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:

Resilience

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.

Timeliness

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).

Integrity

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.

Accuracy

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.

Complexity

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