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Network Working Group                                       P. SrisureshRequest for Comments: 2391                           Lucent TechnologiesCategory: Informational                                           D. Gan                                                  Juniper Networks, Inc.                                                             August 1998Load 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 inRFC 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 inSrisuresh & 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 ensureSrisuresh & 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 used2.1. 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 telnetSrisuresh & 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 space3.2. 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 NAPTSrisuresh & 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 insection 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 WANSrisuresh & 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 (andSrisuresh & 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 insection 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 aSrisuresh & 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.netSrisuresh & Gan              Informational                     [Page 17]

RFC 2391                         LSNAT                       August 1998Full 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]