rfc3049.txt

最新的RFC

Network Working Group                                          J. Naugle
Request for Comments: 3049                             K. Kasthurirangan
Category: Standards Track                                            IBM
                                                              G. Ledford
                                                      Zephyr Development
                                                            January 2001


             TN3270E Service Location and Session Balancing

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document discusses the implementation of Service Location
   Protocol (SLP) and session balancing with a TN3270E emulator in a
   client server implementation with a TN3270E server.

   Application program developer's can locate TN3270E services and load
   balance among those services (3270 host sessions), by using this SLP
   support.

Table of Contents

   1. Introduction and Terminology .................................  2
      1.1 Terminology ..............................................  2
   2. An Overview of RFC 2165 ......................................  3
      2.1 SLP Agents ...............................................  3
      2.2 Service Agents ...........................................  3
      2.3 User Agents ..............................................  4
   3. TN3270E Server Environment and Load ..........................  4
      3.1 TnN3270E Server Load .....................................  4
   4. TN3270E Client Configuration .................................  6
      4.1 SLP Scope ................................................  6
      4.2 DA-Discovery Time-Out ....................................  6
      4.3 SA-Discovery Time-Out ....................................  7
   5. TN3270E Client Implementation Information ....................  7
      5.1 Overview .................................................  7



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      5.2 How to Obtain List of TN3270E Servers Supporting SLP .....  8
      5.3 TN3270E Sample Client Flow ...............................  9
         5.3.1 Open the SLP connection .............................  9
         5.3.2 Query the list of TN3270E servers ...................  9
         5.3.3 Forward Looking Example using SLPv2 ................. 10
         5.3.4 Determine loading of each TN3270E server ............ 10
      5.4 Recommendations .......................................... 11
   6. Sample Trace Flow of SLP and Session Balancing ............... 11
   7. Service Templates and Service Registration ................... 12
      7.1 The TN3270E Service Type Template ........................ 12
      7.2 The Server Service Template .............................. 16
      7.3 Template Contact Information ............................. 17
      7.4 Security Considerations .................................. 17
      7.5 Sample TN3270 Service Registration Message ............... 18
      7.6 Sample Server Service Registration Message ............... 19
   8. References ................................................... 19
   9. Authors' Addresses ........................................... 20
   10. Full Copyright Statement .................................... 21

1. Introduction and Terminology

   This document will provide information on Service Location Protocol
   implementation to discover TN3270E servers in a network and session
   balance among those servers.  This implementation follows the
   standards track RFC 2165, Service Location Protocol [1] but also
   provides some examples when using Service Location Protocol version 2
   to be forward looking.  Service Location Protocol version 2 is
   documented in RFC 2608 [4] and RFC 2609 [2].

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119  [1].

   Session Balance - This refers to the ability of TN3270E client to use
   server load information to establish a TN3270E connection to the
   TN3270E server with the least load at that time.  The purpose is to
   distribute the connection of TN3270E sessions among more than one
   TN3270E server, and one server will not be excessively loaded.  The
   term "load balance" is a more general term, with respect to server
   load, and in this document we are focusing on the TN3270E session
   connections to least loaded servers.

   SNA Gateway - A Systems Network Architecture (SNA) gateway allows
   multiple LAN-attached workstations to access SNA hosts through one or
   more physical connections to one or more hosts.  A SNA gateway acts
   as a protocol converter between workstations attached to a LAN and a



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   WAN host line.  It typically would support the SNA protocols LU 0, 1,
   2, 3, and dependent LU 6.2 (APPC).  SNA gateways typically include a
   TN3270E server capability.

   LU Pool - The Logical Units (LUs) defined in the gateway can be
   dedicated to a particular workstation or pooled among multiple
   workstations.  Pooling allows workstations to share common Logical
   Units (LUs), which increases the efficiency of the LUs and reduces
   the configuration and startup requirements at the host.  When a
   client connects to the gateway, the gateway retrieves an LU from the
   pool to establish a session.  The LU is returned to the pool for
   access by other workstations when the session is ended.

   Commserver Service Type Template - Commserver service type is defined
   as an SNA Gateway server as previously defined above in this
   terminology section.  A template describing the attributes for this
   service type is in section 7.2.

2. An overview of RFC 2165

   RFC 2165, Service Location Protocol (SLP) [1], provides an automatic
   way for clients to discover services within an administrative domain.

   These services have various attributes associated with them from
   which a client can base a service selection.  The basic design
   involves the use of three agent types.  These are: User Agents
   (UA's), Service Agents (SA's) and Directory Agents (DA's).

2.1 SLP Agents

   User Agents are used to query Service Agents or Directory Agents.
   They acquire/request service information based upon the desired
   attributes and service needed for the user application.

   Service Agents represent a specific service and advertise service
   information.

   Directory Agents act as a central collection point for service
   registration information by Service Agents which is later requested
   by "user agents" in "intranets".

2.2 Service Agents

   The service registers itself with the service agent so that the SA
   can start advertising this information over the network.  The process
   of registration consists of the service giving the SA all relevant
   configuration information and attribute tag/value list pairs specific
   to this service.  The Service template is an abstract schema that



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   applies to the service type.  The service template for TN3270E is
   shown later, contains the URL which is the address of the server with
   the port, which should be used to connect to it.  The URL also
   contains the service type which in this case is TN3270.  The template
   also contains all the other attributes associated with this service.

2.3 User Agents

   The User Agent working on the TN3270E client's behalf retrieves
   service information from the Service Agent(s) or a Directory Agent.
   Based on the gathered information and required attributes the TN3270E
   client or user can decide whether or not to connect with a particular
   server.  Based on the service advertisements from various TN3270E
   servers, the client looks at the load attribute and can decide to
   connect to the least loaded server.  If by the time it connects to
   that particular TN3270E server, the server becomes unavailable it can
   try connecting to the next server in its list (ie: the second least
   loaded server whose advertisement was retrieved by the client/user
   agent).

3. TN3270E Server Environment and Load

   TN3270E Servers are pervasive in today's networked environment.  SLP
   provides emulator clients with a way to discover TN3270E servers in
   the network and session balance among the servers.  The TN3270E
   servers could be distributed across different SNA gateways with
   different connection methods to hosts.  The use of LU pools provides
   an easy way for administrators to provide users access to hosts.
   Administrators can add users to LU pools that have pre-configured
   LU's with specific attributes, like LU types and model types.

   These LU pools would typically have LUs from several different
   gateways assigned, and as members of the LU pool make TN3270E session
   connections, they would be making connections to different TN3270E
   servers, with different load factors, so that session balancing could
   be accomplished.  The use of LU pools is not a requirement for SLP
   and session balancing.  A TN3270E client could obtain a session by
   using SLP and session balancing to locate the least loaded server in
   the network.  On a service request a wild card "*" could be used when
   asking for LUPOOL if the emulator doesn't care which device types are
   supported in given pools or if it can assume given pools support only
   certain device types.

3.1 TN3270E Server Load

   TN3270E servers providing load information, SHOULD include number of
   sessions available, not in current use, as part of the calculation in
   determining the total load for the server.  There can be other



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   factors that might have an effect on server load.  An example would
   be if a server is not dedicated to only SNA traffic, and is handling
   other processes, like file services and print services, etc.  It is
   beyond the scope of this document to standardize the method of
   individual server load calculations.  Different vendors server's may
   calculate load information based upon factors they consider
   important, and methods for calculating load may change over time.

   If the TN3270E server coexists in a network with other TN server
   implementations using SLP for session balancing, TN3270E server load
   could be adjusted to compensate for differences in load calculations.

   One way to allow TN3270E server administrators to compensate for
   differences in implementations of calculating server load measurement
   is to provide the ability to modify the load calculation on the
   TN3270E server.  An element of control can be provided by allowing
   the administrator to modify the load measurement, by using an
   integral number between 0 and 100 (100 being the highest) to change
   the load.  This load measurement acts as an additional factor on the
   server's actual load calculation, so that the administrator could
   bias up or down, the likelihood of that server being selected by a
   TN3270E client.

   Load MUST be defined as one of the attributes for the TN3270E server.
   The Load attribute provided at the server will allow clients to
   determine which server to make a connection.  If a UA provides only a
   Service Type, in an Attribute Request,  then the reply includes all
   attributes and all values for that Service Type, and Load would be
   included.  Attribute Requests MAY include a select clause, so you
   could be returned just load information.  For more information on
   Attribute Requests refer to Service Location Protocol [1].

   An application could issue a Service Request to locate a TN3270E
   server.  Then an application designed to perform least-load location
   of a TN3270E service, could issue a series of Attribute Requests to
   obtain the load measurement of each server specified with a URL.  It
   would specify a select clause similar to the one below to receive
   only load information.

   URL = service:tn3270://9.37.51.254:23 Attribute filter = LOAD

   The attribute LOAD would be returned along with its value.  The
   application could then issue other Attribute Request calls for each
   URL.







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   The application would then select the least loaded server as a
   connection target.  If it tries to connect to a server and that
   connection fails, it could then try to connect to the next least-
   loaded server.

4. TN3270E Client Configuration

4.1 SLP Scope

   Scope is a parameter used to control and manage access by clients to
   servers in a network.  It is the same as the Service Location
   Protocol scope defined in RFC 2165 [1].  The control scope provides
   is necessary for two reasons:

   As your network, the number of clients, and the number of servers
   grow, it becomes necessary to partition access to those servers by
   the growing number of clients in order to reduce overall traffic on
   the network.  It allows administrators to organize users and servers
   into administrative groups.

   The meaning of the values of scope is defined by the administrator of
   the network.  These values can represent any entity.  Commonly, they
   fall along either departmental, geographical, or organizational
   lines.

   Each TN3270E server can be assigned to a single scope or multiple
   scopes.  TN3270E clients using these servers can be configured for a
   single specific scope.  If TN3270E clients are not configured with a
   scope they MUST use the scope "default".

   SLP Service Agents and Directory Agents (DA) need to reside in the
   network that support the TN3270E server with configured scopes.

   Attribute information for Service Types pertaining to a specific
   scope can be obtained from Directory Agents (DA).  The DA will not
   return a result unless the requested scope matches.

   For more information on SLP scope refer to Service Location Protocol
   [1].

4.2 DA Discovery time-out

   The DA Discovery time-out value, is used to control how long the SLP
   API must wait to discover Directory Agents (DAs) in the network.  The
   discovery request is a multicast, and the amount of time required to
   gather all DA responses might vary depending on many factors.  If
   there are no DAs in the network, this time-out value can be set to




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   zero to indicate that no DA discovery is to be done.  The time-out is
   expressed in milliseconds.  Time-out intervals and default values
   should be handled as described in RFC 2165 [1].

4.3 SA Multicast time-out

   The SA Multicast time-out value, is used to control how long the SLP
   API must wait to discover services, attributes, or service types in a
   network without at least one DA that supports the scope of the
   request.  In this situation, these requests are multicast and the
   User Agent waits the time-out value to gather the multiple responses
   that are returned.  The time-out is expressed in milliseconds. Time-
   out intervals and default values should be handled as described in
   RFC 2165 [1].

5. TN3270E Client Implementation Information

5.1 Overview

   A TN3270E client that implements TN3270E SLP session balancing does
   not need to configure an IP Host Address or TCP Port for the TN3270E
   server it desires to connect to.  Instead, the IP Host Address and
   TCP Port of the least loaded TN3270E server is discovered by using
   the SLP session balancing described in this document.

   The discovery of the least loaded TN3270E server is done entirely
   outside of and before the TN3270E telnet negotiation.  Once the IP
   Host address and TCP Port of the least loaded TN3270E server is
   discovered, the TN3270E client can then start normal TN3270E telnet
   negotiation.

   The TN3270E client MUST allow for configuration of the following
   parameters.  These SLP specific configuration items are covered by
   configuration parameters in the SLP API [5].

   Enable SLP Session Balancing

   This configuration parameter indicates whether or not SLP session
   balancing is enabled.  If it is enabled the following three
   configuration parameters MUST also be configurable.  If this
   parameter is disabled, SLP session balancing is not supported and
   normal TN3270E telnet negotiation is performed.

   Scope Name The scope name is a text string that specifies a group of
   TN3270E servers.  The scope name can be used to identify groups of
   TN3270E servers in a departmental or geographic setting.  For





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