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Auto-Configuration of a Network of Hybrid Unicast/Multicast DNS-Based Service Discovery Proxy Nodes

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Author Markus Stenberg
Last updated 2015-03-05
Replaces draft-stenberg-homenet-dnssd-hybrid-proxy-zeroconf
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Homenet Working Group                                        M. Stenberg
Intended status: Standards Track                           March 5, 2015
Expires: September 6, 2015

 Auto-Configuration of a Network of Hybrid Unicast/Multicast DNS-Based
                     Service Discovery Proxy Nodes


   This document describes how a proxy functioning between Unicast DNS-
   Based Service Discovery and Multicast DNS can be automatically
   configured using an arbitrary network-level state sharing mechanism.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 6, 2015.

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   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements language . . . . . . . . . . . . . . . . . . . .   3
   3.  Hybrid proxy - what to configure  . . . . . . . . . . . . . .   3
     3.1.  Conflict resolution within network  . . . . . . . . . . .   4
     3.2.  Per-link DNS-SD forward zone names  . . . . . . . . . . .   4
     3.3.  Reasonable defaults . . . . . . . . . . . . . . . . . . .   5
       3.3.1.  Network-wide unique link name (scheme 1)  . . . . . .   5
       3.3.2.  Node name (scheme 2)  . . . . . . . . . . . . . . . .   5
       3.3.3.  Link name (scheme 2)  . . . . . . . . . . . . . . . .   5
   4.  TLVs  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  DNS Delegated Zone TLV  . . . . . . . . . . . . . . . . .   5
     4.2.  Domain Name TLV . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Node Name TLV . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Desirable behavior  . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  DNS search path in DHCP requests  . . . . . . . . . . . .   7
     5.2.  Hybrid proxy  . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Hybrid proxy network zeroconf daemon  . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative references  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative references  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Example configuration  . . . . . . . . . . . . . . .  10
     A.1.  Used topology . . . . . . . . . . . . . . . . . . . . . .  10
     A.2.  Zero-configuration steps  . . . . . . . . . . . . . . . .  10
     A.3.  TLV state . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.4.  DNS zone  . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.5.  Interaction with hosts  . . . . . . . . . . . . . . . . .  12
   Appendix B.  Implementation . . . . . . . . . . . . . . . . . . .  12
   Appendix C.  Why not just proxy Multicast DNS?  . . . . . . . . .  13
     C.1.  General problems  . . . . . . . . . . . . . . . . . . . .  13
     C.2.  Stateless proxying problems . . . . . . . . . . . . . . .  14
     C.3.  Stateful proxying problems  . . . . . . . . . . . . . . .  14
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Section 3 ("Hybrid Proxy Operation") of [I-D.cheshire-dnssd-hybrid]
   describes how to translate queries from Unicast DNS-Based Service
   Discovery described in [RFC6763] to Multicast DNS described in
   [RFC6762], and how to filter the responses and translate them back to
   unicast DNS.

   This document describes what sort of configuration the participating
   hybrid proxy servers require, as well as how it can be provided using

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   any network-wide state sharing mechanism such as link-state routing
   protocol or Home Networking Control Protocol [I-D.ietf-homenet-hncp].
   The document also describes a naming scheme which does not even need
   to be same across the whole covered network to work as long as the
   specified conflict resolution works.  The scheme can be used to
   provision both forward and reverse DNS zones which employ hybrid
   proxy for heavy lifting.

   This document does not go into low level encoding details of the
   Type-Length-Value (TLV) data that we want synchronized across a
   network.  Instead, we just specify what needs to be available, and
   assume every node that needs it has it available.

   We go through the mandatory specification of the language used in
   Section 2, then describe what needs to be configured in hybrid
   proxies and participating DNS servers across the network in
   Section 3.  How the data is exchanged using arbitrary TLVs is
   described in Section 4.  Finally, some overall notes on desired
   behavior of different software components is mentioned in Section 5.

2.  Requirements language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Hybrid proxy - what to configure

   Beyond the low-level translation mechanism between unicast and
   multicast service discovery, the hybrid proxy draft
   [I-D.cheshire-dnssd-hybrid] describes just that there have to be NS
   records pointing to hybrid proxy responsible for each link within the
   covered network.

   In zero-configuration case, choosing the links to be covered is also
   non-trivial choice; we can use the border discovery functionality (if
   available) to determine internal and external links.  Or we can use
   some other protocol's presence (or lack of it) on a link to determine
   internal links within the covered network, and some other signs
   (depending on the deployment) such as DHCPv6 Prefix Delegation (as
   described in [RFC3633]) to determine external links that should not
   be covered.

   For each covered link we want forward DNS zone delegation to an
   appropriate node which is connected to a link, and running hybrid
   proxy.  Therefore the links' forward DNS zone names should be unique
   across the network.  We also want to populate reverse DNS zone
   similarly for each IPv4 or IPv6 prefix in use.

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   There should be DNS-SD browse domain list provided for the network's
   domain which contains each physical link only once, regardless of how
   many nodes and hybrid proxy implementations are connected to it.

   Yet another case to consider is the list of DNS-SD domains that we
   want hosts to enumerate for browse domain lists.  Typically, it
   contains only the local network's domain, but there may be also other
   networks we may want to pretend to be local but are in different
   scope, or controlled by different organization.  For example, a home
   user might see both home domain's services (TBD-TLD), as well as
   ISP's services under

3.1.  Conflict resolution within network

   Any naming-related choice on node may have conflicts in the network
   given that we require only distributed loosely synchronized database.
   We assume only that the underlying protocol used for synchronization
   has some concept of precedence between nodes originating conflicting
   information, and in case of conflict, the higher precedence node MUST
   keep the name they have chosen.  The one(s) with lower precedence
   MUST either try different one (that is not in use at all according to
   the current link state information), or choose not to publish the
   name altogether.

   If a node needs to pick a different name, any algorithm works,
   although simple algorithm choice is just like the one described in
   Multicast DNS[RFC6762]: append -2, -3, and so forth, until there are
   no conflicts in the network for the given name.

3.2.  Per-link DNS-SD forward zone names

   How to name the links of a whole network in automated fashion?  Two
   different approaches seem obvious:

   1.  Unique link name based - (unique-link).(domain).

   2.  Node and link name - (link).(unique-node).(domain).

   The first choice is appealing as it can be much more friendly
   (especially given manual configuration).  For example, it could mean
   just and for a simple home network.
   The second choice, on the other hand, has a nice property of being
   local choice as long as node name can be made unique.

   The type of naming scheme to use can be left as implementation
   option.  And the actual names themselves SHOULD be also overridable,
   if the end-user wants to customize them in some way.

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3.3.  Reasonable defaults

   Note that any manual configuration, which SHOULD be possible, MUST
   override the defaults provided here or chosen by the creator of the

3.3.1.  Network-wide unique link name (scheme 1)

   It is not obvious how to produce network-wide unique link names for
   the (unique-link).(domain) scheme.  One option would be to base it on
   type of physical network layer, and then hope that the number of the
   networks won't be significant enough to confuse (e.g. "lan", or

   The network-wide unique link names should be only used in small
   networks.  Given a larger network, after conflict resolution,
   identifying which link is '' may be challenging.

3.3.2.  Node name (scheme 2)

   Our recommendation is to use some short form which indicates the type
   of node it is, for example, "".  As the name is
   visible to users, it should be kept as short as possible.  In theory
   even more exact model could be helpful, for example, "openwrt-".  In practice providing some other
   records indicating exact node information (and access to management
   UI) is more sensible.

3.3.3.  Link name (scheme 2)

   Recommendation for (link) portion of (link).(node).(domain) is to use
   physical network layer type as base, or possibly even just interface
   name on the node if it's descriptive enough.  For example,
   "" and "" may be
   good enough.

4.  TLVs

   To implement this specification fully, support for following three
   different TLVs is needed.  However, only the DNS Delegated Zone TLVs
   MUST be supported, and the other two SHOULD be supported.

4.1.  DNS Delegated Zone TLV

   This TLV is effectively a combined NS and A/AAAA record for a zone.
   It MUST be supported by implementations conforming to this
   specification.  Implementations SHOULD provide forward zone per link
   (or optimizing a bit, zone per link with Multicast DNS traffic).

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   Implementations MAY provide reverse zone per prefix using this same
   mechanism.  If multiple nodes advertise same reverse zone, it should
   be assumed that they all have access to the link with that prefix.
   However, as noted in Section 5.3, mainly only the node with highest
   precedence on the link should publish this TLV.


   o  Address field is IPv6 address (e.g. 2001:db8::3) or IPv4 address
      mapped to IPv6 address (e.g. ::FFFF: where the
      authoritative DNS server for Zone can be found.  If the address
      field is all zeros, the Zone is under global DNS hierarchy and can
      be found using normal recursive name lookup starting at the
      authoritative root servers (This is mostly relevant with the S bit

   o  S-bit indicates that this delegated zone consists of a full DNS-SD
      domain, which should be used as base for DNS-SD domain enumeration
      (that is, (field)._dns-sd._udp.(zone) exists).  Forward zones MAY
      have this set.  Reverse zones MUST NOT have this set.  This can be
      used to provision DNS search path to hosts for non-local services
      (such as those provided by ISP, or other manually configured
      service providers).

   o  B-bit indicates that this delegated zone should be included in
      network's DNS-SD browse list of domains at b._dns-
      sd._udp.(domain).  Local forward zones SHOULD have this set.
      Reverse zones SHOULD NOT have this set.

   o  L-bit indicates that this delegated zone should be included in the
      network's DNS-SD legacy browse list of domains at lb._dns-
      sd._udp.(DOMAIN-NAME).  Local forward zones SHOULD have this bit
      set, reverse zones SHOULD NOT.

   o  Zone is the label sequence of the zone, encoded according to
      section 3.1.  ("Name space definitions") of [RFC1035].  Note that
      name compression is not required here (and would not have any
      point in any case), as we encode the zones one by one.  The zone
      MUST end with an empty label.

   In case of a conflict (same zone being advertised by multiple parties
   with different address or bits), conflict should be addressed
   according to Section 3.1.

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4.2.  Domain Name TLV

   This TLV is used to indicate the base (domain) to be used for the
   network.  If multiple nodes advertise different ones, the conflict
   resolution rules in Section 3.1 should result in only the one with
   highest precedence advertising one, eventually.  In case of such
   conflict, user SHOULD be notified somehow about this, if possible,
   using the configuration interface or some other notification
   mechanism for the nodes.  Like the Zone field in Section 4.1, the
   Domain Name TLV's contents consist of a single DNS label sequence.

   This TLV SHOULD be supported if at all possible.  It may be derived
   using some future DHCPv6 option, or be set by manual configuration.
   Even on nodes without manual configuration options, being able to
   read the domain name provided by a different node could make the user
   experience better due to consistent naming of zones across the

   By default, if no node advertises domain name TLV, hard-coded default
   (TBD) should be used.

4.3.  Node Name TLV

   This TLV is used to advertise a node's name.  After the conflict
   resolution procedure described in Section 3.1 finishes, there should
   be exactly zero to one nodes publishing each node name.  The contents
   of the TLV should be a single DNS label.

   This TLV SHOULD be supported if at all possible.  If not supported,
   and another node chooses to use the (link).(node) naming scheme with
   this node's name, the contents of the network's domain may look
   misleading (but due to conflict resolution of per-link zones, still

   If the node name has been configured manually, and there is a
   conflict, user SHOULD be notified somehow about this, if possible,
   using the configuration interface or some other notification
   mechanism for the nodes.

5.  Desirable behavior

5.1.  DNS search path in DHCP requests

   The nodes following this specification SHOULD provide the used
   (domain) as one item in the search path to it's hosts, so that DNS-SD
   browsing will work correctly.  They also SHOULD include any DNS
   Delegated Zone TLVs' zones, that have S bit set.

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5.2.  Hybrid proxy

   The hybrid proxy implementation SHOULD support both forward zones,
   and IPv4 and IPv6 reverse zones.  It SHOULD also detect whether or
   not there are any Multicast DNS entities on a link, and make that
   information available to the network zeroconf daemon (if implemented
   separately).  This can be done by (for example) passively monitoring
   traffic on all covered links, and doing infrequent service
   enumerations on links that seem to be up, but without any Multicast
   DNS traffic (if so desired).

   Hybrid proxy nodes MAY also publish it's own name via Multicast DNS
   (both forward A/AAAA records, as well as reverse PTR records) to
   facilitate applications that trace network topology.

5.3.  Hybrid proxy network zeroconf daemon

   The daemon should avoid publishing TLVs about links that have no
   Multicast DNS traffic to keep the DNS-SD browse domain list as
   concise as possible.  It also SHOULD NOT publish delegated zones for
   links for which zones already exist by another node with higher

   The daemon (or other entity with access to the TLVs) SHOULD generate
   zone information for DNS implementation that will be used to serve
   the (domain) zone to hosts.  Domain Name TLV described in Section 4.2
   should be used as base for the zone, and then all DNS Delegated Zones
   described in Section 4.1 should be used to produce the rest of the
   entries in zone (see Appendix A.4 for example interpretation of the
   TLVs in Appendix A.3.

6.  Security Considerations

   There is a trade-off between security and zero-configuration in
   general; if used network state synchronization protocol is not
   authenticated (and in zero-configuration case, it most likely is
   not), it is vulnerable to local spoofing attacks.  We assume that
   this scheme is used either within (lower layer) secured networks, or
   with not-quite-zero-configuration initial set-up.

   If some sort of dynamic inclusion of links to be covered using border
   discovery or such is used, then effectively service discovery will
   share fate with border discovery (and also security issues if any).

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7.  IANA Considerations

   This document has no actions for IANA.

8.  References

8.1.  Normative references

              Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service
              Discovery", draft-cheshire-dnssd-hybrid-01 (work in
              progress), January 2014.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, February 2013.

8.2.  Informative references

              Stenberg, M., Barth, S., and P. Pfister, "Home Networking
              Control Protocol", draft-ietf-homenet-hncp-03 (work in
              progress), January 2015.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

8.3.  URIs


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Appendix A.  Example configuration

A.1.  Used topology

   Let's assume home network that looks like this:

       | CER |
    [1]/    \[2]
      /      \
   +-----+ +-----+
   | IR1 |-| IR2 |
   +-----+ +-----+
    |[3]|   |[4]|

   We're not really interested about links [0], [1] and [2], or the
   links between IRs.  Given the optimization described in Section 4.1,
   they should not produce anything to network's Multicast DNS state
   (and therefore to DNS either) as there isn't any Multicast DNS
   traffic there.

   The user-visible set of links are [3] and [4]; each consisting of a
   LAN and WLAN link.  We assume that ISP provides 2001:db8:1234::/48
   prefix to be delegated in the home via [0].

A.2.  Zero-configuration steps

   Given implementation that chooses to use the second naming scheme
   (link).(node).(domain), and no configuration whatsoever, here's what
   happens (the steps are interleaved in practice but illustrated here
   in order):

   1.  Network-level state synchronization protocol runs, nodes get
       effective precedences.  For ease of illustration, CER winds up
       with 2, IR1 with 3, and IR2 with 1.

   2.  Prefix delegation takes place.  IR1 winds up with
       2001:db8:1234:11::/64 for LAN and 2001:db8:1234:12::/64 for WLAN.
       IR2 winds up with 2001:db8:1234:21::/64 for LAN and
       2001:db8:1234:22::/64 for WLAN.

   3.  IR1 is assumed to be reachable at 2001:db8:1234:11::1 and IR2 at

   4.  Each node wants to be called 'node' due to lack of branding in
       drafts.  They announce that using the node name TLV defined in

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       Section 4.3.  They also advertise their local zones, but as that
       information may change, it's omitted here.

   5.  Conflict resolution ensues.  As IR1 has precedence over the rest,
       it becomes "node".  CER and IR2 have to rename, and (depending on
       timing) one of them becomes "node-2" and other one "node-3".  Let
       us assume IR2 is "node-2".  During conflict resolution, each node
       publishes TLVs for it's own set of delegated zones.

   6.  CER learns ISP-provided domain "" using DHCPv6
       domain list option defined in [RFC3646].  The information is
       passed along as S-bit enabled delegated zone TLV.

A.3.  TLV state

   Once there is no longer any conflict in the system, we wind up with
   following TLVs (NN is used as abbreviation for Node Name, and DZ for
   Delegated Zone TLVs):

   (from CER)
   DZ {s=1,zone=""}

   (from IR1)
   NN {name="node"}

   DZ {address=2001:db8:1234:11::1, b=1,
   DZ {address=2001:db8:1234:11::1,

   DZ {address=2001:db8:1234:11::1, b=1,
   DZ {address=2001:db8:1234:11::1,

   (from IR2)
   NN {name="node-2"}

   DZ {address=2001:db8:1234:21::1, b=1,
   DZ {address=2001:db8:1234:21::1,

   DZ {address=2001:db8:1234:21::1, b=1,
   DZ {address=2001:db8:1234:21::1,

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A.4.  DNS zone

   In the end, we should wind up with following zone for (domain) which
   is in this case, available at all nodes, just based on
   dumping the delegated zone TLVs as NS+AAAA records, and optionally
   domain list browse entry for DNS-SD:

   b._dns_sd._udp PTR lan.node
   b._dns_sd._udp PTR wlan.node

   b._dns_sd._udp PTR lan.node-2
   b._dns_sd._udp PTR wlan.node-2

   node AAAA 2001:db8:1234:11::1
   node-2 AAAA 2001:db8:1234:21::1

   node NS node
   node-2 NS node-2 NS NS NS NS

   Internally, the node may interpret the TLVs as it chooses to, as long
   as externally defined behavior follows semantics of what's given in
   the above.

A.5.  Interaction with hosts

   So, what do the hosts receive from the nodes?  Using e.g.  DHCPv6 DNS
   options defined in [RFC3646], DNS server address should be one (or
   multiple) that point at DNS server that has the zone information
   described in Appendix A.4.  Domain list provided to hosts should
   contain both "" (the hybrid-enabled domain), as well as
   the externally learned domain "".

   When hosts start using DNS-SD, they should check both b._dns-, as well as for
   list of concrete domains to browse, and as a result services from two
   different domains will seem to be available.

Appendix B.  Implementation

   There is an prototype implementation of this draft at hnetd github
   repository [1] which contains variety of other homenet WG-related
   things' implementation too.

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Appendix C.  Why not just proxy Multicast DNS?

   Over the time number of people have asked me about how, why, and if
   we should proxy (originally) link-local Multicast DNS over multiple

   At some point I meant to write a draft about this, but I think I'm
   too lazy; so some notes left here for general amusement of people
   (and to be removed if this ever moves beyond discussion piece).

C.1.  General problems

   There are two main reasons why Multicast DNS is not proxyable in the
   general case.

   First reason is the conflict resolution depends on the RRsets staying
   constant.  That is not possible across multiple links (due to e.g.
   link-local addresses having to be filtered).  Therefore, conflict
   resolution breaks, or at least requires ugly hacks to work around.

   A simple, but not really working workaround for this is to make sure
   that in conflict resolution, propagated resources always loses.
   Given that the proxy function only removes records, the result SHOULD
   be consistently original set of records winning.  Even with that, the
   conflict resolution will effectively cease working, allowing for two
   instances of same name to exist (as both think they 'own' the name
   due to locally seen higher precedence).

   Given some more extra logic, it is possible to make this work by
   having proxies be aware of both the original record sets, and
   effectively enforcing the correct conflict resolution results by (for
   example) passing the unfiltered packets to the losing party just to
   make sure they renumber, or by altering the RR sets so that they will
   consistently win (by inserting some lower rrclass/rrtype records).
   As the conflicts happen only in rrclass=1/rrtype=28, it is easy
   enough to add e.g. extra TXT record (rrtype 16) to force precedence
   even when removing the later rrtype 28 record.  Obviously, this new
   RRset must never wind up near the host with the higher precedence, or
   it will cause spurious renaming loops.

   Second reason is timing, which is relatively tight in the conflict
   resolution phase, especially given lossy and/or high latency

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C.2.  Stateless proxying problems

   In general, typical stateless proxy has to involve flooding, as
   Multicast DNS assumes that most messages are received by every host.
   And it won't scale very well, as a result.

   The conflict resolution is also harder without state.  It may result
   in Multicast DNS responder being in constant probe-announce loop,
   when it receives altered records, notes that it's the one that should
   own the record.  Given stateful proxying, this would be just a
   transient problem but designing stateless proxy that won't cause this
   is non-trivial exercise.

C.3.  Stateful proxying problems

   One option is to write proxy that learns state from one link, and
   propagates it in some way to other links in the network.

   A big problem with this case lies in the fact that due to conflict
   resolution concerns above, it is easy to accidentally send packets
   that will (possibly due to host mobility) wind up at the originator
   of the service, who will then perform renaming.  That can be
   alleviated, though, given clever hacks with conflict resolution

   The stateful proxying may be also too slow to occur within the
   timeframe allocated for announcing, leading to excessive later
   renamings based on delayed finding of duplicate services with same

   A work-around exists for this though; if the game doesn't work for
   you, don't play it.  One option would be simply not to propagate ANY
   records for which conflict has seen even once.  This would work, but
   result in rather fragile, lossy service discovery infrastructure.

   There are some other small nits too; for example, Passive Observation
   Of Failure (POOF) will not work given stateful proxying.  Therefore,
   it leads to requiring somewhat shorter TTLs, perhaps.

Appendix D.  Acknowledgements

   Thanks to Stuart Cheshire for the original hybrid proxy draft and
   interesting discussion in Orlando, where I was finally convinced that
   stateful Multicast DNS proxying is a bad idea.

   Also thanks to Mark Baugher, Ole Troan, Shwetha Bhandari and Gert
   Doering for review comments.

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Author's Address

   Markus Stenberg
   Helsinki  00930


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