Network Working Group T. Dreibholz
Internet-Draft University of Duisburg-Essen
Intended status: Standards Track X. Zhou
Expires: July 13, 2008 Hainan University
January 10, 2008
Definition of a Delay Measurement Infrastructure and Delay-Sensitive
Least-Used Policy for Reliable Server Pooling
draft-dreibholz-rserpool-delay-01.txt
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Abstract
This document contains the definition of a delay measurement
infrastructure and a delay-sensitive Least-Used policy for Reliable
Server Pooling.
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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 [5] 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 [8].
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 [4].
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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 [6]
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 [7]. 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 [7]), the maximum delay before
acknowledging a packet can be set to 0ms (i.e. "acknowledge as soon
as possible"). 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.
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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.
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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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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o Load: Current load of the pool element.
o Load DPF: The LoadDPF setting of the PE.
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.
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4. Reference Implementation
The reference implementation based on the RSerPool prototype
implementation rsplib can be found at [10].
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5. Security Considerations
This document does not identify security requirements beyond those
already documented in the ENRP and ASAP protocol specifications.
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6. IANA Considerations
This document does not require additional IANA actions beyond those
already identified in the ENRP and ASAP protocol specifications.
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7. References
7.1. Normative References
[1] Tuexen, M., "Architecture for Reliable Server Pooling",
draft-ietf-rserpool-arch-10 (work in progress), July 2005.
[2] Stewart, R., "Aggregate Server Access Protocol (ASAP)",
draft-ietf-rserpool-asap-13 (work in progress), February 2006.
[3] Stewart, R., "Endpoint Handlespace Redundancy Protocol (ENRP)",
draft-ietf-rserpool-enrp-13 (work in progress), February 2006.
[4] Stewart, R., "Aggregate Server Access Protocol (ASAP) and
Endpoint Handlespace Redundancy Protocol (ENRP) Parameters",
draft-ietf-rserpool-common-param-10 (work in progress),
February 2006.
[5] Tuexen, M. and T. Dreibholz, "Reliable Server Pooling
Policies", draft-ietf-rserpool-policies-02 (work in progress),
February 2006.
[6] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V.
Paxson, "Stream Control Transmission Protocol", RFC 2960,
October 2000.
[7] Stewart, R., "Sockets API Extensions for Stream Control
Transmission Protocol (SCTP)", draft-ietf-tsvwg-sctpsocket-14
(work in progress), December 2006.
7.2. Informative References
[8] 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.
[9] 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.
[10] Dreibholz, T., "Thomas Dreibholz's RSerPool Page",
URL: http://tdrwww.exp-math.uni-essen.de/dreibholz/rserpool/.
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[11] Dreibholz, T. and E. Rathgeb, "On the Performance of Reliable
Server Pooling Systems", Proceedings of the 30th IEEE Local
Computer Networks Conference, November 2005.
[12] Dreibholz, T. and E. Rathgeb, "Implementing the Reliable Server
Pooling Framework", Proceedings of the 8th IEEE International
Conference on Telecommunications, June 2005.
[13] Dreibholz, T., Zhou, X., and E. Rathgeb, "A Performance
Evaluation of RSerPool Server Selection Policies in Varying
Heterogeneous Capacity Scenarios", Proceedings of the 33rd IEEE
EuroMirco Conference on Software Engineering and Advanced
Applications, August 2007.
[14] Dreibholz, T. and E. Rathgeb, "An Application Demonstration of
the Reliable Server Pooling Framework", Proceedings of the 24th
IEEE Infocom, March 2005.
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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@exp-math.uni-essen.de
URI: http://www.exp-math.uni-essen.de/~dreibh/
Xing Zhou
Hainan University, College of Information Science and Technology
Renmin Road 58
570228 Haikou, Hainan
China
Phone: +86-898-66279141
Email: xing.zhou@uni-due.de
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