Internet DRAFT - draft-kempf-srvloc-notify


Service Location                                        James Kempf
INTERNET DRAFT                                          Sun Microsystems
January, 2001                                           Jason Goldschmidt
                                                        Sun Microsystems

                Notification and Subscription for SLP

                        Status of This Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are work-
ing documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also dis-
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Internet-Drafts are draft documents valid for a maximum of six
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The Service Location Protocol provides mechanisms whereby service agent
clients can advertise and user agent clients can query for services.
The design is very much demand-driven, so that user agents only obtain
service information when they specifically ask for it. There exists
another class of user agent applications, however, that requires
notification when a new service appears or disappears. In the RFC 2608
design, these applications are forced to poll the network to catch
changes. In this document, we describe a protocol for allowing such
clients to be notified when a change occurs, removing the need for

1. Introduction

The Service Location Protocol (SLP)[1] provides a mechanism

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for service agent (SA) clients to advertise network services and for
user agent (UA) clients to find them. The mechanism is demand-driven.
UAs obtain service information by actively querying for it, and do not
obtain any information unless they do so. While this design satisfies
the requirements for most applications, there are some applications
that require more timely information about the appearance or
disappearance in the services of interest.

Ideally, these applications would like to be notified when a new service
comes up or when a service disappears. In order to obtain this
information with SLP as described in RFC 2608, such applications must
poll the network to periodically refresh their local cache of available
service advertisements.

An example of such a client is a desktop GUI that wants to display
network service icons as soon as they appear to provide users with an
accurate picture of all services available to them.

Because polling is inefficient and wasteful of network and processor
resources, we would like to provide these applications a mechanism
whereby they can be explicitly notified of changes. In this document,
we describe a scalable mechanism allowing UAs to be notified of changes
in service availability.

2. Notation Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
this document are to be interpreted as described in RFC 2119 [2].

3. Terminology

In this section, we present some additional terminology beyond
that in [1] and [3].

Notification - A message sent to an interested agent informing that
               agent that a service has appeared or disappeared.

Subscription - A request to be informed about changes in service
               availability for a particular service type and scopes.

4. Design Considerations

The primary design consideration in a notification protocol for SLP
is that we would like it to exhibit the same high degree of scalability
and robustness that the base SLP protocol exhibits. Notification should
work in small networks with only a few SAs, as well as large enterprise
networks with thousands of SAs and hundreds of DAs. Small networks

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should not be required to deploy DAs in order to receive the benefits
of notification. We also want to assure that notification in large
networks does not cause heavy processing loads to fall on any one
particular SLP agent. This requires that the task of notification be
distributed rather than centralized, to avoid loading down one agent
with doing all the notification work. Finally, we would like the
notification scheme to be robust in the face of DA failures, just as
the base SLP design is.

An important consideration is that the UA clients obtain notifications
of SA events in a timely fashion. If a UA has subscribed to notification
for a particular service type, the UA should receive such notification
regardless of the state of intervening DAs. SLP is transparent
with respect to DAs supporting a particular scope; that is, a UA can
use any DA with a particular scope and expect to get the same service
advertisements. Notifications should exhibit the same property. Whether
or not a UA receives a notification should not depend on the DA to
which they happen to connect. This preserves the DAs' identity as a pure

Another goal is that the notification messages contain enough
information about the triggering event that the UA can determine
whether or not it is of interest in the large majority of cases without
having to issue another SLP request a priori. The UA may, of course,
issue an SLP request for related reasons, but it should not have to
issue a request to obtain more information on the event that triggered
the notification in most cases. This reduces the amount of network
traffic related to the event.

In order to simplify implementation, we would like to use similar
mechanisms for notification in large and small networks. The mechanisms
are not identical, obviously, but we want to avoid having radically
different mechanisms that require completely separate implementations.
Having similar mechanisms reduces the amount of code in UA and SA

A minor goal is to make use of existing SLP message types and mechanisms
wherever possible. This reduces the amount of code necessary to
implement the notification mechanism, because much code can be reused
between the base SLP and the notification mechanism. In particular, we
expect to make use of the SLP extension mechanism in certain cases
to support subscription.

5. Notification Design Description

In order to support scalability, we split the design into two parts. A
small network design is used when no DAs are present in the network.
A large network design is used in networks with DAs. The following

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subsections describe the two designs.

5.1 Small Network Design

In networks without DAs, UAs are notified by an SA when the SA
initially appears, and when the SA disappears. This allows UAs to know
about the list of service types the SA supports. In small networks,
there is no centralized agent available to administer subscriptions for
newly appearing SAs. This rules out any kind of subscription design
in which a UA subscribes to notifications for a particular service
type in particular scopes of interest, because a newly appearing SA
can't tell whether or not there are any subscriptions without a
centralizing agent to tell it.

As a result, SAs perform notification when they come on line and
prior to shutting down regardless of their scope or service type,
if they are capable of performing notification. This means that a UA
receives notification of all types of changes for all scopes and
service types, and consequently must be prepared to filter out those
changes in which it is not interested (other scopes, other service

The design requires SAs to perform notification by IP multicasting
(or broadcasting in IPv4 if multicast is not available) SLP SrvReg or
SrvDereg messages using the multicast transmit algorithm described in
Section 9.0. The port number for notifications is not the default SLP
port, because that port is only accessible to privileged users on some
operating systems, but rather the port 1847, as assigned by IANA.

In IPv4, the SA peforms multicast on the SLP multicast address
(, default TTL 255) and is administratively scoped in
the same manner as SLP [4]. IPv4 UAs interested in notification join
the multicast group and listen on port 1847. In IPv6,
the multicast is performed to the scoped IPv6 addresses for the service
type advertised, as described in [8]. The SA advertises on all addresses
up to and including the largest multicast scope that it supports.
IPv6 UAs interested in notification join the multicast groups
corresponding to the multicast scopes and service type in which they
are interested and listen on port 1847. For example, an IPv6 UA that
has access to site local scope and is interested in a service type
whose hash is 42, calculated according to the algorithm in [8], joins
the groups FF01:0:0:0:0:0:10042 through FF05:0:0:0:0:0:10042.

5.2 Large Network Design

In networks with DAs, a DA supporting a particular scope can act as
an intermediary for administering UA subscriptions. A subscription
consists of a service type and a collection of scopes. A UA interested

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in being notified about changes in a particular service type attaches
the Subscribe extension to a SrvRqst message sent to the DA. The DA
obtains multicast group addresses for notification based on the
algorithm described in Section 8.0 and puts them into a NotifyAt
extension which it attaches to the SrvRply. The UA listens on the group
addresses in the reply for notifications.

When a new subscription comes in, existing SAs are informed about
the subscription using the following procedure. The DA compares the
service type and scopes in the new subscription against a list of
existing subscriptions. If no previous subscription has
the same service type and scopes, the DA MUST multicast a DAAdvert,
using the multicast transmit algorithm described in Section 9.0, and
MUST include the NotifyAt extension with the multicast group addresses
for notification. If an existing subscription covers the same service
type and scopes as the new subscription, the DA MUST NOT multicast a

A DA MUST keep track of subscriptions it has arranged as well as
subscriptions arranged by other DAs in any scopes with which the DA is
configured. To avoid multiple multicast NotifyAt messages, a DA MUST
wait a random amount of time, uniformly distributed between 0 and 3
seconds before sending the multicast DAAdvert with NotifyAt. During
this period, the DA MUST listen for NotifyAt messages that match the
one from the new subscription. If a matching NotifyAt is detected, the
DA MUST not multicast.

When a new SA registers with a DA that has existing subscriptions,
the new SA is informed of notifications it should perform using
the following procedure. If the service type and scopes in
the new SA's SrvReg messages match an existing subscription,
a NotifyAt containing the multicast addresses for notification
MUST be included in the SrvAck. If the SA doesn't support
notification, it simply ignores the extension. If the service type
and scopes in the new SA's SrvReg do not match any existing
subscriptions, the DA MUST NOT include a NotifyAt.

The DA itself MUST also perform notification, according to the
multicast transmit algorithm, when a service advertisement times out.
Time-out of a service advertisement results in the DA multicasting a
SrvDereg for the deregistered URL. This allows interested UAs to
be informed of the service advertisement's demise even if the
SA has disappeared without deregistering. A DA MUST NOT perform
notification when it receives a SrvReg from an SA, however, that is
the job of the SA.

As in small networks, notification is performed primarily by SAs.
If an SA receives a DAAdvert or SrvAck with a NotifyAt extension

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and the following conditions are met:

        1. The SA supports notification.

        2. The SA's service type matches the service type in the
           NotifyAt extension.

        3. The SA's scopes match one of the scopes of the NotifyAt

then the SA saves the multicast addresses that correspond to the scopes
and service types it supports. The SA MUST perform notification
immediately after the SA has performed the SrvReg or SrvDereg with the
DA. An SA that has detected a DA in its scopes MUST NOT multicast
any notifications unless it receives a NotifyAt extension in a SrvAck
with service type and scopes matching the SA's service type and

6. Subscribe Extension

The Subscribe extension has the following format:

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 |    Extension Type = 0x0004    |        Extension Length       |
 | Ex. Len. (ct) | Abs. Type Fl. |

The scope list and service type of the extension are taken from the
accompanying SrvRqst. The abstract type flag indicates whether the
UA is interested in hearing from all SAs advertising concrete
instances of an abstract type [3], and is only of interest if
the service type in the SrvRqst is a concrete type. If the flag is 1,
the UA is interested in hearing from all SAs advertising concrete types
having the same abstract type as the type of the SrvRqst. If the
flag is 0, the UA is only interested in hearing from SAs supporting
the particular concrete type in the SrvRqst. If the service type in
the accompanying SrvRqst is not a concrete type, the flag is

7. NotifyAt Extension

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  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 |    Extension Type = 0x0005    |        Extension Length       |
 | Ext. Len (ct) |  Subscription Lifetime        |SGL List Len.  \
 |SGL L. Len (ct)|       Scope/Group List                        \
 |  Length of Service Type Name  |        Service Type Name      \

The service type name is in the same format as in the SrvRqst. The
scope/group list is a list of scope names and multicast group
addresses. The following ABNF [5] syntax describes the list:

        sglist          = sgitem / sgitem "," sglist
        sgitem          = scope-name ":" ip-addr
        ip-addr         = ipv4-number | ipv6-number
        scope-name      =  ; See RFC 2608 for the format of scope names.
        ipv4-number     =  1*3DIGIT 3("." 1*3DIGIT)
        ipv6-number     = ;See RFC 2373 [9] Section 2.2

An example of a scope/group list for IPv4 is:


An example of a scope/group listfor IPv6 is:


The scope/group list gives the multicast addresses to use for
notifications involving the service type for the given scopes.

The service type name can be a simple type name, an abstract type
name, or a concrete type name. If the name is an abstract type name,
all SAs advertising the abstract type MUST notify. If the name is a
concrete or simple type name, ONLY those SAs advertising the simple
or concrete type MUST notify, others MUST NOT notify. A DA that
receives a subscription for a concrete type with the abstract type
flag set, MUST include the abstract type name in all the NotifyAt
messages it sends. If the DA receives a subscription for a concrete
type with the abstract type flag not set, the DA MUST NOT include the
abstract type, but rather MUST include the concrete type name.

There are three cases in which an agent may receive a NotifyAt
extension: in a SrvRply returned to a UA, in a multicast DAAdvert, and
in a SrvAck returned to an SA. The three subsections below describe

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the response in each of these cases.

7.1 NotifyAt received with SrvRply

When a UA sends a SrvRqst with a Subscribe extension, the DA responds
with a SrvRply including a NotifyAt. The DA MUST NOT unicast a
NotifyAt to a UA with any other message and MUST NOT send a
NotifyAt unless a SrvRqst with a Subscribe extension was received.

The UA responds by setting up a multicast listener to the group
addresses included in the extension on the SLP notification port
1847. The UA MAY also want to note the expiration lifetime of the
subscription assigned by the DA, and reissue a  subscription before
the lifetime expires.

7.2 NotifyAt received with Multicast DAAdvert

The DA multicasts a NotifyAt with a DAAdvert using the multicast
transmit algorithm when a UA has requested notification and the scopes
and service type in the subscription were not previously seen. This
message informs existing SAs having the service type and scopes in the
announcement that they should multicast notifications when they
shut down.

A receiving SA participating in notification responds by noting the
multicast address if the service type and scopes match. When
the SA is about to go down, the SA MUST first unicast a SrvDereg
without attribute tag list to its DAs (as per standard SLP), then it
MUST multicast the same SrvDereg message according to the multicast
transmit algorithm. The SA MUST cease performing notification when the
subscription lifetime expires, unless a subsequent NotifyAt is received
prolonging the subscription.

A UA that is performing passive DA detection will naturally also receive
the extension, but the UA SHOULD ignore the extension.

7.3 NotifyAt received with SrvAck

An SA can receive a NotifyAt with a SrvAck when it first comes up
and registers itself with a DA. If the DA has any subscriptions from
UAs for the service type and scopes represented by the SA, it MUST
return a NotifyAt with the SrvAck.

The SA upon receiving the NotifyAt immediately multicasts the
same SrvReg it sent to the DA, according to the multicast transmit
algorithm. The SA MUST only perform the multicast algorithm once,
even if it registers with more than one DA and receives the NotifyAt
in reply from more than one. Prior to its demise and after

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deregistering with a DA, the SA MUST notify with the same SrvDereg,
as described in Section 7.2.

8. Multicast Address Allocation

Enterprise networks that allow SLP notification SHOULD deploy the
Multicast Address Allocation Architecture (MAAA) including
administratively scoped multicast and Multicast Address Dynamic
Client Allocation Protocol (MADCAP) [6].

If it is not possible to obtain a multicast address for use in
SLP notifications, the SLP multicast address is used.

If the MAAA infrastructure is deployed, DAs and SAs obtain their scope
configuration from MADCAP, because the SLP scopes are the same as the
MADCAP scopes. Each SLP scope MUST correspond to a multicast scope name,
in the sense of [6]. In such a case, a DA allocates, using MADCAP, a
new multicast group address for each new service type/scope pair to
which a UA subscribes. The allocation is made by MADCAP from the
multicast address range for the scope. In this way, only those UAs
interested in the service type and scopes in the subscription receive
the multicast notification. The DA sets up the lease on the multicast
address to correspond with the duration of the subscription. If the
MADCAP server runs out of addresses, the SLP multicast group is used
as a last resort.

For example, if the multicast scope has an address range of
through, the notification group address for service type
X in scope A could be and for service type Y in scope B
could be

9. Multicast Transmit Algorithm

The DA and SAs use a multicast transmit algorithm similar to that used
for discovering services in SLP, described in RFC 2608 [1], except
the agent performing the notification doesn't wait for replies.
The agent performing the notification transmits a notification message
repeatedly over a period of 15 seconds, backing off exponentially
on the duration of the time interval between the multicasts.
The rationale for this algorithm is to limit the duration and scope of
the multicast announcement while still repeating the announcement
enough times to increase the probability that one message gets

For an SA, a notification message is either a SrvReg or a SrvDereg
message, depending on whether the SA is registering a new service or
deregistering a service. When a new service is registered, the
SrvReg message MUST have the fresh bit set in the SLP header. The

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entire list of attributes for the service SHOULD be included. The
SrvDereg message MUST NOT include an attribute tag list. Notifications
MUST NOT be transmitted at any other time, to minimize multicast

Since a SrvReg could contain attribute lists of arbitrary length, the
message could potentially overflow the packet MTU for UDP. If an
attribute list causes a packet MTU overflow, the SA MUST set the
overflow bit in the SLP header. The attribute list in the
notification message MUST be formatted so that a UA can use the
attributes even if an overflow occurs. If a UA needs more attributes
than are transmitted in the notification message, it can contact the
SA (if no DA is present) or the DA for the attributes it needs.

A DA multicasts a DAAdvert when a subscription comes in containing
a service type and scopes that do not match any on the DA's list of
known subscriptions. The same algorithm MUST be used. If the
combination of the DA attributes and the NotifyAt message cause the
DAAdvert to overflow a UDP packet, DA attributes MUST be truncated
to allow the NotifyAt to fit and the overflow bit MUST be set in the
header. An SA knows that the purpose of the message is to inform it
of a new subscription rather than for passive advertisement, because
of the extension, and it can therefore ignore the DA attribute list
field if the overflow bit is set in the header. A DA also transmits a
SrvDereg message when a service advertisement is deregistered due to
timeout, following the same rules as for an SA.

10.0 DA Disappearance

Robustness to DA failure is an important goal of the design.
When a DA disappears due to unforeseen circumstances, subscription
information from UAs is lost. UAs continue to get notifications from
existing SAs. However, new SAs will not be informed of the subscription
unless other DAs also have the subscription information. Because a UA
may not discover a new DA until it tries to perform an active request,
the UA could potentially miss the appearance of new services. For this
reason, UAs that are concerned about receiving notification of
absolutely every service that appears SHOULD issue subscriptions to
every newly discovered DA that supports the scopes it supports.
Similarly, if a DA disappears through controlled shutdown, a UA
performing passive discovery can detect the shutdown and reissue the
subscription to an alternate DA.

On the SA side, when a DA goes down, existing SAs continue to notify
until the subscription expires. Before ceasing to notify, an SA MUST
determine whether the DA is still active and, if not, verify with
another DA whether the subscription has been extended. If no other DA
is available, the SA MUST ignore the subscription expiration time and

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continue notifying until a new DA is discovered. When a new DA
is discovered the SA must send a new SrvReg to the DA, according
to RFC 2608 [1]. The replying SrvAck contains a NotifyAt extension
if the UA has renewed its subscription with the DA. If the SrvAck does
not contain a NotifyAt message the SA MUST continue to notify until the
subscription expires. If a UA is interested in continuing the
notification, it renews the subscription with the new DA prior to the
expiration  of the old one, and so the SA is informed to continue

Note that this procedure still does not inform SAs that come up between
the time a newly booted DA comes up and the time the UA has renewed its
subscription with the newly booted DA. If this situation is of concern,
multiple DAs can be used to assure that all subscriptions are covered
when a DA goes down.

11. Network Administration Considerations

In SLP networks with DAs as described in RFC 2608, the only multicast
is the SrvRqst for DAAdverts performed during active DA discovery, and
unsolicited DAAdverts sent periodically by the DA for passive
discovery. There is no multicast involved in UA queries or SA
registrations. This allows network administrators to set up DAs for a
particular collection of IP subnets and confine all service discovery
traffic to unicast between the SA and UA clients and the DA.
Administratively scoped multicast can additionally be used to limit
the extent of active DA discovery and passive DA advertising. The
amount of multicast involved is not high and DHCP DA and scope
configuration can be used to limit which DAs a particular UA or SA
client sees, or to inhibit multicast entirely so that UAs and SAs only
use configured DAs.

With notification, however, multicast traffic involving events in SAs
becomes available. Because DAs request multicast addresses based on
scope and service type, the multicast associated with particular events
should only propagate to those subnets in which UAs and SAs of the same
scope are interacting. Routers should be configured with administrative
multicast scoping to limit multicast. If DAs are not deployed (or the
MAAA is not deployed), however, the amount of multicast on the SLP
multicast address when notifications are being used could quickly
become very large. Therefore, it is crucial that DAs supporting
notification be deployed in large networks where UA clients are
interested in notification.

12. Security Considerations

The SrvReg and SrvDereg messages contain authentication blocks for
all SLP SPIs supported by the DAs with which the SA registers. Since

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these SPIs are necessarily the same as those that UAs can verify,
a UA receiving a multicast notification is in a position to verify
the notification. It does so by selecting the authentication block
or blocks that it can verify. If authentication fails, either due
to lack of an authentication block, or lack of the proper SPI, the
UA simply discards the notification. In a network without DAs,
the SPIs of the UA and SA must also match.

13. IANA Considerations

The SLP Notification services use the IANA-assigned port number
of 1847. The SLP extension identifiers assigned by IANA are
0x0004 for Subscribe and 0x0005 for NotifyAt.

14. Acknowledgements

The authors would like to thank Charles Perkins, of Nokia, and
Erik Guttman and Jonathan Wood, of Sun Microsystems, for their
stimulating discussion and suggestions during the initial phases
of the subscription/notification design. We would also like to
thank Erik for his intense scrutiny of the specification during
the later phases. His comments were instrumental in refining the
design. Shivaun Albright, of HP, motivated simplification of the
protocol to focus on initial registration and deregistration only.
Vaishali Mithbaokar implemented the simplified protocol.

15. References

[1] E. Guttman, C. Perkins, J. Veizades, and M. Day.
Service Location Protocol.
RFC 2608, July 1999.

[2] S. Bradner.
Key Words for Use in RFCs to Indicate Requirement Levels.
RFC 2119, March 1997.

[3] E. Guttman, C. Perkins, J. Kempf
Service Templates and service:  Schemes.
RFC 2609, July 1999.

[4] D. Meyer.
Administratively Scoped IP Multicast.
RFC 2365, July 1998.

[5] D. Crocker and P. Overell.
Augmented BNF for Syntax Specifications: ABNF.
RFC 2234, November 1997.

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[6] Hanna, S., B. Patel, M. Shah
Multicast Address Dynamic Client Allocation Protocol (MADCAP).
RFC 2730, December 1999.


[8] Guttman, E. Service Location Protocol Modifications for IPv6.
draft-ietf-svrloc-ipv6-11.txt, a work in progress.

[9] Hinden, R., and Deering, S. IP Version 6 Addressing Architecture.
RFC 2375, July 1997.

16. Full Copyright Statement}

Copyright (C) The Internet Society (1997).  All Rights Reserved.

This document and translations of it may be copied and furnished to
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or assist in its implementation may be prepared, copied, published and
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provided that the above copyright notice and this paragraph are
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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

17. Author's Addresses

             James Kempf               Jason Goldschmidt
             Sun Microsystems          Sun Microsystems
             UMPK15-214                UMPK17-202
             901 San Antonio Rd.       901 San Antonio Rd.
             Palo Alto, CA 94040       Palo Alto, CA 94040
             USA                       USA

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   Phone:    +1 650 786 5890           +1 650 786 3502

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