A Differentiated Services Code Point (DSCP) for Capacity-Admitted Traffic
draft-ietf-tsvwg-admitted-realtime-dscp-07
The information below is for an old version of the document that is already published as an RFC.
| Document | Type | RFC Internet-Draft (tsvwg WG) | |
|---|---|---|---|
| Authors | James Polk , Fred Baker , Martin Dolly | ||
| Last updated | 2015-10-14 (Latest revision 2010-03-08) | ||
| Replaces | draft-baker-tsvwg-admitted-voice-dscp | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Reviews | |||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | RFC 5865 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Magnus Westerlund | ||
| Send notices to | (None) |
draft-ietf-tsvwg-admitted-realtime-dscp-07
Transport Working Group F. Baker
Internet-Draft J. Polk
Updates: 4542,4594 Cisco Systems
(if approved) M. Dolly
Intended status: Standards Track AT&T Labs
Expires: Sept 8, 2010 March 8, 2010
DSCP for Capacity-Admitted Traffic
draft-ietf-tsvwg-admitted-realtime-dscp-07
Abstract
This document requests one Differentiated Services Code Point (DSCP)
from the Internet Assigned Numbers Authority (IANA) for a class of
real-time traffic. This class conforms to the Expedited Forwarding
Per Hop Behavior. It is also admitted using a CAC procedure
involving authentication, authorization, and capacity admission.
This differs from a real-time traffic class conforming to the
Expedited Forwarding Per Hop Behavior but not subject to capacity
admission or subject to very coarse capacity admission.
Legal
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FOR A PARTICULAR PURPOSE.
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Copyright Notice
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than English.
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].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Problem . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Candidate Implementations of the Admitted Telephony
Service Class . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Potential implementations of EF in this model . . . . . . 7
2.2. Capacity admission control . . . . . . . . . . . . . . . 8
2.3. Recommendations on implementation of an Admitted
Telephony Service Class . . . . . . . . . . . . . . . . . 10
3. Summary: changes from RFC 4594 . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
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6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document requests one Differentiated Services Code Point (DSCP)
from the Internet Assigned Numbers Authority (IANA) for a class of
real-time traffic. This class conforms to the Expedited Forwarding
[RFC3246] [RFC3247] Per Hop Behavior. It is also admitted using a
CAC procedure involving authentication, authorization, and capacity
admission. This differs from a real-time traffic class conforming
to the Expedited Forwarding Per Hop Behavior but not subject to
capacity admission or subject to very coarse capacity admission.
It also recommends that certain classes of video described in
[RFC4594] be treated as requiring capacity admission as well.
Real-time traffic flows have one or more potential congestion points
between the endpoints. Reserving capacity for these flows is
important to application performance. All of these applications
have low tolerance to jitter (aka delay variation) and loss, as
summarized in Section 2, and most (except for multimedia
conferencing) have inelastic flow behavior from Figure 1 of
[RFC4594]. Inelastic flow behavior and low jitter/loss tolerance
are the service characteristics that define the need for admission
control behavior.
One of the reasons behind this is the need for classes of traffic
that are handled under special policies. Service providers need to
distinguish between special-policy traffic and other classes,
particularly the existing VoIP services that perform no capacity
admission or only very coarse capacity admission and can exceed
their allocated resources.
The requested DSCP applies to the Telephony Service Class described
in [RFC4594].
Since video classes have not had the history of mixing admitted and
non-admitted traffic in the same Per-Hop Behavior (PHB) as has
occurred for EF, an additional DSCP code point is not recommended
within this document for video. Instead, the recommended "best
practice" is to perform admission control for all traffic in three
of [RFC4594]'s video classes: the
o Interactive Real-Time Traffic (CS4, used for Video conferencing
and Interactive gaming),
o Broadcast TV (CS3) for use in a video on demand context, and
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o AF4 Multimedia Conferencing (video conferencing).
Other video classes are believed to not have the current problem of
confusion with unadmitted traffic and therefore would not benefit
from the notion of a separate DSCP for admitted traffic. Within an
ISP and on inter-ISP links (i.e. within networks whose internal
paths are uniform at hundreds of megabits per second or faster), one
would expect all of this traffic to be carried in the Real-Time
Traffic (RTP) Class described in [RFC5127].
1.1. Definitions
The following terms and acronyms are used in this document.
PHB: A Per-Hop-Behavior (PHB) is the externally observable
forwarding behavior applied at a Differentiated Services
compliant node to a DS behavior aggregate [RFC2475]. It may be
thought of as a program configured on the interface of an
Internet host or router, specified in terms of drop
probabilities, queuing priorities or rates, and other handling
characteristics for the traffic class.
DSCP: The Differentiated Services Code Point (DSCP), as defined in
[RFC2474], is a value which is encoded in the DS field, and which
each DS Node MUST use to select the PHB which is to be
experienced by each packet it forwards [RFC3260]. It is a 6-bit
number embedded into the 8-bit TOS field of an IPv4 datagram or
the Traffic Class field of an IPv6 datagram.
CAC: Call Admission Control includes concepts of authorization and
capacity admission. "Authorization" refers to any procedure that
identifies a user, verifies the authenticity of the
identification, and determines whether the user is authorized to
use the service under the relevant policy. "Capacity Admission"
refers to any procedure that determines whether capacity exists
supporting a session's requirements under some policy.
In the Internet, these are separate functions, while in the PSTN
they and call routing are carried out together.
UNI: A User/Network Interface (UNI) is the interface (often a
physical link or its virtual equivalent) that connects two
entities that do not trust each other, and in which one (the
user) purchases connectivity services from the other (the
network).
Figure 1 shows two user networks connected by what appears to
each of them to be a single network ("The Internet", access to
which is provided by their service provider) that provides
connectivity services to other users.
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UNIs tend to be the bottlenecks in the Internet, where users
purchase relatively low amounts of bandwidth for cost or service
reasons, and as a result are most subject to congestion issues
and therefore issues requiring traffic conditioning and service
prioritization.
NNI: A Network/Network Interface (NNI) is the interface (often a
physical link or its virtual equivalent) that connects two
entities that trust each other within limits, and in which the
two are seen as trading services for value. Figure 1 shows three
service networks that together provide the connectivity services
that we call "the Internet". They are different administrations
and are very probably in competition, but exchange contracts for
connectivity and capacity that enable them to offer specific
services to their customers.
NNIs may not be bottlenecks in the Internet if service providers
contractually agree to provision excess capacity at them, as they
commonly do. However, NNI performance may differ by ISP, and the
performance guarantee interval may range from a month to a much
shorter period. Furthermore, a peering point NNI may not have
contractual performance guarantees or may become overloaded under
certain conditions. They are also policy-controlled interfaces,
especially in BGP. As a result, they may require traffic
prioritization policy.
Queue: There are multiple ways to build a multi-queue scheduler.
Weighted Round Robin (WRR) literally builds multiple lists and
visits them in a specified order, while a calendar queue (often
used to implement Weighted Fair Queuing, or WFQ) builds a list
for each time interval and queues at most a stated amount of data
in each such list for transmission during that time interval.
While these differ dramatically in implementation, the external
difference in behavior is generally negligible when they are
properly configured. Consistent with the definitions used in the
Differentiated Services Architecture [RFC2475], these are treated
as equivalent in this document, and the lists of WRR and the
classes of a calendar queue will be referred to uniformly as
"queues".
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_.--------.
,-'' `--.
,-' `-.
,-------. ,',-------. `.
,' `. ,',' `. `.
/ User \ UNI / / Service \ \
( Network +-----+ Network ) `.
\ / ; \ / :
`. ,' ; `. .+ :
'-------' / '-------' \ NNI \
; \ :
; "The Internet" \ ,-------. :
; +' `. :
UNI: User/Network Interface / Service \ |
| ( Network ) |
NNI: Network/Network Interface \ / |
: +. ,' ;
: / '-------' ;
: / ;
,-------. \ ,-------. / NNI /
,' `. : ,' `+ ;
/ User \ UNI / Service \ ;
( Network +-----+ Network ) ,'
\ / \ \ / /
`. ,' `.`. ,' ,'
'-------' `.'-------' ,'
`-. ,-'
`--. _.-'
`--------''
Figure 1: UNI and NNI interfaces
1.2. Problem
In short, the Telephony Service Class described in [RFC4594] permits
the use of capacity admission in implementing the service, but
present implementations either provide no capacity admission
services or do so in a manner that depends on specific traffic
engineering. In the context of the Internet backbone, the two are
essentially equivalent; the edge network depends on specific
engineering by the service provider that might not be present,
especially in a mobile environment.
However, services are being requested of the network that would
specifically make use of capacity admission, and would distinguish
among users or the uses of available Voice-over-IP or Video-over-IP
capacity in various ways. Various agencies would like to provide
services as described in section 2.6 of [RFC4504] or in [RFC4190].
This requires the use of capacity admission to differentiate among
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users to provide services to them that are not afforded to
non-capacity admitted customer-to-customer IP telephony sessions.
2. Candidate Implementations of the Admitted Telephony Service Class
2.1. Potential implementations of EF in this model
There are at least two possible ways to implement isolation between
the Capacity Admitted PHB and the Expedited Forwarding PHB in this
model. They are to implement separate classes as a set of
o Multiple data plane traffic classes, each consisting of a policer
and a queue, and the queues enjoying different priorities, or
o Multiple data plane traffic classes, each consisting of a policer
but feeding into a common queue or multiple queues at the same
priority.
We will explain the difference, and describe in what way they differ
in operation. The reason this is necessary is that there is current
confusion in the industry.
The multi-priority model is shown in Figure 2. In this model,
traffic from each service class is placed into a separate priority
queue. If data is present in more than one queue, traffic from one
of them will always be selected for transmission. This has the
effect of transferring jitter from the higher priority queue to the
lower priority queues, and reordering traffic in a way that gives
the higher priority traffic a smaller average queuing delay. Each
queue must have its own policer, however, to protect the network
from errors and attacks; if a traffic class thinks it is carrying a
certain data rate but an abuse sends significantly more, the effect
of simple prioritization would not preserve the lower priorities of
traffic, which could cause routing to fail or otherwise impact an
SLA.
.
policers priorities |`.
Admitted EF <=> ----------||----+ `.
high| `.
Unadmitted EF <=> ----------||----+ .'-----------
. medium .'
rate queues |`. +-----+ .' Priority
AF1------>||----+ `. / low |' Scheduler
| `. /
AF2------>||----+ .'-+
| .'
CS0------>||----+ .' Rate Scheduler
|' (WFQ, WRR, etc)
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Figure 2: Implementation as a data plane priority
The multi-policer model is shown in Figure 3. In this model,
traffic from each service class is policed according to its SLA
requirements, and then placed into a common priority queue. Unlike
the multi-priority model, the jitter experienced by the traffic
classes in this case is the same, as there is only one queue, but
the sum of the traffic in this higher priority queue experiences
less average jitter than the elastic traffic in the lower priority.
policers priorities .
Admitted EF <=> -------\ |`.
--||----+ `.
Unadmitted EF <=> -------/ high| `.
. | .'--------
rate queues |`. +-----+ .'
AF1------>||----+ `. / low | .' Priority
| `. / |' Scheduler
AF2------>||----+ .'-+
| .'
CS0------>||----+ .' Rate Scheduler
|' (WFQ, WRR, etc)
Figure 3: Implementation as a data plane policer
The difference between the two operationally is, as stated, the
issues of loss due to policing and distribution of jitter.
If the two traffic classes are, for example, voice and video,
datagrams containing video data can be relatively large (often of
variable sizes up to the path MTU) while datagrams containing voice
are relatively small, on the order of only 40 to 200 bytes,
depending on the codec. On lower speed links (less than 10 MBPS),
the jitter introduced by video to voice can be disruptive, while at
higher speeds the jitter is nominal compared to the jitter
requirements of voice. At access network speeds, therefore,
[RFC4594] recommends separation of video and voice into separate
queues, while at optical speeds [RFC5127] recommends that they use a
common queue.
If, on the other hand, the two traffic classes are carrying the same
type of application with the same jitter requirements, then giving
one preference in this sense does not benefit the higher priority
traffic and may harm the lower priority traffic. In such a case,
using separate policers and a common queue is a superior approach.
2.2. Capacity admission control
There are at least six major ways that capacity admission is done or
has been proposed to be done for real-time applications. Each will
be described below, then Section 3 will judge which ones are likely
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to meet the requirements of the Admitted Telephony service class.
These include:
o Drop Precedence used to force sessions to voluntarily exit,
o Capacity admission control by assumption or engineering,
o Capacity admission control by call counting,
o End-point capacity admission performed by probing the network,
o Centralized capacity admission control via bandwidth broker, and
o Distributed capacity admission control using protocols such as
RSVP or NSIS.
The problem with dropping traffic to force users to hang up is that
it affects a broad class of users - if there is capacity for N calls
and the N+1 calls are active, data is dropped randomly from all
sessions to ensure that offered load doesn't exceed capacity. On
very fast links, that is acceptable, but on lower speed links it can
seriously affect call quality. There is also a behavioral issue
involved here, in which users who experience poor quality calls tend
to hang up and call again, making the problem better - then worse.
The problem with capacity admission by assumption, which is widely
deployed in today's VoIP environment, is that it depends on the
assumptions made. One can do careful traffic engineering to ensure
needed bandwidth, but this can also be painful, and has to be
revisited when the network is changed or network usage changes.
The problem with call counting based admission control is it gets
exponentially worse the farther you get from the control point
(e.g., it lacks sufficient scalability out into the network).
There are two fundamental problems with depending on the endpoint to
perform capacity admission; it may not be able to accurately measure
the impact of the traffic it generates on the network, and it tends
to be greedy (e.g., it doesn't care). If the network operator is
providing a service, he must be able to guarantee the service, which
means that he cannot trust systems that are not controlled by his
network.
The problem with capacity controls via a bandwidth broker is
centralized servers lack far away awareness, and also lack effective
real-time reaction to dynamic changes in all part of the network
at all instances of time.
The problem with mechanisms that do not enable the association of a
policy with the request is that they do not allow for multi-policy
services, which are becoming important.
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The operator's choice of admission procedure MUST, for this DSCP,
ensure the following:
o The actual links that a session uses have enough bandwidth to
support it.
o New sessions are refused admission if there is inadequate
bandwidth under the relevant policy.
o If multiple policies are in use in a network, that the user is
identified and the correct policy applied.
o Under periods of network stress, the process of admission of new
sessions does not disrupt existing sessions, unless the service
explicitly allows for disruption of calls.
2.3. Recommendations on implementation of an Admitted Telephony
Service Class
When coupled with adequate AAA and capacity admission procedures as
described in Section 2.2, either of the two PHB implementations
described in Section 2.1 is sufficient to provide the services
required for an Admitted Telephony service class. If preemption is
required, as described in section 2.3.5.2 of [RFC4542], this
provides the tools for carrying out the preemption. If preemption is
not in view, or if used in addition to preemptive services, the
application of different thresholds depending on call precedence has
the effect of improving the probability of call completion by
admitting preferred calls at a time that other calls are being
refused. Routine and priority traffic can be admitted using the
same DSCP value, as the choice of which calls are admitted is
handled in the admission procedure executed in the control plane,
not the policing of the data plane.
On the point of what protocols and procedures are required for
authentication, authorization, and capacity admission, we note that
clear standards do not exist at this time for bandwidth brokers,
NSIS has not been finalized at this time and in any event is limited
to unicast sessions, and that RSVP has been standardized and has the
relevant services. We therefore RECOMMEND the use of a protocol,
such as RSVP, at the UNI. Procedures at the NNI are business
matters to be discussed between the relevant networks, and are
I RECOMMENDED but NOT REQUIRED.
3. Summary: changes from RFC 4594
To summarize, there are two changes to [RFC4594] discussed in this
document:
Telephony class: The Telephony Service Class in RFC 4594 does not
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involve capacity admission, but depends on application layer
admission that only estimates capacity, and that through static
engineering. In addition to that class, a separate Admitted
Telephony Class is added which performs capacity admission
dynamically.
Video classes: Capacity admission is added to three video classes.
These are the Interactive Real-Time Traffic class, Broadcast TV
class when used for video on demand, and the Multimedia
Conferencing class.
4. IANA Considerations
This note requests that IANA assign a DSCP value to a second EF
traffic class consistent with [RFC3246] and [RFC3247] in the
"Differentiated Services Field Codepoints" registry. It implements
the Telephony Service Class described in [RFC4594] at lower speeds
and is included in the Real Time Treatment Aggregate [RFC5127] at
higher speeds. The recommended code point value should be from pool
1 within the dscp-registry. This document RECOMMENDS retaining a
parallel with the existing EF code point (101110) by assigning a
value for the code point of 101100 -- keeping the (left to right)
first 4 binary values the same in both. The code point described
within this document should be referred to as VOICE-ADMIT. Here is
the recommended addition to the Pool 1 Codepoint registry:
Sub-registry: Pool 1 Codepoints
Reference: [RFC2474]
Registration Procedures: Standards Action
Registry:
Name Space Reference
--------- ------- ---------
VOICE-ADMIT 101100 [this document]
This traffic class REQUIRES the use of capacity admission, such as
RSVP services together with AAA services, at the User/Network
Interface (UNI); the use of such services at the NNI is at the
option of the interconnected networks.
5. Security Considerations
A major requirement of this service is effective use of a signaling
Protocol, such as RSVP, with the capabilities to identify its user
either as an individual or as a member of some corporate entity, and
assert a policy such as "normal", "routine" or some level of
"priority".
This capability, one has to believe, will be abused by script
kiddies and others if the proof of identity is not adequately strong
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or if policies are written or implemented improperly by the
carriers. This goes without saying, but this section is here for it
to be said...
Much of the security considerations from RFC 3246 [RFC3246] applies
to this document, as well as the security considerations in RFC
2474 and RFC 4542. RFC 4230 [RFC4230] analyzes RSVP, providing some
gap analysis to the NSIS WG as they started their work. Keep in mind
that this document is advocating RSVP at the UNI only, while RFC
4230 discusses (mostly) RSVP from a more complete point of view
(i.e., e2e and edge2edge). When considering the RSVP aspect of this
document, understanding Section 6 of RFC 4230 is a good source of
information.
6. Acknowledgements
Kwok Ho Chan, Georgios Karagiannis, Dan Voce, and Bob Briscoe
commented and offered text. The impetus for including Video in the
discussion, which initially only targeted voice, is from Dave
McDysan.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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,
December 1998.
[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, March 2002.
7.2. Informative References
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC3247] Charny, A., Bennet, J., Benson, K., Boudec, J., Chiu, A.,
Courtney, W., Davari, S., Firoiu, V., Kalmanek, C., and K.
Ramakrishnan, "Supplemental Information for the New
Definition of the EF PHB (Expedited Forwarding Per-Hop
Behavior)", RFC 3247, March 2002.
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[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[RFC4190] Carlberg, K., Brown, I., and C. Beard, "Framework for
Supporting Emergency Telecommunications Service (ETS) in
IP Telephony", RFC 4190, November 2005.
[RFC4504] Sinnreich, H., Lass, S., and C. Stredicke, "SIP Telephony
Device Requirements and Configuration", RFC 4504,
May 2006.
[RFC4542] Baker, F. and J. Polk, "Implementing an Emergency
Telecommunications Service (ETS) for Real-Time Services
in the Internet Protocol Suite", RFC 4542, May 2006.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
August 2006.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
DiffServ Service Classes", RFC 5127, February 2008.
[RFC4230] H. Tschofenig, R. Graveman, "RSVP Security Properties",
RFC4230, December 2005
Authors' Addresses
Fred Baker
Cisco Systems
Santa Barbara, California 93117
USA
Phone: +1-408-526-4257
Email: fred@cisco.com
James Polk
Cisco Systems
Richardson, Texas 75082
USA
Phone: +1-817-271-3552
Email: jmpolk@cisco.com
Martin Dolly
AT&T Labs
Middletown Township, New Jersey 07748
USA
Phone: +1-732-420-4574
Email: mdolly@att.com
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