Internet Engineering Task Force Baker
Internet-Draft
Intended status: Informational Finzi
Expires: December 1, 2018 Frances
Lochin
Mifdaoui
ISAE-SUPAERO
May 30, 2018
Priority Switching Scheduler
draft-finzi-priority-switching-scheduler-02
Abstract
We detail the implementation of a network scheduler that aims at
isolating time constrained and elastic traffic flows from best-effort
traffic. This scheduler inherits from the priority scheduler (PS)
but dynamically changes the priority of one or several queues. Usual
implementations of rate scheduler schemes (such as WRR, DRR, ...) do
not allow to efficiently guarantee the capacity dedicated to both AF
and BE classes as they mostly provide soft bounds. This means
excessive margin is used to ensure the capacity requested and this
impacts the number of additional users that could be accepted in the
network. To cope with this issue, this memo presents a credit based
scheduler mechanism called Priority Switching Scheduler (PSS) that
allows a more predictable output rate per traffic class.
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 http://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 December 1, 2018.
Baker, et al. Expires December 1, 2018 [Page 1]
Internet-Draft Priority Switching Scheduler May 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
(http://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. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.3. Priority Switching Scheduler in a nutshell . . . . . . . 3
2. Priority Switching Scheduler . . . . . . . . . . . . . . . . 4
2.1. Specification . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Implementation . . . . . . . . . . . . . . . . . . . . . 7
3. Usecase: benefit of using PSS in a Diffserv core network . . 9
3.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. New service offered . . . . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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 Queuing, Weighted Round Robin
or Deficit Round Robin. However, with these well-known solutions,
the output rate of a given queue depends on the amount of traffic
crossing other queues. Our proposal aims at reducing the uncertainty
of the output rate of selected queues, we call them in the following
controlled queues. Additionally, compared to previous cited schemes,
this solution is simpler to implement mainly because it does not
require a virtual clock, and more flexible thanks to the wide
possibilities offered by the setting of different priorities.
Baker, et al. Expires December 1, 2018 [Page 2]
Internet-Draft Priority Switching Scheduler May 2018
1.2. 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 [RFC2119].
1.3. Priority Switching Scheduler in a nutshell
_____________________
| p_low[i] p_high[i] |
------|_____________________|
sets() | ^
_________|__ |
PSS controlled | | | | selects()
queue i ------------>| p[i]= v | |
| | credit[i]
. | . | ^
. | . | | updates()
. | . | |
non-active | |------------------> output
PSS queue j ------------>| p[j] | traffic
| | for any queue q:
. | . | p[q]: priority
. | . | p_low[q]: low priority
. | . | p_high[q]: high priority
|____________|
Priority Scheduler
Figure 1: PSS in a nutshell
As illustrated in Figure 1, the principle of PSS is based on the use
of credit counters (detailed in the following) to change the priority
of one or several queues. The idea follows a proposal made by the
TSN Task group named Burst Limiting Shaper [BLS]. For each
controlled queue i, each priority denoted p[i], changes between two
values denoted p_low[i] and p_high[i], depending on the associated
credit counter, i.e., credit[i]. Then a Priority Scheduler is used
for the dequeuing process, e.g., among the queues with available
traffic, the first packet of the queue with the highest priority is
dequeued.
The main idea is that changing the priorities adds fairness to the
Priority Scheduler. Depending on the credit counter parameters, the
amount of capacity available to a controlled queue is bounded between
a minimum and a maximum value. Consequently, good parameterization
is very important to prevent starvation of lower priority queues.
Baker, et al. Expires December 1, 2018 [Page 3]
Internet-Draft Priority Switching Scheduler May 2018
The service obtained for the controlled queue with the switching
priority is more predictable and corresponds to the minimum between a
desired capacity and the residual capacity left by higher priorities.
The impact of the input traffic sporadicity from higher classes is
thus transfered to non-active PSS queues with a lower priority.
Finally, PSS offers much flexibility as both i) controlled queues
with a guaranteed capacity (when two priorities are set), ii) and
queues scheduled with a simple Priority Scheduler (when only one
priority is set) can conjointly be enabled.
2. Priority Switching Scheduler
2.1. Specification
The PSS algorithm defines for each queue q a low priority, p_low[q],
and a high priority, p_high[q]. Each PSS controlled queue q with
p_high[q] < p_low[q] is associated to a credit counter credit[q]
which manages the priority switching. Each credit counter is defined
by:
o a minimum level: 0;
o a maximum level: LMs[q];
o a resume level: LRs[q]
o a reserved capacity: BWs[q]
o an idle slope: Iidle[q] = C * BWs[q];
o a sending slope: Isend[q] = C - Iidle[q];
The available capacity is mostly impacted by the guaranteed capacity
BWs[q]. Hence BWs[q] should be set to the desired capacity plus a
margin taking into account the additional packet due to non-premption
as explained below:
the value of LMs[q] can negatively impact on the guaranteed available
capacity. The maximum level determines the size of the maximum
sending windows, i.e, the maximum uninterrupted transmission time of
the controlled queue packets before a priority switching. The impact
of the non-premption is as a function of the value of LMs[q]. The
smaller the LMs[q], the larger the impact of the non-premption is.
For example, if the number of packets varies between 4 and 5, the
variation of the output traffic is around 25% (i.e. going from 4 to 5
corresponds to a 25% increase). If the number of packets sent varies
between 50 and 51, the variation of the output traffic is around 2%.
Baker, et al. Expires December 1, 2018 [Page 4]
Internet-Draft Priority Switching Scheduler May 2018
The credit allows to keep track of the packet transmissions.
However, there are two cases keeping track of the transmission raises
an issue: when the credit is saturated at LMs[q] or at 0. In both
cases, packets are transmitted without gained or consumed credit.
Nevertheless, the resume level can be used to decrease the times when
the credit is saturated at 0. If the resume level is 0, then as soon
as the credit reaches 0, the priority is switched and the credit
saturates at 0 due to the non-preemption of the current packet. On
the contrary, if LRs[q]>0, then during the transmission of the non-
preempted packet, the credit keeps on decreasing before reaching 0 as
illustrated in Figure 2.
Hence, the proposed value for LRs[q] is LRs[q]= Lmax(MC(q))*BWs[q],
with MC(q) the queues such as k in MC(q) -> p_low[q] > (p_low[k] or
p_high[k])> p_high[q], and Lmax(qs) the maximum size of the queues
qs. With this value, there is no credit saturation at 0 due to non-
preemption.
Finally, we propose to use the following parameters of a controlled
queue q:
o BWs[q]= desired_BWs[q] + 1/(N-1)
o LMs[q]= (N-1) * Lmax(q) * (1 - BWs[q])
o LRs[q]= Lmax(MC(q)) * BWs[q]
with N the maximum number of packet of queue q set uninterrupted
(taking into account the non-preemption) and desired_BWs[q] the
percentage of desired available capacity.
A similar parameter setting is described in [Globecom17], to
transform WRR parameter into PSS parameters, in the specific case of
3-classes DiffServ architecture.
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 LMs[q];
* if p[q] = p_low[q] and credit reaches LRs[q];
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];
Baker, et al. Expires December 1, 2018 [Page 5]
Internet-Draft Priority Switching Scheduler May 2018
o when the credit reaches LMs[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 2 and Figure 3 show two examples of credit and priority
changes of a given queue q. First, in Figure 2, we show an example
when the controlled queue q sends its traffic continuously until the
priority change. Then other traffic is also sent uninterruptedly
until the priority changes back. In Figure 3, we propose a more
complex behaviour. First, this figure shows when a packet with a
priority higher than p_high[q] is available, this packet is sent
before the traffic of class q. Secondly, when no traffic with a
priority lower than p_low[q] is available, then traffic of queue q
can be sent. This highlight the non-blocking nature of PSS and that
p[q] = p_high[q] (resp. p[q] = p_low[q]) does not necessarily mean
that traffic of queue q is being sent (resp. not being sent).
^ credit[q]
| | |
|p_high[q]| p_low[q] | p_high[q]
LMs[q]|- - - - -++++++- - - - - - - |- - - -+++
| +| |+ | +
| Isend + | | + Iidle | +
| [q] + | | + [q] | +
| + | | + | +
| + | | + | +
| + | | + | +
LRs[q]| + | | + |+
0 |-+- - - -|- - |- - - - - - - +- - - - - >
| | time
@@@@@@@@@@@@@@@@oooooooooooooo@@@@@@@@@@
@ queue q traffic o other traffic
Figure 2: First example of queue q credit and priority behaviors
Baker, et al. Expires December 1, 2018 [Page 6]
Internet-Draft Priority Switching Scheduler May 2018
^ credit[q]
| |
| p_high[q] | p_low[q]
LMs[q]+ - - - - - - - - - - - -++++ - - - - - - -+
| +| |+ +
| ++ + | | + +
| + | + + | | + +
| ++ + | + | | +
| +| + + | | | | |
| + | + | | | | |
LRs[q]+--+--|-----|----|---|---|--|------|-------
0 +-+- -| - - |- - |- -|- -|- |- - - |- - - - >
| | | | | | time
@@@@@@oooooo@@@@@oooo@@@@@@@@oooooo@@@@@@@
@ queue q traffic o other traffic
Figure 3: Second example of queue q credit and priority behaviors
Finally, for the dequeuing process, a Priority Scheduler selects the
appropriate packet using the current p[q] values, e.g., among the
queues with available traffic, the first packet of the queue with the
highest priority is dequeued.
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 packet of the controlled queue q 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 LRs[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, highest
priority first (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 LMs[q], the
Baker, et al. Expires December 1, 2018 [Page 7]
Internet-Draft Priority Switching Scheduler May 2018
priority changes to its low value (lines 18 and 19). Finally, the
packet is dequeued (line 22).
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.BWs[q],0)
6 dtime[q] = currentTime
7 if credit[q]<LRs[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_low[q] then:
16 credit[q] = min(LMs[q],
credit[q]+size(head(q)).(1-BWs[q]))
17 timerDQ[q] = currentTime+size(head(q))/C
18 if credit >= LMs[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 break
24 end if
25 end for
Figure 4: 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.
Baker, et al. Expires December 1, 2018 [Page 8]
Internet-Draft Priority Switching Scheduler May 2018
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
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 (DF). 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 (DF) 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. TCP
traffic increases until reaching the capacity of the bottleneck due
to its opportunistic nature of fetching the full remaining capacity.
In particular, this behaviour could lead to starve the DF class.
To prevent a starvation and ensure to both DF and AF a minimum
service rate, the router architecture proposed in [RFC5865] uses a
rate scheduler between AF and DF 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 DF.
Indeed, the residual capacity shared by AF and DF classes is directly
impacted by the EF traffic variation. As a consequence, the AF and
DF 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
Baker, et al. Expires December 1, 2018 [Page 9]
Internet-Draft Priority Switching Scheduler May 2018
(PSS). Figure 5 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
capacity than a rate scheduler. This leaves more remaining capacity
that can be guaranteed to other users.
priorities
________________________________
queues | |\
Admitted EF--->||-----+ p[AEF]= 1 | \
| | \
Unadmitted EF--->||-----+ p[UEF]= 2 | \
| | \
AF1--->||-----+ p_high[AF1]=3 and p_low[AF1]= 6| PSS ---
| | /
AF2--->||-----+ p_high[AF2]=4 and p_low[AF2]= 7| /
| | /
CS0--->||-----+ p[CS0]= 5 | /
|________________________________|/
Figure 5: 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 DF (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 DF 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 scheduling are available in
[Globecom17]
Baker, et al. Expires December 1, 2018 [Page 10]
Internet-Draft Priority Switching Scheduler May 2018
4. Security Considerations
There are no specific security exposure with PSS that would extend
those inherent in default FIFO queuing or in static priority
scheduling systems. However, following the DiffServ usecase proposed
in this memo and in particular the illustration of the integration of
PSS as a possible implementation of the architecture proposed in
[RFC5865], most of the security considerations from [RFC5865] and
more generally from the differentiated services architecture
described in [RFC2475] still hold.
5. Acknowledgements
This document was the result of collaboration and discussion among a
large number of people. In particular the authors wish to thank
Nicolas Kuhn and David Black for reviewing this draft. At last but
not least, a very special thanks to Fred Baker for his help.
6. References
6.1. 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/info/rfc2119>.
6.2. Informative References
[BLS] Gotz, F-J., "Traffic Shaper for Control Data Traffic
(CDT)", IEEE 802 AVB Meeting , 2012.
[Globecom17]
Finzi, A., Lochin, E., Mifdaoui, A., and F. Frances,
"Improving RFC5865 Core Network Scheduling with a Burst
Limiting Shaper", Globecom , 2017, <acceptedPaper>.
[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>.
Baker, et al. Expires December 1, 2018 [Page 11]
Internet-Draft Priority Switching Scheduler May 2018
[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
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
Baker, et al. Expires December 1, 2018 [Page 12]
Internet-Draft Priority Switching Scheduler May 2018
Ahlem Mifdaoui
ISAE-SUPAERO
10 Avenue Edouard Belin
Toulouse 31400
France
Email: ahlem.mifdaoui@isae-supaero.fr
Baker, et al. Expires December 1, 2018 [Page 13]