Priority Switching Scheduler
draft-finzi-priority-switching-scheduler-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
|
|
|---|---|---|---|
| Authors | Fred Baker , Anaïs Finzi , Fabrice Frances , Emmanuel Lochin , Ahlem Mifdaoui | ||
| Last updated | 2018-03-05 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-finzi-priority-switching-scheduler-01
Internet Engineering Task Force Baker
Internet-Draft
Intended status: Informational Finzi
Expires: September 6, 2018 Frances
Lochin
Mifdaoui
ISAE-SUPAERO
March 5, 2018
Priority Switching Scheduler
draft-finzi-priority-switching-scheduler-01
Abstract
We detail the implementation of a network scheduler that switches the
priority of one or several queues. This scheduler aims at carrying
and isolating time constrained and elastic traffic flows from best-
effort traffic. We claim that the usual implementations with rate
schedulers (such as WRR, DRR,...) do not allow to efficiently
quantify the reserved capacity of the different classes. By using
this credit based scheduler mechanism called Priority Switching
Scheduler, we provide a more predictable available capacity.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 6, 2018.
Copyright Notice
Copyright (c) 2018 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
Provisions Relating to IETF Documents
Baker, et al. Expires September 6, 2018 [Page 1]
Internet-Draft Priority Switching Scheduler March 2018
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Context and Motivation . . . . . . . . . . . . . . . . . 2
1.2. Priority Switching Scheduler in a nutshell . . . . . . . 2
2. Priority Switching Scheduler . . . . . . . . . . . . . . . . 3
2.1. Specification . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Implementation . . . . . . . . . . . . . . . . . . . . . 5
3. Usecase: benefit of using PSS in a Diffserv core network . . 6
3.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. New service offered . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
1.1. Context and Motivation
To share the capacity offered by a link, many fair schedulers have
been developed, such as Weighted Fair Queueing, Deficit Round Robin.
However, with these solutions, the capacity available to a class is
strongly impacted by the other classes. With this new scheduler
denoted Priority Switching Scheduler (PSS), we wish to reduce this
impact on some classes and provide them with a more predictable
output rate, reporting the impact on other classes. Additionally,
compared to well-known schemes, this solutions is simpler to
implement as it does not require a virtual clock, and more flexible
thanks to the many possibilities offered by the setting of different
priorities.
1.2. Priority Switching Scheduler in a nutshell
The principle of PSS is the use of credit counters to change the
priority of one or several queues. For each switching queue q, its
priority, denoted p[q], switches between two values, depending on its
associated credit counter. Then classic Priority Scheduler is used
for the dequeuing process.
Baker, et al. Expires September 6, 2018 [Page 2]
Internet-Draft Priority Switching Scheduler March 2018
The main idea is that changing the priorities adds fairness to the
Priority Scheduler. Depending on its credit counter parameters, the
amount of capacity available to a queue is bounded between a minimum
and a maximum value. Consequently, good parameterization is very
important to prevent starvation of lower priority queues.
The new service we seek to obtain for the queue with the switching
priority is more predictable: the minimum between a desired capacity
and the residual capacity left by higher priorities. The impact of
the input variations of higher classes is passed down to lower
priority classes.
Finally, this new solutions offers much flexibility as we can have
both i) queues with a reserved capacity (when two priorities are
set), ii) and queues scheduled with a simple Priority Scheduler (when
only one priority is set).
2. Priority Switching Scheduler
2.1. Specification
The PSS defines for each queue q a low priority, p_low[q], and a high
priority, p_high[q]. For each queue q with p_high[q] > p_low[q], to
manage the priority switching a credit counter is defined with:
o a minimum level: 0;
o a maximum level: LM[q];
o a resume level: LR[q]
o a reserved capacity: BW[q]
o an idle slope: Iidle[q]=C*BW[q];
o a sending slope: Isend[q]=C-Iidle[q];
The priority change depends on the credit counter as follows:
o initially, the credit counter starts at 0;
o the change of priority p[q] of queue q occurs in two cases:
* if p[q]=p_high[q] and the credit reaches LM[q];
* if p[q]=p_low[q] and credit reaches LR[q];
Baker, et al. Expires September 6, 2018 [Page 3]
Internet-Draft Priority Switching Scheduler March 2018
o when a packet of queue q is transmitted, the credit increases with
a rate Isend[q], else the credit decreases with a rate Iidle[q];
o when the credit reaches LM[q], it remains at this level until the
end of the transmission of the current packet;
o when the credit reaches 0, it remains at this level until the
start of the transmission of a queue q packet.
Figure 1 and Figure 2 show two examples of credit and priority
behaviors of a queue q.
^ credit[q]
| | |
|p_high[q]| p_low[q] | p_high[q]
LM +---------|-------------------|------------
| _ '| |` | '
|Isend| ' | | ` | '
| [q] ' | | ` _ | '
| ' | | ` |Iidle| '
| ' | | ` [q]| '
| ' | | ` | '
LR + ' | | ` | '
0 +.........|....|..............|..........>
| | time
@@@@@@@@@@@@@@@@oooooooooooooo@@@@@@@@@@
@ queue q traffic o other traffic
Figure 1: First example of queue q credit and priority behaviors
Baker, et al. Expires September 6, 2018 [Page 4]
Internet-Draft Priority Switching Scheduler March 2018
^ credit[q]
| |
| p_high[q] | p_low[q]
LM + -----------------------|------------------- '
| '| |` '
| '|` ' | | ` '
| [q] ' | ` | | ` '
| '` ' | | | | `
| '| ` ' | | | | |
| ' | | | | | | |
LR +--'--|-----|----|---|---|--|------|--------
0 +-'---|-----|----|---|---|--|------|-------->
| | | | | | time
@@@@@@oooooo@@@@@oooo@@@@@@@@oooooo@@@@@@@
@ queue q traffic o other traffic
Figure 2: Second example of queue q credit and priority behaviors
Finally, for the dequeuing process, a Priority Scheduler selects the
appropriate frame using the current p[q] values.
2.2. Implementation
The new dequeuing algorithm is presented in the PSS Algorithm. The
credit of each queue q, denoted credit[q], and the dequeuing timer
denoted timerDQ[q] are initialized to zero. The initial priority is
set to the high value p_high[q]. First, for each queue with
p_high[q] > p_low[q], the difference between the current time and the
time stored in timerDQ[q], is computed (lines 2 and 3). The duration
dtime[q] represents the time elapsed since the last credit update,
during which no shaped packet was sent, we call this the idle time.
Then, if dtime[q]>0, the credit is updated by removing the credit
gained during the idle time that just occurred (lines 4 and 5).
Next, timerDQ[q] is set to the current time to keep track of the time
the credit is last updated (line 6). If the credit reaches LR[q],
the priority changes to its high value (lines 7 and 8). Then, with
the updated priorities, the priority scheduler performs as usual:
each queue is checked for dequeuing (lines 12 and 13). When a queue
q is selected with p_high[q] > p_low[q], the credit expected to be
consumed is added to credit[q] variable (line 16). The time taken
for the packet to be dequeued is added to the variable timerDQ[q]
(lines 13 and 14) so the transmission time of the packet will not be
taken into account in the idle time dtime[q] (line 2). If the credit
reaches LM[q], the priority changes to its low value (lines 18 and
19). Finally, the packet is dequeued (line 22).
Baker, et al. Expires September 6, 2018 [Page 5]
Internet-Draft Priority Switching Scheduler March 2018
Inputs: credits, timerDQs, C, LMs,LRs,BWs,p_highs, p_lows
1 currentTime=getCurrentTime()
2 for each queue q with p_high[q] > p_low[q] do:
3 dtime[q]=currentTime-timerDQ[q]
4 if dtime[q]>0 then:
5 credit[q]=max(credit[q]-dtime[q].C.BW[q],0)
6 dtime[q]=currentTime
7 if credit[q]<LR[q] and p[q]=p_low[q] then:
8 p[q]=p_high[q]
9 end if
10 end if
11 end for
12 for each priority level pl, highest first do:
13 if length(queue(pl))>0 then:
14 q=queue(pl)
15 if p_high[q] > p_high[q] then:
16 credit[q]=min(LM[q], credit[q]+size(head(q)).(1-BW[q]))
17 timerDQ[q]=currentTime+size(head(q))/C
18 if credit >=LM[q] and p[q]=p_high[q] then:
19 p[q]=p_low[q]
20 end if
21 end if
22 dequeue(head(q))
23 end if
24 end for
Figure 3: PSS algorithm
PSS algorithm also implements the following functions:
o getCurrentTime() uses a timer to return the current time;
o queue(pl) returns the queue associated to priority pl;
o head(q) returns the first packet of queue q;
o size(f) returns the size of packet f;
o dequeue(f) activates the dequeing event of packet f.
3. Usecase: benefit of using PSS in a Diffserv core network
3.1. Motivation
The DiffServ architecture defined in [RFC4594] and [RFC2475] proposes
a scalable mean to deliver IP quality of service (QoS) based on
handling traffic aggregates. This architecture follows the
Baker, et al. Expires September 6, 2018 [Page 6]
Internet-Draft Priority Switching Scheduler March 2018
philosophy that complexity should be delegated to the network edges
while simple functionalities should be located in the core network.
Thus, core devices only perform differentiated aggregate treatments
based on the marking set by edge devices.
Keeping aside policing mechanisms that might enable edge devices in
this architecture, a DiffServ stateless core network is often used to
differentiate time-constrained UDP traffic (e.g. VoIP or VoD) and
TCP bulk data transfer from all the remaining best-effort (BE)
traffic called default traffic (DE). The Expedited Forwarding (EF)
class is used to carry UDP traffic coming from time-constrained
applications (VoIP, Command/Control, ...); the Assured Forwarding
(AF) class deals with elastic traffic as defined in [RFC4594] (data
transfer, updating process, ...) while all other remaining traffic is
classified inside the default (DE) best-effort class.
The first and best service is provided to EF as the priority
scheduler attributes the highest priority to this class. The second
service is called assured service and is built on top of the AF class
where elastic traffic such as TCP traffic, is intended to achieve a
minimum level of throughput. Usually, the minimum assured throughput
is given according to a negotiated profile with the client. The
throughput increases as long as there are available resources and
decreases when congestion occurs. As a matter of fact, a simple
priority scheduler is insufficient to implement the AF service. Due
to its opportunistic nature of fetching the full remaining capacity,
TCP traffic increases until reaching the capacity of the bottleneck.
In particular, this behaviour could lead to starve the DE class.
To prevent this and ensure to both DE and AF a minimum service rate,
the router architecture proposed in [RFC5865] uses a rate scheduler
between AF and DE classes to share the residual capacity left by the
EF class. Nevertheless, one drawback of using a rate scheduler is
the high impact of EF traffic on AF and DE. Indeed, the residual
capacity shared by AF and DE classes is directly impacted by the EF
traffic variation. As a consequence, the AF and DE class services
are difficult to predict in terms of available capacity and latency.
To overcome these limitations and make AF service more predictable,
we propose here to use the newly defined Priority Switching Scheduler
(PSS). Figure 4 shows an example of the Data Plane Priority core
network router presented in [RFC5865] modified with a PSS. The EF
queues have the highest priorities to offer the best service to real-
time traffic. The priority changes set the AF priorities either
higher (3,4) or lower (6,7) than CS0 (5), leading to capacity
sharing. Another example with only 3 queues is described in
[Globecom17]. Thank to the increase predictability, for the same
minimum guaranteed rate, the PSS reserves a lower percentage of the
Baker, et al. Expires September 6, 2018 [Page 7]
Internet-Draft Priority Switching Scheduler March 2018
capacity than a rate scheduler. This leaves more remaining capacity
that can be guaranteed to other users.
priorities
________
queues | `
Admitted EF----->||-----+ 1 `
| `
Unadmitted EF----->||-----+ 2 `
| `
AF1----->||-----+ 3 or 6 PSS -----------
| '
AF2----->||-----+ 4 or 7 '
| '
CS0----->||-----+ 5 '
|________'
Figure 4: PSS applied to Data Plane Priority (we borrow the syntax
from RCF5865)
3.2. New service offered
The new service we seek to obtain is:
o for EF, the full capacity of the output link;
o for AF the minimum between a desired capacity and the residual
capacity left by EF;
o for DE (CS0), the residual capacity left by EF and AF.
As a result, the AF class has a more predictable available capacity,
while the unpredictability is reported on the DE class. With good
parametrization, both classes also have a minimum rate ensured.
Parameterization and simulations results concerning the use of a
similar scheme for core network scehduling are available in
[Globecom17]
4. Security Considerations
TODO
5. Normative References
Baker, et al. Expires September 6, 2018 [Page 8]
Internet-Draft Priority Switching Scheduler March 2018
[Globecom17]
Finzi, A., Lochin, E., Mifdaoui, A., and F. Frances,
"Improving RFC5865 Core Network Scheduling with a Burst
Limiting Shaper", Globecom , 2017, <acceptedPaper>.
[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>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
<https://www.rfc-editor.org/info/rfc5865>.
Authors' Addresses
Fred Baker
Santa Barbara, California 93117
USA
Email: FredBaker.IETF@gmail.com
Anais Finzi
ISAE-SUPAERO
10 Avenue Edouard Belin
Toulouse 31400
France
Phone: 0033561338735
Email: anais.finzi@isae-supaero.fr
Baker, et al. Expires September 6, 2018 [Page 9]
Internet-Draft Priority Switching Scheduler March 2018
Fabrice Frances
ISAE-SUPAERO
10 Avenue Edouard Belin
Toulouse 31400
France
Email: fabrice.frances@isae-supaero.fr
Emmanuel Lochin
ISAE-SUPAERO
10 Avenue Edouard Belin
Toulouse 31400
France
Email: emmanuel.lochin@isae-supaero.fr
Ahlem Mifdaoui
ISAE-SUPAERO
10 Avenue Edouard Belin
Toulouse 31400
France
Email: ahlem.mifdaoui@isae-supaero.fr
Baker, et al. Expires September 6, 2018 [Page 10]