One document matched: draft-ietf-tsvwg-admitted-realtime-dscp-05.txt
Differences from draft-ietf-tsvwg-admitted-realtime-dscp-04.txt
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: May 5, 2009 November 1, 2008
DSCP for Capacity-Admitted Traffic
draft-ietf-tsvwg-admitted-realtime-dscp-05
Status of this Memo
By submitting this Internet-Draft, each author represents that any
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This Internet-Draft will expire on May 5, 2009.
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
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involving authentication, authorization, and capacity admission.
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 . . . . . . . . . . . . . . . . 9
2.3. Recommendations on implementation of an Admitted
Telephony Service Class . . . . . . . . . . . . . . . . . 10
3. Summary: changes from RFC 4594 . . . . . . . . . . . . . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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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.
These applications have one or more potential congestion points
between the endpoints. Reserving capacity for them 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, such as the non-preemptive
Emergency Telecommunication Service, the US Department of Defense's
Assured Service (which is similar to Multi-Level Precedence and
Preemption [ITU.MLPP.1990] procedure), or e-911, in addition to
normal routine calls that use call admission. It is possible to use
control plane protocols to generally restrict session admission such
that admitted traffic should receive the desired service, and the
policy (e.g. Routine, National Security or Emergency Preparedness
[NS/EP] communications, e-911, etc) need not be signaled in a DSCP.
However, 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.
Instead, the recommended "best practice" is to perform admission
control for all traffic in three of [RFC4594]'s video classes: the
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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
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 or faster), one would expect all
of this traffic to be carried in the Real Time Traffic 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 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).
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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.
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 enqueues 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 may 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].
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This requires the use of capacity admission to differentiate among
users (which might be 911 call centers, other offices with
preferential service contracts, or individual users gaining access
with special credentials) 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 both queues, 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 queue, 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.
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.
policers priorities |`.
Unadmitted EF <=> ----------||----+ `.
high| `.
Admitted EF <=> ----------||----+ .'-----------
. medium .'
rate queues |`. +-----+ .' Priority
AF1------>||----+ `. / low |' Scheduler
| `. /
AF2------>||----+ .'-+
| .'
CS0------>||----+ .' Rate Scheduler
|' (WFQ, WRR, etc)
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 .
Unadmitted EF <=> -------\ |`.
--||----+ `.
Admitted 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 conaining 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
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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
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 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
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 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
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to be greedy (eg., it doesn't care). If the network operator is
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
network.
The problem with mechanisms that don't enable the association of a
policy with the request is that they don't allow for multi-policy
services, which are becoming important.
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
It is the belief of the authors that either PHB implementation
described in Section 2.1, if coupled with adequate AAA and capacity
admission procedures as described in Section 2.2, are 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 at this time exist for bandwidth brokers, NSIS
has not at this time been finalized and in any event is limited to
unicast sessions, and that RSVP has been standardized and has the
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relevant services. We therefore recommend the use of RSVP at the
UNI. Procedures at the NNI are business matters to be discussed
between the relevant networks, and are 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
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 value for the code point is 101100,
paralleling the EF code point, which is 101110. The code point
should be referred to as VOICE-ADMIT.
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 "routine" or "priority".
This capability, one has to believe, will be abused by script kiddies
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and others if the proof of identity is not adequately strong 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...
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.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
August 2006.
7.2. Informative References
[ITU.MLPP.1990]
International Telecommunications Union, "Multilevel
Precedence and Preemption Service", ITU-T Recommendation
I.255.3, 1990.
[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
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Definition of the EF PHB (Expedited Forwarding Per-Hop
Behavior)", RFC 3247, March 2002.
[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.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
DiffServ Service Classes", RFC 5127, February 2008.
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
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Martin Dolly
AT&T Labs
Middletown Township, New Jersey 07748
USA
Phone: +1-732-420-4574
Email: mdolly@att.com
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