Least-Common Scope Communications
draft-mudric-6man-lcs-02

Document Type Active Internet-Draft (individual)
Authors Dusan Mudric  , Alexandre Petrescu 
Last updated 2020-11-14
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6MAN Working Group                                             D. Mudric
Internet-Draft                                                     Ciena
Updates: RFC5014, RFC6724 (if approved)                      A. Petrescu
Intended status: Standards Track                               CEA, LIST
Expires: May 18, 2021                                  November 14, 2020

                   Least-Common Scope Communications
                        draft-mudric-6man-lcs-02

Abstract

   This draft formulates a security problem statement.  The problem
   arises when a Host uses its Global Unicast Address (GUA) to
   communicate with another Host situated on the same link.

   To address this problem, we suggest to select and use addresses of a
   least scope that are common.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 18, 2021.

Copyright Notice

   Copyright (c) 2020 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Least Common Scope Communications . . . . . . . . . . . . . .   3
   4.  LL Address Resolution . . . . . . . . . . . . . . . . . . . .   3
   5.  Sending algorithm with LL Address . . . . . . . . . . . . . .   7
   6.  Other Issues with LL Address Resolution . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  ChangeLog  . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Problem Statement

   Sockets listening on a global addresses are exposed to attacks.
   RFC6724 Rule 8 selects a candidate address with the smallest scope.
   Applications don't always have LL candidate address.  They usually
   have a GUA address.  If GUA is on a local link, an application will
   open a socket using GUA.  To avoid using GUA on the local link, a
   sender needs to find a destination LL address.  Currently SASA
   algorithm (RFC 6724 "Default Address Selection for Internet Protocol
   Version 6 (IPv6)") cannot use the smallest common scope, given
   destination GUA.

   For security reasons, hosts should use an address with the smallest
   scope.  To avoid these attacks, the host should use LL or ULA
   addresses.

   These security reasons, in more detail, are described next.  There is
   a security problem when a Host uses (one of) its Global Unicast
   Address(es) (GUA) to communicate to another Host situated on the same
   link.  The problem appears even if that second Host uses its link-
   local address (LL) for this communication.

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   The problem is that the Host that uses the GUA to actively
   communicate with another Host situated on the same link opens a
   globally reachable entry point in its operating system kernel.  This
   entry point appears when the GUA is assigned to a socket structure.
   Were that address an LL, and not a GUA, that entry would not be
   globally reachable.

   To realize communications between Hosts on the same link, it is
   sufficient to rather use LL addresses on both Hosts.

   When a Host uses a GUA to communicate to another Host situated on the
   same link, it unnecessarily becomes an easy attack target.  The
   attacker might be situated anywhere in the Internet (globally).

3.  Least Common Scope Communications

   It is recommended that a Host that needs to communicate with another
   Host that is situated in a particular scope, to use addresses of same
   scope, or of the least common scope.

   For example, two Hosts situated on the same link should ideally use
   LL addresses to communicate to each other.  An interpretation
   suggests that, given GUA and ULA, a least common 'scope' is the ULA
   scope (even though, formally, both ULA and GUA are of same global
   scope).  But the global unicast addresses (GUAs) should not be used
   for two Hosts on the same link: the global scope is unnecessarily
   large; it unnecessarily opens doors to attacks.

4.  LL Address Resolution

   The operation of resolving an LL address (LL address resolution) is
   to find the link-local address that is assigned to the same interface
   as a GUA (or an ULA).  This operation can be realized in several
   manners.

   In one manner, the pair [GUA or ULA address; LL address] is stored in
   a distributed file such as the Active Directory or the DNS.  The
   resolution operation is to query that file to find the LL address
   that corresponds to a GUA or ULA address.  There are some issues to
   be considered.  For example, typically the LL address is not assigned
   neither by DHCPv6 nor by RA (it is self formed by a Host when the
   interface is put up by using a universally known prefix "fe80::/10")
   then how would DNS get that LL address?  Another example is: how to
   query DNS to request the LL address corresponding to an AAAA entry?
   (it is known how to query DNS to obtain the AAAA of an FQDN, but not
   the LL of an AAAA).

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   In another manner, the operation of resolving a link-local address
   (LL address resolution) is performed within the context of selecting
   source and destination addresses within a Host.  In that context, the
   following steps occur:

      1.  Given multiple destination addresses, the DASA selects GUA and
      ULA destination.  The term 'DASA' designates the Destination
      Address Selection Algorithm.

      2.  The LL address resolution operation is performed for these GUA
      and ULA.

      3.  The GUA and the LL addresses are given as input to the SASA.
      The term 'SASA' stands for Source Address Selection Algorithm.
      The SASA selects LL.

   To facilitate LL communication on the local link, given a destination
   GUA or ULA:

   o  Prior to SASA, a host needs to check if a destination is ON-LINK

   o  for ON-LINK destination, a host needs to resolve the GUA or ULA
      destination address into a destination host LL address,

   o  a socket needs to open a port for the source LL address, and

   o  send packets to the destination LL address.

   If both GUA and ULA destinations are known, and ULA destination is
   not on the link, SASA SHOULD use ULA address.

   For the purposes of this document, Link Local (LL) address resolution
   is the process through which a host determines the Link Local address
   of a neighbor which is on the local subnet, given only neighbor's GUA
   or ULA IPv6 address (this 'address resolution' term is different than
   typical 'ND' term, or than the RFC4861 'address resolution' term
   which resolves an IP address into a MAC address).  LL address
   resolution is performed only on addresses that are determined to be
   on-link and for which the sender does not know the corresponding Link
   Local address.  Once the target LL address is learned, the
   communication sockets use LL addresses and are not exposed to
   security attacks.

   For LL address resolution, 'L' flag is added to NS message.  The
   Target-Address, TA, field in the NS message contains the address of
   the target of the solicitation (e.g., a host GUA or ULA address).
   The 'L' flag is added to Neighbor Solicitation Message, for LL
   address request

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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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |     Type      |     Code      |          Checksum             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |L|                         Reserved                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
         +                                                               +
         |                                                               |
         +                       Target Address                          +
         |                                                               |
         +                                                               +
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |   Options ...
         +-+-+-+-+-+-+-+-+-+-+-+-

         IP Fields:

           Source Address
                      If L bit is set, either LL address assigned to the interface from
                      which this message is sent or (if Duplicate Address
                      Detection is in progress
                      [ADDRCONF rfc4861]) the
                      unspecified address.

           Destination Address
                      Either the solicited-node multicast address
                      corresponding to the target GUA or ULA address, or the target
                      GUA or ULA address.

          ICMP Fields:
            L         Link Local flag.  When set, the L-bit indicates that
                      the sender is requesting Link Local address from the target.

                         Figure 1: NS with 'L' bit

   After receiving the Neighbor Solicitation message, the target returns
   its Link Local address in the Target Link-Local Address Option in a
   unicast Neighbor Advertisement, NA, message.

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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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |     Type      |     Code      |          Checksum             |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |R|S|O|                     Reserved                            |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                                                               |
          +                                                               +
          |                                                               |
          +                       Target Address                          +
          |                                                               |
          +                                                               +
          |                                                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |   Options ...
          +-+-+-+-+-+-+-+-+-+-+-+-

         IP Fields:

           Source Address
                      If NS L bit is set, LL address of the same GUA target
                      interface is provided

         Possible options:

              Target Link-Local address
                      The Link Local address of the same GUA target, the sender of NA.
                      This option MUST be included if NS L bit is set and LL
                      is available.

              Type           4 (Target Link Local address)

              Length         16 bytes

              Link Local Address: e.g. fe80:0:0:0:aa:bb:cc:dd

       Receivers MUST silently ignore this option if they do not recognize it
       and continue processing the message.

                  Figure 2: NA for LL address resolution

   The request for comments number 5014 [RFC5014], which treats about
   socket APIs, needs to be updated to use the given destination GUA or
   ULA addresses for ON-LINK determination, prior to SASA address
   selection; it also needs to be be updated to specify to send packets
   using LL address while talking to ON-LINK destinations.

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5.  Sending algorithm with LL Address

   A sender application can choose to use LL for on-link communication.
   That request can be passed via a socket API to ND.  ND should set NS
   'L' bit to indicate the LL address resolution is required and use of
   LL for the on-link communication, if a destination host returns it.

   If a destination host is listening on GUA only for a particular
   application, and this algorithm is supported, the host should disable
   LL address resolution by not returning LL address in NA.  By default,
   the LL address resolution should be disabled.  Otherwise, a sender
   would send a packet to destination LL address and there is no socket
   listening on that address.  LL address resolution should be enabled
   when all socket APIs are ready to support LL sockets (open one socket
   for GUA and one for LL and after LL address resolution, NA with LL is
   returned, close GUA socket) or all sockets are bound to ANY address.

   The process starts with an application requesting a socket to send a
   packet to GUA destination.  First step is a destination address
   selection and the sequence goes to the LL address resolution, step 4:

      1st: A sending application should have an option to request LL vs.
      GUA communication, when opening a socket to GUA destination, that
      might be on a local link.  Socket API should have this option and
      use it to initiate LL address resolution.

      2nd: Host should choose destination address, if multiple GUA and
      ULA are provided

      3rd: Host should choose a source address, for the selected
      destination address

      4th: Host should choose a next hop, and outgoing interface, based
      on the source address prefix

      5th: If a destination is on-link, the host should resolve
      destination GUA into destination LL.  Step 5 is further broken
      down into:

         5.1st: Sender creates a neighbour cache entry for GUA.

         5.2nd: Sender sends NS, with L bit set, to GUA.

         5.3rd: Sender receives NA with link-layer and LL addresses

         5.4th: Sender updates GUA cache entry with the link-layer
         address

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         5.5th: Sender creates a neighbour cache entry for destination
         LL address and sets the destination link-layer address of the
         destination host

         6th: Sender transmits a packet to link-layer address of the
         destination host, using destination host LL address as IPv6
         packet destination address

         7th: Application sending to GUA should obtain the SASA address
         (which is now LL address) for the further negotiations (e.g.
         SIP needs to negotiate media addresses by sending re-INVITE).

         8th: Sender closes the socket listening on GUA and opens a
         socket listening on LL.

6.  Other Issues with LL Address Resolution

   If the Host 'switches' the destination address of an ongoing flow,
   between the GUA and the LL, there might interruptions in
   communications.  The 'switching' behaviour depends on the
   application.  Some applications (e.g. a particular application using
   the SIP protocol) the destination address is selected prior to
   opening the socket dedicated to streaming the media data.  In such an
   application, a hard outage (e.g. interface down), might involve the
   creation of a new socket, and thus interruptions in media streaming.
   The question of maintaining an ongoing communication upon 'switching'
   between a GUA and an LL destination address is valid, for certain
   applications.

   Multiple DNS aspects, for the resolution operation.  Which LL address
   corresponds to a GUA?. How would DNS get that LL address?

7.  Security Considerations

   Security

8.  IANA Considerations

   IANA

9.  Contributors

   Contributors.

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10.  Acknowledgements

   Mark Smith, Eduard Vasilenko.

11.  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/info/rfc2119>.

   [RFC5014]  Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
              Socket API for Source Address Selection", RFC 5014,
              DOI 10.17487/RFC5014, September 2007,
              <https://www.rfc-editor.org/info/rfc5014>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

Appendix A.  ChangeLog

   The changes are listed in reverse chronological order, most recent
   changes appearing at the top of the list.

   -00: initial version, with Dusan's comments.

Authors' Addresses

   Dusan Mudric
   Ciena

             ,

               Canada

   Phone:
             +1-613-670-2425

   Email:
             dmudric@ciena.com

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   Alexandre Petrescu
   CEA, LIST

               CEA Saclay

               Gif-sur-Yvette
             ,
               Ile-de-France

               91190

               France

   Phone:
             +33169089223

   Email:
             Alexandre.Petrescu@cea.fr

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