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RserPool Working Group                           J. Loughney (ed.)
INTERNET DRAFT                                         M. Stillman
                                                         M. Tuexen
                                                        Siemens AG
                                                            Q. Xie
                                                        R. Stewart
                                                            L. Ong

Expires May 21, 2001                              November 21, 2001

          Comparison of Protocols for Reliable Server Pooling

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of [RFC2026].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This document compares some related protocols, which overlap the
   Reliable Server Pooling problem space.  This document discusses the
   applicability of these protocols for Reliable Server Pooling. It
   intends to show why these protocols are not sufficient to accomplish
   the task of reliable server pooling.

Internet-Draft     Comparison of Protocols for RSerPool   Nov 21, 2001

1 Introduction.......................................................3
 1.1 Overview........................................................3
 1.2 Terminology.....................................................3
2 Relation to Other Solutions........................................4
 2.1 CORBA...........................................................4
 2.2 DNS.............................................................5
  2.2.1 Requirements.................................................5
  2.2.2 Technical Issues.............................................5
  2.2.3 Name/Address Resolution......................................7
 2.3 Service Location Protocol (SLP..................................8
  2.3.1 mSLP.........................................................9
3 Comparison Against Requirements....................................9
4 Security Concerns..................................................10
5 Acknowledgements...................................................10
6 References.........................................................10
7 Authors' Addresses.................................................11
Full Copyright Statement.............................................12

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1 Introduction

1.1 Overview

   In creating a solution to provide reliable server pools [RSER-ARCH],
   there are a number of existing protocols, which appear to have
   similar properties as to what RSerPool is trying to accomplish.
   This document discusses why these protocols are not sufficient to
   meet the requirements of Reliable Server Pooling [RSER-REQ].

   This study does not intend to be complete, rather intends to
   highlight several protocols which working group members have

1.2 Terminology

   This document uses the following terms:

   Operation scope -    The part of the network visible to pool users
                        by a specific instance of the reliable server
                        pooling protocols.

   Pool -               A collection of servers providing the same
                        application functionality. Also called a Server

   Pool handle -        A logical pointer to a pool. Each server pool
                        will be identifiable in the operation scope of
                        the system by a unique pool handle or "name".
                        Also called a Pool Name.

   Pool element -       A server entity having registered to a pool.

   Pool User -          A server pool user.

   Pool Element Handle - A logical pointer to a particular pool element
                        in a pool, consisting of the name of the pool
                        and a destination transport address of the pool
                        element.  Also called an Endpoint Handle.

   Name Space -         A cohesive structure of pool names and
                        relations that may be queried by an internal or
                        external agent.

   Name Server -        Entity which the responsible for managing and
                        maintaining the name space within the RSerPool
                        operation scope.

1.3 Abbreviations

   DA:    Directory Agent in SLP.

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   DPE:   Distributed Processing Environment

   CORBA: Common Object Request Broker Architecture.

   OMG:   Object Management Group

   PE:    Pool element

   PU:    Pool user

   SA:    Service Agent in SLP.

   SLP:   Service Location Protocol.

   UA:    User Agent in SLP.

2 Relation to Other Solutions

   This section is intended to discuss the applicability of some
   existing solutions with regards to Reliable Server Pooling
   requirements [RSER-REQ].  The protocols discussed have been
   suggested as possibly overlapping with the problems space of


   Often referred to as a Distributed Processing Environment (DPE),
   CORBA was mainly designed to provide location transparency for
   distributed applications. However, the following limitations may
   exist when applying CORBA to the design of real time fault-tolerant

   CORBA has not been focused on high availability. The recent
   development of a high availability version of CORBA by OMG may be a
   step in the right direction towards improving this situation.
   Nevertheless, the maturity, implementability, and real-time
   performance of the design is yet to be proven.

   CORBA's distribution model encourages an object-based view, i.e.,
   each communication endpoint is normally an object. This level of
   granularity is likely to be somewhat inefficient for designing real-
   time fault-tolerant applications.

   CORBA, in general, has a large signature that makes the use of it a
   challenge in real-time environments. Small devices with limited
   memory and CPU resource (e.g., H.323 or SIP terminals) will find
   CORBA hard to fit in.

   CORBA has lacked easily usable support for the asynchronous
   communication model, and this may be an issue in many applications.
   An improved API for asynchronous communication has been added to the
   CORBA standards recently, but many, if not most, CORBA

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   implementations do not yet support it. There is as yet insufficient
   user experience with it to make conclusions regarding this feature's

2.2 DNS

   This section will answer the question why DNS is not appropriate as
   the sole solution for RSerPool. In addition, it highlights specific
   technical differences between RSerPool and DNS.

   During the 49th IETF December 13, 2000 plenary meeting Randy Bush
   presented a talk entitled "The DNS Today: Are we overloading the
   Saddlebags on an Old Horse?" This talk underlined the issue that DNS
   is currently overloaded with extraneous tasks and has the potential
   to break down entirely due to a growing number of feature

   One requirement to any solution proposed by RSerPool would be to
   avoid any additional requirements for DNS in order to support
   Reliable Server Pooling. Interworking between DNS and RSerPool will
   be considered so that additional burdens to DNS will not be added.

2.2.1 Requirements

   Any solution for RSerPool should meet certain requirements [RSER-
   REQ].  These requirements are related to DNS.

      Servers should be able to register to (become PEs) and deregister
      from a server pool transparently without an interruption in

      The RSerPool mechanisms must be able to support different server
      selection mechanisms. These are called server pool policies.

      The RSerPool architecture must be able to detect server failure
      quickly and be able to perform failover without service

      Server pools are identified by pool handles. These pool handles
      are only valid inside the operation scope. Interoperability
      between different namespaces has to be provided by other

2.2.2 Technical Issues

   This section discusses the relationship between DNS and the
   requirements for RserPool. Host Resolver Problems

   A major issue that prevents the use of DNS as part of the RSerPool
   solution the issue is the architecture of host resolvers. These are

Loughney (editor)                                             [Page 5]

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   stub resolvers - which means that they require their local DNS
   servers to do recursion for them.

   In turn, this implies that setting TTL low or 0 will dramatically
   increase the load not only on the authoritative DNS servers - but
   also on these third party servers.

   A secondary effect of this is that the authoritative DNS will not
   know the IP address of the DNS client - only the IP address of the
   local DNS. This affects the ability to do global load balancing

   There is no way to get around these issues unless you all hosts
   would be full resolvers. Putting full resolvers on newer hosts isn't
   sufficient because the issues would still exist for all the legacy
   systems, which will form the bulk of the host population for years
   to come. The solution is not to use third party servers.

   Additionally, if the client can contact the server directly, then
   the server knows the real IP address of the client. Since there is
   no third party involved, the caching TTL can be set as low as
   desired (even to zero). That will increase load on the server, but
   nowhere else.

   Finally, DNS is based on a recursion. This recursion presents
   certain difficulties for RSerPool. Even if a host resolver is not a
   stub resolver, it has to go to another full resolver where 2
   possibilities exists: either the mapping name-IP address is found or
   it has to do another recursive resolution of the name, staring from
   that intermediate resolver, until there is a cache hit in one of the
   intermediate resolvers or it is resolved by its root resolver (or
   home DNS server).

   This process of recursion means that there is no end-to-end
   communication between the host and its server where the name-to-IP
   mapping resides. That also means that a lot of timers are running in
   intermediate systems. Any updating of the transient status of the
   pool element or of the pool may need to be propagated through the
   DNS. Dynamic Registration

   Registration / de-registration of servers is needed. It can be done
   with DNS by NOTIFY/IXFR. However, frequent updates and replication
   are incompatible.  This is not a DNS problem per se, but it has an
   effect on DNS as it is deployed.

   RSerPool MUST allow software server entities to register themselves
   with a name server dynamically. They can also de-register themselves
   for purposes of preventative maintenance or can be de-registered by
   a name server that believes the server entity is no longer

Loughney (editor)                                             [Page 6]

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   operational. This is a dynamic approach, which is coordinated
   through servers in the pool and among RSerPool name servers. Load Balancing

   RFC 2782 itself points out some of the limitations of using DNS SRV
   for load balancing between servers.

         Weight is only intended for static, not dynamic, server
         selection. Using SRV weight for dynamic server selection would
         require assigning unreasonably short TTLs to the SRV RRs,
         which would limit the usefulness of the DNS caching mechanism,
         thus increasing overall network load and decreasing overall

   Based on this, DNS can only really support stochastic load
   balancing, redirecting clients to servers randomly as various caches
   in various resolvers expire at random (although small) intervals.
   DNS offers excellent network scalability but poor control over load

   As mentioned previously, the issue of doing DNS-based dynamic load
   balancing on short time scales will have impacts on third parties,
   due to the presence of stub resolvers. Heartbeating / Status Monitoring

   DNS does not incorporate an application layer heartbeat.
   Heartbeating would dramatically boost traffic levels, and given the
   unavoidable third party dependencies of DNS, the resulting loading
   would be unacceptable. It is passive in the sense that it does not
   monitor or store information on the state of the host such as
   whether the host is up or down or what kind of load it is currently

   RSerPool SHOULD monitor the state of each server entity on various
   hosts on a continual basis and can collect several state variables
   including up/down state and current load. If a server is no longer
   operational, eventually it will be dropped from the list of
   available servers maintained by the name server, so that subsequent
   application name queries will not resolve to this server address.

2.2.3 Name/Address Resolution

   The technical requirement for DNS name/address resolution is that
   given a name, find a host associated with this name and return its
   IP address(es). In other words, in DNS we have the following

     Name       a host machine

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     Address(es) IP address(es) to reach a (hardware) host machine

   The technical requirement for RSerPool name/address resolution is
   that given a name (or pool handle), find a server pool associated
   with this name and return a list of transport addresses (i.e., IP
   addresses plus port numbers) for reaching a set of currently
   operational servers inside the pool. In other words, in RSerPool we
   have the following mapping:

     Name       a handle to a server pool, which is often distributed
                across multiple host machines

     Address    IP addresses and port numbers to reach a set of
                functionally identical (software) server entities.

2.3 Service Location Protocol (SLP

   SLP is comprised of three components: User Agents (UA), Service
   Agents (SA) and Directory Agents (DA). User agents work on the
   user's behalf to contact a service. The UA retrieves service
   information from service agents or directory agents. A service agent
   works on behalf of one or more services to advertise services. A
   directory agent collects service advertisements.

   The directory agent of SLP functions simply acts as a cache and is
   passive in this regard. The directory agent is optional and SLP can
   function without it. It is incumbent upon the servers to update the
   cache as necessary by reregistering. The directory server is not
   required in small networks as the user agents can contact service
   agents directly using multicast. Unicast queries to SAs are possible
   subsequent to the UA having discovered them. User agents are
   encouraged to locate a directory at regular intervals if they can't
   find one initially, otherwise they can detect DAs by listening
   passively for DA advertisements.

   The most fundamental difference between SLP and RSerPool is that SLP
   is service-oriented while RSerPool is communication-oriented. More
   specifically, what SLP provides to its user is a mapping function
   from a name of a service to the location of the service provider, in
   the form of a URL string. The availability of the service provider
   is outside of the scope of SLP. How a service is accessable can be
   described by the SLP attribute list associated with the service URL.
   SLP is essentially a discovery protocol, not a transport protocol.
   Therefore, the granularity of SLP operation is at application
   service level.

   In contrast, RSerPool provides to its user is a mapping function
   from a communication destination name to a set of routable and
   reachable transport addresses that leads to a group of distributed
   software server entities registered under that name that
   collectively represent the named communication destination. With
   respect to SLP, this information could be represented in SLP

Loughney (editor)                                             [Page 8]

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   attributes. RserPool, however, also has the responsibility of
   reliably delivering a user message to one of these server entities.

   What service(s) a group of servers are providing at the application
   level or whether the group is just a component of an application
   service provider is out of the scope of RSerPool. In other words,
   the granularity of RSerPool operation is at communication server
   entity level.

   Moreover, RSerPool requires a distributed fault-tolerance and real-
   time translation services. SLP does not state either of these as
   design requirements and thus does not attempt to fulfill them. In
   addition, SLP defines optional security features, which support
   authentication and integrity. SLP requires IPSec to fully meet the
   Security Requirements of RserPool.

   The SLP directory agent does not support fault tolerance or
   robustness in contrast to the name servers, which do support it.
   The name servers also monitor the state of the servers, which are
   registered in the pool, but the SLP directory agents do not perform
   this function.

   SLP relies on multicast for some functionality and in RSerPool
   multicast is optional.

   In summary, SLP meets some of the requirements needed for the name
   service portion of RSerPool, but would require modifications to
   fully support the requirements for the name service.  SLP does not
   address the transport of user messages.

2.3.1 mSLP

   SLP alone does not fulfill RSERPOOL update requirements for
   timeliness.  This is achieved through mesh-enhancements to the
   Service Location Protocol (mSLP) [MSLP].

   These enhancements make it possible for SAs to know of only a subset
   of all DAs.  Mesh-enhanced SAs need only forward their registrations
   to only one mesh-enhanced DA.  The mesh takes care of forwarding the
   message to the other DAs.

3 Comparison Against Requirements

   This section attempts to create a comparison table to compare the
   protocols which have been suggested as applicable to the RserPool

Loughney (editor)                                             [Page 9]

Internet-Draft     Comparison of Protocols for RSerPool   Nov 21, 2001

                                | CORBA | DNS | SLP | ASAP | ENRP |
   Robustness                   |   Y   |  Y  |  Y  |  Y   |  Y   |
   Failover Support             |   Y   |  P  |  P  |  Y   |  Y   |
   Communication Model          |   N   |  P  |  Y  |  Y   |  Y   |
   Processing Power             |   N   |  Y  |  Y  |  Y   |  Y   |
   Support of RSerPool          |   N   |  Y  |  N  |  N   |  N   |
    Unaware Clients             |       |     |     |      |      |
   Registering and              |   N   |  P  |  P  |  Y   |  Y   |
    Deregistering               |       |     |     |      |      |
   Naming                       |   Y   |  Y  |  Y  |  Y   |  Y   |
   Name Resolution only to      |   Y   |  N  |  Y  |  Y   |  Y   |
    Active Elements             |       |     |     |      |      |
   Server Selection Policies    |   Y   |  P  |  P  |  P   |  P   |
   Timing Requirements and      |   N   |  N  |  Y  |  Y   |  Y   |
    Scaling                     |       |     |     |      |      |
   Scalability                  |   N   |  Y  |  Y  |  Y   |  Y   |
   Security - General           |   N   |  P  |  P  |  P   |  P   |
   Security - Name Space        |   N   |  P  |  P  |  P   |  P   |
    Services                    |       |     |     |      |      |

   Y = Yes, meets requirement
   P = Partially meets requirement
   N = No, does not meet requirement
   N/A = Not applicable

4 Security Concerns

   This type of non-protocol document does not directly affect the
   security of the Internet.

5 Acknowledgements

   The authors would like to thank Bernard Aboba, Erik Guttman, Matt
   Holdrege, Christopher Ross and Werner Vogels for their invaluable
   comments and suggestions.

6 References

Loughney (editor)                                             [Page 10]

Internet-Draft     Comparison of Protocols for RSerPool   Nov 21, 2001

   [ASAP]         Xie, Q, Stewart, R. R., "Aggregate Server Access
                  Protocol (ASAP)", draft-ietf-rserpool-asap-00.txt,
                  June, 2001.  A work in progress.

   [ENRP]         Xie, Q, Stewart, R. R., "Endpoint Name Resolution
                  Protocol (ENRP)", draft-ietf-rserpool-enrp-00.txt,
                  June, 2001.  A work inprogress.

   [MSLP]         Zhao, W., "mSLP - Mesh-enhanced Service Location
                  Protocol", draft-zhao-slp-da-interaction-12.txt,
                  July, 2001.  A work in progress.

   [RSER-ARCH]    Tuexen, M. et al., "Requirements for Reliable Server
                  Pooling" <draft-ietf-rserpool-arch-00.txt>, Work in
                  Progress, April 2001.

   [RSER-REQ]     Tuexen, M. et al., "Requirements for Reliable Server
                  Pooling" <draft-ietf-rserpool-reqts-03.txt>, Work in
                  Progress, May 2001.

   [RFC793]       J. B. Postel, "Transmission Control Protocol", RFC
                  793, September 1981.

   [RFC959]       J. B. Postel, J. Reynolds, "File Transfer Protocol
                  (FTP)", RFC 959, October 1985.

   [RFC2026]      S. Bradner, "The Internet Standards Process -
                  Revision 3", RFC 2026, October 1996.

   [RFC2608]      E. Guttman et al., "Service Location Protocol,
                  Version 2", RFC 2608, June 1999.

   [RFC2719]      L. Ong et al., "Framework Architecture for Signaling
                  Transport", RFC 2719, October 1999.

   [RFC2782]      A. Gulbrandsen et al., "A DNS RR for specifying the
                  location of services (DNS SRV)", RFC 2782, February

   [RFC2960]      R. R. Stewart et al., "Stream Control Transmission
                  Protocol", RFC 2960, November 2000.

7 Authors' Addresses

   John Loughney
   Nokia Research Center
   PO Box 407
   FIN-00045 Nokia Group
   Email: john.loughney@nokia.com

   Maureen Stillman

Loughney (editor)                                             [Page 11]

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   127 W. State Street
   Ithaca, NY 14850
   Email: maureen.stillman@nokia.com

   Michael Tuexen
   Siemens AG
   ICN WN CS SE 51
   D-81359 Munich
   Email: Michael.Tuexen@icn.siemens.de

   Qiaobing Xie
   Motorola, Inc.
   1501 W. Shure Drive, #2309
   Arlington Heights, Il 60004
   Email: qxie1@email.mot.com

   Randall Stewart
   Cisco Systems, Inc.
   24 Burning Bush Trail
   Crystal Lake, Il 60012
   Email: rrs@cisco.com

   Lyndon Ong
   10480 Ridgeview Court
   Cupertino, CA 95014
   Email: lyong@ciena.com

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Loughney (editor)                                             [Page 13]

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