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Versions: 00 01 02 03 04 05 06 07 08 09 10 11                           
Network Working Group                                   J. Loughney, Ed.
Internet-Draft                                                     Nokia
Expires: January 7, 2006                               A. Silverton, Ed.
                                                             M. Stillman
                                                                  Q. Xie
                                                              R. Stewart
                                                            July 6, 2005

          Comparison of Protocols for Reliable Server Pooling

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on January 7, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document compares protocols that may be applicable for the

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   Reliable Server Pooling problem space.  This document discusses the
   usage and applicability of these protocols for the Reliable Server
   Pooling architecture.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.3   Abbreviations  . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Relation to Other Technologies . . . . . . . . . . . . . . . .  4
     2.1   CORBA  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2   DNS  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       2.2.1   Requirements . . . . . . . . . . . . . . . . . . . . .  5
       2.2.2   Technical Issues . . . . . . . . . . . . . . . . . . .  6
     2.3   Dynamic Delegation Discovery System (DDDS) and URI . . . .  9
     2.4   Service Location Protocol (SLP)  . . . . . . . . . . . . . 12
       2.4.1   Introduction . . . . . . . . . . . . . . . . . . . . . 12
       2.4.2   What to Use  . . . . . . . . . . . . . . . . . . . . . 12
       2.4.3   Summary of SLP Issues  . . . . . . . . . . . . . . . . 13
     2.5   L4/L7 Switching  . . . . . . . . . . . . . . . . . . . . . 15
       2.5.1   Introduction . . . . . . . . . . . . . . . . . . . . . 15
       2.5.2   L4 Switching . . . . . . . . . . . . . . . . . . . . . 15
       2.5.3   L7 Switching . . . . . . . . . . . . . . . . . . . . . 16
       2.5.4   Summary  . . . . . . . . . . . . . . . . . . . . . . . 18
     2.6   ASAP and ENRP  . . . . . . . . . . . . . . . . . . . . . . 19
       2.6.1   ASAP . . . . . . . . . . . . . . . . . . . . . . . . . 19
       2.6.2   ENRP . . . . . . . . . . . . . . . . . . . . . . . . . 20
   3.  Comparison Against Requirements  . . . . . . . . . . . . . . . 20
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     7.1   Normative References . . . . . . . . . . . . . . . . . . . 22
     7.2   Non-Normative References . . . . . . . . . . . . . . . . . 22
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23
       Intellectual Property and Copyright Statements . . . . . . . . 25

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1.  Introduction

1.1  Overview

   In creating a solution to provide reliable server pools [1], 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 [2].

   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:

   Operational 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.

   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.

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

   ENRP Server: Entity which is responsible for managing and maintaining
      the handle-space within the RSerPool operational scope.

1.3  Abbreviations

   DA: Directory Agent in SLP.

   DPE: Distributed Processing Environment.

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   CORBA: Common Object Request Broker Architecture.

   OMG: Object Management Group.

   ORB: Object Request Broker.

   PE: Pool Element.

   PU: Pool User.

   SA: Service Agent in SLP.

   SLP: Service Location Protocol.

   UA: User Agent in SLP.

   TTL: Time to live in DNS.

2.  Relation to Other Technologies

   This section is intended to discuss the applicability of some
   existing solutions with regards to Reliable Server Pooling
   requirements [2].  The protocols discussed have been suggested as
   possibly overlapping with the problems space of RSerPool.

2.1  CORBA

   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 used in fighter aircraft and weapon
   systems.  Additionally, CORBA has been implemented 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 in size.

   CORBA, a DPE, sits above the communications layer that is the purview
   of most IETF work, specifically, RSerPool.  In a conceptual model of
   a middleware stack for highly available clustering, CORBA is
   considered an application service and not a messaging or clustering
   service.  A distributed application may utilize a CORBA ORB for
   location transparency at the application layer, and the ORB may in

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   turn utilize RSerPool for its communications layer.  For example, a
   real-time CORBA ORB such as Tau, would benefit from having its
   interfaces extended to take advantage of RSerPool concepts.

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

   RSerPool and DNS are protocols with very different objectives.
   RSerPool is designed to provide a range of services up to the point
   of relieving an application of the overhead of maintaining a session
   with an active server.  DNS was not intended to handle such a range
   of functions.  DNS may, however, be able to handle some of the lower
   range of RSerPool functionality.

   One requirement of 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 [2].
   These requirements are discussed below in relation 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

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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 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

   There is no way to get around these issues unless one requires all
   hosts to 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

   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

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   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 (i.e., PEs) 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 an ENRP 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 ENRP servers.  Load Balancing

   RFC 2782 [3] 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 reliability.

   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 balance.

   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

   RSerPool working group has agreed that one of its main design goals
   for RSerPool is "...performance for supporting real-time
   applications", as reflected in RFC 3237 [2].  An example of such
   real-time applications would be the IP-based call control
   applications in a 3G cellular network.

   To achieve this goal, it is felt critical that RSerPool monitors the
   state of each server entity on various hosts on a continual basis and

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   collects 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 ENRP
   server, so that subsequent application name queries will not resolve
   to this server address.

   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

   It is not entirely impossible to make DNS utilize the assistance of
   an external heartbeat function/protocol for this monitoring purpose.
   However, to achieve the degree of real-time performance RSerPool is
   seeking, one would most likely need a tight coupling of this external
   function to the DNS operation.  This in turn would likely result in
   substantial modification of the existing DNS, which is what we want
   to avoid.  Name/Address Resolution Granularity

   The technical requirement for DNS name/address resolution is
   basically 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 mapping:

        Name     ; a host machine


        Address  ; IP address(es) to reach the 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
   address(es) plus port number) for reaching a set of currently
   operational servers inside the pool.  In other words, in RSerPool we
   have the following mapping:

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        Name     ; a handle to a server pool


        Address  ; transport address(es) (i.e., IP plus port number) to
                 ; reach a set of functionally identical server entities
                 ; (software processes). These software entities can be
                 ; distributed across multiple host machines.

   Without significant extensions, the current DNS would have difficulty
   achieving this type of name mapping.  Lack of Support for Real-time Fault Detection and Recovery

   Even if we could somehow overcome the aforementioned shortcomings of
   DNS in terms of providing the name resolution service to RSerPool, we
   still would not have the support for real-time fault detection and
   recovery (i.e., failover) which is a key requirement in RSerPool.

   To meet this requirement, a mechanism would need to be in place that
   would detect the unreachability of a message recipient and re-direct
   or re-route a user message to an alternate recipient in the same
   destination pool in real-time or semi-real-time.  DNS currently
   contains no such mechanism.  Lack of Support for Redundancy Models

   Server pooling as defined in RSerPool requires support for different
   redundancy arrangements or models depending on the needs of the
   specific application.  Commonly used models in practice includes N+M,
   N-active, etc.  These models basically define how a PE behaves when
   another PE in the same pool fails and it is often critical for the
   application to have full control over this behavior of each PE in the
   pool.  Without major extensions, it seems difficult for DNS to
   support such redundancy models.

2.3  Dynamic Delegation Discovery System (DDDS) and URI

   In this section, we discuss the difficulties for RSerPool to make use
   of DDDS and URI as building blocks for its distributed pool handle
   database (i.e., RSerPool handle-space).

   RFC 3401 [5] defines DDDS as an abstract algorithm for applying
   dynamically retrieved string transformation rules to an application-
   unique string.  DDDS has been found useful for URI Resolution, ENUM
   telephone number to URI resolution, and the NAPTR DNS resource

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   As stated in [5], DDDS is "used to implement lazy binding of strings
   to data, in order to support dynamically configured delegation
   systems.  The DDDS functions by mapping some unique string to data
   stored within a DDDS Database by iteratively applying string
   transformation rules until a terminal condition is reached."

   In order to discuss the applicability of DDDS/URI in RSerPool, we
   need first talk about some fundamental characteristics of pool
   handles or names in RSerPool.

   It is important to note that, handles or names in RSerPool are meant
   to identify pools comprising of _generic_ communications nodes.
   Those nodes in reality will be some kind of servers - service
   applications that runs on some networked machines.  However, it is
   very important to note that the working group has never had the
   intention to go beyond the "server of generic IP applications" in its
   pool definition.  Nor has it seen the need to categorize the types of
   the service applications for the purpose of RSerPool.  This is
   fundamentally different from the assumption behind URI as well as the
   Service Location Protocol (to be discussed below).

   With the above noted, here are some additional characteristics of
   RSerPool handles/names:

   1.  RSerPool handles have only local significance, i.e., there is no
       requirement for pool handles to be globally unique.

       This is because the first tier of applications we envision for
       RSerPool is those tightly coupled local systems that can use
       RSerPool to make its components highly available.  For example, a
       3G radio access network that contains charging server, call
       controller, media server, etc. where RSerPool can make those
       currently singleton elements into pools and thus gain high
       availability.  This type of local systems can be as compact as a
       bunch of server blades located in a single high performance
       chassis or a group of closely located boxes in a central office
       or in a few closely located buildings.  The use of RSerPool in
       such scenarios can be totally transparent to the outside world.

       For example, a SIP phone may be talking to a softswitch without
       knowing that the call control elements inside the softswitch are
       a bunch of RSerPool-enabled pools.  In such cases, the pool names
       has no need to be globally unique (there is even no need for the
       outside to know they exist).  They only need to be unique within
       the softswitch itself.

       We also have considered the possibility of supporting larger
       scale (even global) deployment cases of RSerPool.  In the

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       requirement, we indicate we want RSerPool to be able to do that
       but we have made it clear that RSerPool will not be able to do
       that by itself.  Instead, it will rely on existing external
       infrastructures (e.g., DNS, possibly URN/DDDS) to bridge locally
       scoped RSerPool clouds into a larger scale deployment.

   2.  RSerPool handles have no need for supporting any structure/

       As merely locally significant identifiers for distinguishing
       pools of generic communication nodes, we consider adding
       structure/syntax to RSerPool handle definition will buy us
       nothing but will have real negative impact on the performance and
       increase the implementation complexity.  The only recommend we
       have made so far is to use NULL-terminated ascii string for the
       pool handles.  This seems to meet our needs nicely.

   3.  RSerPool handles are relatively dynamic.

       We consider that the pools may change relatively frequently; they
       may come and go as the system re-adjusts its capacity or
       configuration.  We do not envision the handles to be long lived.

   4.  RSerPool handles are not for human readers.

       Unlike UNIs (URN/URL), we do not envision RSerPool handles to
       appear in e-mails, web documents, etc. for human viewing.
       Probably the only case where RSerPool handles will be read by a
       human is in a log file or configuration file, just like other
       system configuration parameters.

   Due to the aforementioned characteristics of RSerPool handles, we do
   not see the benefit for directly using URNs/URIs for RSerPool
   handles.  The two rather fundamental requirements (per RFC 1737) that
   brought us URNs - the global scope and persistence - do not apply to
   RSerPool handles.  Moreover, as mentioned above, we see little
   benefit of making RSerPool handles human-readable or parsable in free

   In the future, there may be the possibility that URNs and the
   associated infrastructure (e.g., DNS, DDDS) to play a vital role when
   we start to consider wide area or global deployment scenarios for
   RSerPool.  For instance, a SIP client device that is looking for
   certain network resource will start with a URN that is eventually
   resolved to a pool handle that is then passed to an RSerPool
   namespace server, which in turn will resolve the pool name to a list
   of reachable/routable transport addresses of the server instances.

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   Similarly, since RSerPool handles are not global and have no
   structure/syntax, we do not see that using DDDS inside RSerPool can
   bring meaningful benefits.

2.4  Service Location Protocol (SLP)

2.4.1  Introduction

   SLP [4] 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 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

2.4.2  What to Use

   Figure 1 shows how SLP might be realized to provide RSerPool endpoint
   name resolution (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 |
                            +----+        +----+

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   Figure 1: RSerPool entities employing SLP for ENR services


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

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

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

   o  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.

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

   o  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).

   o  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.

   o  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) [6].

   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.

2.4.3  Summary of SLP Issues

   A fundamental difference between SLP and RSerPool is that SLP is a
   protocol that focuses on the service level, while RSerPool is at the

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   communication level.  More specifically, what SLP provides to its
   user is a mapping function from a name of a _service_ (e.g., b/w
   printing, color printing, faxing) 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
   accessible 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, what RSerPool provides to its user is a mapping function
   from a communication destination name (i.e., a pool handle) to a set
   of routable and reachable transport addresses that leads to a group
   of distributed software server entities registered under that name.
   RSerPool has NO intention to understand or convey to its user what is
   the service (e.g., printing, faxing, document scanning) the named
   pool is providing at the application level.  In other words, the
   responsibility of RSerPool is only to reliably deliver a user message
   to one of those server entities in the destination pool.

   In theory, information such as transport addresses and their
   reachability could be represented in SLP attributes.  Currently, mSLP
   would need changes, for example it was designed to scale to ~10 DAs
   not ~100 DAs.  Additionally, SLP is currently designed to run on top
   of UDP and TCP.  If SCTP support is 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.

   Other difficulties and shortcomings for using SLP to implement
   RSerPool include:

   o  Due to the fact that the resolution granularity of SLP is at the
      service level, it relies on a syntax rich scheme to define
      services (e.g., printers > color printers > color printers with
      720+ resolution, etc).  This implies that SLP implementation will
      need to perform syntax analysis, filtering, and parsing when a
      name is queried, and will need to dynamically search its name
      space to identify which entities are to be included in the
      response to a specific query.  This type of complicated processing
      and searching for each query may severely limit the performance of

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      SLP in a real-time world which is a key requirement of RSerPool.

   o  Without major extensions, SLP will not be able to provide a
      solution for real-time or semi-real-time fault detection and
      recovery.  This is partially because SLP is a discovery protocol,
      not a communication protocol.

2.5  L4/L7 Switching

2.5.1  Introduction

   This section discusses L4 and L7 switching techniques and their
   relation to the RSerPool architecture [2].  Since these technologies
   are highly proprietary, it is difficult to discuss these techniques
   in a thorough manner.

   In both cases, the deployment of these techniques is dependent upon
   the type of switching equipment deployed and breaks the end-to-end
   communication model required by RSerPool.  These devices provide a
   specialized service intended to address a few network challenges,
   e.g., web caching and web cache load balancing, firewall load
   balancing, web server scaling, and streaming media load balancing.
   They are not robust methods for providing network reliability or
   highly reliable and highly available location transparent server
   clustering as required by RSerPool.

   The following sections will provide an overview and example of each
   technique and an accounting for key RSerPool architectural
   requirements not met.  See Section 3 for a more detailed accounting
   of requirements compliance.

2.5.2  L4 Switching

   L4 devices make switching decisions based on the TCP or UDP port
   numbers of the packet in transit.  Example

   Web caching is an example of L4 switching.  The topology requires the
   introduction of an L4 capable switch in series with an existing
   network connection and L2/L3 switch.  This is of particular use to
   web cache configurations where, for example, all traffic destined for
   port 80 (HTTP) could be redirected to a web cache or distributed by
   the switch across a number of web caches to achieve load balancing.
   The L4 switch can react to a failed cache and cease to send traffic
   to that device by automatically detecting that it is unreachable.
   This is all accomplished without any configuration on a client

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   An equally compelling use for this type of switching is load
   balancing across firewalls.  If the firewalls are employing stateful
   packet inspection for TCP connections, then the L4 switch must track
   which packets belong to which connection and see that all such
   packets are switched to the same firewall.

   An L4 switch is incapable of differentiating between packets
   containing cacheable objects and non-cacheable objects, therefore, L7
   devices, which look inside packets, are deployed where such
   determinations must be made.  In general, anytime that knowledge of
   the application level data is required to make a switching decision,
   L7 devices must be deployed.  Technical Issues

   The more general behavior of L4 switching, redirecting traffic based
   on destination UDP or TCP ports, is similar to a function provided by
   RSerPool.  Where it differs in this regard is that L4 switching is
   dependent upon the network infrastructure and not peer-to-peer or
   end-to-end as required by RSerPool.

   L4 switching meets the requirement of forwarding to active elements
   only, as a switch will detect unreachable PEs, but does not provide
   for the necessary registration and deregistration of PEs or
   resolution by name.  L4 switches require the manual configuration of
   access control lists to determine switching behavior.  This is
   achieved in RSerPool by more flexible means and without any
   dependence on specialized network equipment.

   Most of the features of ASAP [7] and ENRP [8] are not met by a device
   employing L4 switching techniques.  See the comparison table in
   Section 5.  Security Issues

   It is not clear that L4 switching introduces any new security
   concerns.  In fact, in a two-port security model, where secure
   RSerPool services are provided on one port, and similar, but insecure
   services, on another, L4 switching could be used to direct traffic to
   a secure or insecure PE or ENRP server as necessary.

2.5.3  L7 Switching

   As previously mentioned, L7 switching was developed to differentiate
   between the type of objects being directed by network switches.  In
   the L4 case, the devices cannot differentiate between the types of

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   data, only the destination of the packets containing that data.  L7
   switches look at the application layer of a packet in transit to
   determine what type of object is contained within.  Example

   For an L7 switch to do this, it is necessary to intercept data
   midstream.  In the case of HTTP, which is carried over TCP, the L7
   switch must break the TCP handshake when a new request is made to the
   server attached to the switch.  This process begins during the
   initialization of the TCP connection and before the higher level
   protocol, i.e., HTTP, sends its request.  The switch acts as the
   server during the TCP SYN, SYN ACK, ACK handshake between that server
   and the client.  Once the HTTP request is issued by the client and
   the switch decides that this is non-cacheable data that should be
   directed to the server as opposed to a web cache, the L7 switch sets
   up a second connection with the actual server through an additional
   three-way handshake.  The switch will forward the client's request to
   the server and for the duration of this connection, must graft the
   client-switch and switch-server connections together by modifying IP
   addresses and TCP ports on the fly.  Cacheable data is handled
   similarly, but is redirected to groups of web caches as opposed to
   the web servers.  Technical Issues

   It is not clear that L7 switching adds anything, as a general
   mechanism, beyond what is provided by L4 switching, towards providing
   a sufficient RSerPool architecture.

   While this technique can be very valuable as a means to scale web
   servers, it is apparent that it takes a significant amount of work on
   the part of the switch to realize these gains.  The nature of this
   method also requires that for each type of application traffic
   handled, a custom software module must be written and added to the
   switch operating system.  It is not known, due to the proprietary
   nature of these devices, if this can be done by the end user and/or
   added dynamically to deployed systems.

   It may be possible to write custom modules for a switch, given the
   appropriate access to hardware and software, to provide some type of
   enhanced reliability in a controlled network.  But, it is the aim of
   RSerPool to provide a general mechanism that is widely deployable and
   highly portable.  L7 switching requires a significant amount of
   development to customize each of the endpoint switches to which PUs
   and PEs may be attached.

   Also of concern is the compatibility of SCTP with L7 techniques.  The

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   interception and subsequent splicing of sessions may nullify some of
   the inherent benefits of SCTP and certainly add additional and
   unnecessary complexity and latency to the transport layer.  ASAP and
   ENRP along with the multi-homing and stream based behavior of SCTP
   provide more benefit than custom L7 switching would provide and at a
   significantly lower cost.  Security Issues

   While L7 switches do provide some robustness to TCP-based DoS attacks
   directed at servers by requiring a proper three-way handshake, and
   they can be used to redirect encrypted traffic to certain servers
   better capable of processing that traffic, they may break the
   security model of RSerPool.

   It may not be possible to make all the routing and switching
   decisions necessary to support RSerPool services without knowing more
   than just the destination address and port of a packet.  The
   necessary extended attributes are not elements of L4 or L7 switching,
   but are instead, parameters of ASAP and ENRP.  As the ENRP traffic is
   encrypted in RSerPool, the L7 devices would not be able to extract
   the necessary session layer data without becoming potential third
   party security liabilities.

2.5.4  Summary

   The L4/L7 switching techniques, being network oriented services, are
   not able to provide the communications session oriented behavior
   required by RSerPool.

   Adequate support for naming, as well as registration and
   deregistration services, is not provided by these devices.  RSerPool
   requires a fault tolerant name service as well as the ability to
   register and deregister PEs in real-time.  To accomplish this with
   L4/L7 switching, one would need to define a standard protocol to
   allow the switches to communicate amongst themselves and, perhaps,
   implement a co-resident name server on the switch.

   The RSerPool communication model is broken as these mechanisms are
   deployed on switch hardware as opposed to end devices such as PEs,
   PUs, and ENRP servers.  This implies a significant requirement for
   processing power and a lack of support for mobility.  It is unlikely
   that one could or would build L4/L7 behavior into end devices and
   RSerPool requires peer-to-peer functionality.

   The equipment needed to deploy such solutions can be an order of
   magnitude more expensive per port than a traditional L2/L3 device and
   oftentimes must be deployed in addition to L2/L3 hardware.  It has

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   been observed that L4/L7 devices show poor performance when acting as
   an L2/L3 switch.  This combined requirement for network
   infrastructure is not appropriate for RSerPool.

   L4/L7 switching while clearly good in certain areas, lacks the
   ability to provide a robust framework for location transparent
   clustering capable of scaling in size and performance from web server
   or other Internet applications to real-time telecommunications
   infrastructure.  There are a host of concerns with the ability of
   these techniques to meet critical RSerPool requirements in the areas
   of flexibility, adaptability, timing, security, etc.  The amount of
   effort required to achieve RSerPool functionality across L4/L7
   switches would amount to implementing RSerPool, as it is currently
   defined, on those very switches.

2.6  ASAP and ENRP

   ASAP [7] and ENRP [8] are being developed in the RSerPool working
   group.  Even though they are separate protocols, they are designed to
   work together.

2.6.1  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.  If multiple endpoints register under a the
   same name, a server pool is effectively created.  It also allows
   dynamic system scalability - members of a server pool can be added or
   removed at any time without interrupting the service.

   ASAP monitors the reachability of the Pool Elements in order to
   provide fault tolerance.  To support real-time or semi-real-time
   fault detection and recovery, ASAP makes use of the peer reachability
   feedback from either the transport layer (such as SCTP) or the upper
   layer protocol and re-send (or failover) user messages to alternate
   PEs in the destination pool.  Load sharing and redundancy model
   support is provided in ASAP at the message sender side.  ASAP allows
   extensions to be made in the future to accommodate new load sharing
   policies and redundancy models.

   ASAP supports the "keepalive" monitoring of PEs by the ENRP server
   and session failover, in which a set of application messages are
   defined as a "session" and ASAP provides best-effort transmission of
   all the messages in the "session" to the same PE in the destination

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   pool.  For some classes of service, ASAP can provide failover for the
   remaining message in the "session" to an alternate PE if the first PE

2.6.2  ENRP

   ENRP defines procedures and message formats of a pool registry
   service (or name 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 also protocol mechanisms for detecting and
   removing unreachable Pool Elements.

   Within the operational scope of RSerPool, ENRP defines the procedures
   and message formats of a distributed, fault-tolerant registry service
   for storing, bookkeeping, retrieving, and distributing pool operation
   and membership information.  This is to avoid the name service itself
   becoming a single point of failure in the system.

   ENRP itself is dynamically scalable, meaning that new ENRP servers
   can be added and existing servers can be removed as needed.  This
   feature can be used to achieve zero planned downtime upgrade of a
   system - a common requirement for many mission critical applications.

   ENRP is not designed to scale Internet wide.  It uses a flat name
   space model to gain performance.  Other protocols, such as DNS could
   be used to bridge small ENRP name spaces to create a large scale name

3.  Comparison Against Requirements

   This section attempts to create a comparison table to compare the
   technologies and protocols which have been suggested as applicable to
   the RSerPool architecture.

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                                   | CORBA  | DNS | SLP | ENRP | L4/L7 |
      Robustness                   |   Y    |  Y  |  Y  |  Y   |   Y   |
      Failover Support             |   Y    |  P  |  P  |  Y   |   P   |
      Communication Model          |   N    |  P  |  Y  |  Y   |   N   |
      Processing Power             |   N    |  Y  |  Y  |  Y   |   N   |
      Support of RSerPool          |   N    |  Y  |  N  |  N   |   N   |
       Unaware Clients             |        |     |     |      |       |
      Registering and              |   N    |  P  |  P  |  Y   |   N   |
       Deregistering               |        |     |     |      |       |
      Naming                       |   Y    |  Y  |  Y  |  Y   |   N   |
      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   |   P   |
       Scaling                     |        |     |     |      |       |
      Scalability                  |   N    |  Y  |  Y  |  Y   |   Y   |
      Security - General           |   Y    |  P  |  P  |  P   |   P   |
      Security - Name Space        |   P    |  P  |  P  |  P   |   N   |
       Services                    |        |     |     |      |       |

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

   Table1: Comparison Against Requirements

4.  Security Considerations

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

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5.  IANA Considerations

   This document creates no considerations for IANA.

6.  Acknowledgements

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

7.  References

7.1  Normative References

   [1]  Tuexen, M., Xie, Q., Stewart, R., Shore, M., Loughney, J., and
        A. Silverton, "Architecture for Reliable Server Pooling",
        draft-ietf-rserpool-arch-09 (work in progress), February 2005.

   [2]  Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L., Loughney,
        J., and M. Stillman, "Requirements for Reliable Server Pooling",
        RFC 3237, January 2002.

7.2  Non-Normative References

   [3]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
        specifying the location of services (DNS SRV)", RFC 2782,
        February 2000.

   [4]  Guttman, E., Perkins, C., Veizades, J., and M. Day, "Service
        Location Protocol, Version 2", RFC 2608, June 1999.

   [5]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        One: The Comprehensive DDDS", RFC 3401, October 2002.

   [6]  Zhao, W., Schulzrinne, H., and E. Guttman, "Mesh-enhanced
        Service Location Protocol (mSLP)", RFC 3528, April 2003.

   [7]  Stewart, R., Xie, Q., Stillman, M., and M. Tuexen, "Aggregate
        Server Access Protocol (ASAP)", draft-ietf-rserpool-asap-09
        (work in progress), June 2004.

   [8]  Xie, Q., Stewart, R., and M. Stillman, "Enpoint Name Resolution
        Protocol (ENRP)", draft-ietf-rserpool-enrp-09 (work in
        progress), July 2004.

   [9]  Bradner, S., "The Internet Standards Process -- Revision 3",
        BCP 9, RFC 2026, October 1996.

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Authors' Addresses

   John Loughney (editor)
   Nokia Research Center
   PO Box 407
   Nokia Group  FIN-00045

   Email: john.loughney@nokia.com

   Aron J. Silverton (editor)
   Motorola, Inc.
   1301 East Algonquin Road
   Mail Drop 2246
   Schaumburg, IL  60196

   Phone: +1 847-576-8747
   Email: aron.j.silverton@motorola.com

   Maureen Stillman
   127 W. State Street
   Ithaca, NY  14850

   Phone: +1 607-273-0724
   Email: maureen.stillman@nokia.com

   Qiaobing Xie
   Motorola, Inc.
   1501 W. Shure Drive, #2309
   Arlington Heights, IL  60004

   Phone: +1 847-632-3028
   Email: qxie1@email.mot.com

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   Randall R. Stewart
   Cisco Systems, Inc.
   8725 West Higgins Road
   Suite 300
   Chicago, IL  60631

   Phone: +1 815-477-2127
   Email: rrs@cisco.com

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