One document matched: draft-pierce-ieprep-pref-treat-examples-02.txt
Differences from draft-pierce-ieprep-pref-treat-examples-01.txt
Internet Engineering Task Force Mike Pierce
Internet Draft Artel
draft-pierce-ieprep-pref-treat-examples-02.txt Don Choi
January 2004 DISA
Expires July 2004
Examples for Provision of Preferential Treatment in Voice over IP
draft-pierce-ieprep-pref-treat-examples-02.txt
Status of this memo
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Copyright
Copyright (C) Internet Society 2004. All rights reserved.
Reproduction or translation of the complete document, but not of
extracts, including this notice, is freely permitted.
Abstract
Assured Service refers to the set of capabilities used to ensure
that mission critical communications are setup and remain connected.
[Pierce] describes the requirements, one of which is to provide
preferential treatment to higher priority calls. IEPS refers to a
set of capabilities used to provide a higher probability of call
completion to emergency calls made by authorized personnel, usually
from ordinary telephones. This also requires some form of
preferential treatment. This informational memo describes some of
the methods which may be applied to provide that preferential
treatment.
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Table of Contents
0. Changes............................................................2
1. Introduction.......................................................3
2. Background.........................................................4
3. Potential Preferential Treatments..................................4
3.1. Reservation of Network Resources...............................4
3.1.1. RSVP....................................... ................4
3.1.2. MPLS........................................ ...............5
3.2. Use of Higher Call Acceptance Limits...........................6
3.3. Preferential Queuing of Signaling Messages.....................7
3.4. Preferential Queuing of User Data Packets......................7
3.5. Discarding of Packets using DiffServ...........................7
3.5.1. Treatment for Signaling Packets.............. ..............8
3.5.2. Treatment for Voice Packets................... .............9
3.6. Preemption of One or More Existing Calls......................10
4. Preemption of Some of the Resources Being Used....................11
4.1. Preemption of the Reservation.................................11
4.2. Others........................................................11
5. Security Considerations...........................................11
6. IANA Considerations...............................................11
7. References........................................................11
8. Authors' Addresses................................................13
A Overview of existing priority mechanisms..........................13
A.1 IPv4..........................................................13
A.2 DiffServ......................................................13
A.3 IntServ/RSVP..................................................14
A.4 MPLS..........................................................14
A.5 SIP...........................................................14
B Possible Protocol Approaches......................................15
B.1 Precedence Level Marking......................................15
B.2 Authentication/Authorization..................................16
B.2.1 Architecture.................................... ...........17
B.2.2 Authentication Procedures........................ ..........17
B.2.3 Authorization Procedures.......................... .........17
B.3 Preferential Treatment........................................17
B.4 Diversion if Not Answered.....................................18
B.5 Notification to Preempted Party...............................18
B.6 Acknowledge by Preempted Party................................19
B.7 Protection of Signaling Information from Disclosure...........19
B.8 Accounting....................................................19
B.9 Call Control Signaling Precedence.............................19
B.10 Interworking..................................................20
0. Changes
This draft was originally submitted under SIPPING, but now being
submitted under IEPREP to focus consideration and discussion in that
WG in conjunction with the related discussions for IEPS.
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(SIPPING) -00 Initial version based on material removed from draft-
pierce-sipping-assured-service-01.
(IEPREP) -00 Added references to IEPREP in Intro. Update references.
add details about packet dropping procedure.
(IEPREP) -01 Updated references
(IEPREP) -02 Added Annexes from requirements draft.
1. Introduction
The requirements for Assured Service in support of networks
requiring precedence treatment for certain calls is are described in
[Pierce]. One of those requirements is Preferential treatment, which
is the following:
It must be possible to provide preferential treatment to higher
precedence calls in relation to lower precedence calls. Examples of
preferential treatments are:
. reservation of network resources for precedence calls
. usage of higher Call Acceptance limits for higher precedence
calls
. preferential queuing of signaling messages based on precedence
level
. preferential queuing of user data packets based on precedence
level
. discarding of packets of lower precedence call
. preemption of one or more existing calls of lower precedence
level
. preemption of some of the resources being used by a call of
lower precedence level
. preemption of the reservation of resources being held for other
traffic
Several current drafts describe the requirements for provision of
the International Emergency Preparedness Scheme (IEPS). This service
requires some types of preferential treatment for these calls, which
can be viewed as a subset of the requirements for Assured Service
listed above. These requirements include:
. higher probability of call completion
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. lower probability of premature disconnect
. distinguish IEPS data packets from other types of VoIP Packets
in order to give them "priority".
. alternate path routing
This informational memo describes some ways in which the above
listed preferential treatments may be provided by utilizing current
or new capabilities.
Two annexes provide background information on current support within
various IETF defined protocols for preferential treatment and
describe some of the possible approaches to providing additional
preferential treatment capabilities.
2. Background
The requirement for Precedence Level marking of a call setup attempt
using SIP [RFC3261] will be met by the proposed Resource Priority
header [Resource]. The value carried in this header represents the
relative precedence level of the call, and is used to control any of
the following described procedures for providing Preferential
Treatment.
3. Potential Preferential Treatments
The requirement to provide preferential treatment to calls may be
met by applying any combination of the following procedures:
3.1. Reservation of Network Resources
This procedure involves pre-reserving certain network resources
during periods when no higher precedence traffic is present so as to
be prepared to handle a given level of high precedence traffic in
the case of an emergency. While this method is already used in the
circuit switched environment, it is less than desirable since it
requires a tradeoff between the amount of wasted resources during
non-emergency periods and the amount of emergency traffic which can
be handled using those reserved facilities.
IETF defined QoS mechanisms for packet-mode operation offer some
improvement to this situation by allowing the amount of reserved
resources to be adjusted.
3.1.1. RSVP
RSVP may be used to establish multiple trunk groups between
switching points, with each trunk group serving a different
precedence level of calls. Each trunk group would be sized based on
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the number of simultaneous calls of that precedence level to be
supported. (In this context, a trunk group refers to a facility
which can support a certain number of voice connections at a certain
Quality of Service level. As noted later, the number of connections
can be increased with a corresponding decease in the QoS level.)
With TE, the reserved sizes of these trunk groups could be adjusted
during times of emergency.
No preemption of these trunk groups is needed. However, reducing the
size of a group to near zero would prevent further calls from using
it while allowing existing calls to continue.
3.1.2. MPLS
MPLS may be used to establish the equivalent of dedicated trunk
groups between switching entities, enterprise network, etc. Each of
these "trunk groups" could exist to support a specific precedence
level of traffic between two points and could be setup using the
procedures of CR-LDP [RFC3212] or RSVP-TE [RFC3209]. These support
the signaling of the required five levels of precedence.
3.1.2.1. Constraint-based LSP Setup using LDP
CR-LDP [RFC3212] defines an extension to LDP to provide a
constraint-based routing using MPLS. One of the constraints is based
on the notion of a "priority" level for the new setup. It includes
the signaling of a setup priority and a holding priority with the
value of each being 0-7 (0 is the highest priority). When setting up
an LSP as a trunk group to carry the traffic of one of the expected
precedence levels defined in [Pierce], the following mapping would
be used:
+------------------+------------------------+
| Assured Service | RFC3212 Preemption TLV |
| Precedence +-----------+------------+
| Level | SetPrio | HoldPrio |
+------------------+-----------+------------+
| Routine | 4 | 0 |
| Priority | 3 | 0 |
| Immediate | 2 | 0 |
| Flash | 1 | 0 |
| Flash Override | 0 | 0 |
+------------------+-----------+------------+
This mapping prevents any preemption of a trunk group for the
establishment of another. Rather, it is expected that trunk groups
for all precedence levels would be initially created and remain.
Only their allocated size might be changed.
If actual preemption were desired, the appropriate HoldPrio values
would be used.
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3.1.2.2. RSVP-TE: Extensions to RSVP for LSP Tunnels
As an alternative to LDP, RSVP-TE [RFC3209] defines the use of RSVP
with extensions to perform the label distribution for MPLS. It also
includes the same setup and holding priorities as defined in CR-LDP
[RFC3212]. When using RSVP as the label distribution protocol, the
same mapping shown above for LDP would be used.
3.2. Use of Higher Call Acceptance Limits
One aspect of preferential treatment may be provided, without
resorting to preemption of calls, by allowing higher precedence
calls to be setup even when they result in exceeding the engineered
traffic limit on a facility (on an MPLS LSR, for example).
This procedure presumes the existence of a call acceptance function
which is aware of the traffic loading on various links and entities,
and compares these against some thresholds before allowing the
establishment of a new call (packet flow).
For example, the limits for call acceptance for new calls could be
set as depicted in the following table, where the engineered
capacity of a route or facility is "x". A new call of each
precedence level would be allowed only if the current load is within
the limit shown:
+------------------+-----------+
| Precedence Level | Capacity |
| | limit of |
+------------------+-----------+
| Routine | .9x |
| Priority | .95x |
| Immediate | x |
| Flash | 1.1x |
| Flash-override | 1.2x |
+------------------+-----------+
Explanation of table: In this example, a new Flash call is allowed
to be setup if the current traffic load for all traffic on the
facility is less than 1.2x. In the example shown in this table,
Routine traffic is always prevented from using the last 10% of the
capacity. The choice of the multipliers would be based on an
analysis of the tradeoff between getting the high precedence level
call through vs. sacrificing its QoS. It would depend on the voice
encoding algorithms typically used and the end user expectations.
This procedure is based on a requirement that Flash override calls
should "never" be blocked. (In a probability-based system, there is
no such thing as "never".) In the circuit-switched environment this
could only be guaranteed by having as many circuits as there might
be Flash override calls. For IP-based service, there is no fixed
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number of "circuits" on any facility. The "x" referred to above is
only an engineering limit based on a guarantee for the provision of
a certain QoS for normal traffic, i.e., Routine and Priority. This
"x" may be thought of as the number of "circuits" for normal
traffic. It is preferable to allow the setup of additional higher
precedence calls with reduced QoS rather than blocking their setup.
For example, while a particular facility may support 100 normal
calls (Routine and Priority) at the guaranteed QoS, it might support
130 flash-override calls at a reduced, yet acceptable, QoS (due to
packet loss) when in an emergency situation.
Since the packet preferential treatment using Diff-Serv described in
Section 3.5 could result in the discard or loss of the packets for
the lower precedence calls, the higher precedence calls could still
be provided a sufficient QoS even though they may have caused the
engineered capacity of the route to be exceeded. The lower
precedence calls will then experience higher packet discard rates or
queuing delay times. If the discard rate or delay for these lower
precedence calls is excessive, the end user will experience poor QoS
and will likely disconnect, thereby freeing up the resources.
3.3. Preferential Queuing of Signaling Messages
There is no plan to apply preferential queuing to signaling messages
(ahead of other signaling messages), just as this was not done in
the circuit switched network. No advantage can be shown for such a
procedure and it would only aggravate the problem of out-of-order
messages.
3.4. Preferential Queuing of User Data Packets
It is not expected that priority queuing of user data packets (ahead
of other user data packets of the same type) would provide a useful
capability.
3.5. Discarding of Packets using DiffServ
Within DiffServ, Assured Forwarding [RFC2597] provides four classes
and three drop precedences for each class (12 DSCP code points). One
of these classes could be used for the signaling messages for
session establishment and release. AF is not considered as being
appropriate for audio.
Expedited Forwarding [RFC3246] defines a single class (DSCP code
point) and operation, but does not include multiple drop precedences
as AF does. The intention of EF is to "provide low loss, latency and
jitter" and is understood to be intended for traffic such as speech,
although RFC 3246 does not explicitly mention speech or voice.
However, speech is less susceptible to loss than the signaling
traffic and, under some traffic situations, will constitute a much
larger portion of the overall load. Therefore, multiple drop
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precedences to alleviate overload are more appropriate to EF than
they are to AF.
The result of this use of DiffServ classes is that voice packets are
always given priority over the signaling packets and all voice
packets are treated the same. While this is the desired behavior in
many cases, it is not desired in those cases in which a limited
sized facility could become completely occupied by voice traffic
(using EF). In this situation, further signaling messages (using
AF), including those to setup new high precedence calls and those to
release low precedence calls, would be lost or excessively delayed.
Therefore, it is necessary to reserve a small capacity for use by
the AF class which serves the signaling traffic as described in
Section 2.10 of EF [RFC3246].
For that portion of the capacity using EF for voice, the required
preferential treatment for the five call precedence levels may be
provided by the use of multiple drop precedence (probability) levels
for packets. The procedures for these drop precedence levels would
be the same as defined currently for the three levels for each class
in AF [RFC2597].
Five such levels for packet marking, using DSCPs, are needed to
provide the required functionality. In the absence of "standardized"
DSCP values, local values could be assigned. Based on the
definitions for AF, these levels are referred to here as:
. Very low (i.e., lowest probability of being dropped)
. Low
. Medium
. High
. Very high (i.e., highest probability of being dropped)
The following possible mappings are shown to illustrate the concept
of using DiffServ codepoints to assist in the provision of
preferential treatment to the individual packets which make up the
information transfer (both the connection setup signaling and the
voice transfer) of an Assured Service call.
3.5.1. Treatment for Signaling Packets
Consideration could be given to utilization of different drop
precedences for the signaling messages associated with different
precedence sessions. However, using SS#7 in the PSTN as a basis, it
might also be meaningful to provide different drop precedences based
on the type of message rather than only based on the precedence of
the call. For example, for routine traffic, those messages which
cause the release of sessions could be given a lower drop precedence
than those which set up new sessions in order to allow such releases
to take place properly under overload conditions. High precedence
calls, on the other hand could use a lower drop precedence level for
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session setup messages than those of routine precedence calls. The
following table shows what is defined for SS#7 [T1.111], including
High Probability of Completion [T1.631] and MLPP [T1.619], and what
might be used for SIP.
(Note: The highest SS#7 Congestion Priority Level, i.e., "3", is the
last to be dropped during congestion.)
(Refer to RFC 3398 for mapping of ISUP to SIP messages.)
+---------------------------------+-----------------------------+
| SS#7 | SIP |
+--------------------+------------+----------------+------------+
| Message | Congestion | Message | Drop |
| | Priority | | Precedence |
| | Level | | Level |
+--------------------+------------+----------------+------------+
| Network management | 3 | ? | low |
| ANM | 2 | 200 OK (INVITE)| medium |
| RLC | 2 | 200 OK (BYE) | (note) |
| IAM (MLPP) | 1 or 2 | INVITE (AS) | low/medium |
| IAM (HPC) | 1 | INVITE (IEPS) | low |
| ACM | 1 | 18x | medium |
| CPG | 1 | 100 Trying/18x | medium |
| REL | 1 | BYE | low |
| IAM (normal) | 0 | INVITE (normal)| high |
| Others | 0 | | |
+--------------------+------------+----------------+------------+
Note: For SIP, unless noted otherwise, all ACKs should have the same
preferential treatment as the message they are acknowledging.
3.5.2. Treatment for Voice Packets
This example is for the case of the use of DiffServ to provide the
packet forwarding preferential treatment through multiple drop
precedence levels. It uses the Multi-Level Expedited Forwarding Per
Hop Behavior [Silverman]. Each packet containing user data (voice)
is marked with a unique DiffServ codepoint to indicate one of the
following levels and resulting treatment:
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+--------------+--------------------+-----------------+
| Precedence | Indication in user | Drop if current |
| Level | voice packets | queue is more |
| +-------+------------+ than -- % full |
| | Class | Drop | (note 1) |
| | | precedence | |
+--------------+-------+------------+-----------------+
|Routine | MLEF | Very high | 80% |
|Priority | MLEF | High | 90% |
|Immediate | MLEF | Medium | 100% |
|Flash | MLEF | Low | 110% |
|Flash Override| MLEF | Very low | 120% |
+--------------+-------+------------+-----------------+
All voice traffic is then served by a single instance of MLEF, and
served by a single (strict FIFO) queue. This results is an equal
treatment in terms of delay variation (often called "jitter") for
all precedence levels for those packets which are delivered, but
achieves this by selective packet discard. The discard may use a
simple tail dropping algorithm as shown in the above table or a form
of "Random Early Detection" as described in [RFC2309] to drop some
packets before the queue actually reaches the fill shown above.
However, since the packets in this queue are not using TCP and can
not be bursty or "aggressive", there appears to be no advantage
gained by the complexity of early detection and random dropping.
Note 1: The "queue full" here refers to the engineered limit, that
is, the limit which needs to be applied in order to meet the
requirements of the EF PHB and the desired QoS. Since this
calculation is based on probabilities of achieving a certain target
QoS, it can be temporarily exceeded as described in the following
section.
3.6. Preemption of One or More Existing Calls
The procedures described above for use of higher call acceptance
limits and selective discard of voice packets based on the
precedence level of the call reduce or eliminate the need to perform
preemption of existing calls within the IP domain. The statistical
nature of packet transmission makes it possible to "squeeze" an
additional high precedence call into an already "full" facility, as
illustrated in the previous section. It should be noted that, in the
extreme case, these procedures would result in the same effect as
preemption, since the resources of the lower precedence calls would
be so severely degraded (via packet loss) that communication would
be impossible and the users would likely disconnect.
Because each packet flow arrives at somewhat regular intervals, it
is expected that, when packet loss is occurring due to discard, that
the loss will not be random across all flows using the DSCP with the
highest discard probability. Rather, losses will likely be bursty on
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each flow, with most discards being on one flow for many consecutive
packets.
When interworking with circuit switched portions of the
telecommunications network, preemption procedures are still required
within transport facilities which are based on fixed numbers of
circuits. In some cases, this preemption results in specific
procedures being applied in the packet portion, such as
notifications of preemption and forced disconnect of a session.
4. Preemption of Some of the Resources Being Used
The "preemption" of some of the resources being used by lower
precedence traffic may be accomplished through the packet discard
described above.
4.1. Preemption of the Reservation
Based on traffic engineering, the amount of resources allocated to
reserved paths (e.g., MPLS or RSVP) could be adjusted. For example,
when an emergency situation occurs, the need for more resources to
support higher priority traffic could be recognized. The existing
LSPs could be changed using the procedures of [RFC3214] to allow the
size of those LSPs supporting the higher priority traffic to be
increased while others are decreased.
4.2. Others
There may be other procedures which could be used to provide the
required preferential treatments.
5. Security Considerations
The security considerations are covered in [Pierce].
6. IANA Considerations
It is not expected that there will be any IANA involvement in
support of provision of Preferential Treatment for Assured Service
beyond what is described in [Resource].
7. References
[RFC2205] "Resource ReSerVation Protocol (RSVP)", R. Braden, et al,
September 1997
[RFC2309] "Recommendations on Queue Management and Congestion
Avoidance", B. Braden, April 1998.
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[RFC2597] "Assured Forwarding PHB Group", J. Heinanen, et al, June
1999.
[RFC2702] RFC 2702, "Requirements for Traffic Engineering Over
MPLS", D. Awduche, et al, September 1999.
[RFC2751] "Signaled Preemption Priority Policy Element", D. Thaler,
January 2000.
[RFC3209] "RSVP-TE: Extensions to RSVP for LSP Tunnels", D. Awduche,
December 2001.
[RFC3212] "CR-LDP: Constraint-based LSP Setup using LDP", B.
Jamoussi, et al, January 2002.
[RFC3214] "LSP Modification Using CR-LDP", J. Ash, et al, January
2002.
[RFC3246] "An Expedited Forwarding PHB", B. Davie, et al, March
2002.
[RFC3261] "SIP: Session Initiation Protocol", J. Rosenberg, et al,
June 2002.
[RFC3313] RFC 3313, "Private SIP Extensions for Media
Authorization", W. Marshall, May 2002.
[T1.111] ANSI T1.111-2001, "Signalling System No. 7 (SS7) - Message
Transfer Part".
[T1.523] ANSI T1.523-2001, "Telecommunications Glossary".
[T1.619] ANSI T1.619-1992 (R1999) "ISDN - Multi-Level Precedence and
Preemption (MLPP) Service Capability".
[T1.631] ANSI T1.631-1993 (R1999) "Telecommunications - Signalling
System No. 7 (SS7) - High Probability of Completion (HPC) Network
Capability".
[Aboda] draft-ietf-aaa-transport-12, "Authentication, Authorization
and Accounting (AAA) Transport Profile", Bernard Aboda, January
2003. (in editor's queue)
[Johnston] draft-ietf-sipping-service-examples-05, "Session
Initiation Protocol Service Examples", A. Johnston, et al, August
2003.
[Pierce] draft-pierce-ieprep-assured-service-req-02, "Requirements
for Assured Service Capabilities in Voice over IP", Jan 2004
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[Resource] draft-ietf-sip-resource-priority-01, "SIP Communications
Resource Priority Header", Henning Schulzrinne and James Polk, July
2003.
[Silverman] draft-silverman-mlefphb-02, "Multi-Level Expedited
Forwarding Per Hop Behavior (MLEF PHB", Steve Silverman, et al, June
2003.
8. Authors' Addresses
Michael Pierce
Artel
1893 Preston White Drive
Reston, VA 20191
Phone: +1 410.817.4795
Email: pierce1m@ncr.disa.mil
Don Choi
DISA
5600 Columbia Pike
Falls Church, VA 22041-2717
Phone: +1 703.681.2312
Email: choid@ncr.disa.mil
A Overview of existing priority mechanisms
Current support within various IETF defined protocols and ongoing
initiatives is not considered to be sufficient for Assured Services
requirements. This informative Annex details some of these.
This Annex is being included here for reference, but is not intended
to be included as this draft moves forward for approval as an RFC.
A.1 IPv4
Although support for the traditional five precedence levels was
included in the TOS field of IPv4 from the earliest days, support
for this field is not universal, and it only provides packet level
priority. It does not provide call setup priority or control of call
retention.
A.2 DiffServ
Within DiffServ, Assured Forwarding [RFC 2597] provides four classes
and three drop precedences for each class. One of these classes
could be used for the signaling messages for session establishment
and release. The multiple drop precedences could be used for various
signaling messages, as is being done with the equivalent call
control messages in ISUP for SS#7. However, AF is not considered to
be appropriate for voice.
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Expedited Forwarding [RFC 3246] is intended for voice, but it treats
all such voice packets the same. It does not define multiple drop
precedences as AF does so there is no way to provide preferential
treatment to packets associated with higher priority calls.
A.3 IntServ/RSVP
Although RSVP includes mention of preemption of existing
reservations in favor of other higher priority ones, it does not
provide detailed procedures for doing so. In principle, it should be
straightforward to do so. However, it is not believed that the
procedures required for establishment of a path using RSVP, and the
additional procedures that would be necessary for preemption of an
existing path, would allow this to be useful for the provision of
Assured Service capabilities to individual calls.
A.4 MPLS
Since MPLS is fundamentally a means to emulate circuit-mode
operation by establishment of a "path" which then functions like a
"connection", the principles of priority and preemption could be
applied to the setup and retention of this path the same as they are
in the circuit-mode environment. RFC 2702 describes the requirements
for such capabilities as applied to "traffic trunks". However, it
uses the term "traffic trunk" to refer to a facility which is
established to carry an aggregate of traffic, i.e., many telephone
calls. This is the equivalent of a "trunk group" in standard
telephony terminology [T1.523]. Because of the extensive procedures
that are required to establish and remove such a Label Switched
Path, it is believed that this prevents MPLS from being used to
setup paths for individual calls.
MPLS may be applicable for the establishment of the equivalent of
dedicated trunk groups between switching entities. Each of these
"trunk groups" or "traffic trunks" could exist to support a specific
precedence level of traffic between two points and could be setup
using the procedures defined in [RFC3212] or those in [RFC3209].
These documents allow the signaling of the required five levels of
precedence as well as separate setup and holding priorities.
A.5 SIP
SIP [RFC3261] defines four tokens for priority levels (non-urgent,
normal, urgent, and emergency), however, they are not intended to be
used to control call setup nor do they equate to the levels required
for Assured Service.
The proposed Resource Priority Header [Resource] provides for the
five precedence levels required for per call marking.
Security is discussed in [RFC3261] and many drafts, but it has been
recognized in various Working Group discussions that security for
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all aspects of call control needs to be considered in a unified
manner. Security for each individual component of call setup and
release can not be designed separately.
The procedure being proposed for authorization of call set-up and
media allocation [RFC3313] appears to be too time consuming to
expect it to occur each time a user attempts to place a telephone
call, especially a high-priority one. The probable delay in
establishing this authorization would be contrary to the goals and
requirements for Assured Service. Use of this type of procedure
would require that preferential treatments also be applied to all
message interactions and proxy processing of the sequence of
messages required for the authorization. Overloads of the proxies
responsible for the Call Authorization would prevent or unacceptably
delay setup of the high precedence call.
B Possible Protocol Approaches
The following identify possible approaches to meeting the
requirements stated above. This Annex is included in the draft to
stimulate discussion on ways of meeting the requirements, but is not
expected to be included in the final version when it is advanced
toward RFC status.
B.1 Precedence Level Marking
The approaches to be used for precedence level marking are different
for each of the following cases:
A. Individual call setup:
There needs to be a definition of a field to be carried in SIP which
identifies the precedence levels of 0-4 for the call (session)
setup. Currently, MLPP uses five values which have specific meanings
as currently defined in applicable standard [T1.619, I.225.3]) and
the definition of the field in SIP may reflect these meanings.
However, it is preferable to provide easy support for other network
applications which utilize a different number of levels or different
meanings.
The specification may allow more than 5 levels. There is no need for
the 65k levels defined in [RFC2751] nor is there currently a
requirement to carry separate preemption and defending priorities
[RFC2751] or separate setup and holding priorities [RFC3212,
RFC3209].
The proposed Resource Priority Header [Resource] is expected to
satisfy this requirement.
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B. Packet forwarding:
To support preferential treatment on the packet transfer level, the
current lack of any priority mechanism within the single Expedited
Forwarding class of DiffServ will need additional capabilities to
provide the required functionality. Just as Assured Forwarding
includes multiple drop precedences for each class, there should be
multiple drop precedences for EF, which is intended for voice. In
fact, voice transport is more tolerable to dropped packets than many
of the intended uses of AF classes.
(It should be emphasized again that such multiple "drop precedence"
levels for EF would not provide an actual priority forwarding
mechanisms per packet, e.g., priority queuing of some packets ahead
of other within that EF class, just as there is no such capability
included within the AF PHB definition.)
In order to provide the required preferential treatments for the
five call precedence levels, it is required to provide five
corresponding drop precedence levels for the voice packet handling.
The proposed MLEF PHB [Silverman] provides this capability.
C. Trunk group establishment:
For MPLS, RFC 2702 defines the idea of a "traffic trunk" for which a
priority may be signaled by the label distribution protocol in order
to establish telephony "trunk groups". If LDP is used for label
distribution, the priority defined in [RFC3212] should be used. If
RSVP is used for label distribution, the priority defined in
[RFC3209] should be used.
It should be noted that the traditional definition of a "trunk
group" does not include the notion of a "priority" associated with a
trunk group. The priority is only associated with individual calls
placed on that trunk group. It is possible that the routing logic
could reserve a trunk group for higher priority traffic, but this is
also not the normal application, since it wastes facilities during
periods when very little high priority traffic exists and it can not
support the heavier load of high priority traffic when conditions
cause such a high volume.
B.2 Authentication/Authorization
This draft uses the following definitions from draft-ietf-aaa-
transport-12 [Aboda]:
. Authentication: The act of verifying a claimed identity, in the
form of a pre-existing label from a mutually known name space,
as the originator of a message (message authentication) or as
the end-point of a channel (entity authentication).
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. Authorization: The act of determining if a particular right, such
as access to some resource, can be granted to the presenter of a
particular credential.
B.2.1 Architecture
In many other cases besides call setup for Assured Service it is
also necessary to perform authentication and authorization.
Appropriate security mechanisms have already been defined which may
be used.
Refer to [pierce2] for a discussion of the architecture required to
support the authentication/authorization requirements in a network
providing the Assured Service capability.
B.2.2 Authentication Procedures
It is essential that a framework for security for SIP be established
that allows a security association to be established between a
user's terminal and their dedicated SIP proxy at the time of an
initial registration. This initial registration, which includes
authentication, may require an extensive number of messages and
interactions with numerous network elements, including a Policy
Server, and may require a rather large time as a password is
verified. This registration and authentication would normally be
done when a terminal is turned on, activated, or places the first
call. It is not performed for each call. This reduces the need to
apply preferential treatment procedures to the authentication
process.
For the purpose of Assured Service provision, as with other SIP-
based services, it is expected that Authentication may be performed
based on the entry of an ID and password or the use of terminal
resident biometrics (e.g., iris scan) so that permission to use the
services can be associated with an individual, not a device. Once
registration is done, this permission is then associated with the
device.
B.2.3 Authorization Procedures
The procedures for authorization per call should consist only of
added fields/information within the normal messages used for basic
call setup as defined for SIP [RFC3261]. It should not require
additional messages to be added to the call setup sequence.
B.3 Preferential Treatment
The preferential treatments would not be standardized unless they
require signaling between network elements. Currently, most
treatments envisioned are local matters within a proxy or router.
Consideration of preferential treatments depends on the case:
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A. Per call:
Preemption of existing calls, if done, would require coordination
between network elements, and therefore protocol standards,
especially if distinct actions are expected to reserve the preempted
resources for setup of the higher precedence call.
It is not expected that network initiated preemption of calls
(sessions) within the IP environment will be necessary. Instead,
sufficient preferential treatment can be provided by applying higher
call admission control limits and lower drop precedence procedures
to higher precedence calls. Examples of these procedures are shown
in [Pierce1].
B. Packet Level:
Current capabilities of DiffServ, with additional code points for
drop precedences within an EF-type class, will provide the necessary
preferential treatments regarding voice packet transfer, including
indications of discard priority. It will also be necessary to define
new capabilities to provide the necessary preferential treatment for
call control signaling.
C. MPLS/RSVP Paths:
There should be no need for preemption of MPLS/RSVP established
traffic trunks (trunk groups) as described in [RFC2702] and
[RFC2205]. The required capability should be provided by mechanisms
to reduce the traffic engineering limits placed on lower priority
trunk groups (even by reducing to zero) to allow the capacity to be
used for the establishment of higher priority calls in other traffic
engineered traffic trunks.
B.4 Diversion if Not Answered
It is expected that Diversion would be based on procedures that are
developed for a Call Forwarding on No Answer type service. However,
it should not be dependent on a timing performed by the original
calling or called parties' devices, but rather, be the function of a
proxy serving the called party as shown in the proposed Service
Examples [Johnston].
B.5 Notification to Preempted Party
Notification to the preempted party should follow whatever is done
for notifications for any network-initiated release. Since it is
expected that actual call preemption will only be needed in the
circuit mode environment, the gateway between it and the IP
environment should deal with such preemption by application of the
required notification (in-band) to party on the IP side.
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B.6 Acknowledge by Preempted Party
Acknowledge by the preempted party (before connection of a new call)
should follow the same general procedures as used for normal call
presentation, that is, the new call is acknowledged (answered)
before any audio is transferred in either direction between end
users. (Note that this does not refer to the transfer of "media"
between the terminals, only the transfer of actual audio between the
persons using the terminals.)
B.7 Protection of Signaling Information from Disclosure
See Section 7.
B.8 Accounting
This draft uses the following definition from draft-ietf-aaa-
transport-12 [Aboda]:
Accounting: The act of collecting information on resource usage for
the purpose of trend analysis, auditing, billing, or cost
allocation.
Call detail records (CDR) can be maintained by the proxy, since it
knows which users are authorized to place Assured Service calls and
knows when they do. Since not actually done to support billing
functions, it is not expected that a record of call duration is
required.
B.9 Call Control Signaling Precedence
Adequate precedence (and preferential treatment) can be provided to
call control signaling, with respect to user data carried by EF, by
utilization of a single AF class (single queue) for all call control
signaling. A weighted queue serving algorithm is then required to
guarantee that this queue receives a minimum percentage of the
bandwidth of the outgoing facility, if it needs it, regardless of
the volume of "higher priority" packers (such as voice in an "EF"
queue). It is expected that this percentage for call control
signaling would be less than 5% of the total bandwidth.
To address the situation in which the signaling traffic exceeds the
minimum guaranteed and there is excessive traffic, thereby blocking
some call control messages, the multiple drop precedence capability
of AF would suffice. Relative drop precedences for SIP messages can
be modeled on those use in ISUP.
In order to address the other side of the problem - preventing long
call control messages from adversely affecting the performance of
the voice packets - it may be necessary to utilize a packet
segmentation scheme.
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B.10 Interworking
Interworking requirements can be met in the following manner:
Within a single network which supports two or more priority schemes,
the network operator determines the relative priority and
preferential treatments to apply. For example, the operator may
decide that the IEPS priority for "Authorized Emergency" will fall
between the Assured Service levels of Immediate and Flash.
At the gateway between two networks which support two different
priority schemes, the operator of the gateway determines and
performs the mapping between the two schemes. For example, for the
priority schemes defined for Assured Service (in the Defense
Switched Network or DSN) and assuming 3 levels (normal, public
emergency, and authorized emergency) for IEPS (in the public
network), the mappings could be:
From DSN To public network
------------ ---------------------
Routine --> Normal
Priority --> Normal
Immediate --> Normal
Flash --> Authorized Emergency
Flash Override --> Authorized Emergency
and
From Public network TO DSN
-------------------- -------
Normal --> Routine
Public Emergency --> Immediate
Authorized Emergency --> Flash
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Mike Pierce Expires July 2004 [Page 21]
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