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rfc:rfc2391

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

Srisuresh & Gan Informational [Page 1] RFC 2391 LSNAT August 1998

 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

Srisuresh & Gan Informational [Page 2] RFC 2391 LSNAT August 1998

 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

Srisuresh & Gan Informational [Page 3] RFC 2391 LSNAT August 1998

 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.

Srisuresh & Gan Informational [Page 4] RFC 2391 LSNAT August 1998

 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.

Srisuresh & Gan Informational [Page 5] RFC 2391 LSNAT August 1998

 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.

Srisuresh & Gan Informational [Page 6] RFC 2391 LSNAT August 1998

 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.

Srisuresh & Gan Informational [Page 7] RFC 2391 LSNAT August 1998

 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.

Srisuresh & Gan Informational [Page 8] RFC 2391 LSNAT August 1998

 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

Srisuresh & Gan Informational [Page 9] RFC 2391 LSNAT August 1998

 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

Srisuresh & Gan Informational [Page 10] RFC 2391 LSNAT August 1998

 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.

Srisuresh & Gan Informational [Page 11] RFC 2391 LSNAT August 1998

 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

Srisuresh & Gan Informational [Page 12] RFC 2391 LSNAT August 1998

 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.

Srisuresh & Gan Informational [Page 13] RFC 2391 LSNAT August 1998

 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.

Srisuresh & Gan Informational [Page 14] RFC 2391 LSNAT August 1998

 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

Srisuresh & Gan Informational [Page 15] RFC 2391 LSNAT August 1998

    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.

Srisuresh & Gan Informational [Page 16] RFC 2391 LSNAT August 1998

 [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

Srisuresh & Gan Informational [Page 17] RFC 2391 LSNAT August 1998

Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Srisuresh & Gan Informational [Page 18]

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