Network Working Group J. Kempf
Request for Comments: 3105 NTT DoCoMo USA Labs
Category: EXPerimental G. Montenegro
Sun Microsystems
October 2001
Finding an RSIP Server with SLP
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
IESG Note
The IESG notes that the set of documents describing the RSIP
technology imply significant host and gateway changes for a complete
implementation. In addition, the floating of port numbers can cause
problems for some applications, preventing an RSIP-enabled host from
interoperating transparently with existing applications in some cases
(e.g., IPsec). Finally, there may be significant operational
complexities associated with using RSIP. Some of these and other
complications are outlined in section 6 of the RFC3102, as well as
in the Appendices of RFC3104. Accordingly, the costs and benefits
of using RSIP should be carefully weighed against other means of
relieving address shortage.
Abstract
This document contains an SLP service type template that describes
the advertisements made by RSIP servers for their services. Service
Location Protocol (SLP) is an IETF standards track protocol
specifically designed to allow clients to find servers offering
particular services. Since RSIP (Realm Specific IP) clients require
a mechanism to discover RSIP servers, SLP is a natural match for a
solution. The service type template is the basis for an Internet
Assigned Numbers Authority (IANA) standard definition of the
advertisements offered by RSIP servers, an important step toward
interoperability.
Table of Contents
1. IntrodUCtion ............................................... 2
2. Notation Conventions ....................................... 2
3. Terminology ................................................ 2
4. Using SLP for RSIP Service Discovery ....................... 3
5. Using Scopes for Server Provisioning ....................... 4
6. Load Balancing ............................................. 6
7. The RSIP Service Type Template ............................. 7
8. Security Considerations .................................... 9
9. Summary .................................................... 9
References ..................................................... 9
Authors' Addresses ............................................. 10
Full Copyright Statement ....................................... 11
1. Introduction
Realm Specific IP (RSIP) [7] enables an RSIP client in one realm to
borrow addresses and other resources from another realm. It does so
by engaging in an RSIP protocol [1] exchange with an RSIP server.
The RSIP protocol requires the RSIP server to have a permanent
presence on both realms.
There are a variety of traditional ways an RSIP client could go about
locating the appropriate RSIP server. However, Service Location
Protocol (SLP) [2][11] is an IETF standards track protocol
specifically designed to facilitate location of services and their
servers by clients. SLP provides a number of features that simplify
locating RSIP servers. In this document, we describe how RSIP
clients can use SLP to discover RSIP servers.
2. Notation Conventions
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 [4].
3. Terminology
We reproduce here some SLP terminology from [2] for readers
unfamiliar with SLP.
User Agent (UA)
A process working on the user's behalf to establish contact with
some service. The UA retrieves service information from the
Service Agents or Directory Agents.
Service Agent (SA)
A process working on behalf of one or more services to advertise
the services and their capabilities.
Directory Agent (DA)
A process which collects service advertisements. There can only
be one DA present per given host.
Scope
A set of services, typically making up a logical administrative
group.
Service Advertisement
A URL, attributes, and a lifetime (indicating how long the
advertisement is valid), providing service Access information and
capabilities description for a particular service.
4. Using SLP for RSIP Service Discovery
SLP provides the framework in which RSIP clients and servers make
contact. Here is a description of how an RSIP server and client find
each other using SLP:
1. The RSIP server implements a SLP SA while the RSIP client
implements an SLP UA.
2. The RSIP SA constructs a service advertisement consisting of a
service URL, attributes and a lifetime. The URL has service type
"service:rsip", and attributes defined according to the template
in Section 7.
3. If an SLP DA is found, the SA contacts the DA and registers the
advertisement. If no DA is found, the SA maintains the
advertisement itself, answering multicast UA queries directly.
4. When the RSIP client requires contact information for an RSIP
server, the UA either contacts the DA using unicast or the SA
using multicast. The UA includes a query based on the attributes
to indicate the characteristics of the server it requires.
5. Once the UA has the host name or address of the RSIP server as
well as the port number, it can begin negotiation using the RSIP
protocol.
This procedure is exactly the same for any client/server pair
implementing SLP and is not specific to RSIP.
Many protocols use a variety of traditional methods for service
discovery. These methods include static configuration, purpose-build
protocols for discovery, special features in the protocol itself, DNS
SRV RRs [5], or DHCP [6]. SLP provides a number of advantages over
these traditional methods:
1. Discovery of services using SLP is dynamic, whereas many of the
traditional methods only allow static or weakly dynamic (i.e.,
difficult to update) discovery. Clients only discover services
that are actually active with SLP. Furthermore, if subsequent to
initial discovery a server goes down, the client can reissue an
SLP query and oBTain a new server. On the server side, no
databases must be updated to provide dynamic discovery, the
servers advertise themselves.
2. SLP requires no third party configuration. Only the server
offering the service and the client seeking it are required to
know the details for the particular service type.
3. SLP allows clients to specify the attributes describing the
desired server. A client discovers servers that meet a set of
specific requirements. This reduces the amount of network traffic
involved in selecting a server when many possible choices are
available.
4. SLP contains a number of scaling mechanisms (DAs, scopes,
multicast convergence algorithm), that facilitate deployment in
large enterprise networks as well as in smaller networks.
5. Using Scopes for Server Provisioning
One particular design feature of SLP that is useful for RSIP is
scopes. Scopes in SLP are a mechanism for provisioning access to
particular service advertisements. An administrator assigns UAs and
SAs to particular scopes to assure that UAs only find SAs in those
scopes. Scopes are not an access control mechanism for the service
itself, however. UAs from outside the scope can still access
services in a particular scope (unless the service itself provides
for access control), they just won't be able to find the services
using SLP.
Scopes are useful for RSIP service advertisement provisioning because
they allow a system administrator to tie particular RSIP clients to
specific RSIP servers. For example, consider the network
architecture described in Section 4.2.1 of [7]. RSIP clients are
recommended to find "the nearest" RSIP server, but exactly how that
should be arranged is left unspecified. SLP provides a way for
system administrators to precisely specify which realm an RSIP client
resides in, by tying the realm to an SLP scope. The diagram from
Section 14.1 is reproduced here, with SLP scopes included to
illustrate how clients could be directed to the right RSIP servers.
+-----------+
RSIP
server +---- 10.0.0.0/8
B SLP Scope: B
+-----+-----+
10.0.1.0/24
+-----------+ (149.112.240.0/25)
149.112.240.0/24 RSIP +--+
----------------+ server SLP Scope: A
A +--+
+-----------+ 10.0.2.0/24
(149.112.240.128/25)
+-----+-----+
RSIP
server +---- 10.0.0.0/8
C SLP Scope: C
+-----------+
Clients on the upper 10.0.0.0/8 network are configured to use SLP
scope B, while clients on the lower 10.0.0.0/8 network are configured
to use SLP scope C. RSIP servers B and C (as clients of server A)
use SLP to locate RSIP server A, as do other RSIP clients on the
10.0.1.0/24 and 10.0.2.0/24 subnets. Within these two subnets, all
clients have their scopes configured to be A.
Note that specifying a particular SLP scope for RSIP clients does not
restrict the SLP scope for other services advertised by SLP. SLP UAs
can be configured for multiple scopes, so the scope configured for
printing may be different from the scope configured for RSIP service.
Since SLP scopes are configured through a DHCP option [8], along with
the IP address, system administrators can easily switch a cluster of
machines from one realm to another by simply changing the scope and
IP address assignments on the DHCP server. For example, in the above
architecture, suppose a system administrator wanted to remove RSIP
server B so that clients on the upper 10.0.0.0/8 subnet were directly
on subnet 10.0.1.0/24. These clients now communicate with RSIP
server A. By simply changing the address assignments and scope
configuration of these clients on the DHCP server, the realm can be
effectively switched.
6. Load Balancing
While SLP itself contains no specific provision for load balancing,
load balancing can easily be implemented using SLP. The only
requirement is that the service type template specify an attribute
indicating server load. In the case of RSIP, the service type
template in Section 7 contains such an attribute. The attribute
indicates the number of RSIP client sessions currently being
supported by the server.
In order to perform load balancing, the RSIP server must update its
service advertisement periodically as new connections are accepted.
An RSIP client seeking to find the server having the lightest load
performs the following series of SLP operations.
1. As in Section 4, the client issues an SLP service request and
collects all the returned service URLs.
2. For each service URL, the client performs an SLP attribute request
for the attribute LOAD. The integer load figures are returned.
3. The client sorts through the returned load figures and selects the
URL having the least number of connections. The client
establishes its RSIP session with that server.
Because of network delays, this procedure does not guarantee that a
client will always obtain a connection with the lightest loaded
server, but it does provide a high probability that the selected
server is more lightly loaded.
A similar procedure is used in [9] to load balance access to TN3270E
telnet servers.
7. The RSIP Service Type Template
Name of submitters: James Kempf <james@docomolabs-usa.com>
Gabriel Montenegro <gab@sun.com>
Language of service template: en
Security Considerations:
RSIP clients can use Service Location Protocol to find RSIP
servers having particular security characteristics. If secure
access to such information is required, SLP security should be
used.
Template text:
----------------------template begins here -------------------------
template-type = rsip
template-version = 0.0
template-description=
The service:rsip type provides advertisements for clients seeing
realm-specific IP (RSIP) servers. RSIP servers use the Realm
Specific IP protocol to manage addresses and other resources
from one realm on behalf of a client in another realm.
template-url-syntax=
;No additional URL path information required. An example service
;URL for an RSIP server is: service:rsip://gateway.mydomain:4455
ipsec-support = BOOLEAN O
#True if the server supports IPSEC as per [10]
ike-support = BOOLEAN O
#True if the server supports IKE as per [10]
tunnel-type = STRING L M O
IP-IP
#The tunneling methods supported by the RSIP server. Clients
#should include this attribute in a query so that they obtain a
#server offering a tunneling method for which they have
#support. Default is IP-IP. The values are currently
#restricted to IP-IP, L2TP, GRE and NONE. A server can support
#multiple tunnel types.
IP-IP,L2TP,GRE,NONE
transport = STRING L M O
TCP
#Transport used by the RSIP protocol itself.
TCP,UDP
load = INTEGER O
#If the server supports load balancing, this attribute should be
#set to an integer from 0 to 100. 0 is the lowest indication of
#load and 100 the highest. Clients can query for this attribute
#and obtain load information, from which they can make an
#intelligent decision about which server to use.
----------------------template ends here ---------------------------
8. Security Considerations
Service type templates provide information that is used to interpret
information obtained by clients through SLP. If the RSIP template is
modified or if a false template is distributed, RSIP servers may not
correctly register themselves, or RSIP clients may not be able to
interpret service information.
SLP provides an authentication mechanism for UAs to assure that
service advertisements only come from trusted SAs [2]. If trust is
an issue, particularly with respect to the information sought by the
client about IPSEC and IKE support, then SLP authentication should be
enabled in the network.
9. Summary
This document describes how SLP can be used by RSIP clients to find
RSIP servers. A service type template for an RSIP SLP service type
is presented. In addition, a few techniques for provisioning access
to service advertisements for particular gateway servers, and for
load balancing using SLP were provided. The result should allow RSIP
service provisioning that is considerably more dynamic and robust
than when traditional service discovery mechanisms are used.
References
[1] Borella, M., Grabelsky, D., Lo, J. and K. Taniguchi, "Realm
Specific IP: Protocol Specification", RFC3103, April 2001.
[2] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, version 2", RFC2608, July 1999.
[3] Guttman, E, Perkins, C. and J. Kempf, "Service Templates and
service: Schemes", RFC2609, July 1999.
[4] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[5] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
location of services (DNS SRV)", RFC2052, October 1996.
[6] Droms, R., "Dynamic Host Configuration Protocol", RFC2131,
March 1997.
[7] Borella, M., Lo, J., Grabelsky, D. and G. Montenegro, "Realm
Specific IP: Framework", RFC3102, October 2001.
[8] Perkins, C. and E. Guttman, "DHCP Options for Service Location
Protocol", RFC2610, July 1999.
[9] Naugle, J., Kasthurirangan, K. and G. Ledford, "TN3270E Service
Location and Session Balancing", RFC3049, January 2001.
[10] Montenegro, G. and M. Borella, "RSIP Support for End-to-end
IPSEC", RFC3104, October 2001.
[11] E. Guttman, "Service Location Protocol: Automatic Discovery of
IP Network Services," IEEE Internet Computing, July/August 1999.
Available at: http://computer.org/internet/ic1999/w4toc.htm
Authors' Addresses
Questions about this document may be directed to:
James Kempf
NTT DoCoMo USA Labs
181 Metro Drive, Suite 300
San Jose, CA
95110
Phone: 408-451-4711
Email: james@docomolabs-usa.com
Gabriel E. Montenegro
Sun Microsystems
Laboratories, Europe
29, chemin du Vieux Chene
38240 Meylan
FRANCE
Phone: +33 476 18 80 45
EMail: gab@sun.com
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