One document matched: draft-ietf-rap-signaled-priority-00.txt
Internet Draft Shai Herzog
Expiration: Apr. 1999 IPHighway
File: draft-ietf-rap-signaled-priority-00.txt
Preemption Priority Policy Element
November 18, 1998
Status of this Memo
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Abstract
This document describes a preemption priority policy element for use by
policy based admission protocols (such as [RSVP] and [COPS]).
Preemption priority defines a relative importance (rank) within the set
of flows competing to be admitted into the network. Rather than
admitting flows by order of arrival (First Come First Admitted) network
nodes may consider priorities to preempt some previously admitted low
priority flows in order to make room for a newer, high-priority flow.
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Table of Contents
Abstract...............................................................1
Table of Contents......................................................2
1.Introduction.........................................................3
2.Scope and Applicability..............................................3
3.Policy Element Format:...............................................4
4.Priority Merging Issues..............................................5
5.Priority Merging Strategies..........................................6
5.1.Take priority of highest QoS.......................................6
5.2.Take highest priority..............................................7
5.3.Force error on heterogeneous merge.................................7
6.Error Processing.....................................................7
7.Security Considerations..............................................8
8.Example..............................................................8
8.1.Computing Merged Priority..........................................8
8.2.Translation (Compression) of Priority Elements.....................9
9.References...........................................................9
10. Author Information.................................................9
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1. Introduction
Traditional Capacity based Admission Control (CAC) indiscriminately
flows until capacity is exhausted. Policy based Admission Control (PAC)
on the other hand attempts to minimize the significance of order of
arrival and use policy based criteria instead.
One of the more popular policy criteria is the rank of importance of a
flow relative to the others competing for admition into/through a
network node. Preemption Priority takes effect only when a set of flows
attempting admission through a node represents overbooking of resources
such that based on CAC some would have to be rejected. Preemption
priority criteria help the node select the most important flows
(highest priority) for admission, while rejecting the low priority
ones.
Network nodes which support preemption should consider priorities to
preempt some previously admitted low priority flows in order to make
room for a newer, high-priority flow.
This document describe the format and applicability of the Preemption
Priority represented as a policy element in [RSVP-EXT].
2. Scope and Applicability
The Framework document for Policy-based Admission Control [Fwk]
describes the various components that participate in policy decision
making (i.e., PDP, PEP and LPD). The emphasis of PREEMPTION_PRI
elements is to be simple and get processed quick enough such that they
could be implemented internally within a node’s LDP (Local Decision
Point).
Certain base assumptions are made in the usage model for PREEMPTION_PRI
elements:
- They are created by PDPs
In a model where PDPs control the periphery (boarder routers), PDPs
reduce their entire set of relevant policy rules into a single
priority criteria. This priority as expressed in the PREEMPTION_PRI
element can then be communicated to in-cloud core nodes, which have
LPDs but no controlling PDP.
- They are processed by LDPs
PREEMPTION_PRI elements are interpreted and are forwarded locally
within in-cloud core nodes (in their LDP modules). Policy ignorant
nodes (PINs) may interpret these objects, and forward them as is, or
they can perform local merging and forward an equivalent merged
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PREEPMTION_PRI policy element. In both cases, whether for
interpretation only or for merging, LDPs must follow the merging
strategy as specified in the policy element itself.
- They are enforced by PEPs
PREEMPTION_PRI elements interact with a node’s traffic control
module (and capacity admission control) to enforce priorities, and
to perform preemption of previously admitted flows if the need
arises.
3. Policy Element Format:
The format of Policy Data objects is defined in [RSVP-EXT]. A single
Policy Data object may contain one or more policy elements, each
representing a different (and perhaps orthogonal) policy.
The format of Preemption Priority policy element is as follows:
+-------------+-------------+-------------+-------------+
| Length (12) | P-Type = PREEMPTION_PRI |
+------+------+-------------+-------------+-------------+
| Flags | M. Strategy | Error Code | Reserved(0) |
+------+------+-------------+-------------+-------------+
| Preemption Priority | Defending Priority |
+------+------+-------------+-------------+-------------+
Length: 16 bits
Always 12. The overall length of the policy element, in bytes.
P-Type: 16 bits
PREEMPTION_PRI Preemption Priority policy element, as registered
with IANA.
Flags: 8 bits
Reserved (always 0).
Merge Strategy: 8 bit
1 Take priority of highest QoS: recommended
2 Take highest priority: aggressive
3 Error (fail) on heterogeneous merge
Reserved: 8 bits
Error code: 8 bits
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0 No error. Value used for all regular PP elements.
1 Preemption This previously admitted flow was preempted
2 Heterogeneous This PP element encountered heterogeneous merge
Reserved: 8 bits
Always 0.
Preemption Priority: 16 bit (0..2^16)
The priority of the new flow compared with the defending priority of
previously admitted flows. Higher values represent higher Priority.
Defending Priority: 16 bits
Once a flow was admitted, the preemption priority becomes
irrelevant. Instead, its defending priority is used to compare with
the preemption priority of new flows.
For any specific flow, its preemption priority must always be less
or equal to the defending priority. A wide gap between preemption
and defending priority provides leeway for added stability: moderate
preemption priority makes it harder for a flow to preempt others,
but once it succeeded, the higher defending priority makes it easier
for the flow to escape preemption itself. This mechanism provides
some order dependency, although it is not absolute, as is the case
for CAC.
4. Priority Merging Issues
Consider the case where two reservations merge:
F1: QoS=High, Priority=Low
F2: QoS=Low, Priority=High
F1+F2= F3: QoS=High, Priority=???
Quite clearly, the merged reservation F3 should have QoS=Hi, but what
Priority should it assume? Several negative side-effects have been
identified that can affect such a merger:
Free-Riders:
If F3 assumes Priority=High, the result is that F1 managed to get a
free ride for his high QoS, assuming high priority that was only
intended to the low QoS F2. If one associates costs as a function of
QoS and priority, F1 receives an “expensive” priority without having to
“pay” for it.
Denial of Service:
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If F3 assumes Priority=Low, the merged flow could be preempted or fail
even though F2 presented high priority.
Denial of service is also a mirror image of the free-rider problem;
given competition for resources among the flows, if one flow receives
undeserving high priority it should be able to preempt another
deserving flow (hence one free-rider turns out to be another’s denial
of service).
Instability:
The combination of preemption priority, killer reservation and blockade
state [RSVP] may increase the instability of admitted flows where a
reservation may be preempted, reinstated, and preempted again
periodically.
5. Priority Merging Strategies
In merging situations LDPs receive multiple preemption elements for a
merged flow and must compute the priority of the merged flow according
to the following rules:
a. Preemption priority and defending priority are computed separately,
irrespective of each other.
b. All priority elements are examined according to their merging
strategy to decide whether they should participate in the merged
result (as specified bellow).
c. Take the highest priority of all those who participate.
d. A node may wish to reduce the number of PP elements it forwards by
translating multiple PP elements into their equivalent one. In this
case, it can only combine participating PP elements of the same
merging strategy into one (by taking the highest priority amongst
them). This effectively compress the number of forwarded PP elements
at most to the number of different merging strategies, regardless of
the number of receivers (See the example in Section 8.2).
The remainder of this section describes the different merging
strategies the can be specified in the PREEMTION_PRI element.
5.1. Take priority of highest QoS
The PP element would participate in the merged reservation only if it
belongs to a flow that contributed to the merged QoS level (i.e., that
its QoS requirement does not constitute a subset another reservation.)
A simple way to determine whether a flow contributed to the merged QoS
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result is to compute the merged QoS with and without it and to compare
the results (although this is clearly not the most efficient method).
The thinking behind this approach is that the highest QoS flow is the
one dominating the merged reservation and as such, its priority should
dominate it as well. This approach is the most amiable to the
prevention of priority distortions such as free-riders and denial of
service.
This is a recommended merging strategy.
5.2. Take highest priority
All PP elements participate in the merged reservation.
This strategy allows all receivers to participate and contribute to the
preemption priority even if they don’t contribute to the merged
reservation. It is therefor highly subject to free-riders and its
mirror image, denial of service.
This is not a recommended method, but may be simpler to implement.
5.3. Force error on heterogeneous merge
A PP element with this strategy cannot participate in a merged
reservation if any other flow in the merged reservation has a QoS level
that is substantially different from its own. The determination of what
is “substantially different” QoS level is up to the local node.
The thinking behind this approach is that most cases encounter
homogenous receivers that use similar QoS levels. Furthermore, it
assumes the use of PREEMPTION_PRI in the heterogeneous case is too
complicated to deal with and should not be permitted.
This strategy lends itself to denial of service, when a single receiver
specifying a non-compatible QoS level may cause denial for all other
receivers of the merged reservation.
6. Error Processing
A PREEMPTION_PRI error object is sent back toward the appropriate
receivers when an error involving PP elements occur.
Preemption
When a previously admitted flow is preempted, a copy of the PREEMPTING
flow’s PP element is sent downstream to the receiver (with the
preemption error set). This allows receivers (or PDPs) to construct a
higher priority PP element that may cause the flow to be re-instated.
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Heterogeneity
When a flow F1 with Heterogeneous Error merging strategy set in its PP
element encounters heterogeneity the PP element is sent back toward
receivers with the appropriate error code set.
7. Security Considerations
The integrity of PREEMPTION_PRI is guaranteed, as any other policy
element, by the encapsulation into a Policy Data object [RSVP-EXT].
Further security mechanism are not warranted, especially considering
that preemption priority aims to provide simple and quick guidance to
routers within a trusted zone or at least single zone (no zone
boundaries are crossed).
8. Example
The following examples describe the computation of merged priority
elements as well as the translation (compression) of PP elements.
8.1. Computing Merged Priority
r1
/ QoS=Hi (Pr=3, St=Highest QoS)
/
s1-----A---------B--------r2 QoS=Low (Pr=4, St=Highest PP)
\ \
\ \ QoS=Low (Pr=7, St=Highest QoS)
r4 r3
QoS=Low (Pr=9, St=Error)
Example 1: merging Preemption Priority elements
Example one describes a multicast scenario with one sender and four
receivers each with each own PP element definition.
r1, r2 and r3 merge in B. The resulting priority is 4.
Reason: The PP of r3 doesn’t participate (since r3 is not contributing
to the merged QoS) and the priority is the highest of the PP from r1
and r2.
r1, r2, r3 and r4 merge in A. The resulting priority is again 4: r4
doesn’t participate because its own QoS=Low is incompatible with the
other (r1) QoS=High. An error PP should be sent back to r4 telling it
that its PP element encountered heterogeneity.
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8.2. Translation (Compression) of Priority Elements
Given this set of participating PP elements, the following compression
can take place at the merging node:
From:
(Pr=3, St=Highest QoS)
(Pr=7, St=Highest QoS)
(Pr=4, St=Highest PP)
(Pr=9, St=Highest PP)
(Pr=6, St=Highest PP)
To:
(Pr=7, St=Highest QoS)
(Pr=9, St=Highest PP)
9. References
[RSVP] Braden, R. ed., "Resource ReSerVation Protocol (RSVP) -
Functional Specification." Internet-Draft, draft-ietf-rsvp-
spec-16.txt, June 1997.
[COPS] Boyle, J., Cohen, R., Durham, D., Herzog, S., Raja,n R.,
Sastry, A., "The COPS (Common Open Policy Service) Protocol",
Internet-Draft <draft-ietf-rap-cops-02.txt>, Aug. 1998.
[RSVP-EXT] Herzog, S. "RSVP Extensions for Policy Control", Internet-
Draft, draft-ietf-rap-rsvp-ext-00.txt, Apr. 1998.
[Fwk] R. Yavatkar, D. Pendarakis, R. Guerin. "A Framework for Policy
Based Admission Control", Internet-Draft <draft-ietf-rap-
framework-00.txt>, November, 1997.
10. Author Information
Shai Herzog, IPHighway
Parker Plaza, Suite 1500
400 Kelby St.
Fort-Lee, NJ 07024
(201) 585-0800
herzog@iphighway.com
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