📄 rfc3049.txt
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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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RFC 3049 TN3270E Location & Balancing January 2001
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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RFC 3049 TN3270E Location & Balancing January 2001
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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RFC 3049 TN3270E Location & Balancing January 2001
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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RFC 3049 TN3270E Location & Balancing January 2001
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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RFC 3049 TN3270E Location & Balancing January 2001
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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RFC 3049 TN3270E Location & Balancing January 2001
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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