





DNSEXT Working Group                                        Levon Esibov
INTERNET-DRAFT                                             Bernard Aboba
Category: Standards Track                                    Dave Thaler
<draft-ietf-dnsext-mdns-01.txt>                                Microsoft
6 July 2001


                             Multicast DNS

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet- Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Copyright Notice

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

Abstract

Today, with the rise of home networking, there are an increasing number
of ad-hoc networks operating without a DNS server. In order to allow DNS
name resolution in such environments, the use of a multicast DNS is
proposed.














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Table of Contents

1.     Introduction ..........................................    3
2.     Name resolution using multicast DNS ...................    3
   2.1       Behavior of the sender and responder ............    3
3.     Usage model ...........................................    4
4.     Sequence of events ....................................    8
5.     Conflict resolution ...................................    8
   5.1       Considerations for multiple interfaces ..........   10
6.     IANA considerations ...................................   11
7.     ARPA domain considerations ............................   11
8.     References ............................................   12
9.     Security considerations ...............................   13
ACKNOWLEDGMENTS ..............................................   13
AUTHORS' ADDRESSES ...........................................   14
Intellectual Property Statement ..............................   14
Full Copyright Statement .....................................   15


































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1.  Introduction

Multicast DNS enables DNS name resolution in the scenarios when
conventional DNS name resolution is not possible. Namely, when there are
no DNS servers available on the network or available DNS servers do not
provide name resolution for the names of the hosts on the local network.
The latter case, for example, corresponds to a scenario when a network
that doesn't have a DNS server is connected to the Internet through an
ISP and the network hosts are configured with the ISP's DNS server for
the name resolution. The ISP's DNS server provides the name resolution
for the names registered on the Internet, but doesn't provide name
resolution for the names of the hosts on the network.

This document discusses multicast DNS, an extension to the DNS protocol
which consists of a single change to the method of use, and no change to
the format of DNS packets.

Service discovery in general as well as discovery of DNS servers using
mDNS in particular is outside of the scope of this document, as is name
resolution over non-multicast capable media.

In this document, the key words "MAY", "MUST,  "MUST  NOT", "optional",
"recommended",  "SHOULD",  and  "SHOULD  NOT",  are to be interpreted as
described in [1].

2.  Name resolution using Multicast DNS

This extension to the DNS protocol consists of a single change to the
method of use, and no change to the current format of DNS packets.
Namely, this extension allows multicast DNS queries to be sent to and
received on port 53.

The messages are sent using the LINKLOCAL addresses [2] for IPv4 and
IPv6 (below referred as the LINKLOCAL address), which are yet to be
assigned by IANA. LINKLOCAL addresses are used to prevent propagation of
multicast DNS traffic across routers, potentially flooding the network.

Propagation of multicast DNS packets within the local subnet is
considered sufficient to enable DNS name resolution in small adhoc
networks. The assumption is that if a network has a router, then the
network either has a DNS server or the router can function as a DNS
proxy, possibly implementing dynamic DNS.

In the future, mDNS may be defined to support greater than LINKLOCAL
multicast scope.  This would occur if LINKLOCAL mDNS deployment is
successful, the assumption that mDNS is not needed in multiple subnets
proves incorrect, and multicast routing becomes ubiquitous.  For
example, it is not clear that this assumption will be valid in large



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adhoc networking scenarios.

Once we have experience in mDNS deployment in terms of administrative
issues, usability and impact on the network it will be possible
reevaluate which multicast scopes are appropriate for use with mDNS.

2.1.  Behavior of the sender and responder

For the purpose of this document a host that sends a multicast query is
called a "sender", while a host that listens to (but not necessarily
responds to) a multicast query is called "responder". A host configured
to be a "responder" may also be a "sender". A host configured to not be
a "responder" cannot be a "sender".

2.1.1.  Behavior of senders

A sender sends multicast DNS query for any legal Type of resource record
(e.g. A, PTR, etc.) for a name within the ".local.arpa." domain to the
LINKLOCAL address. The RD (Recursion Desired) bit MUST NOT be set. If a
responder receives a query with the header containing RD set bit, the
responder MUST ignore the RD bit.

If the multicast query is not positively resolved ("positively resolved"
refers in this document to a response with the RCODE set to 0) during a
limited amount of time, then a sender MAY repeat the transmission of a
query in order to assure themselves that the query has been received by
a host capable of responding to the query.

Repetition MUST NOT be attempted more than 3 times and SHOULD NOT be
repeated more often than once per second to reduce unnecessary network
traffic. The delay between attempts should be randomised so as to avoid
synchronisation effects.

2.1.2.  Behavior of responders

A responder listens on port 53 on the LINKLOCAL address. Responders MUST
respond to multicast queries to those and only those names for which
they are authoritative. As an example, computer
"host.example.com.local.arpa." is authoritative for the domain
"host.example.com.local.arpa.". On receiving a multicast DNS A record
query for the name "host.example.com.local.arpa." such a host responds
with A record(s) that contain IP address(es) in the RDATA of the record.

In conventional DNS terminology a DNS server authoritative for a zone is
authoritative for all the domain names under the zone root except for
the branches delegated into separate zones. Contrary to conventional DNS
terminology, a responder is authoritative only for the zone root. For
example the host "host.example.com.local.arpa." is not authoritative for



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the name "child.host.example.com.local.arpa." unless the host is
configured with multiple names, including "host.example.com.local.arpa."
and "child.host.example.com.local.arpa.". The purpose of limiting the
name authority scope of a responder is to prevent complications that
could be caused by coexistence of two or more hosts with the names
representing child and parent (or grandparent) nodes in the DNS tree,
for example, "host.example.com.local.arpa." and
"child.host.example.com.local.arpa.".

In this example (unless this limitation is introduced) a multicast query
for an A record for the name "child.host.example.com.local.arpa." would
result in two authoritative responses: name error received from
"host.example.com.local.arpa.", and a requested A record - from
"child.host.example.com.local.arpa.". To prevent this ambiguity,
multicast enabled hosts could perform a dynamic update of the parent (or
grandparent) zone with a delegation to a child zone. In this example a
host "child.host.example.com.local.arpa." would send a dynamic update
for the NS and glue A record to "host.example.com.local.arpa.", but this
approach significantly complicates implementation of multicast DNS and
would not be acceptable for lightweight hosts.

A response to a multicast query is composed in exactly the same manner
as a response to the unicast DNS query as specified in [4]. Responders
MUST never respond using cached data, and the AA (Authoritative Answer)
bit MUST be set. The response is sent to the sender via unicast. A
response to an mDNS query MUST have RCODE set to zero, since mDNS
responders MUST NOT send error replies in response to mDNS queries.

If a TC (truncation) bit is set in the response, then the sender MAY use
the response if it contains all necessary information, or the sender MAY
discard the response and resend the query over TCP or using EDNS0 with
larger window using the unicast address of the responder. The RA
(Recursion Available) bit in the header of the response MUST NOT be set.
Even if the RA bit is set in the response header, the sender MUST ignore
it.

2.1.3.  mDNS addressing

For IPv4 LINKLOCAL addressing, section 2.4 of [6] lays out the rules
with respect to source address selection, TTL settings, and acceptable
source/destination address combinations. IPv6 LINKLOCAL addressing is
described in [9]. mDNS queries and responses MUST obey the rules laid
out in these documents.

In composing an mDNS response, the responder MUST set the Hop Limit
field in the IPv6 header and the TTL field in IPv4 header of the
multicast DNS response to 255. The sender MUST verify that the Hop Limit
field in IPv6 header and TTL field in IPv4 header of each response to



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the multicast DNS query is set to 255. If it is not, then sender MUST
ignore the response.

   Implementation note:

   In the sockets API for IPv4, the IP_TTL and IP_MULTICAST_TTL socket
   options are used to specify the TTL of outgoing unicast and multicast
   packets. The IP_RECVTTL socket option is available on some platforms
   to receive the IPv4 TTL of received packets with recvmsg(). RFC 2292
   specifies similar options for specifying and receiving the IPv6 Hop
   Limit.

2.1.4.  Use of DNS TTL

The responder should use a pre-configured TTL [5] value in the records
returned in the multicast DNS query response. Due to the TTL
minimalization necessary when caching an RRset, all TTLs in an RRset
MUST be set to the same value.  In the additional and authority section
of the response the responder includes the same records as a DNS server
would insert in the response to the unicast DNS query.

2.1.5.  No/multiple responses

The sender MUST anticipate receiving no replies to some multicast
queries, in the event that no responders are available within the
multicast scope, or in the event that no positive non-null responses
exist for the transmitted query.

If no positive response is received, a resolver treats it as a response
that no records of the specified type and class for the specified name
exist (NXRRSET).

The sender MUST anticipate receiving multiple replies to the same
multicast query, in the event that several multicast DNS enabled
computers receive the query and respond with valid answers.  When this
occurs, the responses MAY first be concatenated, and then treated in the
same manner that multiple RRs received from the same DNS server would,
ordinarily.

3.  Usage model

A host configured to be an mDNS "responder" MUST also be configured as a
"sender". A host not configured as a "responder" MUST NOT be a "sender".

Multicast DNS usage is determined by the domain search configuration as
well as by special treatment of the ".local.arpa." namespace.  Any host
whose domain search configuration contains the ".local.arpa." domain is
configured to behave as "responder". The sender treats queries for



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".local.arpa." as a special case. The domain search list can be
configured manually or automatically via a DHCP option [3].

A sender MUST NOT send a unicast query for names ending with the
".local.arpa." suffix except when:

  a. A sender repeats a query over TCP after it received a response
     to the previous multicast query with the TC bit set, or

  b. The sender's cache contains an NS resource record that enables
     the sender to send a query directly to the hosts
     authoritative for the name in the query.

It is not expected that a host named "host.example.com." will be
manually configured to have the additional name
"host.example.com.local.arpa." when it is configured to use multicast
DNS. Instead, a responder with a name "host.example.com." configured
with ".local.arpa." suffix in its domain search configuration is
authoritative for the name "host.example.com.local.arpa.". For example,
when a responder with the name "host.example.com." receives an A type
query for the name "host.example.com.local.arpa." it authoritatively
responds to the query.

If ".local.arpa" is not in the domain search list, then multicast DNS
MUST NOT be used. This implies that the host will neither listen on the
DNS LINKLOCAL multicast address, nor will it send queries to that
address. An auto-configured host will typically have ".local.arpa" first
in its search list so that it will be enabled to use multicast DNS.
Typically an enterprise host will not have ".local.arpa" in its search
list at all so that it will not use multicast DNS.

The same host MAY use multicast DNS queries for the resolution of names
ending with ".local.arpa.", and unicast DNS queries for resolution of
all other names. When a user or application requests a DNS client to
resolve a dot-terminated name that contains a ".local.arpa" suffix, the
query for such a name MUST be multicast and the name SHOULD NOT be
concatenated with any suffix.

If a DNS server is running on a host, the host MUST NOT listen for
multicast DNS queries, to prevent the host from listening on port 53 and
intercepting DNS queries directed to a DNS server.  By default, a DNS
server MUST NOT listen to multicast DNS queries. For a DNS server, the
presence of  ".local.arpa." in the domain search list MUST NOT enable
multicast DNS.







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4.  Sequence of events

The sequence of events for multicast DNS usage is as follows:

1. If a sender needs to resolve a query for a name
  "host.example.com.local.arpa", then it sends a multicast query to the
   LINKLOCAL multicast address.

2. A responder responds to this query only if it is authoritative
   for the domain name "host.example.com.local.arpa". The responder sends
   a response to the sender via unicast over UDP.

3. Upon the reception of the response, the sender verifies that the Hop
   Limit field in IPv6 header or TTL field in IPv4 header (depending on
   the protocol used) of the response is set to 255. If the destination
   address is a LINKLOCAL address, then the sender verifies use of a
   LINKLOCAL source address. If these conditions are met, then the sender
   uses and caches the returned response. If not, then the sender ignores
   the response and continues waiting for the response.

5.  Conflict resolution

There are some scenarios when multiple responders MAY respond to the
same query. There are other scenarios when only one responder may
respond to a query. Resource records for which the latter queries are
submitted are referred as UNIQUE throughout this document. The
uniqueness of a resource record depends on a nature of the name in the
query and type of the query. For example it is expected that:

   - multiple hosts may respond to a query for a SRV type record
   - multiple hosts may respond to a query for an A type record for a
     cluster name (assigned to multiple hosts in the cluster)
   - only a single host may respond to a query for an A type record for
     a hostname.

Every responder that responds to a multicast DNS query and/or dynamic
update request AND includes a UNIQUE record in the response:

   1. MUST verify that there is no other host within the scope of the
      multicast DNS query propagation that can return a DNS record
      for the same name, type and class.
   2. MUST NOT include a UNIQUE resource record in the
      response without having verified its uniqueness.

Where a host is configured to respond to multicast DNS queries on more
than one interface, the host MUST verify resource record uniqueness on
each interface for each UNIQUE resource record that could be used on
that interface. To accomplish this, the host MUST multicast a dynamic



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DNS update request as specified in RFC 2136 [11] for each new UNIQUE
resource record. Uniqueness verification is carried out when the host:

  - starts up or
  - is configured to respond to the multicast DNS queries on
    some interface or
  - is configured to respond to the multicast DNS queries using
    additional UNIQUE DNS records.

Below we describe the data to be specified in the dynamic update
request:

Header section
     contains values according to [RFC 2136].

Zone section
     The zone name in the zone section MUST be set to the name of the
     UNIQUE record. The zone type in the zone section MUST be set to
     SOA. The zone class in the zone section MUST be set to the class of
     the UNIQUE record.

Prerequisite section
     This section MUST contain a record set whose semantics are
     described in RFC 2136 [11], Section 2.4.3 "RRset Does Not Exist",
     requesting that RRs with the NAME and TYPE of the UNIQUE record do
     not exist.

Update section
     This section MUST be left empty.

Additional section
     This section is set according to RFC 2136.

When a host that owns a UNIQUE record receives a dynamic update request
that requests that the UNIQUE resource record set does not exist, the
host MUST respond via unicast with the YXRRSET error, according to the
rules described in Section 3 of RFC 2136 [11].

After client receives an YXRRSET response to its dynamic update request
that a UNIQUE resource record does not exist, the host MUST not use the
UNIQUE resource record in responses to multicast queries and dynamic
update requests.

Note that this name conflict detection mechanism doesn't prevent name
conflicts when previously partitioned segments are connected by a
bridge.  In such a situation, name conflicts are detected when a sender
receives more than one response to its multicast DNS query.  In this
case, the sender sends the first response that it received to all



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responders that responded to this query except the first one, using
unicast. A host that receives a query response containing a UNIQUE
resource record that it owns, even if it didn't send such a query, MUST
verify that no other host within the multicast DNS scope is
authoritative for the same name, using the dynamic DNS update request
mechanism described above.

Based on the result, the host detects whether there is a name conflict
and acts as described above.

5.1.  Considerations for Multiple Interfaces

A multi-homed host may elect to configure multicast DNS on only one of
its active interfaces. In many situations this will be adequate.
However, should a host wish to configure multicast DNS on more than one
of its active interfaces, there are some additional precautions it MUST
take. Implementers who are not planning to support multicast DNS on
multiple interfaces simultaneously may skip this section.

A multi-homed host checks the uniqueness of UNIQUE records as described
in Section 5.

The situation is illustrated in figure 1 below:

     ----------  ----------
      |      |    |      |
     [A]    [myhost]   [myhost]

   Figure 1. LINKLOCAL name conflict

In this situation, the multi-homed myhost will probe for, and defend,
its host name on both interfaces. A conflict will be detected on one
interface, but not the other, and as a result, the multi-homed myhost
will not be able to respond with a host RR for "myhost".

Since names are only unique per-link, hosts on different links could be
using the same name.  If an mDNS client sends requests over multiple
interfaces, and receives replies from more than one, the result returned
to the client is defined by the implementation.  The situation is
illustrated in figure 2 below.

     ----------  ----------
      |      |    |     |
     [A]    [myhost]   [A]


   Figure 2. Off-segment name conflict




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If host myhost is configured to use mDNS on both interfaces, it will
send mDNS queries on both interfaces.  When host myhost sends a query
for the host RR for name "A" it will receive a response from hosts on
both interfaces.  Host myhost will then forward a response from the
first responder to the second responder, who will attempt to verify the
uniqueness of host RR for its name, but will not discover a conflict,
since the conflicting host resides on a different subnet. Therefore it
will continue using its name.  This illustrates that the proposed name
conflict resolution mechanism does not support resolution of conflicts
between hosts on different subnets. This problem can also occur with
unicast DNS when a multi-homed host is connected to two different
networks with separated name spaces. It is not the intent of this
document to address the issue of uniqueness of names within DNS.

6.  IANA Considerations

Authors will contact IANA to reserve LINKLOCAL IPv4 and IPv6 addresses.

7.  ARPA domain considerations

This document specifies the use of a new sub-domain of the "ARPA"
domain.  According to Section 2.1 of the ARPA Guidelines [12], this
specification requires description and justification.

The 'local.arpa' domain is used to distinguish a local namespace.  This
namespace differs from others in the following respects:

    - Name servers responding to requests for names in this
      domain have different rules concerning authority.  As
      explained in Section 2.1, mDNS servers have limited
      scope of authority, not extending to sub-domains of
      domain they are authoritative for.

    - DNS servers SHOULD NOT forward queries for domain names
      in the local.arpa domain - if the server cannot answer
      the query from its own database, it should reply with a
      non-authoritative NXDOMAIN.

    - Hosts may derive their own names in this namespace,
      independent of centralized authorization and registration
      (as defined in section 3 and section 5).

    - There is no delegation or administrative structure to
      sub-domains of '.local.arpa'.

How protocol objects are mapped into lookup keys:

      Names are associated with resources which can be requested



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      according to the DNS protocol.  However, recursive lookup
      is impossible.  Further, mDNS specifies only the use of
      multicast to transmit these requests.

8.  References

[1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

[2]  Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
     2365, July 1998.

[3]  Aboba, B., "DHCP Domain Search Option", Internet draft (work in
     progress), draft-aboba-dhc-domsearch-02.txt, June 2001.

[4]  Mockapetris, P., "Domain Names - Implementation and Specification",
     RFC 1035, November 1987.

[5]  Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC
     1034, November, 1987.

[6]  Cheshire, S., Aboba, B., "Dynamic Configuration of IPv4 Link-Local
     Addresses", Internet draft (work in progress), draft-ietf-zeroconf-
     ipv4-linklocal-03.txt, June 2001.

[7]  Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
     Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

[8]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
     Specification", RFC 2460, December 1998.

[9]  Hinden, R., Deering, S., "IP Version 6 Addressing Architecture",
     RFC 2373, July 1998.

[10] Information technology - Telecommunications and information
     exchange between systems - Local and metropolitan area networks -
     Specific Requirements Part 11:  Wireless LAN Medium Access Control
     (MAC) and Physical Layer (PHY) Specifications, IEEE Std.
     802.11-1997, 1997.

[11] Vixie, P., Thomson, S., Rekhter, Y., Bound, J., "Dynamic Updates in
     the Domain Name System (DNS UPDATE)", RFC 2136, April 1997.

[12] Huston, G., "Management Guidelines & Operational Requirements for
     the Internet Infrastructure Domain ("ARPA")", Internet draft (work
     in progress), draft-iab-arpa-02.txt,  May 2001.





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9.  Security Considerations

This draft does not prescribe a means of securing the multicast DNS
mechanism. It is possible that hosts will allocate conflicting names for
a period of time, or that non-conforming hosts will attempt to deny
service to other hosts by allocating the same name. Such attacks also
allow nodes to receive packets destined for other nodes. The protocol
reduces the exposure to such threats in the absence of authentication by
ignoring multicast DNS query response packets received from off-link
senders.

In all received responses, the Hop Limit field in IPv6 and the TTL field
in IPv4 are verified to contain 255, the maximum legal value.  Since
routers decrement the Hop Limit on all packets they forward, received
packets containing a Hop Limit of 255 must have originated from a
neighbor. Packets destined for a LINKLOCAL address are verified to have
been sent from a LINKLOCAL source address.

These threats are most serious in wireless networks such as 802.11,
since attackers on a wired network will require physical access to the
home network, while wireless attackers may reside outside the home.
Link-layer security will serve to secure mDNS against the above threats
if it is available. For example, where 802.11 "Wired Equivalency
Privacy" (WEP) [10] is implemented, a casual attacker is likely to be
deterred from gaining access to the home network.

The  mechanism specified in this draft does not require use of the
DNSSEC, which means that the responses to the multicast DNS queries may
not be authenticated. If a network contains a "signed key distribution
center" for all (or at least some) of the DNS zones that the responders
are authoritative for, then hosts on such a network are configured with
the key for the top zone, "local.arpa." (hosted by "signed keys
distribution center") and may use DNSSEC for authentication of the
responders using DNSSEC.

Acknowledgments

This work builds upon original work done on multicast DNS by Bill
Manning and Bill Woodcock.  The authors gratefully acknowledge their
contribution to the current specification.  Constructive input has also
been received from Mark Andrews, Stuart Cheshire, Robert Elz, James
Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig, Thomas Narten,
Erik Nordmark, Sander Van-Valkenburg and Tomohide Nagashima.








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Authors' Addresses

Levon Esibov
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

EMail: levone@microsoft.com

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: +1 (425) 936-6605
EMail: bernarda@microsoft.com

Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: +1 (425) 703-8835
EMail: dthaler@microsoft.com

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Full Copyright Statement

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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 "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

Expiration Date

This memo is filed as <draft-ietf-dnsext-mdns-01.txt>,  and  expires
January 15, 2002.
























Esibov, Aboba & Thaler       Standards Track                   [Page 15]


