Network Working Group Q. Xie
Internet-Draft Motorola
Expires: May 9, 2005 L. Yarroll
TimeSys Corporation
November 08, 2004
RSERPOOL Redundancy-model Policy
draft-xie-rserpool-redundancy-model-03.txt
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Abstract
This document defines RSERPOOL Redundancy-model Member Selection
Policy parameter and the related procedures. This policy is designed
to be flexible and capable of supporting a wide range of advanced
redundancy models found in some high availability systems. The
design uses the extensibility in RSERPOOL pool load sharing policy.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Redundancy-model Member Selection Policy Parameter . . . . . . 3
4. Member Selection Procedures under Redundancy-model Policy . . 5
5. Examples for Implementing Various Redundancy Models . . . . . 6
5.1 2N Redundancy with Active/Standby Protection . . . . . . . 6
5.2 N+M Redundancy with Active/Standby Protection . . . . . . 6
5.3 N-way Redundancy with Active/Standby Protection . . . . . 7
5.4 N-way Redundancy with Active/Active Protection . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 9
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1. Introduction
The pool member selection policy in RSERPOOL design [2][3] is both
flexible and extensible. By observing the pool's member selection
policy a pool user (PU) can accomplish a wide range of different load
distribution schemes and communications strategies across the pool
elements (PE) in the pool.
Some most basic load distribution schemes, such as the round-robin
and least used policies, are already defined in ASAP [1]. In this
document, we define another member selection policy -
Redundancy-model policy - in order to support a variety of more
advanced redundancy models used in some high availability systems.
1.1 Definitions
This document uses the following terms:
Pool (or server pool): See [3];
Pool element (PE): See [3];
Pool user (PU): See [3];
ENRP handlespace (or handlespace): See [3];
ENRP handlespace server (or ENRP server): See [3];
2. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
[5].
3. Redundancy-model Member Selection Policy Parameter
This defines a new Member Selection Policy TLV structure that SHOULD
be used to indicate the role of a PE in a high availability
redundancy model. This definition follows the member selection
policy extension rules described in Section 3.6? of [6]
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x6 | Length = variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Policy=0x5 | (reserved) | HA Role | Service State |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Backup PE Identifier Parameter (optional) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Type, Length, and Policy fields are defined in Section 3.6? in
[6], with the addition of the following new Policy value:
Value Policy
----- ---------
0x5 HA Redundancy-model support
Other fields are defined as follows:
High Availability (HA) role: 8 bits (unsigned integer)
Indicating the role of the corresponding PE in the HA
redundancy model. The following HA roles are currently
defined:
Value HA state
----- ---------
0x1 No role
0x2 Standby
0x3 Active
0x4 Dual role
other values are reserved by IETF and MUST NOT be used.
Service State: 8 bit (unsigned integer)
Indicating the current service state elected by the
corresponding PE. The following service states are currently
defined:
Value Service state
----- -------------
0x1 In service
0x2 Stopping
other values are reserved by IETF and MUST NOT be used.
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Backup PE Identifier Parameter:
This TLV is used in some redundancy models to indicate the
designate backup of this PE. The format of PE Identifier
Parameter is defined in [6]
More detailed description of the HA states and Service states is
given in the following sections.
4. Member Selection Procedures under Redundancy-model Policy
When sending to a pool of Redundancy-model policy, a PU MUST use the
following rules to select the recipient of the message:
1. When sending a new message for the first time to the pool, the PU
SHOULD select a PE whose HA role is either "Active" or "Dual
role" and service state is "In service." If more than one PEs in
the pool meets these criteria, the PU SHOULD select one of them
as the recipient of this message in a round-robin fashion.
2. When the PU is re-sending a message which was previously sent to
a PE (PE_A) in the pool but failed because PE_A is found
unreachable (i.e., a fail-over case), the PU SHOULD select a
qualified backup PE as the message recipient, by following the
steps below:
A. First check if PE_A has a designated backup PE, whose
presence and PE Id are indicated by the presence of the
"Backup PE Identifier Parameter" in the Member Selection
Policy TLV of PE_A.
B. If no backup is designated by PE_A, the PU SHOULD select a PE
whose HA role is either "Standby" or "Dual role", and service
state is "In service." If no such PE can be found in the
pool, the PU SHOULD abort re-sending the message and notify
the application about the failure. If multiple PEs exist in
the pool that meet these criteria, the PU SHOULD select one
of them in a round-robin fashion.
C. If a backup (PE_B) is designated by PE_A and exists in the
pool and its service state is "In service", the PU SHOULD
select PE_B as the recipient of the message being re-sent,
regardless of the HA state of PE_B.
D. If a backup (PE_B) is designated by PE_A but does not exist
in the pool (i.e., the PE Id in the "Backup PE Identifier
Parameter" in PE_A dose not match any PE in the pool), the PU
SHOULD abort re-sending the message and notify the
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application about the failure.
E. If a backup (PE_B) is designated by PE_A and exists in the
pool but its service state is NOT "In service", the PU SHOULD
attempt to find a qualified backup for PE_B, by recursively
invoking the above steps.
In the case where the failed PE does not have a designated backup
and the sending PU has succeeded in finding a qualified backup PE
for the failed PE using the above procedures, the PU MAY keep a
record of its finding and use the same backup for all subsequent
fail-over events that may occur on the same failed PE.
5. Examples for Implementing Various Redundancy Models
5.1 2N Redundancy with Active/Standby Protection
In this redundancy model there are two PEs registered in the pool;
one has its HA role set to Active and the other's set to Standby.
The active PE is providing the service while the standby PE is
prepared to take over the active role should the active PE fail.
In this redundancy model, the active PE does not have to have a
designated backup, i.e., it does not need to include a "Backup PE
Identifier Parameter" in its selection policy TLV when registering.
5.2 N+M Redundancy with Active/Standby Protection
This redundancy model allows flexible active-standby relationships to
be defined between the PEs in a pool. A pool with the N+M redundancy
model consists of N PEs registered with their HA role set to Active
(active PEs) and M PEs registered with their HA role set to Standby
(standby PEs).
Normally, the value of M is less than or equal to N, which means that
a single PE can act as a standby for several active PEs at a time. A
common use of the N+M redundancy model is N+1 redundancy. This is a
special case of the N+M redundancy model. In the pool using N+1
redundancy, a single PE acts as a standby for all active PEs.
In operation, the active PEs are provided the service while the
standby PEs are prepared to become backup to any of the active PEs,
should one or more of them fails.
In this redundancy model, the active PEs do not have to have a
designated backup, i.e., they do not need to include a "Backup PE
Identifier Parameter" in their selection policy TLV when registering.
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5.3 N-way Redundancy with Active/Standby Protection
In this redundancy model, there are N (an even number) PEs in the
pool. The PEs are paired to form N/2 active/standby pairs. In each
pair, the active PE is providing the service while the standby PE is
prepared to take over the active role should the active PE fail.
Load is distributed across the N/2 active PEs.
To implement such a pool, at registration each active PE needs to set
its HA role to Active and indicate its designated backup by including
a "Backup PE Identifier Parameter" in its selection policy TLV that
contains the PE id of the backup. The standby PEs simply register
with their HA role set to Standby and omit the "Backup PE Identifier
Parameter."
5.4 N-way Redundancy with Active/Active Protection
In this redundancy model there are one or more PEs in the pool, and
are registered with their HA role set to Dual role. All PEs in the
pool are actively providing service and are prepared to backup others
at the same time.
Note, this redundancy model can also be implemented (in some cases
more efficiently) by using other active/active member selection
policies such as the Round Robin and Weighted Round Robin policies
(see Section 4.5.2? [1]).
6. IANA Considerations
A new RSERPOOL Member Selection Policy type (HA Redundancy-model
support: 0x5) will need to be registered with IANA. Additionally,
the IETF reserved values specified in this document will need to be
managed by IANA.
7. Security Considerations
TBD
8. Acknowledgements
The authors wish to thank Randy Presuhn and others for their review
and comments.
9 Normative References
[1] Stewart, R., Xie, Q., Stillman, M. and M. Tuexen, "Aggregate
Server Access Protocol (ASAP)", draft-ietf-rserpool-asap-10
(work in progress), October 2004.
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[2] Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L., Loughney,
J. and M. Stillman, "Requirements for Reliable Server Pooling",
RFC 3237, January 2002.
[3] Tuexen, M., Xie, Q., Stewart, R., Shore, M. and J. Loughney,
"Architecture for Reliable Server Pooling",
draft-ietf-rserpool-arch-08 (work in progress), October 2004.
[4] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Stewart, R., Xie, Q., Stillman, M. and M. Tuexen, "Aggregate
Server Access Protocol (ASAP) and Endpoint Name Resolution
(ENRP) common parameters document",
draft-ietf-rserpool-common-param-07 (work in progress), October
2004.
Authors' Addresses
Qiaobing Xie
Motorola, Inc.
1501 W. Shure Drive, 2-F9
Arlington Heights, IL 60004
US
Phone: +1-847-632-3028
EMail: qxie1@email.mot.com
La Monte H.P. Yarroll
TimeSys Corporation
925 Liberty Avenue
Pittsburgh, PA 15122
USA
Phone: +1-412-325-6391
EMail: piggy@acm.org
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