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
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
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
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.
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.
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
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.
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.
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) | +-------------+---------------+-------------+--------------+
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
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
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.
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
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
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) ))
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.
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
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.
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:
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.
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.
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.
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.