Network Working Group P. Lei
Internet-Draft Cisco Systems, Inc.
Intended status: Informational L. Ong
Expires: October 30, 2007 Ciena Corporation
M. Tuexen
Muenster Univ. of Applied Sciences
April 28, 2007
An Overview of Reliable Server Pooling Protocols
draft-ietf-rserpool-overview-01.txt
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Aggregate Server Access Protocol (ASAP) Overview . . . . . . . 5
2.1. Pool Initialization . . . . . . . . . . . . . . . . . . . 5
2.2. Pool Entity Registration . . . . . . . . . . . . . . . . . 5
2.3. Pool Entity Selection . . . . . . . . . . . . . . . . . . 6
2.4. Endpoint Keepalive . . . . . . . . . . . . . . . . . . . . 6
2.5. Failover Services . . . . . . . . . . . . . . . . . . . . 6
2.5.1. Cookie Mechanism . . . . . . . . . . . . . . . . . . . 6
2.5.2. Business Card Mechanism . . . . . . . . . . . . . . . 7
3. Endpoint Nameserver Redundancy Protocol (ENRP) Overview . . . 7
3.1. Initialization . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Server Discovery and Home Server Selection . . . . . . . . 7
3.3. Server Pool Maintenance . . . . . . . . . . . . . . . . . 8
4. Example Scenarios . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Example Scenario Using RSerPool Resolution Service . . . . 8
4.1.1. Standalone Mode . . . . . . . . . . . . . . . . . . . 8
4.1.2. Pool Mode . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Example Scenario Using RSerPool Session Services . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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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 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 dependancies on
intermediate equipment
o separation of session availability/failover functionality from
application itself
o facilitate different server selection policies
o facilitate 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:
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.......................
------ . +-------+ .
/ ENRP \ . | | .
/---->| Server | . | PE 1 | .
| /--- \______/ . | | .
| | . +-------+ .
| | . .
| | . server pool .
| V . .
+-------+ . +-------+ .
| | . | | .
| PU 1 |-----------------.------| PE 2 | .
| | . | | .
+-------+ . +-------+ .
. .
. +-------+ .
. | | .
. | 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 deregisters out of the pool using the Aggregate Server Access
Protocol ASAP [2].
Each server pool is identified by a unique byte string called the
pool handle. The pool handle allows a mapping from the pool to a
specific Pool Element located by its IP address and port. The pool
handle is what is specified by the Pool User (PU) when it attempts to
access a server in the pool, again using ASAP. Both IPv4 and IPv6 PE
addresses are supported.
To resolve the pool handle to the address necessary to access a Pool
Element, the PU consults an entity called the Endpoint haNdlespace
Redundancy Protocol (ENRP) server. This server may be a standalone
server supporting many PUs or a part of the PU itself, however it is
envisioned that ENRP servers provide a fully distributed and fault-
tolerant registry service using ENRP [3] to maintain synchronization
of data concerning the pool handle mapping space.
Rserpool provides a number of tools to aid client migration between
servers on server failure: it allows the client to identify
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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.
The requirements for the Reliable Server Pooling effort are defined
in RFC3237 [1].
This document provides an overview of the RSerPool protocol suite,
specifically the Aggregate Server Access Protocol ASAP [2] and the
Endpoint Nameserver Redundancy Protocol ENRP [3].
In addition to the protocol specifications, there is a common
parameter format specification COMMON [4] for both protocols, as well
as a security threat analysis SEC [5].
2. Aggregate Server Access Protocol (ASAP) Overview
ASAP is a straight-forward implementation of a set of mechanisms
identified as necessary for support of the creation and maintenance
of pools of redundant servers. These mechanisms include:
o registration of a new server for the server pool
o deregistration of an existing server from the pool
o resolution of a pool 'handle' to a server or list of servers
o liveness detection for servers in the pool
o failover mechanisms for handling 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
deregisters. 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.
2.2. Pool Entity Registration
A new server joins an existing pool by sending a Registration message
in ASAP to an ENRP server, indicating the 'handle' of the pool that
it wishes to join, a pool identifier for itself (chosen randomly)
information about it's lifetime in the pool, and what transport
protocols and selection policies it supports. The ENRP server that
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it first contacts is called its Home ENRP server, and maintains a
list of subscriptions by the PE as well as performing 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
genereates a Handle Resolution message in ASAP 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 in
ASAP. The Resolution 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 supports
a few simply defined policies and allows the use of external server
selection input for more complex policies.
2.4. Endpoint Keepalive
ENRP servers monitor the status of pool elements using the ASAP Keep
Alive message. A Pool Element responds to the ASAP Keep Alive
message with an Ack response.
In addition, an endpoint can notify its home ENRP server that the PE
the endpoint was using has become unresponsive by sending the ENRP
server an Endpoint Unreachable message.
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 fail over the PU sends this last
received cookie to the new PE. This method provides a simple way of
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state sharing between the PEs. Please note that the old PE should
sign the cookie and the receiving PE should verify the 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 Nameserver Redundancy Protocol (ENRP) Overview
A server pool can be supported by one or more ENRP servers. If
multiple ENRP servers are used to support a single pool then the ENRP
protocol is used between the ENRP servers in order to maintain a
distributed, fault-tolerant real-time registry service. ENRP servers
communicate with each other in order to exchange information 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 to determine other ENRP servers from the responses
and select one as Mentor. A Mentor can be any peer ENRP server that
it selects to provide current data about the pool.
It then requests the most current data about the pool handlespace
from its Mentor server and unpacks received Handle Table Response
messages into its local database.
It is then 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 need only register with one
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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 setup
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. Server Pool Maintenance
PE failure detection, keepalive, etc. TBD
4. Example Scenarios
4.1. Example Scenario Using RSerPool Resolution Service
4.1.1. Standalone Mode
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.
4.1.2. Pool Mode
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:
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 gethostbyname()) to translate from a hostname to an IP
address, the application will invoke an RSerPool service
primitive GETPRIMARYSERVER that takes as input a pool handle, 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.
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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 the service
primitive 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.).
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(s).
2. The server should invoke the DEREGISTER service primitive to
remove itself from the server pool when shutting down.
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
[2]). Finally, when failures occur, these are reported to the pool
via signaling present in ASAP [2] and ENRP [3], 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 are used, all communication
between the pool user and the pool element is mediated by the
RSerPool framework, including session establishment and teardown, and
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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, 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
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
(described in Section 2.5.2) 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.
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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. Security Considerations
This document does not identify security requirements beyond those
already documented in the ENRP and ASAP protocol specifications.
6. IANA Considerations
This document does not require additional IANA actions beyond those
already identified in the ENRP and ASAP protocol specifications.
7. Acknowledgements
The authors wish to thank Maureen Stillman, Qiaobing Xie, Randall
Stewart, Scott Bradner, and many others for their invaluable
comments.
8. Normative References
[1] Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L., Loughney,
J., and M. Stillman, "Requirements for Reliable Server Pooling",
RFC 3237, January 2002.
[2] Stewart, R., "Aggregate Server Access Protocol (ASAP)",
draft-ietf-rserpool-asap-15 (work in progress), January 2007.
[3] Stewart, R., "Endpoint Handlespace Redundancy Protocol (ENRP)",
draft-ietf-rserpool-enrp-15 (work in progress), January 2007.
[4] Stewart, R., "Aggregate Server Access Protocol (ASAP) and
Endpoint Handlespace Redundancy Protocol (ENRP) Parameters",
draft-ietf-rserpool-common-param-11 (work in progress),
October 2006.
[5] Stillman, M., "Threats Introduced by Rserpool and Requirements
for Security in response to Threats",
draft-ietf-rserpool-threats-06 (work in progress),
November 2006.
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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
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