Internet DRAFT - draft-dang-queuing-with-multiple-cyclic-buffers
draft-dang-queuing-with-multiple-cyclic-buffers
Network Working Group B. Liu
Internet-Draft J. Dang, Ed.
Intended status: Standards Track Huawei
Expires: August 26, 2021 February 22, 2021
A Queuing Mechanism with Multiple Cyclic Buffers
draft-dang-queuing-with-multiple-cyclic-buffers-00
Abstract
This document presents a queuing mechanism with multiple cyclic
buffers.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Terminology & Abbreviations . . . . . . . . . . . . . . . . . 2
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2
5. Queuing with Multiple Cyclic Buffers . . . . . . . . . . . . 3
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
9. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Network forwarding with bounded latency and zero congestion loss is
important for various industrial applications. The DetNet working
group draft "DetNet Bounded
Latency"[draft-ietf-detnet-bounded-latency] describes requirements
for queuing mechanisms. Among the referenced queuing mechanisms,
Cyclic Queuing and Forwarding (CQF) requires no per-flow dynamic
state at core nodes, which is scalable when the number of flows
grows. To cope with long link delay, more than two cyclic buffers
can be used.
This document discusses the details of the cyclic queuing mechanisms.
We propose a queuing model and mechanism with multiple cyclic
buffers, which can improve bandwidth utilization without sacrificing
latency and jitter.
2. 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.
3. Terminology & Abbreviations
CQF: Cyclic Queuing and Forwarding
T_c: the length of a cycle
4. Problem Statement
IEEE 802.1Q[IEEE8021TSN] defines CQF. As described in "DetNet
Bounded Latency"[draft-ietf-detnet-bounded-latency], CQF with two
synchronized buffers works as follows. All nodes keep a same cycle
starting time. In a cycle x, Node A sends all packets in a buffer to
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Node B. In the same cycle x, Node B uses a buffer to accumulate all
packets from A, and at the same time sends out the packets that have
already been buffered in cycle x-1. In the next cycle x+1, Node B
sends out all the packets that are received from Node A in cycle x.
If a packet traverses h hops, the maximum latency is (h+1)T_c, and
the minimum latency is (h-1)T_c. The jitter (latency variation)
bound is 2T_c.
| cycle x | cycle x+1 |
Node A +-----------+-----------+
\
\packet
\receiving
\
| V | cycle x+1 |
Node B +-----------+-----------+
cycle x \packets
\sending
\
\
V
Figure-1: CQF with two synchronized buffers
The major disadvantages of CQF with two synchronized buffers are as
follows. First, packets are sent and received in a same cycle by
upstream and downstream nodes, respectively. So the link propagation
delay must be smaller than the T_c. This prohibits the method from
being used with long links, such as in WAN and MAN scenarios.
Otherwise T_c must be larger than the link delay, resulting in high
latency, jitter and buffer upper bound. Second, when the method is
applicable, the sum of link delay, output delay, preemption delay and
processing delay takes a portion of T_c, called dead time in
[draft-ietf-detnet-bounded-latency], which cannot be used to send
packets with deterministic services. This results in the conflict
between good bandwidth utilization rate and good latency and jitter
bound.
5. Queuing with Multiple Cyclic Buffers
This document proposes a cyclic queuing model that decouples link
propagation delay with cycle length T_c. The model is shown in
Figure-2.
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| cycle x | cycle x+1 |
Node A +-----------+-----------+
\ `
packets \ ` packets
in \ ` in
cycle x \ ` cycle x+1
\ `
\ `
\ `
\ `
\ `
\ `
\ `
\ `
\ `
\ `
V v
| cycle y-1 | cycle y | cycle y+1 | cycle y+2 |
Node B +-----------+-----------+-----------+-----------+
\ `
\ `
V v
Figure-2: Proposed model for cyclic queuing
In this model, the cycle starting time points of different nodes can
be either synchronized or not. However, the "phase difference"
between neighbor nodes should be stable, and the variation should be
bounded.
Since link delay and T_c are decoupled, a small T_c can be used with
arbitrary link length without sacrificing bandwidth utilization. Any
time range in a cycle can be used for deterministic traffic. In
Figure-2, the last packet of the deterministic packets sent in cycle
x by Node A is received by Node B in its local cycle y (all
variations and jitters are considered). Node B stores those packets
in a buffer associated with cycle y, and transmits the packets in
this buffer in cycle y+1. The first packet of cycle x+1 may also be
received by Node B in cycle y. So two or even more receiving buffers
are needed simultaneously.
Since there is variation in link delay and output delay, the last
packet in cycle x and the first packet in cycle x+1 may be difficult
to distinguish to Node B, especially when the bandwidth utilization
is high. To resolve the ambiguity, a cycle label can be put in a
packet, which identifies which cycle the packet belongs with.
Packets in different cycles carry different cycle labels. So Node B
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can unambiguously distinguish the packet's sending cycle and map it
in a correct local buffer even when the bandwidth is fully utilized.
There can be multiple ways to map a cycle label in a packet to a
local cyclic buffer. For example, an ordered pair of neighboring
nodes can learn a cycle mapping table. Node B may receive packets
from different upstream nodes that carry different cycle labels.
Node B can use the mapping table to swap the labels to a same local
cycle label, and put the packets into a same local buffer. Or, an
upstream node can swap a local label to a downstream label before
transmitting the packet. Another way is to put a label stack in the
packet, so every hop just pops a label and maps it to a local buffer.
Further study is required to define how cycle labels are formated and
processed and how mapping tables are learned.
6. Security Considerations
TBD
7. IANA Considerations
TBD
8. Acknowledgements
TBD
9. Normative References
[draft-ietf-detnet-bounded-latency]
"DetNet Bounded Latency", <https://tools.ietf.org/html/
draft-ietf-detnet-bounded-latency>.
[IEEE8021TSN]
"IEEE 802.1 Time-Sensitive Networking (TSN) Task Group",
<http://www.ieee802.org/1/>.
[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>.
Authors' Addresses
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Bingyang Liu
Huawei
No.156 Beiqing Road
Beijing, P.R. China 100095
China
Email: liubingyang@huawei.com
Joanna Dang (editor)
Huawei
No.156 Beiqing Road
Beijing, P.R. China 100095
China
Email: dangjuanna@huawei.com
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