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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

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   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

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

   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

   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

       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

   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

   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

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

   Phone: +1 773 695-8201
   Email: peterlei@cisco.com

   Lyndon Ong
   Ciena Corporation
   PO Box 308
   Cupertino, CA  95015

   Email: Lyong@Ciena.com

   Michael Tuexen
   Muenster Univ. of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt

   Email: tuexen@fh-muenster.de

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