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RFC 2391 - Load Sharing using IP Network Address Translation (LS


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Network Working Group                                       P. Srisuresh
Request for Comments: 2391                           Lucent Technologies
Category: Informational                                           D. Gan
                                                  Juniper Networks, Inc.
                                                             August 1998

       Load Sharing using IP Network Address Translation (LSNAT)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Preface

   This document combines the idea of address translation described in
   RFC 1631 with real-time load share algorithms to introduce Load Share
   Network Address Translators(or, simply LSNATs). LSNATs would
   transparently offload network load on a single server and distribute
   the load across a pool of servers.

Abstract

   Network Address Translators (NATs) translate IP addresses in a
   datagram, transparent to end nodes, while routing the datagram. NATs
   have traditionally been been used to allow private network domains to
   connect to Global networks using as few as one globally unique IP
   address.  In this document, we extend the use of NATs to offer Load
   share feature, where session load can be distributed across a pool of
   servers, instead of directing to a single server.  Load sharing is
   beneficial to service providers and system administrators alike in
   grappling with scalability of servers with increasing session load.

1. Introduction

   Traditionally, Network Address Translators, or simply NATs were used
   to connect private network domains to globally unique public domain
   IP networks. Applications originate in private domains and NATs would
   transparently translate datagrams belonging to these applications in

   either direction. This document combines the characteristic of
   transparent address translation with real-time load share algorithms
   to introduce Load Share Network Address Translators.

   The problem of Load sharing or Load balancing is not new and goes
   back many years. A variety of techniques were applied to address the
   problem.  Some very ad-hoc and platform specific and some employing
   clever schemes to reorder DNS resource records. REF [11] uses DNS
   zone transfer program in name servers to periodically shuffle the
   order of resource records for server nodes based on a pre-determined
   load balancing algorithm. The problem with this approach is that
   reordering time periods can be very large on the order of minutes and
   does not reflect real-time load variations on the servers.  Secondly,
   all hosts in the server pool are assumed to have equal capability to
   offer all services. This may not often be the case. In addition,
   there may be requirement to support load balancing for a few specific
   services only. The load share approach outlined in this document
   addresses both these concerns and offers a solution that does not
   require changes to clients or servers and one that can be tailored to
   individual services or for all services.

   For the reminder of this document, we will refer to NAT routers that
   provide load sharing support as LSNATs. Unlike traditional NATs,
   LSNATs are not required to operate between private and public domain
   routing realms alone. LSNATs also operate in a single routing realm
   and provide load sharing functionality.

   The need for Load sharing arises when a single server is not able to
   cope with increasing demand for multiple sessions simultaneously.
   Clearly, load sharing across multiple servers would enhance
   responsiveness and scale well with session load. Popular applications
   inundating servers would include Web browsers, remote login, file
   transfer and mail applications.

   When a client attempts to access a server through an LSNAT router,
   the router selects a node in server pool, based on a load share
   algorithm and redirect the request to that node. LSNATs pose no
   restriction on the organization and rearrangement of nodes in server
   pool. Nodes in a pool may be replaced, new nodes may be added and
   others may be in transition. Changes of this kind to server pool can
   be shielded from client nodes by making LSNAT router the focal point
   for change management.

   There are limitations to using LSNATs.  Firstly, it is mandatory that
   all requests and responses pertaining to a session between a client
   and server be routed via the same LSNAT router. For this reason, we
   recommend LSNATs to be operated on a single border router to a stub
   domain in which the server pool would be confined.  This would ensure

   that all traffic directed to servers from clients outside the domain
   and vice versa would necessarily traverse the LSNAT border router.
   Later in the document, we will examine a special case of LSNAT setup,
   which gets around the topological constraint on server pool. Another
   limitation of LSNATs is the inability to switch loads between hosts
   in the midst of sessions. This is because LSNATs measure load in
   granularity of sessions. Once a session is assigned to a host, the
   session cannot be moved to a different host till the end of that
   session. Other limitations, inherent to NATs, as outlined in REF [1]
   are also applicable to LSNATs.

   As with traditional NATs, LSNATs have the disadvantage of taking away
   the end-to-end significance of an IP address. The major advantage,
   however, is that it can be installed without changes to clients or
   servers.

2. Terminology and concepts used

2.1. TU ports, Server ports, Client ports

   For the reminder of this document, we will refer TCP/UDP ports
   associated with an IP address simply as "TU ports".

   For most TCP/IP hosts, TU port range 0-1023 is used by servers
   listening for incoming connections. Clients trying to initiate a
   connection typically select a TU port in the range of 1024-65535.
   However, this convention is not universal and not always followed. It
   is possible for client nodes to initiate connections using a TU port
   number in the range of 0-1023, and there are applications listening
   on TU port numbers in the range of 1024-65535.

   A complete list of TU port services may be found in REF [2].  The TU
   ports used by servers to listen for incoming connections are called
   "Server Ports" and the TU ports used by clients to initiate a
   connection to server are called "Client Ports".

2.2. Session flow vs. Packet flow

   Connection or session flows are different from packet flows. A
   session flow  indicates the direction in which the session was
   initiated with reference to a network port. Packet flow is the
   direction in which the packet has traversed with reference to a
   network port.  A session flow is uniquely identified by the direction
   in which the first packet of that session traversed.

   Take for example, a telnet session. The telnet session consists of
   packet flows in both inbound and outbound directions. Outbound telnet
   packets carry terminal keystrokes from the client and inbound telnet

   packets carry screen displays from the telnet server.  Performing
   address translation for a telnet session would involve translation of
   incoming as well as outgoing packets belonging to that session.

   Packets belonging to a TCP/UDP  session are uniquely identified by
   the tuple of (source IP address, source TU port, target IP address,
   target TU port). ICMP sessions that correlate queries and responses
   using query id are uniquely identified by the tuple of (source IP
   address, ICMP Query Identifier, target IP address). For lack of
   well-known ways to distinguish, all other types of sessions are
   lumped together and distinguished by the tuple of (source IP address,
   IP protocol, target IP address).

2.3. Start of session for TCP, UDP and others

   The first packet of every TCP session tries to establish a session
   and contains connection startup information. The first packet of a
   TCP session may be recognized by the presence of SYN bit and absence
   of ACK bit in the TCP flags. All TCP packets, with the exception of
   the first packet must have the ACK bit set.

   The first packet of every session, be it a TCP session, UDP session,
   ICMP query session or any other session, tries to establish a
   session.  However, there is no deterministic way of recognizing the
   start of a UDP session or any other non-TCP session.

   Start of session is significant with NATs, as a state describing
   translation parameters for the session is established  at the start
   of session. Packets pertaining to the session cannot undergo
   translation, unless a state is established by NAT at the start of
   session.

2.4. End of session for TCP, UDP and others

   The end of a TCP session is detected when FIN is acknowledged by both
   halves of the session or when either half receives RST bit in TCP
   flags field. Within a short period (say, a couple of seconds) after
   one of the session partners sets RST bit, the session can be safely
   assumed to have been terminated.

   For all other types of session, there is no deterministic way of
   determining the end of session unless you know the application
   protocol. Many heuristic approaches are used to terminate sessions.
   You can make the assumption that TCP sessions that have not been used
   for say, 24 hours, and non-TCP sessions that have not been used for
   say, 1 minute,  are terminated. Often this assumption works, but
   sometimes it doesn't. These idle period session timeouts may vary
   considerably across the board and may be made user configurable.

   Another way to handle session terminations is to timestamp sessions
   and keep them as long as possible and retire the longest idle session
   when it becomes necessary.

2.5. Basic Network Address Translation (Basic NAT)

   Basic NAT is a method by which hosts in a private network domain are
   allowed access to hosts in the external network transparently.  A
   block of external addresses are set aside for translating addresses
   of private hosts as the private hosts originate sessions to
   applications in external domain. Once an external address is bound by
   the NAT device to a specific private address, that address binding
   remains in place for all subsequent sessions originating from the
   same private host. This binding may be terminated when there are no
   sessions left to use the binding.

2.6. Network Address Port Translation (NAPT)

   Network Address Port Translation(NAPT) is a method by which hosts in
   a private network domain are allowed simultaneous access to hosts in
   the external network transparently using a single registered address.
   This is made possible by multiplexing transport layer identifiers of
   private hosts into the transport identifiers of the single assigned
   external address. For this reason, only the applications based on TCP
   and UDP protocols are supported by NAPT. ICMP query based
   applications are also supported as the ICMP header carries a query
   identifier that is used to corelate responses with requests.
   Sessions other than TCP, UDP and ICMP query type are simply not
   permitted from local nodes, serviced by a NAPT router.

2.7. Load share

   Load sharing for the purpose of this document is defined as the
   spread of session load amongst a cluster of servers  which are
   functionally similar or the same.  In other words, each of the nodes
   in cluster can support a client session equally well with no
   discernible difference in functionality. Once a node is assigned to
   service a session, that session is bound to that node till
   termination. Sessions are not allowed to swap between nodes in the
   midst of session.

   Load sharing may be applicable for all services, if all hosts in
   server cluster carry the capability to carry out all services.
   Alternately, load sharing may be limited to one or more specific
   services alone and not to others.

   Note, the term "Session load" used in the context of load share is
   different from the term "system load" attributed to hosts by way of
   CPU, memory and other resource usage on the system.

3. Overview of Load sharing

   While both traditional NATs and LSNATs perform address translations,
   and provide transparent connectivity between end nodes, there are
   distinctions between the two. Traditional NATs initiate translations
   on outbound sessions, by binding a private address to a global
   address (basic NAT) or by binding a tuple of private address and
   transport identifier (such as TCP/UDP port or ICPM query ID) to a
   tuple of global address and transport identifier. LSNATs, on the
   other hand, initiate translations on inbound sessions, by binding
   each session represented by a tuple such as (client address, client
   TU port, virtual server address, server TU port) to one of server
   pool nodes, selected based on a real-time load-share algorithm. A
   virtual server address is a globally unique IP address that
   identifies a physical server or a group of servers that can provide
   similar or same functionality.

   For the reminder of this document, we will refer traditional NATs
   simply as NATs and refer LSNATs exclusively in the context of load
   share, without implying traditional NAT functionality.

   LSNATs are not limited to operate between private and public domain
   routing realms. LSNATs may operate within a single routing realm with
   globally unique IP addresses, just as well as between private and
   public network domains. The only requirement is that server pool be
   confined to a stub domain, accessible to clients outside the domain
   through a single LSNAT border router. However, as you will notice
   later, this topology limitation on server pool can be overcome under
   certain configurations.

   Load Share NAT operates as follows. A client attempts to access a
   server by using the server virtual address. The LSNAT router
   transparently redirects the request to one of the hosts in server
   pool, selected using a real-time load sharing algorithm. Multiple
   sessions may be initiated from the same client, and each session
   could be directed to a different host based on load balance across
   server pool hosts at the time. If load share is desired for just a
   few specific services, the configuration on LSNAT could be defined to
   restrict load share for just the services desired.

   In the case where virtual server address is same as the interface
   address of an LSNAT router, server applications (such as telnet) on
   LSNAT router must be disabled for external access on that address.
   This is the limitation to using address owned by LSNAT router as the
   virtual server address.

   Load share NAT operation is also applicable during individual server
   upgrades as follows. Say, a server, that needs to be upgraded is
   statically mapped to a backup server on the inbound.  Subsequent to
   this mapping, new session requests to the original server would be
   redirected by LSNAT to the backup server.  As an extension, it is
   also possible to statically map a specific TU port service on a
   server to that of  backup sever.

   We illustrate the operation of LSNAT in the following subsections,
   where  (a) servers are confined to a stub domain, and belong to
   globally unique address space as shared by clients, (b) servers are
   confined to private address space stub domain, and (c) servers are
   not restrained by any topological limitations.

3.1 Operation of LSNAT in a globally unique address space

   In this section, we will illustrate the operation of LSNAT in a
   globally unique address space. The border router with LSNAT enabled
   on WAN link would perform load sharing and address translations for
   inbound sessions. However, sessions outbound from the hosts in server
   pool will not be subject to any type of translation, as all nodes
   have globally unique IP addresses.

   In the example below, servers S1 (172.85.0.1), S2(172.85.0.2) and
   S3(172.85.0.3) form a server pool, confined to a stub domain. LSNAT
   on the border router is enabled on the WAN link, such that the
   virtual server address S(172.87.0.100) is mapped to the server pool
   consisting of hosts S1, S2 and S3. When a client 198.76.29.7
   initiates a HTTP session to the virtual server S, the LSNAT router
   examines the load on hosts in server pool and selects a host, say S1
   to service the request. The transparent address and TU port
   translations performed by the LSNAT router become apparent as you
   follow the down arrow line. IP packets on the return path go through
   similar address translation. Suppose, we have another client
   198.23.47.2 initiating telnet session to the same virtual server S.
   The LSNAT would determine that host S3 is a better choice to service
   this session as S1 is busy with a session and redirect the session to
   S3. The second session redirection path is delineated with colons.
   The procedure continues for any number of sessions the same way.

   Notice that this requires no changes to clients or servers. All the
   configuration and mapping necessary would be limited just to the
   LSNAT router.

                                   \ | /
                                 +---------------+
                                 |Backbone Router|
                                 +---------------+
                               WAN |
                                   |
         Stub domain border .......|.........
                                   |
   {s=198.76.29.7, 2745, v         |            {s=198.23.47.2,  3200,
    d=172.87.0.100, 80 } v         |             d=172.87.0.100, 23 }
                         v +------------------+ :
                         v |Border Router with| :
                         v |LSNAT enabled on  | :
                         v |WAN interface     | :
                         v +------------------+ :
                         v       |              :
                         v       |  LAN         :
                   ------v----------------------:---
   {s=198.76.29.7, 2745, v |            |         |:{s=198.23.47.2, 3200,
    d=172.85.0.1,  80  }   |         |         |  d=172.85.0.3,  23 }
                         +--+      +--+       +--+
                         |S1|      |S2|       |S3|
                         |--|      |--|       |--|
                        /____\    /____\     /____\
                    172.85.0.1   172.85.0.2  172.85.0.3

    Figure 1: Operation of LSNAT in Globally unique address space

3.2. Operation of LSNAT in conjunction with a private network

   In this section, we will illustrate the operation of LSNAT in
   conjunction with NAT on the same router. The NAT configuration is
   required for translation of outbound sessions and could be either
   Basic NAT or NAPT.  The illustration below will assume NAPT on the
   outbound and LSNAT on the inbound on WAN link.

   Say, an organization has a private IP network and a WAN link to
   backbone router. The private network's stub router is assigned a
   globally valid address on the WAN link and the remaining nodes in the
   organization have IP addresses that have only local significance. The
   border router is NAPT configured on the outbound allowing access to
   external hosts, using the single registered IP address.

   In addition, say the organization has servers S1 (10.0.0.1),
   S2(10.0.0.2) and S3 (10.0.0.3) that form a pool to provide inbound
   access to external clients. This is made possible by enabling LSNAT
   on the WAN link of the border router, such that virtual server
   address S(198.76.28.4) is mapped to the server pool consisting of
   hosts S1, S2 and S3. When an external client 198.76.29.7 initiates a
   HTTP session to the virtual server S, the LSNAT router examines load
   on hosts in server pool and selects a host, say S1 to service the
   request. The transparent address  and TU port translations performed
   by the LSNAT router are apparent as you follow the down arrow line.
   IP packets on the return path go through similar address translation.
   Suppose, we have another client 198.23.47.2 initiating telnet session
   to the same address. The LSNAT would determine that host S3 is a
   better choice to service this session as S1 is busy with a session
   and redirect the session to S3. The second session redirection path
   is delineated with colons. The procedure continues for any number of
   sessions the same way.

                                   \ | /
                                 +---------------+
                                 |Backbone Router|
                                 +---------------+
                               WAN |
                                   |
        Stub domain border ........|.........
                                   |
   {s=198.76.29.7, 2745, v         |           {s=198.23.47.2, 3200,
    d=198.76.28.4, 80   }v         |           :d=198.76.28.4, 23 }
                         v+-------------------+:
                         v|Border Router with |:
                         v|  LSNAT and NAPT   |:
                         v|enabled on WAN link|:
                         v+-------------------+:
                         v      |              :
                         v      |  LAN         :
                   ------v---------------------:------
   {s=198.76.29.7, 2745, v |            |       | : {s=198.23.47.2, 3200,
    d=10.0.0.1,    80  }   |         |       |    d=10.0.0.3,    23 }
                         +--+      +--+     +--+
                         |S1|      |S2|     |S3|
                         |--|      |--|     |--|
                        /____\    /____\   /____\
                       10.0.0.1  10.0.0.2  10.0.0.3

     Figure 2: Operation of LSNAT, in coexistence with NAPT

   Once again, notice that this requires no changes to clients or
   servers.  The translation is completely transparent to end nodes.
   Address mapping on the LSNAT performs load sharing and address
   translations for inbound sessions. Sessions outbound from hosts in
   server pool are subject to NAPT. Both NAT and LSNAT co-exist with
   each other in the same router.

3.3. Load Sharing with no topological restraints on servers

   In this section, we will illustrate a configuration in which load
   sharing can be accomplished on a router without enforcing topological
   limitations on servers. In this configuration, virtual server address
   will be owned by the router that supports load sharing. I.e., virtual
   server address will be same as address of one of the interfaces of
   load share router. We will distinguish this configuration from LSNAT
   by referring this as "Load Share Network Address Port Translation"
   (LS-NAPT). Routers that support the LS-NAPT configuration will be
   termed "LS-NAPT routers", or simply LS-NAPTs.

   In an LSNAT router, inbound TCP/UDP sessions, represented by the
   tuple of (client address, client TU port, virtual server address,
   service port) are translated into a tuple of (client address, client
   TU port, selected server address, service port). Translation is
   carried out on all datagrams pertaining to the same session, in
   either direction. Whereas, LS-NAPT router would translate the same
   session into a tuple of (virtual server address, virtual server TU
   port, selected server, service port). Notice that LS-NAPT router
   translates the client address and TU port with the address and TU
   port of virtual server, which is same as the address of one of its
   interfaces. By doing this, datagrams from clients as well as servers
   are forced to bear the address of LS-NAPT router as the destination
   address, thereby guaranteeing that the datagrams would necessarily
   traverse the LS-NAPT router. As a result, there is no need to require
   servers to be under topological constraints.

   Take for example, figure 1 in section 3.1. Let us say the router on
   which load sharing is enabled is not just a border router, but can be
   any kind of router. Let us also say that the virtual server address S
   (172.87.0.100) is same as the address of WAN link and LS-NAPT is
   enabled on the WAN interface. Figure 3 summarizes the new router
   configuration.

   When a client 198.76.29.7 initiates a HTTP session to the virtual
   server address S (i.e., address of the WAN interface), the LS-NAPT
   router examines load on hosts in server pool and selects a host, say
   S1 to service the request. Appropriately, the destination address is
   translated to be S1 (172.85.0.1). Further, original client address
   and TU port are replaced with the address and TU port of the WAN

   link.  As a result, destination addresses as well as source address
   and source TU port are translated when the packet reaches S1, as can
   be noticed from the down-arrow path. IP packets on the return path go
   through similar translation. The second client 198.23.47.2 initiating
   telnet session to the same virtual server address S is load share
   directed to S3. This packet once again undergoes LS-NAPT translation,
   just as with the first client. The data path and translations can be
   noticed following the colon line. The procedure continues for any
   number of sessions the same way. The translations made to datagrams
   in either direction are completely transparent to end nodes.

                                   \ | /
                              +---------------+
                              |   Router      |
                              +---------------+
                            WAN |
                                |
                                |
   {s=198.76.29.7, 2745, v      |                {s=198.23.47.2, 3200,
    d=198.76.28.4, 80   }v      | 198.76.28.4  :d=198.76.28.4, 23 }
                         v +----------------+  :
                         v | A Router with  |  :
                         v | LS-NAPT enabled|  :
                            v | on WAN link    |  :
                         v +----------------+  :
                         v               |     :
                         v          LAN  |     :
                   ------v---------------------:------
   {s=198.76.28.4, 7001, v|             |        |:{s=198.76.28.4,7002,
    d=172.85.0.1,   80 }  |          |        |  d=172.85.0.3,  23 }
                        +--+       +--+      +--+
                        |S1|       |S2|      |S3|
                        |--|       |--|      |--|
                       /____\     /____\    /____\
                     172.85.0.1 172.85.0.2 172.85.0.3

     Figure 3: LS-NAPT configuration on a router

   As you will notice, datagrams from clients as well as servers are
   forced to be directed to the router, because they use WAN interface
   address of router as the destination address in their datagrams. With
   the assurance that all packets from clients and servers would
   traverse the router, there is no longer a requirement for servers to
   be confined to a stub domain and for LSNAT to be enabled only on
   border router to the stub domain.

   The LS-NAPT configuration described in this section involves more
   translations and hence is more complex compared to LSNAT
   configurations described in the previous sections. While the
   processing is complex, there are benefits to this configuration.
   Firstly, it breaks down restraints on server topology. Secondly, it
   scales with bandwidth expansion for client access. Even if Service
   providers have one link today for client access, the LS-NAPT
   configuration allows them to expand to more links in the future
   guaranteeing the same LS-NAPT load share service on newer links.

   The configuration is not without its limitations. Server applications
   (such as telnet) on the router box would have to be disabled for the
   interface address assigned to be virtual server address. Load sharing
   would be limited to TCP and UDP applications only. Maximum
   concurrently allowed sessions would be limited by the maximum allowed
   TCP/UDP client ports on the same address. Assuming that ports 0-1023
   must be set aside as well-known service ports, that would leave a
   maximum of 63K TCP client ports and 63K of UDP client ports on the
   LS-NAPT router to communicate with each load-share server.  As a
   result, LS-NAPT routers will not be able to concurrently support more
   than a maximum of (63K * count of Load-share servers) TCP sessions
   and (63K * count of Load-share servers) UDP sessions.

4.0. Translation phases of a session in LSNAT router.

   As with NATs, LSNATs must monitor the following three phases in
   relation to Address translation.

4.1. Session binding:

   Session binding is the phase in which an incoming session is
   associated with the address of a host in server pool. This
   association essentially sets the translation parameters for all
   subsequent datagrams pertaining to the session. For addresses that
   have static mapping, the binding happens at startup time. Otherwise,
   each incoming session is dynamically bound to a different host based
   on a load sharing algorithm.

4.2. Address lookup and translation:

   Once session binding is established for a connection setup, all
   subsequent packets belonging to the same connection will be subject
   to session lookup for translation purposes.

   For outbound packets of a session, the source IP address (and source
   TU port, in case of TCP/UDP sessions) and related fields (such as IP,
   TCP, UDP and ICMP header checksums) will undergo translation. For
   inbound packets of a session, the destination IP address (and

   destination TU port, in case of TCP/UDP sessions) and related fields
   such as IP, TCP, UDP and ICMP header checksums) will undergo
   translation.

   The header and payload modifications made to IP datagrams subject to
   LSNAT will be exactly same as those subject to traditional NATs,
   described in section 5.0 of REF [1]. Hence, the reader is urged to
   refer REF [1] document for packet translation process.

4.3. Session unbinding:

   Session unbinding is the phase in which a server node is no longer
   responsible for the session. Usually, session unbinding happens when
   the end of session is detected.  As described in the terminology
   section, it is not always easy to determine end of session.

5. Load share algorithms

   Many algorithms are available to select a host from a pool of servers
   to service a new session. The load distribution is based primarily on
   (a) cost of accessing the network on which a  server resides and load
   on the network interface used to access the server, and (b)resource
   availability and system load on the server. A variety of policies can
   be adapted to distribute sessions across the servers in a server
   pool.

   For simplicity, we will consider two types algorithms, based on
   proximity between server nodes and LSNAT router. The higher the cost
   of access to a sever, the farther the proximity of server is assumed
   to be. The first kind of algorithms will assume that all server pool
   members are at equal or nearly equal proximity to LSNAT router and
   hence the load distribution can be based solely on resource
   availability or system load on remote servers. Cost of network access
   will be  considered irrelevant. The second kind would assume that all
   server pool members have equal resource availability and the criteria
   for selection would be proximity to servers. In other words, we
   consider algorithms which take into account the cost of network
   access.

5.1. Local Load share algorithms

   Ideally speaking, the selection process would have precise knowledge
   of real-time resource availability and system load for each host in
   server pool, so that the selection of host with maximum unutilized
   capacity would be the obvious choice. However, this is not so easy to
   achieve.

   We consider here two kinds of heuristic approaches to monitor session
   load on server pool members. The first kind is where the load share
   selector tracks system load on individual servers in non-intrusive
   way.  The second kind is where the individual members actively
   participate in communicating with the load share selector, notifying
   the selector of their load capacity.

   Listed below are the most common selection algorithms adapted in the
   non-intrusive category.

   1. Round-Robin algorithm
      This is the simplest scheme, where a host is selected simply on a
      round robin basis, without regard to load on the host.

   2. Least Load first algorithm
      This is an improvement over round-robin approach, in that, the
      host with least number of sessions bound to it is selected to
      service a new session. This approach is not without its caveats.
      Each session is assumed to be as resource consuming as any other
      session, independent of the type of service the session represents
      and all hosts in server pool are assumed to be equally
      resourceful.

   3. Least traffic first algorithm
      A further improvement over the previous algorithm would be to
      measure system load by tracking packet count or byte count
      directed from or to each of the member hosts over a period of
      time. Although packet count is not the same as system load, it is
      a reasonable approximation.

   4. Least Weighted Load first approach
      This would be an enhancement to the first two. This would allow
      administrators to assign (a) weights to sessions, based on likely
      resource consumption estimates of session types and (b) weights to
      hosts based on resource availability.

      The sum of all session loads by weight assigned to a server,
      divided by weight of server would be evaluated to select the
      server with least weighted load to assign for each new session.
      Say, FTP sessions are assigned 5 times the weight(5x) as a telnet
      session(x), and server S3 is assumed to be 3 times as resourceful
      as server S1. Let us also say that S1 is assigned 1 FTP session
      and 1 telnet session, whereas S3 is assigned 2 FTP sessions and 5
      telnet sessions. When a new telnet session need assignment, the
      weighted load on S3 is evaluated to be (2*5x+5*x)/3 = 5x, and the
      load on S1 is evaluated to be (1*5x+1*x) = 6x. Server S3 is
      selected to bind the new telnet session, as the weighted load on
      S3 is smaller than that of S1.

   5. Ping to find the most responsive host.
      Till now, capacity of a member host is determined exclusively by
      the LSNAT using heuristic approaches. In reality, it is impossible
      to predict system capacity from remote, without interaction with
      member hosts. A prudent approach would be to periodically ping
      member hosts and measure the response time to determine how busy
      the hosts really are. Use the response time in conjunction with
      the heuristics to select the host most appropriate for the new
      session.

   In the active category, we involve individual member hosts in
   resource utilization monitoring process. An agent software on each
   node would notify the monitoring agent on resource availability.
   Clearly, this would imply having an application program (one that
   does not consume significant resources, by itself) to run on each
   member node. This strategy of involving member hosts in system load
   monitoring is likely to yield the most optimal results in the
   selection process.

5.2. Distributed Load share algorithms

   When server nodes are distributed geographically across different
   areas and cost to access them vary widely, the load share selector
   could use that information in selecting a server to service a new
   session. In order to do this, the load share selector would need to
   consult the routing tables maintained by routing protocols such as
   RIP and OSPF to find the cost of accessing a server.

   All algorithms listed below would be non-intrusive kind where the
   server nodes do not actively participate in notifying the load share
   selector of their load capacity.

   1. Weighted Least Load first algorithm
      The selection criteria would be based on (a) cost of access to
      server, and (b) the number of sessions assigned to server.  The
      product of cost and session load for each server would be
      evaluated to select the server with least weighted load for each
      new session. Say, cost of accessing server S1 is twice as much as
      that of server S2. In that case, S1 will be assigned twice as much
      load as that of S2 during the distribution process. When a server
      is not accessible due to network failure, the cost of access is
      set to infinity and hence no further load can be assigned to that
      server.

   2. Weighted Least traffic first algorithm
      An improvement over the previous algorithm would be
      to measure network load by tracking packet count or byte
      count directed from or to each of the member hosts over a

      period of time. Although packet count is not the same as
      system load, it is a reasonable approximation. So, the
      product of cost and traffic load (over a fixed duration)
      for each server would be evaluated to select the server
      with least weighted traffic load for each new session.

6. Dead host detection

   As sessions are assigned to hosts, it is important to detect the
   live-ness of the hosts. Otherwise, sessions could simply be black-
   holed into a dead host. Many heuristic approaches are adopted.
   Sending pings periodically would be one way to determine the live-
   ness. Another approach would be to track datagrams originating from a
   member host in response to new session assignments.  If no response
   is detected in a few seconds, declare the server dead and do not
   assign new sessions to this host. The server can be monitored later
   again after a long pause (say, in the order of a few minutes) by
   periodically reassigning new sessions and monitoring response times
   and so on.

7. Miscellaneous

   The IETF has been notified of potential intellectual Property Rights
   (IPR) issues with the technology described in this document.
   Interested people are requested to look in the IETF web page
   (http://www.ietf.org) under the Intellectual property Rights Notices
   section for the current information.

8. Security Considerations

   All security considerations associated with NAT routers, described in
   REF [1] are applicable to LSNAT routers as well.

REFERENCES

   [1] Egevang, K. and P. Francis, "The IP Network Address Translator
       (NAT)", RFC 1631, May 1994.

   [2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
       October 1994.  See also: http://www.iana.org/numbers.html

   [3] Braden, R., "Requirements for Internet Hosts -- Communication
       Layers", STD 3, RFC 1122, October 1989.

   [4] Braden, R., "Requirements for Internet Hosts -- Application and
       Support", STD 3, RFC 1123, October 1989.

   [5] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
       June 1995.

   [6] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD
       9, RFC 959, October 1985.

   [7] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
       September 1981.

   [8] Postel, J., "Internet Control Message (ICMP) Specification", STD
       5, RFC 792, September 1981.

   [9] Postel, J., "User Datagram Protocol (UDP)", STD 6, RFC 768,
       August 1980.

   [10] Mogul, J., and J. Postel, "Internet Standard Subnetting
        Procedure", STD 5, RFC 950, August 1985.

   [11] Brisco, T., "DNS Support for Load Balancing", RFC 1794, April
        1995.

Authors' Addresses

   Pyda Srisuresh
   Lucent Technologies
   4464 Willow Road
   Pleasanton, CA 94588-8519
   U.S.A.

   Voice: (925) 737-2153
   Fax:   (925) 737-2110
   EMail: suresh@ra.lucent.com

   Der-hwa Gan
   Juniper Networks, Inc.
   385 Ravensdale Drive.
   Mountain View, CA 94043
   U.S.A.

   Voice: (650) 526-8074
   Fax:   (650) 526-8001
   EMail: dhg@juniper.net

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