TCP modifications for Congestion Exposure
draft-ietf-conex-tcp-modifications-01

Abstract

Congestion Exposure (ConEx) is a mechanism by which senders inform the network about the congestion encountered by previous packets on the same flow. This document describes the necessary modifications to use ConEx with the Transmission Control Protocol (TCP).

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on September 11, 2012.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Sender-side Modifications
• 3. Accounting congestion
• 3.1. ECN
• 3.1.1. Accurate ECN feedback
• 3.1.2. Classic ECN support
• 3.2. Loss Detection with/without SACK
• 4. Setting the ConEx IPv6 Bits
• 4.1. Setting the E and the L Bit
• 4.2. Credit Bits
• 5. Timeliness of the ConEx Signals
• 6. Acknowledgements
• 7. IANA Considerations
• 8. Security Considerations
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Revision history
• Authors' Addresses

1. Introduction

Congestion Exposure (ConEx) is a mechanism by which senders inform the network about the congestion encountered by previous packets on the same flow. This document describes the necessary modifications to use ConEx with the Transmission Control Protocol (TCP). The ConEx signal is based on loss or ECN marks [RFC3168] as a congestion indication. This congestion information is retrived by the sender based on existing feedback mechanisms from the receiver to the sender in TCP.

With standard TCP without Selective Acknowledgments (SACK) [RFC2018] the actual number of losses is hard to detect, thus we recommend to enable SACK when using ConEx. However, we discuss both cases, with and without SACK support, later on.

Explicit Congestion Notification (ECN) is defined in such a way that only a single congestion signal is guaranteed to be delivered per Round-trip Time (RTT) from the receiver to the sender. For ConEx a more accurate feedback signal would be beneficial. Such an extension to ECN is defined in a seperate document [draft-kuehlewind-conex-accurate-ecn], as it can also be useful for other mechanisms, as e.g. [DCTCP] or whenever the congestion control reaction should be proportional to the expirienced congestion. ConEx also works with classic ECN but it is less accurate when multiple congestion markings occur within on RTT.

ConEx is currently/will be defined as an destination option for IPv6. The use of four bits have been defined, namely the X (ConEx-capable), the L (loss experienced), the E (ECN experienced) and C (credit) bit.

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 [RFC2119].

2. Sender-side Modifications

A ConEx sender MUST negotitate for both SACK and ECN or the more accurate ECN feedback in the TCP handshake if these TCP extension are available at the sender. Depending on the capability of the receiver, the following operation modes exist:

• Full-ConEx (SACK and accurate ECN feedback)
• accECN-ConEx (no SACK but accurate ECN feedback)
• ECN-ConEx (no SACK and no accurate ECN feedback but 'classic' ECN)
• SACK-ECN-ConEx (SACK and 'classic' instead of accurate ECN)
• SACK-ConEx (SACK but no ECN at all)
• Basic-ConEx (neither SACK nor ECN)

A ConEx sender MUST expose congestion to the network according to the congestion information received by ECN or based on loss information provided by the TCP feedback loop. A TCP sender SHOULD account congestion byte-wise (and not packet-wise). A sender MUST mark subsequent packets (after the congestion notification) with the respective ConEx bit in the IP header.

With SACK only the number of lost bytes is known, but not the number of packets carrying these bytes. With classic ECN only an indication is given that a marking occured which is not giving an exact number of bytes nor packets. As network congestion is usually byte-congestion, the exact number of bytes should be taken into account if available to make the ConEx signal as exact as possible.

The congestion accounting based on different operation modes is described in the next section and the handling of the IPv6 bits itself in the subsequent section afterwards.

3. Accounting congestion

A TCP sender SHOULD account congestion byte-wise (and not packet-wise) based the congestion information received by ECN or loss detection provided by TCP. For this purpose a TCP sender will maintain two different counters for number outstanding bytes that need to be ConEx marked either with the E bit or the L Bit.

The outstanding bytes accounted based on ECN feedback information are maintained in the congestion exposure gauge (CEG). The accounting of these bytes from the ECN feedback is explained in more detail next.

The outstanding bytes for congestion indications based on loss are maintained in the loss exposure gauge (LEG) and the accounting is explained in subsequent to the CEG accounting.

The subtraction of bytes which have been ConEx marked from both counters is explained in the next section.

Usually all byte of an IP packet must be accounted. If we assume equal sized packets or at least equally distributed packet sizes the sender MAY only account the TCP payload bytes, as the ConEx marked packets as well as the original packets causing the congestion will both contain about the same number of headers. Otherwise the sender MUST take the headers into account. A sender which sends different sized packets with unequally distributed packet sizes should know about reason to do so and thus may be able to reconstruct the exact number of headers based on this information. Otherwise if no additional information is available the worse case number of headers SHOULD be estimated in a conservative way based on a minimum packet size (of all packets sent in the last RTT).

3.1. ECN

A receiver can support the accurate ECN feedback scheme, the 'classic' ECN or neither. In the case ECN is not supported at all, the transport is not ECN-capable and no ECN marks will occur, thus the E bit will never be set. In the other cases a ConEx sender MUST maintain a gauge for the number of outstanding bytes that has to be ConEx marked with the E bit, the congestion exposure gauge (CEG).

ECN is an IP/TCP mechanism that allows network nodes to mark packets instead of (early) dropping them if congestion occurs with the Congestion Experienced (CE) mark. As soon as a CE mark is seen at the receiver, with classic ECN it will feed this information back to the sender by setting the Echo Congestion Expierenced (ECE) bit in the TCP header until a a packet with Congestion Window Reduced (CWR) bit in the TCP is receiver to acknowledge the reception of the congestion notification. The sender sets the CWR bit in the TCP header once when the first ECE of a congestion notification is received.

The CEG is increased when ECN information is received from an ECN-capable receiver supporting the 'classic' ECN scheme or the accurate ECN feedback scheme. When the ConEx sender receives an ACK indicating one or more segments were received with a CE mark, CEG is increased by the appropriate number of bytes. The two cases, depending on the receiver capability, are discussed in the following sections.

3.1.1. Accurate ECN feedback

With a more accurate ECN feedback scheme either the number of marked packets/received CE marks or directly the number of marked bytes is known. In the later case the CEG can directly be increased by the number of marked bytes. Otherwise if D is assumed to be the number of marks, the gauge CEG has to be increased by the amount of bytes sent which were marked:

CEG += min( SMSS*D, acked_bytes )

3.1.2. Classic ECN support

A ConEx sender that communicates with a classic ECN receiver (conforming to [RFC3168] or [RFC5562]) MAY run in one of these modes:

• Full compliance mode:
  
  The ConEx sender fully conforms to all the semantics of the ECN signaling as defined by [RFC5562]. In this mode, only a single congestion indication can be signaled by the receiver per RTT. Whenever the ECE flag toggles from "0" to "1", the gauge CEG is increased at maximum by the SMSS:
  
  CEG += min(SMSS, acked_bytes)
  
  Note that under severe congestion, a session adhering to these semantics may not provide enough ConEx marks. This may cause appropriate sanctions by an audit device in a ConEx enabled network.
• Simple compatibility mode:
  
  The sender will set the CWR permanently to force the receiver to signal only one ECE per CE mark. Unfortunately, in a high congestion situation where all packets are CE marled over a certain period of time, the use of delayed ACKs [RFC5681], as it is usually done today, will prevent a feedback of every CE mark. With an ACK rate of m, about m-1/m CE indications will not be signaled back by the receiver (e.g. 50% with M=2 for delayed ACKs). Thus, in this mode the ConEx sender MUST increase CEG as if M congestion notification were received for each received ECE signal:
  
  CEG += min(M*SMSS, acked_bytes + (M-1)*SMSS)
  
  In case of a congestion event with low congestion (that means when only a very smaller number of packets get marked), the sender might miss the whole congestion event. In average the sender will sent sufficient ConEx marks due to the scheme proposed above but these ConEx marks might be timely shifted. Regarding congestion control it is not a general problem to miss a congestion event as by chance a marking scheme in the network node might also miss a certain flow. Even if then no other flow is reacting, the congestion level will increase and it will get more likely that the congestion feedback is delivered. But to provide a fair share over time, a TCP sender could react more strong when receiving a ECN feedback signal. This of course depends on the congestion control used. A TCP sender using this scheme MUST take the impact on congestion control into account.
• Advanced compatibility mode:
  
  More sophisticated heuristics, such as a phase locked loop, to set CWR only on those data segments, that will actually trigger an (delayed) ACK, could extract congestion notifications more timely. A ConEx sender MAY choose to implement such an heuristic. In addition, further heuristics SHOULD be implemented, to determine the value of each ECE notification. E.g. for each consecutive ACK received with the ECE flag set, CEG should be increased by min( M*SSMS, acked_bytes). Else if the predecessor ACK was received with the ECE flag cleared, CEG need only be increase at maximum by one SMSS:
  
  if previous_marked: CEG += min( M*SSMS, acked_bytes)
  else: CEG += min(SMSS, acked_bytes)
  
  This heuristic is conservative during more serious congestion, and more relaxed at low congestion levels.

3.2. Loss Detection with/without SACK

For all the data segments that are determined by a ConEx sender as lost, an identical number of IP bytes MUST be be sent with the ConEx L bit set. Loss detection typically happens by use of duplicate ACKs, or the firing of the retransmission timer. A ConEx sender MUST maintain a loss exposure gauge (LEG), indicating the number of outstanding bytes that must be sent with the ConEx L bit. When a data segment is retransmitted, LEG will be increased by the size of the TCP payload packet containing the retransmission, assuming equal sized segments such that the retransmitted packet will have the same number of header as the original ones. When sending subsequent segments, the ConEx L bit is set as long as LEG is positive, and LEG is decreased by the size of the sent TCP payload with the ConEx L bit set.

Any retransmission may be spurious. To accommodate that, a ConEx sender SHOULD make use of heuristics to detect such spurious retransmissions (e.g. F-RTO [RFC5682], DSACK [RFC3708], and Eifel [RFC3522], [RFC4015]). When such a heuristic has determined, that a certain number of packets were retransmitted erroneously, the ConEx sender should subtract the payload size of these TCP packets from LEG.

Note that the above heuristics delays the ConEx signal by one segment, and also decouples them from the retransmissions themselves, as some control packets (e.g. pure ACKs, window probes, or window updates) may be sent in between data segment retransmissions. A simpler approach would be to set the ConEx signal for each retransmitted data segment. However, it is important to remember, that a ConEx signal and TCP segments do not natively belong together.

If SACK is not available or SACK information has been reseted for any reason, spurious retransmission are more likely. In this case it might be valuable to slightly delay the ConEx loss feedback until a spurious retransmission might be detected. But the ConEx signal MUST NOT be delayed more than one RTT.

4. Setting the ConEx IPv6 Bits

ConEx is currently/will be defined as an destination option for IPv6. The use of four bits have been defined, namely the X (ConEx-capable), the L (loss experienced), the E (ECN experienced) and C (credit) bit.

By setting the X bit a packet is marked as ConEx-capable. All packets carrying payload MUST be marked with the X bit set including retransmissions. No congestion feedback information are available about control packets as pure ACKs which are not carrying any payload. Thus these packet should not be taken into account when determining ConEx information. These packet MUST carry a ConEx Destination Option with the X bit unset.

By setting the X bit a packet is marked as ConEx-capable. All packets carrying payload MUST be marked with the X bit set including retransmissions. About control packets as pure ACKs which are not carrying any payload no congestion feedback information is available thus these packet should not be taken into account when determining ConEx information. These packet MUST carry a ConEx Destination Option with the X bit unset.

4.1. Setting the E and the L Bit

As long as the CEG or LEG counter is positive, ConEx-capable packets MUST be marked with E or L respectively, and the CEG or LEG counter is decreased by the TCP payload bytes carried in this packet. If the CEG or LEG counter is negative, the respective counter SHOULD be reset to zero within one RTT after it was decreased the last time or one RTT after recovery if no further congestion occured.

4.2. Credit Bits

The ConEx abstract mechanism requires that the transport SHOULD signal sufficient credit in advance to cover any reasonably expected congestion during its feedback delay. To be very conservative the number of credits would need to equal the number of packets in flight, as every packet could get lost or congestion marked. With a more moderate view, only an increase in the sending rate should cause congestion.

For TCP sender using the [RFC5681] congestion control algorithm, we recommend to only send credit in Slow Start, as in Congestion Avoidance an increase of one segment per RTT should only cause a minor amount of congestion marks (usually at max one). If a more aggressive congestion control is used, a sufficient amount of credits need to be set.

In TCP Slow Start the sending rate will increase exponentially and that means double every RTT. Thus the number of credits should equal half the number of packets in flight in every RTT. Under the assumption that all marks will not get invalid for the whole Slow Start phase, marks of a previous RTT have to be summed up. Thus the marking of every fourth packet will allow sufficient credits in Slow Start as it can be seen in Figure 1.

RTT1	|------XC------>|
	|------X------->|
	|------X------->|   credit=1  in_flight=3
	|		|
RTT2	|------X------->|
	|------XC------>|
	|------X------->|
	|------X------->|
	|------X------->|
	|------XC------>|   credit=3  in_flight=6
	|		|
RTT3	|------X------->|
	|------X------->|
	|------X------->|
	|------XC------>|
	|------X------->|
	|------X------->|
	|------X------->|
	|------XC------>|
	|------X------->|
	|------X------->|
	|------X------->|
	|------XC------>|   credit=6  in_flight=12
	|	.	|
	|	:	|
			

5. Timeliness of the ConEx Signals

ConEx signals will anyway be evaluated with a slight time delay of about one RTT by a network node. Therefore, it is not absolutely necessary to immediately signal ConEx bits when they become known (e.g. L and E bits), but a sender SHOULD sent the ConEx signaling with the next available packet. In cases where it is preferable to slightly delay the ConEx signal, the sender MUST NOT delay the ConEx signal more than one RTT.

Multiple ConEx bits may become available for signaling at the same time, for example when an ACK is received by the sender, that indicates that at least one segment has been lost, and that one or more ECN marks were received at the same time. This may happen during excessive congestion, where buffer queues overflow and some packets are marked, while others have to be dropped nevertheless. Another possibility when this may happen are lost ACKs, so that a subsequent ACK carries summary information not previously available to the sender.

6. Acknowledgements

The authors would like to thank Bob Briscoe who contributed with this initial ideas and valuable feedback.

7. IANA Considerations

This document does not have any requests to IANA.

8. Security Considerations

With some of the advanced ECN compability modes it is possible to miss congestion notifications. Thus a sender will not decrese its sending rate. If the congestion is persistent, the likelihood to receive a congestion notification increases. In the worst case the sender will still react correctly to loss. This will prevent a congestion collapse.

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Revision history

RFC Editior: This section is to be removed before RFC publication.

00 ... initial draft, early submission to meet deadline.

01 ... refined draft, updated LEG "drain" from per-packet to RTT-based.

Authors' Addresses