Internet DRAFT - draft-briscoe-tsvwg-l4s-diffserv
draft-briscoe-tsvwg-l4s-diffserv
Transport Area Working Group B. Briscoe
Internet-Draft CableLabs
Intended status: Informational November 1, 2018
Expires: May 5, 2019
Interactions between Low Latency, Low Loss, Scalable Throughput (L4S)
and Differentiated Services
draft-briscoe-tsvwg-l4s-diffserv-02
Abstract
L4S and Diffserv offer somewhat overlapping services (low latency and
low loss), but bandwidth allocation is out of scope for L4S.
Therefore there is scope for the two approaches to complement each
other, but also to conflict. This informational document explains
how the two approaches interact, how they can be arranged to
complement each other and in which cases one can stand alone without
needing the other.
Status of This Memo
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This Internet-Draft will expire on May 5, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Document Roadmap . . . . . . . . . . . . . . . . . . . . 4
2. Architectural Comparison of L4S and Diffserv . . . . . . . . 4
2.1. Overlaps between L4S and Diffserv . . . . . . . . . . . . 4
2.2. Differences between L4S and Diffserv . . . . . . . . . . 4
3. Low Latency Diffserv Classes within a DualQ Bandwidth Pool . 5
4. DualQ Bandwidth Pool within a Hierarchy of (Diffserv)
Bandwidth Queues . . . . . . . . . . . . . . . . . . . . . . 9
4.1. DualQ Complemented by an Assured Bandwidth Service . . . 10
4.2. DualQ Complemented by a Guaranteed Low Latency Service . 12
4.3. DualQ Complemented by a Scavenger Service . . . . . . . . 13
5. Coupling More than Two AQMs within a Bandwidth Pool . . . . . 14
6. Applicability of Coupled AQM to Global Diffserv PHBs . . . . 14
7. Best Practice for Classification and Marking . . . . . . . . 16
7.1. Never Re-Mark a DSCP . . . . . . . . . . . . . . . . . . 16
7.2. Classification Order . . . . . . . . . . . . . . . . . . 16
7.2.1. Classification Order: Problem . . . . . . . . . . . . 17
7.2.2. Classification Order: Solutions . . . . . . . . . . . 17
8. Policing and Traffic Conditioning . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . 18
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
13.1. Normative References . . . . . . . . . . . . . . . . . . 18
13.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The Low Latency Low Loss Scalable throughput (L4S) Internet service
[I-D.ietf-tsvwg-l4s-arch] provides a new Internet service that could
eventually replace best efforts, but with ultra-low queuing delay and
loss. A structure called the Dual-Queue Coupled AQM manages to
provide the L4S service alongside a second queue for Classic Internet
traffic, but without prejudging the bandwidth allocations between
them. L4S is orthogonal to allocation of bandwidth, so it can be
complemented by various bandwidth allocation approaches without
prejudging which one.
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The Differentiated Services (Diffserv) architecture [RFC2475]
provides for various service classes, some defined globally, others
defined locally per network domain. Certain of these service classes
offer low latency and low loss, as well as differentiated allocation
of bandwidth.
Thus, L4S and Diffserv offer somewhat overlapping services (low
latency and low loss), but bandwidth allocation is out of scope for
L4S. Therefore there is scope for the two approaches to complement
each other, but also to conflict. This informational document
explains how the two approaches interact, how they can be arranged to
complement each other and in which cases one can stand alone without
needing the other.
1.1. Terminology
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]. In this
document, these words will appear with that interpretation only when
in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
Classic service: The 'Classic' service is intended for all the
congestion control behaviours that currently co-exist with TCP
Reno [RFC5681] (e.g. TCP Cubic, Compound, SCTP, etc).
Low-Latency, Low-Loss and Scalable (L4S) service: The 'L4S' service
is intended for traffic from scalable congestion control
algorithms such as Data Centre TCP [RFC8257]. But it is also more
general--it will allow a set of congestion controls with similar
scaling properties to DCTCP to evolve.
Both Classic and L4S services can cope with a proportion of
unresponsive or less-responsive traffic as well (e.g. DNS, VoIP,
etc).
Pure L4S: L4S without unresponsive traffic.
Scalable Congestion Control: See [I-D.ietf-tsvwg-l4s-arch] for
definition.
Classic Congestion Control: See [I-D.ietf-tsvwg-l4s-arch] for
definition.
DualQ: Abbreviation for Dual-Queue Coupled AQM
[I-D.ietf-tsvwg-aqm-dualq-coupled], which is not a specific AQM,
but a framework for coupling two AQMs in order to provide L4S
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service while doing no harm to 'Classic' traffic from traditional
sources.
ECN field: The Explicit Congestion Notification field [RFC3168] in
the IP header (v4 or v6). [RFC8311] has relaxed some of the
restrictions that RFC 3168 placed on the use of ECN, in order to
enable experiments like L4S, among others.
Site: A home, mobile device, small enterprise or campus, where the
network bottleneck is typically the access link to the site. Not
all network arrangements fit this model but it is a useful, widely
applicable generalisation.
1.2. Document Roadmap
{ToDo}
2. Architectural Comparison of L4S and Diffserv
This section compares the L4S architecture [I-D.ietf-tsvwg-l4s-arch]
with the Diffserv architecture [RFC2475].
L4S uses an identifier [I-D.ietf-tsvwg-ecn-l4s-id] in the ECN field
in IP packet headers that is orthogonal to the Diffserv field
[RFC2474]. This is because the two approaches can either overlap or
complement each other, as outlined in the following two subsections.
2.1. Overlaps between L4S and Diffserv
L4S provides a low queuing latency, low loss Internet Service.
Specific Diffserv service classes also provide low latency and low
loss.
This means that it is possible to mix traffic from certain Diffserv
classes in the same queue as L4S traffic (see Section 3).
2.2. Differences between L4S and Diffserv
Bandwidth allocation: L4S is orthogonal to allocation of bandwidth,
so it can be complemented by various bandwidth allocation
approaches without prejudging which one. In contrast, with
Diffserv it was never possible to completely separate control of
latency and loss from allocation of bandwidth. The only
bandwidth-related aspect of L4S is that it ensures that the
capacity seeking behaviour of end-systems can scale with
increasing flow rate.
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Differentiation vs. General improvement: Diffserv concerns give and
take of bandwidth, latency and loss between traffic classes. In
contrast, the separation of L4S from Classic traffic in separate
queues concerns incremental deployment of a general improvement in
latency and loss, without taking from the other queue.
Open vs. closed loop control: The Diffserv architecture requires the
source to keep traffic within a contract and, failing that, it has
mechanisms to enforce the contract. In this respect, Diffserv is
an open-loop control system that is primarily concerned with
keeping traffic within capacity limits. Nonetheless, there is an
element of closed-loop control in Diffserv. The weighted AQM
(e.g. WRED) used for Assured Forwarding [RFC2597] expects traffic
to seek to fill capacity and exploits the response to feedback of
congestion controllers at traffic sources (closed-loop).
Nonetheless, the Diffserv architecture still provides for traffic
conditioners that tag traffic that is outside the bandwidth
contract for each AF class (open-loop). Then out-of-contract
traffic can be discarded if it would otherwise lead to congestion.
L4S uses a similar closed-loop mechanism to the weighted AQM used
in Diffserv AF in order to ensure roughly equal per-flow
throughput between the L4S and Classic queues. That is, L4S
relies on the source's closed-loop response to feedback, not any
open-loop obligation of each source to keep within a traffic
contract. With L4S, any enforcement of per-flow throughput
(whether open-loop or closed) is set aside as a separate issue
that may or may not be addressed by separate mechanisms, dependent
on policy.
Per bottleneck vs. per domain: L4S can be independently and
incrementally deployed at certain bottlenecks. In contrast a
Diffserv system is domain-based consisting of the per-hop
behaviour of interior nodes and the traffic conditioning behaviour
of boundary nodes, which have to be deployed as a coordinated
whole.
Degree of multiplexing: Diffserv components such as traffic
conditioning are less applicable in access networks where
statistical multiplexing is low, whereas L4S was initially
designed for access networks, but is also applicable at larger
pinch-points (e.g. public peerings).
3. Low Latency Diffserv Classes within a DualQ Bandwidth Pool
The experimental Dual-Queue Coupled AQM
[I-D.ietf-tsvwg-aqm-dualq-coupled] consists of a pair of queues. One
provides a low latency low loss service but both have full access to
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the same pool of bandwidth. When Diffserv was defined no mechanism
like this was available that could provide low latency without also
requiring bandwidth controls. All Diffserv's mechanisms for low
latency and low loss use some form of priority over bandwidth, then
apply a bandwidth constraint to prevent the lower priority traffic
from being starved.
This Diffserv bandwidth constraint has a flip side - it can also
provide a bandwidth assurance. However, in turn, bandwidth assurance
has both positive and negative aspects. It certainly prevents other
traffic encroaching on the bandwidth of the low latency class, but it
also carves off a partition within which low latency sessions are
more prone to encroach on each other.
The DualQ offers an alternative where low latency traffic can access
the whole pool of bandwidth (in effect, the largest possible
bandwidth constraint). This is expected to be preferred by many
network operators and users who would rather not set a bandwidth
limit for their low latency traffic - particularly at links in access
networks where the very low level of flow multiplexing makes the
bandwidth shares of different traffic classes nearly impossible to
predict. Nonetheless, if a bandwidth partition is required for
bandwidth assurance purposes, it can still be provided separately
(see Section 4).
The DualQ classifies packets with the ECN field set to ECT(1) or CE
into the low latency low loss (L) queue. The L queue maintains a low
latency low loss service primarily because an L4S source paces its
packets and is linearly responsive to ECN markings, which earns it
the right to set the ECT(1) codepoint [I-D.ietf-tsvwg-ecn-l4s-id]
[RFC8311].
Nonetheless, a low level of non-L4S traffic can share the L queue
without compromising the low latency and low loss of the service.
Certain existing Diffserv classes are already intended as low latency
and low loss services. An operator could use the DualQ instead of
traditional Diffserv queues to give a few of these classes the
benefit of low latency and access to the whole pool of bandwidth.
However, that would only be safe for those Diffserv service classes
that would not risk ruining the low latency of the service.
Therefore, an operator must take care to only classify a Diffserv
traffic class into the L queue if it is expected to send smoothly
without multi-packet bursts. Below we give examples of classes that
should (and should not) be safe to mix into the L queue.
Table 1 lists the Diffserv service classes that have been allocated
global use Diffserv codepoints (DSCPs) from Pool 1. They are
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described in RFC 4594 [RFC4594] and its updates ([RFC5865] and
[I-D.ietf-tsvwg-le-phb] so far). An operator that only deploys a
DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] but not the relevant
Diffserv PHBs could classify those with an 'L' in the 'Coupled Queue'
column (or local use DSCPs with similar characteristics) into its L
queue, irrespective of the setting of the ECN field.
+--------------------+-------------+--------+-------+---------------+
| Service Class Name | DSCP Name | DSCP | AQM? | Coupled Queue |
+--------------------+-------------+--------+-------+---------------+
| Network Control{1} | CS7 | 111000 | Y & N | L if L4S |
| Network Control | CS6 | 110000 | Y & N | L if L4S |
| OAM | CS2 | 010000 | Y & N | L if L4S |
| Signalling | CS5 | 101000 | N | L if L4S{2} |
| Telephony | EF | 101110 | N | L |
| RFC 5865 | Voice-Admit | 101100 | N | L{3} |
| R-T Interactive | CS4 | 100000 | N | L if L4S{4} |
| MM Conferencing | AF4n | 100nn0 | Y | L if L4S |
| Broadcast Video | CS3 | 011000 | N | L if L4S{4} |
| MM Streaming | AF3n | 011nn0 | Y | L if L4S |
| Low Latency Data | AF2n | 010nn0 | Y | L if L4S |
| High Thru'put Data | AF1n | 001nn0 | Y | L if L4S{5} |
| Standard | BE/DF/CS0 | 000000 | Y | L if L4S |
| Low Priority Data | LE{6} | 000001 | Y | L if L4S{7} |
+--------------------+-------------+--------+-------+---------------+
Some service class names have been abbreviated to fit. Abbreviations
are expanded in RFC 4594 or its updates. For the assured forwarding
(AF) DSCP names, the digit 'n' represents 1, 2 or 3 and the
corresponding binary digits 'nn' in the DSCP value represent 01,10 or
11. The 'Coupled Queue' column is explained in the text.
Table 1: Mapping of RFC4594 Diffserv Service Classes in a Coupled AQM
Notes for Table 1:
{1}: Reserved by RFC 2474 [RFC2474].
{2}: Superficially, CS5 is a candidate for classification into the
L queue irrespective of its ECN field, given application
signalling is bursty but usually lightweight. However, at
least one major equipment vendor uses CS5 by default to
indicate unresponsive broadcast video traffic (to which RFC
4594 allocates CS3).
{3}: Voice-Admit [RFC5865] could be given priority over Expedited
Forwarding (EF) [RFC3246].
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{4}: The Real-Time Interactive and Broadcast Video service classes
(or any equivalent local-use classes) are intended for
inelastic traffic. Therefore they would not be expected to
mark themselves as ECN-capable. If they did they would be
claiming to be elastic and therefore eligible for
classification into the L queue (subject to any policing).
These classes should not be classified into the L queue on the
basis of DSCP alone, because high bandwidth unresponsive
traffic with potentially variable rate is not compatible with
the L4S service.
{5}: High Throughput Data (or any equivalent local-use class) might
use the L4S service because of its support for scalable
congestion control.
{6}: [I-D.ietf-tsvwg-le-phb] updates RFC 4594 to deprecate using
CS1 for Lower Effort (LE).
{7}: If a packet is marked LE and ECT(1) and the operator has
solely provided a DualQ, this recommends that the packet is
classified into the L queue. This could result in LE traffic
competing for bandwidth with other classes of traffic in the L
queue, but at least it should not harm the latency of other
traffic. This is because the ECT(1) marking means the source
"MUST" use a scalable congestion control
[I-D.ietf-tsvwg-ecn-l4s-id], but the LE marking only means it
"SHOULD" use an LBE congestion control
[I-D.ietf-tsvwg-le-phb].
Those classes with an 'L' in the 'DualQ-Coupled' column would not be
expected to have the ECT(1) codepoint set because they are generally
unresponsive to congestion. Nonetheless, they could coexist in the
same queue as L4S traffic because traffic in all of these classes is
expected to arrive smoothly, not in bursts of more than a few
packets. Therefore an operator could configure a DualQ Coupled AQM
to classify such packets into the L queue solely based on their DSCP,
irrespective of their ECN codepoint [I-D.ietf-tsvwg-ecn-l4s-id].
Otherwise, [I-D.ietf-tsvwg-ecn-l4s-id] requires that any other DSCP
has no effect on classification into the L queue. Thus a packet of
any other DSCP will not be classified into the L queue unless it
carries an ECT(1) or CE codepoint in the ECN field. This is shown as
'L if L4S' in in the 'DualQ-Coupled' column of Table 1.
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4. DualQ Bandwidth Pool within a Hierarchy of (Diffserv) Bandwidth
Queues
The DualQ Coupled AQM offers an L queue that provides low latency low
loss service but it pools bandwidth with the Classic (C) service as
if they shared a single FIFO. As explained earlier, unlike previous
Diffserv low latency mechanisms, the L queue can offer low latency
without needing to limit its bandwidth.
Typically the DualQ will be able to use all the bandwidth available
to a customer site, e.g. a household, a campus or a mobile node, as a
single pool. However, this section considers scenarios where the
network operator might want to carve off a fraction of a site's
bandwidth for other purposes, for instance:
1. to ensure that a particularly demanding application (e.g. a
virtual reality session) survives even if excess traffic
overloads the remainder of the site's bandwidth;
2. to give guaranteed low latency to a particular application (e.g.
industrial process control), if the statistically assured low
latency of the L queue is insufficiently stable;
3. to provide a bandwidth scavenger service that will have no effect
on any other applications at the site, but will scavenge any
unused bandwidth, for instance to transfer backups or large data
sets.
In all cases, it is assumed that the DualQ has to be able to borrow
back any of the carved off bandwidth that is unused by the other
service.
The following three subsections present solutions for each of the
above scenarios. Depending on the reader's viewpoint, each scenario
can be seen as:
o either taking a queue within an existing Diffserv hierarchy and
splitting it into L4S and Classic queues;
o or building a queuing hierarchy around a pre-existing dual L4S/
Classic queue.
In each case, the DualQ remains as an indivisible 'atomic' component
as if it were a single queue with a single pool of bandwidth (but
that can either be used for low latency or classic service).
The three examples represent the three main ways that this queue-like
'atom' can be included in a hierarchy of other queues. Without loss
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of generality only one other queue complements the DualQ in each
case, but it would be straightforward to extend the examples with
more queues.
Although these examples are framed in the context of IP and Diffserv,
similar queuing hierarchies could be constructed at a lower layer, as
long as it supported a similar capability to ECN and a similar
Traffic Class identifier to Diffserv.
4.1. DualQ Complemented by an Assured Bandwidth Service
Figure 1 shows a DualQ complemented by an additional queue to add a
bandwidth assured service. It is assumed that the operator
classifies certain packets into the assured bandwidth queue, perhaps
by class of service, source address or 5-tuple flow ID.
---------+--+
Assured b/w | |-----------.
---------+--+ \ Weighted
w\.-.scheduler
, -----------++ ( )--->
| L .->||---. /`-'
DualQ | -------/---++ c\.-. /
b/w < (Coupling ( )--'
pool | ----+--\----+ /`-'Conditional
| C | \ |---' priority
` ----+-------+ scheduler
Figure 1: How to Complement a DualQ with an Assured Bandwidth Service
The DualQ is used as if it were an indivisible 'atomic' component,
unchanged from its original description in
[I-D.ietf-tsvwg-aqm-dualq-coupled]:
o The outputs of the AQMs in the two queues (L and C) are coupled
together so that L4S sources leave enough space for C packets so
that all 'standard' flows get roughly equal throughput;
o A scheduler recombines the outputs of the two queues, giving
conditional priority to L packets (the condition prevents
starvation of the C queue if any L traffic misbehaves).
A weighted scheduler, e.g. weighted round robin (WRR), is used to
combine the outputs of the assured bandwidth queue and the DualQ. It
is configured with weight w for the assured bandwidth queue. Then,
packets requesting assured bandwidth will have priority access to
fraction w of the link capacity. However, whenever the assured
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bandwidth queue is idle or under-utilized, the DualQ can borrow the
balance of the bandwidth. Likewise the assured bandwidth queue can
borrow more than fraction w if the DualQ under-utilizes its remaining
share.
Note that a weighted scheduler such as WRR can be used to implement
the conditional priority scheduler between the L and C queues.
However, the system will not work as intended if the two weighted
schedulers in series are replaced by a single three-input weighted
scheduler. This is because, whenever one queue under-uses its
weighted share, a weighted scheduler allows the other queue to borrow
unused capacity. Whenever traffic is present in the C queue, the
coupling ensures that L traffic makes space for it by underutiliizing
its share of the first scheduler. If the assured bandwidth queue was
also served by the same scheduler, the assured bandwidth service
would continually borrow the spare capacity left by the L queue that
was intended for the C queue.
The assured bandwidth service could itself also support applications
using low latency low loss and scalable throughput (L4S). This would
be done by serving assured bandwidth traffic with a DualQ (Figure 2)
and, as usual, confining legacy queue-building traffic to the C
queue.
, -----------++ Conditional
| L .->||---. priority
Assured | -------/---++ c\.-.scheduler
b/w < (Coupling ( )--.
| ----+--\----+ /`-' \
| C | \ |---' \ Weighted
` ----+-------+ w\.-.scheduler
( )--->
, -----------++ /`-'
| L .->||---. /
DualQ | -------/---++ c\.-. /
b/w < (Coupling ( )--'
pool | ----+--\----+ /`-'Conditional
| C | \ |---' priority
` ----+-------+ scheduler
Figure 2: How to Complement a DualQ with an Assured Bandwidth Service
that also Supports L4S
The symmetry of Figure 2 reveals that both DualQs actually have
assured bandwidth. Nonetheless, the label 'Assured bandwidth' is
only really meaningful from a per-application perspective if the
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traffic classified into that DualQ is limited to a small number of
application sessions at any one time.
4.2. DualQ Complemented by a Guaranteed Low Latency Service
Figure 3 shows a DualQ complemented by an additional queue to add a
guaranteed latency service. It is assumed that the operator
classifies certain packets into the guaranteed latency queue, perhaps
by class of service, source address or 5-tuple flow ID.
o Token bucket
| o |rate/burst limiter
|___|
|___| -----------++
Guaranteed low latency||-----------.
-----------++ \ Priority
1\.-.scheduler
, -----------++ ( )--->
| L .->||---. /`-'
DualQ | -------/---++ c\.-. /
b/w < (Coupling ( )--'
pool | ----+--\----+ /`-'Conditional
| C | \ |---' priority
` ----+-------+ scheduler
Figure 3: How to Complement a DualQ with a Guaranteed Low Latency
Service
As in all the previous example, the DualQ is used as if it were an
indivisible 'atomic' component.
A strict priority scheduler is used to combine the outputs of the
guaranteed latency queue and the DualQ. Guaranteed low latency
traffic is shown as subject to a token bucket that limits rate and
tightly limits burst size, which ensures that:
o Excessive guaranteed latency traffic cannot abuse its priority and
cause the DualQ to starve;
o Guaranteed latency traffic cannot ruin its own latency guarantees
- it has to keep to a the traffic contract enforced by the token
bucket.
In a traditional Diffserv architecture, the token bucket would be
deployed at the ingress network edge, to limit traffic at each entry
point. Alternatively, the token bucket could be deployed directly in
front of the queue, where it would only limit the total traffic from
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all entry points to the network. For an access link into a network,
these two alternative would amount to the same thing.
Whenever the guaranteed latency queue is idle or under-utilized, the
DualQ can borrow the balance of the bandwidth. However, the
guaranteed latency queue cannot borrow more than the token bucket
allows, even if the DualQ under-utilizes its remaining share.
4.3. DualQ Complemented by a Scavenger Service
Figure 3 shows a DualQ complemented by an additional queue to add a
bandwidth scavenger service. It is assumed that the operator
classifies certain packets into the scavenger queue, probably by
class of service, e.g. the global-use Lower Effort (LE) Diffserv
codepoint [I-D.ietf-tsvwg-le-phb].
, -----------++ Conditional
| L .->||---. priority
DualQ | -------/---++ c\.-.scheduler
b/w < (Coupling ( )--.
pool | ----+--\----+ /`-' \ Priority
| C | \ |---' 1\.-.scheduler
` ----+-------+ ( )--->
/`-'
-+-----------+ /
Bandwidth|scavenger |-----------'
-+-----------+
Figure 4: How to Complement a DualQ with a Bandwidth Scavenger
Service
As in all the previous example, the DualQ is used as if it were an
indivisible 'atomic' component.
A strict priority scheduler is used to combine the outputs of the
DualQ and the scavenger service. Section 2 of
[I-D.ietf-tsvwg-le-phb] suggests alternative mechanisms.
Whenever the DualQ is idle or under-utilized, the scavenger service
can borrow the balance of the bandwidth. In contrast to the previous
guaranteed latency example, no rate limiter is needed on the DualQ
because, by definition, the scavenger service is expected to starve
if the higher priority service is using all the capacity.
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5. Coupling More than Two AQMs within a Bandwidth Pool
The Diffserv Assured Forwarding (AF) classes of service [RFC2597] use
an AQM with differently weighted outputs, e.g. WRED, to provide
weighted congestion feedback to the transport layer. Flows
classified to use a higher weight AQM each take more of the available
capacity, because the weighted AQM has fooled their congestion
controller into detecting that the bottleneck is more lightly loaded.
A similar mechanism can be used to add throughput differentiation to
either or both of the queues within a DualQ. Figure 5 illustrates an
example with an AQM offering three weights within the L queue, where
L1 gets the highest throughput per flow. It would be a matter of
operator policy to choose which of the three L4S AQMs the Classic AQM
would couple to. If it were coupled to L3, then C and L3 flows would
get roughly equal throughput, while L2 and L1 flows would get more.
, -----------++
| L1 ||
| L2 ||--.
| L3 .-> || \
DualQ | -----/-----++ c\.-.
b/w < ( Coupling ( )--->
pool | ----+\------+ /`-'Conditional
| C | \ |---' priority
` ----+-------+ scheduler
Figure 5: Coupling the Classic AQM to Multiple L4S AQMs
Note: this structure seems straightforward to implement, but the
authors are not aware of any implementation or evaluation of AQMs
that are both weighted and coupled to other AQMs.
6. Applicability of Coupled AQM to Global Diffserv PHBs
As has been explained, Diffserv always divides up bandwidth and
divides up latency along the same lines as a consequence, whereas the
DualQ Coupled AQM solely provides latency separation without
bandwidth separation (the idea being that bandwidth separation can be
added if needed, using Diffserv mechanisms).
In this draft so far, various queuing structures have been described
in terms of the way they separate bandwidth and latency. Operators
with existing Diffserv deployments may put the question the other way
round and ask whether the DualQ Coupled AQM can be used to isolate
low latency traffic within the bandwidth allocated to one of the
standardized Diffserv PHBs. For instance:
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o Bandwidth has been allocated to Network Control traffic, but some
BGP speakers have been upgraded to a low latency Scalable TCP
while others still use Classic TCP. However it's not possible to
predict how much bandwidth one or the other needs at any one time.
So it would be useful to isolate the low latency BGP and all the
control signalling from the delay caused by the legacy BGP
speaker, without having to decide how to carve up the Network
Control bandwidth.
o Bandwidth has been allocated to Assured Forwarding (AF) traffic
but it all shares the same WRED queue and therefore all suffers
the same delay. So it would be useful to isolate the AF traffic
that supports low latency congestion control from the rest.
However, again, it is not possible to predict how many flows of
each type there will be at any one time.
Table 2 lists all the PHBs with standardized global-use DSCPs from
[RFC4594] and the right-hand 'Latency Separation?' column identifies
all those that could benefit from an unknowable and variable fraction
of their traffic being separated between ultra-low and regular delay
using a DualQ Coupled AQM. There is no implication that it is
sensible to do this in any of the cases; just that it is possible.
For convenience, the 'Mechanism' column also answers the question
"How do PHBs for the global-use DSCPs map to the scenarios in this
draft?"
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+---------------+-----------+------+---------------+----------------+
| Service Class | DSCP Name | AQM? | Mechanism | Latency |
| Name | | | | Separation? |
+---------------+-----------+------+---------------+----------------+
| Network | CS7 | Y&N | Figure 1 or | Y |
| Control | | | Figure 2 | |
| Network | CS6 | Y&N | Figure 1 or | Y |
| Control | | | Figure 2 | |
| OAM | CS2 | Y&N | Figure 1 or | Y |
| | | | Figure 2 | |
| Signalling | CS5 | N | Figure 1 | N |
| Telephony | EF | N | Section 4.2 | N |
| RFC 5865 | VA | N | Section 4.2 | N |
| R-T | CS4 | N | Figure 2 | Y |
| Interactive | | | | |
| MM | AF4n | Y | Section 5 | Y |
| Conferencing | | | | |
| Broadcast | CS3 | N | Figure 2 | N |
| Video | | | | |
| MM Streaming | AF3n | Y | Section 5 | Y |
| Low Latency | AF2n | Y | Section 5 | Y |
| Data | | | | |
| High Thru'put | AF1n | Y | Section 5 | Y |
| Data | | | | |
| Standard | BE/DF/CS0 | Y | Section 3 | Y |
| Low Priority | LE | Y | Section 4.3 | n/a |
| Data | | | | |
+---------------+-----------+------+---------------+----------------+
Table 2: Applicability of a Coupled AQM to RFC4594 Diffserv PHBs
7. Best Practice for Classification and Marking
7.1. Never Re-Mark a DSCP
It is not a DualQ's job to alter Diffserv codepoints to attempt to
make other downstream AQMs classify selected packets in certain ways.
Each DualQ Coupled AQM is independently (but hopefully consistently)
configured to select certain DSCPs for classification into the L
queue. It never alters the DSCP nor the ECN codepoint (except
setting CE to indicate that congestion was experienced)
[I-D.ietf-tsvwg-aqm-dualq-coupled].
7.2. Classification Order
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7.2.1. Classification Order: Problem
The above wide range of possible structures raises the question of
which order it would be more efficient for classifier rules to take:
DSCP before ECN, ECN before DSCP or some hybrid.
On the one hand, for a structure like that in Figure 1 it would make
sense to classify on DSCP first, then ECN. Otherwise, if packets
were classified on ECN first, an extra merge stage would be required
because the assured bandwidth queue handles all ECN codepoints for a
particular DSCP.
On the other hand, for a structure like that in Figure 5 it would
make sense to classify on ECN first, then DSCP. Otherwise, again an
extra merge stage would be needed, because the C queue handles all
DSCPs but only some ECN codepoints.
A hybrid of these two scenarios would be possible, for instance where
the L queue in Figure 1 was further broken down into three weighted
AQMs, as in Figure 5. In this case, the ideal matching order would
be DSCP, ECN, DSCP.
7.2.2. Classification Order: Solutions
Probably the most straightforward solution would be to classify in a
single stage over all 8 octets of the IPv6 Traffic Class field or the
former IPv4 TOS octet, irrespective of the boundary between the 6-bit
DS field and the 2-bit ECN field [RFC3260]. As long as hardware
supports this, it will be possible because all the inputs to the
queues are at the same level of hierarchy, even though the outputs
form a multi-level hierarchy of schedulers in some cases.
Pre-existing classifier hardware might consider the 6-bit and 2-bit
fields as separate. Then it would seem most efficient for the order
of the classifiers to depend on the structure of the queues being
classified (given the structure has to have been designed before the
classifiers are designed).
8. Policing and Traffic Conditioning
{ToDo: L4S latency policing is discussed in the Security
Considerations section of [I-D.ietf-tsvwg-l4s-arch]. This section
will compare Diffserv traffic conditioning with L4S latency
policing.}
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9. IANA Considerations
This specification contains no IANA considerations.
10. Security Considerations
{ToDo}
11. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group mailing list
<tsvwg@ietf.org>, and/or to the authors.
12. Acknowledgements
Thanks to Greg White, David Black, Wes Eddy and Gorry Fairhurst for
their useful discussions prior to this -00 draft.
13. References
13.1. 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>.
13.2. Informative References
[I-D.ietf-tsvwg-aqm-dualq-coupled]
Schepper, K., Briscoe, B., Bondarenko, O., and I. Tsang,
"DualQ Coupled AQMs for Low Latency, Low Loss and Scalable
Throughput (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-07
(work in progress), October 2018.
[I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K. and B. Briscoe, "Identifying Modified
Explicit Congestion Notification (ECN) Semantics for
Ultra-Low Queuing Delay (L4S)", draft-ietf-tsvwg-ecn-l4s-
id-03 (work in progress), July 2018.
[I-D.ietf-tsvwg-l4s-arch]
Briscoe, B., Schepper, K., and M. Bagnulo, "Low Latency,
Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture", draft-ietf-tsvwg-l4s-arch-03 (work in
progress), October 2018.
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[I-D.ietf-tsvwg-le-phb]
Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
draft-ietf-tsvwg-le-phb-06 (work in progress), October
2018.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[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>.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<https://www.rfc-editor.org/info/rfc3246>.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
<https://www.rfc-editor.org/info/rfc3260>.
[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>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
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[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>.
[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
October 2017, <https://www.rfc-editor.org/info/rfc8257>.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.
Appendix A. Open Issues
o The Abstract promises "in which cases one can stand alone without
needing the other". That's in the text, but not highlighted as
such.
o Document Roadmap TBA
o Mapping to 802.11 user priorities (or LTE QCIs)? Not strictly
within the scope, but perhaps desirable to add, or at least to
mention how L4S (experimental) would affect RFC8325 which gives
(standards track) mappings between Diffserv and 802.11.
o Identify L4S-friendly rate policers
o Comparison between L4S policing and Diffserv traffic conditioning
is TBA
o Security Considerations are TBA (largely depends on the previous
bullet)
Author's Address
Bob Briscoe
CableLabs
UK
Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
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