Internet Research Task Force (IRTF)                         S. Symington
Request for Comments: 6259                         The MITRE Corporation
Category: Experimental                                          May 2011
ISSN: 2070-1721

         Delay-Tolerant Networking Previous-Hop Insertion Block

Abstract

   This document defines an extension block for use with the Delay-
   Tolerant Networking (DTN) Bundle Protocol.  This Previous-Hop
   Insertion Block (PHIB) extension block is designed to be inserted by
   a forwarding node to provide the endpoint identifier (EID) of an
   endpoint of which the forwarding node is a member so that this EID
   may be conveyed to the next-hop receiving node.  Knowledge of an EID
   of an endpoint of which a previous-hop node is a member may be
   required in some circumstances to support certain routing protocols
   (e.g., flood routing).  If this EID cannot be provided by the
   convergence layer or other means, the PHIB defines the mechanism
   whereby the EID can be provided with the bundle.  Each PHIB is always
   removed from the bundle by the receiving node so that its presence
   within the bundle is limited to exactly one hop.  This document
   defines the format and processing of this PHIB.  This document is a
   product of the Delay-Tolerant Networking Research Group and has been
   reviewed by that group.  No objections to its publication as an RFC
   were raised.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Research Task
   Force (IRTF).  The IRTF publishes the results of Internet-related
   research and development activities.  These results might not be
   suitable for deployment.  This RFC represents the consensus of the
   Delay-Tolerant Networking Research Group of the Internet Research
   Task Force (IRTF).  Documents approved for publication by the IRSG
   are not a candidate for any level of Internet Standard; see Section 2
   of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6259.

Copyright Notice

   Copyright (c) 2011 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................4
   2. Previous-Hop Insertion Block Format .............................4
   3. Previous-Hop Insertion Block Processing .........................6
      3.1. Bundle Transmission ........................................6
      3.2. Bundle Forwarding ..........................................6
      3.3. Bundle Reception ...........................................7
   4. Security Considerations .........................................8
   5. IANA Considerations .............................................9
   6. References ......................................................9
      6.1. Normative References .......................................9
      6.2. Informative References .....................................9

1.  Introduction

   This document defines an extension block for use with the Bundle
   Protocol [RFC5050] within the context of a Delay-Tolerant Networking
   architecture [RFC4838].  The DTN Bundle Protocol defines the bundle
   as its protocol data unit and defines "bundle blocks" to carry data
   of different types.  This document defines an optional bundle block
   called a Previous-Hop Insertion Block (PHIB).

   The PHIB is inserted into a bundle by a forwarding node to provide
   the endpoint identifier (EID) of an endpoint of which the forwarding
   node is a member so that this EID may be conveyed to the next-hop
   receiving node.  (Hereafter, an EID of an endpoint of which a node is
   a member will be referred to as an "M-EID" of that node.  A node may
   have one or more M-EIDs, depending on the number of endpoints to
   which it belongs.  An EID of a singleton endpoint of which a node is
   a member will be referred to as a "singleton M-EID" of that node.)
   In situations where there is a requirement that the receiving node be
   able to determine an M-EID of a forwarding node, but the M-EID of the
   forwarding node cannot be inferred by the receiving node through
   existing mechanisms, the forwarding node must explicitly provide this

   M-EID in the bundle.  This specification defines the mechanism
   whereby a node can insert such an M-EID into a bundle before
   forwarding it to the bundle's next hop.

   This previous-hop M-EID information may be used in some circumstances
   to support various routing protocols.  For example, the PHIB could be
   helpful when implementing flood routing because each receiving node
   could use the PHIB to determine which EID to exclude from the list of
   adjacent nodes to which it forwards received bundles as it does its
   part in flooding the bundle.  A node will flood the bundle to all
   neighboring nodes except for the node from which it received the
   bundle, as identified in the PHIB.

   The PHIB could also be used in conjunction with the Bundle
   Authentication Block (BAB) of the DTN Bundle Security Protocol
   [DTNBSP] to provide the security-source EID for the BAB.  The PHIB
   can be used to carry the BAB's security-source EID instead of
   conveying this EID using a reference in the BAB's EID-reference field
   or including the EID as part of the BAB's key-information parameters.

   In many situations, a node that receives a bundle may be able to
   infer an M-EID of the node that forwarded the bundle.  In some
   situations, however, no M-EID will be able to be inferred by the
   receiving node.  For example, if tunneling DTN bundles across some
   portion of the DTN network, it is not possible for the node at the
   receiving end of the tunnel to determine from the convergence layer
   the M-EID of the node at the sending end of the tunnel.  The node at
   the receiving end of the tunnel will receive an encapsulating bundle
   from one of its adjacent nodes, and it may be able to tell the M-EID
   of this adjacent node using the convergence-layer protocol.  However,
   the node at the sending end of the tunnel is most likely not adjacent
   to the node at the receiving end of the tunnel, so in order for the
   node at the receiving end of the tunnel to be able to learn the M-EID
   of the node at the sending end of the tunnel, which is the previous-
   hop node of the tunneled bundle, the M-EID must be provided within
   the tunneled bundle.  In this case, the PHIB is the vehicle for
   enabling the node at the sending end of the tunnel to provide its
   M-EID to the node at the receiving end of the tunnel.

   EIDs may be presented in two ways within the PHIB.  If the M-EID of
   the forwarding node is already in the Dictionary field of the
   bundle's primary bundle block, the PHIB MAY identify this EID using
   its Block EID-reference count and EID-reference field.  Otherwise,
   the PHIB MUST identify this EID by providing the EID in its block-
   type-specific data field.  These two alternative ways of presenting
   EIDs in the PHIB are further discussed in Section 3.

   The lifetime of the PHIB is always exactly one hop in the DTN.  If a
   bundle containing a PHIB is received, the receiving node is assured
   that this PHIB was inserted by the previous node, assuming all nodes
   are operating correctly; likewise, this PHIB is not retained with the
   bundle when the bundle is forwarded.  If the bundle is forwarded with
   a PHIB, this PHIB MUST identify an M-EID of the forwarding node.

   This document defines the format and processing of the PHIB.  The
   capabilities described in this document are OPTIONAL for deployment
   with the Bundle Protocol.  Bundle Protocol implementations claiming
   to support the PHIB MUST be capable of:

   o  generating a PHIB and inserting it into a bundle,

   o  receiving bundles containing a PHIB and making the information
      contained in this PHIB available for use, e.g., in forwarding
      decisions, and

   o  deleting a PHIB from a bundle

   as defined in this document.

1.1.  Requirements Language

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

2.  Previous-Hop Insertion Block Format

   The PHIB uses the Canonical Bundle Block Format as defined in the
   Bundle Protocol Specification [RFC5050].  That is, the PHIB is
   comprised of the following elements, which are defined as in all
   bundle protocol blocks except the primary bundle block.  Note that
   Self-Delimiting Numeric Value (SDNV) encoding is also described in
   the Bundle Protocol Specification:

   o  Block-type code (one byte) - The block-type code for the PHIB is
      0x05.

   o  Block processing control flags (SDNV) - The following block
      processing control flag MUST be set:

         Discard block if it can't be processed

   o  Block EID-reference count and EID-references (optional) -
      composite field defined in [RFC5050] containing a count of EID-
      references (expressed as an SDNV) followed by an EID-reference
      (expressed as a pair of SDNVs).

      Whether or not this field is allowed in the PHIB is determined by
      whether or not an M-EID of the node inserting the PHIB is already
      in the Dictionary field of the primary bundle block (e.g., whether
      an M-EID of the inserting node is also an M-EID of the bundle's
      source, current custodian, or report-to endpoint, or is the same
      as some other endpoint in the dictionary that is referenced by
      another block in the bundle).

      If an M-EID of the inserting node is already in the dictionary,
      this field MAY be present in the PHIB.  If this field is present
      in the PHIB, the value of the EID-reference count MUST be one,
      meaning that the field contains exactly one EID-reference, which
      MUST be a reference to an M-EID of the inserting node.  Presence
      of this field MUST be indicated by a set "block contains an EID-
      reference field" flag in the block processing control flags.

      If no M-EID of the inserting node is in the dictionary, this field
      MUST NOT be present in the PHIB, which MUST be indicated by an
      unset "block contains an EID-reference field" flag in the block
      processing control flags.

   o  Block data length (SDNV) - If this value is zero, there are no
      block-type-specific data fields.  In this case, the M-EID of the
      inserting node must be in the dictionary and it MUST be referenced
      in the Block EID-reference count and EID-references field as
      described above.

   o  Block-type-specific data fields (optional) as follows:

      *  Inserting Node's EID Scheme Name - A null-terminated array of
         bytes that comprises the scheme name of an M-EID of the node
         inserting this PHIB.

      *  Inserting Node's EID SSP - A null-terminated array of bytes
         that comprises the scheme-specific part (SSP) of an M-EID of
         the node inserting this PHIB.

      If the Block EID-reference count and EID-references field is not
      present in the PHIB, the above two EID scheme name and SSP block-
      type-specific data fields MUST be present.  If the Block EID-
      reference count and EID-references field is present in the PHIB,
      the above two EID scheme name and SSP block-type-specific data
      fields MUST NOT be present.

   The structure of a PHIB is as follows:

   PHIB Format:
   +----+------------+--------------------------------- -+-------------+
   |type|flags (SDNV)|EID-ref count and list (comp) (opt)|length (SDNV)|
   +----+------------+-----------------------------------+-------------+
   | Inserting Node EID Scheme Name (opt)| Inserting Node EID SSP (opt)|
   +-------------------------------------------------------------------+

                                 Figure 1

3.  Previous-Hop Insertion Block Processing

   The following are the processing steps that a bundle node must take
   relative to generation, reception, and processing of a PHIB.

3.1.  Bundle Transmission

   When an outbound bundle is created per the parameters of the bundle
   transmission request, this bundle MAY include one or more PHIBs.
   Whether or not PHIBs are included is a local bundle agent
   configuration option and may be influenced by other factors, such as
   the routing protocol in use.

3.2.  Bundle Forwarding

   Before forwarding a bundle, the node SHALL delete all PHIBs that were
   in the bundle when it was received (if any).  As described in the
   Bundle Protocol Specification, the node MAY delete all strings
   (scheme names and SSPs) in the bundle's dictionary to which no
   endpoint ID references in the bundle currently refer (if any).

   The node MAY insert one or more PHIBs into the bundle before
   forwarding it, as dictated by local policy.  If there are already
   strings (scheme names and SSPs) in the bundle's dictionary that
   denote the M-EID of the inserting node, the PHIB MAY reference these
   strings and, if it does, it MUST NOT include any block-type-specific
   data fields.  The inserting node MUST NOT insert strings into the
   bundle's dictionary in order that they may be referenced by only the
   PHIB.  If the PHIB is constructed such that it does not reference any
   strings from the dictionary, the inserting node MUST include the
   scheme name and SSP of one of its M-EIDs as the PHIB's block-type-
   specific data fields.

   The node that is inserting a PHIB into the bundle may have more than
   one endpoint in which it is a member.  The choice of which M-EID to
   insert into the PIB SHALL be made as follows:

   o  If the inserting node is a member of exactly one singleton
      endpoint, the node may insert at most one PHIB into the bundle and
      the EID of this singleton endpoint is what MUST be inserted into
      the PHIB.

   o  If the inserting node is a member of more than one singleton
      endpoint, then:

         If the inserting node has a priori knowledge of the URI schemes
         supported by the next-hop node and if the inserting node has
         one or more singleton M-EIDs that are expressible in one or
         more of those URI schemes, then the inserting node MAY insert
         one or more PHIBs into the bundle being forwarded.  The EIDs in
         the inserted PHIBs MUST be unique, they MUST be singleton
         M-EIDs of the inserting node, and they MUST be expressed in URI
         schemes supported by the next-hop node.  Mechanisms for
         determining what URI schemes are supported by particular next-
         hop neighbors are not defined here.

         If the inserting node has one or more singleton M-EIDs that are
         expressible in the same URI scheme as the destination of the
         bundle that is being forwarded, then the inserting node MAY
         insert one or more PHIBs into the bundle being forwarded.  The
         EIDs in the inserted PHIBs MUST be unique, they MUST be
         singleton M-EIDs of the inserting node, and they MUST be
         expressed in the destination URI scheme of the bundle.

         Else, if the inserting node has neither a singleton M-EID that
         is expressible in a URI scheme known to be supported by the
         next-hop node nor a singleton M-EID that is expressible in the
         same URI scheme as the destination of the bundle that is being
         forwarded, then the inserting node MAY insert one or more PHIBs
         into the bundle being forwarded.  The EIDs in the inserted
         PHIBs MUST be unique, and they MUST be singleton M-EIDs of the
         inserting node.

3.3.  Bundle Reception

   If the bundle includes a PHIB, the EID identified in the PHIB SHALL
   be made available for use at the receiving node (e.g., in forwarding
   decisions or, if the receiving node is the bundle-destination, the
   EID may be made available to the receiving application; whether or
   not it is made available to the receiving application is an
   implementation matter).  If the EID is identified both by a reference
   in the PHIB's block EID-reference count and EID-references field and
   by a scheme name and SSP in the block-type-specific fields, the PHIB
   is not considered to be well-formed.  In the case of reception of
   such an ill-formed PHIB, if the identified EIDs are the same, the

   receiving node MAY process the PHIB as if it were well-formed.
   However, if the identified EIDs differ, the receiving node MUST NOT
   process the PHIB and must take action on the PHIB as specified by the
   PHIB's block processing flags.

4.  Security Considerations

   The DTN Bundle Security Protocol [DTNBSP] defines security-related
   blocks to provide hop-by-hop bundle authentication and integrity,
   end-to-end integrity, and end-to-end confidentiality of bundles or
   parts of bundles, as well as a set of ciphersuites that may be used
   to calculate security-results carried in these security blocks.  The
   PHIB will not be encrypted when using the PCB-RSA-AES128-PAYLOAD-PIB-
   PCB ciphersuite with the Payload Confidentiality Block (PCB) to
   provide end-to-end confidentiality.  This ciphersuite only allows for
   payload and Payload Integrity Block (PIB) encryption.  If encryption
   of the PHIB block is desired, the Extension Security Block (ESB)
   could be used for this purpose.

   All ciphersuites that use the strict canonicalization algorithm
   [DTNBSP] to calculate and verify security-results (e.g., many hop-by-
   hop authentication ciphersuites) apply to all blocks in the bundle,
   and so would apply to bundles that include an optional PHIB and would
   include that block in the calculation of their security-result.  In
   particular, bundles including the optional PHIB would have their
   integrity protected in their entirety for the extent of a single hop,
   from a forwarding node to an adjacent receiving node, using the
   Bundle Authentication Block (BAB) with the BAB-HMAC (Hashed Message
   Authentication Code) ciphersuite defined in the Bundle Security
   Protocol Specification.

   Ciphersuites that use the mutable canonicalization algorithm to
   calculate and verify security-results (e.g., the PIB-RSA-SHA256
   ciphersuite and most end-to-end authentication ciphersuites used with
   the PIB) will (correctly) omit the PHIB from their calculation.  The
   fact that several different instantiations of this PHIB block may be
   added to and deleted from the bundle as the bundle transits the
   network will not interfere with end-to-end security protection when
   using ciphersuites that use mutable canonicalization.

   As stated above, the BAB can be used to ensure the integrity of the
   PHIB.  Nodes receiving bundles with PHIBs should be aware, however,
   that forwarding nodes that insert PHIBs might lie about the EIDs of
   endpoints of which they are members.  Lying in this way could provide
   a mechanism for subverting routing strategies that base routing
   decisions on EID information in the PHIB.

   Note that if some Bundle Protocol implementation does not support the
   PHIB but does not properly implement the "Discard block if it can't
   be processed" flag, then a PHIB may unexpectedly persist for longer
   than a single hop.

5.  IANA Considerations

   This specification allocates a codepoint from the "Bundle Block
   Types" registry defined in [RFC6255] (see Section 2):

   Additional Entry for the Bundle Block Type Codes Registry:
     +-------+----------------------------------------+----------------+
     | Value | Description                            + Reference      |
     +-------+----------------------------------------+----------------+
     |   5   | Previous-Hop Insertion Block           + This document  |
     +-------+----------------------------------------+----------------+

6.  References

6.1.  Normative References

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

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, November 2007.

   [RFC6255]  Blanchet, M., "Delay-Tolerant Networking (DTN) Bundle
              Protocol IANA Registries", RFC 6255, May 2011.

6.2.  Informative References

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, April 2007.

   [DTNBSP]   Symington, S., Farrell, S., Weiss, H., and P. Lovell,
              "Bundle Security Protocol Specification", RFC 6257,
              May 2011.

Author's Address

   Susan Flynn Symington
   The MITRE Corporation
   7515 Colshire Drive
   McLean, VA  22102
   US

   Phone: +1 (703) 983-7209
   EMail: susan@mitre.org
   URI:   http://mitre.org/