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RserPool Working Group                           J. Loughney (ed.)
INTERNET DRAFT                                         M. Stillman
                                                            Q. Xie
                                                        R. Stewart

Issued:    March 1, 2002
Expires:   September 1, 2002

           Comparison of Protocols for Reliable Server Pooling

Status of this Memo

   This document is an Internet-Draft and is subject to 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

   Distribution of this memo is unlimited.

   Copyright (C) The Internet Society 2002.  All Rights Reserved.


   This document compares protocols that may be applicable for Reliable
   Server Pooling problem space.  This document discusses the usage and
   applicability of these protocols for Reliable Server Pooling.

Internet-Draft     Comparison of Protocols for RSerPool     Mar 1, 2002

1 Introduction........................................................3
  1.1 Overview........................................................3
  1.2 Terminology.....................................................3
2 Relation to Other Solutions.........................................4
  2.1 CORBA...........................................................4
  2.2 DNS.............................................................4
   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 Introduction.................................................8
   2.3.2 What to Use..................................................8
   2.3.3 Summary......................................................9
3 ASAP and ENRP......................................................10
  3.1 ASAP...........................................................10
  3.2 ENRP...........................................................11
4 Comparison Against Requirements....................................11
5 Security Concerns..................................................12
6 Acknowledgements...................................................12
7 References.........................................................12
8 Authors' Addresses.................................................13
Full Copyright Statement.............................................13

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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 the applicability of these protocols in
   meeting the requirements of Reliable Server Pooling [RFC3237].

   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

   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 [RFC3237].  The protocols discussed have been suggested
   as possibly overlapping with the problems space of RSerPool.


   Often referred to as a Distributed Processing Environment (DPE),
   CORBA was mainly designed to provide location transparency for
   distributed applications. CORBA's distribution model encourages an
   object-based view, i.e., each communication endpoint is normally an

   CORBA has a number of variants, such as fault-tolerant CORBA, Real-
   time CORBA, etc.  CORBA has been used in a number of situations, for
   example, Real-time CORBA has been usedin fighter aircraft and weapon
   systems.  Additionally, CORBA has been implentented in a wide range
   of devices, from attack submarines to Palm Pilots - the MICO open
   source ORB has been ported to the Palm Pilot, and the client-only
   application is 45 KB.

   Currently, the applicability of CORBA is unclear, and interaction
   with other Internet protocols, such as AAA, IPsec and Ipv6 may be

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

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   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
   [RFC3237].  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
   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

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   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
   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,

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         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

     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

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

2.3 Service Location Protocol (SLP)

2.3.1 Introduction

   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.

2.3.2 What to Use

   Figure 1 shows how SLP might be realized to provide ENR services:

          Pool User (PU)         ENR Service       Pool Endpoint (PE)
          +-------------+                              +---------+
          | APPLICATION |                              | SERVICE |
        +-+-------------+-+                        +---+---------+---+
        |ASAP/RSERPOOL API| <--------------------> |ASAP/RSERPOOL API|
        +-+----+--------+-+      +----------+      +-+--------+----+-+
          |    | SLP UA | <----> |  SLP DA  | <----> | SLP SA |    |
          |    +----+---+        +------+---+        +--------+    |
          |SCTP     |UDP|        | SCTP |UDP|        |UDP|    SCTP |
          +---------+---+        +------+---+        +---+---------+
                                   /       \
                                  /  mesh   \
                             +----+        +----+
                             | DA |--------| DA |
                             +----+        +----+

         Figure 1:  RSERPOOL entities employing SLP for ENR services


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    * Each box constitutes a host (running a PU, PE or ENR server
      'stack'), though one host could support more than one of these

    * As far as the Application is concerned, it is using a framework
      for exchanging messages with services reliably.

    * As far as the Service is concerned, it is making itself available
      to a reliable server pool by interacting with the framework API.

    * The ASAP/RSERPOOL API obtains endpoint name resolution data in a
      timely and robust manner and uses it to determine how to route
      PU requests to PEs.

    * The ENR service function is performed using SLP.  The PU employs
      a SLP UA to obtain information from a SLP DA.

    * The ENR service function is performed using SLP.  The PU employs
      a SLP UA to obtain information from a SLP DA.

    * The PE employs a SLP SA to register information with a SLP DA.
      As the SLP SA is 'mesh-enhanced,' it only registers with one DA
      of this type (as long as it detects that this DA is alive &
      responsive & returns 'OK' results).

    * The SLP DA is part of a mesh.  It will forward PE state to other
      DAs in the mesh.  For example, it will forward the registrations
      the SLP SA made on behalf of the PE on right of Figure 1.

    * SCTP is used for communication between entities.  Multicast UDP
      is used by SLP entities for active and passive discovery.  While
      the RSERPOOL architecture cannot rely upon multicast mechanisms,
      it can profit from them when these are present in the network

   SLPv2 will be needed, but SLPv2 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.

2.3.3 Summary

   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

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

   Currently, mSLP would need changes, for example it was designed to
   scale to ~10 DAs not ~100 DAs.  Additionally, SLP is currently to
   run on top of UDP and TCP.  If SCTP support was needed, some
   additional specification work would be needed.

   SLP security makes no attempt to address the confidentiality of data
   transmitted between SLP agents. To properly address this concern,
   SLP agents would need to establish secure communication with each
   other.  This would be achieved through the use of IPSec
   Encapsulating Security Payload.

   Server discovery, however, is something which SLP does well, and if
   used for RserPool, this would be useful.


   ASAP [ASAP] and ENRP [ENRP] are being developed the RserPool working
   group.  Even though they are separate protocols, they are designed
   to work together.

3.1 ASAP

   ASAP uses a name-based addressing model which isolates a logical
   communication endpoint from its IP address(es), thus effectively
   eliminating the binding between the communication endpoint and its
   physical IP address(es) which normally constitutes a single point of
   failure. In addition, ASAP defines each logical communication
   destination as a pool, providing full transparent support for
   server-pooling and load sharing. It also allows dynamic system
   scalability - members of a server pool can be added or removed at
   any time without interrupting the service.

   ASAP is not designed to scale Internet wide.  It uses a flat, peer-
   to-peer addressing model.  Other protocols, such as DNS could be
   used to bridge the pools of servers.  It uses a name-based
   addressing model to logically isolate the communication endpoint
   from its IP address(es).  If multiple endpoints register under a the
   same name, a server pool is effectively created.  ASAP is used to

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   select one Pool Element, based on a number of criteria, such as load
   sharing.  ASAP monitors the reacability of the Pool Elements in
   order to provide fault tolerance.

3.2 ENRP

   ENRP defines procedures and message formats of a distributed fault-
   tolerant registry service for storing, bookkeeping, retrieving, and
   distributing pool operation and membership information.  It allows
   Pool Elements to be dynamically added, updated and removed from
   service.  There are protocol mechanisms for detecting and removing
   unreachable Pool Elements.

4 Comparison Against Requirements

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

                                | 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      |   P    |  N  |  Y  |  Y   |  Y   |
    Scaling                     |        |     |     |      |      |
   Scalability                  |   N    |  Y  |  Y  |  Y   |  Y   |
   Security - General           |   Y    |  P  |  P  |  P   |  P   |
   Security - Name Space        |   P    |  P  |  P  |  P   |  P   |
    Services                    |        |     |     |      |      |

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

5 Security Concerns

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

6 Acknowledgements

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

7 References

   [ASAP]         Xie, Q, Stewart, R. R., "Aggregate Server Access
                  Protocol (ASAP)", Work in progress.

   [ENRP]         Xie, Q, Stewart, R. R., "Endpoint Name Resolution
                  Protocol (ENRP)", Work inprogress.

   [MSLP]         Zhao, W., "mSLP - Mesh-enhanced Service Location
                  Protocol" Work in progress.

   [RSER-ARCH]    Tuexen, M. et al., "Requirements for Reliable Server
                  Pooling" Work in Progress.

   [RFC3237]      Tuexen, M. et al., "Requirements for Reliable Server
                  Pooling",  RFC3237, January 2002.

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

   [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

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

8 Authors' Addresses

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

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

   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

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   The limited permissions granted above are perpetual and will not be
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