Network Working Group T. Dreibholz
Internet-Draft University of Duisburg-Essen
Intended status: Experimental X. Zhou
Expires: January 6, 2010 Hainan University
July 5, 2009
Definition of a Delay Measurement Infrastructure and Delay-Sensitive
Least-Used Policy for Reliable Server Pooling
draft-dreibholz-rserpool-delay-04.txt
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Abstract
This document contains the definition of a delay measurement
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infrastructure and a delay-sensitive Least-Used policy for Reliable
Server Pooling.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Delay-Measurement Infrastructure . . . . . . . . . . . . . . . 3
2.1. Quantification of Distance . . . . . . . . . . . . . . . . 3
2.2. Distance Measurement Environment . . . . . . . . . . . . . 4
3. Distance-Sensitive Least-Used Policy . . . . . . . . . . . . . 4
3.1. Description . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. ENRP Server Considerations . . . . . . . . . . . . . . . . 5
3.3. Pool User Considerations . . . . . . . . . . . . . . . . . 5
3.4. Pool Member Selection Policy Parameter . . . . . . . . . . 5
4. Reference Implementation . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
Reliable Server Pooling defines protocols for providing highly
available services. PEs of a pool may be distributed over a large
geographical area, in order to provide redundancy in case of
localized disasters. But the current pool policies defined in
[RFC5356] do not incorporate the fact of distances (i.e. delay)
between PU and PE. This leads to a low performance for delay-
sensitive applications.
1.1. Scope
This draft defines a delay measurement infrastructure for ENRP
servers to add delay information into the handlespace. Furthermore,
a delay-sensitive Least-Used policy is defined. Performance
evaluations can be found in [KiVS2007].
1.2. Terminology
The terms are commonly identified in related work and can be found in
the Aggregate Server Access Protocol and Endpoint Handlespace
Redundancy Protocol Common Parameters document [RFC5354].
1.3. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Delay-Measurement Infrastructure
This section describes the necessary delay measurement infrastructure
for the policy later defined in Section 3. It has to be provided as
part of the ENRP servers.
2.1. Quantification of Distance
Measuring delay for SCTP associations is easy: the SCTP protocol
[RFC4960] already calculates a smoothed round-trip time (RTT) for the
primary path. This RTT only has to be queried via the standard SCTP
API as defined in [I-D.ietf-tsvwg-sctpsocket]. By default, the
calculated RTT has a small restriction: a SCTP endpoint waits up to
200ms before acknowledging a packet, in order to piggyback the
acknowledgement chunk with payload data. In this case, the RTT would
include this latency. Using the option SCTP_DELAYED_ACK_TIME (see
[I-D.ietf-tsvwg-sctpsocket]), the maximum delay before acknowledging
a packet can be set to 0ms (i.e. "acknowledge as soon as possible").
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After that, the RTT approximately consists of the network latency
only. Then, using the RTT, the end-to-end delay between two
associated components is approximately 0.5*RTT.
In real networks, there may be negligible delay differences: for
example, the delay between a PU and PE #1 is 5ms and the latency
between the PU and PE #2 is 6ms. From the service user's
perspective, such minor delay differences may be ignored and are
furthermore unavoidable in Internet scenarios. Therefore, the
distance parameter between two components A and B is defined as
follows:
Distance = DistanceStep * round( (0.5*RTT) / DistanceStep )
That is, the distance parameter is defined as the nearest integer
multiple of the constant DistanceStep for the measured delay (i.e.
0.5*RTT).
2.2. Distance Measurement Environment
In order to define a distance-aware policy, it is first necessary to
define a basic rule: PEs and PUs choose "nearby" ENRP servers. Since
the operation scope of RSerPool is restricted to a single
organization, this condition can be met easily by appropriately
locating ENRP servers.
o A Home ENRP server can measure the delay of the ASAP associations
to its PE. As part of its ENRP updates to other ENRP servers, it
can report this measured delay together with the PE information.
o A non-Home-ENRP server receiving such an update simply adds the
delay of the ENRP association with the Home ENRP server to the
PE's reported delay.
Now, each ENRP server can approximate the distance to every PE in the
operation scope using the equation in Section 2.1.
Note, that delay changes are propagated to all ENRP servers upon PE
re-registrations, i.e. the delay information (and the approximated
distance) dynamically adapts to the state of the network.
3. Distance-Sensitive Least-Used Policy
In this section, a distance-sensitive Least Used policy is defined,
based on the delay-measurement infrastructure introduced in
Section 2.
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3.1. Description
The Least Used with Distance Penalty Factor (LU-DPF) policy uses load
information provided by the pool elements to select the lowest-loaded
pool elements within the pool. If there are multiple elements having
lowest load, the nearest PE should be chosen.
3.2. ENRP Server Considerations
The ENRP server SHOULD select at most the requested number of pool
elements. Their load values SHOULD be the lowest possible ones
within the pool and their distances also SHOULD be lowest. Each
element MUST NOT be reported more than once to the pool user. If
there is a choice of equal-loaded and equal-distanced pool elements,
round robin selection SHOULD be made among these elements. The
returned list of pool elements MUST be sorted by load value in
ascending order (1st key) and distance in ascending order (2nd key).
3.3. Pool User Considerations
The pool user should try to use the pool elements returned from the
list in the order returned by the ENRP server. A subsequent call for
handle resolution may result in the same list. Therefore, it is
RECOMMENDED for a pool user to request multiple entries in order to
have a sufficient amount of feasible backup entries available.
3.4. Pool Member Selection Policy Parameter
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0x6 | Length = 0x14 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Policy Type = 0x40000010 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Load |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Load DPF |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Distance |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Load: Current load of the pool element.
o Load DPF: The LoadDPF setting of the PE.
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o Distance: The approximated distance in milliseconds.
* Between PE and Home ENRP server: The distance SHOULD be set to
0.
* Between Non-Home ENRP server and Home ENRP server: The delay
measured on the ASAP association between Home ENRP server and
PE.
* Between ENRP server and PU: The sums of the measured delays on
the ASAP association and the ENRP association to the Home ENRP
server.
4. Reference Implementation
The RSerPool reference implementation RSPLIB can be found at
[RSerPoolPage]. It supports the functionalities defined by
[RFC5351], [RFC5352], [RFC5353], [RFC5354] and [RFC5356] as well as
the options [I-D.dreibholz-rserpool-asap-hropt],
[I-D.dreibholz-rserpool-enrp-takeover] and of course the option
defined by this document. An introduction to this implementation is
provided in [Dre2006].
5. Security Considerations
Security considerations for RSerPool systems are described by
[RFC5355].
6. IANA Considerations
This document does not require additional IANA actions beyond those
already identified in the ENRP and ASAP protocol specifications.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5351] Lei, P., Ong, L., Tuexen, M., and T. Dreibholz, "An
Overview of Reliable Server Pooling Protocols", RFC 5351,
September 2008.
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[RFC5352] Stewart, R., Xie, Q., Stillman, M., and M. Tuexen,
"Aggregate Server Access Protocol (ASAP)", RFC 5352,
September 2008.
[RFC5353] Xie, Q., Stewart, R., Stillman, M., Tuexen, M., and A.
Silverton, "Endpoint Handlespace Redundancy Protocol
(ENRP)", RFC 5353, September 2008.
[RFC5354] Stewart, R., Xie, Q., Stillman, M., and M. Tuexen,
"Aggregate Server Access Protocol (ASAP) and Endpoint
Handlespace Redundancy Protocol (ENRP) Parameters",
RFC 5354, September 2008.
[RFC5355] Stillman, M., Gopal, R., Guttman, E., Sengodan, S., and M.
Holdrege, "Threats Introduced by Reliable Server Pooling
(RSerPool) and Requirements for Security in Response to
Threats", RFC 5355, September 2008.
[RFC5356] Dreibholz, T. and M. Tuexen, "Reliable Server Pooling
Policies", RFC 5356, September 2008.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
7.2. Informative References
[I-D.ietf-tsvwg-sctpsocket]
Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P.
Lei, "Sockets API Extensions for Stream Control
Transmission Protocol (SCTP)",
draft-ietf-tsvwg-sctpsocket-19 (work in progress),
February 2009.
[KiVS2007]
Dreibholz, T. and E. Rathgeb, "On Improving the
Performance of Reliable Server Pooling Systems for
Distance-Sensitive Distributed Applications",
Proceedings of the 15. ITG/GI Fachtagung Kommunikation in
Verteilten Systemen, February 2007.
[Dre2006] Dreibholz, T., "Reliable Server Pooling -- Evaluation,
Optimization and Extension of a Novel IETF Architecture",
Ph.D. Thesis University of Duisburg-Essen, Faculty of
Economics, Institute for Computer Science and Business
Information Systems, URL: http://
duepublico.uni-duisburg-essen.de/servlets/DerivateServlet/
Derivate-16326/Dre2006-final.pdf, March 2007.
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[RSerPoolPage]
Dreibholz, T., "Thomas Dreibholz's RSerPool Page",
URL: http://tdrwww.iem.uni-due.de.de/dreibholz/rserpool/.
[I-D.dreibholz-rserpool-asap-hropt]
Dreibholz, T., "Handle Resolution Option for ASAP",
draft-dreibholz-rserpool-asap-hropt-04 (work in progress),
January 2009.
[I-D.dreibholz-rserpool-enrp-takeover]
Dreibholz, T. and X. Zhou, "Takeover Suggestion Flag for
the ENRP Handle Update Message",
draft-dreibholz-rserpool-enrp-takeover-01 (work in
progress), January 2009.
Authors' Addresses
Thomas Dreibholz
University of Duisburg-Essen, Institute for Experimental Mathematics
Ellernstrasse 29
45326 Essen, Nordrhein-Westfalen
Germany
Phone: +49-201-1837637
Fax: +49-201-1837673
Email: dreibh@iem.uni-due.de
URI: http://www.iem.uni-due.de/~dreibh/
Xing Zhou
Hainan University, College of Information Science and Technology
Renmin Avenue 58
570228 Haikou, Hainan
China
Phone: +86-898-66279141
Email: zhouxing@hainu.edu.cn
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