Internet Draft Shai Herzog
Expiration: March 2000 IPHighway
File: draft-ietf-rap-signaled-priority-04.txt
Signaled Preemption Priority Policy Element
September 24, 1999
Status of this Memo
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Abstract
This document describes a preemption priority policy element for use by
signaled 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 Stateless Policy ...................................................4
4 Policy Element Format ..............................................4
5 Priority Merging Issues ............................................6
5.1 Priority Merging Strategies .....................................7
5.1.1 Take priority of highest QoS ...................................7
5.1.2 Take highest priority ..........................................8
5.1.3 Force error on heterogeneous merge .............................8
5.2 Modifying Priority Elements .....................................9
6 Error Processing ...................................................9
7 IANA Considerations ...............................................10
8 Security Considerations ...........................................10
9 References ........................................................11
10 Author Information ..............................................11
Appendix A: Example .................................................12
A.1 Computing Merged Priority ......................................12
A.2 Translation (Compression) of Priority Elements .................13
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1 Introduction
Traditional Capacity based Admission Control (CAC) indiscriminately
admits new flows until capacity is exhausted (First Come First
Admitted). Policy based Admission Control (PAC) on the other hand
attempts to minimize the significance of order of arrival and use
policy based admission criteria instead.
One of the more popular policy criteria is the rank of importance of a
flow relative to the others competing for admission into 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 describes 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 [RAP]
describes the various components that participate in policy decision
making (i.e., PDP, PEP and LDP). The emphasis of PREEMPTION_PRI
elements is to be simple, stateless, and light-weight 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 PEPs at the periphery of the policy
domain (e.g., in border routers), PDPs reduce sets of relevant policy
rules into a single priority criterion. This priority as expressed in
the PREEMPTION_PRI element can then be communicated to downstream
PEPs of the same policy domain, which have LDPs but no controlling
PDP.
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- They can be processed by LDPs
PREEMPTION_PRI elements are processed by LDPs of nodes that do not
have a controlling PDP. LDPs may interpret these objects, forward
them as is, or perform local merging to forward an equivalent merged
PREEMPTION_PRI policy element. LDPs must follow the merging strategy
that was encoded by PDPs in the PREEMPTION_PRI objects. (Clearly, a
PDP, being a superset of LDP, may act as an LDP as well).
- They are enforced by PEPs
PREEMPTION_PRI elements interact with a node's traffic control module
(and capacity admission control) to enforce priorities, and preempt
previously admitted flows when the need arises.
3 Stateless Policy
Signaled Preemption Priority is stateless (does not require past
history or external information to be interpreted). Therefore, when
carried in COPS messages for the outsourcing of policy decisions, these
objects are included as COPS Stateless Policy Data Decision objects
(see [COSP, COPS-RSVP]).
4 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 |
+------+------+-------------+-------------+-------------+
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Length: 16 bits
Always 12. The overall length of the policy element, in bytes.
P-Type: 16 bits
PREEMPTION_PRI = 3
This value is registered with IANA, see Section 7.
Flags: 8 bits
Reserved (always 0).
Merge Strategy: 8 bit
1 Take priority of highest QoS: recommended
2 Take highest priority: aggressive
3 Force Error on heterogeneous merge
Reserved: 8 bits
Error code: 8 bits
0 NO_ERROR Value used for regular PREEMPTION_PRI elements
1 PREEMPTION This previously admitted flow was preempted
2 HETEROGENEOUS This element encountered heterogeneous merge
Reserved: 8 bits
Always 0.
Preemption Priority: 16 bit (unsigned)
The priority of the new flow compared with the defending priority of
previously admitted flows. Higher values represent higher Priority.
Defending Priority: 16 bits (unsigned)
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.
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For any specific flow, its preemption priority must always be less
than or equal to the defending priority. A wide gap between
preemption and defending priority provides 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 avoid preemption itself. This provides a mechanism for
balancing between order dependency and priority.
5 Priority Merging Issues
Consider the case where two RSVP reservations merge:
F1: QoS=High, Priority=Low
F2: QoS=Low, Priority=High
F1+F2= F3: QoS=High, Priority=???
The merged reservation F3 should have QoS=Hi, but what Priority should
it assume? Several negative side-effects have been identified that may
affect such a merger:
Free-Riders:
If F3 assumes Priority=High, then F1 got a free ride, 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:
If F3 assumes Priority=Low, the merged flow could be preempted or fail
even though F2 presented high priority.
Denial of service is virtually the inverse of the free-rider problem.
When flows compete for resources, if one flow receives undeserving high
priority it may 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.
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5.1 Priority Merging Strategies
In merging situations LDPs may receive multiple preemption elements and
must compute the priority of the merged flow according to the following
rules:
a. Preemption priority and defending priority are merged and computed
separately, irrespective of each other.
b. Participating priority elements are selected.
All priority elements are examined according to their merging
strategy to decide whether they should participate in the merged
result (as specified bellow).
c. The highest priority of all participating priority elements is
computed.
The remainder of this section describes the different merging
strategies the can be specified in the PREEMPTION_PRI element.
5.1.1 Take priority of highest QoS
The PREEMPTION_PRI 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
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 reasoning for 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.
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5.1.2 Take highest priority
All PREEMPTION_PRI elements participate in the merged reservation.
This strategy disassociates priority and QoS level, and therefore is
highly subject to free-riders and its inverse image, denial of service.
This is not a recommended method, but may be simpler to implement.
5.1.3 Force error on heterogeneous merge
A PREEMPTION_PRI element may participate in a merged reservation only
if all other flows in the merged reservation have the same QoS level
(homogeneous flows).
The reasoning for this approach assumes that the heterogeneous case is
relatively rare and too complicated to deal with, thus it better be
prohibited.
This strategy lends itself to denial of service, when a single receiver
specifying a non-compatible QoS level may cause denial of service for
all other receivers of the merged reservation.
Note: The determination of heterogeneous flows applies to QoS level
only (FLOWSPEC values), and is a matter for local (LDP) definition.
Other types of heterogeneous reservations (e.g. conflicting reservation
styles) are handled by RSVP and are unrelated to this PREEMPTION_PRI
element.
This is a recommended merging strategy when reservation homogeneity is
coordinated and enforced for the entire multicast tree. It is more
restrictive than Section 5.1.1, but is easier to implement.
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5.2 Modifying Priority Elements
When POLICY_DATA objects are protected by integrity, LDPs should not
attempt to modify them. They must be forwarded as-is or else their
security envelope would be invalidated. In other cases, LDPs may modify
and merge incoming PREEMPTION_PRI elements to reduce their size and
number according to the following rule:
Merging is performed for each merging strategy separately.
There is no known algorithm to merge PREEMPTION_PRI element of
different merging strategies without loosing valuable information that
may affect OTHER nodes.
- For each merging strategy, the highest QoS of all participating
PREEMPTION_PRI elements is taken and is placed in an outgoing
PREEMPTION_PRI element of this merging strategy.
- This approach effectively compresses the number of forwarded
PREEMPTION_PRI elements to at most to the number of different
merging strategies, regardless of the number of receivers (See the
example in Appendix A.2).
6 Error Processing
A PREEMPTION_PRI error object is sent back toward the appropriate
receivers when an error involving PREEMPTION_PRI elements occur.
PREEMPTION
When a previously admitted flow is preempted, a copy of the preempting
flow's PREEMPTION_PRI element is sent back toward the PDP that
originated the preempted PREEMPTION_PRI object. This PDP, having
information on both the preempting and the preempted priorities may
construct a higher priority PREEMPTION_PRI element in an effort to re-
instate the preempted flow.
Heterogeneity
When a flow F1 with Heterogeneous Error merging strategy set in its
PREEMPTION_PRI element encounters heterogeneity the PREEMPTION_PRI
element is sent back toward receivers with the Heterogeneity error code
set.
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7 IANA Considerations
Following the policies outlined in [IANA-CONSIDERATIONS],
Standard RSVP Policy Elements (P-type values) are assigned by IETF
Consensus action\t as described in [RSVP-EXT].
P-Type PREEMPTION_PRI is assigned the value 3.
8 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 mechanisms are not warranted, especially considering
that preemption priority aims to provide simple and quick guidance to
routers within a trusted zone or at least a single zone (no zone
boundaries are crossed).
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9 References
[RSVP-EXT] Herzog, S. "RSVP Extensions for Policy Control", Internet-
Draft, draft-ietf-rsvp-ext-02.txt, Jan. 1999.
[COPS-RSVP] Boyle, J., Cohen, R., Durham, D., Herzog, S., Raja, R.,
Sastry, A., "COPS usage for RSVP" Internet-Draft, draft-ietf-
rap-cops-rsvp-02.txt, Jan 1999.
[RAP] Yavatkar, R., et al., "A Framework for Policy Based Admission
Control",IETF <draft-ietf-rap-framework-02.txt>, Jan., 1999.
[COPS] Boyle, J., Cohen, R., Durham, D., Herzog, S., Raja,n R.,
Sastry, A., "The COPS (Common Open Policy Service) Protocol",
IETF <draft-ietf-rap-cops-05.txt>, Jan. 1999.
[RSVP] Braden, R. ed., "Resource ReSerVation Protocol (RSVP) -
Functional Specification.", IETF RFC 2205, Proposed Standard,
Sep. 1997.
[IANA-CONSIDERATIONS] Alvestrand, H. and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", RFC 2434,
October 1998.
10 Author Information
Shai Herzog, IPHighway
Parker Plaza, 16 floor
400 Kelby St.
Fort-Lee, NJ 07024
(201) 585-0800
herzog@iphighway.com
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Appendix A: Example
The following examples describe the computation of merged priority
elements as well as the translation (compression) of PREEMPTION_PRI
elements.
A.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 PREEMPTION_PRI element definition.
r1, r2 and r3 merge in B. The resulting priority is 4.
Reason: The PREEMPTION_PRI of r3 doesn't participate (since r3 is not
contributing to the merged QoS) and the priority is the highest of the
PREEMPTION_PRI 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 PREEMPTION_PRI should be sent back to r4
telling it that its PREEMPTION_PRI element encountered heterogeneity.
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A.2 Translation (Compression) of Priority Elements
Given this set of participating PREEMPTION_PRI 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)
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