messages from the client to the server
should be protected and reliable otherwise an attacker may compromise
FEC rate adaptation. Indeed, an attacker could either modify the
values indicated by the client or drop signaling messages.
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In case of FEC below the transport, the aggregate rate of source and
repair packets may exceed the rate at which a congestion control
mechanism allows an application to send. This could result in an
application obtaining more than its fair share of the network
capacity.
10. Informative References
[BEYONDJAIN]
Ware (et al.), R., "Beyond Jain's Fairness Index: Setting
the Bar For The Deployment of Congestion Control
Algorithms", HotNets '19 10.1145/3365609.3365855, 2019.
[CTCP] Kim (et al.), M., "Network Coded TCP (CTCP)",
arXiv 1212.2291v3, 2013.
[I-D.briscoe-tsvarea-fair]
Briscoe, B., "Flow Rate Fairness: Dismantling a Religion",
Work in Progress, Internet-Draft, draft-briscoe-tsvarea-
fair-02, 11 July 2007, <https://www.ietf.org/archive/id/
draft-briscoe-tsvarea-fair-02.txt>.
[I-D.detchart-nwcrg-tetrys]
Detchart, J., Lochin, E., Lacan, J., and V. Roca, "Tetrys,
an On-the-Fly Network Coding protocol", Work in Progress,
Internet-Draft, draft-detchart-nwcrg-tetrys-08, 17 October
2021, <https://www.ietf.org/archive/id/draft-detchart-
nwcrg-tetrys-08.txt>.
[I-D.ietf-quic-datagram]
Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
Datagram Extension to QUIC", Work in Progress, Internet-
Draft, draft-ietf-quic-datagram-10, 4 February 2022,
<https://www.ietf.org/archive/id/draft-ietf-quic-datagram-
10.txt>.
[I-D.singh-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://www.ietf.org/archive/id/draft-
singh-rmcat-adaptive-fec-03.txt>.
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[I-D.swett-nwcrg-coding-for-quic]
Swett, I., Montpetit, M., Roca, V., and F. Michel, "Coding
for QUIC", Work in Progress, Internet-Draft, draft-swett-
nwcrg-coding-for-quic-04, 9 March 2020,
<https://www.ietf.org/archive/id/draft-swett-nwcrg-coding-
for-quic-04.txt>.
[NCTCP] Sundararajan (et al.), J., "Network Coding Meets TCP:
Theory and Implementation", IEEE
INFOCOM 10.1109/JPROC.2010.2093850, 2009.
[QUIC-FEC] Michel (et al.), F., "QUIC-FEC: Bringing the benefits of
Forward Erasure Correction to QUIC", IFIP
Networking 10.23919/IFIPNetworking.2019.8816838, 2019.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758,
DOI 10.17487/RFC3758, May 2004,
<https://www.rfc-editor.org/info/rfc3758>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.
[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <https://www.rfc-editor.org/info/rfc5109>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC6297] Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort
Transport Protocols", RFC 6297, DOI 10.17487/RFC6297, June
2011, <https://www.rfc-editor.org/info/rfc6297>.
[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, DOI 10.17487/RFC6356, October 2011,
<https://www.rfc-editor.org/info/rfc6356>.
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[RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>.
[RFC8406] Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek,
F., Ghanem, S., Lochin, E., Masucci, A., Montpetit, M-J.,
Pedersen, M., Peralta, G., Roca, V., Ed., Saxena, P., and
S. Sivakumar, "Taxonomy of Coding Techniques for Efficient
Network Communications", RFC 8406, DOI 10.17487/RFC8406,
June 2018, <https://www.rfc-editor.org/info/rfc8406>.
[RFC8680] Roca, V. and A. Begen, "Forward Error Correction (FEC)
Framework Extension to Sliding Window Codes", RFC 8680,
DOI 10.17487/RFC8680, January 2020,
<https://www.rfc-editor.org/info/rfc8680>.
[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>.
[RFC8699] Islam, S., Welzl, M., and S. Gjessing, "Coupled Congestion
Control for RTP Media", RFC 8699, DOI 10.17487/RFC8699,
January 2020, <https://www.rfc-editor.org/info/rfc8699>.
[TENTET] Lochin, E., "On the joint use of TCP and Network Coding",
NWCRG session IETF 100, 2017.
Authors' Addresses
Nicolas Kuhn
CNES
Email: nicolas.kuhn.ietf@gmail.com
Emmanuel Lochin
ENAC
Email: emmanuel.lochin@enac.fr
Francois Michel
UCLouvain
Email: francois.michel@uclouvain.be
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Michael Welzl
University of Oslo
Email: michawe@ifi.uio.no
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