NWCRG N. Kuhn Internet-Draft CNES Intended status: Informational E. Lochin Expires: October 21, 2021 ENAC F. Michel UCLouvain M. Welzl University of Oslo April 19, 2021 Coding and congestion control in transport draft-irtf-nwcrg-coding-and-congestion-08 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 21, 2021. Kuhn, et al. Expires October 21, 2021 [Page 1] Internet-Draft Coding and congestion April 2021 Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Separate channels, separate entities . . . . . . . . . . 4 2.2. Relation between transport layer and application requirements . . . . . . . . . . . . . . . . . . . . . . 6 2.3. Scope of the document concerning transport multipath and multi-streams applications . . . . . . . . . . . . . . . 7 2.4. Types of coding . . . . . . . . . . . . . . . . . . . . . 8 2.5. Fairness, a policy concern . . . . . . . . . . . . . . . 9 3. FEC above the transport . . . . . . . . . . . . . . . . . . . 9 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 . . . . . . . . . . . . . . . . 11 3.5. On partial ordering . . . . . . . . . . . . . . . . . . . 11 3.6. On partial reliability . . . . . . . . . . . . . . . . . 12 3.7. On multipath transport . . . . . . . . . . . . . . . . . 12 4. FEC within the transport . . . . . . . . . . . . . . . . . . 12 4.1. Fairness and impact on non-coded flows . . . . . . . . . 13 4.2. Congestion control and recovered symbols . . . . . . . . 13 4.3. Interactions between congestion control and coding rates 13 4.4. On useless repair symbols . . . . . . . . . . . . . . . . 13 4.5. On partial ordering . . . . . . . . . . . . . . . . . . . 14 4.6. On partial reliability . . . . . . . . . . . . . . . . . 14 4.7. On transport multipath . . . . . . . . . . . . . . . . . 14 5. FEC below the transport . . . . . . . . . . . . . . . . . . . 14 5.1. Fairness and impact on non-coded flows . . . . . . . . . 15 5.2. Congestion control and recovered symbols . . . . . . . . 16 5.3. Interactions between congestion control and coding rates 16 5.4. On useless repair symbols . . . . . . . . . . . . . . . . 16 5.5. On partial ordering . . . . . . . . . . . . . . . . . . . 16 Kuhn, et al. Expires October 21, 2021 [Page 2] Internet-Draft Coding and congestion April 2021 5.6. On partial reliability . . . . . . . . . . . . . . . . . 16 5.7. On transport multipath . . . . . . . . . . . . . . . . . 16 6. Research recommendations and questions . . . . . . . . . . . 17 6.1. Activities related to congestion control and coding . . . 17 6.2. Open research questions . . . . . . . . . . . . . . . . . 17 6.2.1. Parameter derivation . . . . . . . . . . . . . . . . 17 6.2.2. New signaling methods and fairness . . . . . . . . . 18 6.3. Recommendations and advice for evaluating coding mechanisms . . . . . . . . . . . . . . . . . . . . . . . 18 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 9. Security Considerations . . . . . . . . . . . . . . . . . . . 19 10. Informative References . . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 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 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 Kuhn, et al. Expires October 21, 2021 [Page 3] Internet-Draft Coding and congestion April 2021 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. Separate channels, separate entities Figure 1 presents 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 packets have been repaired, 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) 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 ---| | +------+ +------+ +------+ +------+ | | source and/or ---->| | | | ----- repair symbols ---->| | | FEC | | FEC | | | <--- info: repaired symbols --| | +------+ +------+ Figure 1: Notations and separate channels Inside a host, the CC and FEC entities can be regarded as conceptually separate: Kuhn, et al. Expires October 21, 2021 [Page 4] Internet-Draft Coding and congestion April 2021 | ^ | ^ | source | coding |packets | sending | packets | rate |requirements | rate (or v | v | window) +---------------+source +-----------------+ | FEC |and/or | CC | | |repair | |network | |symbols | |packets +---------------+==> +-----------------+==> ^ ^ | signaling about | network | losses and/or | information | repaired symbols Figure 2: Separate entities (sender-side) | | | source and/or | network | repair symbols | packets v v +---------------+ +-----------------+ | FEC |signaling | CC | | |repaired | |network | |symbols | |information +---------------+==> +-----------------+==> Figure 3: Separate entities (receiver-side) Figure 2 and Figure 3 provide more details than Figure 1. Some elements are introduced: o '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 (e.g., TCP can estimate the latency and the available capacity at the bottleneck). o '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. o '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'. o 'signaling repaired symbols' (input control plane for the FEC): refers to the information a FEC sender can obtain from a FEC Kuhn, et al. Expires October 21, 2021 [Page 5] Internet-Draft Coding and congestion April 2021 receiver about the performance of the FEC solution as seen by the receiver. o '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). o '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. o '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 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. 2.2. 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]: o 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. o 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, FEC in CC, FEC below CC), the impact of reliable transport on the FEC reliability mechanisms is different. Kuhn, et al. Expires October 21, 2021 [Page 6] Internet-Draft Coding and congestion April 2021 2.3. Scope of the document concerning transport multipath and multi- streams applications The application layer may be composed of several streams above FEC and transport layers instances. The transport layer may exploit a multipath mechanism. The different streams could exploit different paths between the sender and the receiver or not. 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 4), and (2) multiple application layer streams as input packets above a combination of FEC and multipath (Mpath) or single path (Spath) transport layers (Figure 5). In Figure 4, each of stream 1 and stream 2 are considered in the scope of the document. In Figure 5, the case with single path transport is considered in the scope of this document but the cases with mutipath transport is not. The case of multiple application level streams above multiple transport layers instances is out of the scope of the document and not further described. +---------------+ +---------------+ | Stream 1 | | Stream 2 | +---------------+ +---------------+ +---------------+ +---------------+ | FEC | |Mpath Transport| +---------------+ +---------------+ +---------------+ +-----+ +-----+ |Mpath Transport| |Flow1|...|FlowM| +---------------+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ |Flow1|...|FlowM| | FEC |...| FEC | +-----+ +-----+ +-----+ +-----+ Figure 4: Transport multipath and single stream applications Kuhn, et al. Expires October 21, 2021 [Page 7] Internet-Draft Coding and congestion April 2021 +-------+ +-------+ +-------+ +-------+ +-------+ +-------+ |Stream1|...|StreamM| |Stream1|...|StreamM| |Stream1|...|StreamM| +-------+ +-------+ +-------+ +-------+ +-------+ +-------+ +-------------------+ +-------------------+ +-------------------+ | | | FEC | | Mpath Transport | | FEC | +-------------------+ +-------------------+ | above/in/below | | Spath Transport | +-------------------+ +-------------------+ | | | Mpath Transport | | FEC | +-------------------+ +-------------------+ +-------------------+ +-------------------+ +-----+ +-----+ +-----+ +-----+ | Flow | |Flow1| ... |FlowM| |Flow1| ... |FlowM| +-------------------+ +-----+ +-----+ +-----+ +-----+ Figure 5: Transport single path, transport multipath and multi-stream applications 2.4. 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): o 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. o 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. o 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. o Partial reliability: The receiver cannot deliver source symbols that could not have been decoded to the upper layer. For a fixed Kuhn, et al. Expires October 21, 2021 [Page 8] Internet-Draft Coding and congestion April 2021 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. 2.5. Fairness, a policy concern 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. 3. FEC above the transport Kuhn, et al. Expires October 21, 2021 [Page 9] Internet-Draft Coding and congestion April 2021 | 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 | network|Transport | |(incl. CC) | information| | | |network <==| | | |packets | | +-------------+==> +-------------+ SENDER RECEIVER Figure 6: FEC above the transport Figure 6 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 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 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. Kuhn, et al. Expires October 21, 2021 [Page 10] Internet-Draft Coding and congestion April 2021 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. 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). 3.4. On useless repair symbols 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. 3.5. On partial ordering 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. Kuhn, et al. Expires October 21, 2021 [Page 11] Internet-Draft Coding and congestion April 2021 3.6. On partial reliability 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. 3.7. On multipath transport 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| | repaired| |FEC| |CC| | | +---+ +--+ | symbols| +---+ +--+ | | | <==| | | |network network| | | |packets information| | +------------+ ==> <==+------------+ SENDER RECEIVER Figure 7: FEC in the transport Figure 7 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 Kuhn, et al. Expires October 21, 2021 [Page 12] Internet-Draft Coding and congestion April 2021 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. 4.1. Fairness and impact on non-coded flows 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. 4.2. Congestion control and recovered symbols 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. 4.3. Interactions between congestion control and coding rates 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. 4.4. On useless repair symbols The sender may exploit the information given by the receiver to reduce the number of useless repair symbols and the resulting goodput reduction. Kuhn, et al. Expires October 21, 2021 [Page 13] Internet-Draft Coding and congestion April 2021 4.5. On partial ordering 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. 4.6. On partial reliability 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 2.4. 4.7. On transport multipath 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 repaired| | | |symbols symbols| | | |==> <==| | +--------------+ +--------------+ SENDER RECEIVER Figure 8: FEC below the transport Kuhn, et al. Expires October 21, 2021 [Page 14] Internet-Draft Coding and congestion April 2021 Figure 8 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. 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 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 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. 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]). Kuhn, et al. Expires October 21, 2021 [Page 15] Internet-Draft Coding and congestion April 2021 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 actually used and useless symbols, and adjust the coding rate. 5.5. On partial ordering 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. 5.6. On partial reliability 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. 5.7. On transport multipath 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. Kuhn, et al. Expires October 21, 2021 [Page 16] Internet-Draft Coding and congestion April 2021 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: o For the FEC above transport case: [RFC8680]. o For the FEC within transport case: [I-D.swett-nwcrg-coding-for-quic], [QUIC-FEC], [RFC5109]. o 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. 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 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 ?" Kuhn, et al. Expires October 21, 2021 [Page 17] Internet-Draft Coding and congestion April 2021 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 ?" 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 repaired 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 Kuhn, et al. Expires October 21, 2021 [Page 18] Internet-Draft Coding and congestion April 2021 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. 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. 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", draft-briscoe-tsvarea-fair-02 (work in progress), July 2007. [I-D.detchart-nwcrg-tetrys] Detchart, J., Lochin, E., Lacan, J., and V. Roca, "Tetrys, an On-the-Fly Network Coding protocol", draft-detchart- nwcrg-tetrys-06 (work in progress), December 2020. Kuhn, et al. Expires October 21, 2021 [Page 19] Internet-Draft Coding and congestion April 2021 [I-D.ietf-quic-datagram] Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable Datagram Extension to QUIC", draft-ietf-quic-datagram-01 (work in progress), August 2020. [I-D.swett-nwcrg-coding-for-quic] Swett, I., Montpetit, M., Roca, V., and F. Michel, "Coding for QUIC", draft-swett-nwcrg-coding-for-quic-04 (work in progress), March 2020. [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. [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, . [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340, March 2006, . [RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error Correction", RFC 5109, DOI 10.17487/RFC5109, December 2007, . [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, . [RFC6297] Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort Transport Protocols", RFC 6297, DOI 10.17487/RFC6297, June 2011, . [RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion Control for Multipath Transport Protocols", RFC 6356, DOI 10.17487/RFC6356, October 2011, . Kuhn, et al. Expires October 21, 2021 [Page 20] Internet-Draft Coding and congestion April 2021 [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, . [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, . [RFC8680] Roca, V. and A. Begen, "Forward Error Correction (FEC) Framework Extension to Sliding Window Codes", RFC 8680, DOI 10.17487/RFC8680, January 2020, . [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, . [RFC8699] Islam, S., Welzl, M., and S. Gjessing, "Coupled Congestion Control for RTP Media", RFC 8699, DOI 10.17487/RFC8699, January 2020, . [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@cnes.fr Emmanuel Lochin ENAC Email: emmanuel.lochin@enac.fr Francois Michel UCLouvain Email: francois.michel@uclouvain.be Kuhn, et al. Expires October 21, 2021 [Page 21] Internet-Draft Coding and congestion April 2021 Michael Welzl University of Oslo Email: michawe@ifi.uio.no Kuhn, et al. Expires October 21, 2021 [Page 22]