Internet DRAFT - draft-shi-ccwg-advanced-ecn
draft-shi-ccwg-advanced-ecn
Congestion Control Working Group H. Shi, Ed.
Internet-Draft T. Zhou
Intended status: Standards Track Huawei
Expires: 11 January 2024 10 July 2023
Advanced Explicit Congestion Notification
draft-shi-ccwg-advanced-ecn-00
Abstract
This document proposes Advanced Explicit Congestion Notification
mechanism enabling host to obtain the congestion information at the
bottleneck. The sender sets the congestion information collection
command in the packet header indicating the network device to update
the congestion information field per hop. The receiver carries the
updated congestion information back to the sender in the ACK. The
sender then leverage the rich congestion information to do congestion
control.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Congestion Control
Working Group Working Group mailing list (ccwg@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/ccwg/.
Source for this draft and an issue tracker can be found at
https://github.com/VMatrix1900/draft-ccwg-advanced-ecn.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 January 2024.
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Copyright Notice
Copyright (c) 2023 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. AECN header format and encapsulation . . . . . . . . . . . . 4
4. Example: HPCC with AECN . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Traditionally, congestion control has depended on implicit congestion
detection by the host, where hosts gauge congestion primarily through
packet loss or variations in round-trip times. Explicit Congestion
Notification (ECN) represents a substantial improvement, as it
facilitates network devices to explicitly signal congestion to the
endpoints before packet loss occurs. Low Latency, Low Loss, Scalable
throughput (L4S) leverages ECN to meticulously control the queuing
delay. It uses ECN markings to maintain low queuing delays and avoid
bufferbloat. However, ECN is limited by the use of a single bit of
information. This limitation constrains the granularity of
congestion information that can be conveyed. L4S's requirement for
more detailed congestion signals demands an enhanced utilization of
ECN, which could involve employing additional bits for a more precise
representation of congestion levels and better control over delay and
throughput in contemporary network environments.
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HPCC[I-D.draft-an-ccwg-hpcc] leverages more extensive congestion
signals from the network by utilizing in-band telemetry, which
facilitates the gathering of detailed load information from each
switch it traverses. This enhanced approach enables HPCC to make
more informed decisions on controlling network congestion and
converge fast. However, one caveat associated with this approach is
that HPCC utilizes an append mode for in-band telemetry. In append
mode, as the packet traverses the network, it accumulates data from
each switch, which consequently increases the size of the packet.
This growth in packet size can potentially lead to issues such as
exceeding the Maximum Transmission Unit (MTU) size which makes it
unsuitable for the internet. Another caveat is that each sender need
to repeat the computation to get the bottleneck information even if
they shares the same path.
This document defines Advanced ECN which expands the 1 bit congestion
notification to multiple bits and enables network device to update
the congestion information per hop. When the packet arrives at the
receiver, the congestion information field will reflect the
congestion status of the path. By offloading the congestion
information calculation to the network device, the computing burden
of the endpoint can be reduced.
1.1. Terminology
* ECN: Explicit Congestion Notification
* AECN: Advanced Explicit Congestion Notification
* HPCC: High Precision Congestion Control[I-D.draft-an-ccwg-hpcc]
* DRE: Discounting Rate Estimator[CONGA]
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. Overview
Figure 1 shows the overview procedure of AECN. First the sender MUST
marks the packet with AECN command and initial Congestion Info(called
AECN header, see Section 3). The AECN Command specified what kind of
the congestion information that the endpoint intend to collect from
network devices. As the packet traverses through the network, each
router MUST update the Congestion Info field based on the AECN
command and the router's local load condition. Upon reaching the
receiver, the updated congestion information within the packet is
extracted and then communicated back to the sender, typically using
the transport protocol's acknowledgment mechanism. The sender, now
equipped with the congestion information reflective of the packet's
journey, uses this data to make informed adjustments to its sending
rate.
pkt+ pkt+ pkt+
AECN Command+ AECN Command+ AECN Command+
+------+Congestion Info0+-------+Congestion Info1+-------+Congestion Info2+--------+
|Sender|===============>|Router1|===============>|Router2|===============>|Receiver|
+------+ Link-1 +-------+ Link-2 +-------+ Link-3 +--------+
/|\ |
| |
+--------------------------------------------------------------------------+
ACKs
Figure 1: Overview of Advanced ECN
3. AECN header format and encapsulation
Figure 2 shown the format of AECN. The AECN header SHOULD be
encapsulated in IPv6 extension header[RFC8200] such as SRH, Hop by
Hop Options Header etc.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Congestion Info Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Congestion Info Data |
~ .... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: AECN header format
where:
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Flags: An 8-bit field. The Bit 7 of Flags indicates the Congestion
Info is customized and used only in limited domain such as Data
center network. If the Bit 7 is 0, the Congestion Info Type is a
bitmap. Other bits are reserved.
Congestion Info Type: A 24-bit map that specifies the present
Congestion Info Data. Supported Congestion Info Data is listed in
Table 1. Note that it is possible for multiple Congestion Info Data
to coexist in one packet.
Congestion Info Data: A variable length field including the
congestion information data. Router MUST update this field based on
local load status.
+=====+=========================+========+===========+
| Bit | Congestion Info Data | Length | Operation |
+=====+=========================+========+===========+
| 0 | Inflight Ratio | 8 | Max |
+-----+-------------------------+--------+-----------+
| 1 | DRE | 8 | Max |
+-----+-------------------------+--------+-----------+
| 2 | Queue Utilization Ratio | 8 | Max |
+-----+-------------------------+--------+-----------+
| 3 | Queue Delay | 8 | Add |
+-----+-------------------------+--------+-----------+
| 4 | Congested Hops | 8 | Add |
+-----+-------------------------+--------+-----------+
Table 1: Congestion Info Data
4. Example: HPCC with AECN
HPCC calculates the inflight ratio of each link(represent the link
utilization of the link) from the collected raw load information
carried in the INT. Then maximum inflight ratio along the path is
identified and used to adjust the sending rate. The formula to
calculate the inflight ratio of each link is shown below:
txRate = (txBytes_1 - txBytes_2)/(t_1-t_2)
inflight ratio = qlen/(B*T) + txRate/B
where: txBytes: link total transmitted bytes associated with
timestamp ts
qlen: link queue length
B: link bandwidth
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T: Baseline RTT
Leveraging AECN, the router participates in calculation of the
maximum inflight ratio. Each router MUST calculate the inflight
ratio of the down link and then compare it to the one in the AECN
header and keep the larger one. When the packet arrives at the
endpoint, the Congestion Info field of the AECN header already
contains the maximum inflight ratio. The sending rate adjustment
algorithm remains unchanged. By allowing routers to conduct these
calculations, the computing overhead is reduced for the endpoint.
Since the update of value is in-place, the packet size remains
unchanged regardless of the hops count.
5. Security Considerations
TBD.
6. IANA Considerations
TBD.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/rfc/rfc8200>.
7.2. Informative References
[CONGA] Alizadeh, M., Edsall, T., Dharmapurikar, S., Vaidyanathan,
R., Chu, K., Fingerhut, A., Lam, V., Matus, F., Pan, R.,
Yadav, N., and G. Varghese, "CONGA: distributed
congestion-aware load balancing for datacenters",
Proceedings of the 2014 ACM conference on SIGCOMM,
DOI 10.1145/2619239, August 2014,
<https://doi.org/10.1145/2619239>.
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[I-D.draft-an-ccwg-hpcc]
An, Q., Gao, J., Anubolu, S., Pan, R., Lee, J., Gafni, B.,
Shpigelman, Y., Tantsura, J., and G. Caspary, "HPCC++:
Enhanced High Precision Congestion Control", Work in
Progress, Internet-Draft, draft-an-ccwg-hpcc-00, 30 June
2023, <https://datatracker.ietf.org/doc/html/draft-an-
ccwg-hpcc-00>.
Authors' Addresses
Hang Shi (editor)
Huawei
Beijing
China
Email: shihang9@huawei.com
Tianran Zhou
Huawei
Email: zhoutianran@huawei.com
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