ECN

Explicit Congestion Notification (ECN) is a mechanism by which network nodes can mark IP packets instead of dropping them to indicate incipient congestion to the endpoints. Receivers with an ECN-capable transport protocol feed back this information to the sender. ECN was originally specified for TCP in such a way that only one feedback signal can be transmitted per Round-Trip Time (RTT). More recently defined mechanisms like Congestion Exposure (ConEx), Data Center TCP (DCTCP), or Low Latency, Low Loss, and Scalable Throughput (L4S) need more accurate ECN feedback information whenever more than one marking is received in one RTT. This document updates the original ECN specification defined in RFC 3168 by specifying a scheme that provides more than one feedback signal per RTT in the TCP header, called More Accurate ECN (AccECN). Given TCP header space is scarce, it allocates a reserved header bit previously assigned to the ECN-nonce. It also overloads the two existing ECN flags in the TCP header. The resulting extra space is additionally exploited to feed back the IP-ECN field received during the TCP connection establishment. Supplementary feedback information can optionally be provided in two new TCP Option alternatives, which are never used on the TCP SYN. The document also specifies the treatment of this updated TCP wire protocol by middleboxes.

The purpose of this document is to guide the design of congestion notification in any lower-layer or tunnelling protocol that encapsulates IP. The aim is for explicit congestion signals to propagate consistently from lower-layer protocols into IP. Then, the IP internetwork layer can act as a portability layer to carry congestion notification from non-IP-aware congested nodes up to the transport layer (L4). Specifications that follow these guidelines, whether produced by the IETF or other standards bodies, should assure interworking among IP-layer and lower-layer congestion notification mechanisms. This document is included in BCP 89 and updates the single paragraph of advice to subnetwork designers about Explicit Congestion Notification (ECN) in Section 13 of RFC 3819 by replacing it with a reference to this document.

Explicit Congestion Notification (ECN) allows a forwarding element to notify downstream devices, including the destination, of the onset of congestion without having to drop packets. This can improve network efficiency through better congestion control without packet drops. This document extends ECN to TRansparent Interconnection of Lots of Links (TRILL) switches, including integration with IP ECN, and provides for ECN marking in the TRILL header extension flags word (RFC 7179).

RFC 6040 on "Tunnelling of Explicit Congestion Notification" made the rules for propagation of Explicit Congestion Notification (ECN) consistent for all forms of IP-in-IP tunnel. This specification updates RFC 6040 to clarify that its scope includes tunnels where two IP headers are separated by at least one shim header that is not sufficient on its own for wide-area packet forwarding. It surveys widely deployed IP tunnelling protocols that use such shim headers and updates the specifications of those that do not mention ECN propagation (including RFCs 2661, 3931, 2784, 4380 and 7450, which specify L2TPv2, L2TPv3, Generic Routing Encapsulation (GRE), Teredo, and Automatic Multicast Tunneling (AMT), respectively). This specification also updates RFC 6040 with configuration requirements needed to make any legacy tunnel ingress safe.

This document describes the L4S architecture, which enables Internet applications to achieve low queuing latency, low congestion loss, and scalable throughput control. L4S is based on the insight that the root cause of queuing delay is in the capacity-seeking congestion controllers of senders, not in the queue itself. With the L4S architecture, all Internet applications could (but do not have to) transition away from congestion control algorithms that cause substantial queuing delay and instead adopt a new class of congestion controls that can seek capacity with very little queuing. These are aided by a modified form of Explicit Congestion Notification (ECN) from the network. With this new architecture, applications can have both low latency and high throughput.

The architecture primarily concerns incremental deployment. It defines mechanisms that allow the new class of L4S congestion controls to coexist with 'Classic' congestion controls in a shared network. The aim is for L4S latency and throughput to be usually much better (and rarely worse) while typically not impacting Classic performance.

This specification defines the protocol to be used for a new network service called Low Latency, Low Loss, and Scalable throughput (L4S). L4S uses an Explicit Congestion Notification (ECN) scheme at the IP layer that is similar to the original (or 'Classic') ECN approach, except as specified within. L4S uses 'Scalable' congestion control, which induces much more frequent control signals from the network, and it responds to them with much more fine-grained adjustments so that very low (typically sub-millisecond on average) and consistently low queuing delay becomes possible for L4S traffic without compromising link utilization. Thus, even capacity-seeking (TCP-like) traffic can have high bandwidth and very low delay at the same time, even during periods of high traffic load.

The L4S identifier defined in this document distinguishes L4S from 'Classic' (e.g., TCP-Reno-friendly) traffic. Then, network bottlenecks can be incrementally modified to distinguish and isolate existing traffic that still follows the Classic behaviour, to prevent it from degrading the low queuing delay and low loss of L4S traffic. This Experimental specification defines the rules that L4S transports and network elements need to follow, with the intention that L4S flows neither harm each other's performance nor that of Classic traffic. It also suggests open questions to be investigated during experimentation. Examples of new Active Queue Management (AQM) marking algorithms and new transports (whether TCP-like or real time) are specified separately.

This specification defines a framework for coupling the Active Queue Management (AQM) algorithms in two queues intended for flows with different responses to congestion. This provides a way for the Internet to transition from the scaling problems of standard TCP-Reno-friendly ('Classic') congestion controls to the family of 'Scalable' congestion controls. These are designed for consistently very low queuing latency, very low congestion loss, and scaling of per-flow throughput by using Explicit Congestion Notification (ECN) in a modified way. Until the Coupled Dual Queue (DualQ), these Scalable L4S congestion controls could only be deployed where a clean-slate environment could be arranged, such as in private data centres.

This specification first explains how a Coupled DualQ works. It then gives the normative requirements that are necessary for it to work well. All this is independent of which two AQMs are used, but pseudocode examples of specific AQMs are given in appendices.

Active Queue Management (AQM) mechanisms allow for burst tolerance while enforcing short queues to minimise the time that packets spend enqueued at a bottleneck. This can cause noticeable performance degradation for TCP connections traversing such a bottleneck, especially if there are only a few flows or their bandwidth-delay product (BDP) is large. The reception of a Congestion Experienced (CE) Explicit Congestion Notification (ECN) mark indicates that an AQM mechanism is used at the bottleneck, and the bottleneck network queue is therefore likely to be short. Feedback of this signal allows the TCP sender-side ECN reaction in congestion avoidance to reduce the Congestion Window (cwnd) by a smaller amount than the congestion control algorithm's reaction to inferred packet loss. Therefore, this specification defines an experimental change to the TCP reaction specified in RFC 3168, as permitted by RFC 8311.

This memo updates RFC 3168, which specifies Explicit Congestion Notification (ECN) as an alternative to packet drops for indicating network congestion to endpoints. It relaxes restrictions in RFC 3168 that hinder experimentation towards benefits beyond just removal of loss. This memo summarizes the anticipated areas of experimentation and updates RFC 3168 to enable experimentation in these areas. An Experimental RFC in the IETF document stream is required to take advantage of any of these enabling updates. In addition, this memo makes related updates to the ECN specifications for RTP in RFC 6679 and for the Datagram Congestion Control Protocol (DCCP) in RFCs 4341, 4342, and 5622. This memo also records the conclusion of the ECN nonce experiment in RFC 3540 and provides the rationale for reclassification of RFC 3540 from Experimental to Historic; this reclassification enables new experimental use of the ECT(1) codepoint.

This Informational RFC describes Data Center TCP (DCTCP): a TCP congestion control scheme for data-center traffic. DCTCP extends the Explicit Congestion Notification (ECN) processing to estimate the fraction of bytes that encounter congestion rather than simply detecting that some congestion has occurred. DCTCP then scales the TCP congestion window based on this estimate. This method achieves high-burst tolerance, low latency, and high throughput with shallow- buffered switches. This memo also discusses deployment issues related to the coexistence of DCTCP and conventional TCP, discusses the lack of a negotiating mechanism between sender and receiver, and presents some possible mitigations. This memo documents DCTCP as currently implemented by several major operating systems. DCTCP, as described in this specification, is applicable to deployments in controlled environments like data centers, but it must not be deployed over the public Internet without additional measures.

The goal of this document is to describe the potential benefits of applications using a transport that enables Explicit Congestion Notification (ECN). The document outlines the principal gains in terms of increased throughput, reduced delay, and other benefits when ECN is used over a network path that includes equipment that supports Congestion Experienced (CE) marking. It also discusses challenges for successful deployment of ECN. It does not propose new algorithms to use ECN nor does it describe the details of implementation of ECN in endpoint devices (Internet hosts), routers, or other network devices.

This document describes an optional extension for Two-Way Active Measurement Protocol (TWAMP) allowing the monitoring of the Differentiated Service Code Point and Explicit Congestion Notification fields with the TWAMP-Test protocol.

Explicit Congestion Notification (ECN) is a mechanism where network nodes can mark IP packets, instead of dropping them, to indicate congestion to the endpoints. An ECN-capable receiver will feed this information back to the sender. ECN is specified for TCP in such a way that it can only feed back one congestion signal per Round-Trip Time (RTT). In contrast, ECN for other transport protocols, such as RTP/UDP and SCTP, is specified with more accurate ECN feedback. Recent new TCP mechanisms (like Congestion Exposure (ConEx) or Data Center TCP (DCTCP)) need more accurate ECN feedback in the case where more than one marking is received in one RTT. This document specifies requirements for an update to the TCP protocol to provide more accurate ECN feedback.

This memo specifies how Explicit Congestion Notification (ECN) can be used with the Real-time Transport Protocol (RTP) running over UDP, using the RTP Control Protocol (RTCP) as a feedback mechanism. It defines a new RTCP Extended Report (XR) block for periodic ECN feedback, a new RTCP transport feedback message for timely reporting of congestion events, and a Session Traversal Utilities for NAT (STUN) extension used in the optional initialisation method using Interactive Connectivity Establishment (ICE). Signalling and procedures for negotiation of capabilities and initialisation methods are also defined. [STANDARDS-TRACK]

This document redefines how the explicit congestion notification (ECN) field of the IP header should be constructed on entry to and exit from any IP-in-IP tunnel. On encapsulation, it updates RFC 3168 to bring all IP-in-IP tunnels (v4 or v6) into line with RFC 4301 IPsec ECN processing. On decapsulation, it updates both RFC 3168 and RFC 4301 to add new behaviours for previously unused combinations of inner and outer headers. The new rules ensure the ECN field is correctly propagated across a tunnel whether it is used to signal one or two severity levels of congestion; whereas before, only one severity level was supported. Tunnel endpoints can be updated in any order without affecting pre-existing uses of the ECN field, thus ensuring backward compatibility. Nonetheless, operators wanting to support two severity levels (e.g., for pre-congestion notification -- PCN) can require compliance with this new specification. A thorough analysis of the reasoning for these changes and the implications is included. In the unlikely event that the new rules do not meet a specific need, RFC 4774 gives guidance on designing alternate ECN semantics, and this document extends that to include tunnelling issues. [STANDARDS-TRACK]

The proposal in this document is Experimental. While it may be deployed in the current Internet, it does not represent a consensus that this is the best possible mechanism for the use of Explicit Congestion Notification (ECN) in TCP SYN/ACK packets.

This document describes an optional, experimental modification to RFC 3168 to allow TCP SYN/ACK packets to be ECN-Capable. For TCP, RFC 3168 specifies setting an ECN-Capable codepoint on data packets, but not on SYN and SYN/ACK packets. However, because of the high cost to the TCP transfer of having a SYN/ACK packet dropped, with the resulting retransmission timeout, this document describes the use of ECN for the SYN/ACK packet itself, when sent in response to a SYN packet with the two ECN flags set in the TCP header, indicating a willingness to use ECN. Setting the initial TCP SYN/ACK packet as ECN-Capable can be of great benefit to the TCP connection, avoiding the severe penalty of a retransmission timeout for a connection that has not yet started placing a load on the network. The TCP responder (the sender of the SYN/ACK packet) must reply to a report of an ECN-marked SYN/ACK packet by resending a SYN/ACK packet that is not ECN-Capable. If the resent SYN/ACK packet is acknowledged, then the TCP responder reduces its initial congestion window from two, three, or four segments to one segment, thereby reducing the subsequent load from that connection on the network. If instead the SYN/ACK packet is dropped, or for some other reason the TCP responder does not receive an acknowledgement in the specified time, the TCP responder follows TCP standards for a dropped SYN/ACK packet (setting the retransmission timer). This memo defines an Experimental Protocol for the Internet community.

RFC 3270 defines how to support the Diffserv architecture in MPLS networks, including how to encode Diffserv Code Points (DSCPs) in an MPLS header. DSCPs may be encoded in the EXP field, while other uses of that field are not precluded. RFC 3270 makes no statement about how Explicit Congestion Notification (ECN) marking might be encoded in the MPLS header. This document defines how an operator might define some of the EXP codepoints for explicit congestion notification, without precluding other uses. [STANDARDS-TRACK]

There have been a number of proposals for alternate semantics for the Explicit Congestion Notification (ECN) field in the IP header RFC 3168. This document discusses some of the issues in defining alternate semantics for the ECN field, and specifies requirements for a safe coexistence in an Internet that could include routers that do not understand the defined alternate semantics. This document evolved as a result of discussions with the authors of one recent proposal for such alternate semantics. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.

This document contains the profile for Congestion Control Identifier 3, TCP-Friendly Rate Control (TFRC), in the Datagram Congestion Control Protocol (DCCP). CCID 3 should be used by senders that want a TCP-friendly sending rate, possibly with Explicit Congestion Notification (ECN), while minimizing abrupt rate changes. [STANDARDS-TRACK]

This note describes the Explicit Congestion Notification (ECN)-nonce, an optional addition to ECN that protects against accidental or malicious concealment of marked packets from the TCP sender. It improves the robustness of congestion control by preventing receivers from exploiting ECN to gain an unfair share of network bandwidth. The ECN-nonce uses the two ECN-Capable Transport (ECT)codepoints in the ECN field of the IP header, and requires a flag in the TCP header. It is computationally efficient for both routers and hosts. This memo defines an Experimental Protocol for the Internet community.

This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header. [STANDARDS-TRACK]

This memo presents a performance study of the Explicit Congestion Notification (ECN) mechanism in the TCP/IP protocol using our implementation on the Linux Operating System. This memo provides information for the Internet community.