RFC1108

Network Working Group S. Kent Request for Comments: 1108 BBN Communications Obsoletes: RFC 1038 November 1991

                      U.S. Department of Defense
              Security Options for the Internet Protocol

Status of this Memo

  This RFC specifies an IAB standards track protocol for the Internet
  community, and requests discussion and suggestions for improvements.
  Please refer to the current edition of the "IAB Official Protocol
  Standards" for the standardization state and status of this protocol.
  Distribution of this memo is unlimited.

Abstract

  This RFC specifies the U.S. Department of Defense Basic Security
  Option and the top-level description of the Extended Security Option
  for use with the Internet Protocol.  This RFC obsoletes RFC 1038
  "Revised IP Security Option", dated January 1988.

1. DoD Security Options Defined

  The following two internet protocol options are defined for use on
  Department of Defense (DoD) common user data networks:

  CF  CLASS  #  TYPE  LENGTH   DESCRIPTION

  1     0    2   130   var.    DoD Basic Security:  Used to carry the
                               classification level and protection
                               authority flags.

  1     0    5   133   var.    DoD Extended Security:  Used to carry
                               additional security information as
                               required by registered authorities.

  CF = Copy on Fragmentation

2. DoD Basic Security Option

  This option identifies the U.S. classification level at which the
  datagram is to be protected and the authorities whose protection
  rules apply to each datagram.

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  This option is used by end systems and intermediate systems of an
  internet to:

       a.  Transmit from source to destination in a network standard
       representation the common security labels required by computer
       security models,

       b.  Validate the datagram as appropriate for transmission from
       the source and delivery to the destination,

       c.  Ensure that the route taken by the datagram is protected to
       the level required by all protection authorities indicated on
       the datagram.  In order to provide this facility in a general
       Internet environment, interior and exterior gateway protocols
       must be augmented to include security label information in
       support of routing control.

  The DoD Basic Security option must be copied on fragmentation.  This
  option appears at most once in a datagram.  Some security systems
  require this to be the first option if more than one option is
  carried in the IP header, but this is not a generic requirement
  levied by this specification.

  The format of the DoD Basic Security option is as follows:

     +------------+------------+------------+-------------//----------+
     |  10000010  |  XXXXXXXX  |  SSSSSSSS  |  AAAAAAA[1]    AAAAAAA0 |
     |            |            |            |         [0]             |
     +------------+------------+------------+-------------//----------+
       TYPE = 130     LENGTH   CLASSIFICATION         PROTECTION
                                    LEVEL              AUTHORITY
                                                         FLAGS

                   FIGURE 1.  DoD BASIC SECURITY OPTION FORMAT

2.1. Type

  The value 130 identifies this as the DoD Basic Security Option.

2.2. Length

  The length of the option is variable.  The minimum length of the
  option is 3 octets, including the Type and Length fields (the
  Protection Authority field may be absent).  A length indication of
  less than 3 octets should result in error processing as described in
  Section 2.8.1.

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2.3. Classification Level

       Field Length:  One Octet

  This field specifies the (U.S.) classification level at which the
  datagram must be protected.  The information in the datagram must be
  protected at this level.  The field is encoded as shown in Table 1
  and the order of values in this table defines the ordering for
  comparison purposes.  The bit string values in this table were chosen
  to achieve a minimum Hamming distance of four (4) between any two
  valid values.  This specific assignment of classification level names
  to values has been defined for compatibility with security devices
  which have already been developed and deployed.

  "Reserved" values in the table must be treated as invalid until such
  time they are assigned to named classification levels in a successor
  to this document.  A datagram containing a value for this field which
  is either not in this table or which is listed as "reserved" is in
  error and must be processed according to the "out-of-range"
  procedures defined in section 2.8.1.

  A classification level value from the Basic Security Option in a
  datagram may be checked for equality against any of the (assigned)
  values in Table 1 by performing a simple bit string comparison.
  However, because of the sparseness of the classification level
  encodings, range checks involving a value from this field must not be
  performed based solely using arithmetic comparisons (as such
  comparisons would encompass invalid and or unassigned values within
  the range).  The details of how ordered comparisons are performed for
  this field within a system is a local matter, subject to the
  requirements set forth in this paragraph.

                   Table 1.  Classification Level Encodings

                        Value              Name

                       00000001   -   (Reserved 4)
                       00111101   -   Top Secret
                       01011010   -   Secret
                       10010110   -   Confidential
                       01100110   -   (Reserved 3)
                       11001100   -   (Reserved 2)
                       10101011   -   Unclassified
                       11110001   -   (Reserved 1)

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2.4. Protection Authority Flags

       Field Length:  Variable

  This field identifies the National Access Programs or Special Access
  Programs which specify protection rules for transmission and
  processing of the information contained in the datagram.  Note that
  protection authority flags do NOT represent accreditation
  authorities, though the semantics are superficially similar.  In
  order to maintain architectural consistency and interoperability
  throughout DoD common user data networks, users of these networks
  should submit requirements for additional Protection Authority Flags
  to DISA DISDB, Washington, D.C.  20305-2000, for review and approval.
  Such review and approval should be sought prior to design,
  development or deployment of any system which would make use of
  additional facilities based on assignment of new protection authority
  flags.  As additional flags are approved and assigned, they will be
  published, along with the values defined above, in the Assigned
  Numbers RFC edited by the Internet Assigned Numbers Authority (IANA).

       a.  Field Length: This field is variable in length.  The low-
       order bit (Bit 7) of each octet is encoded as "0" if it is the
       final octet in the field or as "1" if there are additional
       octets.  Initially, only one octet is required for this field
       (because there are fewer than seven authorities defined), thus
       the final bit of the first octet is encoded as "0".  However,
       minimally compliant implementations must be capable of
       processing a protection authority field consisting of at least 2
       octets (representing up to 14 protection authorities).
       Implementations existing prior to the issuance of this RFC, and
       which process fewer protection authority than specified here,
       will be considered minimally compliant so long as such
       implementations process the flags in accordance with the RFC.
       This field must be a minimally encoded representation, i.e., no
       trailing all-zero octets should be emitted.  If the length of
       this field as indicated by this extensible encoding is not
       consistent with the length field for the option, the datagram is
       in error and the procedure described in Section 2.8.1 must be
       followed.  (Figure 2 illustrates the relative significance of
       the bits within an octet).

                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
         High-order  |   |   |   |   |   |   |   |   |  Low-order
                     +---+---+---+---+---+---+---+---+

                        Figure 2.  Significance of Bits

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       b.  Source Flags: The first seven bits (Bits 0 through 6) in
       each octet are flags.  Each flag is associated with an
       authority.  Protection Authority flags currently assigned are
       indicated in Table 2.  The bit corresponding to an authority is
       "1" if the datagram is to be protected in accordance with the
       rules of that authority.  More than one flag may be present in a
       single instance of this option if the data contained in the
       datagram should be protected according to rules established by
       multiple authorities.  Table 3 identifies a point of contact for
       each of the authorities listed in Table 2.  No "unassigned" bits
       in this or other octets in the Protection Authority Field shall
       be considered valid Protection Authority flags until such time
       as such bits are assigned and the assignments are published in
       the Assigned Numbers RFC.  Thus a datagram containing flags for
       unassigned bits in this field for this option is in error and
       must be processed according to the "out-of-range" procedures
       defined in section 2.8.1.

       Two protection authority flag fields can be compared for
       equality (=) via simple bit string matching.  No relative
       ordering between two protection authority flag fields is
       defined.  Because these flags represent protection authorities,
       security models such as Bell-LaPadula do not apply to
       interpretation of this field.  However, the symbol "=<" refers
       to set inclusion when comparing a protection authority flag
       field to a set of such fields.  Means for effecting these tests
       within a system are a local matter, subject to the requirements
       set forth in this paragraph.

                     Table 2 - Protection Authority Bit Assignments

                               BIT
                              NUMBER     AUTHORITY

                                0        GENSER

                                1        SIOP-ESI

                                2        SCI

                                3        NSA

                                4        DOE

                             5, 6        Unassigned

                                7        Field Termination Indicator

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               Table 3 - Protection Authority Points of Contact

               AUTHORITY             POINT OF CONTACT

               GENSER                Designated Approving Authority
                                     per DOD 5200.28

               SIOP-ESI              Department of Defense
                                     Organization of the
                                     Joint Chiefs of Staff
                                     Attn: J6
                                     Washington, DC  20318-6000

               SCI                   Director of Central Intelligence
                                     Attn: Chairman, Information
                                     Handling Committee, Intelligence
                                     Community Staff
                                     Washington, D.C. 20505

               NSA                   National Security Agency
                                     9800 Savage Road
                                     Attn: T03
                                     Ft. Meade, MD 20755-6000

               DOE                   Department of Energy
                                     Attn:  DP343.2
                                     Washington, DC  20545

2.5. System Security Configuration Parameters

  Use of the Basic Security Option (BSO) by an end or intermediate
  system requires that the system configuration include the parameters
  described below.  These parameters are critical to secure processing
  of the BSO, and thus must be protected from unauthorized
  modification.  Note that compliant implementations must allow a
  minimum of 14 distinct Protection Authority flags (consistent with
  the Protection Authority field size defined in Section 2.4) to be set
  independently in any parameter involving Protection Authority flag
  fields.

       a. SYSTEM-LEVEL-MAX: This parameter specifies the highest
       Classification Level (see Table 1) which may be present in the
       classification level field of the Basic Security Option in any
       datagram transmitted or received by the system.

       b. SYSTEM-LEVEL-MIN: This parameter specifies the lowest
       Classification Level (see Table 1) which may be present in the
       classification level field of the Basic Security Option in any

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       datagram transmitted by the system.

       c. SYSTEM-AUTHORITY-IN:  This parameter is a set, each member of
       which is a Protection Authority flag field.  The set enumerates
       all of the Protection Authority flag fields which may be present
       in the Protection Authority field of the Basic Security Option
       in any datagram received by this system.  A compliant
       implementation must be capable of representing at least 256
       distinct Protection Authority flag fields (each field must be
       capable of representing 14 distinct Protection Authority flags)
       in this set.  Each element of the enumerated set may be a
       combination of multiple protection authority flags.

       Set elements representing multiple Protection Authorities are
       formed by ORing together the flags that represent each
       authority.  Thus, for example, a set  element representing
       datagrams to be protected according to NSA and SCI rules might
       be represented as "00110000" while an element representing
       protection mandated by NSA, DOE and SIOP-ESI might be
       represented as "01011000".  (These examples illustrate 8-bit set
       elements apropos the minimal encodings for currently defined
       Protection Authority flags.  If additional flags are defined
       beyond the first byte of the Protection Authority Field, longer
       encodings for set elements may be required.)

       It is essential that implementations of the Internet Protocol
       Basic Security Option provide a convenient and compact way for
       system security managers to express which combinations of flags
       are allowed.  The details of such an interface are outside the
       scope of this RFC, however, enumeration of bit patterns is NOT a
       recommended interface.  As an alternative, one might consider a
       notation of the form COMB(GENSER,NSA,SCI)+COMB(SIOP-ESI,NSA,SCI)
       in which "COMB" means ANY combination of the flags referenced as
       parameters of the COMB function are allowed and "+" means "or".

       d. SYSTEM-AUTHORITY-OUT:  This parameter is a set, each member
       of which is a Protection Authority flag field.  The set
       enumerates all of the Protection Authority flag fields which may
       be present in the Protection Authority field of the Basic
       Security Option in any datagram transmitted by this system.  A
       compliant implementation must be capable of representing at
       least 256 distinct Protection Authority flag fields in this set.
       Explicit enumeration of all authorized Protection Authority
       field flags permits great flexibility, and in particular does
       not impose set inclusion restrictions on this parameter.

  The following configuration parameters are defined for each network
  port present on the system.  The term "port" is used here to refer

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  either to a physical device interface (which may represent multiple
  IP addresses) or to a single IP address (which may be served via
  multiple physical interfaces).  In general the former interpretation
  will apply and is consistent with the Trusted Network Interpretation
  of the Trusted Computer Systems Evaluation Criteria (TNI) concept of
  a "communications channel" or "I/O device."  However, the latter
  interpretation also may be valid depending on local system security
  capabilities.  Note that some combinations of port parameter values
  are appropriate only if the port is "single level," i.e., all data
  transmitted or received via the port is accurately characterized by
  exactly one Classification Level and Protection Authority Flag field.

       e. PORT-LEVEL-MAX: This parameter specifies the highest
       Classification Level (see Table 1) which may be present in the
       classification level field of the Basic Security Option in any
       datagram transmitted or received by the system via this network
       port.

       f. PORT-LEVEL-MIN: This parameter specifies the lowest
       Classification Level (see Table 1) which may be present in the
       classification level field of the Basic Security Option in any
       datagram transmitted by the system via this network port.

       g. PORT-AUTHORITY-IN:  This parameter is a set each member of
       which is a Protection Authority flag field.  The set enumerates
       all of the Protection Authority flag fields which may be present
       in the Protection Authority field of the Basic Security Option
       in any datagram received via this port.  A compliant
       implementation must be capable of representing at least 256
       distinct Protection Authority flag fields in this set.

       h. PORT-AUTHORITY-OUT:  This parameter is a set each member of
       which is a Protection Authority flag field.  The set enumerates
       all of the Protection Authority flag fields which may be present
       in the Protection Authority field of the Basic Security Option
       in any datagram transmitted via this port.  A compliant
       implementation must be capable of representing at least 256
       distinct Protection Authority flag fields in this set.

       i. PORT-AUTHORITY-ERROR:  This parameter is a single Protection
       Authority flag field assigned to transmitted ICMP error messages
       (see Section 2.8).  The PORT-AUTHORITY-ERROR value is selected
       from the set of values which constitute PORT-AUTHORITY-OUT.
       Means for selecting the PORT-AUTHORITY-ERROR value within a
       system are a local matter subject to local security policies.

       j. PORT-IMPLICIT-LABEL:  This parameter specifies a single
       Classification Level and a Protection Authority flag field

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       (which may be null) to be associated with all unlabelled
       datagrams received via the port.  This parameter is meaningful
       only if PORT-BSO-REQUIRED-RECEIVE = FALSE, otherwise receipt of
       an unlabelled datagram results in an error response.

       k. PORT-BSO-REQUIRED-RECEIVE:  This parameter is a boolean which
       indicates whether all datagrams received via this network port
       must contain a Basic Security Option.

       l. PORT-BSO-REQUIRED-TRANSMIT:  This parameter is a boolean
       which indicates whether all datagrams transmitted via this
       network port must contain a Basic Security Option.   If this
       parameter is set to FALSE, then PORT-BSO-REQUIRED-RECEIVE should
       also be set to FALSE (to avoid communication failures resulting
       from asymmetric labelling constraints).

  In every intermediate or end system, the following relationship must
  hold for these parameters for all network interfaces.  The symbol
  ">=" is interpreted relative to the linear ordering defined for
  security levels specified in Section 2.3 for the "LEVEL" parameters,
  and as set inclusion for the "AUTHORITY" parameters.

          SYSTEM-LEVEL-MAX >= PORT-LEVEL-MAX >=
                  PORT-LEVEL-MIN >= SYSTEM-LEVEL-MIN

          SYSTEM-AUTHORITY-IN >= PORT-AUTHORITY-IN
                           and
          SYSTEM-AUTHORITY-OUT >= PORT-AUTHORITY-OUT

2.6. Configuration Considerations

  Systems which do not maintain separation for different security
  classification levels of data should have only trivial ranges for the
  LEVEL parameters, i.e., SYSTEM-LEVEL-MAX = PORT-LEVEL-MAX = PORT-
  LEVEL-MIN = SYSTEM-LEVEL-MIN.

  Systems which do maintain separation for different security
  classification levels of data may have non-trivial ranges for the
  LEVEL parameters, e.g., SYSTEM-LEVEL-MAX >= PORT-LEVEL-MAX >= PORT-
  LEVEL-MIN >= SYSTEM-LEVEL-MIN.

2.7. Processing the Basic Security Option

  For systems implementing the Basic Security Option, the parameters
  PORT-BSO-REQUIRED-TRANSMIT and PORT-BSO-REQUIRED-RECEIVE are used to
  specify the local security policy with regard to requiring the
  presence of this option on transmitted and received datagrams,
  respectively, on a per-port basis.  Each datagram transmitted or

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  received by the system must be processed in accordance with the per-
  port and system-wide security parameters configured for the system.

  Systems which process only Unclassified data may or may not be
  configured to generate the BSO on transmitted datagrams.  Such
  systems also may or may not require a BSO to be present on received
  datagrams.  However, all systems must be capable of accepting
  datagrams containing this option, irrespective of whether the option
  is processed or not.

  In general, systems which process classified data must generate this
  option for transmitted datagrams.  The only exception to this rule
  arises in (dedicated or system high [DoD 5200.28]) networks where
  traffic may be implicitly labelled rather than requiring each
  attached system to generate explicit labels.  If the local security
  policy permits receipt of datagrams without the option, each such
  datagram is presumed to be implicitly labelled based on the port via
  which the datagram is received.  A per-port parameter (PORT-
  IMPLICIT-LABEL) specifies the label to be associated with such
  datagrams upon receipt.  Note that a datagram transmitted in response
  to receipt of an implicitly labelled datagram, may, based on local
  policy, require an explicit Basic Security Option.

2.7.1. Handling Unclassified Datagrams

  If an unmarked datagram is received via a network port for which
  PORT-BSO-REQUIRED = FALSE and PORT-IMPLICIT-LABEL = UNCLASSIFIED (NO
  FLAGS), the datagram shall be processed as though no Protection
  Authority Flags were set.  Thus there are two distinct, valid
  representations for Unclassified datagrams to which no Protection
  Authority rules apply (an unmarked datagram as described here and a
  datagram containing an explicit BSO with Classification Level set to
  Unclassified and with no Protection Authority flags set).  Note that
  a datagram also may contain a Basic Security Option in which the
  Classification Level is Unclassified and one or more Protection
  Authority Field Flags are set.  Such datagrams are explicitly
  distinct from the equivalence class noted above (datagrams marked
  Unclassified with no Protection Authority field flags set and
  datagrams not containing a Basic Security Option).

2.7.2. Input Processing

  Upon receipt of any datagram a system compliant with this RFC must
  perform the following actions.  First, if PORT-BSO-REQUIRED-RECEIVE =
  TRUE for this port, then any received datagram must contain a Basic
  Security Option and a missing BSO results in an ICMP error response
  as specified in Section 2.8.1.  A received datagram which contains a
  Basic Security Option must be processed as described below.  This

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  algorithm assumes that the IP header checksum has already been
  verified and that, in the course of processing IP options, this
  option has been encountered.  The value of the Classification Level
  field from the option will be designated "DG-LEVEL" and the value of
  the Protection Authority Flags field will be designated "DG-
  AUTHORITY."

  Step 1. Check that DG-LEVEL is a valid security classification level,
          i.e., it must be one of the (non-reserved) values from Table
          1.  If this test fails execute the out-of-range procedure in
          Section 2.8.1.

  Step 2. Check that PORT-LEVEL-MAX >= DG-LEVEL.  If this test fails,
          execute out-of-range procedure specified in Section 2.8.2.

  Step 3. Check that DG-AUTHORITY =< PORT-AUTHORITY-IN.  If this test
          fails, execute out-of-range procedure specified in Section
          2.8.2.

2.7.3. Output Processing

  Any system which implements the Basic Security Option must adhere to
  a fundamental rule with regard to transmission of datagrams, i.e., no
  datagram shall be transmitted with a Basic Security Option the value
  of which is outside of the range for which the system is configured.
  Thus for every datagram transmitted by a system the following must
  hold: PORT-LEVEL-MAX >= DG-LEVEL >= PORT-LEVEL-MIN and DG-AUTHORITY
  =< PORT-AUTHORITY-OUT.  It is a local matter as to what procedures
  are followed by a system which detects at attempt to transmit a
  datagram for which these relationships do not hold.

  If a port is configured to allow both labelled and unlabelled
  datagrams (PORT-BSO-REQUIRED-TRANSMIT = FALSE) to be transmitted, the
  question arises as to whether a label should be affixed.  In
  recognition of the lack of widespread implementation or use of this
  option, especially in unclassified networks, this RFC recommends that
  the default be transmission of unlabelled datagrams.  If the
  destination requires all datagrams to be labelled on input, then it
  will respond with an ICMP error message (see Section 2.8.1) and the
  originator can respond by labelling successive packets transmitted to
  this destination.

  To support this mode of operation, a system which allows transmission
  of both labelled and unlabelled datagrams must maintain state
  information (a cache) so that the system can associate the use of
  labels with specific destinations, e.g., in response to receipt of an
  ICMP error message as specified in Section 2.8.1.  This requirement
  for maintaining a per-destination cache is very much analogous to

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  that imposed for processing the IP source route option or for
  maintaining first hop routing information (RFC 1122).  This RFC does
  not specify which protocol module must maintain the per-destination
  cache (e.g., IP vs.  TCP or UDP) but security engineering constraints
  may dictate an IP implementation in trusted systems.  This RFC also
  does not specify a cache maintenance algorithm, though use of a timer
  and activity flag may be appropriate.

2.8. Error Procedures

  Datagrams received with errors in the Basic Security Option or which
  are out of range for the network port via which they are received,
  should not be delivered to user processes.  Local policy will specify
  whether logging and/or notification of a system security officer is
  required in response to receipt of such datagrams.  The following are
  the least restrictive actions permitted by this protocol.  Individual
  end or intermediate systems, system administrators, or protection
  authorities may impose more stringent restrictions on responses and
  in some instances may not permit any response at all to a datagram
  which is outside the security range of a host or system.

  In all cases, if the error is triggered by receipt of an ICMP, the
  ICMP is discarded and no response is permitted (consistent with
  general ICMP processing rules).

2.8.1.Parameter Problem Response

  If a datagram is received with no Basic Security Option and the
  system security configuration parameters require the option on the
  network port via which the datagram was received, an ICMP Parameter
  Problem Missing Option (Type = 12, Code = 1) message is transmitted
  in response.  The Pointer field of the ICMP should be set to the
  value "130" to indicate the type of option missing.  A Basic Security
  Option is included in the response datagram with Clearance Level set
  to PORT-LEVEL-MIN and Protection Authority Flags set to PORT-
  AUTHORITY-ERROR.

  If a datagram is received in which the Basic Security Option is
  malformed (e.g., an invalid Classification Level Protection Authority
  Flag field), an ICMP Parameter Problem (Type = 12, Code = 0) message
  is transmitted in response.  The pointer field is set to the
  malformed Basic Security Option.  The Basic Security Option is
  included in the response datagram with Clearance Level set to PORT-
  LEVEL-MIN and Protection Authority Flags set to PORT-AUTHORITY-ERROR.

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2.8.2. Out-Of-Range Response

  If a datagram is received which is out of range for the network port
  on which it was received, an ICMP Destination Unreachable
  Communication Administratively Prohibited (Type = 3, Code = 9 for net
  or Code = 10 for host) message is transmitted in response.  A Basic
  Security Option is included in the response datagram with Clearance
  Level set to PORT-LEVEL-MIN and Protection Authority Flags set to
  PORT-AUTHORITY-ERROR.

2.9. Trusted Intermediary Procedure

  Certain devices in an internet may act as intermediaries to validate
  that communications between two hosts are authorized.  This decision
  is based on the knowledge of the accredited security levels of the
  hosts and the values in the DoD Basic Security Option.  These devices
  may receive IP datagrams which are in range for the intermediate
  device, but are not within the accredited range either for the source
  or for the destination.  In the former case, the datagram should be
  treated as described above for an out-of-range option.  In the latter
  case, an ICMP Destination Unreachable Communication Administratively
  Prohibited (Type = 3, Code = 9 for net or Code = 10 for host)
  response should be transmitted. The security range of the network
  interface on which the reply will be sent determines whether a reply
  is allowed and at what level it will be sent.

3. DoD Extended Security Option

  This option permits additional security labelling information, beyond
  that present in the Basic Security Option, to be supplied in an IP
  datagram to meet the needs of registered authorities.  Note that
  information which is not labelling data or which is meaningful only
  to the end systems (not intermediate systems) is not appropriate for
  transmission in the IP layer and thus should not be transported using
  this option.  This option must be copied on fragmentation.  Unlike
  the Basic Option, this option may appear multiple times within a
  datagram, subject to overall IP header size constraints.

  This option may be present only in conjunction with the Basic
  Security Option, thus all systems which support Extended Security
  Options must also support the Basic Security Option.  However, not
  all systems which support the Basic Security Option need to support
  Extended Security Options and support for these options may be
  selective, i.e., a system need not support all Extended Security
  Options.

  The top-level format for this option is as follows:

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            +------------+------------+------------+-------//-------+
            |  10000101  |  000LLLLL  |  AAAAAAAA  |  add sec info  |
            +------------+------------+------------+-------//-------+
             TYPE = 133      LENGTH     ADDITIONAL      ADDITIONAL
                                       SECURITY INFO     SECURITY
                                        FORMAT CODE        INFO

                  FIGURE 3.  DoD EXTENDED SECURITY OPTION FORMAT

3.1. Type

  The value 133 identifies this as the DoD Extended Security Option.

3.2. Length.

  The length of the option, which includes the "Type" and "Length"
  fields, is variable.  The minimum length of the option is 3 octets.

3.3. Additional Security Info Format Code

       Length:  1 Octet

  The value of the Additional Security Info Format Code identifies the
  syntax and semantics for a specific "Additional Security Information"
  field.  For each Additional Security Info Format Code, an RFC will be
  published to specify the syntax and to provide an algorithmic
  description of the processing required to determine whether a
  datagram carrying a label specified by this Format Code should be
  accepted or rejected.  This specification must be sufficiently
  detailed to permit vendors to produce interoperable implementations,
  e.g., it should be comparable to the specification of the Basic
  Security Option provided in this RFC.  However, the specification
  need not include a mapping from the syntax of the option to human
  labels if such mapping would cause distribution of the specification
  to be restricted.

  In order to maintain the architectural consistency of DoD common user
  data networks, and to maximize interoperability, each activity should
  submit its plans for the definition and use of an Additional Security
  Info Format Code to DISA DISDB, Washington, D.C.  20305-2000 for
  review and approval.  DISA DISDB will forward plans to the Internet
  Activities Board for architectural review and, if required, a cleared
  committee formed by the IAB will be constituted for the review
  process.  Once approved, the Internet Assigned Number authority will
  assign an Additional Security Info Format Code to the requesting
  activity, concurrent with publication of the corresponding RFC.

  Note: The bit assignments for the Protection Authority flags of the

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RFC 1108 U.S. DOD Security Option November 1991

  Basic Security Option have no relationship to the "Additional
  Security Info Format Code" of this option.

3.4. Additional Security Information.

       Length:  Variable

  The Additional Security Info field contains the additional security
  labelling information specified by the "Additional Security Info
  Format Code" of the Extended Security Option.  The syntax and
  processing requirements for this field are specified by the
  associated RFC as noted above.  The minimum length of this field is
  zero.

3.5. System Security Configuration Parameters

  Use of the Extended Security Option requires that the intermediate or
  end system configuration accurately reflect the security parameters
  associated with communication via each network port (see Section 2.5
  as a guide).  Internal representation of the security parameters
  implementation dependent.  The set of parameters required to support
  processing of the Extended Security Option is a function of the set
  of Additional Security Info Format Codes supported by the system.
  The RFC which specifies syntax and processing rules for a registered
  Additional Security Info Format Code will specify the additional
  system security parameters required for processing an Extended
  Security Option relative to that Code.

3.6. Processing Rules

  Any datagram containing an Extended Security Option must also contain
  a Basic Security Option and receipt of a datagram containing the
  former absent the latter constitutes an error.  If the length
  specified by the Length field is inconsistent with the length
  specified by the variable length encoding for the Additional Security
  Info field, the datagram is in error.  If the datagram is received in
  which the Additional Security Info Format Code contains a non-
  registered value, the datagram is in error.  Finally, if the
  Additional Security Info field contains data inconsistent with the
  defining RFC for the Additional Security Info Format Code, the
  datagram is in error.  In any of these cases, an ICMP Parameter
  Problem response should be sent as per Section 2.8.1.  Any additional
  error processing rules will be specified in the defining RFC for this
  Additional Security Info Format Code.

  If the additional security information contained in the Extended
  Security Option indicates that the datagram is within range according
  to the security policy of the system, then the datagram should be

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RFC 1108 U.S. DOD Security Option November 1991

  accepted for further processing.  Otherwise, the datagram should be
  rejected and the procedure specified in Section 2.8.2 should be
  followed (with the Extended Security Option values set apropos the
  Additional Security Info Format Code port security parameters).

  As with the Basic Security Option, it will not be possible in a
  general internet environment for intermediate systems to provide
  routing control for datagrams based on the labels contained in the
  Extended Security Option until such time as interior and exterior
  gateway routing protocols are enhanced to process such labels.

References

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

Security Considerations

  The focus of this RFC is the definition of formats and processing
  conventions to support security labels for data contained in IP
  datagrams, thus a variety of security issues must be considered
  carefully when making use of these options.  It is not possible to
  address all of the security considerations which affect correct
  implementation and use of these options, however the following
  paragraph highglights some of these issues.

  Correct implementation and operation of the software and hardware
  which processes these options is essential to their effective use.
  Means for achieving confidence in such correct implementation and
  operation are outside of the scope of this RFC.  The options
  themselves incorporate no facilities to ensure the integrity of the
  security labels in transit (other than the IP checksum mechanism),
  thus appropriate technology must be employed whenever datagrams
  containing these options transit "hostile" communication
  environments.  Careful, secure management of the configuration
  variables associated with each system making use of these options is
  essential if the options are to provide the intended security
  functionality.

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RFC 1108 U.S. DOD Security Option November 1991

Author's Address

  Stephen Kent
  BBN Communications
  150 CambridgePark Drive
  Cambridge, MA  02140

  Phone: (617) 873-3988

  Email: [email protected]

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