|Title||Signaled Preemption Priority Policy Element
Network Working Group S. Herzog
Request for Comments: 2751 IPHighway
Category: Standards Track January 2000
Signaled Preemption Priority Policy Element
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document describes a preemption priority policy element for use
by signaled policy based admission protocols (such as [RSVP] and
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-
Table of Contents
1 Introduction .....................................................2
2 Scope and Applicability ..........................................3
3 Stateless Policy .................................................3
4 Policy Element Format ............................................4
5 Priority Merging Issues ..........................................5
5.1 Priority Merging Strategies ...................................6
5.1.1 Take priority of highest QoS .................................6
5.1.2 Take highest priority ........................................7
5.1.3 Force error on heterogeneous merge ...........................7
5.2 Modifying Priority Elements ...................................7
6 Error Processing .................................................8
7 IANA Considerations ..............................................8
8 Security Considerations ..........................................8
9 References .......................................................9
10 Author Information .............................................9
Appendix A: Example ...............................................10
A.1 Computing Merged Priority ....................................10
A.2 Translation (Compression) of Priority Elements ...............11
Full Copyright Statement ..........................................12
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
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
Certain base assumptions are made in the usage model for
- 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.
- 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
- 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 |
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
Preemption Priority: 16 bit (unsigned)
The priority of the new flow compared with the defending priority
of previously admitted flows. Higher values represent higher
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.
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:
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).
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
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
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
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
This is a recommended merging strategy.
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
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
The reasoning for this approach assumes that the heterogeneous case
is relatively rare and too complicated to deal with, thus it better
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
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.
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.
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.
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
7 IANA Considerations
Following the policies outlined in [IANA-CONSIDERATIONS], Standard
RSVP Policy Elements (P-type values) are assigned by IETF Consensus
action 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).
[RSVP-EXT] Herzog, S., "RSVP Extensions for Policy
Control", RFC 2750, January 2000.
[COPS-RSVP] Boyle, J., Cohen, R., Durham, D., Herzog, S.,
Raja, R. and A. Sastry, "COPS usage for RSVP",
RFC 2749, January 2000.
[RAP] Yavatkar, R., et al., "A Framework for Policy
Based Admission Control", RFC 2753, January
[COPS] Boyle, J., Cohen, R., Durham, D., Herzog, S.,
Raja, R. and A. Sastry, "The COPS (Common Open
Policy Service) Protocol", RFC 2748, January
[RSVP] Braden, R., ed., et al., "Resource ReSerVation
Protocol (RSVP) - Functional Specification",
RFC 2205, September 1997.
[IANA-CONSIDERATIONS] Alvestrand, H. and T. Narten, "Guidelines for
Writing an IANA Considerations Section in
RFCs", BCP 26, RFC 2434, October 1998.
10 Author Information
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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
A.1 Computing Merged Priority
/ 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)
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
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:
(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)
(Pr=7, St=Highest QoS)
(Pr=9, St=Highest PP)
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