Transport Working Group F. Baker
Internet-Draft J. Polk
Expires: August 11, 2005 Cisco Systems
February 7, 2005
MLEF Without Capacity Admission Does Not Satisfy MLPP Requirements
draft-ietf-tsvwg-mlef-concerns-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The Defense Information Systems Agency of the United States
Department of Defense, with its contractors, has proposed a service
architecture for military (NATO and related agencies) telephone
systems. This is called the Assured Service, and is defined in two
documents: "Architecture for Assured Service Capabilities in Voice
over IP" and "Requirements for Assured Service Capabilities in Voice
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over IP". Responding to these are three documents: "Extending the
Session Initiation Protocol Reason Header to account for Preemption
Events", "Communications Resource Priority for the Session Initiation
Protocol", and the "Multi-Level Expedited Forwarding Per Hop
Behavior" (MLEF PHB). MLEF, as currently defined, has serious
problems, which this draft seeks to discuss.
In short, our concern is that the Assured Service attempts to
implement MLPP in the Internet Architecture, but fails due to its
proposed implementation. It operates on the premise that packet
loss, rather than call loss, is sufficiently analogous to MLPP's
services for military use, and that if a caller cannot make himself
clear on the telephone, the caller will hang up and perform another
task. But the current TDM environment has trained the military
caller to expect that low call quality is a fault in the telephone
system, not an indication of the presence of higher priority calls.
The logical expectation is not that the caller will hang up and go
away; it is, especially under stressful conditions, that he or she
will hang up and call again.
MLEF does not satisfy the MLPP requirements for end user experience.
It can cause a breakdown in communications, increasing the likelihood
of grave consequences especially at times of crisis.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Multi-Level Preemption and Precedence . . . . . . . . . . 4
1.2 Multi-Level Expedited Forwarding . . . . . . . . . . . . . 6
2. The problem with MLEF . . . . . . . . . . . . . . . . . . . . 7
2.1 Codecs are not infinitely resilient to loss . . . . . . . 8
2.1.1 Issues with variable rate codecs . . . . . . . . . . . 9
2.2 MLEF induced packet loss severely impacts voice
quality for any affected class . . . . . . . . . . . . . . 9
2.3 Packet loss happens in tactical situations . . . . . . . . 10
2.4 MLEF increases end to end delay, can add jitter, and
can interfere with other traffic classes . . . . . . . . . 10
2.5 MLEF induced loss triggers congestive collapse . . . . . . 11
2.6 MLEF gives no preemption feedback notification . . . . . . 11
3. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 13
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1 Normative References . . . . . . . . . . . . . . . . . . . 17
7.2 Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . 20
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1. Overview
The Defense Information Systems Agency of the United States
Department of Defense, with is contractors, has proposed a service
architecture for military (NATO and related agencies) telephone
systems. This is called the Assured Service, and is defined in two
documents: [I-D.pierce-ieprep-assured-service-arch] and
[I-D.pierce-ieprep-assured-service-req]. Responding to these are
three documents: [I-D.ietf-sipping-reason-header-for-preemption],
[I-D.ietf-sip-resource-priority], and
[I-D.silverman-diffserv-mlefphb] (MLEF PHB). MLEF, as currently
defined, has serious problems, which this draft seeks to discuss.
1.1 Multi-Level Preemption and Precedence
Let us discuss the problem that MLEF is intended to solve and the
architecture of the system. The Assured Service is designed as an IP
implementation of an existing ITU-T/NATO/DoD telephone system
architecture known as Multilevel Precedence and Preemption
[ITU.MLPP.1990][ANSI.MLPP.Spec][ANSI.MLPP.Supplement], or MLPP. MLPP
is an architecture for a prioritized call handling service such that
in times of emergency in the relevant NATO and DoD commands, the
relative importance of various kinds of communications is strictly
defined, allowing higher priority communication at the expense of
lower priority communications. These priorities, in descending
order, are:
Flash Override Override: used by the Commander in Chief, Secretary of
Defense, and Joint Chiefs of Staff, Commanders of combatant
commands when declaring the existence of a state of war.
Commanders of combatant commands when declaring Defense Condition
One or Defense Emergency or Air Defense Emergency and other
national authorities that the President may authorize in
conjunction with Worldwide Secure Voice Conferencing System
conferences. Flash Override Override cannot be preempted.
Flash Override: used by the Commander in Chief, Secretary of Defense,
and Joint Chiefs of Staff, Commanders of combatant commands when
declaring the existence of a state of war. Commanders of
combatant commands when declaring Defense Condition One or Defense
Emergency and other national authorities the President may
authorize. Flash Override cannot be preempted in the DSN.
Flash: reserved generally for telephone calls pertaining to command
and control of military forces essential to defense and
retaliation, critical intelligence essential to national survival,
conduct of diplomatic negotiations critical to the arresting or
limiting of hostilities, dissemination of critical civil alert
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information essential to national survival, continuity of federal
government functions essential to national survival, fulfillment
of critical internal security functions essential to national
survival, or catastrophic events of national or international
significance.
Immediate: reserved generally for telephone calls pertaining to
situations that gravely affect the security of national and allied
forces, reconstitution of forces in a post-attack period,
intelligence essential to national security, conduct of diplomatic
negotiations to reduce or limit the threat of war, implementation
of federal government actions essential to national survival,
situations that gravely affect the internal security of the
nation, Civil Defense actions, disasters or events of extensive
seriousness having an immediate and detrimental effect on the
welfare of the population, or vital information having an
immediate effect on aircraft, spacecraft, or missile operations.
Priority: reserved generally for telephone calls requiring
expeditious action by called parties and/or furnishing essential
information for the conduct of government operations.
Routine: designation applied to those official government
communications that require rapid transmission by telephonic means
but do not require preferential handling.
The rule in MLPP is that more important calls override less important
calls when congestion occurs within a network. Station based
preemption is used when a more important call needs to be placed to
either party in an existing call. Trunk based preemption is used
when trunk bandwidth needs to be reallocated to facilitate a higher
precedence call over a given path in the network. In both station
and trunk based preemption scenarios, preempted parties are
positively notified, via preemption tone, that their call can no
longer be supported. The same preemption tone is used regardless of
whether calls are terminated for the purposes of station of trunk
based preemption. The remainder of this discussion focuses on trunk
based preemption issues.
MLPP is built as a proactive system in which callers must assign one
of the precedence levels listed above at call initiation; this
precedence level cannot be changed throughout that call. If
preemption is not assigned by a user at call initiation time, routine
is assumed. If there is end to end capacity to place a call, any
call may be placed at any time. However, when any trunk (in the
circuit world) or interface (in an IP world) reaches utilization
capacity, a choice must be made as to which call continues. The
system will seize the trunks or bandwidth necessary to place the more
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important calls in preference to less important calls by preempting
an existing call (or calls) of lower precedence to permit a higher
precedence call to be placed.
More than one call might properly be preempted if more trunks or
bandwidth is necessary for this higher precedence call. A video call
(perhaps of 384 KBPS, or 6 trunks) competing with several lower
precedence voice calls is a good example of this situation.
1.2 Multi-Level Expedited Forwarding
The [RFC2475] defines a capability for systems to identify traffic
they originate or qualify using [RFC2474]. These DSCP values trigger
the application of a policy in the network called a Per Hop Behavior,
or PHB.
The Multi-Level Expedited Forwarding (MLEF) PHB builds on the
[RFC3246] PHB (EF). Like EF, it posits that sufficient bandwidth is
present to support the service, and therefore places correctly marked
traffic into a low jitter queue, with a form of traffic policing at
the ingress to the queue. It differs from EF in two fundamental
ways. First, while there is generally assumed to be enough capacity
for VoIP traffic in the general case, the probability of having
insufficient capacity is sufficiently high to force network
administration to think carefully about whose traffic is most
important. To deal with this issue, the Assured Service architecture
not only identifies call precedence in the SIP/H.323 signalling to
enable an endpoint to preempt a call in favor of a higher precedence
incoming call, but MLEF marks VoIP traffic with code points
corresponding to the various MLPP precedence levels, and assigns them
different loss probabilities comparable to the behavior of the
[RFC2597] (AF). Existing non-IP MLPP networks have five or more
precedence levels, therefore five or more different MLEF code points
are required. It is assumed that an SLA will be required between
MLPP networks with differing numbers of precedence levels.
The intended effect is to permit - during congestion - a higher
precedence call to reduce the call quality of lower precedence calls
by dropping packets that exceed the total rate assigned to the
aggregate. It assumes that the loss rate is in fact nominal, and
that the users of lower precedence calls will simply go away as their
call quality fades. There is no other active feedback like that in
Section 1.1 to users who experience this loss of quality.
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2. The problem with MLEF
The problem with MLEF, in a nutshell, is that it implements a
different service than MLPP, and that service has a very different
effect. The basic function of MLPP is to cause some number of lower
precedence calls to be dropped, or not started, so that
o higher precedence calls get placed,
o remaining lower precedence calls stay at acceptable quality,
o parties on pre-empted calls receive clear feedback on why their
call is being dropped (e.g., due to pre-emption as opposed to
circuit failure or other trivial cause).
MLEF fails to achieve the second and third functions. Instead, MLEF
can create a situation where all lower precedence calls experience
reduced call quality, potentially becoming unintelligible, and thus
destroying most of the usefulness of the communications system.
[G711.2] considers a MOS/PESQ score below 3.6 to be "poor" and a MOS
score below 3.1 to be "bad". The effect of MLEF is to disrupt voice
quality (reduce MOS/PESQ scores below 3.6 and at times below 3.1) on
all calls at routine precedence and potentially other calls at the
Priority or Immediate precedence, causing their users to be unable to
conduct their business or to do so with increased difficulty.
The logical expectation of a military caller, who understands the
behavior of MLPP, who cannot place a call or whose call is clearly
preempted is that he or she will perform another task and retry the
call later. The logical expectation of a military caller is that
he/she either gets good service or no service, because that is what
he/she has gotten in the existing TDM environment. The logical
expectation of a caller who experiences degraded voice quality is not
that they will hang up and go away, however.
In a time of crisis, the rational expectation is that the caller will
attempt to continue using the service or will hang up and call again
fairly quickly since they have no (MLPP-like) audible signal
indicating that the call was preempted by lack of available
bandwidth, and since they are operating under stress. For all lower
precedence calls, in the worst case, MLEF creates congestive collapse
- 100% utilization with zero effectiveness of communication for all
calls of a certain class.
Within MLEF, there is a belief that congestion occurrences will
always be brief in time; that it is better to have momentary
interruptions in service (similar to cellular or mobile phone
service) than out right preemption events (where both parties are
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informed of the event audibly). No accounting or analysis has been
done to show that congestion events in times of military emergency
will be milliseconds to seconds long (analogous to cell phone quality
service), verses seconds to minutes (or even hours) long. The
existence of the MLPP service itself argues against this assumption:
if congestion was routinely momentary, then returning a fast busy and
expecting the calling party to call again, or simply queuing the call
until bandwidth became available, would be sufficient.
It is possible that, in an MLEF world, the commander might give the
order to "launch the fleet", but the fleet be unable to place the
order to "raise the anchor", as the latter order is given by a more
junior officer whose call precedence level may be disrupted.
It is clear that MLEF falls short and does not satisfy the MLPP
requirements for end user experience. MLEF will cause breakdown in
communications increasing the likelihood of grave consequences
especially at times of crisis.
Following subsections provide more detail on the impacts of packet
loss, codec issues and users' experience in and MLEF environment.
2.1 Codecs are not infinitely resilient to loss
The issue of concern results from the nature of real time traffic and
the effect of packet loss on known codecs.
One of the world's most common and well known codecs is G.711; it is
the codec used in standard circuit switch voice networks throughout
the PSTN. Numerous [G711.1][G711.2][G711.3][G711.4][G711.5] exist
depicting the effect of traffic loss on G.711 in ATM and IP packet
switched environments. While they differ in the details of their
findings, they generally agree that a random packet loss rate on the
order of 1-2% has a serious effect on voice quality, and higher
packet loss rates essentially place speech beyond comprehension by
the human listener. [G711.2] states that "the packet loss rate of 5%
seems to be almost the quality threshold (low boundary) of the "poor"
QoS class", which is to say the boundary between 'poor', where most
users find it disruptive, and 'bad', where all users find it
disruptive.
The resilience of G.729A and the [RFC3951] (ILBC) have also been
studied in [ILBC]. G.729A is another common VoIP codec, which
provides a lower amount of generated bandwidth and has better
resilience than G.711. ILBC generates more bandwidth than G.729A,
and less than G.711, but includes with that traffic a variable
quantity of forward error correction data, which can be used in lossy
environments to further improve voice quality in the presence of
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loss. However, like G.711, these codecs also have limits on their
resilience. In the presence of 15% loss, the ILBC reportedly loses
enough voice quality that it can be difficult to understand what it
said. [G711.3] indicates that G.729 systems drop to a MOS score
below 3.0 with 2% packet loss.
2.1.1 Issues with variable rate codecs
G.729A and ILBC are examples of codecs which increase their
throughput to carry forward error correction data when they are
experiencing loss, a behavior referred to as "protection coding".
This behavior - increasing offered load in situations where offered
load may be triggering the problem - has an additional characteristic
that will interact poorly with MLEF. Understand that this is not a
criticism of the codecs per se; as far as we know, the codecs are
fine codecs. But this characteristic has a serious side-effect in
MLEF environments.
ILBC generates on the order of 31.2 KBPS of traffic in the 20 ms
mode. However, when additional protection coding used (as in [ILBC])
in response to RTCP reports of a high level of loss, it increases its
Forward Error Correction, expanding the bandwidth of the packets to
meet acceptable voice quality to the receiving end. This protective
feature of iLBC is the result of piggybacking additional copies of
what it calls critical voice samples in other packets of other voice
samples (this is how the bandwidth increases - the effective payload
for a series of packets increases by a factor of 2). ILBC with
protection will increase its bandwidth requirements from the no
protection rate of 31.2 KBPS to 35.6 KBPS in times of a packet loss
rate of 26%. ILBC further increases its bandwidth requirement to
45.6 KBPS (to raise a PESQ-MOS value from 2.38 to 3.0) in times where
30% of packets are lost.
Thus, in any situation where a codec using protection coding
experiences difficulty due to lack of available bandwidth in an MLEF
service discipline, it can be expected to compound the difficulty.
2.2 MLEF induced packet loss severely impacts voice quality for any
affected class
While MLEF protects flows for highest priority calls, it worsens the
quality of service for all others. In a case where a large number of
higher precedence calls are being placed, such as at the "Flash"
level, this may include calls at lower but still non-routine
precedences, such as at the "Priority" level.
Telephone systems are generally provisioned with enough bandwidth for
10% or less of their customers or potential users to simultaneously
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place calls. In a small office with 250 persons in it, this means
that the ISDN access to the PSTN is often a single T-1 line, and for
larger offices a corresponding level of bandwidth is generally
available. If an Internet connection has enough bandwidth for 20
VoIP sessions, the simultaneous placement of 20 calls represents a
100% load that should be carried with at most nominal loss, but 21
calls represents a ~5% overload, and ~5% data loss may be expected to
be distributed evenly over all calls; in other words, each call may
be expected to experience 5% loss. Thus, in such a case, the
placement of a single call may be the difference between 20 routine
calls operating normally and 21 calls operating with a seriously
degraded MOS score. In larger installations, corresponding ratios
apply. In a network which protects some calls from loss, there is no
magic: the total loss will be the same, and will be concentrated on
those calls least protected.
In emergency situations, especially in command and control centers
such as the US Pentagon, a situation where the center is under attack
or where the command is given to go to war can easily result in a
high percentage of the senior staff needing to place such calls.
Under such cases, even calls at the "Priority" or "Immediate"
precedence level would be adversely affected.
2.3 Packet loss happens in tactical situations
MLEF is being considered in tactical deployments such as WIN-T, and
faces the same kinds of concerns. In radio environments, and in
mobile networks, a certain level of loss is normal. However,
bandwidth is usually limited. Any tactical situation which would
place a large number of soldiers on the telephone simultaneously can
be expected to result in congestive loss.
2.4 MLEF increases end to end delay, can add jitter, and can interfere
with other traffic classes
The MLEF PHB depends on the build-up of a queue for its operation:
when an MLEF queue becomes deep, traffic of the lowest precedence
starts to experience loss. This is contrary to the behavior of an EF
[RFC3246] queue, which is either engineered with a priority scheduler
or higher bandwidth than it actually uses in order to limit induced
delay. If the MLEF PHB is used in a priority queue, since it depends
on maintanence of a queue, it can lock out traffic classes of lower
priority for arbitrary periods. If it depends on WFQ/WRR, it will
force other classes to interleave, in cases waiting for a quantum of
traffic from each competing class before sending the next voice
packet, which affects voice quality for all call precedence levels.
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2.5 MLEF induced loss triggers congestive collapse
The fundamental effect of non-negligible loss of traffic in a
precedence class, therefore, is the disruption of all calls in that
precedence class, especially if protection-based codecs are in use.
This is, definitively, congestive collapse - 100% utilization with
zero effectiveness of communication for all calls of a certain class.
When a call experiences congestion when MLEF is in use, the ILBC
codec (taking one example analyzed in [ILBC] ) will start replicating
voice samples to include in other RTP payload packets, increasing the
bandwidth required for just that one call. This will further congest
the network, causing ILBC to add more voice samples to other RTP
payloads in other packets, further congesting the network. If a
substantial number of calls in the same MLPP precedence level are
performing this same codec protection function, the network bandwidth
grows exponentially within that MLPP precedence level. This will
cause, as mentioned before, all calling parties within a MLPP level
to experience packet loss, disrupting or destroying the ability to
communicate, with no preemption indication to any one party.
Existing behavior would be to hang up and try again, because MLPP
domain personnel are trained to recognize a preemption event and know
that the system is experiencing congestion due to some emergency.
There is no such indication, in an MLEF environment, so it is
reasonable to conclude that some or most calling parties will merely
hang up and try again. The problem at this point is that MLEF does
not (and cannot) provide feedback to application layer multimedia
signaling protocols to inform those protocols that a new call attempt
is not such a good idea; nor will there be anything to prevent a new
call from being set up to the previous party (provided there is
enough bandwidth available for signaling packets within the network
through some mechanism such as CBWFQ. With the new call set up, and
the network too congested to transmit enough media packets
end-to-end, no calls within that MLPP level will function properly,
and no one will receive the proper feedback as to what is occurring.
2.6 MLEF gives no preemption feedback notification
One attribute of the current MLPP service is that when a user's call
is preempted, the user is told, via an audible signal, of the event.
In such a case, the user can be expected to find other tasks for a
period of time and try again later. However, that is not a typical
human response - especially the response of a human in an agitated
state of mind - to a noisy connection. The more typical response is
to hope that the circuit will improve as others vacate their calls,
or to hang up and call again in an attempt to "get another circuit".
As such, the MLEF PHB fails to signal to the user that sufficient
bandwidth is simply not available to support his call, so that the
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user can be expected to respond to the situation in a different way.
There are three ways this can fail:
o If a call is placed when there is insufficient bandwidth, the
system does not give definitive feedback,
o If another call is set-up into a priority level that is at
capacity, the bandwidth for all calls at that level (and below)
are reduced, and there is no signal to any call parties indicating
this
o If policy is changed during a call, resulting in the necessity to
drop one or more calls, there is no signal.
A measurement-based counterpart to the MLPP procedure has been
proposed, in which calls experiencing significant loss treat this as
a signal from the network and drop the call. But if all calls at a
precedence level are experiencing loss, many and perhaps all calls at
the precedence level would be dropped by this heuristic; if many
calls are vying for service, the effect would be rolling call
disruption - a set of calls would be established, additional calls
would be established disrupting that class of calls, many of the
disrupted calls would drop, and then more of the competing calls
would be established - only to be disrupted when the first set
redialed.
This procedure would still require a forward looking mechanism, for
each precedence class, to disallow new calls, to prevent this rolling
call disruption.
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3. Recommendation
Considering the nature of real-time traffic and the effect of packet
loss on known codecs, it is clear that degradation of voice quality
in an MLEF environment for lower precedence calls will be severe if
no form of bandwidth and routing-aware Call Admission Control (CAC)
is used. Even the advances in codec technology do not fix the
problem, and could make it worse.
The authors cannot in good conscience recommend its deployment as it
stands. It will protect the calls placed by senior officers and
constitutional officials, but it does not provide the same service
that MLPP provides to those who respond to their orders, and
therefore seriously and negatively affects the likelihood that those
orders will be efficiently disseminated and carried out. Considering
the environment this proposed mechanism is for, the potential
attractiveness for other environments, and that the effects could and
should compound upon themselves, the worst case scenario includes
loss of life due to communications failure. Nothing done here should
enhance this possibility.
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4. IANA Considerations
IANA is not called upon to do anything with this document.
If this document is published as an RFC, the RFC Editor should remove
this section during the process of publication.
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5. Security Considerations
This document exposes a problem, but it proposes neither a protocol
nor a procedure. As such, it does not directly affect the security
of the Internet.
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6. Acknowledgements
This document was developed with the knowledge and input of many
people, far too numerous to be mentioned by name. They include at
least Alan Duric, Christopher Eagan, Francois Le Faucheur, Haluk
Keskiner, Julie Ann Connery, Marty Egan, Mike Pierce, Mike Tibodeau,
Pete Babendreier, Rohan Mahy, Scott Bradner, Scott Morrison, and
Subha Dhesikan.
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7. References
7.1 Normative References
[I-D.pierce-ieprep-assured-service-arch]
Pierce, M. and D. Choi, "Architecture for Assured Service
Capabilities in Voice over IP",
Internet-Draft draft-pierce-ieprep-assured-service-arch-02, January 2004
.
[I-D.pierce-ieprep-assured-service-req]
Pierce, M. and D. Choi, "Requirements for Assured Service
Capabilities in Voice over IP",
Internet-Draft draft-pierce-ieprep-assured-service-req-02,
January 2004.
[I-D.silverman-diffserv-mlefphb]
Silverman, S., "Multi-Level Expedited Forwarding Per Hop
Behavior (MLEF PHB)",
Internet-Draft draft-silverman-diffserv-mlefphb-03,
February 2004.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
7.2 Informative References
[ANSI.MLPP.Spec]
American National Standards Institute, "Telecommunications
- Integrated Services Digital Network (ISDN) - Multi-Level
Precedence and Preemption (MLPP) Service Capability",
ANSI T1.619-1992 (R1999), 1992.
[ANSI.MLPP.Supplement]
American National Standards Institute, "MLPP Service
Domain Cause Value Changes", ANSI ANSI T1.619a-1994
(R1999), 1990.
[G711.1] Viola Networks, "Netally VoIP Evaluator", January 2003,
<http://www.sygnusdata.co.uk/white_papers/viola/netally_vo
ip_sample_report_preliminary.pdf>.
[G711.2] ETSI Tiphon, "ETSI Tiphon Temporary Document 64", July
1999,
<http://docbox.etsi.org/tiphon/tiphon/archives/1999/05-990
7-Amsterdam/14TD113.pdf>.
Baker & Polk Expires August 11, 2005 [Page 17]
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[G711.3] Nortel Networks, "Packet Loss and Packet Loss
Concealment", 2000,
<http://www.nortelnetworks.com/products/01/succession/es/c
ollateral/tb_pktloss.pdf>.
[G711.4] Clark, A., "Modeling the Effects of Burt Packet Loss and
Recency on Subjective Voice Quality", 2000,
<http://www.telchemy.com/references/tech_papers/iptel2001.
pdf>.
[G711.5] Cisco Systems, "Understanding Codecs: Complexity, Hardware
Support, MOS, and Negotiation", 2003,
<http://www.cisco.com/en/US/tech/tk652/tk701/technologies_
tech_note09186a00800b6710.shtml#mos>.
[I-D.ietf-sip-resource-priority]
Schulzrinne, H. and J. Polk, "Communications Resource
Priority for the Session Initiation Protocol (SIP)",
Internet-Draft draft-ietf-sip-resource-priority-05,
October 2004.
[]
Polk, J., "Extending the Session Initiation Protocol
Reason Header for Preemption Events",
Internet-Draft draft-ietf-sipping-reason-header-for-preemption-02
, August 2004.
[ILBC] Chen, M. and M. Murthi, "On The Performance Of ILBC Over
Networks With Bursty Packet Loss", July 2003.
[ITU.MLPP.1990]
International Telecommunications Union, "Multilevel
Precedence and Preemption Service (MLPP)",
ITU-T Recommendation I.255.3, 1990.
[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.
[RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
[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.
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Internet-Draft MLEF Considered Harmful February 2005
[RFC3326] Schulzrinne, H., Oran, D. and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, December 2002.
[RFC3951] Andersen, S., Duric, A., Astrom, H., Hagen, R., Kleijn, W.
and J. Linden, "Internet Low Bit Rate Codec (iLBC)",
RFC 3951, December 2004.
Authors' Addresses
Fred Baker
Cisco Systems
1121 Via Del Rey
Santa Barbara, California 93117
USA
Phone: +1-408-526-4257
Fax: +1-413-473-2403
Email: fred@cisco.com
James Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, Texas 75082
USA
Phone: +1-469-255-5208
Email: jmpolk@cisco.com
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