





Network Working Group                                        M. StJohns
Request for Comments: 5570                                   Consultant
Category: Informational                                     R. Atkinson
                                                       Extreme Networks
                                                              G. Thomas
                                               US Department of Defense
                                                              July 2009


        Common Architecture Label IPv6 Security Option (CALIPSO)

Abstract

   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.

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

IESG Note

   This RFC specifies the use of an IPv6 hop-by-hop option.  The IESG
   notes that general deployment of protocols with hop-by-hop options
   are problematic, and the development of such protocols is
   consequently discouraged.  After careful review, the IETF has
   determined that a hop-by-hop option is an appropriate solution for
   this specific limited environment and use case.  Furthermore, the
   mechanism specified in this RFC is only applicable to closed IP
   networks.  It is unsuitable for use and ineffective on the global
   public Internet.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.





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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.








































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Table of Contents

   1. Introduction ....................................................4
      1.1. History ....................................................4
      1.2. Intent and Applicability ...................................6
      1.3. Deployment Examples ........................................7
   2. Definitions .....................................................9
      2.1. Domain of Interpretation ...................................9
      2.2. Sensitivity Level .........................................10
      2.3. Compartment ...............................................10
      2.4. Releasability .............................................11
      2.5. Sensitivity Label .........................................16
      2.6. Import ....................................................17
      2.7. Export ....................................................17
      2.8. End System ................................................18
      2.9. Intermediate System .......................................18
      2.10. System Security Policy ...................................19
   3. Architecture ...................................................19
   4. Defaults .......................................................24
   5. Format .........................................................26
      5.1. Option Format .............................................27
      5.2. Packet Word Alignment Considerations ......................30
   6. Usage ..........................................................31
      6.1. Sensitivity Label Comparisons .............................31
      6.2. End System Processing .....................................34
      6.3. Intermediate System Processing ............................37
      6.4. Translation ...............................................40
   7. Architectural and Implementation Considerations ................41
      7.1. Intermediate Systems ......................................42
      7.2. End Systems ...............................................43
      7.3. Upper-Layer Protocols .....................................43
   8. Security Considerations ........................................46
   9. IANA Considerations ............................................48
      9.1. IP Option Number ..........................................48
      9.2. CALIPSO DOI Values Registry ...............................49
   10. Acknowledgments ...............................................50
   11. References ....................................................50
      11.1. Normative References .....................................50
      11.2. Informative References ...................................50












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1.  Introduction

   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 [RFC791] [RFC1038] [RFC1108].
   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.  Some banks use MLS
   technology to restrict sensitive information, for example information
   about mergers and acquisitions.  This IPv6 option, like its IPv4
   predecessors, is only intended for deployment 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 [RFC1038].
   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 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, wire-speed.






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   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 Application-Specific Integrated Circuits (ASICs);
   supporting more complex packet formats usually requires 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 the 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 [RFC1825]
   [RFC4301] [RFC4302] [RFC4303].  For MLS IPv6 deployments where the
   use of AH or ESP is practical, the 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, 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 packet.

   Existing Layer 3 Virtual Private Network (VPN) technology can't solve
   the set of issues addressed by this specification, for several
   independent reasons.  First, in a typical deployment, many labeled
   packets will flow from an MLS End System through some set of networks
   to a receiving MLS End System.  The received per-packet label is used
   by the receiving MLS End System 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 MLS End Systems, nor in typical single-level End
   System operating systems, but instead typically are only implemented
   in routers.  Adding a Layer 3 VPN implementation to the networking
   stack of an MLS End System would be a great deal more work than
   adding this IPv6 option to that same MLS End System.  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



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   essential if one wanted to obviate this IPv6 option.  Substantial new
   standards development, along with significant new implementation work
   in End Systems, 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 End
   System (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 and 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 organizations for at least a decade now.

   In particular, the authors believe that this mechanism is suitable
   for deployment in United Nations (UN) peace-keeping operations, in
   North Atlantic Treaty Organisation (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.

   Because of the unusually severe adverse consequences (e.g., loss of
   life, loss of very large sums of money) likely if a packet labeled
   with this IPv6 Option were to escape onto the global public Internet,
   organizations deploying this mechanism have unusually strong
   incentives to configure security controls to prevent labeled 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.




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   However, to ensure interoperability of both End Systems and
   Intermediate Systems within such a labeled 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", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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]
   [MLOSPP].  In this type of deployment, all IP packets carried within
   the private internetwork are labeled, the IP routers apply mandatory
   access controls (MAC) based on the packet labels and the sensitivity
   ranges configured into the routers, all End Systems include packet
   Sensitivity Labels in each originated packet, and all End Systems
   apply Mandatory Access Controls to each received packet.  Packets
   received by a router or End System 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 End Systems
   with non-MLS operating systems are present on certain subnetworks of
   the private internetwork.  By definition, these non-MLS End Systems
   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



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   example of "system high" operation.  In this scenario, each
   subnetwork that contains any single-level End Systems has one single
   "default" Sensitivity Label that applies to all single-level systems
   on that IP subnetwork.  Because those non-MLS End Systems 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" End Systems that are to be forwarded
   off subnet.  While the CALIPSO IPv6 option is marked as "ignore if
   unrecognized", there are some deployed IPv6 End Systems with bugs.
   Users can't fix these operating system bugs; some users need to be
   able to integrate their existing IPv6 single-level End Systems to
   have a useful overall MLS deployment.  So, for packets destined for
   IP subnetworks containing single-level End Systems, 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) exclusive-or 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 Network Address Translation (NAT), Network
   Address and Port Translation (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 isolated from the global
   Internet, so operators of MLS networks are not concerned about
   external penetration (e.g., by worms, viruses, or the like).
   Similarly, all users of the MLS network have been cleared using some
   process specific to that organization, 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.



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2.  Definitions

   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 describe 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 Secure
   circles 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 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
   organization (e.g., US Department of Defense (DoD), US Department of
   Energy (DoE), UK Ministry of Defence (MoD), North Atlantic Treaty
   Organisation (NATO), United Nations (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 [DoD5200.28] and [DoD5200.1-R] documents, which define how to
   mark and protect data with the US Department of Defense (DoD)
   [DoD5200.28] [DoD5200.1-R].





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   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 organization
   (e.g., NATO) might create a DOI, while a given member nation or
   organization (e.g., UK MoD) might be using that multi-national DOI
   (possibly along with other DOIs created by others) within its private
   networks.  To provide a different example, the United States 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 organization 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 Internet Security Association and Key Management
   Protocol (ISAKMP) / Internet Key Exchange (IKE) Domain of
   Interpretation.  It is important not to confuse the two different
   concepts, even though the terms might superficially appear to be
   similar.

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



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   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.

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 behavior 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 behavior 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 REL XYZ (e.g.,
   COMPANY CONFIDENTIAL REL XYZ).

   However, unlike the compartment example above, COMPANY CONFIDENTIAL
   dominates COMPANY CONFIDENTIAL REL XYZ.  This 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.





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   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 CONFIDENTIAL REL XYZ, COMPANY
   CONFIDENTIAL REL ABLE, or COMPANY CONFIDENTIAL REL XYZ/ABLE can
   access this information.

2.4.2.  Releasability Encoding

   Individual bits in this option's Compartment Bitmap field MAY be used
   to encode "releasability" 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 in Section 2.5.1 and 2.5.2) to determine whether or not a
   packet's label is in-range for an interface.  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.






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

   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, and 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:
            CONFIDENTIAL RELEASABLE AC

   Therefore, this interface has a minimum Releasability of 0101.

   Let the MAXIMUM label for that router interface be:
            TOP SECRET NOT RELEASABLE



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   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:
          (CONFIDENTIAL <= CONFIDENTIAL <= TOP SECRET)
          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:
          (CONFIDENTIAL <= CONFIDENTIAL <= TOP SECRET)
          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:
          (CONFIDENTIAL <= SECRET <= TOP SECRET)
          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.

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.




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   Now, let us consider an equivalent electronic example implemented and
   deployed as outlined above.  In this, we consider two 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.
   Users also report that various operational complexities can arise
   from this approach.

   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 to associate
   multiple clearances with each given user ID because that level of
   detail is beyond the scope of an IP labeling 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 results would not have been different than those
   described here.

   Several MLS network deployments connect MLS End Systems both to a
   labeled national network and also to a labeled coalition network
   simultaneously.  Depending on whether the data is labeled 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).





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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.

   End Systems 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 receive.

   End Systems 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".

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,





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     3) A equals B (e.g., A=SECRET, B=SECRET);
        A can read/write B,

     exclusive-or

     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.  Sensitivity Label 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.

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:












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           Source Port
           Destination Port
           Transport Protocol
           Application Protocol
           Application Information
           End System
           Subnetwork
           Network
           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 cognizant 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 has defined the term Node to include both
   Intermediate Systems and End Systems [RFC2460].

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.  An Intermediate System might also be called a
   "gateway", "guard", or "router" in some user communities.

   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.

   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.







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2.10.  System Security Policy

   A System Security Policy (SSP) consists of a Sensitivity Label and
   the organizational security policies associated with content labeled
   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 Systems Security
   Officer (ISSO) of the site or organization covered by that SSP.

3.  Architecture

   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 labeling schemes also exist
   [CW87].  For example, one global electrical equipment company has a
   formal security policy that defines six 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 and
   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
   organizations 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".

   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



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   appropriate "downgrade" privilege manually verifies the information
   is permitted to be downgraded before s/he manually relabels (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 information.

   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 specifications for (1) encoding
   Sensitivity Labels in packets, and (2) how such Sensitivity Labels
   are to be interpreted and enforced at the IP layer.  This document
   recognizes 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] [MLOSPP].

   This specification does not preclude an End System capable of
   providing labeled 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 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





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   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 it.

   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 practice.

   Many existing MLS deployments already use (and operationally need to
   use) more than one DOI concurrently.  User feedback from early
   versions of this specification indicates that it is common at present
   for a single 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 organization) 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




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   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 two CALIPSO DOIs concurrently.  Support for more
   than two concurrent CALIPSO DOIs is encouraged.  This requirement to
   support at least two 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,

   AND

        2) Publish a document with a table of equivalencies that
           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 that are a subset of the full table of
   equivalences.  Actual administration of inter-DOI agreements is
   outside the scope of this document.





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   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.  Selection of the Export DOI is described in detail
   in Section 6.2.1.

   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 interface.

   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.  One or more Sensitivity Label ranges are
   associated with each physical network interface.  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 interfaces.

   A given logical network interface might be associated with more than
   one physical interface.  For example, a switch/router might have two
   separate Ethernet ports that are associated with the same Virtual
   Local Area Network (VLAN), where that one VLAN mapped to a single
   IPv6 subnetwork [IEEE802.1Q].

   A given physical network interface might have more than one
   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.

   One or more Sensitivity Label ranges are associated with each logical
   network interface.  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.





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   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 authorized to
   provide on-the-fly relabeling of packets at the boundaries between
   complete systems of End Systems within a single DOI.  Typically, such
   systems require specific certification(s) and accreditation(s) before
   deployment or use.

4.  Defaults

   This Section describes the default behavior of CALIPSO-compliant End
   Systems and Intermediate Systems.  Implementers MAY implement
   configuration knobs to vary from this behavior, provided that the
   default behavior (i.e., if the system administrator does not
   explicitly change the configured behavior of the device) is as
   described below.  If implementers choose to implement such
   configuration knobs, the configuration parameters and the behaviors
   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 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 interface.

   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-labeled packet
   traverses, not merely at the end points of a labeled IP session.  As



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   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 unlabeled packet is received from a node that does not support
   CALIPSO Sensitivity Labels (i.e., unable to assign Sensitivity Labels
   itself) and the packet is destined for a node that supports CALIPSO
   Sensitivity Labels, then the receiving intermediate system needs to
   insert a Sensitivity Label.  This Sensitivity Label MUST 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, then the maximum Sensitivity Label of the
   interface that received the unlabeled packet MUST 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
   link.

   If a packet is to be sent to a node that is defined to not be
   Sensitivity Label aware, from a node that is label aware, then the
   Sensitivity Label MAY be 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 labeled,
   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: In some IPv4 MLS network deployments that exist as of the
   publication date, if a first-hop router receives an unlabeled IPv4
   packet, the router inserts an appropriate Sensitivity Label into that
   IPv4 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.







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5.  Format

   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 tunneled contains at most one
   CALIPSO label.  In the special case where (1) a labeled IPv6 datagram
   is tunneled inside another labeled 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 (UNCLASSIFIED).

   If the inner IPv6 packet is tunneled 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 labeled cleartext IP packet that has been
   encapsulated (tunneled) inside another labeled IP packet.  This is
   true whether one has IPv6 tunneled in IPv6, IPv4 tunneled in IPv6, or
   IPv6 tunneled 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 an 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, encode, and implement in more general terms.








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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 Section 4.2 of RFC 2460, the Option Type field of this option must
   have 4n+2 alignment [RFC2460].

   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 unlabeled 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 three
   exceptions are described elsewhere in this document.

   If the option type is not recognized by a node examining the packet,
   the option is ignored.  However, all implementations of this
   specification MUST be able to recognize 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
   [RFC2460].  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 to apply MLS Mandatory Access Controls
   to those packets.

   The diagram below shows the CALIPSO option along with the required
   (first) two 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)      |
   +-------------+---------------+-------------+--------------+






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5.1.1.  Option Type Field

   This field contains an unsigned 8-bit value.  Its value is 00000111
   (binary).

   Nodes that do not recognize 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 routers that do not support the CALIPSO forward this
   packet normally, even though those routers do not recognize the
   CALIPSO 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 one octet in size.  Its
   minimum value is eight (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 calculation.

5.1.3.  Compartment Length Field

   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



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   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.

5.1.5.  Domain of Interpretation Field

   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.

   NOTE WELL: The Domain Of Interpretation value where all 4 octets
   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.

5.1.6.  Sensitivity Level Field

   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 [RFC1662].  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




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   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
   specifications.

5.1.8.  Compartment Bitmap Field

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

   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
   devices.

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.








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   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.

6.  Usage

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

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 (MACs) to network traffic entering or leaving the system.

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

     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) Sensitivity Level.

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

   AND



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        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
            that 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.

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

   AND EITHER

        2.   The Sensitivity Level of M is less than the
             Sensitivity Level of LO.

             (M.LEV < LO.LEV)

   OR

        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)

   AND EITHER




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        2A.  M's Sensitivity Level is above HI's Sensitivity Level.

             (M.LEV > HI.LEV)

   OR

        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.

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) ))








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        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.

6.2.1.  Export

   An End System that 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 selected.

        b) Else, if there are tables defining a specific default
           DOI for the specific destination End System address
           or for the network address, then that DOI is selected.

        c) Else, if 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.

   NOTE WELL: A connection-oriented transport-layer protocol session
   (e.g., Transmission Control Protocol (TCP) session, Stream Control
   Transmission Protocol (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.





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   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 labeled 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 recognized 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, such as ENOTREACH).  This fault represents
   (1) incorrect configuration of either the Intermediate System or of
   the End System or (2) correct operation for a node that is not
   permitted to send IPv6 packets with that DOI through that
   Intermediate System.

   When an MLS End System 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, then



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        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:

        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 unrecognized 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 with a Sensitivity Label within the
             permitted range is accepted for further processing.

             A datagram with 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 with 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.



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             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.

        5.   Once the datagram has been accepted, the receiving
             system MUST use the import Sensitivity Label and DOI
             to associate the appropriate internal Sensitivity Label
             with the data in the received datagram.  This label
             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 Application-Specific Integrated Circuit (ASIC) or Field
   Programmable Gate Array (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-labeled packet, an Intermediate System must:





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        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
            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 unrecognized 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
            happened.

        5.  Verify the Sensitivity Label within the CALIPSO
            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 rejected datagram with 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.





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            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.

   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
   paragraphs.

6.3.2.  Translation by Intermediate Systems

   It is at this point, after input processing and before output
   processing, that translation of the CALIPSO from one DOI to another
   DOI takes place in an Intermediate System, if at all.  Section 6.4
   describes the two possible approaches to 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.




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        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.

            If the cause of the dropped packet is that the
            DOI is prohibited or unrecognized, 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
            unlabeled packet has been dropped because the
            packet is required to be labeled, 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 that provides on-the-fly relabeling is said to "translate"
   from one DOI to another.  There are basically two ways a datagram can
   be relabeled:

   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, exclusive-or 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" (Section 6.2.2) and "Export" (Section 6.2.1) sections
   above.

   The remainder of this section describes the second method, which is
   direct relabeling.  The choice of which method to use for relabeling
   is an implementation decision outside the scope of this document.





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   A system that 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 passed the Intermediate System input processing
   and input validation described in Section 6.3.1, and has been
   accepted as valid, the CALIPSO in that datagram may be relabeled.  To
   determine the new Sensitivity Label, first determine the new output
   DOI.

   The selection of the output DOI may be based on any of Incoming DOI,
   Incoming Sensitivity Label, Destination End System, Destination
   Network, Destination Subnetwork, Sending Interface, or Receiving
   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 has been relabeled, the Intermediate System output
   procedures described in Section 6.3.3 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.

7.  Architectural and Implementation Considerations

   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
   implementing an MLS End System, is available from a range of sources
   [TCSEC] [TNI] [CMW] [CC] [ISO-15408] [MLOSPP].



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   Because the usual 5-tuple (i.e., Source IP address, Destination IP
   address, Transport protocol, Source Port, and Destination Port) do
   not necessarily uniquely identify a flow within a labeled 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 labeled MLS network deployment using such
   application protocols.  For example, Resource Reservation Protocol
   (RSVP), Session Initiation Protocol (SIP), and Session Description
   Protocol (SDP) might be impacted by this.

   A number of Commercial-Off-The-Shelf (COTS) applications (e.g.,
   Remote Access Dial-In User Service (RADIUS), Hyper-Text Transfer
   Protocol (HTTP), and Transport-Layer Security (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
   router.  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 an 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, Routing Information Base (RIB) or Forwarding
   Information Base (FIB) calculations, their routing protocols, or
   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 categorized and handled by Intermediate Systems as if
   they were a single flow.

   Router performance is optimized 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



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   should make it more practical to create custom logic (e.g., in
   Verilog) with support for this option and the associated Mandatory
   Access Controls.

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 an MLS End
   System 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 [MLOSPP].

   In a typical MLS End System, 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 End System
   operating system.  The MLS End System 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 that exists in multiple virtual networks to
   have identical TCP connections, each one existing in a different
   virtual network.



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   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] [RFC793].  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 End System
   implementations of UDP and IPv6, a UDP listener might be bound only
   to a particular UDP port on its End System -- without binding to a
   particular remote IP address or local IP address.

   UDP can be used with unicast or with multicast.  Some existing UDP
   End System 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 behavior of a given
   system should remain the same (so that application behavior 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 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.




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   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 that exists in multiple virtual networks to
   have identical SCTP connections, each one existing in a different
   virtual network.

   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 communication
   sessions.

   In single-level End Systems, 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 End Systems, in the one-to-many mode, the SCTP
   session state for 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 End Systems, 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 End Systems, 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.

   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 session's 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



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   "A:X", then that packet will not be able to use the "U"-only network
   interface.  Hence, an 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.

8.  Security Considerations

   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 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 or not AH is used.

   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.



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   It is anticipated that if AH is being used with a symmetric
   authentication algorithm, then not only the recipient End System, 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 "labeled 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-
   labeled 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.

   NOTE WELL: The "not modified during transit" bit for IPv6 option
   types was really created to be the "include in AH calculations"
   signal.  There was no other reason to define that bit in IPv6.

   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 labeled 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



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   message to the intended destination node.  In turn, this can mean
   operational faults (e.g., fibre cut, misconfiguration) in a labeled
   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 Internet.

   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] [MLOSPP].

   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-labeled packet might travel either through a Tunnel-mode ESP
   (with encryption) VPN tunnel that connects two or more MLS-labeled
   network segments.  Alternatively, a CALIPSO-labeled 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 labeled packet before transmission onto the
   link and decrypt the labeled 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.  IANA Considerations

9.1.  IP Option Number

   An IPv6 Option Number [RFC2460] has been registered for CALIPSO.

      HEX             BINARY
                act   chg   rest
      ---       ---   ---   -----
        7        00     0   00111          CALIPSO






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   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 recognize the option type.  The third bit indicates that the
   Option Data must not change en route.

   This document is listed as the reference document.

9.2.  CALIPSO DOI Values Registry

   IANA has created 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                     Organization 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
                                    organizations following the
                                    Expert Review procedure
                                    [RFC5226].

      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.





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10.  Acknowledgments

   This document is directly derived from an Internet-Draft titled "Son
   of IPSO (SIPSO)" written by Mike StJohns circa 1992.  Various changes
   have been made since then, primarily to support IPv6 instead of IPv4.
   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 authors 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 authors would like to thank the IESG as a whole for providing
   feedback on earlier versions of this document.

11.  References

11.1.  Normative References

   [RFC1662]     Simpson, W., Ed., "PPP in HDLC-like Framing", STD 51,
                 RFC 1662, July 1994.

   [RFC2460]     Deering, S. and R. Hinden, "Internet Protocol, Version
                 6 (IPv6) Specification", RFC 2460, December 1998.

   [RFC5226]     Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section in RFCs", BCP 26, RFC
                 5226, May 2008.

11.2.  Informative References

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

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




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

   [DoD5200.1-R]
                 US Department of Defense, "Information Security Program
                 Regulation", DoD 5200.1-R, 17 January 1997.

   [DoD5200.28]  US Department of Defense, "Security Requirements for
                 Automated Information Systems," Directive 5200.28, 21
                 March 1988.

   [MLOSPP]      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 Standards 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.

   [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.

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

   [IEEE802.1Q]  IEEE, "Virtual Bridged Local Area Networks", IEEE
                 Standard for Local and metropolitan area networks,
                 802.1Q - 2005, ISBN 0-7381-4876-6, IEEE, New York, NY,
                 USA, 19 May 2006.

   [RFC791]      Postel, J., "Internet Protocol", STD 5, RFC 791,
                 September 1981.

   [RFC793]      Postel, J., "Transmission Control Protocol", STD 7, RFC
                 793, September 1981.




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   [RFC1038]     St. Johns, M., "Draft revised IP security option", RFC
                 1038, January 1988.

   [RFC1108]     Kent, S., "U.S. Department of Defense Security Options
                 for the Internet Protocol", RFC 1108, November 1991.

   [RFC1825]     Atkinson, R., "Security Architecture for the Internet
                 Protocol", RFC 1825, August 1995.

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

   [RFC4301]     Kent, S. and K. Seo, "Security Architecture for the
                 Internet Protocol", RFC 4301, December 2005.

   [RFC4302]     Kent, S., "IP Authentication Header", RFC 4302,
                 December 2005.

   [RFC4303]     Kent, S., "IP Encapsulating Security Payload (ESP)",
                 RFC 4303, December 2005.

Authors' Addresses

   Michael StJohns
   Germantown, MD
   USA

   EMail: mstjohns@comcast.net


   Randall Atkinson
   Extreme Networks
   3585 Monroe Street
   Santa Clara, CA
   USA 95051

   EMail: rja@extremenetworks.com
   Phone: +1 (408)579-2800


   Georg Thomas
   US Department of Defense
   Washington, DC
   USA







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