TN3270E Working Group Jim Naugle
Internet Draft: Kashturi Kasthurangan
<draft-ietf-tn3270e-service-loc-00.txt> IBM
Expiration Date: November 98 Gregg Ledford
Zephyr Development
May 98
TN3270E Service Location and Session Balancing
draft-ietf-tn3270e-service-loc-00.txt
Status of This Memo
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Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document discusses the implementation of Service Location
Protocol 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 Service Location Protocol support.
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1. Table of Contents
Status of This Memo
Abstract
1. Table of Contents
2. Introduction
3. Overview of RFC 2165
3.1 SLP Agents
3.2 SLP Relationships
3.3 User Agents
3.4 SLP Relationships
4. TN3270E Server Environment and Load
4.1 TN3270E Server Load
5. TN3270E Client Configuration
5.1 SLP Scope
5.2 Directory Agent Discovery Time-out
5.3 Service Agent Multicast Time-out
6. Service Registration and Service Templates
7. TN3270E Client Implementation
7.1 Overview
7.2 How to Obtain List of TN3270E Servers Supporting SLP
7.3 TN3270E Sample Client Flow
7.3.1 Open the SLP connection
7.3.2 Query the list of TN3270E servers
7.3.3 Determine loading of each TN3270E server
7.3.4 Closing the SLP connection
7.3.5 Freeing memory structures
7.4 Recommendations
8. Sample Trace Flow of SLP and Session Balancing
9. Service Registration and Service Templates
10. References
11. Author's Addresses
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2. Introduction
This document will provide information on Service Location Protocol
implementation to discover TN3270E servers in a network and session
balance among those servers. The implementation follows the
standards track RFC 2165, Service Location Protocol [1].
3. 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 on. 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).
3.1 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".
3.2 Service Agents
The Service Agent is defined as a process working on behalf of one
or more services to advertise service information. Service
information is a collection of attributes and configuration
information associated with a single service. The Service Agents
advertise service information for either one or a collection of
service instances. The service itself is a process or system
providing a facility to the network, which in our case would be the
TN3270 server.
The service registers itself with the service agent so that the SA
can start advertising this information over the network. The process
of registrations consists of the service giving the SA all relevant
configuration information and attributes. The format of this
information is called the Service template. The service template for
TN3270 which is given below contains the URL which is the address of
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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 of the service like
scope, load etc.
3.3 User Agents
The User Agent working on the TN3270 client's behalf retrieves
service information from the Service Agent or a Directory Agent and
based on the required attributes decides to connect or not with a
particular server. Based on the service advertisements from various
TN3270 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 TN3270 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).
3.4 SLP Relationships
The diagram below taken from RFC 2165 [1] helps illustrate the agent
relationships described above.
+---------------+ we want this info: +-----------+
| Application | - - - - - - - - - - - -> | Service |
+---------------+ +-----------+
/|\ | |
| +-------------+ |
| | |
\|/ \|/ \|/
+---------------+ +-----------+ +----------------+
| User Agent |<-------->| Service | | Service |
+---------------+ | Agent | | Agent which |
| +-----------+ | does not reply |
| | | to UA requests |
| \|/ +----------------+
| +-------------+ |
+------------------>| Directory |<----------+
| Agent |
+-------------+ ___________
/|\ / Many other\
+------------>| SA's |
\___________/
4. 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
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different connection methods to hosts. The use of LU pools provides
a 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.
(LUPOOL=xyz\t*)
4.1 TN3270E Server Load
TN3270E servers providing load information, typically will include
number of sessions available, not in current use, as part of the
calculation in determining load for that server. There can be other
factors that might have an effect on server load. For example, 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 discuss how individual server load
calculations should be determined.
If the TN3270E server coexists in a network with other TN server
implementations using SLP for load 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 is defined as one of the attributes for the TN3270E server. If
a UA provides only a Service Type, in an Attribute Request, then
the reply includes all attributes and all values for that Service
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Type, and Load would be included. Attribute Requests can include a
select clause, so you could just be returned information on load for
each server. For more information on Attribute Requests refer to
Service Location Protocol Internet draft [4].
An application could issue a SL_GetService request to locate a
TN3270E server. Then an application designed to perform least-load
location of a TN3270E service, could issue a series of SL_GetAttrs
calls directed to each URL to obtain the load measurement of each
server. 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 SL_GetAttrs calls to each URL.
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.
5. TN3270E Client Configuration
5.1 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
in to 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 scope or scopes. TN3270E
clients using these servers can be configured for a single specific
scope or be unscoped. Different scopes can be assigned for
different services provided on the server if desired.
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SLP Service Agents and Directory Agents need to reside in the
network that support the TN3270E server with configured scopes. If
you plan to allow clients to locate TN3270E server services based on
scopes, keep in mind how scope relates to the network as a whole. If
there are unscoped services in a network where scopes are also used,
then these services are eligible to satisfy any scoped requests,
which can potentially put a burden on those service agents and
directory agents that support the unscoped services. For this
reason, we recommend that every reachable server either have a
scope, or no server has a scope defined. If directory agents are to
be used in the site network (for upward scaling), then they should
be configured to handle the same scopes as is set for the TN3270E
servers. In addition, if unscoped services are to be used in
networks with directory agents, at least one unscoped directory
agent should be set up.
Discovery - the scope value can be discovered using the SL_GetAttrs
API by issuing an unscoped attribute request for a service type with
an attribute filter of "SCOPE". This API returns a list of scopes
for services currently active in the network. The list can be
displayed for user selection.
For more information on SLP scope refer to Service Location Protocol
Internet draft [4].
5.2 DA-Discovery time-out
The DA Discovery time-out value, a parameter on the SLP_Open API, 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 zero to indicate that no DA
discovery is to be done. The time-out is expressed in milliseconds.
5.3 SA Multicast time-out
The SA Multicast time-out value. a parameter on the SL_Open API, 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 SLP API waits the time-out value to
gather the multiple responses that are returned. The time-out is
expressed in milliseconds.
6. Service Registration and Service Templates
The Service Location Protocol uses the ''service:'' URL scheme name
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to define URLs called ''service: URLs''. These schemes provide a
way for clients to obtain configuration information that is needed
to establish a 3270 session through the Telnet server. The Service
Location Protocol provides for service: URLs to be registered and
discovered.
Service Registration
These service registrations contain a service: URL, and possible
attributes associated with that service. The service registration
templates are shown in section 9 below for the communications server
and the TN3270E server.
Service Templates
Service templates are documents defining in a formal way the
attributes associated with that service that a client may want to
use. For more information on service templates please refer to,
Service Templates and service: Schemes. [3]. The service templates
are shown in section 9 for the communications server service and the
TN3270E service.
7. TN3270E Client Implementation Information
7.1 Overview
A TN3270E client that implements TN3270E SLP load balancing does not
need to configure an IP Host Address for the TN3270E server it
desires to connect to. Instead, the IP Host Address of the least
loaded TN3270E server is discovered by using the SLP load 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 of the least loaded TN3270E server is discovered, the
TN3270E client can then start normal TN3270E telnet negotiation.
The TN3270E client should allow for configuration of the
following parameters:
Enable SLP Load Balancing
This configuration parameter indicates whether or not SLP load
balancing is enabled. If it is enabled the following three
configuration parameters must also be configured. If this
parameter is disabled, SLP load 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
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servers. The scope name can be used to identify groups of TN3270E
servers in a departmental or geographic setting. For example, if the
scope name is Building-D, then the SLP load balancing would search
all TN3270E servers in the Building-D scope to find the least loaded
TN3270E server.
If the scope name is blank, then the scope name is not used. This is
referred to as unscoped. All TN3270E servers, with or without scope
names, can be used to satisfy the request for least loaded TN3270E
servers. Because of the amount of network overhead that this
involves, this is not recommended.
DA Discovery Time Out Value
This value is specified in milliseconds and is fully described
in section 5.2 of this document.
SA Multicast Time Out Value
This value is specified in milliseconds and is fully described
in section 5.3 of this document.
7.2 How to obtain the list of TN3270E servers supporting SLP
A TN3270E client that implements SLP load balancing uses API calls
to obtain the list of TN3270E servers supporting SLP load balancing.
These API calls are described in "An API for Service Location" [2].
The following API calls are a subset of those described in "An API
for Service Location" [2] and can be used with TN3270E SLP session
balancing.
SL_Open
SL_GetService
SL_GetAttrs
SL_FreeResult
SL_Close
7.3 TN3270E Sample Client Flow
7.3.1 Open the SLP connection
The TN3270E client must first open the SLP library using the SL_Open
API call. The SA multicast time out and DA discovery time out values
are passed as parameters to the SL_OPEN API call.
7.3.2 Query the list of TN3270E servers
The TN3270E client then queries for the list of TN3270E servers
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supporting SLP. This is done by using the SL_GetService API call.
The request string contains information that determines which type
of TN3270E servers that this client desires to connect to. The
request string can contain the scope name, pool name, session type
and 3270 screen size.
The string has the following format:
TN3270/<scope name>/LUPOOL == <pool name><TAB><device type>
The <scope name> is the name of the scope that is configured
for the TN3270E client. If the scope is blank or null (unscoped
request), then the scope is not inserted into the request string.
The <pool name> is a 1 to 8 character upper case string that
indicates the name of the pool that the TN3270E client desires
to connect to. For SLP load balancing, the same pool name must
be configured on different TN3270E servers.
The <TAB> is the '/t' tab character which is hexadecimal 0x09.
The <device type> can be any of the following:
3270DSC for TN3270E device type IBM-3287-1
3270002 for TN3270E device types IBM-3278-2 and IBM-3278-2-E
3270003 for TN3270E device types IBM-3278-3 and IBM-3278-3-E
3270004 for TN3270E device types IBM-3278-4 and IBM-3278-4-E
3270005 for TN3270E device types IBM-3278-5 and IBM-3278-5-E
* for TN3270E device type IBM-DYNAMIC
Example:
For a TN3270E client searching for TN3270E servers in the
ENGINEERING scope for a model 2 screen size and LUPOOL name pool2,
the following request string would be constructed:
"TN3270/ENGINEERING/LUPOOL == pool2<TAB>3270002"
Note: The " characters before and after the string are not
part of the request string.
7.3.3 Determine loading of each TN3270E server
Once the list of TN3270E servers and their respective IP Host
addresses is constructed, each of the TN3270E servers must be
interrogated to find the loading of each TN3270E server. Then the
TN3270E client can determine which TN3270E server is least loaded,
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and use that IP Host address to connect to.
The TN3270E client uses these load values to determine which TN3270E
server is least loaded. The TN3270E client then uses the TN3270E
server's IP Host address to start normal Telnet TN3270E negotiation.
7.4 Recommendations
It is recommended that the TN3270E client display the IP host
address that is being used for the TN3270E connection. This gives
the user knowledge of which TN3270E server the session is connected
to. For example, the IP host address could be displayed in the
Windows status bar.
It is also recommended that the TN3270E client display the resource
name that is returned by the TN3270E server after connection and
TN3270E negotiation is completed. This gives the user knowledge of
which LU resource name in the LUPOOL the session is connected to.
For example, the resource name could be displayed in the Windows
status bar, or even in the 3270 OIA line.
In the event that after the TN3270E client has determined the least
loaded server and the connection to that server fails for some
reason, the connection should be closed and an attempt made to
connect to other TN3270E servers in the list of least loaded
servers.
For example, a TN3270E server may reject a connection to a specific
pool if the pool is full, or if the device type does not match what
is available in the pool. If this occurs, then an attempt to other
least loaded TN3270E servers should be performed.
8. Sample Trace Flow of SLP and Session Balancing
This sample trace flow is provided for informational purposes only.
SLP: SL_GetService Request: TN3270//LUPOOL == POOL2 3270002/
SLP: service:tn3270://206.109.45.139:1366, LOAD = 8
SLP: service:tn3270://206.109.45.140:1366, LOAD = 26
Connecting to 206.109.45.139...
TerminalType=NVT
Connection established
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Recv <- DO TN3270E
Send -> WILL TN3270E
TerminalType=TN3270E
Recv <- SEND DEVICE_TYPE
Send -> DEVICE_TYPE REQUEST IBM-3278-2-E CONNECT POOL2
Recv <- DEVICE_TYPE IS IBM-3278-2-E CONNECT TN8003
Send -> FUNCTIONS REQUEST BIND_IMAGE SYSREQ
Recv <- FUNCTIONS IS BIND_IMAGE SYSREQ
9. Service Registration Messages and Service Templates
See TN3270E Service Location and Session Balancing Templates
Internet Draft for submitted templates to IANA for registration. [5]
Communications Server Service Template
URL:service:commserver://<addr-spec>:<port-number>
The communications server service type is registered whenever the
communications software is loaded on the server. It describes
generic attributes of the communications server. These attributes
are also repeated on the other service types provided.
Release = <version/release>
This is the version and release level of the commserver advertising
services. Its format is vv.rr.mm where "vv" is the major version
number, "rr" is the minor version number, and "mm" is the
modification level. All numbers are padded on the left with zeroes
to two characters.
Example: version 3, release 0, mod level 0 is "03.00.00"
Platform = <platform>
This is the network operating system platform underlying the
advertising service. The defined values are:
IW Server uses the IntranetWare network operating system
NT Server uses the Microsoft NT operating system
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OS2 Server uses the OS2 operating system
AIX Server uses the AIX operating system
Protocol = <protocol>
This is the protocol(s) supported by the server providing this
service. The defined values are:
IP Server supports client connections over IP (TCP/IP or
UDP/IP)
IPX Server supports client connections over IPX (SPX/IPX)
Server name = <server name>
This is the name of the server that was configured during
installation. This value has meaning only for the IW platform.
Communications Server Service Registration Message
URL:service:commserver://<addr-spec>:<port-number>
Attributes:
[(SCOPE=<string>),]
(RELEASE=03.00.00),
(PLATFORM=IW),
(PROTOCOL=IP),
(SERVERNAME=<string>)
TN3270 Service Template
URL: service:tn3270://<addr-spec>:<port-number>
The tn3270 service provides 3270 gateway access to an SNA network
via the TN3270 protocol. The attributes reflect the types of 3270
devices, LU Pools, and load information available on the server.
Load = <server_load>:
This is the load balancing quantity to use in determining the least
loaded comm server to attach to for the service. The range of valid
values is an integral 0 to 100 with 0 indicating the lowest
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possible load and 100 the highest.
LU Pool = <pool_name>,
<pool_name>/t<dev-type> ,
<pool_name>/t<dev_type>, ...
<pool_name>/t<dev-type>
Identifies the LU pool names of LU pools available for use on this
service with the associated device types supported in each pool.
Each value is a record where the first token is the pool name of the
pool and the second token is a device type supported in that pool.
A pool name without a device type indicates that LUs of unknown type
are included in the pool. Records associated with a given pool name
are repeated for each supported device type. A given pool is
included in a registration request if any PU profile that
contributes at least one LU to the pool is active on the server. The
range of valid dev_types are:
dev_type Meaning
3270002 Lu Type 2 Model 2
3270003 Lu Type 2 Model 3
3270004 Lu Type 2 Model 4
3270005 Lu Type 2 Model 5
3270DSC Printer LU
BIND, DATA, RESPONSES, SCS, SYSREQ
These keyword attributes describe the TN3270e functions supported by
this service.
BIND The server supports the SNA bind image function
DATA The non-SNA 3270 data stream is supported by server
RESPONSES The server supports SNA response mode
SCS The server supports SNA 3270 SCS data stream
SYSREQ The SYSREQ keyboard key is supported on server
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They are present in the service advertisement if the functions they
describe are available.
Security = <security>
This field will contain the security technique supported by the
server. The actual values are currently being defined in the
Internet standards body.
RFC1576, RFC1646, RFC1647
The RFC numbers that document features supported by the service.
Current RFC's for TN3270 include 1576, 1646, and 1647.
TN3270 Service Registration Message
URL: service:tn3270://<addr-spec>:<port-number>
Attributes:
[(SCOPE=<string>),]
(RELEASE=03.00.00),
(PLATFORM=IW),
(PROTOCOL=IP),
(SERVERNAME=<string>),
(LOAD=<integer 0 to 100>),
[(LUPOOL=pool-name0/tANY,
pool-name1/tdevice_type1,
pool-name2/tdevice-type2, ...
pool-namen/tdevice-typen)]
BIND,
DATA,
RESPONSES,
SCS,
SYSREQ,
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(SECURITY=NONE),
RFC1576,
RFC1646,
RFC1647,
10. References
Request For Comments (RFC) and Internet Draft documents are
available from <URL:ftp://ftp.internic.net> and numerous mirror
sites.
[1] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan. Service
Location Protocol. RFC 2165, July 1997.
[2] J. Kempf, E. Guttman, Don Provan. An API for Service Location.
(work in progress), March 1998.
[3] E. Guttman, C. Perkins, J. Kempf. Service Templates and
service: Schemes. draft-ietf-svrloc-service-scheme-07.txt
February 98 (work in progress)
[4] E. Guttman, C. Perkins, J. Veizades, Michael Day.
Service Location Protocol,
draft-ietf-svrloc-protocol-v2-01.txt October 1997
(work in progress)
[5] J. Naugle, K. Kasthurirangan. TN3270E Service Location and
Session Balancing Templates. May 1997 Individual Contribution
11. Authors' addresses
Jim Naugle
IBM
P.O. Box 12195
Research Triangle Park, N.C. 27709-2195
USA
Phone: (919) 254-8789
EMail: jnaugle@us.ibm.com
Kathuri Kasthurirangan
IBM
P.O. Box 12195
Research Triangle Park, N.C. 27709-2195
USA
Phone: (919) 254-5721
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Gregg Ledford
Zephyr Development Corporation
8 Greenway Plaza Suite 1400
Houston, Texas 77046
USA
Phone: (713) 623-0089
Email: gledford@zephyrcorp.com
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