draft-ietf-tsvwg-admitted-realtime-dscp-05.txt   draft-ietf-tsvwg-admitted-realtime-dscp-06.txt 
Transport Working Group F. Baker Transport Working Group F. Baker
Internet-Draft J. Polk Internet-Draft J. Polk
Updates: 4542,4594 Cisco Systems Updates: 4542,4594 Cisco Systems
(if approved) M. Dolly (if approved) M. Dolly
Intended status: Standards Track AT&T Labs Intended status: Standards Track AT&T Labs
Expires: May 5, 2009 November 1, 2008 Expires: June 4, 2010 December 4, 2009
DSCP for Capacity-Admitted Traffic DSCP for Capacity-Admitted Traffic
draft-ietf-tsvwg-admitted-realtime-dscp-05 draft-ietf-tsvwg-admitted-realtime-dscp-06
Abstract
This document requests one Differentiated Services Code Point (DSCP)
from the Internet Assigned Numbers Authority (IANA) for real-time
traffic classes similar to voice conforming to the Expedited
Forwarding Per Hop Behavior, and admitted using a call admission
procedure involving authentication, authorization, and capacity
admission.
This document also recommends that certain classes of video traffic
described in RFC 4594 and which have similar requirements be changed
to require admission using a Call Admission Control (CAC) procedure
involving authentication, authorization, and capacity admission.
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any This Internet-Draft is submitted to IETF in full conformance with
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have been or will be disclosed, and any of which he or she becomes
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This Internet-Draft will expire on May 5, 2009. This Internet-Draft will expire on June 4, 2010.
Abstract Copyright Notice
This document requests one Differentiated Services Code Point (DSCP) Copyright (c) 2009 IETF Trust and the persons identified as the
from the Internet Assigned Numbers Authority (IANA) for real-time document authors. All rights reserved.
traffic classes similar to voice conforming to the Expedited
Forwarding Per Hop Behavior, and admitted using a call admission
procedure involving authentication, authorization, and capacity
admission.
This document also recommends that certain classes of video traffic This document is subject to BCP 78 and the IETF Trust's Legal
described in RFC 4594 and which have similar requirements be changed Provisions Relating to IETF Documents
to require admission using a Call Admission Control (CAC) procedure (http://trustee.ietf.org/license-info) in effect on the date of
involving authentication, authorization, and capacity admission. publication of this document. Please review these documents
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Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Problem . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Problem . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Candidate Implementations of the Admitted Telephony 2. Candidate Implementations of the Admitted Telephony
Service Class . . . . . . . . . . . . . . . . . . . . . . . . 7 Service Class . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Potential implementations of EF in this model . . . . . . 7 2.1. Potential implementations of EF in this model . . . . . . 7
2.2. Capacity admission control . . . . . . . . . . . . . . . . 9 2.2. Capacity admission control . . . . . . . . . . . . . . . 9
2.3. Recommendations on implementation of an Admitted 2.3. Recommendations on implementation of an Admitted
Telephony Service Class . . . . . . . . . . . . . . . . . 10 Telephony Service Class . . . . . . . . . . . . . . . . . 10
3. Summary: changes from RFC 4594 . . . . . . . . . . . . . . . . 11 3. Summary: changes from RFC 4594 . . . . . . . . . . . . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12 7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12 7.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
1. Introduction 1. Introduction
This document requests one Differentiated Services Code Point (DSCP) This document requests one Differentiated Services Code Point (DSCP)
from the Internet Assigned Numbers Authority (IANA) for a class of from the Internet Assigned Numbers Authority (IANA) for a class of
real-time traffic. This class conforms to the Expedited Forwarding real-time traffic. This class conforms to the Expedited Forwarding
[RFC3246] [RFC3247] Per Hop Behavior. It is also admitted using a [RFC3246] [RFC3247] Per Hop Behavior. It is also admitted using a
CAC procedure involving authentication, authorization, and capacity CAC procedure involving authentication, authorization, and capacity
admission. This differs from a real-time traffic class conforming to admission. This differs from a real-time traffic class conforming
the Expedited Forwarding Per Hop Behavior but not subject to capacity to the Expedited Forwarding Per Hop Behavior but not subject to
admission or subject to very coarse capacity admission. capacity admission or subject to very coarse capacity admission.
It also recommends that certain classes of video described in It also recommends that certain classes of video described in
[RFC4594] be treated as requiring capacity admission as well. [RFC4594] be treated as requiring capacity admission as well.
These applications have one or more potential congestion points These applications have one or more potential congestion points
between the endpoints. Reserving capacity for them is important to between the endpoints. Reserving capacity for them is important to
application performance. All of these applications have low application performance. All of these applications have low
tolerance to jitter (aka delay variation) and loss, as summarized in tolerance to jitter (aka delay variation) and loss, as summarized in
Section 2, and most (except for multimedia conferencing) have Section 2, and most (except for multimedia conferencing) have
inelastic flow behavior from Figure 1 of [RFC4594]. Inelastic flow inelastic flow behavior from Figure 1 of [RFC4594]. Inelastic flow
skipping to change at page 3, line 38 skipping to change at page 3, line 38
One of the reasons behind this is the need for classes of traffic One of the reasons behind this is the need for classes of traffic
that are handled under special policies, such as the non-preemptive that are handled under special policies, such as the non-preemptive
Emergency Telecommunication Service, the US Department of Defense's Emergency Telecommunication Service, the US Department of Defense's
Assured Service (which is similar to Multi-Level Precedence and Assured Service (which is similar to Multi-Level Precedence and
Preemption [ITU.MLPP.1990] procedure), or e-911, in addition to Preemption [ITU.MLPP.1990] procedure), or e-911, in addition to
normal routine calls that use call admission. It is possible to use normal routine calls that use call admission. It is possible to use
control plane protocols to generally restrict session admission such control plane protocols to generally restrict session admission such
that admitted traffic should receive the desired service, and the that admitted traffic should receive the desired service, and the
policy (e.g. Routine, National Security or Emergency Preparedness policy (e.g. Routine, National Security or Emergency Preparedness
[NS/EP] communications, e-911, etc) need not be signaled in a DSCP. [NS/EP] communications, e-911, etc) need not be signaled in a DSCP.
However, service providers need to distinguish between special-policy However, service providers need to distinguish between
traffic and other classes, particularly the existing VoIP services special-policy traffic and other classes, particularly the existing
that perform no capacity admission or only very coarse capacity VoIP services that perform no capacity admission or only very coarse
admission and can exceed their allocated resources. capacity admission and can exceed their allocated resources.
The requested DSCP applies to the Telephony Service Class described The requested DSCP applies to the Telephony Service Class described
in [RFC4594]. in [RFC4594].
Since video classes have not had the history of mixing admitted and Since video classes have not had the history of mixing admitted and
non-admitted traffic in the same Per-Hop Behavior (PHB) as has non-admitted traffic in the same Per-Hop Behavior (PHB) as has
occurred for EF, an additional DSCP code point is not recommended. occurred for EF, an additional DSCP code point is not recommended.
Instead, the recommended "best practice" is to perform admission Instead, the recommended "best practice" is to perform admission
control for all traffic in three of [RFC4594]'s video classes: the control for all traffic in three of [RFC4594]'s video classes: the
o Interactive Real-Time Traffic (CS4, used for Video conferencing o Interactive Real-Time Traffic (CS4, used for Video conferencing
and Interactive gaming), and Interactive gaming),
o Broadcast TV (CS3) for use in a video on demand context, and o Broadcast TV (CS3) for use in a video on demand context, and
o AF4 Multimedia Conferencing (video conferencing). o AF4 Multimedia Conferencing (video conferencing).
Other video classes are believed to not have the current problem of Other video classes are believed to not have the current problem of
confusion with unadmitted traffic and therefore would not benefit confusion with unadmitted traffic and therefore would not benefit
from the notion of a separate DSCP for admitted traffic. Within an 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 ISP and on inter-ISP links (i.e. within networks whose internal
are uniform at hundreds of megabits or faster), one would expect all paths are uniform at hundreds of megabits or faster), one would
of this traffic to be carried in the Real Time Traffic Class expect all of this traffic to be carried in the Real Time Traffic
described in [RFC5127]. (RTP) Class described in [RFC5127].
1.1. Definitions 1.1. Definitions
The following terms and acronyms are used in this document. The following terms and acronyms are used in this document.
PHB: A Per-Hop-Behavior (PHB) is the externally observable PHB: A Per-Hop-Behavior (PHB) is the externally observable
forwarding behavior applied at a Differentiated Services compliant forwarding behavior applied at a Differentiated Services
node to a DS behavior aggregate [RFC2475]. It may be thought of compliant node to a DS behavior aggregate [RFC2475]. It may be
as a program configured on the interface of an Internet host or thought of as a program configured on the interface of an
router, specified drop probabilities, queuing priorities or rates, Internet host or router, specified drop probabilities, queuing
and other handling characteristics for the traffic class. priorities or rates, and other handling characteristics for the
traffic class.
DSCP: The Differentiated Services Code Point (DSCP), as defined in DSCP: The Differentiated Services Code Point (DSCP), as defined in
[RFC2474], is a value which is encoded in the DS field, and which [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 each DS Node MUST use to select the PHB which is to be
by each packet it forwards [RFC3260]. It is a 6-bit number experienced by each packet it forwards [RFC3260]. It is a 6-bit
embedded into the 8-bit TOS field of an IPv4 datagram or the number embedded into the 8-bit TOS field of an IPv4 datagram or
Traffic Class field of an IPv6 datagram. the Traffic Class field of an IPv6 datagram.
CAC: Call Admission Control includes concepts of authorization and CAC: Call Admission Control includes concepts of authorization and
capacity admission. "Authorization" refers to any procedure that capacity admission. "Authorization" refers to any procedure that
identifies a user, verifies the authenticity of the identifies a user, verifies the authenticity of the
identification, and determines whether the user is authorized to identification, and determines whether the user is authorized to
use the service under the relevant policy. "Capacity Admission" use the service under the relevant policy. "Capacity Admission"
refers to any procedure that determines whether capacity exists refers to any procedure that determines whether capacity exists
supporting a session's requirements under some policy. supporting a session's requirements under some policy.
In the Internet, these are separate functions, while in the PSTN In the Internet, these are separate functions, while in the PSTN
they and call routing are carried out together. they and call routing are carried out together.
UNI: A User/Network Interface (UNI) is the interface (often a UNI: A User/Network Interface (UNI) is the interface (often a
physical link or its virtual equivalent) that connects two physical link or its virtual equivalent) that connects two
entities that do not trust each other, and in which one (the user) entities that do not trust each other, and in which one (the
purchases connectivity services from the other (the network). user) purchases connectivity services from the other (the
network).
Figure 1 shows two user networks connected by what appears to each Figure 1 shows two user networks connected by what appears to
of them to be a single network ("The Internet", access to which is each of them to be a single network ("The Internet", access to
provided by their service provider) that provides connectivity which is provided by their service provider) that provides
services to other users. connectivity services to other users.
UNIs tend to be the bottlenecks in the Internet, where users UNIs tend to be the bottlenecks in the Internet, where users
purchase relatively low amounts of bandwidth for cost or service purchase relatively low amounts of bandwidth for cost or service
reasons, and as a result are most subject to congestion issues and reasons, and as a result are most subject to congestion issues
therefore issues requiring traffic conditioning and service and therefore issues requiring traffic conditioning and service
prioritization. prioritization.
NNI: A Network/Network Interface (NNI) is the interface (often a NNI: A Network/Network Interface (NNI) is the interface (often a
physical link or its virtual equivalent) that connects two physical link or its virtual equivalent) that connects two
entities that trust each other within limits, and in which the two entities that trust each other within limits, and in which the
are seen as trading services for value. Figure 1 shows three two are seen as trading services for value. Figure 1 shows three
service networks that together provide the connectivity services service networks that together provide the connectivity services
that we call "the Internet". They are different administrations that we call "the Internet". They are different administrations
and are very probably in competition, but exchange contracts for and are very probably in competition, but exchange contracts for
connectivity and capacity that enable them to offer specific connectivity and capacity that enable them to offer specific
services to their customers. services to their customers.
NNIs may not be bottlenecks in the Internet if service providers NNIs may not be bottlenecks in the Internet if service providers
contractually agree to provision excess capacity at them, as they contractually agree to provision excess capacity at them, as they
commonly do. However, NNI performance may differ by ISP, and the commonly do. However, NNI performance may differ by ISP, and the
performance guarantee interval may range from a month to a much performance guarantee interval may range from a month to a much
shorter period. Furthermore, a peering point NNI may not have shorter period. Furthermore, a peering point NNI may not have
contractual performance guarantees or may become overloaded under contractual performance guarantees or may become overloaded under
certain conditions. They are also policy-controlled interfaces, certain conditions. They are also policy-controlled interfaces,
especially in BGP. As a result, they may require traffic especially in BGP. As a result, they may require traffic
prioritization policy. prioritization policy.
Queue: There are multiple ways to build a multi-queue scheduler. Queue: There are multiple ways to build a multi-queue scheduler.
Weighted Round Robin (WRR) literally builds multiple lists and Weighted Round Robin (WRR) literally builds multiple lists and
visits them in a specified order, while a calendar queue (often visits them in a specified order, while a calendar queue (often
used to implement Weighted Fair Queuing, or WFQ) builds a list for used to implement Weighted Fair Queuing, or WFQ) builds a list
each time interval and enqueues at most a stated amount of data in for each time interval and queues at most a stated amount of data
each such list for transmission during that time interval. While in each such list for transmission during that time interval.
these differ dramatically in implementation, the external While these differ dramatically in implementation, the external
difference in behavior is generally negligible when they are difference in behavior is generally negligible when they are
properly configured. Consistent with the definitions used in the properly configured. Consistent with the definitions used in the
Differentiated Services Architecture [RFC2475], these are treated Differentiated Services Architecture [RFC2475], these are treated
as equivalent in this document, and the lists of WRR and the as equivalent in this document, and the lists of WRR and the
classes of a calendar queue will be referred to uniformly as classes of a calendar queue will be referred to uniformly as
"queues". "queues".
_.--------. _.--------.
,-'' `--. ,-'' `--.
,-' `-. ,-' `-.
skipping to change at page 6, line 41 skipping to change at page 6, line 41
`-. ,-' `-. ,-'
`--. _.-' `--. _.-'
`--------'' `--------''
Figure 1: UNI and NNI interfaces Figure 1: UNI and NNI interfaces
1.2. Problem 1.2. Problem
In short, the Telephony Service Class described in [RFC4594] permits In short, the Telephony Service Class described in [RFC4594] permits
the use of capacity admission in implementing the service, but the use of capacity admission in implementing the service, but
present implementations either provide no capacity admission services present implementations either provide no capacity admission
or do so in a manner that depends on specific traffic engineering. services or do so in a manner that depends on specific traffic
In the context of the Internet backbone, the two are essentially engineering. In the context of the Internet backbone, the two are
equivalent; the edge network depends on specific engineering by the essentially equivalent; the edge network depends on specific
service provider that may not be present, especially in a mobile engineering by the service provider that may not be present,
environment. especially in a mobile environment.
However, services are being requested of the network that would However, services are being requested of the network that would
specifically make use of capacity admission, and would distinguish specifically make use of capacity admission, and would distinguish
among users or the uses of available Voice-over-IP or Video-over-IP among users or the uses of available Voice-over-IP or Video-over-IP
capacity in various ways. Various agencies would like to provide capacity in various ways. Various agencies would like to provide
services as described in section 2.6 of [RFC4504] or in [RFC4190]. services as described in section 2.6 of [RFC4504] or in [RFC4190].
This requires the use of capacity admission to differentiate among This requires the use of capacity admission to differentiate among
users (which might be 911 call centers, other offices with users (which might be 911 call centers, other offices with
preferential service contracts, or individual users gaining access preferential service contracts, or individual users gaining access
skipping to change at page 7, line 33 skipping to change at page 7, line 31
o Multiple data plane traffic classes, each consisting of a policer o Multiple data plane traffic classes, each consisting of a policer
but feeding into a common queue or multiple queues at the same but feeding into a common queue or multiple queues at the same
priority. priority.
We will explain the difference, and describe in what way they differ We will explain the difference, and describe in what way they differ
in operation. The reason this is necessary is that there is current in operation. The reason this is necessary is that there is current
confusion in the industry. confusion in the industry.
The multi-priority model is shown in Figure 2. In this model, The multi-priority model is shown in Figure 2. In this model,
traffic from each service class is placed into a separate priority traffic from each service class is placed into a separate priority
queue. If data is present in both queues, traffic from one of them queue. If data is present in more than one queue, traffic from one
will always be selected for transmission. This has the effect of of them will always be selected for transmission. This has the
transferring jitter from the higher priority queue to the lower effect of transferring jitter from the higher priority queue to the
priority queue, and reordering traffic in a way that gives the higher lower priority queue, and reordering traffic in a way that gives the
priority traffic a smaller average queuing delay. Each queue must higher priority traffic a smaller average queuing delay. Each queue
have its own policer, however, to protect the network from errors and must have its own policer, however, to protect the network from
attacks; if a traffic class thinks it is carrying a certain data rate errors and attacks; if a traffic class thinks it is carrying a
but an abuse sends significantly more, the effect of simple certain data rate but an abuse sends significantly more, the effect
prioritization would not preserve the lower priorities of traffic, of simple prioritization would not preserve the lower priorities of
which could cause routing to fail or otherwise impact an SLA. traffic, which could cause routing to fail or otherwise impact an
SLA.
. .
policers priorities |`. policers priorities |`.
Unadmitted EF <=> ----------||----+ `. Admitted EF <=> ----------||----+ `.
high| `. high| `.
Admitted EF <=> ----------||----+ .'----------- Unadmitted EF <=> ----------||----+ .'-----------
. medium .' . medium .'
rate queues |`. +-----+ .' Priority rate queues |`. +-----+ .' Priority
AF1------>||----+ `. / low |' Scheduler AF1------>||----+ `. / low |' Scheduler
| `. / | `. /
AF2------>||----+ .'-+ AF2------>||----+ .'-+
| .' | .'
CS0------>||----+ .' Rate Scheduler CS0------>||----+ .' Rate Scheduler
|' (WFQ, WRR, etc) |' (WFQ, WRR, etc)
Figure 2: Implementation as a data plane priority Figure 2: Implementation as a data plane priority
The multi-policer model is shown in Figure 3. In this model, traffic The multi-policer model is shown in Figure 3. In this model,
from each service class is policed according to its SLA requirements, traffic from each service class is policed according to its SLA
and then placed into a common priority queue. Unlike the multi- requirements, and then placed into a common priority queue. Unlike
priority model, the jitter experienced by the traffic classes in this the multi-priority model, the jitter experienced by the traffic
case is the same, as there is only one queue, but the sum of the classes in this case is the same, as there is only one queue, but
traffic in this higher priority queue experiences less average jitter the sum of the traffic in this higher priority queue experiences
than the elastic traffic in the lower priority. less average jitter than the elastic traffic in the lower priority.
policers priorities . policers priorities .
Unadmitted EF <=> -------\ |`. Admitted EF <=> -------\ |`.
--||----+ `. --||----+ `.
Admitted EF <=> -------/ high| `. Unadmitted EF <=> -------/ high| `.
. | .'-------- . | .'--------
rate queues |`. +-----+ .' rate queues |`. +-----+ .'
AF1------>||----+ `. / low | .' Priority AF1------>||----+ `. / low | .' Priority
| `. / |' Scheduler | `. / |' Scheduler
AF2------>||----+ .'-+ AF2------>||----+ .'-+
| .' | .'
CS0------>||----+ .' Rate Scheduler CS0------>||----+ .' Rate Scheduler
|' (WFQ, WRR, etc) |' (WFQ, WRR, etc)
Figure 3: Implementation as a data plane policer Figure 3: Implementation as a data plane policer
The difference between the two operationally is, as stated, the The difference between the two operationally is, as stated, the
issues of loss due to policing and distribution of jitter. issues of loss due to policing and distribution of jitter.
If the two traffic classes are, for example, voice and video, If the two traffic classes are, for example, voice and video,
datagrams conaining video data can be relatively large (often of datagrams containing video data can be relatively large (often of
variable sizes up to the path MTU) while datagrams containing voice variable sizes up to the path MTU) while datagrams containing voice
are relatively small, on the order of only 40 to 200 bytes, depending are relatively small, on the order of only 40 to 200 bytes,
on the codec. On lower speed links (less than 10 MBPS), the jitter depending on the codec. On lower speed links (less than 10 MBPS),
introduced by video to voice can be disruptive, while at higher the jitter introduced by video to voice can be disruptive, while at
speeds the jitter is nominal compared to the jitter requirements of higher speeds the jitter is nominal compared to the jitter
voice. At access network speeds, therefore, [RFC4594] recommends requirements of voice. At access network speeds, therefore,
separation of video and voice into separate queues, while at optical [RFC4594] recommends separation of video and voice into separate
speeds [RFC5127] recommends that they use a common queue. 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 If, on the other hand, the two traffic classes are carrying the same
type of application with the same jitter requirements, then giving type of application with the same jitter requirements, then giving
one preference in this sense does not benefit the higher priority one preference in this sense does not benefit the higher priority
traffic and may harm the lower priority traffic. In such a case, traffic and may harm the lower priority traffic. In such a case,
using separate policers and a common queue is a superior approach. using separate policers and a common queue is a superior approach.
2.2. Capacity admission control 2.2. Capacity admission control
There are at least six major ways that capacity admission is done or There are at least six major ways that capacity admission is done or
skipping to change at page 9, line 36 skipping to change at page 9, line 34
o Capacity admission control by call counting, o Capacity admission control by call counting,
o End-point capacity admission performed by probing the network, o End-point capacity admission performed by probing the network,
o Centralized capacity admission control via bandwidth broker, and o Centralized capacity admission control via bandwidth broker, and
o Distributed capacity admission control using protocols such as o Distributed capacity admission control using protocols such as
RSVP or NSIS. RSVP or NSIS.
The problem with capacity admission by assumption, which is widely
reployed 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 dropping traffic to force users to hang up is that 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 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 and the N+1 calls are active, data is dropped randomly from all
sessions to ensure that offered load doesn't exceed capacity. On sessions to ensure that offered load doesn't exceed capacity. On
very fast links, that is acceptable, but on lower speed links it can very fast links, that is acceptable, but on lower speed links it can
seriously affect call quality. 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 There are two fundamental problems with depending on the endpoint to
perform capacity admission; it may not be able to accurately measure perform capacity admission; it may not be able to accurately measure
the impact of the traffic it generates on the network, and it tends the impact of the traffic it generates on the network, and it tends
to be greedy (eg., it doesn't care). If the network operator is 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 providing a service, he must be able to guarantee the service, which
means that he can't trust systems that are not controlled by his means that he cannot trust systems that are not controlled by his
network. network.
The problem with mechanisms that don't enable the association of a The problem with capacity controls via a bandwidth broker is
policy with the request is that they don't allow for multi-policy 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. services, which are becoming important.
The operator's choice of admission procedure must, for this DSCP, The operator's choice of admission procedure MUST, for this DSCP,
ensure the following: ensure the following:
o The actual links that a session uses have enough bandwidth to o The actual links that a session uses have enough bandwidth to
support it. support it.
o New sessions are refused admission if there is inadequate o New sessions are refused admission if there is inadequate
bandwidth under the relevant policy. bandwidth under the relevant policy.
o If multiple policies are in use in a network, that the user is o If multiple policies are in use in a network, that the user is
identified and the correct policy applied. identified and the correct policy applied.
o Under periods of network stress, the process of admission of new o Under periods of network stress, the process of admission of new
sessions does not disrupt existing sessions, unless the service sessions does not disrupt existing sessions, unless the service
explicitly allows for disruption of calls. explicitly allows for disruption of calls.
2.3. Recommendations on implementation of an Admitted Telephony Service 2.3. Recommendations on implementation of an Admitted Telephony
Class Service Class
It is the belief of the authors that either PHB implementation It is the belief of the authors that either PHB implementation
described in Section 2.1, if coupled with adequate AAA and capacity described in Section 2.1, if coupled with adequate AAA and capacity
admission procedures as described in Section 2.2, are sufficient to admission procedures as described in Section 2.2, are sufficient to
provide the services required for an Admitted Telephony service provide the services required for an Admitted Telephony service
class. If preemption is required, as described in section 2.3.5.2 of class. If preemption is required, as described in section 2.3.5.2
[RFC4542], this provides the tools for carrying out the preemption. of [RFC4542], this provides the tools for carrying out the
If preemption is not in view, or if used in addition to preemptive preemption. If preemption is not in view, or if used in addition to
services, the application of different thresholds depending on call preemptive services, the application of different thresholds
precedence has the effect of improving the probability of call depending on call precedence has the effect of improving the
completion by admitting preferred calls at a time that other calls probability of call completion by admitting preferred calls at a
are being refused. Routine and priority traffic can be admitted time that other calls are being refused. Routine and priority
using the same DSCP value, as the choice of which calls are admitted traffic can be admitted using the same DSCP value, as the choice of
is handled in the admission procedure executed in the control plane, which calls are admitted is handled in the admission procedure
not the policing of the data plane. executed in the control plane, not the policing of the data plane.
On the point of what protocols and procedures are required for On the point of what protocols and procedures are required for
authentication, authorization, and capacity admission, we note that authentication, authorization, and capacity admission, we note that
clear standards do not at this time exist for bandwidth brokers, NSIS clear standards do not exist at this time for bandwidth brokers,
has not at this time been finalized and in any event is limited to NSIS has not been finalized at this time and in any event is limited
unicast sessions, and that RSVP has been standardized and has the to unicast sessions, and that RSVP has been standardized and has the
relevant services. We therefore recommend the use of RSVP at the relevant services. We therefore recommend the use of RSVP at the
UNI. Procedures at the NNI are business matters to be discussed UNI. Procedures at the NNI are business matters to be discussed
between the relevant networks, and are recommended but not required. between the relevant networks, and are recommended but not required.
3. Summary: changes from RFC 4594 3. Summary: changes from RFC 4594
To summarize, there are two changes to [RFC4594] discussed in this To summarize, there are two changes to [RFC4594] discussed in this
document: document:
Telephony class: The Telephony Service Class in RFC 4594 does not Telephony class: The Telephony Service Class in RFC 4594 does not
involve capacity admission, but depends on application layer involve capacity admission, but depends on application layer
admission that only estimates capacity, and that through static admission that only estimates capacity, and that through static
engineering. In addition to that class, a separate Admitted engineering. In addition to that class, a separate Admitted
Telephony Class is added which performs capacity admission Telephony Class is added which performs capacity admission
dynamically. dynamically.
Video classes Capacity admission is added to three video classes. Video classes: Capacity admission is added to three video classes.
These are the Interactive Real-Time Traffic class, Broadcast TV These are the Interactive Real-Time Traffic class, Broadcast TV
class when used for video on demand, and the Multimedia class when used for video on demand, and the Multimedia
Conferencing class. Conferencing class.
4. IANA Considerations 4. IANA Considerations
This note requests that IANA assign a DSCP value to a second EF This note requests that IANA assign a DSCP value to a second EF
traffic class consistent with [RFC3246] and [RFC3247] in the traffic class consistent with [RFC3246] and [RFC3247] in the
"Differentiated Services Field Codepoints" registry. It implements "Differentiated Services Field Codepoints" registry. It implements
the Telephony Service Class described in [RFC4594] at lower speeds the Telephony Service Class described in [RFC4594] at lower speeds
and is included in the Real Time Treatment Aggregate [RFC5127] at and is included in the Real Time Treatment Aggregate [RFC5127] at
higher speeds. The recommended value for the code point is 101100, higher speeds. The recommended value for the code point is 101100,
paralleling the EF code point, which is 101110. The code point paralleling the EF code point, which is 101110. The code point
should be referred to as VOICE-ADMIT. should be referred to as VOICE-ADMIT.
This traffic class requires the use of capacity admission such as This traffic class requires the use of capacity admission such as
RSVP services together with AAA services at the User/Network RSVP services together with AAA services at the User/Network
Interface (UNI); the use of such services at the NNI is at the option Interface (UNI); the use of such services at the NNI is at the
of the interconnected networks. option of the interconnected networks.
5. Security Considerations 5. Security Considerations
A major requirement of this service is effective use of a signaling A major requirement of this service is effective use of a signaling
protocol such as RSVP, with the capabilities to identify its user protocol such as RSVP, with the capabilities to identify its user
either as an individual or as a member of some corporate entity, and either as an individual or as a member of some corporate entity, and
assert a policy such as "routine" or "priority". assert a policy such as "routine" or "priority".
This capability, one has to believe, will be abused by script kiddies This capability, one has to believe, will be abused by script
and others if the proof of identity is not adequately strong or if kiddies and others if the proof of identity is not adequately strong
policies are written or implemented improperly by the carriers. This or if policies are written or implemented improperly by the
goes without saying, but this section is here for it to be said... carriers. This goes without saying, but this section is here for it
to be said...
6. Acknowledgements 6. Acknowledgements
Kwok Ho Chan, Georgios Karagiannis, Dan Voce, and Bob Briscoe Kwok Ho Chan, Georgios Karagiannis, Dan Voce, and Bob Briscoe
commented and offered text. The impetus for including Video in the commented and offered text. The impetus for including Video in the
discussion, which initially only targeted voice, is from Dave discussion, which initially only targeted voice, is from Dave
McDysan, McDysan.
7. References 7. References
7.1. Normative References 7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS "Definition of the Differentiated Services Field (DS
skipping to change at page 13, line 19 skipping to change at page 13, line 14
[RFC4190] Carlberg, K., Brown, I., and C. Beard, "Framework for [RFC4190] Carlberg, K., Brown, I., and C. Beard, "Framework for
Supporting Emergency Telecommunications Service (ETS) in Supporting Emergency Telecommunications Service (ETS) in
IP Telephony", RFC 4190, November 2005. IP Telephony", RFC 4190, November 2005.
[RFC4504] Sinnreich, H., Lass, S., and C. Stredicke, "SIP Telephony [RFC4504] Sinnreich, H., Lass, S., and C. Stredicke, "SIP Telephony
Device Requirements and Configuration", RFC 4504, Device Requirements and Configuration", RFC 4504,
May 2006. May 2006.
[RFC4542] Baker, F. and J. Polk, "Implementing an Emergency [RFC4542] Baker, F. and J. Polk, "Implementing an Emergency
Telecommunications Service (ETS) for Real-Time Services in Telecommunications Service (ETS) for Real-Time Services
the Internet Protocol Suite", RFC 4542, May 2006. in the Internet Protocol Suite", RFC 4542, May 2006.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of [RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
DiffServ Service Classes", RFC 5127, February 2008. DiffServ Service Classes", RFC 5127, February 2008.
Authors' Addresses Authors' Addresses
Fred Baker Fred Baker
Cisco Systems Cisco Systems
Santa Barbara, California 93117 Santa Barbara, California 93117
USA USA
skipping to change at page 14, line 4 skipping to change at page 13, line 37
Phone: +1-408-526-4257 Phone: +1-408-526-4257
Email: fred@cisco.com Email: fred@cisco.com
James Polk James Polk
Cisco Systems Cisco Systems
Richardson, Texas 75082 Richardson, Texas 75082
USA USA
Phone: +1-817-271-3552 Phone: +1-817-271-3552
Email: jmpolk@cisco.com Email: jmpolk@cisco.com
Martin Dolly Martin Dolly
AT&T Labs AT&T Labs
Middletown Township, New Jersey 07748 Middletown Township, New Jersey 07748
USA USA
Phone: +1-732-420-4574 Phone: +1-732-420-4574
Email: mdolly@att.com Email: mdolly@att.com
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