TCP Maintenance and Minor Extensions (tcpm) M. Kühlewind, Ed.
Internet-Draft University of Stuttgart
Intended status: Informational R. Scheffenegger
Expires: January 12, 2014 NetApp, Inc.
July 11, 2013

Problem Statement and Requirements for a More Accurate ECN Feedback


Explicit Congestion Notification (ECN) is an IP/TCP mechanism where network nodes can mark IP packets instead of dropping them to indicate congestion to the end-points. An ECN-capable receiver will feedback this information to the sender. ECN is specified for TCP in such a way that only one feedback signal can be transmitted per Round-Trip Time (RTT). Recently, new TCP mechanisms like ConEx or DCTCP need more accurate ECN feedback information in the case where more than one marking is received in one RTT. This documents specifies requirement for different ECN feedback scheme in the TCP header to provide more than one feedback signal per RTT.

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

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 January 12, 2014.

Copyright Notice

Copyright (c) 2013 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 ( 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

Explicit Congestion Notification (ECN) [RFC3168] is an IP/TCP mechanism where network nodes can mark IP packets instead of dropping them to indicate congestion to the end-points. An ECN-capable receiver will feedback this information to the sender. ECN is specified for TCP in such a way that only one feedback signal can be transmitted per Round-Trip Time (RTT). This is sufficient for current congestion control mechanisms, as only one reduction in sending rate is performed per RTT independent of the number of ECN congestion marks. But recently proposed mechanisms like Congestion Exposure (ConEx) or DCTCP [Ali10] need more accurate ECN feedback information in the case where more than one marking is received in one RTT to work correctly.

The following scenarios should briefly show where the accurate feedback is needed or provides additional value:

A Standard (RFC5681) TCP sender that supports ConEx:

In this case the congestion control algorithm still ignores multiple marks per RTT, while the ConEx mechanism uses the extra information per RTT to re-echo more precise congestion information.
A sender using DCTCP congestion control without ConEx:

The congestion control algorithm uses the extra info per RTT to perform its decrease depending on the number of congestion marks.
A sender using DCTCP congestion control and supports ConEx:

Both the congestion control algorithm and ConEx use the accurate ECN feedback mechanism.
A standard TCP sender (using RFC5681 congestion control algorithm) without ConEx:

No accurate feedback is necessary here. The congestion control algorithm still react only on one signal per RTT. But it is best to have one generic feedback mechanism, whether it is used or not.

This document summarizes the requirements for a new more accurate ECN feedback scheme. While a new feedback scheme should still deliver identical performance as classic ECN, this document also clarifies what has to be taken into consideration in addition. Thus the listed requirements should be addressed in the specification of a more accurate ECN feedback scheme. Moreover, as a large set of proposals already exists, a few high level design choices are sketched and briefly discussed, to demonstrate some of the benefits and drawbacks of each of these potential schemes.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

We use the following terminology from [RFC3168] and [RFC3540]:

The ECN field in the IP header:

The ECN flags in the TCP header:

In this document, the ECN feedback scheme as specified in [RFC3168] is called the 'classic ECN' and any new proposal the 'more accurate ECN feedback' scheme. A 'congestion mark' is defined as an IP packet where the CE codepoint is set. A 'congestion event' refers to one or more congestion marks belong to the same overload situation in the network (usually during one RTT). A TCP segment with the acknowledgment flag set is simply called ACK.

2. Overview ECN and ECN Nonce in IP/TCP

ECN requires two bits in the IP header. The ECN capability of a packet is indicated when either one of the two bits is set. An ECN sender can set one or the other bit to indicate an ECN-capable transport (ECT) which results in two signals, ECT(0) and ECT(1). A network node can set both bits simultaneously when it experiences congestion. When both bits are set the packet is regarded as "Congestion Experienced" (CE).

In the TCP header the first two bits in byte 14 are defined for the use of ECN. The TCP mechanism for signaling the reception of a congestion mark uses the ECN-Echo (ECE) flag in the TCP header. To enable the TCP receiver to determine when to stop setting the ECN-Echo flag, the CWR flag is set by the sender upon reception of the feedback signal. This leads always to a full RTT of ACKs with ECE set. Thus any additional CE markings arriving within this RTT can not signaled back anymore.

ECN-Nonce [RFC3540] is an optional addition to ECN that is used to protect the TCP sender against accidental or malicious concealment of marked or dropped packets. This addition defines the last bit of byte 13 in the TCP header as the Nonce Sum (NS) bit. With ECN-Nonce a nonce sum is maintain that counts the occurrence of ECT(1) packets.

  0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
|               |           | N | C | E | U | A | P | R | S | F |
| Header Length | Reserved  | S | W | C | R | C | S | S | Y | I |
|               |           |   | R | E | G | K | H | T | N | N |

Figure 1: The (post-ECN Nonce) definition of the TCP header flags

3. Requirements

The requirements of the accurate ECN feedback protocol, for the use of e.g. Conex or DCTCP, are to have a fairly accurate (not necessarily perfect), timely and protected signaling. This leads to the following requirements, which should be discussed for any proposed more accurate ECN feedback scheme:


The ECN feedback signal is carried within the ACK. TCP ACKs can get lost. Moreover, delayed ACKs are mostly used with TCP. That means in most cases only every second data packet triggers an ACK. In a high congestion situation where most of the packets are marked with CE, an accurate feedback mechanism must still be able to signal sufficient congestion information. Thus the accurate ECN feedback extension has to take delayed ACK and ACK loss into account.

The CE mark is induced by a network node on the transmission path and echoed by the receiver in the TCP ACK. Thus when this information arrives at the sender, its naturally already about one RTT old. With a sufficient ACK rate a further delay of a small number of ACK can be tolerated but with large delays this information will be out dated due to high dynamic in the network. TCP congestion control which introduces parts of these dynamics operates on a time scale of one RTT. Thus the congestion feedback information should be delivered timely (within one RTT).

With ECN Nonce, a misbehaving receiver or network node can be detected with good probability. If the accurate ECN feedback is reusing the NS bit, it is encouraged to ensure integrity at least as good as ECN Nonce. If this is not possible, alternative approaches should be provided how a mechanism using the accurate ECN feedback extension can re-ensure integrity or give strong incentives for the receiver and network node to cooperate honestly.

Classic ECN feeds back one congestion notification per RTT, as this is supposed to be used for TCP congestion control which reduces the sending rate at most once per RTT. The accurate ECN feedback scheme has to ensure that if a congestion events occurs at least one congestion notification is echoed and received per RTT as classic ECN would do. Of course, the goal of this extension is to reconstruct the number of CE markings (more) accurately and in the best case even to reconstruct the (exact) number of payload bytes that a CE marked packet was carrying. However, a sender should not assume to get the exact number of congestion markings or marked bytes in all situations.

Of course, the more accurate ECN feedback can also be used, even if only one ECN feedback signal per RTT is need. The implementation should be as simple as possible and only a minimum of additional state information should be needed.

4. Design Approaches

All discussed approaches aim to provide accurate ECN feedback information as long as no ACK loss occurs and the congestion rate is reasonable. Otherwise the proposed schemes have different resilience characteristics depending on the number of used bits for the encoding. While classic ECN provides a reliable (inaccurate) feedback of a maximum of one congestion signal per RTT, the proposed schemes do not implement any acknowledgement mechanism.

4.1. Re-use of ECN/NS Header Bits

The three ECN/NS header, ECE, CWR and NS are re-used (not only for additional capability negotiation during the TCP handshake exchange but) to signal the current value of an CE counter at the receiver. This approach only provides a limited resilience against ACK lost depending of the number of used bits.

There are several codings proposed so far: An one bit scheme sends one ECE for each CE received (while the CWR could be used to introduce redundant information in next ACK to increase the robustness against ACK loss). An 3 bit counter scheme uses all three bits for continuously feeding the three most significant bits of a CE counter back. An 3 bit codepoint scheme encodes either a CE counter or an ECT(1) counter in 8 codepoints.

The proposed schemes provides accumulated information on ECN-CE-marking feedback, similar to the number of acknowledged bytes in the TCP header. Due to the limited number of bits the ECN feedback information will wrap-around more often (than the acknowledgement). Thus with a smaller number of ACK losses it is already possible to loose feedback information. The resilience could be increased by introducing redundancy, e.g. send each counter increase twice or more times. Of course any of these additional mechanisms will increasee the complexity. If the congestion rate is larger that the ACK rate (multiplied with the number of feedback information that can be signaled per ACK), the congestion information cannot correctly be feed back. Thus an accurate ECN feedback mechanism needs to be able to also cover the worst case situation where every packet is CE marked. This can potentially be realized by dynamically adapt the ACK rate and redundancy which again increases complexity and also potentially the signaling overhead. For all schemes, an integrity check is only provided if ECN Nonce can be supported.

4.2. Use of Other Header Bits

As seen in Figure 1, there are currently three unused flag bits in the TCP header. The proposed 3 bit or codepoint schemes could be extended by one or more bits, to add higher resilience against ACK loss. The relative gain would be proportionally higher resilience against ACK loss, while the respective drawbacks would remain identical.

Moreover, the Urgent Pointer could be used if the Urgent Flag is not set. As this is often the case, the resiliency could by increased without additional signaling overhead.

4.3. TCP Option

Alternatively, a new TCP option could be introduced, to help maintaining the accuracy and integrity of the ECN feedback between receiver and sender. Such an option could provide higher resilience and even more information. E.g. ECN for RTP/UDP provides explicit the number of ECT(0), ECT(1), CE, non-ECT marked and lost packets. However, deploying new TCP options has its own challenges. Moreover, to actually achieve a high resilience, this option would need to be carried by either all or a large number ACKs. Thus this approach would introduce considerable signaling overhead while ECN feedback is not such a critical information (as in the worst case, loss will still be available to provide a strong congestion feedback signal). Anyway, such a TCP option could also be used in addition to a more accurate ECN feedback scheme in the TCP header or in addition to classic ECN, only when available and needed.

5. Acknowledgements

6. IANA Considerations

This memo includes no request to IANA.

7. Security Considerations

If this scheme is used as input for congestion control, the respective algorithm might not react appropriately if ECN feedback information got lost. As those schemes should still react appropriately to loss, this drawback can not lead to a congestion collapse though.

Providing wrong feedback information could otherwise lead to throttling of certain connections. This problem is identical in the classic ECN feedback scheme and should be addressed by an additional integrity check like ECN Nonce.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001.
[RFC3540] Spring, N., Wetherall, D. and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, June 2003.

8.2. Informative References

[I-D.briscoe-tsvwg-re-ecn-tcp] Briscoe, B., Jacquet, A., Moncaster, T. and A. Smith, "Re-ECN: Adding Accountability for Causing Congestion to TCP/IP", Internet-Draft draft-briscoe-tsvwg-re-ecn-tcp-09, October 2010.
[I-D.kuehlewind-tcpm-accurate-ecn-option] Kuehlewind, M. and R. Scheffenegger, "Accurate ECN Feedback Option in TCP", Internet-Draft draft-kuehlewind-tcpm-accurate-ecn-option-01, July 2012.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S. and K. Ramakrishnan, "Adding Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, June 2009.
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion Control", RFC 5681, September 2009.
[RFC5690] Floyd, S., Arcia, A., Ros, D. and J. Iyengar, "Adding Acknowledgement Congestion Control to TCP", RFC 5690, February 2010.
[Ali10] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel, P., Prabhakar, B., Sengupta, S. and M. Sridharan, "DCTCP: Efficient Packet Transport for the Commoditized Data Center", Jan 2010.

Authors' Addresses

Mirja Kühlewind (editor) University of Stuttgart Pfaffenwaldring 47 Stuttgart, 70569 Germany EMail:
Richard Scheffenegger NetApp, Inc. Am Euro Platz 2 Vienna, 1120 Austria Phone: +43 1 3676811 3146 EMail: