Encrypted DNS Server Redirection
draft-ietf-add-encrypted-dns-server-redirection-00

add                                                              J. Todd
Internet-Draft                                                     Quad9
Intended status: Standards Track                               T. Jensen
Expires: 25 January 2025                                       Microsoft
                                                               C. Mosher
                                                            Innate, Inc.
                                                            24 July 2024

                    Encrypted DNS Server Redirection
           draft-ietf-add-encrypted-dns-server-redirection-00

Abstract

   This document defines Encrypted DNS Server Redirection (EDSR), a
   mechanism for encrypted DNS servers to redirect clients to other
   encrypted DNS servers.  This enables dynamic routing to geo-located
   or otherwise more desirable encrypted DNS servers without modifying
   DNS client endpoint configurations or the use of anycast by the DNS
   server.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   add.github.io/EncDNSServerRedirect/draft-add-encrypted-dns-server-
   redirection.html.  Status information for this document may be found
   at https://datatracker.ietf.org/doc/draft-ietf-add-encrypted-dns-
   server-redirection/.

   Discussion of this document takes place on the add Working Group
   mailing list (mailto:add@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/add/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/add/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-add/EncDNSServerRedirect.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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Table of Contents

   1.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  DNS client behavior . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Discovering redirections  . . . . . . . . . . . . . . . .   4
       3.1.1.  Use of Delegated Credentials  . . . . . . . . . . . .   4
       3.1.2.  Redirection after Discovery Using Resolver IP
               Addresses . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Identifying self-redirections . . . . . . . . . . . . . .   5
     3.3.  Waiting for redirections  . . . . . . . . . . . . . . . .   5
     3.4.  Refreshing redirections . . . . . . . . . . . . . . . . .   5
     3.5.  Multiple redirections . . . . . . . . . . . . . . . . . .   6
     3.6.  Network changes . . . . . . . . . . . . . . . . . . . . .   6
   4.  DNS server behavior . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Ensuring compatibility  . . . . . . . . . . . . . . . . .   7
     4.2.  Dealing with persistent clients . . . . . . . . . . . . .   7
     4.3.  Redirection to servers controlled by third parties  . . .   7
   5.  Deployment Considerations . . . . . . . . . . . . . . . . . .   7
     5.1.  Large trees of redirections . . . . . . . . . . . . . . .   8
     5.2.  Redirection TTLs  . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Including IP addresses in EDSR responses  . . . . . . . .   8
     5.4.  Comparison to Discovery Using Resolver Names  . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     6.1.  Trusting the redirected connection  . . . . . . . . . . .   9
     6.2.  Use with unencrypted DNS  . . . . . . . . . . . . . . . .   9
     6.3.  Use with DDR discovery from IP addresses  . . . . . . . .  10

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   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   8.  Data Flow Considerations  . . . . . . . . . . . . . . . . . .  11
     8.1.  Data Scope  . . . . . . . . . . . . . . . . . . . . . . .  11
     8.2.  Data Visibility . . . . . . . . . . . . . . . . . . . . .  11
     8.3.  Data centralization . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Introduction

   Encrypted DNS Server Redirection (EDSR) is a protocol that allows an
   encrypted DNS resolver whose configuration is well known to clients
   to redirect them to other, more desirable resolvers without having to
   support anycast and without having to configure clients with these
   other resolvers ahead of time.  It uses a similar mechanism to the
   one defined by Section 4 of [RFC9462] to redirect an encrypted DNS
   client from one encrypted DNS resolver to another encrypted DNS
   resolver.  Where DDR uses a threat model that presumes the initial
   DNS traffic could be unencrypted, EDSR only ever applies when the
   initial DNS traffic is already encrypted.

   One example of what makes redirection to another resolver desirable
   is geolocation.  A DNS service may document one or a few well known
   resolver configurations even though it routes traffic to hundreds or
   thousands of resolvers that are closer to the client, reducing
   latency and making DNS resolutions more applicable to the client.

   This document describes only one mode of redirection - Strict Origin
   Redirection.  Previous versions of this draft defined an additional
   mode of redirection that allowed servers to redirect to servers that
   presented a different domain name than the original server.  While
   the scenario's validity has some interest, there is no consensus in
   the WG for how it can be addressed.

3.  DNS client behavior

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3.1.  Discovering redirections

   When a DNS client first opens a connection to an encrypted DNS
   server, it MUST use the Discovery Using Resolver Names mechanism
   defined in Section 5 of [RFC9462] to send a SVCB query for the name
   of the resolver to discover its encrypted DNS configuration.  The DNS
   client SHOULD open a connection to the server returned in the SVCB
   query using the same domain name as the original server and one of
   the IP addresses returned in additional A/AAAA records for the same
   name.  Once a connection has been successfully opened, as
   subsequently described by reaching a suitable server at the end of
   the redirection chain, the client SHOULD close the first connection.

3.1.1.  Use of Delegated Credentials

   If the DNS client's TLS dependency supports Delegated Credentials
   [RFC9345], it SHOULD present the "delegated_credential" TLS extension
   in its ClientHello as described in Section 4.1.1 of [RFC9345] to
   maximize compatibility with EDSR-supporting servers.  This is because
   some server operators MAY redirect to servers controlled by other
   entities which do not have access to its private key but which
   nevertheless have the ability to terminate TLS connections for the
   server's name.

3.1.2.  Redirection after Discovery Using Resolver IP Addresses

   EDSR assumes that the original server is identified by domain name
   from the client's perspective.  Examples include when the client was
   configured with the resolver through endpoint management or DNR
   discovery [RFC9463].  However, when the server was discovered using
   DDR's Discovery Using Resolver IP Addresses Section 4 of [RFC9462],
   this is not the case.  Due to the threat model that mode of DDR
   operates under, where it has to start from an unencrypted resolver,
   the identity of the server used for verification is its IP address.
   The risks involved with using the domain name of resolvers discovered
   by Discovery Using Resolver IP Addresses are further explored in the
   Security Considerations section Section 6.

   When clients use EDSR with a resolver discovered using DDR's
   Discovery Using Resolver IP Addresses Section 4 of [RFC9462], the
   only difference is that the destination server it was redirected to
   MUST be able to claim the IP address of the previous server in its
   SAN field.

   This section applies to both the Verified and Opportunistic forms of
   DDR's Discovery Using Resolver IP Addresses mechanism.

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3.2.  Identifying self-redirections

   If the set of IP addresses that are valid for the server being
   redirected to include the IP address of the current server, the
   client SHOULD ignore the redirection, treating it the same as
   receiving no response or a NODATA response from the SVCB query.
   However, clients receiving preferable encryption parameters as part
   of the SVCB response MAY choose to reconnect to negotiate to upgrade
   to the preferred encryption method.  When doing so, the client SHOULD
   NOT immediately repeat EDSR as the redirection from the server to
   itself has terminated the redirection chain.

3.3.  Waiting for redirections

   The client does not need to wait for the results of the redirection
   discovery query before sending other DNS queries on the connection,
   though they SHOULD gracefully close the connection as soon as it has
   successfully established a connection to the server it was redirected
   to and received or timed out the outstanding queries on the original
   connection.

   See the Deployment Considerations section for reasons a client MAY
   choose to decline a redirection.

3.4.  Refreshing redirections

   If a chain of redirections was followed, the effective TTL of the
   redirection is the minimum of the TTLs encountered along the chain.
   If the effective TTL of the redirection is considered to be too short
   for the client's performance (because it would require frequent
   repetition of EDSR), clients MAY refuse to follow the redirection.
   Servers SHOULD NOT redirect clients in a way that results in short
   effective TTLs.

   When the redirection TTL expires or connectivity to the server the
   client was redirected to fails, the client MUST close the connection
   and return to using the servers it is currently configured to use by
   its local configuration before using EDSR again.  This allows the
   client to honor the intention of whatever configuration method was
   used to provide it with a set of DNS servers to use.

   If the client DNS server remains the same, it SHOULD repeat the EDSR
   mechanism before the effective TTLS expires so that if the same
   redirection is valid, it can avoid needing to tear down the current
   connection by refreshing its effective TTL.

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3.5.  Multiple redirections

   When clients receive more than one valid SVCB response, they SHOULD
   prefer using the redirections that match their configuration (such as
   supported IP address family or desired encrypted DNS protocol) in
   ascending order of the SVCB priority.  Once a successful connection
   is made to a redirected destination, clients MUST discard results for
   other servers.  Entries returned for the same IP address MAY be
   retained for multi-protocol path diversity to what is presumed to be
   the same server.  Later unsuitability of all connections to the
   server MUST result in restarting EDSR.

   Redirections are considered to be a one-to-one relationship (starting
   with one recursive resolver and following its redirections should
   result in one replacement recursive resolver).  It is not expected
   that a stub resolver ends up using more recursive resolvers than it
   was originally configured with when using EDSR.

3.6.  Network changes

   When a client device changes what network it is connected to, it
   SHOULD forget pre-existing redirections and start EDSR over with the
   originally configured resolvers.  This ensures that a resolver which
   redirects clients based on their source network can behave
   accordingly.

   Note that this is unrelated to what resolvers a client is originally
   configured with.  For example, a client which is configured to always
   used the resolvers advertised by DHCP will likely start with
   different original resolvers when changing networks.  How a client is
   configured with DNS resolvers is out of scope for this document.
   EDSR only provides a mechanism for clients to discover redirections
   from resolvers they were previously configured to use.

4.  DNS server behavior

   DNS resolvers who want to redirect clients to other resolvers MUST
   respond to SVCB [RFC9461] queries for their own domain names with
   records that describe the configuration of the destination server.

   Guidance in Section 5 of [RFC9460] to improve performance by
   including additional A/AAAA records with the SVCB response SHOULD be
   followed.

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   If the server knows it supports Discovery Using Resolver IP
   Addresses, or does not know for sure, it MUST be prepared for clients
   to connect without an SNI because clients might have discovered the
   server that way.  Otherwise, if the server knows it does not support
   Discovery Using Resolver IP Addresses, it MAY refuse connections
   without an SNI instead.

   The destination server MAY use Delegated Credentials [RFC9345] if the
   DNS client advertises its support for Delegated Credentials as
   described in Section 4.1.1 of [RFC9345].  This is valid so long as
   the delegated credential is valid for the same domain name used by
   the referring server.  This is an encouraged practice for servers
   which need to redirect clients to servers owned by other entities, as
   is the case with CDN contracts.

4.1.  Ensuring compatibility

   Redirections MUST only redirect to resolvers which support at least
   the same protocol, address family, port, and TLS minimum versions as
   the referring resolver.  This ensures that redirections do not lead
   clients to resolvers that are not compatible with the client.  In
   addition, servers SHOULD avoid redirecting to servers which will also
   redirect clients unless they are actively coordinating to ensure a
   positive client experience.  See the Deployment Considerations
   section for more details.

4.2.  Dealing with persistent clients

   Servers SHOULD be prepared for clients to not follow the redirection
   immediately as connection failures, other technical issues, or even
   client policy affecting server choice may lead to clients being
   unable to follow the redirection promptly or at all.  Servers who are
   redirecting due to being overloaded MAY respond as they normally
   would to overwhelming traffic.

4.3.  Redirection to servers controlled by third parties

   Server operators ought to consider using delegated credentials
   [RFC9345] when they wish to redirect general clients to other servers
   operated by other entities.  This allows the server operator to avoid
   giving access to their domain's private key to third parties but also
   ensure general clients have a secured, same-origin redirection
   experience.

5.  Deployment Considerations

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5.1.  Large trees of redirections

   It is possible for DNS servers to redirect clients to DNS servers
   which also redirect clients.  Clients which are presented with long
   chains of redirections MAY choose to stop following redirections to
   reduce connection thrashing.  DNS server operators SHOULD deploy
   redirection behavior mindfully to avoid long chains of redirection.

   Servers SHOULD ensure their redirections do not create loops, where
   clients are redirected to a server it already encountered earlier in
   the process.  Clients MAY stop following redirections when they
   detect this, but MAY also take a simpler approach, following only a
   maximum number of redirections.

5.2.  Redirection TTLs

   Servers SHOULD provide sufficiently long TTLs for clients to avoid
   the need to constantly repeat EDSR queries.  Server operators should
   be mindful of redirection chains because unless they collaboratively
   control the TTLs of one another's redirections, redirection chains
   will end up with greatly reduced effective TTLs because the client
   will always use the lowest.

5.3.  Including IP addresses in EDSR responses

   If a recursive resolver does not include additional A/AAAA records
   per Section 5 of [RFC9460], stub resolvers might end up failing the
   redirection if the redirection destination name cannot be resolved.
   Additionally, the recursive resolver SHOULD ensure records
   conntaining the same IP version as the existing connection are
   returned (if the stub is currently connected over IPv4, one or more A
   records SHOULD be included, and if the stub is currently connected
   over IPv6, one or more AAAA records SHOULD be included).

5.4.  Comparison to Discovery Using Resolver Names

   Discovery Using Resolver Names as defined in Section 5 of [RFC9345]
   describes discovery of the encrypted DNS configuration for a server
   using its domain name.  The technical mechanism described there and
   EDSR are the same in the context of on-wire behavior; they differ by
   how clients and servers deploy them.

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   For Discovery Using Resolver Names, servers are free to return
   whatever subset of valid configurations for the domain name provided
   by the client it wishes, such as those it sees as valid for the
   client's apparent geolocation.  In the case of EDSR, servers are
   expected to only return configurations if it wants the client to be
   redirected to another resolver.  Servers implementing EDSR SHOULD
   only answer SVCB queries for its own domain name in the EDSR context
   following its requirements.

   For Discovery Using Resolver Names, clients are querying for
   encrypted DNS configurations available for a given server domain
   name.  EDSR does not restrict clients from issuing these queries
   whenever they want.  However, clients ought to consider that querying
   an encrypted DNS server for its own configuration that supports EDSR
   (which is not inherently discoverable by the client) might only
   return configuration it is ok with the client using to immediately
   reconnect.

6.  Security Considerations

6.1.  Trusting the redirected connection

   EDSR does not provide novel authentication or security mechanisms.
   Redirection is trusted by virtue of the server authentication via PKI
   through TLS [RFC5280].  The DNS stub resolver implementing EDSR
   SHOULD use whatever policies it uses for other TLS connections for
   encrypted DNS traffic to determine if a given TLS cert chain is
   trustworthy before proceeding with EDSR.

   EDSR MUST NOT be used with encrypted DNS protocols that are not based
   on TLS.  This scenario will require future standards work.

   EDSR should not introduce any additional security considerations
   beyond use of the original encrypted resolver prior to redirection.
   Because the original connection was trusted, information sent over it
   about a new connection to use should be as trusted.  However, clients
   that wish to time bound vulnerabilities to attackers who compromise
   the original resolver MAY choose to implement a maximum TTL to honor
   on SVCB records that redirect to other servers.

6.2.  Use with unencrypted DNS

   EDSR MUST NOT be used to redirect unencrypted DNS traffic to any
   other resolver.  This use case is called "designation" and is covered
   by Discovery of Designated Resolvers (DDR) as defined in [RFC9462],
   specifically Section 4: "Discovery Using Resolver IP Addresses".  Not
   following that security guidance opens up a DNS client to malicious
   redirection to an attacker-controlled DNS server.  For more

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   information, see Section 7 of [RFC9462].

   EDSR also MUST NOT be used to redirect encrypted DNS traffic to a
   resolver that advertises support for unencrypted DNS.  This would
   reduce the security posture of the client.  Clients MUST NOT follow
   an encrypted DNS redirection and then send unencrypted DNS traffic to
   the new resolver.

6.3.  Use with DDR discovery from IP addresses

   When a resolver is discovered using DDR's Discovery Using Resolver IP
   Addresses mechanism defined in Section 4 of [RFC9462], the server's
   identity used for TLS purposes is its IP address, not its domain
   name.  This means servers and clients MUST use the original server's
   IP address, not the IP address of the previous server in the event of
   redirection chains, in the SAN field of destination servers to
   validate the redirection.

   The reason for this is due to an attack where the DDR SVCB query
   response is modified by an active attacker to have a different domain
   name in its "dohpath" SVCB key.  When the client uses it to issue the
   EDSR query to the (valid) DDR-designated resolver, it will innocently
   forward the query upstream and return the result.  The result may
   even be DNSSEC signed since it was issued by the valid owner of the
   attacker's domain name.  If this redirection is then followed and
   validated with the attacker's domain name, it will succeed and the
   client will have been maliciously redirected to use an attacker's
   server at the low cost of a port 53 attack without breaking
   encryption or compromising the encrypted DNS server DDR designated.

   There is no harm in using the name of the server for the EDSR query
   so long as the validation of the destination server is performed
   using the original IP address and not the name.  This ensures EDSR
   clients can consistently use the domain name of a server for
   redirection discovery.  Use of the DDR-defined SUDN "resolver.arpa"
   was considered and rejected because this would conflate DDR
   configuration and EDSR configuration by placing them in the same
   zone, using the same DNS record type.

7.  Privacy Considerations

   A client MAY choose to not send other name queries until redirection
   is complete, but there should be no privacy issue with sending
   queries to intermediate resolvers before redirection takes place.
   This is because the intermediate resolvers are considered to be
   appropriate destinations by the client even if the resolver wants to
   substitute another resolver for reasons other than name resolution
   results such as latency optimization or load balancing.

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8.  Data Flow Considerations

8.1.  Data Scope

   EDSR does not result in any additional data being shared by the end
   user, as the DNS queries going to the new resolver were already going
   to go to the original resolver.

8.2.  Data Visibility

   EDSR results in a 1:1 replacement of DNS resolvers used (future
   queries sent to the new resolver will not be sent to the original
   resolver anymore).  This means the number of servers which see any
   given query remain the same.

   This is only true if clients only use one redirected DNS server per
   original DNS server.  If the DNS server offers more than one
   redirection, and the client validates and uses two or more of those
   redirections, then there will be greater data visibility (more
   destinations).  This is however entirely within the client's choice
   following their own policy as a redundancy versus volume of exhausted
   data trade-off.

   EDSR requires the redirection to another server to also use encrypted
   DNS, so no third-party will be introduced to the data flow unless the
   encryption is broken.

8.3.  Data centralization

   EDSR can only increase data centralization if multiple resolver
   operators choose to redirect DNS clients to the same, other DNS
   resolver.  To prevent the reduction of their resolution redundancy,
   DNS clients MAY choose to ignore redirections if they find that they
   point to resolvers they are already configured to use, by a previous
   redirection or some other configuration.

9.  IANA Considerations

   This draft has no IANA considerations.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC9345]  Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
              "Delegated Credentials for TLS and DTLS", RFC 9345,
              DOI 10.17487/RFC9345, July 2023,
              <https://www.rfc-editor.org/rfc/rfc9345>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.

   [RFC9461]  Schwartz, B., "Service Binding Mapping for DNS Servers",
              RFC 9461, DOI 10.17487/RFC9461, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9461>.

   [RFC9462]  Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", RFC 9462,
              DOI 10.17487/RFC9462, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9462>.

10.2.  Informative References

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC9463]  Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
              and T. Jensen, "DHCP and Router Advertisement Options for
              the Discovery of Network-designated Resolvers (DNR)",
              RFC 9463, DOI 10.17487/RFC9463, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9463>.

Appendix A.  Acknowledgments

   The authors would like to thank the following individuals for their
   invaluable feedback to this document: Ben Schwartz, Eric Orth, Erik
   Nygren, Manu Bretelle, Ralph Weber, Ted Hardie, Tommy Pauly, Viktor
   Dukhovni, and Vittorio Bertola.

Authors' Addresses

   J. Todd
   Quad9

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   Email: jtodd@quad9.net

   T. Jensen
   Microsoft
   Email: tojens@microsoft.com

   C. Mosher
   Innate, Inc.
   Email: cmosher@gmail.com

Todd, et al.             Expires 25 January 2025               [Page 13]