Internet DRAFT - draft-romo-iccrg-ccid5
draft-romo-iccrg-ccid5
ICCRG Working Group N. Romo
Internet-Draft J. Kim
Intended status: Experimental M. Amend
Expires: 28 April 2022 DT
25 October 2021
Profile for Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 5
draft-romo-iccrg-ccid5-00
Abstract
This document contains the profile for Congestion Control Identifier
5 (CCID 5), BBR-like Congestion Control, in the Datagram Congestion
Control Protocol (DCCP). CCID 5 is meant to be used by senders who
have a strong demand on low latency and require a steady throughput
behavior.
Status of This Memo
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Convention and Notation . . . . . . . . . . . . . . . . . . . 3
3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Relationship with TCP BBR and CCID2 . . . . . . . . . . . 3
3.2. Multiple-path communications . . . . . . . . . . . . . . 4
3.3. Half-Connection Example . . . . . . . . . . . . . . . . . 4
4. Connection Establishment . . . . . . . . . . . . . . . . . . 5
5. Congestion Control on Data Packets . . . . . . . . . . . . . 5
5.1. State machine . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Response to Idle and Application-Limited Periods . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. ProbeRTT phase transitions . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 8
10. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This document contains the profile for Congestion Control Identifier
5, BBR-like Congestion Control, in the Datagram Congestion Control
Protocol (DCCP) [RFC4340]. DCCP uses Congestion Control Identifiers,
or CCIDs, to specify the congestion control mechanism in use on a
half-connection.
The BBR-like Congestion Control CCID5 sends data following the
guidelines and principles of TCP BBR
[I-D.cardwell-iccrg-bbr-congestion-control]. i.e, it estimates the
path characteristics, to later update accordingly the sending data
behavior. It achieves an optimal point of operation by keeping the
amount of data in flight at the BDP (Bandwidth Delay Product) level,
avoiding the abrupt Bandwidth changes typical of loss based
congestion control algorithms.
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2. Convention and Notation
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].
A DCCP half-connection consists of the application data sent by one
endpoint and the corresponding acknowledgements sent by the other
endpoint. The terms "HC-Sender" and "HC-Receiver" denote the
endpoints sending application data and acknowledgements,
respectively. Since CCIDs apply at the level of half-connections, we
abbreviate HC-Sender to "sender" and HC-Receiver to "receiver" in
this document. See [RFC4340] for more discussion
3. Usage
CCID5 congestion control algorithm is aimed to achieve a high
bandwidth and low latency by the active probe of the end-to-end link
capacity. The active probe helps hosts to adjust their sending rates
before a packet loss happens at a buffer on the path. As a result,
the communication path experiences a consistent and low latency by
avoiding unnecessary packet drops at buffers.
Since CCID5 effectively avoids unnecessary packet losses, the spiky
traffic behavior, that is commonly caused by traditional TCP
congestion control mechanisms, is suppressed. This leads to a stable
throughput throughout the connection period and thus yields a higher
throughput than that with a loss-based congestion control mechanism.
Therefore, CCID5 suits applications that require consistent low
latencies and stable high bandwidth. This includes multimedia
streaming, online video gaming, video conferencing, and latency-
sensitive industry applications such as industrial robots and
autonomous vehicles are usage examples of CCID5.
3.1. Relationship with TCP BBR and CCID2
The CCID5 congestion control mechanism closely follows TCP's
[I-D.cardwell-iccrg-bbr-congestion-control]|BBR congestion control
algorithm, replicating the functions intended to estimate the path
characteristics and to determine the pace and the amount of data to
send. However, CCID5 must also comply with the DCCP requirements for
a CCID profile ([RFC4340] Section 10.4) and define how the data is
going to be acknowledged.
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For this purpose, CCID5 implements the format of the ACK packets, the
timing of their generation, and how they are congestion controlled.
CCID5 uses the same ACK format as CCID2, including ACK vectors
containing the same information that can be found in SACK options,
and implements the ACK ratio as ACK congestion control mechanism.
In addition, the different variables and functions used to track
packets in flight, packets acknowledged, and their corresponding
sending and arrival times as well as the function to detect
application-limited periods are replicated from the CCID2
implementation
3.2. Multiple-path communications
CCID 5 congestion control algorithm is adopted from TCP's BBR
congestion control algorithm with a multiple-path communication as a
representative use-case example. Multiple-path communications do not
only target to maximize the link capacity, but also are aimed to
improve the availability on critical situations such as a link
failure. With that regard, MP-DCCP has been proposed. MP-DCCP
extends capabilities of DCCP into multiple concurrent connections. A
study [paper] has shown that CCID5 improves the overall bandwidth and
the end-to-end latency compared to loss-based congestion control
algorithms in an MP-DCCP enabled network. The study has also shown
that the latency difference among multiple paths has an influence on
the overall performance of the communication. A smaller gap among
available paths leads to a higher aggregation performance of the link
capacity. CCID5 is designed to provide a low and stable latency over
each of the available paths and thus has a potential to improve the
multi-path communication performance.
3.3. Half-Connection Example
This example shows the typical progress of a half-connection using
CCID 5's BBR-like Congestion Control, not including connection
initiation and termination. The example is informative, not
normative.
1. The sender transmits DCCP-Data packets, each one of them
identified with a sequence number. The sending behavior is
governed by two control parameters: congestion window and pacing
rate. The congestion window limits the amount of packets in
flight and the pacing rate limits the sending rate.
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2. The Acknowledgment mechanism replicates CCID2 specifications.
Thus, The sender sends an Ack Ratio feature option specifying the
number of data packets to be covered by an Ack packet from the
receiver and consequently, the receiver sends a DCCP-Ack packet
acknowledging the data packets for every Ack Ratio data packets
transmitted by the sender.
3. The sender continues sending DCCP-Data packets. Upon receiving
DCCP-Ack packets, the sender examines their Ack Vectors to learn
about acknowledged and marked or dropped data packets. With the
information of the acknowledged packets, it proceeds to estimate
round-trip times (as TCP does) and the delivery rate, following
the algorithm described in
[I-D.cheng-iccrg-delivery-rate-estimation].
4. The sender uses the round-trip time and delivery rate estimations
to calculate the round-trip propagation delay (RTprop) and the
bottleneck bandwidth (BtlBw) of the path, following the
specifications in ([I-D.cardwell-iccrg-bbr-congestion-control]
Section 4.1) The RTprop and BtlBw are then used to update the
values of the congestion window and pacing rate.
5. As in CCID2, the sender responds to lost or marked DCCP-Ack
packets by modifying the Ack Ratio sent to the receiver and
acknowledges the receiver's acknowledgements at least once per
congestion window.
4. Connection Establishment
The connection establishment is as specified in ([RFC4341] Section 4)
5. Congestion Control on Data Packets
CCID 5 is based on the BBR congestion control mechanisms described in
[I-D.cardwell-iccrg-bbr-congestion-control]. The subsequent
sections, present a general description of such mechanisms and
discuss the considerations to be addressed when used within the DCCP
protocol.
BBR proposes an algorithm based on the characterization of the
network path made through the estimation of the Bottleneck Bandwidth
(BtlBW) and the Round Trip propagation time (RTProp) defined
respectively as the maximum delivered rate and minimum RTT seen by
the sender. The algorithm aims to achieve an optimal point of
operation by fulfilling two conditions
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1. The amount of data inflight must be equal to the Bandwidth Delay
Product (BDP), guaranteeing that buffers are not being filled and
therefore avoiding long delay generation
2. The bottleneck packet arrival must match the BtlBw to ensure its
full utilization.
To match those conditions, the sending data behavior is updated, by
using three control variables: Congestion window (which limits the
amount of data in flight), pacing rate, and send quantum (which
limits the amount of aggregated packets in case of segmentation
offload). The calculation of the control parameters uses as input
the estimated values of BtlBW and RTprop along with two dynamic gain
factors named pacing_gain and cwnd_gain.
The estimation of the path parameters Rtprop and BtlBw follow the
guidelines and pseudo-code described in
[I-D.cheng-iccrg-delivery-rate-estimation] and
[I-D.cardwell-iccrg-bbr-congestion-control]
5.1. State machine
The way the control parameters are updated is governed by the BBR
state machine Illustrated in Figure 1. In the initial Startup state,
the sending rate will increase rapidly until the pipe is detected to
be full. Afterwards, the data rate will be reduced so any possible
queue can be drained, to finally enter into the ProbeBW state, where
the amount of data in flight is slightly increased to probe for more
possible bandwidth available. From any of these states, the
algorithm can jump into the ProbeRTT phase. Here the data inflight
is reduced to probe for lower RTTs. Each state defines specific
values for two dynamic gains: cwnd_gain and pacing_gain, which will
finally be used in the calculation of the aforementioned control
variables.
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|
V
+---> Startup ----+
| | |
| V |
| Drain ----+
| | |
| V |
+---> ProbeBW -----+
| ^ | |
| | | |
| +----+ |
| |
+---- ProbeRTT <---+
Figure 1: BBR State machine
5.2. Response to Idle and Application-Limited Periods
6. Acknowledgements
The Acknowledgement format and its generation mechanism SHOULD follow
the same specifications established for CCID2[RFC4341]. Thus, each
Acknowledgment MUST contain an ACK vector defined with the format
described in ([RFC4340] section 1.3) And its generation frequency
will be controlled by the sender by using the ACK ratio feature.
7. Discussion
7.1. ProbeRTT phase transitions
The transition to and from the probeRTT phase MIGHT imply drastic
changes of the congestion window, thus the synchronization of the ACK
ratio between and receiver SHOULD be handled carefully. When
entering this phase at least one Packet MUST be sent with the new
value of the ACK ratio before the reduction of the congestion window
to 4 packets is executed, otherwise, the receiver MIGHT not be able
to send ACK packets, preventing the sender from updating the
measurement of the RTProp and BtlBW variables and remaining in this
phase longer than required. Following a similar logic, before
leaving the phase and restoring the congestion window value, at least
one packet MUST be sent updating the ack ratio value, otherwise, the
receiver MIGHT not be able to keep the pace to acknowledge the
arriving packets, and the missing ACKs MIGHT trigger a RTO timeout.
In addition to the synchronization of the ACK ratio, the sender and
receiver MUST keep synchronized the Sequence and Acknowledgment
validity windows, as defined in ([RFC4340] section 7.5) This adds an
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additional constraint to the BBR algorithm when leaving the ProbeRTT
phase, as at least one RTT is necessary for the sender to ensure the
synchronization before restoring the congestion window value, causing
again a longer duration of the probeRTT phase. Thus, it might be
necessary to consider the possibility of restoring the congestion
window even if this synchronization has not yet been confirmed by the
arrival of the last Acknowledgement sent by the receiver.
8. IANA Considerations
9. Acknowledgment
10. Informative References
[I-D.cardwell-iccrg-bbr-congestion-control]
Cardwell, N., Cheng, Y., Yeganeh, S. H., and V. Jacobson,
"BBR Congestion Control", Work in Progress, Internet-
Draft, draft-cardwell-iccrg-bbr-congestion-control-00, 3
July 2017, <https://www.ietf.org/archive/id/draft-
cardwell-iccrg-bbr-congestion-control-00.txt>.
[I-D.cheng-iccrg-delivery-rate-estimation]
Cheng, Y., Cardwell, N., Yeganeh, S. H., and V. Jacobson,
"Delivery Rate Estimation", Work in Progress, Internet-
Draft, draft-cheng-iccrg-delivery-rate-estimation-00, 3
July 2017, <https://www.ietf.org/archive/id/draft-cheng-
iccrg-delivery-rate-estimation-00.txt>.
[paper] Romo Moreno, N., Amend, M., Rakocevic, V., Kassler, A.,
and A. Brunstrom, "CCID5 An implementation of the BBR
Congestion Control algorithm for DCCP and its impact over
multi-path scenarios", DOI 10.1145/3472305.3472322, June
2021, <https://doi.org/10.1145/3472305.3472322>.
[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/info/rfc2119>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, DOI 10.17487/RFC4341, March
2006, <https://www.rfc-editor.org/info/rfc4341>.
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Authors' Addresses
Nathalie Romo Moreno
Deutsche Telekom
Deutsche-Telekom-Allee 9
64295 Darmstadt
Germany
Email: nathalie.romo-moreno@telekom.de
Juhoon Kim
Deutsche Telekom
Winterfeldstr. 21
10781 Berlin
Germany
Email: j.kim@telekom.de
Markus Amend
Deutsche Telekom
Deutsche-Telekom-Allee 9
64295 Darmstadt
Germany
Email: Markus.Amend@telekom.de
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