Network Working Group P. Lei
Request for Comments: 5351 Cisco Systems, Inc.
Category: Informational L. Ong
Ciena Corporation
M. Tuexen
Muenster Univ. of Applied Sciences
T. Dreibholz
University of Duisburg-Essen
September 2008
An Overview of Reliable Server Pooling Protocols
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
The Reliable Server Pooling effort (abbreviated "RSerPool") provides
an application-independent set of services and protocols for building
fault-tolerant and highly available client/server applications. This
document provides an overview of the protocols and mechanisms in the
Reliable Server Pooling suite.
Lei, et al. Informational [Page 1]RFC 5351 RSerPool Overview September 2008
Table of Contents
1. Introduction ....................................................3
2. Aggregate Server Access Protocol (ASAP) Overview ................6
2.1. Pool Initialization ........................................6
2.2. Pool Entity Registration ...................................6
2.3. Pool Entity Selection ......................................7
2.4. Endpoint Keep-Alive ........................................7
2.5. Failover Services ..........................................7
2.5.1. Cookie Mechanism ....................................7
2.5.2. Business Card Mechanism .............................8
3. Endpoint Handlespace Redundancy Protocol (ENRP) Overview ........8
3.1. Initialization .............................................8
3.2. Server Discovery and Home Server Selection .................8
3.3. Failure Detection, Handlespace Audit, and Synchronization ..9
3.4. Server Takeover ............................................9
4. Example Scenarios ...............................................9
4.1. Example Scenario Using RSerPool Resolution Service .........9
4.2. Example Scenario Using RSerPool Session Services ..........11
5. Reference Implementation .......................................12
6. Security Considerations ........................................12
7. IANA Considerations ............................................12
8. Acknowledgements ...............................................12
9. References .....................................................13
9.1. Normative References ......................................13
9.2. Informative References ....................................13
Lei, et al. Informational [Page 2]RFC 5351 RSerPool Overview September 2008
1. Introduction
The Reliable Server Pooling (RSerPool) protocol suite is designed to
provide client applications ("pool users") with the ability to select
a server (a "pool element") from among a group of servers providing
equivalent service (a "pool"). The protocols are currently targeted
for Experimental Track.
The RSerPool architecture supports high availability and load
balancing by enabling a pool user to identify the most appropriate
server from the server pool at a given time. The architecture is
defined to support a set of basic goals:
o application-independent protocol mechanisms
o separation of server naming from IP addressing
o use of the end-to-end principle to avoid dependencies on
intermediate equipment
o separation of session availability/failover functionality from the
application itself
o facilitation of different server selection policies
o facilitation of a set of application-independent failover
capabilities
o peer-to-peer structure
The basic components of the RSerPool architecture are shown in
Figure 1 below:
Lei, et al. Informational [Page 3]RFC 5351 RSerPool Overview September 2008
.......................
______ ______ . +-------+ .
/ ENRP \ / ENRP \ . | | .
|Server| <----> |Server|<----------.----->| PE 1 | .
\______/ ENRP \______/ ASAP(1) . | | .
^ . +-------+ .
| . .
| ASAP(2) . Server Pool .
V . .
+-------+ . +-------+ .
| | . | | .
| PU |<---------->. | PE 2 | .
| | PU to PE . | | .
+-------+ . +-------+ .
. .
. +-------+ .
. | | .
. | PE 3 | .
. | | .
. +-------+ .
.......................
Figure 1
A server pool is defined as a set of one or more servers providing
the same application functionality. The servers are called Pool
Elements (PEs). Multiple PEs in a server pool can be used to provide
fault tolerance or load sharing, for example. The PEs register into
and de-register out of the pool at an entity called the Endpoint
haNdlespace Redundancy Protocol (ENRP) server, using the Aggregate
Server Access Protocol (ASAP) [RFC5352] (this association is labeled
ASAP(1) in the figure).
Each server pool is identified by a unique byte string called the
pool handle (PH). The pool handle allows a mapping from the pool to
a specific PE located by its IP address (both IPv4 and IPv6 PE
addresses are supported) and port number. The pool handle is what is
specified by the Pool User (PU) when it attempts to access a server
in the pool. To resolve the pool handle to the address necessary to
access a PE, the PU consults an ENRP server using ASAP (this
association is labeled ASAP(2) in the figure). The space of pool
handles is assumed to be a flat space with limited operational scope
(see RFC 3237 [RFC3237]). Administration of pool handles is not
addressed by the RSerPool protocol documents at this time. The
protocols used between the PU and PE are application-specific. It is
assumed that the PU and PE are configured to support a common set of
protocols for application layer communication, independent of the
RSerPool mechanisms.
Lei, et al. Informational [Page 4]RFC 5351 RSerPool Overview September 2008
RSerPool provides a number of tools to aid client migration between
servers on server failure: it allows the client to identify
alternative servers, either on initial discovery or in real time; it
also allows the original server to provide a state cookie to the
client that can be forwarded to an alternative server to provide
application-specific state information. This information is
exchanged between the PE and PU directly, over the association
labeled PU to PE in the figure.
It is envisioned that ENRP servers provide a fully distributed and
fault-tolerant registry service. They use ENRP [RFC5353] to maintain
synchronization of data concerning the pool handle mapping space.
For PUs and PEs, the ENRP servers are functionally equal. Due to the
synchronization provided by ENRP, they can contact an arbitrary one
for registration/de-registration (PE) or PH resolution (PU). An
illustration containing 3 ENRP servers is provided in Figure 2 below:
______ _____
... / ENRP \ / ENRP \ ...
PEs/PUs <---->|Server| <----> |Server|<----> PEs/PUs
... ASAP \______/ ENRP \______/ ASAP ...
^ ^
| |
| / ENRP \ |
+---->|Server|<----+
ENRP \______/ ENRP
^
| ASAP
v
...
PEs/PUs
...
Figure 2
The requirements for the Reliable Server Pooling framework are
defined in RFC 3237 [RFC3237]. It is worth noting that the
requirements on RSerPool in the area of load balancing
partially overlap with grid computing/high-performance
computing. However, the scope of both areas is completely
different: grid and high-performance computing also cover
topics like managing different administrative domains, data
locking and synchronization, inter-session communication, and
resource accounting for powerful computation services, but the
intention of RSerPool is simply a lightweight realization of
load distribution and session management. In particular, these
functionalities are intended to be used on
Lei, et al. Informational [Page 5]RFC 5351 RSerPool Overview September 2008
systems with small memory and CPU resources only. Any further
functionality is not in the scope of RSerPool and can -- if
necessary -- be provided by the application itself.
This document provides an overview of the RSerPool protocol
suite, specifically the Aggregate Server Access Protocol (ASAP)
[RFC5352] and the Endpoint Handlespace Redundancy Protocol
(ENRP) [RFC5353]. In addition to the protocol specifications,
there is a common parameter format specification [RFC5354] for
both protocols, a definition of server selection rules (pool
policies) [RFC5356], as well as a security threat analysis
[RFC5355].
2. Aggregate Server Access Protocol (ASAP) Overview
ASAP defines a straightforward set of mechanisms necessary to support
the creation and maintenance of pools of redundant servers. These
mechanisms include:
o registration of a new server into a server pool
o de-registration of an existing server from a pool
o resolution of a pool handle to a server or list of servers
o liveness detection for servers in a pool
o failover mechanisms for handling a server failure
2.1. Pool Initialization
Pools come into existence when a PE registers the first instance of
the pool name with an ENRP server. They disappear when the last PE
de-registers. In other words, the starting of the first PE on some
machine causes the creation of the pool when the registration reaches
the ENRP server.
It is assumed that information needed for RSerPool, such as the
address of an ENRP server to contact, is configured into the PE
beforehand. Methods of automating this configuration process are not
addressed at this time.
2.2. Pool Entity Registration
A new server joins an existing pool by sending a Registration message
via ASAP to an ENRP server, indicating the pool handle of the pool
that it wishes to join, a PE identifier for itself (chosen randomly),
information about its lifetime in the pool, and what transport
Lei, et al. Informational [Page 6]RFC 5351 RSerPool Overview September 2008
protocols and selection policy it supports. The ENRP server that it
first contacts is called its Home ENRP server, and maintains a list
of subscriptions by the PE as well as performs periodic audits to
confirm that the PE is still responsive.
Similar procedures are applied to de-register itself from the server
pool (or, alternatively, the server may simply let the lifetime that
it previously registered with expire, after which it is gracefully
removed from the pool).
2.3. Pool Entity Selection
When an endpoint wishes to be connected to a server in the pool, it
generates an ASAP Handle Resolution message and sends this to its
Home ENRP server. The ENRP server resolves the handle based on its
knowledge of pool servers and returns a Handle Resolution Response
message via ASAP. The response contains a list of the IP addresses
of one or more servers in the pool that can be contacted. The
process by which the list of servers is created may involve a number
of policies for server selection. The RSerPool protocol suite
defines a few basic policies and allows the use of external server
selection input for more complex policies.
2.4. Endpoint Keep-Alive
ENRP servers monitor the status of pool elements using the ASAP
Endpoint Keep-Alive message. A PE responds to the ASAP Keep-Alive
message with an Endpoint Keep-Alive Ack response.
In addition, a PU can notify its Home ENRP server that the PE it used
has become unresponsive by sending an ASAP Endpoint Unreachable
message to the ENRP server.
2.5. Failover Services
While maintaining application-independence, the RSerPool protocol
suite provides some simple hooks for supporting failover of an
individual session with a pool element. Generally, mechanisms for
failover that rely on application state or transaction status cannot
be defined without more specific knowledge of the application being
supported. However, some simple mechanisms supported by RSerPool
allow some level of failover that any application can use.
2.5.1. Cookie Mechanism
Cookies may optionally be generated by the ASAP layer and
periodically sent from the PE to the PU. The PU only stores the last
received cookie. In case of failover, the PU sends this last
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RFC 5351 RSerPool Overview September 2008
received cookie to the new PE. This method provides a simple way of
state sharing between the PEs. Please note that the old PE should
sign the cookie, and the receiving PE should verify that signature.
For the PU, the cookie has no structure and is only stored and
transmitted to the new PE.
2.5.2. Business Card Mechanism
A PE can send a business card to its peer (PE or PU) containing its
pool handle and guidance concerning which other PEs the peer should
use for failover. This gives a PE a means of telling a PU what it
identifies as the "next best" PE to use in case of failure, which may
be based on pool considerations, such as load balancing, or user
considerations, such as PEs that have the most up-to-date state
information.
3. Endpoint Handlespace Redundancy Protocol (ENRP) Overview
A set of server pools, which is denoted as a handlespace, is managed
by ENRP servers. Pools are not valid in the whole Internet but only
in smaller domains, called the operational scope. The ENRP servers
use the ENRP protocol in order to maintain a distributed, fault-
tolerant, real-time registry service. ENRP servers communicate with
each other for information exchange, such as pool membership changes,
handlespace data synchronization, etc.
3.1. Initialization
Each ENRP server initially generates a 32-bit server ID that it uses
in subsequent messaging and remains unchanged over the lifetime of
the server. It then attempts to learn all of the other ENRP servers
within the scope of the server pool, either by using a pre-defined
Mentor server or by sending out Presence messages on a well-known
multicast channel in order to determine other ENRP servers from the
responses and select one as Mentor. A Mentor can be any peer ENRP
server. The most current handlespace data is requested by Handle
Table Requests from the Mentor. The received answer in the form of
Handle Table Response messages is unpacked into the local database.
After that, the ENRP server is ready to provide ENRP services.
3.2. Server Discovery and Home Server Selection
PEs can now register their presence with the newly functioning ENRP
server by using ASAP messages. They discover the new ENRP server
after the server sends out an ASAP Server Announce message on the
well-known ASAP multicast channel. PEs only have to register with
Lei, et al. Informational [Page 8]
RFC 5351 RSerPool Overview September 2008
one ENRP server, as other ENRP servers supporting the pool will
synchronize their knowledge about pool elements using the ENRP
protocol.
The PE may have a configured list of ENRP servers to talk to, in the
form of a list of IP addresses, in which case it will start to set up
associations with some number of them and assign the first one that
responds to it as its Home ENRP server.
Alternatively, it can listen on the multicast channel for a set
period, and when it hears an ENRP server, start an association. The
first server it gets up can then become its Home ENRP server.
3.3. Failure Detection, Handlespace Audit, and Synchronization
ENRP servers send ENRP Presence messages to all of their peers in
order to show their liveness. These Presence messages also include a
checksum computed over all PE identities for which the ENRP server is
in the role of a Home ENRP server. Each ENRP server maintains an up-
to-date list of its peers and can also compute the checksum expected
from a certain peer, according to its local handlespace database. By
comparing the expected sum and the sum reported by a peer (denoted as
handlespace audit), an inconsistency can be detected. In such a
case, the handlespace -- restricted to the PEs owned by that peer --
can be requested for synchronization, analogously to Section 3.2.
3.4. Server Takeover
If the unresponsiveness of an ENRP server is detected, the remaining
ENRP servers negotiate which other server takes over the Home ENRP
role for the PEs of the failed peer. After reaching a consensus on
the takeover, the ENRP server taking over these PEs sends a
notification to its peers (via ENRP) as well as to the PEs taken over
(via ASAP).
4. Example Scenarios
4.1. Example Scenario Using RSerPool Resolution Service
RSerPool can be used in a 'standalone' manner, where the application
uses RSerPool to determine the address of a primary server in the
pool, and then interacts directly with that server without further
use of RSerPool services. If the initial server fails, the
application uses RSerPool again to find the next server in the pool.
For pool user ("client") applications, if an ASAP implementation is
available on the client system, there are typically only three
modifications required to the application source code:
Lei, et al. Informational [Page 9]RFC 5351 RSerPool Overview September 2008
1. Instead of specifying the hostnames of primary, secondary,
tertiary servers, etc., the application user specifies a pool
handle.
2. Instead of using a DNS-based service (e.g., the Unix library
function getaddrinfo()) to translate from a hostname to an IP
address, the application will invoke an RSerPool service
primitive provisionally named GETPRIMARYSERVER that takes a pool
handle as input, and returns the IP address of the primary
server. The application then uses that IP address just as it
would have used the IP address returned by the DNS in the
previous scenario.
3. Without the use of additional RSerPool services, failure
detection and failover procedures must be designed into each
application. However, when failure is detected on the primary
server, instead of invoking DNS translation again on the hostname
of a secondary server, the application invokes a service
primitive provisionally named GETNEXTSERVER, which performs two
functions in a single operation.
1. First, it indicates to the RSerPool layer the failure of the
server returned by a previous GETPRIMARYSERVER or
GETNEXTSERVER call.
2. Second, it provides the IP address of the next server that
should be contacted, according to the best information
available to the RSerPool layer at the present time (e.g.,
set of available pool elements, pool element policy in effect
for the pool, etc.).
Note: at the time of this document, a full API for use with RSerPool
protocols has not yet been defined.
For pool element ("server") applications where an ASAP implementation
is available, two changes are required to the application source
code:
1. The server should invoke the REGISTER service primitive upon
startup to add itself into the server pool using an appropriate
pool handle. This also includes the address(es) protocol or
mapping id, port (if required by the mapping), and pooling policy
(or policies).
2. The server should invoke the DEREGISTER service primitive to
remove itself from the server pool when shutting down.
Lei, et al. Informational [Page 10]RFC 5351 RSerPool Overview September 2008
When using these RSerPool services, RSerPool provides benefits that
are limited (as compared to utilizing all services), but nevertheless
quite useful as compared to not using RSerPool at all. First, the
client user need only supply a single string, i.e., the pool handle,
rather than a list of servers. Second, the decision as to which
server is to be used can be determined dynamically by the server
selection mechanism (i.e., a "pool policy" performed by ASAP; see
ASAP [RFC5352]). Finally, when failures occur, these are reported to
the pool via signaling present in ASAP [RFC5352] and ENRP [RFC5353];
other clients will eventually know (once this failure is confirmed by
other elements of the RSerPool architecture) that this server has
failed.
4.2. Example Scenario Using RSerPool Session Services
When the full suite of RSerPool services is used, all communication
between the pool user and the pool element is mediated by the
RSerPool framework, including session establishment and teardown, and
the sending and receiving of data. Accordingly, it is necessary to
modify the application to use the service primitives (i.e., the API)
provided by RSerPool, rather than the transport layer primitives
provided by TCP, Stream Control Transmission Protocol (SCTP), or
whatever transport protocol is being used.
As in the previous case, sessions (rather than connections or
associations) are established, and the destination endpoint is
specified as a pool handle rather than as a list of IP addresses with
a port number. However, failover from one pool element to another is
fully automatic, and can be transparent to the application (so long
as the application has saved enough state in a state cookie):
The RSerPool framework control channel provides maintenance
functions to keep pool element lists, policies, etc. current.
Since the application data (e.g., data channel) is managed by the
RSerPool framework, unsent data (data not yet submitted by
RSerPool to the underlying transport protocol) is automatically
redirected to the newly selected pool element upon failover. If
the underlying transport layer supports retrieval of unsent data
(as in SCTP), retrieved unsent data can also be automatically
re-sent to the newly selected pool element.
An application server (pool element) can provide a state cookie
(described in Section 2.5.1) that is automatically passed on to
another pool element (by the ASAP layer at the pool user) in the
event of a failover. This state cookie can be used to assist the
application at the new pool element in recreating whatever state
is needed to continue a session or transaction that was
Lei, et al. Informational [Page 11]RFC 5351 RSerPool Overview September 2008
interrupted by a failure in the communication between a pool user
and the original pool element.
The application client (pool user) can provide a callback function
that is invoked on the pool user side in the case of a failover.
This callback function can execute any application-specific
failover code, such as generating a special message (or sequence
of messages) that helps the new pool element construct any state
needed to continue an in-process session.
Suppose in a particular peer-to-peer application, PU A is
communicating with PE B, and it so happens that PU A is also a PE
in pool X. PU A can pass a "business card" to PE B identifying it
as a member of pool X. In the event of a failure at A, or a
failure in the communication link between A and B, PE B can use
the information in the business card to contact an equivalent PE
to PU A from pool X.
Additionally, if the application at PU A is aware of some
particular PEs of pool X that would be preferred for B to contact
in the event that A becomes unreachable from B, PU A can provide
that list to the ASAP layer, and it will be included in A's
business card (see Section 2.5.2).
5. Reference Implementation
A reference implementation of RSerPool is available at [RSerPoolPage]
and described in [Dre2006].
6. Security Considerations
This document does not identify security requirements beyond those
already documented in the ENRP and ASAP protocol specifications. A
security threat analysis of RSerPool is provided in THREATS
[RFC5355].
7. IANA Considerations
This document does not require additional IANA actions beyond those
already identified in the ENRP [RFC5353] and ASAP [RFC5352] protocol
specifications.
8. Acknowledgements
The authors wish to thank Maureen Stillman, Qiaobing Xie, Randall
Stewart, Scott Bradner, and many others for their invaluable
comments.
Lei, et al. Informational [Page 12]RFC 5351 RSerPool Overview September 2008
9. References
9.1. Normative References
[RFC3237] Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L.,
Loughney, J., and M. Stillman, "Requirements for
Reliable Server Pooling", RFC 3237, January 2002.
[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., Ed., Gopal, R., Guttman, E., Holdrege,
M., and S. Sengodan, "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.
9.2. Informative References
[RSerPoolPage] Dreibholz, T., "Thomas Dreibholz's RSerPool Page",
<http://tdrwww.iem.uni-due.de/dreibholz/rserpool/>.
[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,
March 2007, <http://duepublico.uni-duisburg-essen.de/
servlets/DerivateServlet/Derivate-16326/
Dre2006-final.pdf>.
Lei, et al. Informational [Page 13]RFC 5351 RSerPool Overview September 2008
Authors' Addresses
Peter Lei
Cisco Systems, Inc.
955 Happfield Dr.
Arlington Heights, IL 60004
US
Phone: +1 773 695-8201
EMail: peterlei@cisco.com
Lyndon Ong
Ciena Corporation
PO Box 308
Cupertino, CA 95015
US
EMail: Lyong@Ciena.com
Michael Tuexen
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
EMail: tuexen@fh-muenster.de
Thomas Dreibholz
University of Duisburg-Essen, Institute for Experimental Mathematics
Ellernstrasse 29
45326 Essen, Nordrhein-Westfalen
Germany
Phone: +49 201 183-7637
Fax: +49 201 183-7673
EMail: dreibh@iem.uni-due.de
URI: http://www.iem.uni-due.de/~dreibh/
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RFC 5351 RSerPool Overview September 2008
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Lei, et al. Informational [Page 15]
