| NWCRG | N. Kuhn |
| Internet-Draft | CNES |
| Intended status: Informational | E. Lochin |
| Expires: January 10, 2021 | ISAE-SUPAERO |
| F. Michel | |
| UCLouvain | |
| M. Welzl | |
| University of Oslo | |
| July 9, 2020 |
Coding and congestion control in transport
draft-irtf-nwcrg-coding-and-congestion-03
FEC coding is a reliability mechanism that is distinct and separate from the loss detection of congestion controls. Using FEC coding can be a useful way to deal with transfer tail losses 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 the sender to detect transfer tail losses (losses that occur at the end of a transfer, where e.g. TCP obtains no more ACKs to quickly repair the loss via retransmission). This would improve the experience of applications using short flows. Another example are networks 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 TCP as a loss-based congestion control; because FEC coding repairs such losses, blindly applying it may easily lead to an implementation that also hides a congestion signal to 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 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 being said, this document does not aim at proposing guidelines for characterizing FEC coding solutions: comparing FEC Schemes without considering congestion control can be relevant if the goal is to compare those schemes only.
The proposed document considers FEC coding at the transport or application layer for an end-to-end unicast data transfer. The typical application scenario that is considered in the current version of the document is a client browsing the web or watching a live video. This memo may be extended to cases with multiple paths.
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, ECN marks, etc.) from the receiver to the sender. The Forward Erasure Correction channel carries repair packets (from the sender to the receiver) and potential information signaling which packets have been repaired (from the receiver to the sender). It is worth pointing out that there are cases where these channels are not separated.
Sender Receiver
+------+ +------+
| | ----- source packets ---->| |
| CC | | CC |
| | <--- network information ---| |
+------+ +------+
+------+ +------+
| | ----- repair packets ---->| |
| FEC | | FEC |
| | <--- info: repaired packets --| |
+------+ +------+
Figure 1: Notations and separate channels
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
| |packets | |packets
+---------------+==> +-----------------+==>
^ ^
| signaling about | network
| losses and/or | information
| repaired packets
Figure 2: Separate entities (sender-side)
| |
| source and/or | packets
| repair packets |
v v
+---------------+ +-----------------+
| FEC |signaling | CC |
| |repaired | |network
| |packets | |information
+---------------+==> +-----------------+==>
repaired packets
Figure 3: Separate entities (client-side)
Figure 2 and Figure 3 provide more details than Figure 1. Some elements are introduced:
The inputs to FEC (packets to work upon, and signaling from the receiver about losses and/or repaired packets) 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:
| source ^ source
| packets | packets
v |
+-------------+ +-------------+
|FEC | signaling|FEC |
| | repaired| |
| | packets| |
| | <==| |
+-------------+ +-------------+
| source ^ ^ source
| and/or | sending | and/or
| repair | rate | repair
| packets | (or window) | packets
v | |
+-------------+ +-------------+
|Transport |source network|Transport |
|(incl. CC) |and/or information| |
| |repair <==| |
| |packets | |
+-------------+==> +-------------+
SERVER CLIENT
Figure 4: FEC above the transport
Figure 4 present an architecture where FEC is on top of the transport.
While CC is in principle independent of other transport functions such as reliability, we note that CC is often embedded in reliable transfer protocols (e.g. TCP). This approach may require that the transport protocol does not implement a fully reliable data transfer service (e.g., based on lost packet retransmission), otherwise this could lead to spurious retransmissions. UDP is an example of a protocol for which this approach is relevant.
The advantage of this approach is that the FEC does not contribute to adding congestion in the network. When a congestion control is implementedd at the transport layer, the repair packets are sent within what the congestion window allows.
| source | sending ^ source
| packets | rate | packets
v v |
+------------+ +------------+
| Transport | | Transport |
| | | |
| +---+ +--+ | signaling| +---+ +--+ |
| |FEC| |CC| | repaired| |FEC| |CC| |
| +---+ +--+ |source packets| +---+ +--+ |
| |and/or <==| |
| |repair network| |
| |packets information| |
+------------+ ==> <==+------------+
SERVER CLIENT
Figure 5: FEC in the transport
Figure 5 presents an architecture where FEC is within the transport. The repair packets are sent within what the congestion window allows, such as in [CTCP].
The advantage of this approach is that it can enable conjoint optimization between the CC and the FEC. Moreover, the transmission of repair packets does not add congestion in potentially congested networks but helps repair lost packets (such as tail losses).
The drawback of this approach is that it may not result in much gains as opposed to classical retransmission mechanisms and it costs bandwidth that could have been exploited to transmit source packets. The coding ratio needs to be carefully designed.
Examples of the solution would be sending repair packets when there is no more data to transmit or preferably send repair packets instead of the following packets in the send buffer.
| 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| |
| |packets packets| |
| |==> <==| |
+--------------+ +--------------+
SERVER CLIENT
Figure 6: FEC below the transport
Figure 6 presents an architecture where FEC is below the transport. The repair packets are sent on top of what is allowed by the congestion control.
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 adding a given percentage of the congestion window as supplementary packets or sending a given amount of repair packets at a given rate. The redundancy flow can be decorrelated from the congestion control that manages source packets: a secondary congestion control could be introduced underneath the FEC layer. The separate congestion control mechanisms could be made to work together while adhering to priorities, as in coupled congestion control for RTP media [I-D.ietf-rmcat-coupled-cc]. Another possibility would be to exploit a lower than best-effort congestion control [RFC6297] for repair packets.
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.