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Common Architecture Label IPv6 Security Option (CALIPSO)

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 5570.
Authors Randall Atkinson , Michael StJohns
Last updated 2020-01-21 (Latest revision 2009-03-06)
RFC stream Internet Engineering Task Force (IETF)
Stream WG state (None)
Document shepherd (None)
IESG IESG state RFC 5570 (Informational)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Tim Polk
Send notices to,
Network Working Group                                        M. StJohns
Internet-Draft                            R. Atkinson, Extreme Networks
draft-stjohns-sipso-11.txt                G. Thomas, US Dept of Defense
Expires: 6 SEP 2009                                        6 March 2009
Intended Status: Informational

                       Common Architecture Label
                          IPv6 Security Option

Status of this Memo

   This is an Internet-Draft.

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

   Internet-Drafts are working documents of the Internet
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     This document describes an optional method for encoding
   explicit packet Sensitivity Labels on IPv6 packets.  It is
   intended for use only within Multi-Level secure (MLS) networking
   environments that are both trusted and trustworthy.

StJohns Et Alia                                                 [Page 1]
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Table of Contents

   1.    INTRODUCTION ........................................3
   1.1.  History .............................................3
   1.2.  Intent ..............................................5
   1.3.  Deployment Examples..................................6
   2.    DEFINITIONS .........................................8
   2.1.  Domain of Interpretation.............................8
   2.2.  Sensitivity Level ...................................9
   2.3.  Compartment ........................................10
   2.4.  Releasability ......................................10
   2.5.  Sensitivity Label ..................................15
   2.6.  Import .............................................17
   2.7   Export .............................................17
   2.8.  End System .........................................18
   2.9.  Intermediate System ................................18
   2.10. System Security Policy .............................18
   3.    ARCHITECTURE........................................18
   4.    DEFAULTS............................................24
   5.    FORMAT..............................................25
   5.1   Option Format ......................................26
   5.2   Packet Word Alignment...............................30
   6.    USAGE...............................................31
   6.1   Sensitivity Label Comparisons.......................31
   6.2   End System Processing ..............................34
   6.3   Intermediate System Processing......................37
   7.1   Intermediate Systems................................42
   7.2   End Systems.........................................43
   7.3   Upper-Layer Protocols...............................43
   8.    SECURITY CONSIDERATIONS.............................47
   9.    IANA CONSIDERATIONS.................................49

StJohns Et Alia                                                 [Page 2]
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        The original IPv4 specification in RFC-791 includes an
   option for labeling the sensitivity of IP packets.  That option
   was revised by RFC-1038 and later by RFC-1108. [RFC-791]
   [RFC-1038] [RFC-1108] Although the IETF later deprecated
   RFC-1108, that IPv4 option continues to be in active use
   within a number of closed Multi-Level Secure (MLS) IP networks.

       One or another IP Sensitivity Label option has been in
   limited deployment for about two decades, most usually in
   governmental or military internal networks.  There are also some
   commercial sector deployments, where corporate security policies
   require Mandatory Access Controls be applied to sensitive data.
   For example, some banks use MLS technology to compartment
   information known to their investment banking staff, so that
   their trading staff is unaware of that information.  This option,
   like its IPv4 predecessors, is nearly always deployed within
   private internetworks, disconnected from the global Internet.
   This document specifies the explicit packet labeling extensions
   for IPv6 packets.

1.1 History

      This document is a direct descendent of RFC-1038 and RFC-1108
   and is a close cousin to the work done in the Commercial IP
   Security Option (CIPSO) Working Group of the Trusted Systems
   Interoperability Group (TSIG).[FIPS-188] The IP Security option
   defined by RFC-1038 was designed with one specific purpose in
   mind: to support the fielding of an IPv4 packet encryption device
   called a BLACKER.[RFC-1038] Because of this, the definitions and
   assumptions in those documents were necessarily focused on the US
   Department of Defense and the BLACKER device.  Today, IP packet
   Sensitivity Labeling is most commonly deployed within Multi-Level
   Secure (MLS) environments, often composed of Compartmented Mode
   Workstations (CMWs) connected via a Local Area Network (LAN).
   So the mechanism defined here is accordingly more general than
   either RFC-1038 or RFC-1108 were.

      Also, the deployment of Compartmented Mode Workstations ran
   into operational constraints caused by the limited, and
   relatively small, space available for IPv4 options.  This caused
   one non-IETF specification for IPv4 packet labeling to have a
   large number of sub-options.  A very unfortunate side-effect of
   having sub-options within an IPv4 label option was that it became

StJohns Et Alia                                                 [Page 3]
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   much more challenging to implement Intermediate System support
   for Mandatory Access Controls (e.g. in a router or MLS guard
   system) and still be able to forward traffic at, or near,

      In the last decade or so, typical Ethernet link speeds have
   changed from 10 Mbps half-duplex to 1 Gbps full-duplex.  The 10
   Gbps full-duplex Ethernet standard is widely available today in
   routers, Ethernet switches, and even in some servers.  The IEEE
   is actively developing standards for both 40 Gbps Ethernet and
   100 Gbps Ethernet as of this writing. Forwarding at those speeds
   typically requires support from ASICs; supporting more complex
   packet formats usually require significantly more gates than
   supporting simpler packet formats.  So the pressure to have a
   single simple option format has only increased in the past
   decade, and is only going to increase in future.

      When IPv6 was initially being developed, it was anticipated
   that the availability of IP Security, in particular the
   Encapsulating Security Payload (ESP) and the IP Authentication
   Header (AH), would obviate the need for explicit packet
   Sensitivity Labels with IPv6. [RFC-1825, IPSEC, AH, ESP]
   For MLS IPv6 deployments where the use of AH or ESP is
   practical, use of AH and/or ESP is recommended.

      However, some applications (e.g. distributed file systems),
   most often those not designed for use with Compartmented Mode
   Workstations or other Multi-Level Secure (MLS) computers,
   multiplex different transactions at different sensitivity levels
   and/or with different privileges over a single IP communications
   session (e.g. with the User Datagram Protocol).  In order to
   maintain data Sensitivity Labeling for such applications, in
   order to be able to implement routing and Mandatory Access
   Control decisions in routers and guards on a per-IP-packet basis,
   and for other reasons, there is a need to have a mechanism for
   explicitly labeling the sensitivity information for each IPv6

      Existing Layer-3 Virtual Private Network technology can't
   solve the set of issues addressed by this specification, for
   several independent reasons.  First, in a typical deployment,
   many labelled packets will flow from a MLS host through some
   set of networks to a receiving MLS host.  The received
   per-packet label is used by the receiving MLS host to determine
   which Sensitivity Label to associate with the user data carried
   in the packet.  Existing Layer-3 VPN specifications do not
   specify any mechanism to carry a Sensitivity Label.  Second,
   existing Layer-3 VPN technologies are not implemented in any

StJohns Et Alia                                                 [Page 4]
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   MLS hosts, nor in typical single-level host operating systems,
   but instead typically are only implemented in routers.  Adding
   a Layer-3 VPN implementation to the networking stack of a MLS
   host would be a great deal more work than adding this IPv6
   option to that same MLS host.  Third, existing Layer-3 VPN
   specifications do not support the use of Sensitivity Labels
   to select a VPN to use in carrying a packet, which function
   is essential if one wanted to obviate this IPv6 option.
   Substantial new standards development, along with significant
   new implementation work in hosts, would be required before
   a Layer-3 VPN approach to these issues could be used.
   Developing such specifications, and then implementing them
   in MLS systems, would need substantially greater effort than
   simply implementing this IPv6 label option in an MLS host
   (or in a label-aware router).  Further, both the MLS user
   community and the MLS implementer community prefer the
   approach defined in this specification.

1.2.  Intent & Applicability

      Nothing in this document applies to any system that does
   not claim to implement this document.

      This document describes a generic way of labeling IPv6
   datagrams to reflect their particular sensitivity.  Provision
   is made for separating data based on domain of interpretation
   (e.g. an agency, a country, an alliance, or a coalition), the
   relative sensitivity (i.e. sensitivity levels), and need-to-know
   or formal access programs (i.e. compartments or categories).

      A commonly used method of encoding Releasabilities as if they
   were Compartments is also described.  This usage does not have
   precisely the same semantics as some formal Releasability
   policies, but existing Multi-Level Secure operating systems do
   not contain operating system support for releasabilities as a
   separate concept from compartments.  The semantics for this sort
   of Releasability encoding is close to the formal policies and has
   been deployed by a number of different organisations for at least
   a decade now.

      In particular, the authors believe that this mechanism is
   suitable for deployment in UN peace-keeping operations, in NATO
   or other coalition operations, in all current US Government MLS
   environments, and for deployment in other similar commercial
   or governmental environments.  This option would not normally
   ever be visible in an IP packet on the global public Internet.

StJohns Et Alia                                                 [Page 5]
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      Because of the unusually severe adverse consequences
   (e.g. loss of life, loss of very large sums of money) likely
   if a packet labelled with this IPv6 Option were to escape
   onto the global public Internet, organisations deploying
   this mechanism are unusually strongly incented to configure
   security controls to prevent labelled packets from ever
   appearing on the global public Internet.  Indeed, a primary
   purpose of this mechanism is to enable deployment of
   Mandatory Access Controls for IPv6 packets.

     However, to ensure interoperability of both hosts and
   intermediate systems within such a labelled deployment of IPv6,
   it is essential to have an open specification for this option.

      This option is NOT designed to be an all-purpose label option
   and specifically does not include support for generic Domain Type
   Enforcement (DTE) mechanisms.  If such a DTE label option is
   desired, it ought to be separately specified and have its own
   (i.e. different) IPv6 option number.

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

1.3 Deployment Examples

      Two deployment scenarios for IP packet sensitivity labels
   are most common.  We should first note that in typical
   deployments, all people having access to an unencrypted link
   are cleared for all unencrypted information traversing that
   link.  Also, MLS system administrators normally have previously
   been cleared to see all of the information processed or stored
   by that MLS system.  This specification does not seek to
   eliminate all potential covert channels relating to this
   IPv6 option.

      In the first scenario, all the connected nodes in a given
   private internetwork are trusted systems that have Multi-Level
   Secure (MLS) operating systems, such as Compartmented Mode
   Workstations (CMWs), that support per-packet sensitivity labels.
   [TCSEC] [TNI] [CMW] [DOD MLOS PP] In this type of deployment,
   all IP packets carried within the private internetwork are
   labelled, the IP routers apply mandatory access controls (MAC)
   based on the packet labels and the sensitivity ranges configured
   into the routers, all hosts include packet sensitivity labels

StJohns Et Alia                                                 [Page 6]
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   in each originated packet, and all hosts apply Mandatory Access
   Controls to each received packet.  Packets received by a router
   or host that have a sensitivity label outside the permitted
   range for the receiving interface (or, in the case of a router,
   outside the permitted range for either the incoming or the
   outgoing interface) are dropped because they violate the
   MAC policy.

      The second scenario is a variation of the first where
   hosts with non-MLS operating systems are present on certain
   subnetworks of the private internetwork.  By definition,
   these non-MLS hosts operate in "system high" mode.
   In "system high" mode, all information on the system is
   considered to have the sensitivity of the most sensitive
   data on the system.  If a system happens to contain data
   only at one sensitivity level, this would also be an
   example of "system high" operation.  In this scenario,
   each subnetwork that contains any single-level hosts has
   one single "default" Sensitivity Label that applies
   to all single-level systems on that IP subnetwork.
   Because those non-MLS hosts are unable to create packets
   containing sensitivity labels and are also unable to apply
   MAC enforcement on received packets, security gateways
   (which might, for example, be label-aware IP routers)
   connected to such subnetworks need to insert sensitivity
   labels to packets originated by the system-high hosts that
   are to be forwarded off subnet.  While the CALIPSO IPv6 option
   is marked as "ignore if unrecognised", there are some deployed
   IPv6 hosts with bugs.  Users can't fix these operating system
   bugs; some users need to be able to integrate their existing
   IPv6 single-level hosts to have a useful overall MLS
   deployment.  So, for packets destined for IP subnetworks
   containing single-level hosts, those last-hop security
   gateways also apply Mandatory Access Controls (MAC) and
   then either drop (if the packet is not permitted on that
   destination subnet) xor remove sensitivity labels and
   forward packets onto those system-high subnetworks
   (if the packet is permitted on that destination subnetwork).

      The authors are not aware of any existing MLS network
   deployments that use a commercial NAT/NAPT or any other
   commercial "middlebox" device.  For example, NAT boxes
   aren't used, unlike practices in some segments of the
   public Internet.

     Similarly, the authors are not aware of any existing MLS
   network deployments that use a commercial firewall.  MLS
   networks normally are both physically and electronically

StJohns Et Alia                                                 [Page 7]
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   isolated from the global Internet, so operators of MLS
   networks are not concerned about external penetration
   (e.g. by worms, viruses, or such like).  Similarly, all users
   of the MLS network have been cleared using some process
   specific to that organisation, and hence are believe to be
   trustworthy.  In a typical deployment, all computers
   connected to the MLS network are in a physically secure room
   or building (e.g. protected by guards with guns).  Electronic
   equipment that enters such a space typically does not leave.
   Items such as USB memory sticks are generally not permitted;
   In fact, often the USB ports on MLS computers have been
   removed or otherwise made inoperable to prevent people
   from adding or removing information.

      Also, for security reasons, content transformation
   in the middle of an MLS network is widely considered
   undesirable, and so is not typically undertaken.
   Hypothetically, if such content transformation were
   undertaken, it would be performed by a certified MLS
   system that has been suitably accredited for that
   particular purpose in that particular deployment.


        This section defines several terms that are important to
   understanding and correctly implementing this specification.
   Because of historical variations in terminology in different
   user communities, several terms have defined synonyms.

        The verb "dominate" is used in this document to descibe
   comparison of two Sensitivity Labels within a given Domain of
   Interpretation.  Sensitivity Label A dominates Sensitivity Label
   B if the Sensitivity Level of A is greater than or equal to the
   Sensitivity Level of B AND the Compartment Set of A is a superset
   (proper or improper) of the Compartment Set of B.  This term has
   been used in multi-level security circiles with this meaning for
   at least two decades.

2.1.  Domain of Interpretation

        A Domain of Interpretation (DOI) is a shorthand way of
   identifying the use of a particular labeling, classification,
   and handling system with respect to data, the computers and
   people who process it, and the networks that carry it.  The

StJohns Et Alia                                                 [Page 8]
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   DOI policies, combined with a particular Sensitivity Label
   (which is defined to have meaning within that DOI) applied
   to a datum or collection of data, dictates which systems,
   and ultimately which persons may receive that data.

        In other words, a label of "SECRET" by itself is not
   meaningful; one also must know that the document or data
   belongs to some specific organisation (e.g. US Dept of Defense,
   US Dept of Energy, UK Ministry of Defence, NATO, UN,
   a specific commercial firm) before one can decide on who
   is allowed to receive the data.

        A CALIPSO DOI is an opaque identifier that is used as a
   pointer to a particular set of policies which define the
   Sensitivity Levels and Compartments present within the DOI, and
   by inference, to the "real world" (e.g. used on paper documents)
   equivalent labels (See "Sensitivity Label" below).  Registering
   or defining a set of real world security policies as a CALIPSO
   DOI results in a standard way of labeling IP data originating
   from End Systems "accredited" or "approved" to operate within
   that DOI and the constraints of those security policies.  For
   example, if one did this for the US Department of Defense, one
   would list all the acceptable labels such as "Secret" and "Top
   Secret", and one would link the CALIPSO DOI to the DOD 5200.28
   and DOD 5200.1R documents which define how to mark and protect
   data with the US Department of Defense (DoD).  [DoD 5200.28,
   DoD 5200.1-R]

      The scope of the DOI is dependent on the organization
   creating it.  In some cases, the creator of the DOI might not
   be identical to a given user of the DOI.  For example,
   a multi-national organisation (e.g. NATO) might create a DOI,
   while a given member nation or organisation (e.g. UK MoD) might
   be using that multi-national DOI (posssibly along with other
   DOIs created by others) within its private networks.  To provide
   a different example, the US might establish a DOI with specific
   meanings which correspond to the normal way it labels classified
   documents and which would apply primarily to the US DOD, but
   those specific meanings might also apply to other associated
   agencies.  A company or other organisation also might establish
   a DOI which applies only to itself.

      NOTE WELL: A CALIPSO Domain of Interpretation is different
   from, and is disjoint from, an ISAKMP/IKE Domain of
   Interpretation.  It is important not to confuse the two
   different concepts, even though the terms might superficially
   appear similar.

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2.2.  Sensitivity Level

      A Sensitivity Level represents a mandatory separation of data
   based on relative sensitivity.  Sensitivity Levels ALWAYS have a
   specific ordering within a DOI.  Clearance to access a specific
   level of data also implies access to all levels whose sensitivity
   is less than that level.  For example, if the A, B, and C are
   levels, and A is more sensitive than B which is in turn more
   sensitive than C (A > B > C), access to data at the B level
   implies access to C as well. As an example, common UK terms for
   a Sensitivity Level include (from low to high) "Unclassified",
   "Restricted", "Confidential", "Secret", and "Most Secret".

   NOTE WELL: A Sensitivity Level is only one component of a
   Sensitivity Label.  It is important not to confuse the two terms.
   The term "Sensitivity Level" has the same meaning as the term
   "Security Level".

2.3.  Compartment

       A Compartment represents a mandatory segregation of data
   based on formal information categories, formal information
   compartments, or formal access programs for specific types of
   data.  For example, a small startup company creates "Finance"
   and "R&D" compartments to protect data critical to its success
   -- only employees with a specific need to know (e.g. the
   accountants and controller for "Finance", specific engineers for
   "R&D") are given access to each compartment.  Each Compartment is
   separate and distinct.  Access to one Compartment does not imply
   access to any other Compartment.  Data may be protected in
   multiple compartments (e.g. "Finance" data about a new "R&D"
   project) at the same time, in which case access to ALL of those
   compartments is required to access the data.  Employees only
   possessing clearance for a given sensitivity level (i.e. without
   having clearance for any specific compartments at that
   sensitivity level) do not have access to any data classified in
   any compartments (e.g. SECRET FINANCE dominates SECRET).

   NOTE WELL: The term "category" has the same meaning as
   "compartment".  Some user communities have used the term
   "category", while other user communities have used the term
   "compartment", but the terms have identical meaning.

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2.4   Releasability

      A Releasability represents a mandatory segregation of data,
   based on a formal decision to release information to others.

      Historically, most MLS deployments handled Releasability
   as if it were an inverted Compartment.  Strictly speaking, this
   provides slightly different semantics and behaviour than a paper
   marked with the same Releasabilities would obtain, because the
   formal semantics of Compartments are different from the formal
   semantics of Releasability.  The differences in behaviour are
   discussed in more detail later in this sub-section.

      In practice, for some years now some relatively large
   MLS deployments have been encoding Releasabilities as if they
   were inverted Compartments.  The results have been tolerable
   and those deployments are generally considered successful by
   their respective user communities.  This description is
   consistent with these MLS deployments, so has significant
   operational experience behind it.

2.4.1 Releasability Conceptual Example

      For example, two companies (ABC and XYZ) are engaging in a
   technical alliance.  ABC labels all information present within
   its enterprise that is to be shared as part of the alliance as

      However, unlike the compartment example above, COMPANY
   means that XYZ employees granted a COMPANY CONFIDENTIAL
   REL XYZ clearance can only access releasable material,
   while ABC employees with a COMPANY CONFIDENTIAL clearance
   can access all information.

      If REL XYZ were managed as a compartment, then users
   granted a COMPANY CONFIDENTIAL REL XYZ clearance would have
   access to all of ABC's COMPANY CONFIDENTIAL material, which
   is undesirable.

      Releasabilities can be combined (e.g. COMPANY
   CONFIDENTIAL REL XYZ/ABLE).  In this case, users possessing
   a clearance of either COMPANY CONFIDENTIAL, COMPANY

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2.4.2  Releasability Encoding

      Individual bits in this option's Compartment Bitmap field
   MAY be used to encode "releaseability" information.  The
   process for making this work properly is described below.

      This scheme is carefully designed so that intermediate
   systems need not know whether a given bit in the Compartment
   Bitmap field represents a compartment or a releasability.
   All that an intermediate system needs to do is apply the usual
   comparison (described elsewhere) to determine whether a packet's
   label is in-range for an interface or not.  This simplifies
   both the configuration and implementation of a label-aware
   intermediate system.

      Unlike bits that represent compartments, bits that
   represent a releasability are "active low".

      If a given releasability bit in the Compartment Bitmap
   field is "0", the information may be released to that community.
   If the compartment bit is "1", the information may not be
   released to that community.

      Only administrative interfaces used to present or construct
   binary labels in human-readable form need to understand the
   distinction between releasability bits and non-releasability
   bits.  Implementers are encouraged to describe Releasability
   encoding in the documentation supplied to users of systems
   that implement this specification.

2.4.2  Releasability Encoding Examples

   For objects, such as IP packets, let bits 0-3 of the Compartment
   Bitmap field be dedicated to controlling releasability to the
   communities A, B, C, and D, respectively.

   Example 1,  Not releasable to any community:
               This is usually how handling restrictions
               such as "No Foreigners (NO FORN)" are encoded.
                   ABCD == 1111

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   Example 2,  Releasable only to community A and community C:
                   ABCD == 0101

   Example 3,  Releasable only to community B:
                   ABCD == 1011

   Example 4,  Releasable to communities A,B,C, & D:
                   ABCD == 0000

   For subjects, such as clearances of users, the same bit encodings
   are used for releasabilities as are used for objects (see above).

   Example 1, clearance not belonging to any community:
              This user can see information belonging
              to any releasability community, since s/he
              is not in any releasability community.
                   ABCD = 1111

   Example 2, clearance belonging to community A and C:
              This user can only see Releasable AC information,
              and cannot see Releasable A information.
                   ABCD == 0101

   Example 3, clearance belonging to community B:
              This user can only see Releasable B information.
                   ABCD == 1011

   Example 4, clearance belongs to communities A,B,C, & D:
              This user can only see Releasable ABCD information,
              and cannot (for example) see Releasable AB or
              Releasable BD information.
                   ABCD == 0000

   Now we consider example comparisons for an IP router that is
   enforcing MAC by using CALIPSO labels on some interface:

   Let the MINIMUM label for that router interface be:

   Therefore, this interface has a minimum Releasability
   of 0101.

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   Let the MAXIMUM label for that router interface be:

   Therefore, this interface has a maximum Releasability
   of 1111.

   For the range comparisons, the bit values for the current
   packet need to be "greater than or equal to" the minimum
   value for the interface AND also the bit values for the
   current packet need to be "less than or equal to" the
   maximum value for the interface, just as with compartment
   comparisons.  The inverted encoding scheme outlined above
   ensures that the proper results occur.

   Consider a packet with label CONFIDENTIAL RELEASABLE AC:
       1) Sensitivity Level comparison:
           so the Sensitivity Level is "within range"
           for that router interface.
       2) Compartment bitmap comparison
           The test is [(0101 >= 0101) AND (0101 <= 1111)],
           so the Compartment bitmap is "within range"
           for that router interface.

   Consider a packet with label CONFIDENTIAL RELEASABLE ABCD:
       1) Sensitivity Label comparison:
           so the Sensitivity Level is "within range"
           for that router interface.
       2) Compartment bitmap comparison
           The test is [(0000 >= 0101) AND (0000 <= 1111)],
           so the Compartment Bitmap is NOT "within range"
           for that router interface.

   Consider a packet with label SECRET NOT RELEASABLE:
       1) Sensitivity Label comparison:
          so the Sensitivity Level is "within range"
          for that router interface.
       2) Compartment bitmap comparison:
          The test is [(1111 >= 0101) AND (1111 <= 1111)],
          so the Compartment bitmap is "within range"
          for that router interface.

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2.4.3  Limitations of this Releasability Approach

      For example, If one considers a person "Jane Doe" who is a
   member of two Releasability communities (A and also B), she is
   permitted to see a paper document that is marked "Releasable A",
   "Releasable B", OR "Releasable AB" -- provided that her Clearance
   and Compartments are in-range for the Sensitivity Level and
   Compartments (respectively) of the paper document.

      Now, let us consider an equivalent electronic example
   implemented and deployed as outlined above.  In this
   we consider 2 Releasability communities (A and B).
   Those bits will be set to 00 for the electronic user-id
   used by user "Jane Doe".

     However, the electronic Releasability approach above will
   ONLY permit her to see information marked as "Releasable AB".
   The above electronic approach will deny her the ability
   to read documents marked "Releasable A" or "Releasable B".
   This is because "Releasable A" is encoded as 01, "Releasable B"
   is encoded as 10, while "Releasable AB" is encoded as 00.
   If one looks at the compartment dominance computation,
   00 dominates 00, but 00 does NOT dominate 01, and 00 also
   does NOT dominate 10.

     Users report that the current situation is tolerable,
   but not ideal, and can lead to various operational complexities.

     Several deployments work around this limitation by assigning
   an electronic user several parallel clearances.  Referring
   to the (fictitious) example above, the user "Jane Doe" might
   have one clearance without any Releasability, another
   separate clearance with Releasability A, and a third separate
   clearance with Releasability B.  While this has implications
   (e.g. a need to be able to associate multiple separate parallel
   clearances with a single user-id) for implementers of MLS
   systems, this specification cannot (and does not) levy any
   requirements that an implementation be able able to associate
   multiple clearances with each given user-id because that
   level of detail is beyond the scope of an IP labelling option.

     Separating the Releasability bits into a separate bitmap
   within the CALIPSO option was seriously considered.  However,
   existing MLS implementations lack operating system support
   for Releasability.  So even if CALIPSO had a separate bitmap
   field, those bits would have been mapped to Compartment
   bits by the sending/receiving nodes, so the operational

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   results would not have been different than those described

     Several MLS network deployments connect MLS hosts both to
   a labelled national network and also to a labelled coalition
   network simultaneously.  Depending on whether the data is
   labelled according to national rules or according to coalition
   rules, the set of Releasability marks will vary.  Some choices
   are likely to lead to more (or fewer) incorrect Releasability
   decisions (although the results of the above Releasability
   encodings are believed to be fail-safe).

2.5.  Sensitivity Label

        A Sensitivity Label is a quadruple consisting of a DOI,
   a Sensitivity Level, a Compartment Set, and a Releasability
   Set.  The Compartment Set may be the empty set if and only
   if no compartments apply.  A Releasability Set may be the
   empty set if and only if no releasabilities apply.  A DOI
   used within an end system may be implicit or explicit
   depending on its use.  CALIPSO Sensitivity Labels always
   have an explicit DOI.  A CALIPSO Sensitivity Label consists
   of a Sensitivity Label in a particular format (defined
   below).  A CALIPSO Sensitivity Label ALWAYS contains an
   explicit DOI value.  In a CALIPSO Sensitivity Label, the
   Compartment Bitmap field is used to encode both the logical
   Compartment Set and also the logical Releasability Set.

      Hosts using operating systems with MLS capabilities
   that also implement IPv6 normally will be able to include
   CALIPSO labels in packets they originate and will be able to
   enforce MAC policy on the CALIPSO labels in any packets they

          Hosts using an operating system that lacks
   Multi-Level Secure capabilities operate in "system high"
   mode.  This means that all data on the system is considered
   to have the Sensitivity Label of the most sensitive data on
   the system.  Such a system normally is neither capable of
   including CALIPSO labels in packets that it originates, nor
   of enforcing CALIPSO labels in packets that it receives.

      Note Well: The term "Security Marking" has the same
   meaning as "Sensitivity Label".

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2.5.1 Sensitivity Label Comparison

        Two Sensitivity Labels (A and B) can be compared. Indeed,
   Sensitivity Labels exist primarily so they can be compared as
   part of a Mandatory Access Control decision.  Comparison is
   critical to determining if a subject (a person, network, etc.)
   operating at one Sensitivity Label (A) should be allowed to
   access an object (file, packet,route, etc) classified at another
   Sensitivity Label (B).  The comparison of two labels (A and B)
   can return one (and only one) of the following results:

     1) A dominates B (e.g. A=SECRET, B=UNCLASSIFIED);
        A can read B,
     2) B dominates A (e.g. A=UNCLASSIFIED, B=SECRET);
        A cannot access B,
     3) A equals B (e.g. A=SECRET, B=SECRET);
        A can read/write B,
     4) A is incomparable to B (e.g. A=SECRET R&D, B=SECRET FINANCE);
        A cannot access B, and also, B cannot access A.

        By definition, if A and B are members of different DOIs,
   the result of comparison is always incomparable.  It is possible
   to overcome this if and only if A and/or B can be translated into
   some common DOI, such that the labels are then interpretable.

2.5.2  Range

        A range is a pair of Sensitivity Labels which indicate
   both a minimum and a maximum acceptable Sensitivity Label for
   objects compared against it.  A range is usually expressed as
   "<minimum> : <maximum>" and always has the property that the
   maximum Sensitivity Label dominates the minimum Sensitivity
   Label.  In turn, this requires that the two Sensitivity Labels
   MUST be comparable.

      A range where <minimum> equals <maximum> may be expressed
   simply as "<minimum>"; in this case, the only acceptable
   Sensitivity Label is <minimum>.

2.6.  Import

        The act of receiving a datagram and translating the
   CALIPSO Sensitivity Label of that packet into the appropriate
   internal (i.e. End System specific) Sensitivity Label.

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2.7.  Export

        The act of selecting an appropriate DOI for an outbound
   datagram, translating the internal (End System specific) label
   into an CALIPSO Sensitivity Label based on that DOI, and sending
   the datagram.  The selection of the appropriate DOI may be based
   on many factors including, but not necessarily limited to:

        Source Port
        Destination Port
        Transport Protocol
        Application Protocol
        Application Information
        End System
        Sending Interface
        System Implicit/Default DOI

      Regardless of the DOI selected, the Sensitivity Label of the
   outbound datagram must be consistent with the security policy
   monitor of the originating system and also with the DOI
   definition used by all other devices cognisant of that DOI.

2.8.  End System

      An End System is a host or router from which a datagram
   originates or to which a datagram is ultimately delivered.
   The IPv6 community often uses the term Node, which includes
   both routers and end systems.  This document sometimes uses
   the term "host" to refer to an end system.

2.9.  Intermediate System

      An Intermediate System (IS) is a node that receives and
   transmits a particular datagram without being either the source
   or destination of that datagram.  This document sometimes uses
   the term "router" or "guard" to refer to an intermediate system.

      So an IPv6 router is one example of an intermediate system.
   A firewall or security guard device that applies security
   policies and forwards IPv6 packets that comply with those
   security policies is another example of an intermediate system.

      An intermediate system may handle ("forward") a datagram
   destined for some other node without necessarily importing or
   exporting the datagram to/from itself.

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      NOTE WELL: Any given system can be both an end system and an
   intermediate system -- which role the system assumes at any given
   time depends on the address(es) of the datagram being considered
   and the address(es) associated with that system.

2.10 System Security Policy

      A System Security Policy (SSP) consists of a Sensitivity Label
   and the organizational security policies associated with content
   labelled with a given security policy.  The SSP acts as a bridge
   between how the organization's Mandatory Access Control (MAC)
   policy is stated and managed and how the network implements that
   policy.  Typically, the SSP is a document created by the
   Information Security administrator of the site or organisation
   covered by that SSP.


      This document describes a convention for labeling an IPv6
   datagram within a particular system security policy.  The labels
   are designed for use within a Mandatory Access Control (MAC)
   system.  A real world example is the security classification
   system in use within the UK Government. Some data held by the
   government is "classified", and is therefore restricted by law
   to those people who have the appropriate "clearances".

      Commercial examples of information labelling schemes also
   exist.[CW87] For example, one global electrical equipment company
   has a formal security policy that defines 6 different Sensitivity
   Levels for its internal data, ranging from "Class 1" to "Class 6"
   information.  Some financial institutions use multiple
   compartments to restrict access to certain information
   (e.g. "mergers & acquisitions", "trading") to those working
   directly on those projects and to deny access to other groups
   within the company (e.g. equity trading).  A CALIPSO Sensitivity
   Label is the network instantiation of a particular information
   security policy, and the policy's related labels,
   classifications, compartments, and releasabilities.

         Some years ago, the Mandatory Access Control (MAC) policy
   for US Government classified information was specified formally
   in mathematical notation.[BL73] As it happens, many other
   organisations or governments have the same basic Mandatory Access
   Control (MAC) policy for information with differing ("vertical")
   Sensitivity Levels.  This document builds upon the formal
   definitions of Bell-LaPadula.[BL73] There are two basic
   principles "no write down" and "no read up".

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      The first rule means that an entity having minimum Sensitivity
   Level X must not be able to write information that is marked with
   a Sensitivity Level below X.  The second rule means that an
   entity having maximum Sensitivity Level X must not be able to
   read information having a Sensitivity Level above X.  In a normal
   deployment, information downgrading ("write down") must not occur
   automatically, and is permitted if and only if a person with
   appropriate "downgrade" privilege manually verifies the
   information is permitted to be downgraded before s/he manually
   re-labels (i.e. "downgrades") the information.  Subsequent to the
   original work by Bell and LaPadula in this area, this formal
   model was extended to also support ("horizontal") Compartments of

      This document extends Bell-LaPadula to accommodate the notion
   of separate Domains of Interpretation (DOI). [BL73] Each DOI
   constitutes a single comparable domain of Sensitivity Labels
   as stated by Bell-LaPadula.  Sensitivity Labels from different
   domains cannot be directly compared using Bell-LaPadula semantics.

      This document is focused on providing standards for encoding
   Sensitivity Labels in packets, as well as certain standards for
   how these labels are to be interpreted and enforced at the IP
   layer.  This document recognises that there are several kinds of
   application processing that occur above the IP layer that
   significantly impact end-to-end system security policy
   enforcement, but are out of scope for this document.  In
   particular, how the network labeling policy is enforced within
   processing in an end system is critical, but is beyond the scope
   of a network (IP) layer Sensitivity Label encoding standard.
   Other specifications exist which discuss such details.  [TCSEC]
   [TNI] [CMW] [ISO-15408] [CC] [DoD MLOS PP]

      This standard does not preclude an End System capable of
   providing labelled packets across some range of Sensitivity
   Labels.  A Compartmented Mode Workstation (CMW) is an example of
   such an End System.[CMW] This is useful if the End System is
   capable of, and accredited to, separate processing across some
   range of Sensitivity Labels.  Such a node would have a range
   associated with it within the network interface connecting the
   node to the network.  As an example, an End System has the range
   "SECRET: TOP SECRET" associated with it in the Intermediate
   System to which the node is attached.  SECRET processing on the
   node is allowed to traverse the network to other "SECRET :
   SECRET" segments of the network, ultimately to a "SECRET :
   SECRET" node.  Likewise, TOP SECRET processing on the node is
   allowed to traverse a network through "TOP SECRET: TOP SECRET"
   segments, ultimately to some "TOP SECRET: TOP SECRET" node.  The

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   node in this case can allow a user on this node to access SECRET
   and TOP SECRET resources, provided the user holds the appropriate
   clearances and has been correctly configured.

      With respect to a given network, each distinct Sensitivity
   Label represents a separate virtual network which shares the same
   physical network.  There are rules for moving information between
   the various virtual networks.  The model we use within this
   document is based on the Bell-LaPadula model, but is extended to
   cover the concept of differing Domains of Interpretation.  Nodes
   that implement this protocol MUST enforce this mandatory
   separation of data.

      CALIPSO provides for both horizontal ("Compartment") and
   vertical ("Sensitivity Level") separation of information, as well
   as separation based on DOI.  The basic rule is that data MUST NOT
   be delivered to a user or system that is not approved to receive

   NOTE WELL: wherever we say "not approved", we also mean
   "not cleared", "not certified", and/or "not accredited"
   as applicable in one's operational community.

        This specification does not enable AUTOMATIC relabeling of
   information, within a DOI or to a different DOI.  That is,
   neither automatic "upgrading" nor automatic "downgrading" of
   information are enabled by this specification.  Local security
   policies might allow some limited downgrading, but this normally
   requires the intervention of some human entity and is usually
   done within an End System with respect to the internal
   Sensitivity Label, rather than on a network or in an
   intermediate-system (e.g. router, guard).  Automatic downgrading
   is not suggested operational practice; further discussion of
   downgrading is outside the scope of this protocol specification.

      Implementers of this specification MUST NOT permit automatic
   upgrading or downgrading of information in the default
   configuration of their implementation.  Implementers MAY add a
   configuration knob that would permit a System Security Officer
   holding appropriate privilege to enable automatic upgrading or
   downgrading of information.  If an implementation supports such a
   knob, the existence of the configuration knob must be clearly
   documented and the default knob setting MUST be that automatic
   upgrading or downgrading is DISABLED.  Automatic information
   upgrading and downgrading is not recommended operational

      Many existing MLS deployments already use (and operationally

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   need to use) more than one DOI concurrently.  User feedback from
   early drafts of this specification indicates that it is common
   at present for a single of network link (i.e. IP subnetwork)
   to carry traffic for both a particular "coalition" (or
   joint-venture) activity and also for the government (or other
   organisation) that owns and operates that particular network
   link.  On such a link, one CALIPSO DOI would typically be used
   for the "coalition" traffic and some different CALIPSO DOI would
   typically be used for non-coalition traffic (i.e. traffic that is
   specific to the government that owns and operates that particular
   network link).  For example, a UK military network that is part
   of a NATO deployment might have and use a UK MoD DOI for
   information originating/terminating on another UK system, while
   concurrently using a different NATO DOI for information
   originating/terminating on a non-UK NATO system.

      Additionally, operational experience with existing MLS systems
   has shown that if a system only supports a single DOI at a given
   time, then it is impossible for a deployment to migrate from
   using one DOI value to a different DOI value in a smooth,
   lossless, zero downtime, manner.

      Therefore, a node that implements this specification MUST be
   able to support at least 2 CALIPSO DOIs concurrently.  Support
   for more than 2 concurrent CALIPSO DOIs is encouraged.  This
   requirement to support at least 2 CALIPSO DOIs concurrently is
   not necessarily an implementation constraint upon MLS operating
   system internals that are unrelated to the network.

      Indeed, use of multiple DOIs is also operationally useful in
   deployments having a single administration that also have very
   large numbers of compartments.  For example, such a deployment
   might have one set of related compartments in one CALIPSO DOI
   and a different set of compartments in a different CALIPSO DOI.
   Some compartments might be present in both DOIs, possibly at
   different bit positions of the compartment bitmap in different
   DOIs.  While this might make some implementations more complex,
   it might also be used to reduce the typical size of the IPv6
   CALIPSO option in data packets.

        Moving information between any two DOIs is permitted
   -- if and only if -- the owners of the DOIs:

        1) Agree to the exchange,


        2) Publish a document with a table of equivalencies that

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           maps the CALIPSO values of one DOI into the other
           and make that document available to security
           administrators of MLS systems within the deployment
           scope of those two DOIs.

      The owners of two DOIs may choose to permit the exchange on
   or between any of their systems, or may restrict exchange to a
   small subset of the systems they own/accredit.  One way
   agreements are permissible, as are agreements which are a subset
   of the full table of equivalences.  Actual administration of
   inter-DOI agreements is outside the scope of this document.

      When data leaves an end system it is "exported" to the
   network, and marked with a particular DOI, Sensitivity Level,
   and Compartment Set. (This triple is collectively termed a
   Sensitivity Label.)  This Sensitivity Label is derived from
   the internal Sensitivity Label (the End System specific
   implementation of a given Sensitivity Label), and the export DOI.
   The export DOI is selected based on a range of parameters
   described in detail later in this document.

      When data arrives at an end system, it is "imported" from the
   network to the End System.  The data from the datagram takes on
   an internal Sensitivity Label based on the Sensitivity Label
   contained in the datagram.  This assumes the datagram is marked
   with a recognizable DOI, there is a corresponding internal
   Sensitivity Label equivalent to the CALIPSO Sensitivity Label,
   and the datagram is "within range" for the receiving logical

      A node has one or more physical interfaces.  Each physical
   interface is associated with a physical network segment used
   to connect the node, router, LAN, or WAN.  Each physical
   network interface has one or more Sensitivity Label ranges
   associated with it.  Sensitivity Label ranges from multiple DOIs
   must be enumerated separately.  Multiple ranges from the same DOI
   are permissible.

      Each node also might have one or more logical network

      A given logical network interface might be associated with more
   than one physical interface.  For example, a layer-3 switch/router
   might have 2 Ethernet ports that are associated with the same VLAN
   (i.e. where that one VLAN mapped to a single IPv6 subnetwork).
   [IEEE 802.1Q]

      A given physical network interface might have more than one

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   associated logical interface.  For example, a node might have 2
   logical network interfaces, each for a different IP subnetwork
   ("super-netting"), on a single physical network interface (e.g.
   on a single Network Interface Card of a personal computer).
   Alternatively, also as an example, a single Ethernet port might
   have multiple Virtual LANs (VLANs) associated with it,
   where each VLAN could be a separate logical network interface.

      Each logical network interface has one or more Sensitivity
   Label ranges associated with it.  Sensitivity Label ranges from
   multiple DOIs must be enumerated separately.  Multiple ranges
   from the same DOI are permissible.  Each range associated with a
   logical interface must fall within a range separately defined for
   the corresponding physical interface.

      There is specific user interest in having IPv6 routers that
   can apply per-logical-interface mandatory access controls based
   on the contents of the CALIPSO Sensitivity Labels in IPv6
   packets.  The authors note that since the early 1990s, and
   continuing through today, some commercial IPv4 router products
   provide MAC enforcement for the RFC-1108 IP Security Option.

      In transit, a datagram is handled based on its CALIPSO
   Sensitivity Label, and is usually neither imported to or exported
   from the various intermediate systems it transits.  There also is
   the concept of "CALIPSO Gateways" which import data from one DOI
   and export it to another DOI such that the effective Sensitivity
   Label is NOT changed, but is merely represented using a different
   DOI.  In other words, such devices would be trustworthy, trusted,
   and authorised to provide on-the-fly re-labeling of packets at
   the boundaries between complete systems of hosts within a single
   DOI.  Typically, such systems require specific certification(s)
   and accreditation(s) before deployment or use.


      This Section describes the default behaviour of CALIPSO
   compliant end nodes and intermediate systems.  Implementers
   MAY implement configuration knobs to vary from this behaviour,
   provided that the default behaviour (i.e. if the system
   administrator does not explicitly change the configured
   behaviour of the device) is as described below.  If implementers
   choose to implement such configuration knobs, the configuration
   parameters and the behaviours that they enable and disable
   SHOULD be documented for the benefit of system administrators
   of those devices.

      Each Intermediate System or End System is responsible for

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   properly interpreting and enforcing the MLS Mandatory Access
   Control policy.  Practically, this means that each node must
   evaluate the label on the inbound packet, ensure that this
   Sensitivity Label is valid (i.e.  within range) for the receiving
   interface, and at a minimum only forward the packet to an
   interface and node where the Sensitivity Label of the packet
   falls within the assigned range of that node's receiving

      Packets with an invalid (e.g. out of range) Sensitivity Label
   for the receiving interface MUST be dropped upon receipt.  A
   Sensitivity Label is valid if and only if the Sensitivity Label
   falls within the range assigned to the transmitting interface on
   the sending system and within the range assigned to the receiving
   interface on the receiving system.  These rules also need
   to be applied by intermediate systems on each hop that a
   CALIPSO-labelled packet traverses, not merely at the end points
   of a labelled IP session.  As an example, it is a violation of the
   default MLS MAC policy for a packet with a higher sensitivity
   level (e.g. "MOST SECRET") to transit a link whose maximum
   sensitivity level is less than that first sensitivity level
   (e.g. "SECRET").

      If an unlabelled packet is received from a node which is
   not aware of CALIPSO Sensitivity Labels (i.e. unable to assign
   Sensitivity Labels itself) and the packet is destined for a node
   which is sensitivity label aware, the receiving intermediate
   system needs to insert a Sensitivity Label.  This Sensitivity
   Label will be equal to the maximum Sensitivity Label assigned
   to the originating node if and only if that is known to the
   receiving node.  If this receiving intermediate system does not
   know which Sensitivity Label is assigned to the originating node,
   the maximum Sensitivity Label of the interface the packet was
   received upon will be inserted.

   [NOTE WELL: The procedure in the preceding paragraph is NOT
   a label upgrade -- because it is not changing an existing
   label; instead, it is simply inserting a Sensitivity Label
   that has the only "safe" value, given that no other
   information is known to the receiving node.  In large-scale
   deployments, it is very unlikely that a given node will have
   any authoritative a priori information about the security
   configuration of any node that is NOT on a directly attached

      If a packet is to be sent to a node which is
   defined to not be Sensitivity Label aware, from a node
   which is label aware, then the Sensitivity Label MAY be

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   removed upon transmission if and only if local security
   policy explicitly permits this.  The originating node is
   still responsible for ensuring that the Sensitivity Label on
   the packet falls within the Sensitivity Label range
   associated with the receiving node.  If the packet will
   traverse more than one subnetwork between origin and
   destination, and those subnetworks are labelled, then the
   packet SHOULD normally contain a Sensitivity Label so that
   the packet will be able to reach the destination and the
   intermediate systems will be able to apply the requisite MAC
   policy to the packet.

   [NOTE WELL that in some IPv4 labelled network deployments that
   exist as of this writing, the first hop router usually will
   insert a Sensitivity Label into a received unlabelled packet
   upon the receipt of that unlabelled packet, in the manner
   described above.  So sending a packet without a label across
   a multiple subnetwork path to a destination does not guarantee
   that the packet will arrive containing no Sensitivity Label.]


      This section describes the format of the CALIPSO option for
   use with IPv6 datagrams.  CALIPSO is an IPv6 hop-by-hop option,
   rather than an IPv6 Destination Option, to ensure that a security
   gateway or router can apply access controls to IPv6 packets based
   on the CALIPSO label carried by the packet.

      An IPv6 datagram that has not been tunnelled contains at most
   one CALIPSO label.  In the special case where (1) a labelled IPv6
   datagram is tunnelled inside another labelled IPv6 datagram AND
   (2) IP Security is NOT providing confidentiality protection for
   the inner packet, the outer CALIPSO Sensitivity Label must have
   the same meaning as the inner CALIPSO Sensitivity Label.  For
   example, it would be invalid to encapsulate an unencrypted IPv6
   packet with a Sensitivity Label of (SECRET, no compartments)
   inside a packet with an outer Sensitivity Label of

      If the inner IPv6 packet is tunnelled inside the Encapsulating
   Security Payload (ESP) and confidentiality is being provided to
   that inner packet, then the outer packet MAY have a different
   CALIPSO Sensitivity Label -- subject to local security policy.

      As a general principle, the meaning of the Sensitivity Labels
   must be identical when one has a labelled cleartext IP packet
   that has been encapsulated (tunnelled) inside another labelled
   IP packet.  This is true whether one has IPv6 tunnelled in IPv6,

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   IPv4 tunnelled in IPv6, or IPv6 tunnelled in IPv4.  This
   is essential to maintaining proper Mandatory Access Controls.

      This option's syntax has been designed with intermediate
   systems in mind.  It is now common for a MLS network deployment
   to contain an intermediate systems acting as a guard (sometimes
   several acting as guards).  Such a guard device needs to be able
   to very rapidly parse the sensitivity label in each packet, apply
   ingress interface MAC policy, forward the packet while aware of
   the packet's Sensitivity Label, and then apply egress interface
   MAC policy.

      At least one prior IP Sensitivity Label option [FIPS-188] used
   a syntax that was unduly complex to parse in IP routers, hence
   that option never was implemented in an IP router.  So there is
   a deliberate effort here to choose a streamlined option syntax
   that is easy to parse, to encode, and to implement in more
   general terms.

5.1.  Option Format

      The CALIPSO option is an IPv6 Hop-by-Hop Option and is
   designed to comply with IPv6 optional header rules.  Following
   the nomenclature of RFC-2460, Section 4.2, the Option Type
   field of this option must have 4n+2 alignment.[RFC-2460]

      The CALIPSO Option Data MUST NOT change en-route, except
   when (1) "DOI translation" is performed by a trusted
   intermediate system, (2) a CALIPSO Option is inserted by
   a trusted intermediate system upon receipt of an unlabelled
   IPv6 packet, or (3) a CALIPSO Option is removed by a last-hop
   trusted intermediate system immediately prior to forwarding
   the packet to a destination node that does not implement
   support for CALIPSO labels.  The details of these 3 exceptions
   are described elsewhere in this document.

      If the option type is not recognised by a node examining the
   packet, the option is ignored.  However, all implementations
   of this specification MUST be able to recognise this option
   and therefore MUST NOT ignore this option if it is present
   in an IPv6 packet.

      This option is designed to comply with the IPv6 optional
   header rules. [RFC-2460] The CALIPSO option is always carried
   in a Hop-By-Hop Option Header, never in any other part of an
   IPv6 packet.  This rule exists because IPv6 routers need to be
   able to see the CALIPSO label so that those routers are able

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   to apply MLS Mandatory Access Controls to those packets.

      The diagram below shows the CALIPSO option along with
   the required (first) 2 fields of the Hop-By-Hop Option Header
   that envelops the CALIPSO option.  The design of the CALIPSO
   option is arranged to avoid the need for 16 bits of padding
   between the HDR EXT LEN field and the start of the CALIPSO
   option.  Also, the CALIPSO Domain of Interpretation field is
   laid out so that it normally will be 32-bit aligned.

   | Next Header | Hdr Ext Len   | Option Type | Option Length|
   |             CALIPSO Domain of Interpretation             |
   | Cmpt Length |  Sens Level   |     Checksum (CRC-16)      |
   |      Compartment Bitmap (Optional; variable length)      |

5.1.1.  OPTION TYPE field

      This field contains an unsigned 8-bit value.  Its value is
   (TO BE ASSIGNED BY IANA; the high order bits of this option
   need to be 000).

      Nodes that do not recognise this option should ignore it.
   In many cases, not all routers in a given MLS deployment
   will contain support for this CALIPSO option.  For
   interoperability reasons, it is important that such label
   unaware routers forward this packet normally, even though
   the router does not recognise the CALIPSO Hop-by-Hop option.

   In the event the IPv6 packet is fragmented, this option MUST be
   copied on fragmentation.  Virtually all users want the choice
   of using the IP Authentication Header (a) to authenticate this
   option and (b) to bind this option to the associated IPv6 packet.

5.1.2.  OPTION LENGTH field

        This field contains an unsigned integer 1 octet in
   size.  It has minimum value is 8 (e.g. when the Compartment
   Bitmap field is absent).  This field specifies the Length of
   the option data field of this option in octets.  The Option
   Type and Option Length fields are not included in the length

StJohns Et Alia                                                [Page 28]
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      This field contains an unsigned 8-bit integer.  The field
   specifies the size of the COMPARTMENT BITMAP field in 32-bit
   words.  The minimum value is zero, which is used only when the
   information in this packet is not in any compartment.  (In that
   situation, the CALIPSO Sensitivity Label has no need for a
   Compartment Bitmap).  Note that measuring the Compartment
   Bitmap field length in 32-bit words permits the header
   to be 64-bit aligned, following IPv6 guidelines, without
   wasting 32-bits.  Using 64-bit words for the size of the
   Compartment Bitmap field length would force 32-bits of
   padding with every option in order to maintain 64-bit
   alignment; wasting those bits in every CALIPSO option
   is undesirable.

      Because this specification represents Releasabilities on the
   wire as inverted Compartments, the size of the COMPARTMENT BITMAP
   field needs to be large enough to hold not only the set of
   logical Compartments, but instead to hold both the set
   of logical Compartments and the set of logical Releasabilities.

      Recall that the overall length of this option MUST follow
   IPv6 optional header rules, including the word alignment rules.
   This has implications for the valid values for this field.  In
   some cases the length of the Compartment Bitmap field might need
   to exceed the number of bits required to hold the sum of the
   logical Compartments and the logical Releasabilities, in order
   to comply with IPv6 alignment rules.


      This field contains an unsigned 32-bit integer.
   IANA maintains a registry with assignments of the DOI values
   used in this field.  The DOI identifies the rules under
   which this datagram must be handled and protected.  The NULL
   DOI, in which this field is all zeros, MUST NOT appear in
   any IPv6 packet on any network.

   contain zero is defined to be the NULL DOI. The NULL DOI has
   no compartments and has a single level whose value and
   CALIPSO representation are each zero.  The NULL DOI MUST NOT
   ever appear on the wire.  If a packet is received containing
   the NULL DOI, that packet MUST be dropped and the event
   SHOULD be logged as a security fault.

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      This contains an unsigned 8-bit value.  This field
   contains an opaque octet whose value indicates the relative
   sensitivity of the data contained in this datagram in the
   context of the indicated DOI.  The values of this field MUST
   be ordered, with 00000000 being the lowest sensitivity level
   and 11111111 being the highest sensitivity level.

      However, in a typical deployment, not all 256
   Sensitivity Levels will be in use.  So the set of valid
   Sensitivity Level values depends upon the CALIPSO DOI in
   use. This sensitivity ordering rule is necessary so that
   intermediate systems (e.g. routers or MLS guards) will be
   able to apply MAC policy with minimal per-packet computation
   and minimal configuration.

5.1.7.  16-bit Checksum Field

      This 16-bit field contains the a CRC-16 checksum as
   defined in Appendix C of RFC-1662.[RFC-1662]  The checksum
   is calculated over the entire CALIPSO option in this packet,
   including option header, zeroed-out checksum field, option
   contents, and any required padding zero bits.

      The checksum MUST always be computed on transmission and
   MUST always be verified on reception.  This checksum only
   provides protection against accidental corruption of the
   CALIPSO option in cases where neither the underlying medium
   nor other mechanisms, such as the IP Authentication Header
   (AH), are available to protect the integrity of this option.

      Note that the checksum field is always required, even
   when other integrity protection mechanisms (e.g. AH) are
   used.  This method is chosen for its reliability and
   simplicity in both hardware and software implementations,
   and because many implementations already support this
   checksum due to its existing use in various IETF

5.1.8.  Compartment Bitmap

        This contains a variable number of 64-bit words.  Each bit
   represents one compartment within the DOI.  Each "1" bit within
   an octet in the "Compartment Bitmap" field represents a separate
   compartment under whose rules the data in this packet must be
   protected.  Hence, each "0" bit indicates that the compartment
   corresponding with that bit is not applicable to the data in this

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   packet.  The assignment of identity to individual bits within a
   Compartment Bitmap for a given DOI is left to the owner of that

        This specification represents a Releasability on the wire
   as if it were an inverted Compartment.  So the Compartment Bitmap
   holds the sum of both logical Releasabilities and also logical
   Compartments for a given DOI value.  The encoding of the
   Releasabilities in this field is described elsewhere in this
   document. The Releasability encoding is designed to permit the
   Compartment Bitmap evaluation to occur without the evaluator
   necessarily knowing the human semantic associated with each bit
   in the Compartment Bitmap.  In turn, this facilitates the
   implementation and configuration of Mandatory Access Controls
   based on the Compartment Bitmap within IPv6 routers or guard

5.2.  Packet Word Alignment considerations

       The basic option is variable length, due to the variable
   length Compartment Bitmap field.

      Intermediate Systems that lack custom silicon processing
   capabilities and most End Systems perform best when processing
   fixed length, fixed location items.  So the IPv6 base specification
   levies certain requirements on all IPv6 optional headers.

     The CALIPSO option must maintain this IPv6 64-bit alignment
   rule for the option overall.  Please note that the Compartment
   Bitmap field has a length in quanta of 32-bit words (e.g. 0 bits,
   32 bits, 64 bits, 96 bits), which permits the overall CALIPSO
   option length to be 64-bit aligned -- without requiring 32-bits
   of NULL padding with every CALIPSO option.


      This section describes specific protocol processing steps
   required for systems that claim to implement or conform with this

6.1.  Sensitivity Label Comparisons

      This section describes how comparisons are made between
   two Sensitivity Labels.  Implementing this comparison correctly
   is critical to the MLS system providing the intended Mandatory
   Access Controls (MAC) to network traffic entering or leaving
   the system.

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      A Sensitivity Label consists of a DOI, a Sensitivity Level,
   and zero or more Compartments.  The following notation will be

     A.DOI  = the DOI portion of Sensitivity Label A
     A.LEV  = the Sensitivity Level portion of Sensitivity Label A
     A.COMP = the Compartments portion of Sensitivity Label A

6.1.1.  "Within Range"

        A Sensitivity Label, "M", is "within range" for a
   particular range "LO:HI" if and only if:

        1.  M, LO, and HI are members of the same DOI.

            (M.DOI == LO.DOI == HI.DOI)

        2.  The range is a valid range.  A given range LO:HI is
              valid if and only if HI dominates LO.

            ((LO.LEV  <= HI.LEV)  &&  (LO.COMP <= HI.COMP))

        3.  The Sensitivity Level of M dominates the low-end (LO)
            Sensitivity Level AND the Sensitivity Level of M is
            dominated by the high-end (HI) Sensitivty Level.

            (LO.LEV <= M.LEV <= HI.LEV)


        4.  The Sensitivity Label M has a compartment set that
            dominates the compartment set contained in the
            Sensitivity Label from the low-end range (LO), and
            which is dominated by the Compartment set contained
            in the high-end Sensitivity Label (HI) from the range.

            (LO.COMP <= M.COMP <= HI.COMP)

6.1.2.  "Less Than" or "Below Range"

        A Sensitivity Label "M" is "less than" some other
   Sensitivity Label "LO" if and only if:

        1.   The DOI for the Sensitivity Label M is identical
             to the DOI for both the low-end and high-end of
             the range.

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             (M.DOI == LO.DOI == HI.DOI)


        2.   The sensitivity level of M is less than the
             sensitivity level of LO.

             (M.LEV < LO.LEV)


        3.   The compartment set of Sensitivity Label M is
             dominated by the compartment set of Sensitivity
             Label LO.

             (M.COMP <= LO.COMP)

   A Sensitivity Label "M" is "below range" for a Sensitivity Label
   "LO:HI", if LO dominates M and LO is not equal to M.

6.1.3.  "Greater Than" or "Above Range"

        A Sensitivity Label, "M" is "greater than" some Sensitivity
   Label "HI" if and only if:

        1.   Their DOI's are identical.

             (M.DOI == HI.DOI)


        2A.  M's sensitivity level is above HI's sensitivity level.

             (M.LEV > HI.LEV)


        2B.  M's compartment set is greater than HI's compartment set.

             (M.COMP > HI.COMP)

   A Sensitivity Label, "M", is "above range" for a Sensitivity Label,
   "LO:HI", if M dominates HI and M is not equal to HI.

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6.1.4.  "Equal To"

        A Sensitivity Label "A" is "equal to" another
   Sensitivity Label "B" if and only if:

        1.  They have the exact same DOI.

            (A.DOI == B.DOI)

        2.  They have identical sensitivity levels.

            (A.LEV == B.LEV)

        3.  Their compartment sets are identical.

            (A.COMP == B.COMP)

6.1.5.  "Disjoint" or "Incomparable"

        A Sensitivity Label "A" is disjoint from another
   Sensitivity Label "B" if any of these conditions are true:

        1.   Their DOI's differ.

             (A.DOI <> B.DOI)

        2.   B does not dominate A, A does not dominate B,
             and A is not equal to B.

             (^( (A < B) || (A > B) || (A == B) ))

        3.   Their compartment sets are disjoint from each
             other; A's compartment set does not dominate B's
             compartment set AND B's compartment set does not
             dominate A's compartment set.

             (^( (A.COMP >= B.COMP) || (A.COMP <= B.COMP) ))

6.2.  End System Processing

        This section describes CALIPSO-related processing for IPv6
   packets imported or exported from an End System claiming to
   implement or conform with this specification.  This document
   places no additional requirements on IPv6 nodes that do not
   claim to implement or conform with this document.

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6.2.1  Export

        An end system which sends data to the network is said to
   "export" it to the network.  Before a datagram can leave an end
   system and be transmitted over a network the following ordered
   steps must occur:

        1. Selection of the export DOI:

        a) If the upper level protocol selects a DOI,
               then that DOI is the one used.

        b) elseIf there are tables defining a specific default
               DOI for the specific destination End System address
               or for the network address, then that DOI is
        c) elseIf there is a specific DOI associated with the
           sending logical interface (i.e. IP address),
               then that DOI is selected.
        d)     Else the default DOI for the system is selected.

        A connection-oriented transport-layer protocol session
     (e.g. TCP session, SCTP session) MUST have the same DOI and
     same Sensitivity Label for the life of that connection.  The
     DOI is selected at connection initiation and MUST NOT change
     during the session.

      A trusted multi-level application that possesses
     appropriate privilege MAY use multiple connection-oriented
     transport-layer protocol sessions with differing Sensitivity
     Labels concurrently.

        Some trusted UDP-based applications (e.g. remote
     procedure call service) multiplex different transactions
     having different sensitivity levels in different packets
     for the same IP session (e.g. IP addresses and UDP ports
     are constant for a given UDP session).  In such cases,
     the Trusted Computing Base MUST ensure that each packet
     is labelled with the correct Sensitivity Label for the
     information carried in that particular packet.

        In the event the End System selects and uses a specific
     DOI and that DOI is not recognised by the originating Node's
     first-hop router, the packet MUST be dropped by the first-hop
     router.  In such a case, the networking API should indicate
     the connection failure (e.g. with some appropriate error,

StJohns Et Alia                                                [Page 35]
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     such as ENOTREACH).  This fault represents either incorrect
     configuration of either the Intermediate System or of the
     End System -- xor correct operation for a node that is not
     permitted to send IPv6 packets with that DOI through that
     Intermediate System.

        When an MLS End ystem is connected to an MLS LAN, it is
     possible that there would be more than one first-hop
     Intermediate System concurrently, with different
     Intermediate Systems having different valid Sensitivity
     Label ranges.  Thoughtful use of the IEEE 802 Virtual
     LAN (VLAN) standard (e.g. with different VLAN IDs
     corresponding to different sensitivity ranges) might
     ease proper system configuration in such deployments.

        2.  Export Labeling:

             Once the DOI is selected, the CALIPSO Sensitivity
        Label and values are determined based on the internal
        sensitivity label and the DOI.  In the event the internal
        sensitivity level does not map to a valid CALIPSO
        Sensitivity Label, then an error SHOULD be returned
        to the upper level protocol and that error MAY be
        logged. No further attempt to send this datagram
        should be made.

        3.  Access Control:

             Once the datagram is marked and the sending logical
        interface is selected (by the routing code), the
        datagram's Sensitivity Label is compared against the
        Sensitivity Label range(s) associated with that logical
        interface.  For the datagram to be sent, the interface
        MUST list the DOI of the datagram Sensitivity Label as
        one of the permissible DOI's and the datagram Sensitivity
        Label must be within range for the range associated with
        that DOI.   If the datagram fails this access test an
        error SHOULD be returned to the upper level protocol
        and MAY be logged.  No further attempt to send this
        datagram should be made.

6.2.2.  Import

        When a datagram arrives at an interface on an end system,
   the receiving end system MUST:

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        1.   Verify the CALIPSO checksum.  Datagrams with
             invalid checksums MUST be silently dropped.
             Such a drop event SHOULD be logged as a security
             fault with an indication of what happened.

        2.   Verify the CALIPSO has a known and valid DOI.
             Datagrams with unrecognised or illegal DOIs MUST
             be silently dropped.  Such an event SHOULD be
             logged as a security fault with an indication
             of what happened.

        3.   Verify the DOI is a permitted one for the receiving
             interface.  Datagrams with prohibited DOI values
             MUST be silently dropped.  Such an event SHOULD
             be logged as a security fault with an indication
             of what happened.

        4.   Verify the CALIPSO Sensitivity Label is within
             the permitted range for the receiving interface:

             NOTE WELL:
                 EACH permitted DOI on an interface has a
             separate table describing the permitted range
             for that DOI.

             A datagram which has a Sensitivity Label within
             the permitted range is accepted for further

             A datagram which has a Sensitivity Label disjoint
             with the permitted range MUST be silently dropped.
             Such an event SHOULD be logged as a security fault,
             with an indication that the packet was dropped
             because of a disjoint Sensitivity Label.  An ICMP
             error message MUST NOT be sent in this case.

             A datagram which has a Sensitivity Label below
             the permitted range MUST be dropped.  This event
             SHOULD be logged as a security fault, with an
             indication that the packet was below range.
             An ICMP error message MUST NOT be sent in this case.

             A datagram with a Sensitivity Label above the
             permitted range MUST be dropped.  This event
             SHOULD be logged as a security fault, with an
             indication that the packet was above range.
             An ICMP error message MUST NOT be sent in this case.

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        5.   Once the datagram has been accepted, the import
             Sensitivity Label and DOI are used to associate
             the appropriate internal Sensitivity Label with
             the data contained in the datagram.  This
             information MUST be carried as part of the
             information returned to the upper-layer protocol.

6.3.  Intermediate System Processing

        This section describes CALIPSO-related processing for IPv6
   packets transiting an IPv6 Intermediate System that claims to
   implement and comply with this specification.  This document
   places no additional requirements on IPv6 Intermediate Systems
   that do not claim to comply or conform with this document.

        The CALIPSO packet format has been designed so that one can
   configure an intermediate system with the minimum sensitivity
   level, maximum sensitivity level, minimum compartment bitmap,
   and maximum compartment bitmap -- and then deploy that system
   without forcing the system to know the detailed human meaning
   of each sensitivity level or compartment bit value.  Instead,
   once the minimum and maximum labels have been configured, the
   intermediate system can apply a simple algorithm to determine
   whether or not a packet is within range for a given interface.
   This design should be straight-forward to implement in ASIC or
   FPGA hardware because the option format is simple and easy to
   parse, and because only a single comparison algorithm (defined
   in this RFC, hence known in advance) is needed.

6.3.1.  Input

        Intermediate Systems have slightly different rules for
   processing marked datagrams than do end systems.  Primarily,
   Intermediate Systems do not IMPORT or EXPORT transit datagrams,
   they just forward them.  Also, in most deployments intermediate
   systems are used to provide Mandatory Access Controls to packets
   traversing more than one subnetwork.

        The following checks MUST occur before any other
   processing.  Upon receiving a CALIPSO-labelled packet,
   an intermediate system must:

        1.  Determine whether or not this datagram is destined
            for (addressed to) this Intermediate System.  If
            so, then the Intermediate System becomes an End
            System for the purposes of receiving this
            particular datagram and the rules for IMPORTing

StJohns Et Alia                                                [Page 38]
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            described above are followed.

        2.  Verify the CALIPSO checksum.  Datagrams with
            invalid checksums MUST be silently dropped.  The
            drop event SHOULD be logged as a security fault
            with an indication of what happened and MAY
            additionally be logged as a network fault.

            NOTE WELL:
            A checksum failure could indicate a general network
            problem (e.g. noise on a radio link) that is
            unrelated to the presence of a CALIPSO option, but
            it also could indicate an attempt by an adversary
            to tamper with the value of a CALIPSO label.

        3.  Verify the CALIPSO has a known and valid DOI.
            Datagrams with unrecognised or illegal DOIs MUST
            be silently dropped.  Such an event SHOULD be
            logged as a security fault with an indication of
            what happened.

        4.  Verify the DOI is a permitted one for the receiving
            interface.  Datagrams with prohibited DOIs MUST be
            silently dropped.  Such a drop SHOULD be logged as
            a security fault with an indication of what

        5.  Verify the Sensitivity Label within the CALIPSO
            is within the permitted range for the receiving

            NOTE WELL:
                 Each permitted DOI on an interface has a
            separate table describing the permitted range
            for that DOI.

            A rejected datagram which has a Sensitivity Label
            below or disjoint with the permitted range MUST be
            silently dropped.  Such an event SHOULD be logged
            as a security fault with an indication of what
            happened.  An ICMP error message MUST NOT be sent
            in this case.

             A rejected datagram with a Sensitivity Label above
             the permitted range MUST be dropped.  The drop
             event SHOULD be logged as a security fault with an
             indication of what happened.  An ICMP error
             message MUST NOT be sent in this case.

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        If and only if all the above conditions are met is the
   datagram accepted by the IPv6 Intermediate System for further
   processing and forwarding.

        At this point, the datagram is within the permitted range
   for the Intermediate System, so appropriate ICMP error messages
   MAY be created by the IP module back to the originating End
   System regarding the forwarding of the datagram.  These ICMP
   messages MUST be created with the exact same Sensitivity Label
   as the datagram causing the error.  Standard rules about
   generating ICMP error messages (e.g. never generate an ICMP error
   message in response to a received ICMP error message) continue
   to apply.  Note that these locally-generated ICMP messages
   must go through the same outbound checks (including MAC checks)
   as any other forwarded datagram as described in the following

6.3.2.  Translation

        It is at this point that TRANSLATION of the CALIPSO
   takes place if at all.  Please see Section 6.4 for a
   discussion of the appropriate processing for Translation.

6.3.3.  Output

        Once the forwarding code has selected the interface
   through which the datagram will be transmitted the following
   takes place:

        1.  If the output interface requires that all packets
            contain a CALIPSO label, then Verify that the packet
            contains a CALIPSO label.

        2.  Verify the DOI is a permitted one for the sending
            interface and that the datagram is within the
            permitted range for the DOI and for the interface.

        3.  Datagrams with prohibited DOIs or with out of range
            Sensitivity Labels MUST be dropped.  Any drop event
            SHOULD be logged as a security fault, including
            appropriate details about which datagram was
            dropped and why.

        4.  Datagrams with prohibited DOIs or out of range
            Sensitivity Labels MAY result in an ICMP "Destination
            Unreachable" error message, depending upon the
            security configuration of the system.

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            If the cause of the dropped packet is that the
            DOI is prohibited or unrecognised, then a reason
            code of "No Route to Host" is used.  If the dropped
            packet's DOI is valid, but the Sensitivity Label
            is out of range, then a reason code of
            "Administratively Prohibited" is used.  If an
            unlabelled packet has been dropped because the
            packet is required to be labelled, then a reason
            code of "Administratively Prohibited" is used.

            In all cases, if an ICMP Error Message is sent,
            then it MUST be sent with the same Sensitivity
            Label as the rejected datagram.

            The choice of whether or not to send an ICMP
            message, if sending an ICMP message for this case
            is implemented, MUST be configurable, and SHOULD
            default to not sending an ICMP message.  Standard
            conditions about generating ICMP error messages
            (e.g. never send an ICMP error message about a
            received ICMP error message) continue to apply.

6.4.  Translation

        A system which provides on-the-fly re-labeling is said
   to "translate" from one DOI to another.  There are basically
   two ways a datagram can be relabelled:

      EITHER the Sensitivity Label can be converted from a
   CALIPSO Sensitivity Label, to an internal Sensitivity Label,
   and then back to a new CALIPSO Sensitivity Label, XOR a
   CALIPSO Sensitivity Label can be directly remapped into a
   new CALIPSO Sensitivity Label.

      The first of these methods is the functional
   equivalent of "importing" the datagram then "exporting" it
   and is covered in detail in the "Import" and "Export"
   sections above.  This section describes direct relabeling.
   The choice of which method to use for relabeling is an
   implementation decision outside the scope of this document.

      A system which provides on-the-fly relabeling
   without importing or exporting is basically a special case
   of the Intermediate System rules listed above.  Translation
   or relabeling takes place AFTER all input checks take place,
   but before any output checks are done.

      Once a datagram has been accepted (passing all the

StJohns Et Alia                                                [Page 41]
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   appropriate checks described in section 5.3), it may be
   relabelled.  To determine the new Sensitivity Label, first
   determine the new DOI.  The selection of the new DOI may be
   INTERFACE, or combinations thereof.  Exact details on how
   the output DOI is selected are implementation dependent,
   with the caveat that it should be consistent and reversible.
   If a datagram from End System A to End System B with DOI X
   maps into DOI Y, then a datagram from B to A with DOI Y
   should map into DOI X.

      Once the output DOI is selected, the output
   Sensitivity Label is determined based on (1) the input DOI
   and input Sensitivity Label and (2) the output DOI.  In the
   event the input Sensitivity Label does not map to a valid
   output Sensitivity Label for the output DOI, then the
   datagram MUST be silently dropped and the drop event SHOULD
   be logged as a security fault.

      Once the datagram is re-labelled, the output
   procedures under Section 5.3 "Intermediate Systems" are
   followed, with the exception that any error that would cause
   an ICMP error message to be generated back to the originating
   End System instead MUST silently drop the datagram without
   sending an ICMP error message.  Such a drop SHOULD be logged
   as a security fault.


       This section contains "implementation considerations";
   it does not contain "requirements".  Implementation experience
   might eventually turn some of them into implementation
   requirements in some future version of this specification.

       This IPv6 option specification is only a small part of
   an overall distributed Multi-Level Secure (MLS) deployment.
   Detailed instructions on how to build a Multi-Level Secure
   (MLS) device are well beyond the scope of this specification.
   Additional information on implementing a Multi-Level Secure
   operating system, for example an MLS host, is available from
   a range of sources. [TCSEC] [TNI] [CMW] [CC] [ISO-15408]
   [DoD MLOS PP]

      Because the usual 5-tuple (i.e. Source IP address,
   Destination IP address, Transport protocol, Source Port, &
   Destination Port) do not necessarily uniquely identify

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   a flow within a labelled MLS network deployment, some
   applications or services might be impacted by multiple flows
   mapping to a single 5-tuple.  This might have unexpected
   impacts in a labelled MLS network deployment using such
   application protocols.  For example, RSVP, SIP, and SDP
   might be impacted by this.

       A number of Commercial-Off-The-Shelf (COTS) applications
   (e.g. RADIUS, HTTP and TLS web content access) have been
   included in MLS network deployments for about two decades,
   without operational difficulties or a need for special
   modifications.  The ability to use these common applications
   demonstrates that the basic Internet architecture remains
   unchanged in an MLS deployment, although certain details
   (e.g. adding labels to IP datagrams) do change.

7.1.  Intermediate Systems

      Historically, RFC-1108 was supported by one commercial
   label-aware IP arouter.  Neither RFC-1038 nor FIPS-188 were
   supported in any commercial IP router, so far as the authors
   are aware.  A label-aware router does not necessarily use
   a MLS operating system.  Instead, a label-aware router might
   use a conventional router operating system, adding extensions
   to permit application of per-logical-interface label-oriented
   Access Control Lists (ACLs) to IP packets entering and
   leaving that router's network interface(s).

      This proposal does not change IP routing in any way.
   Existing label-aware routers do not use Sensitivity Labels
   in path calculations, in RIB or FIB calculations, in their
   routing protocols, or in their packet forwarding decisions.

      Similarly, existing MLS network deployments use many
   protocols or specifications, for example Differentiated
   Services, without modification.  For Differentiated Services,
   this might mean that multiple IP flows (i.e. flows differing
   only in their CALIPSO label value) would be categorised and
   handled by intermediate systems as if they were a single flow.

      Router performance is optimised if there is hardware
   support for applying the Mandatory Access Controls based on
   this label option.  An issue with CIPSO is that the option
   syntax is remarkably complex.[FIPS-188] So this label option
   uses a simplified syntax.  This should make it more practical
   to create custom logic (e.g. in Verilog or VHDL) with support
   for this option and the associated Mandatory Access Controls.

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7.2.  End Systems

      It is possible for a system administrator to create two
   DOIs with different overlapping compartment ranges.  This
   can be used to reduce the size of the IPv6 Sensitivity
   Label option in some deployments.

7.3.  Upper-Layer Protocols

      As CALIPSO is an IP option, this document focuses
   upon the network-layer handling of IP packets containing
   CALIPSO options.  This section provides some discussion
   of some upper-layer protocol issues.

      This section is not a complete specification for
   how a MLS host handles information internally after the
   decision has been made to accept a received IPv6 packet
   containing a CALIPSO option.  Implementers of MLS systems
   might wish also to consult [TCSEC], [TNI], [CMW], [CC],
   [ISO-15408], and [DoD MLOS PP].

      In a typical MLS host, the information received from
   the network (i.e. information not dropped by the Network
   Layer as a result of the CALIPSO processing described in
   this document) is assigned an internal Sensitivity Label
   while inside the host operating system.  The MLS host uses
   the Bell-LaPadula Mandatory Access Control policy [BL73]
   to determine how that information is processed, including
   to which transport-layer sessions or to which applications
   the information is delivered.

      Within this section, we use one additional notation,
   in an attempt to be both clear and concise.  Here, the
   string "W:XY" defines a Sensitivity Label where the
   sensitivity level is W and where X and Y are the only
   compartments enabled, while the string "W::" defines a
   Sensitivity Label where the Sensitivity Level is W and
   there are no compartments enabled.

7.3.1.  TCP-related issues

        With respect to a network, each distinct Sensitivity
   Label represents a separate virtual network which shares
   the same physical network.

      The above statement taken from section 3 has a
   non-obvious, but critical, corollary.  If there are
   separate virtual networks, then it is possible for a system

StJohns Et Alia                                                [Page 44]
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   which exists in multiple virtual networks to have identical
   TCP connections, each one existing in a different virtual

      TCP connections are normally identified by source and
   destination port, and source and destination address.  If
   a system labels datagrams with the CALIPSO option (which
   it must do if it exists in multiple virtual networks -
   e.g. a "Multi-Level Secure" system), then TCP connections
   are identified by source and destination port, source and
   destination address, and an internal Sensitivity Label
   (optionally, a Sensitivity Label range).  This corrects
   a technical error in RFC-793, and is consistent with all
   known MLS operating system implementations. [TNI, RFC-793]
   There are no known currently deployed TCP instances
   that actually comply with this specific detail of RFC-793.

7.3.2.  UDP-related Issues

      Unlike TCP or SCTP, UDP is a stateless protocol,
   at least conceptually.  However, many implementations of UDP
   have some session state (e.g. Protocol Control Blocks in
   4.4 BSD), although the UDP protocol specifications do not
   require any state.

      One consequence of this is that in widely used host
   implementations of UDP and IPv6, a UDP listener might be
   bound only to a particular UDP port on its host -- without
   binding to a particular remote IP address or local IP

      UDP can be used with unicast or with multicast.  Some
   existing UDP host implementations permit a single UDP packet
   to be delivered to more than one listener at the same time.
   Except for the application of Mandatory Access Controls,
   the behaviour of a given system should remain the same
   (so that application behaviour does not change in some
   unexpected way) with respect to delivery of UDP datagrams
   to listeners.

      For example, if a listener on UDP port X has a
   Sensitivity Label range with a minimum of "S:AB" and a
   maximum of "S:AB", then only datagrams with a destination
   of UDP port X and a Sensitivity Label of "S:AB" will be
   delivered to that listener.

      For example, if a listener on UDP port Y has a
   Sensitivity Label range with a minimum of "W::" and a

StJohns Et Alia                                                [Page 45]
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   maximum of "X:ABC" (where X dominates W), then a datagram
   addressed to UDP port Y with a Sensitivity Label of "W:A"
   normally would be delivered to that listener.

7.3.3.  SCTP-related Issues

      With respect to a network, each distinct Sensitivity Label
   represents a separate virtual network which shares the same
   physical network.

      The above statement taken from section 3 has a non-obvious,
   but critical, corollary.  If there are separate virtual
   networks, then it is possible for a system which exists
   in multiple virtual networks to have identical SCTP
   connections, each one existing in a different virtual

      As with TCP, SCTP is a connection- oriented transport
   protocol and has substantial session state.  Unlike TCP,
   SCTP can support session endpoint migration among IP
   addresses at the same end node(s), and SCTP can also
   support both one-to-one and one-to-many communications

     In single-level hosts, in the one-to-one mode, the SCTP
   session state for a single local SCTP session includes the
   set of remote IP addresses for the single remote SCTP
   instance, the set of local IP addresses, the remote SCTP
   port number, and the local SCTP port number.

     In single-level hosts, in the one-to-many mode, the SCTP
   session state for a a single local SCTP instance can have
   multiple concurrent connections to several different remote
   SCTP peers.  There cannot be more than one connection from
   a single SCTP instance to any given remote SCTP instance.
   Thus, in single-level hosts, in the one-to-many mode, the
   local SCTP session state includes the set of remote IP
   addresses, the set of local IP addresses, the remote SCTP
   port number for each remote SCTP instance, and the (single)
   local SCTP port number.

      In MLS hosts, for either SCTP mode, the SCTP session
   state additionally includes the Sensitivity Label for each
   SCTP session.  A single SCTP session, whether in the
   one-to-one mode or in the one-to-many mode, MUST have
   a single Sensitivity Label, rather than a Sensitivity
   Label range.

StJohns Et Alia                                                [Page 46]
Internet-Draft                                                6 MAR 2009

      Unlike TCP, SCTP has the ability to shift an existing
   SCTP session from one endpoint IP address to a different
   IP address that belongs to the same endpoint, when one
   or more endpoints have multiple IP addresses.  If such
   shifting occurs within an MLS deployment, it is important
   that it only move to an IP address with a Sensitivity
   Label range that includes that SCTP sessions own
   Sensitivity Label.

      Further, although a node might be multi-homed, it is
   entirely possible that only one of those interfaces is
   reachable for a given Sensitivity Label value.  For example,
   one network interface on a node might have a Sensitivity
   Label range from "A::" to "B:XY" (where B dominates A),
   while a different network interface on the same node might
   have a Sensitivity Label range from "U::" "U::" (where A
   dominates U).  In that example, if a packet has a CALIPSO
   label of "A:X", then that packet will not be able to use the
   "U"-only network interface.  Hence, a SCTP implementation
   needs to consider the Sensitivity Label of each SCTP instance
   on the local system when deciding which of its own IP
   addresses to communicate to the remote SCTP instance(s)
   for that SCTP instance.  This issue might lead to novel
   operational issues with SCTP sessions.  Implementers ought
   to give special attention to this SCTP-specific issue.

7.3.4.  Security Logging

      This option is recommended for deployment only in
   well-protected private networks that are NOT connected
   to the global Internet.  By definition, such private networks
   are also composed only of trusted systems that are believed
   to be trustworthy.  So the risk of a denial of service attack
   upon the logging implementation is much lower in the intended
   deployment environment than it would have been for general
   Internet deployments.


      This document describes a mechanism for adding explicit
   Sensitivity Labels to IPv6 datagrams.  The primary purpose of
   these labels is to facilitate application of Mandatory Access
   Controls (MAC) in End Systems or Intermediate Systems that
   implement this specification.  As such, correct implementation
   of this mechanism is very critical to the overall security
   of the systems and networks where this mechanisms is deployed.
   Use of high-assurance development techniques is encouraged.
   End users should carefully consider the assurance requirements

StJohns Et Alia                                                [Page 47]
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   of their particular deployment, in the context of that
   deployment's prospective threats.

      A concern is that since this label is used for mandatory
   access controls, some method of binding the Sensitivity Label
   option to the rest of the packet is needed.  Without such
   binding, malicious modification of the Sensitivity Label in a
   packet would go undetected.  So, implementations of this
   Sensitivity Label option MUST also implement support for the IP
   Authentication Header (AH).  Implementations MUST permit the
   system administrator to configure whether AH is used or not.

     ESP with null encryption mechanism can only protect the payload
   of an IPv6 packet, not any Hop-by-Hop Options.  By contrast,
   AH protects all invariant headers and data of an IPv6 packet,
   including the CALIPSO Hop-by-Hop option.  The CALIPSO option
   defined in this document is always an IPv6 Hop-by-Hop option,
   because the CALIPSO option needs to be visible to, and parsable
   by, IPv6 routers and security gateways so that they can apply
   MAC policy to packets.

     It is anticipated that if AH is being used with a symmetric
   authentication algorithm, then not only the recipient host,
   but also one or more security gateways along the path, will
   have knowledge of the symmetric key -- so that AH can be used
   to authenticate the packet, including the CALIPSO label.  In
   this case, all devices knowing that symmetric authentication
   key would need to be trusted.  Alternatively, AH may be used
   with an asymmetric authentication algorithm, so that the
   recipient and any security gateways with knowledge of the
   authentication key can authenticate the packet, including
   the CALIPSO label.

     If AH or ESP are employed to provide "labelled IP Security"
   within some CALIPSO deployment, then the Sensitivity Label of
   the IP Security Association used for a given packet MUST have
   the same meaning as the Sensitivity Label carried in the
   CALIPSO option of that packet, in order that MAC policy can
   and will be correctly applied.

      Because the IP Authentication Header will include the CALIPSO
   option among the protected IPv6 header fields, modification of
   a CALIPSO-labelled packet that also contains an IP Authentication
   Header will cause the resulting packet to fail authentication
   at the destination node for the AH security session.  Therefore,
   CALIPSO labels cannot be inserted, deleted, or translated for
   IPv6 packets that contain an IP Authentication Header.
   (N.B. It might be important to recall that the "not modified

StJohns Et Alia                                                [Page 48]
Internet-Draft                                                6 MAR 2009

   en route" bit for IPv6 option types was really created to be
   the "include in AH calculations" signal and was not designed
   for some other purpose.)  In situations where a modification
   by an intermediate system is required by policy, but is not
   possible due to AH, then the packet MUST be dropped instead.
   If the packet must be dropped for this reason, then an ICMP
   Destination Unreachable error message SHOULD be sent back to
   the originator of the dropped packet with a reason code of
   "Administratively Prohibited".  If the packet can be forwarded
   properly without violating the MLS MAC policy of the
   intermediate system, then (by definition) such a packet
   modification is not required.

        Note that in a number of error situations with labelled
   networking, an ICMP error message MUST NOT be sent in order to
   avoid creating security problems.  In certain other error
   situations, an ICMP error message might be sent.  Such ICMP
   handling details have been described earlier in this document.
   Even if an ICMP error message is sent, it might be dropped along
   the way before reaching its intended destination -- due to MAC
   rules, DOI differences, or other configured security policies
   along the way from the node creating the ICMP error message to
   the intended destination node.  In turn, this can mean
   operational faults (e.g. fibre cut, misconfiguration) in a
   labelled network deployment might be more difficult to identify
   and resolve.

      This mechanism is only intended for deployment in very limited
   circumstances where a set of systems and networks are in a
   well-protected operating environment and the threat of external
   or internal attack on this mechanism is considered acceptable to
   the accreditor of those systems and networks.  IP packets
   containing visible packet labels ought never traverse the public

      This specification does not seek to eliminate all possible
   covert channels.  The TCP specification clarification in
   Section 7.3.1 happens to reduce the bandwidth of a particular
   known covert channel, but is present primarily to clarify
   how networked MLS systems have always been implemented. [TNI]
   [DoD MLOS PP]

      Of course, subject to local security policies, encrypted
   IPv6 packets with CALIPSO labels might well traverse the
   public Internet after receiving suitable cryptographic
   protection.  For example, a CALIPSO labelled packet might
   travel either through a Tunnel-mode ESP (with encryption)
   VPN tunnel that connects two or more MLS labelled network

StJohns Et Alia                                                [Page 49]
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   segments.  Alternatively, a CALIPSO labelled IPv6 packet
   might travel over some external link that has been protected
   by the deployment of evaluated, certified, and accredited
   bulk encryptors that would encrypt the labelled packet
   before transmission onto the link and decrypt the labelled
   packet after reception from the link.

      Accreditors of a given CALIPSO deployment should consider
   not only personnel clearances and physical security issues,
   but also electronic security (e.g. TEMPEST), network security
   (NETSEC), communications security (COMSEC), and other issues.
   This specification is only a small component of an overall
   MLS network deployment.


9.1  IP Option Number

      CALIPSO requires an IPv6 Option Number [RFC-2460]:

      HEX         act  chg  rest
      ----        ---- ---  ----
      TBD3         00   0      TBD4   CALIPSO

      For the IPv6 Option Number, the first two bits indicate
   that the IPv6 node skip over this option and continue
   processing the header if it does not recognise the option
   type.  The third bit indicates that the Option Data must
   not change en-route.

      This document should be listed as the reference document.

9.2 CALIPSO DOI Values Registry

      IANA is requested to create a registry for
   CALIPSO DOI values.  The initial values for the
   CALIPSO DOI registry, shown in colon-separated
   quad format, are as follows:

      DOI Value                     Organisation or Use
      =======================       ============================
      0:0:0:0                       Null DOI.  This ought not
                                    be used on any network;
      0:0:0:1 to 0:255:255:255      For private use among
                                    consenting parties within
                                    private networks;
      1:0:0:0 to 254:255:255:255    For assignment by IANA to

StJohns Et Alia                                                [Page 50]
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                                    organisations following the
                                    Expert Review procedure.
      255:0:0:0 to 255:255:255:255  Reserved to the IETF for
                                    future use by possible
                                    revisions of this specification.

      The CALIPSO DOI value 0:0:0:0 is the Null DOI and
   is not to be used on any network or in any deployment.

      All other CALIPSO DOI values beginning with decimal 0:
   are reserved for private use amongst consenting parties;
   values in this range will not be allocated by IANA to any
   particular user or user community.

      For the CALIPSO DOI values 1:0:0:0 through 254:255:255:255
   (inclusive), IANA should follow the Expert Review procedure
   when DOI Allocation requests are received.

      CALIPSO DOI values beginning with decimal 255 are reserved
   to the IETF for potential future use in revisions of this
   specification.  IESG approval is required for allocation
   of DOI values within that range.


      This document is directly derived from an Internet-Draft
   titled "Son of IPSO (SIPSO)" written by Mike StJohns circa 1992.
   Packet format changes have been made since that draft, primarily
   to comply with IPv6 syntax rules.  The concepts, most definitions,
   and nearly all of the processing rules here are identical to those
   in that earlier document.

      Steve Brenneman, L.C. Bruzenak, James Carlson, Pasi Eronen,
   Michael Fidler, Bob Hinden, Alfred Hoenes, Russ Housley, Suresh
   Krishnan, Jarrett Lu, Dan McDonald, Paul Moore, Joe Nall, Dave
   Parker, Tim Polk, Ken Powell, Randall Stewart, Bill Sommerfeld,
   and Joe Touch (listed in alphabetical order by family name)
   provided specific feedback on earlier versions of this document.

     The editors also would like to thank the several anonymous
   reviewers for their feedback, and particularly for sharing their
   insights into operational considerations with MLS networking.

      The editors would like to thank the IESG as a whole for
   providing feedback on earlier versions of this document.

StJohns Et Alia                                                [Page 51]
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  [BL73]         Bell, D.E. & LaPadula, L.J., "Secure Computer
                 Systems: Mathematical Foundations and Model"
                 Technical Report M74-244, MITRE Corporation,
                 Bedford, MA, May 1973.

  [CW87]         D.D. Clark & D.R. Wilson, "A Comparison of
                 Commercial and Military Computer Security
                 Policies", in Proceedings of the IEEE Symposium
                 on Security & Privacy, pp. 184-194, IEEE
                 Computer Society, Oakland, CA, May 1987.

  [CMW]          US Defense Intelligence Agency,
                 "Compartmented Mode Workstation Evaluation
                 Criteria", Technical Report
                 DDS-2600-6243-91, Washington, DC,
                 November 1991.

  [DOD 5200.1]   US Department of Defense,
                 "DoD Information Security Program",
                 Directive 5200.1, 13 December 1996.

  [DOD 5200.1-R] US Department of Defense,
                 "Information Security Program Regulation",
                 DoD 5200.1-R, 17 January 1997.

  [DoD 5200.28]  US Department of Defense, "Security Requirements
                 for Automated Information Systems,"
                 Directive 5200.28, 21 March 1988.

  [DoD MLOS PP]  US Department of Defense,
                 "Protection Profile for Multi-level
                 Operating Systems in Environments requiring
                 Medium Robustness, Version 1.22, 23 May 2001.

  [ISO-15408]    International Stanards Organisation,
                 "Evaluation Criteria for IT Security",
                 ISO/IEC 15408, 2005.

  [CC]          "Common Criteria for Information Technology
                Security Evaluation", Version 3.1, Revision
                1, CCMB-2006-09-001, September 2006.

  [TCSEC]        US Department of Defense, "Trusted Computer
                 System Evaluation Criteria", DoD 5200.28-STD,
                 26 December 1985.

StJohns Et Alia                                                [Page 52]
Internet-Draft                                                6 MAR 2009

  [TNI]          (US) National Computer Security Center, "Trusted
                 Network Interpretation (TNI) of the Trusted Computer
                 System Evaluation Criteria", NCSC-TG-005,
                 Version 1, 31 July 1987.

  [DoD 8500.1]   US Department of Defense, "Information Assurance",
                 Directive 8500.1, 24 October 2002.

  [FIPS-188]     US National Institute of Standards & Technology,
                 "Standard Security Labels for Information Transfer",
                 Federal Information Processing Standard (FIPS) 188,
                 September 1994.

  [RFC-791]      J. Postel, Internet Protocol, RFC-791,
                 September 1981.

  [RFC-793]      J. Postel, Transmission Control Protocol,
                 RFC-793, September 1981.

  [RFC-1038]     M. StJohns, Draft Revised IP Security Option,
                 RFC-1038, January 1988.

  [RFC-1108]     S. Kent, US DoD Security Options for the
                 Internet Protocol, RFC-1108, November 1991.

  [RFC-1825]     R. Atkinson, Security Architecture for the
                 Internet Protocol, RFC-1825, August 1995.

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

  [IPSEC]       S. Kent & K. Seo, "Security Architecture for the
                Internet Protocol", RFC-4301, December 2005.

  [AH]          S. Kent, "IP Authentication Header", RFC-4302,
                December 2005.

  [ESP]         S. Kent, "IP Encapsulating Security Payload",
                RFC-4303, December 2005.


      [RFC-2460]     S. Deering & R. Hinden, "Internet Protocol
                     Version 6 Specification", RFC-2460,
                     December 1998.

      [RFC-2434]     T. Narten & H. Alvestrand, "Guidelines

StJohns Et Alia                                                [Page 53]
Internet-Draft                                                6 MAR 2009

                     for Writing an IANA Considerations Section",
                     RFC-2434, BCP 26, October 1998.

      [RFC-1662]     W. Simpson, "PPP in HDLC-like Framing",
                     Appendix C, RFC-1662, July 1992.


   M. StJohns
   Germantown, MD

   R. Atkinson
   Extreme Networks
   3585 Monroe Street
   Santa Clara, CA
   USA 95051
   +1 (408)579-2800

   G. Thomas
   US Department of Defense
   Washington, DC

StJohns Et Alia                                                [Page 54]
Internet-Draft                                                6 MAR 2009


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Internet-Draft                                                6 MAR 2009

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StJohns Et Alia                                                [Page 56]