| Internet-Draft | Coding and congestion | March 2021 |
| Kuhn, et al. | Expires 30 September 2021 | [Page] |
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.¶
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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 is a reliability mechanism that is distinct and separate from the loss detection of congestion controls. [RFC5681] defines the loss-based congestion control of TCP; since FEC coding repairs such losses, blindly applying it 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.¶
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.¶
We consider an end-to-end unicast data transfer with FEC coding in the application (above the transport), within the transport or directly below the transport. 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].¶
Figure 1 presents the notations that will be used in this document and introduces the Congestion Control (CC) and Forward Erasure Correction (FEC) channels. The Congestion Control channel carries source packets from a sender to a receiver, and packets signaling information about the network (number of packets received vs. lost, Explicit Congestion Notification (ECN) marks, etc.) from the receiver to the sender. 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 packets have been repaired, loss rate prior and/or after decoding, etc.).¶
SENDER RECEIVER +------+ +------+ | | ----- source packets ---->| | | CC | | CC | | | <--- network information ---| | +------+ +------+ +------+ +------+ | | ----- repair symbols ---->| | | FEC | | FEC | | | <--- info: repaired symbols --| | +------+ +------+
Inside a host, the CC and FEC entities can be regarded as conceptually separate:¶
| ^ | ^ | source | coding |packets | sending | packets | rate |requirements | rate (or v | v | window) +---------------+source +-----------------+ | FEC |and/or | CC | | |repair | |source | |symbols | |packets +---------------+==> +-----------------+==> ^ ^ | signaling about | network | losses and/or | information | repaired symbols
| | | source and/or | packets | repair symbols | v v +---------------+ +-----------------+ | FEC |signaling | CC | | |repaired | |network | |symbols | |information +---------------+==> +-----------------+==>
Figure 2 and Figure 3 provide more details than Figure 1. Some elements are introduced:¶
The inputs to FEC (incoming data packets without repair symbols, and signaling from the receiver about losses and/or repaired 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 repaired blocks may be quite irrelevant to a CC algorithm.¶
The choice of the adequate transport layer may be related to application requirements and the services offered by a transport protocol [RFC8095]:¶
The application layer may be composed of several streams above FEC and transport layers instances. The different streams could exploit different paths between the sender and the receiver or not. The document considers one application layer stream as input packets above a combination of FEC and transport layers. In Figure 4, each of stream 1 and stream 2 are considered in the scope of the document but a combination of stream 1 and stream 2 is not. The case of multiple application level streams above multiple FEC and transport layers instances is out of the scope of the document and not further described.¶
+-----------------------------------------------+ | Application | | | | +-------------------+ +-------------------+ | | | Stream 1 | | Stream 2 | | | +-------------------+ +-------------------+ | | | | +-------------------+ +-------------------+ | | | FEC | |Multipath Transport| | | +-------------------+ +-------------------+ | | | | +-------------------+ +-----+ +-----+ | | |Multipath Transport| |Flow1| ... |FlowM| | | +-------------------+ +-----+ +-----+ | | | | +-----+ +-----+ +-----+ +-----+ | | |Flow1| ... |FlowM| | FEC | ... | FEC | | | +-----+ +-----+ +-----+ +-----+ | +-----------------------------------------------+
[RFC8406] summarizes recommended terminology for Network Coding concepts and constructs. In particular, the document identifies the following coding types (among many others):¶
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 tencoding 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.¶
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.¶
| source ^ source
| packets | packets
v |
+-------------+ +-------------+
|FEC | signaling|FEC |
| | repaired| |
| | symbols| |
| | <==| |
+-------------+ +-------------+
| source ^ ^ source
| and/or | sending | and/or
| repair | rate | repair
| symbols | (or window) | symbols
v | |
+-------------+ +-------------+
|Transport |source network|Transport |
|(incl. CC) |and/or information| |
| |repair <==| |
| |packets | |
+-------------+==> +-------------+
SENDER RECEIVER
Figure 5 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, 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 VoIP or any not-fully reliable services.¶
This approach requires that the transport protocol does not implement a fully reliable data transfer service (e.g., based on lost packet retransmission). QUIC with unreliable datagram extension [I-D.ietf-quic-datagram] is an example of a protocol for which this is relevant. In cases where QUIC traffic is blocked and a fall-back to TCP is proposed, there is a risk for bad interactions between TCP's full reliability and coding schemes. For reliable transfers, coding usage does not guarantee better performance; instead, it would mainly reduce goodput.¶
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.¶
The congestion control mechanism may not be able to differentiate repair symbols from actual source packets. 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.¶
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).¶
The discussion depends on application needs. 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.¶
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.¶
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 2.4.¶
Whether the transport protocol exploits multiple paths or not does not have an impact on the FEC mechanism.¶
| source | sending ^ source
| packets | rate | packets
v v |
+------------+ +------------+
| Transport | | Transport |
| | | |
| +---+ +--+ | signaling| +---+ +--+ |
| |FEC| |CC| | repaired| |FEC| |CC| |
| +---+ +--+ |source symbols| +---+ +--+ |
| |and/or <==| |
| |repair network| |
| |packets information| |
+------------+ ==> <==+------------+
SENDER RECEIVER
Figure 6 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).¶
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.¶
The addition of coding within the transport may impact the congestion control mechanism and hide congestion losses. Specific interaction between congestion controls and coding schemes can be proposed (see Section 4.2, Section 4.3 and Section 4.4). 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.¶
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.¶
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.¶
The sender may exploit the information given by the receiver to reduce the number of useless repair symbols and the resulting goodput reduction.¶
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: reordering mechanisms at the receiver may not be necessary.¶
The application may require partial reliability. In this case, the transport and FEC mechanisms could be conjointly designed. As one example, 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 2.4.¶
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.¶
| source | sending rate ^ source
| packets | (or window) | packets
v v |
+--------------+ +--------------+
|Transport | network|Transport |
|(including CC)| information| |
| | <==| |
+--------------+ +--------------+
| source packets ^ source packets
v |
+--------------+ +--------------+
| FEC |source | FEC |
| |and/or signaling| |
| |repair repaired| |
| |symbols symbols| |
| |==> <==| |
+--------------+ +--------------+
SENDER RECEIVER
Figure 7 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, where it contributes to the total capacity that a link exposes to upper layers. This application of FEC is out of scope of this document. In the scenario considered here, the repair symbols are 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 bitrate 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 repaired packets 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.¶
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]).¶
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].¶
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.¶
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 actually used and useless symbols, and adjust the coding rate.¶
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.¶
The transport or application layer above the FEC channel may require partial reliability only. In this case, FEC may provide an unnecessary service if it is not 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 2.4.¶
The transport may exploit multiple paths without the FEC channel being aware of it. This depends on whether FEC is applied to all subflows or each of the subflows individually. When FEC is applied to all the flows, there is a risk for the coding rate to be inadequate for the characteristics of the individual paths.¶
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.¶
We map activities related to congestion control and coding with the organization presented in this document:¶
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.¶
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 important level 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 ?"¶
Recovering lost symbols may hide congestion losses from the congestion control. Disambiguating 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.¶
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 not collocated, if the FEC is applied only on the last mile, would this raise fairness issues ?"¶
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."¶
Our advice is that new research contributions should be mapped following the organization of this document. Otherwise, this may lead to wrong assumptions on the validity of the proposal and wrong ideas about the relevance of coding for a given use case.¶
The discussion provided in this document aims to encourage the research community to also consider congestion control aspects when proposing and comparing FEC coding solutions in communication systems. As one example, this draft proposes discussions on the impact of the proposed FEC solution on congestion control, especially loss-based congestion control mechanisms. When a research work aims at improving throughput by hiding the packet loss signal from congestion control, 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.¶
Many thanks to Spencer Dawkins, Dave Oran, Carsten Bormann, Vincent Roca and Marie-Jose Montpetit for their useful comments that helped improve the document.¶
This memo includes no request to IANA.¶
FEC and CC schemes can contribute to DoS attacks. This is not specific to this document.¶
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.¶