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RFC 5050 - Bundle Protocol Specification


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Network Working Group                                           K. Scott
Request for Comments: 5050                         The MITRE Corporation
Category: Experimental                                       S. Burleigh
                                          NASA Jet Propulsion Laboratory
                                                           November 2007

                     Bundle Protocol Specification

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and in particular notes that the decision to publish is not
   based on IETF review for such things as security, congestion control,
   or inappropriate interaction with deployed protocols.  The RFC Editor
   has chosen to publish this document at its discretion.  Readers of
   this document should exercise caution in evaluating its value for
   implementation and deployment.  See RFC 3932 for more information.

Abstract

   This document describes the end-to-end protocol, block formats, and
   abstract service description for the exchange of messages (bundles)
   in Delay Tolerant Networking (DTN).

   This document was produced within the IRTF's Delay Tolerant
   Networking Research Group (DTNRG) and represents the consensus of all
   of the active contributors to this group.  See http://www.dtnrg.org
   for more information.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Service Description  . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Implementation Architectures . . . . . . . . . . . . . . .  9
     3.3.  Services Offered by Bundle Protocol Agents . . . . . . . . 11
   4.  Bundle Format  . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Self-Delimiting Numeric Values (SDNVs) . . . . . . . . . . 12
     4.2.  Bundle Processing Control Flags  . . . . . . . . . . . . . 13
     4.3.  Block Processing Control Flags . . . . . . . . . . . . . . 15
     4.4.  Endpoint IDs . . . . . . . . . . . . . . . . . . . . . . . 16
     4.5.  Formats of Bundle Blocks . . . . . . . . . . . . . . . . . 17
       4.5.1.  Primary Bundle Block . . . . . . . . . . . . . . . . . 19
       4.5.2.  Canonical Bundle Block Format  . . . . . . . . . . . . 22
       4.5.3.  Bundle Payload Block . . . . . . . . . . . . . . . . . 23
     4.6.  Extension Blocks . . . . . . . . . . . . . . . . . . . . . 24
     4.7.  Dictionary Revision  . . . . . . . . . . . . . . . . . . . 24
   5.  Bundle Processing  . . . . . . . . . . . . . . . . . . . . . . 24
     5.1.  Generation of Administrative Records . . . . . . . . . . . 25
     5.2.  Bundle Transmission  . . . . . . . . . . . . . . . . . . . 26
     5.3.  Bundle Dispatching . . . . . . . . . . . . . . . . . . . . 26
     5.4.  Bundle Forwarding  . . . . . . . . . . . . . . . . . . . . 27
       5.4.1.  Forwarding Contraindicated . . . . . . . . . . . . . . 28
       5.4.2.  Forwarding Failed  . . . . . . . . . . . . . . . . . . 29
     5.5.  Bundle Expiration  . . . . . . . . . . . . . . . . . . . . 29
     5.6.  Bundle Reception . . . . . . . . . . . . . . . . . . . . . 30
     5.7.  Local Bundle Delivery  . . . . . . . . . . . . . . . . . . 31
     5.8.  Bundle Fragmentation . . . . . . . . . . . . . . . . . . . 32
     5.9.  Application Data Unit Reassembly . . . . . . . . . . . . . 33
     5.10. Custody Transfer . . . . . . . . . . . . . . . . . . . . . 34
       5.10.1. Custody Acceptance . . . . . . . . . . . . . . . . . . 34
       5.10.2. Custody Release  . . . . . . . . . . . . . . . . . . . 35
     5.11. Custody Transfer Success . . . . . . . . . . . . . . . . . 35
     5.12. Custody Transfer Failure . . . . . . . . . . . . . . . . . 35
     5.13. Bundle Deletion  . . . . . . . . . . . . . . . . . . . . . 36
     5.14. Discarding a Bundle  . . . . . . . . . . . . . . . . . . . 36
     5.15. Canceling a Transmission . . . . . . . . . . . . . . . . . 36
     5.16. Polling  . . . . . . . . . . . . . . . . . . . . . . . . . 36
   6.  Administrative Record Processing . . . . . . . . . . . . . . . 37
     6.1.  Administrative Records . . . . . . . . . . . . . . . . . . 37
       6.1.1.  Bundle Status Reports  . . . . . . . . . . . . . . . . 38
       6.1.2.  Custody Signals  . . . . . . . . . . . . . . . . . . . 41
     6.2.  Generation of Administrative Records . . . . . . . . . . . 44
     6.3.  Reception of Custody Signals . . . . . . . . . . . . . . . 44

   7.  Services Required of the Convergence Layer . . . . . . . . . . 44
     7.1.  The Convergence Layer  . . . . . . . . . . . . . . . . . . 44
     7.2.  Summary of Convergence Layer Services  . . . . . . . . . . 45
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 45
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 47
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 47
     10.2. Informative References . . . . . . . . . . . . . . . . . . 47
   Appendix A.  Contributors  . . . . . . . . . . . . . . . . . . . . 49
   Appendix B.  Comments  . . . . . . . . . . . . . . . . . . . . . . 49

1.  Introduction

   This document describes version 6 of the Delay Tolerant Networking
   (DTN) "bundle" protocol (BP).  Delay Tolerant Networking is an end-
   to-end architecture providing communications in and/or through highly
   stressed environments.  Stressed networking environments include
   those with intermittent connectivity, large and/or variable delays,
   and high bit error rates.  To provide its services, BP sits at the
   application layer of some number of constituent internets, forming a
   store-and-forward overlay network.  Key capabilities of BP include:

   o  Custody-based retransmission

   o  Ability to cope with intermittent connectivity

   o  Ability to take advantage of scheduled, predicted, and
      opportunistic connectivity (in addition to continuous
      connectivity)

   o  Late binding of overlay network endpoint identifiers to
      constituent internet addresses

   For descriptions of these capabilities and the rationale for the DTN
   architecture, see [ARCH] and [SIGC].  [TUT] contains a tutorial-level
   overview of DTN concepts.

   This is an experimental protocol, produced within the IRTF's Delay
   Tolerant Networking Research Group (DTNRG) and represents the
   consensus of all of the active contributors to this group.  If this
   protocol is used on the Internet, IETF standard protocols for
   security and congestion control should be used.

   BP's location within the standard protocol stack is as shown in
   Figure 1.  BP uses the "native" internet protocols for communications
   within a given internet.  Note that "internet" in the preceding is
   used in a general sense and does not necessarily refer to TCP/IP.
   The interface between the common bundle protocol and a specific

   internetwork protocol suite is termed a "convergence layer adapter".
   Figure 1 shows three distinct transport and network protocols
   (denoted T1/N1, T2/N2, and T3/N3).

   +-----------+                                         +-----------+
   |   BP app  |                                         |   BP app  |
   +---------v-|   +->>>>>>>>>>v-+     +->>>>>>>>>>v-+   +-^---------+
   |    BP   v |   | ^    BP   v |     | ^    BP   v |   | ^   BP    |
   +---------v-+   +-^---------v-+     +-^---------v-+   +-^---------+
   | Trans1  v |   + ^  T1/T2  v |     + ^  T2/T3  v |   | ^  Trans3 |
   +---------v-+   +-^---------v-+     +-^---------v +   +-^---------+
   | Net1    v |   | ^  N1/N2  v |     | ^  N2/N3  v |   | ^  Net3   |
   +---------v-+   +-^---------v +     +-^---------v-+   +-^---------+
   |         >>>>>>>>^         >>>>>>>>>>^         >>>>>>>>^         |
   +-----------+   +-------------+     +-------------+   +-----------+
   |                      |                   |                      |
   |<--- An internet  --->|                   |<--- An internet  --->|
   |                      |                   |                      |

                  Figure 1: The Bundle Protocol Sits at
                the Application Layer of the Internet Model

   This document describes the format of the protocol data units (called
   bundles) passed between entities participating in BP communications.
   The entities are referred to as "bundle nodes".  This document does
   not address:

   o  Operations in the convergence layer adapters that bundle nodes use
      to transport data through specific types of internets.  (However,
      the document does discuss the services that must be provided by
      each adapter at the convergence layer.)

   o  The bundle routing algorithm.

   o  Mechanisms for populating the routing or forwarding information
      bases of bundle nodes.

2.  Requirements Notation

   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 [RFC2119].

3.  Service Description

3.1.  Definitions

   Bundle -  A bundle is a protocol data unit of the DTN bundle
      protocol.  Each bundle comprises a sequence of two or more
      "blocks" of protocol data, which serve various purposes.  Multiple
      instances of the same bundle (the same unit of DTN protocol data)
      might exist concurrently in different parts of a network --
      possibly in different representations -- in the memory local to
      one or more bundle nodes and/or in transit between nodes.  In the
      context of the operation of a bundle node, a bundle is an instance
      of some bundle in the network that is in that node's local memory.

   Bundle payload -  A bundle payload (or simply "payload") is the
      application data whose conveyance to the bundle's destination is
      the purpose for the transmission of a given bundle.  The terms
      "bundle content", "bundle payload", and "payload" are used
      interchangeably in this document.  The "nominal" payload for a
      bundle forwarded in response to a bundle transmission request is
      the application data unit whose location is provided as a
      parameter to that request.  The nominal payload for a bundle
      forwarded in response to reception of that bundle is the payload
      of the received bundle.

   Fragment -  A fragment is a bundle whose payload block contains a
      fragmentary payload.  A fragmentary payload is either the first N
      bytes or the last N bytes of some other payload -- either a
      nominal payload or a fragmentary payload -- of length M, such that
      0 < N < M.

   Bundle node -  A bundle node (or, in the context of this document,
      simply a "node") is any entity that can send and/or receive
      bundles.  In the most familiar case, a bundle node is instantiated
      as a single process running on a general-purpose computer, but in
      general the definition is meant to be broader: a bundle node might
      alternatively be a thread, an object in an object-oriented
      operating system, a special-purpose hardware device, etc.  Each
      bundle node has three conceptual components, defined below: a
      "bundle protocol agent", a set of zero or more "convergence layer
      adapters", and an "application agent".

   Bundle protocol agent -  The bundle protocol agent (BPA) of a node is
      the node component that offers the BP services and executes the
      procedures of the bundle protocol.  The manner in which it does so
      is wholly an implementation matter.  For example, BPA
      functionality might be coded into each node individually; it might
      be implemented as a shared library that is used in common by any

      number of bundle nodes on a single computer; it might be
      implemented as a daemon whose services are invoked via inter-
      process or network communication by any number of bundle nodes on
      one or more computers; it might be implemented in hardware.

   Convergence layer adapters -  A convergence layer adapter (CLA) sends
      and receives bundles on behalf of the BPA, utilizing the services
      of some 'native' internet protocol that is supported in one of the
      internets within which the node is functionally located.  The
      manner in which a CLA sends and receives bundles is wholly an
      implementation matter, exactly as described for the BPA.

   Application agent -  The application agent (AA) of a node is the node
      component that utilizes the BP services to effect communication
      for some purpose.  The application agent in turn has two elements,
      an administrative element and an application-specific element.
      The application-specific element of an AA constructs, requests
      transmission of, accepts delivery of, and processes application-
      specific application data units; the only interface between the
      BPA and the application-specific element of the AA is the BP
      service interface.  The administrative element of an AA constructs
      and requests transmission of administrative records (status
      reports and custody signals), and it accepts delivery of and
      processes any custody signals that the node receives.  In addition
      to the BP service interface, there is a (conceptual) private
      control interface between the BPA and the administrative element
      of the AA that enables each to direct the other to take action
      under specific circumstances.  In the case of a node that serves
      simply as a "router" in the overlay network, the AA may have no
      application-specific element at all.  The application-specific
      elements of other nodes' AAs may perform arbitrarily complex
      application functions, perhaps even offering multiplexed DTN
      communication services to a number of other applications.  As with
      the BPA, the manner in which the AA performs its functions is
      wholly an implementation matter; in particular, the administrative
      element of an AA might be built into the library or daemon or
      hardware that implements the BPA, and the application-specific
      element of an AA might be implemented either in software or in
      hardware.

   Bundle endpoint -  A bundle endpoint (or simply "endpoint") is a set
      of zero or more bundle nodes that all identify themselves for BP
      purposes by some single text string, called a "bundle endpoint ID"
      (or, in this document, simply "endpoint ID"; endpoint IDs are
      described in detail in Section 4.4 below).  The special case of an
      endpoint that never contains more than one node is termed a
      "singleton" endpoint; every bundle node must be a member of at
      least one singleton endpoint.  Singletons are the most familiar

      sort of endpoint, but in general the endpoint notion is meant to
      be broader.  For example, the nodes in a sensor network might
      constitute a set of bundle nodes that identify themselves by a
      single common endpoint ID and thus form a single bundle endpoint.
      *Note* too that a given bundle node might identify itself by
      multiple endpoint IDs and thus be a member of multiple bundle
      endpoints.

   Forwarding -  When the bundle protocol agent of a node determines
      that a bundle must be "forwarded" to an endpoint, it causes the
      bundle to be sent to all of the nodes that the bundle protocol
      agent currently believes are in the "minimum reception group" of
      that endpoint.  The minimum reception group of an endpoint may be
      any one of the following: (a) ALL of the nodes registered in an
      endpoint that is permitted to contain multiple nodes (in which
      case forwarding to the endpoint is functionally similar to
      "multicast" operations in the Internet, though possibly very
      different in implementation); (b) ANY N of the nodes registered in
      an endpoint that is permitted to contain multiple nodes, where N
      is in the range from zero to the cardinality of the endpoint (in
      which case forwarding to the endpoint is functionally similar to
      "anycast" operations in the Internet); or (c) THE SOLE NODE
      registered in a singleton endpoint (in which case forwarding to
      the endpoint is functionally similar to "unicast" operations in
      the Internet).  The nature of the minimum reception group for a
      given endpoint can be determined from the endpoint's ID (again,
      see Section 4.4 below): for some endpoint ID "schemes", the nature
      of the minimum reception group is fixed - in a manner that is
      defined by the scheme - for all endpoints identified under the
      scheme; for other schemes, the nature of the minimum reception
      group is indicated by some lexical feature of the "scheme-specific
      part" of the endpoint ID, in a manner that is defined by the
      scheme.

   Registration -  A registration is the state machine characterizing a
      given node's membership in a given endpoint.  Any number of
      registrations may be concurrently associated with a given
      endpoint, and any number of registrations may be concurrently
      associated with a given node.  Any single registration must at any
      time be in one of two states: Active or Passive.  A registration
      always has an associated "delivery failure action", the action
      that is to be taken when a bundle that is "deliverable" (see
      below) subject to that registration is received at a time when the
      registration is in the Passive state.  Delivery failure action
      must be one of the following:

      *  defer "delivery" (see below) of the bundle subject to this
         registration until (a) this bundle is the least recently

         received of all bundles currently deliverable subject to this
         registration and (b) either the registration is polled or else
         the registration is in the Active state; or

      *  "abandon" (see below) delivery of the bundle subject to this
         registration.

      An additional implementation-specific delivery deferral procedure
      may optionally be associated with the registration.  While the
      state of a registration is Active, reception of a bundle that is
      deliverable subject to this registration must cause the bundle to
      be delivered automatically as soon as it is the least recently
      received bundle that is currently deliverable subject to the
      registration.  While the state of a registration is Passive,
      reception of a bundle that is deliverable subject to this
      registration must cause delivery of the bundle to be abandoned or
      deferred as mandated by the registration's current delivery
      failure action; in the latter case, any additional delivery
      deferral procedure associated with the registration must also be
      performed.

   Delivery -  Upon reception, the processing of a bundle that has been
      sent to a given node depends on whether or not the receiving node
      is registered in the bundle's destination endpoint.  If it is, and
      if the payload of the bundle is non-fragmentary (possibly as a
      result of successful payload reassembly from fragmentary payloads,
      including the original payload of the received bundle), then the
      bundle is normally "delivered" to the node's application agent
      subject to the registration characterizing the node's membership
      in the destination endpoint.  A bundle is considered to have been
      delivered at a node subject to a registration as soon as the
      application data unit that is the payload of the bundle, together
      with the value of the bundle's "Acknowledgement by application is
      requested" flag and any other relevant metadata (an implementation
      matter), has been presented to the node's application agent in a
      manner consistent with the state of that registration and, as
      applicable, the registration's delivery failure action.

   Deliverability, Abandonment -  A bundle is considered "deliverable"
      subject to a registration if and only if (a) the bundle's
      destination endpoint is the endpoint with which the registration
      is associated, (b) the bundle has not yet been delivered subject
      to this registration, and (c) delivery of the bundle subject to
      this registration has not been abandoned.  To "abandon" delivery
      of a bundle subject to a registration is simply to declare it no
      longer deliverable subject to that registration; normally only
      registrations' registered delivery failure actions cause
      deliveries to be abandoned.

   Deletion, Discarding -  A bundle protocol agent "discards" a bundle
      by simply ceasing all operations on the bundle and functionally
      erasing all references to it; the specific procedures by which
      this is accomplished are an implementation matter.  Bundles are
      discarded silently; i.e., the discarding of a bundle does not
      result in generation of an administrative record.  "Retention
      constraints" are elements of the bundle state that prevent a
      bundle from being discarded; a bundle cannot be discarded while it
      has any retention constraints.  A bundle protocol agent "deletes"
      a bundle in response to some anomalous condition by notifying the
      bundle's report-to endpoint of the deletion (provided such
      notification is warranted; see Section 5.13 for details) and then
      arbitrarily removing all of the bundle's retention constraints,
      enabling the bundle to be discarded.

   Transmission -  A transmission is a sustained effort by a node's
      bundle protocol agent to cause a bundle to be sent to all nodes in
      the minimum reception group of some endpoint (which may be the
      bundle's destination or may be some intermediate forwarding
      endpoint) in response to a transmission request issued by the
      node's application agent.  Any number of transmissions may be
      concurrently undertaken by the bundle protocol agent of a given
      node.

   Custody -  To "accept custody" upon forwarding a bundle is to commit
      to retaining a copy of the bundle -- possibly re-forwarding the
      bundle when necessary -- until custody of that bundle is
      "released".  Custody of a bundle whose destination is a singleton
      endpoint is released when either (a) notification is received that
      some other node has accepted custody of the same bundle; (b)
      notification is received that the bundle has been delivered at the
      (sole) node registered in the bundle's destination endpoint; or
      (c) the bundle is explicitly deleted for some reason, such as
      lifetime expiration.  The condition(s) under which custody of a
      bundle whose destination is not a singleton endpoint may be
      released are not defined in this specification.  To "refuse
      custody" of a bundle is to decide not to accept custody of the
      bundle.  A "custodial node" of a bundle is a node that has
      accepted custody of the bundle and has not yet released that
      custody.  A "custodian" of a bundle is a singleton endpoint whose
      sole member is one of the bundle's custodial nodes.

3.2.  Implementation Architectures

   The above definitions are intended to enable the bundle protocol's
   operations to be specified in a manner that minimizes bias toward any
   particular implementation architecture.  To illustrate the range of
   interoperable implementation models that might conform to this

   specification, four example architectures are briefly described
   below.

   1.  Bundle protocol application server

       A single bundle protocol application server, constituting a
       single bundle node, runs as a daemon process on each computer.
       The daemon's functionality includes all functions of the bundle
       protocol agent, all convergence layer adapters, and both the
       administrative and application-specific elements of the
       application agent.  The application-specific element of the
       application agent functions as a server, offering bundle protocol
       service over a local area network: it responds to remote
       procedure calls from application processes (on the same computer
       and/or remote computers) that need to communicate via the bundle
       protocol.  The server supports its clients by creating a new
       (conceptual) node for each one and registering each such node in
       a client-specified endpoint.  The conceptual nodes managed by the
       server function as clients' bundle protocol service access
       points.

   2.  Peer application nodes

       Any number of bundle protocol application processes, each one
       constituting a single bundle node, run in ad-hoc fashion on each
       computer.  The functionality of the bundle protocol agent, all
       convergence layer adapters, and the administrative element of the
       application agent is provided by a library to which each node
       process is dynamically linked at run time.  The application-
       specific element of each node's application agent is node-
       specific application code.

   3.  Sensor network nodes

       Each node of the sensor network is the self-contained
       implementation of a single bundle node.  All functions of the
       bundle protocol agent, all convergence layer adapters, and the
       administrative element of the application agent are implemented
       in simplified form in Application-Specific Integrated Circuits
       (ASICs), while the application-specific element of each node's
       application agent is implemented in a programmable
       microcontroller.  Forwarding is rudimentary: all bundles are
       forwarded on a hard-coded default route.

   4.  Dedicated bundle router

       Each computer constitutes a single bundle node that functions
       solely as a high-performance bundle forwarder.  Many standard
       functions of the bundle protocol agent, the convergence layer
       adapters, and the administrative element of the application agent
       are implemented in ASICs, but some functions are implemented in a
       high-speed processor to enable reprogramming as necessary.  The
       node's application agent has no application-specific element.
       Substantial non-volatile storage resources are provided, and
       arbitrarily complex forwarding algorithms are supported.

3.3.  Services Offered by Bundle Protocol Agents

   The bundle protocol agent of each node is expected to provide the
   following services to the node's application agent:

   o  commencing a registration (registering a node in an endpoint);

   o  terminating a registration;

   o  switching a registration between Active and Passive states;

   o  transmitting a bundle to an identified bundle endpoint;

   o  canceling a transmission;

   o  polling a registration that is in the passive state;

   o  delivering a received bundle.

4.  Bundle Format

   Each bundle shall be a concatenated sequence of at least two block
   structures.  The first block in the sequence must be a primary bundle
   block, and no bundle may have more than one primary bundle block.
   Additional bundle protocol blocks of other types may follow the
   primary block to support extensions to the bundle protocol, such as
   the Bundle Security Protocol [BSP].  At most one of the blocks in the
   sequence may be a payload block.  The last block in the sequence must
   have the "last block" flag (in its block processing control flags)
   set to 1; for every other block in the bundle after the primary
   block, this flag must be set to zero.

4.1.  Self-Delimiting Numeric Values (SDNVs)

   The design of the bundle protocol attempts to reconcile minimal
   consumption of transmission bandwidth with:

   o  extensibility to address requirements not yet identified, and

   o  scalability across a wide range of network scales and payload
      sizes.

   A key strategic element in the design is the use of self-delimiting
   numeric values (SDNVs).  The SDNV encoding scheme is closely adapted
   from the Abstract Syntax Notation One Basic Encoding Rules for
   subidentifiers within an object identifier value [ASN1].  An SDNV is
   a numeric value encoded in N octets, the last of which has its most
   significant bit (MSB) set to zero; the MSB of every other octet in
   the SDNV must be set to 1.  The value encoded in an SDNV is the
   unsigned binary number obtained by concatenating into a single bit
   string the 7 least significant bits of each octet of the SDNV.

   The following examples illustrate the encoding scheme for various
   hexadecimal values.

   0xABC  : 1010 1011 1100
            is encoded as
            {1 00 10101} {0 0111100}
            = 10010101 00111100

   0x1234 : 0001 0010 0011 0100
          =    1 0010 0011 0100
            is encoded as
            {1 0 100100} {0 0110100}
            = 10100100 00110100

   0x4234 : 0100 0010 0011 0100
          =  100 0010 0011 0100
            is encoded as
            {1 000000 1} {1 0000100} {0 0110100}
            = 10000001 10000100 00110100

   0x7F   : 0111 1111
          =  111 1111
            is encoded as
            {0 1111111}
            = 01111111

                          Figure 2: SDNV Example

   Note: Care must be taken to make sure that the value to be encoded is
   (in concept) padded with high-order zero bits to make its bitwise
   length a multiple of 7 before encoding.  Also note that, while there
   is no theoretical limit on the size of an SDNV field, the overhead of
   the SDNV scheme is 1:7, i.e., one bit of overhead for every 7 bits of
   actual data to be encoded.  Thus, a 7-octet value (a 56-bit quantity
   with no leading zeroes) would be encoded in an 8-octet SDNV; an
   8-octet value (a 64-bit quantity with no leading zeroes) would be
   encoded in a 10-octet SDNV (one octet containing the high-order bit
   of the value padded with six leading zero bits, followed by nine
   octets containing the remaining 63 bits of the value). 148 bits of
   overhead would be consumed in encoding a 1024-bit RSA encryption key
   directly in an SDNV.  In general, an N-bit quantity with no leading
   zeroes is encoded in an SDNV occupying ceil(N/7) octets, where ceil
   is the integer ceiling function.

   Implementations of the bundle protocol may handle as an invalid
   numeric value any SDNV that encodes an integer that is larger than
   (2^64 - 1).

   An SDNV can be used to represent both very large and very small
   integer values.  However, SDNV is clearly not the best way to
   represent every numeric value.  For example, an SDNV is a poor way to
   represent an integer whose value typically falls in the range 128 to
   255.  In general, though, we believe that SDNV representation of
   numeric values in bundle blocks yields the smallest block sizes
   without sacrificing scalability.

4.2.  Bundle Processing Control Flags

   The bundle processing control flags field in the primary bundle block
   of each bundle is an SDNV; the value encoded in this SDNV is a string
   of bits used to invoke selected bundle processing control features.
   The significance of the value in each currently defined position of
   this bit string is described here.  Note that in the figure and
   descriptions, the bit label numbers denote position (from least
   significant ('0') to most significant) within the decoded bit string,
   and not within the representation of the bits on the wire.  This is
   why the descriptions in this section and the next do not follow
   standard RFC conventions with bit 0 on the left; if fields are added
   in the future, the SDNV will grow to the left, and using this
   representation allows the references here to remain valid.

            2                   1                   0
            0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |Status Report|Class of Svc.|   General   |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 3: Bundle Processing Control Flags Bit Layout

   The bits in positions 0 through 6 are flags that characterize the
   bundle as follows:

   0 --   Bundle is a fragment.

   1 --   Application data unit is an administrative record.

   2 --   Bundle must not be fragmented.

   3 --   Custody transfer is requested.

   4 --   Destination endpoint is a singleton.

   5 --   Acknowledgement by application is requested.

   6 --   Reserved for future use.

   The bits in positions 7 through 13 are used to indicate the bundle's
   class of service.  The bits in positions 8 and 7 constitute a two-bit
   priority field indicating the bundle's priority, with higher values
   being of higher priority: 00 = bulk, 01 = normal, 10 = expedited, 11
   is reserved for future use.  Within this field, bit 8 is the most
   significant bit.  The bits in positions 9 through 13 are reserved for
   future use.

   The bits in positions 14 through 20 are status report request flags.
   These flags are used to request status reports as follows:

   14 --   Request reporting of bundle reception.

   15 --   Request reporting of custody acceptance.

   16 --   Request reporting of bundle forwarding.

   17 --   Request reporting of bundle delivery.

   18 --   Request reporting of bundle deletion.

   19 --   Reserved for future use.

   20 --   Reserved for future use.

   If the bundle processing control flags indicate that the bundle's
   application data unit is an administrative record, then the custody
   transfer requested flag must be zero and all status report request
   flags must be zero.  If the custody transfer requested flag is 1,
   then the sending node requests that the receiving node accept custody
   of the bundle.  If the bundle's source endpoint ID is "dtn:none" (see
   below), then the bundle is not uniquely identifiable and all bundle
   protocol features that rely on bundle identity must therefore be
   disabled: the bundle's custody transfer requested flag must be zero,
   the "Bundle must not be fragmented" flag must be 1, and all status
   report request flags must be zero.

4.3.  Block Processing Control Flags

   The block processing control flags field in every block other than
   the primary bundle block is an SDNV; the value encoded in this SDNV
   is a string of bits used to invoke selected block processing control
   features.  The significance of the values in all currently defined
   positions of this bit string, in order from least significant
   position in the decoded bit string (labeled '0') to most significant
   (labeled '6'), is described here.

                        0
            6 5 4 3 2 1 0
           +-+-+-+-+-+-+-+
           |   Flags     |
           +-+-+-+-+-+-+-+

            Figure 4: Block Processing Control Flags Bit Layout

      0 - Block must be replicated in every fragment.

      1 - Transmit status report if block can't be processed.

      2 - Delete bundle if block can't be processed.

      3 - Last block.

      4 - Discard block if it can't be processed.

      5 - Block was forwarded without being processed.

      6 - Block contains an EID-reference field.

   For each bundle whose primary block's bundle processing control flags
   (see above) indicate that the bundle's application data unit is an
   administrative record, the "Transmit status report if block can't be
   processed" flag in the block processing flags field of every other
   block in the bundle must be zero.

   The 'Block must be replicated in every fragment' bit in the block
   processing flags must be set to zero on all blocks that follow the
   payload block.

4.4.  Endpoint IDs

   The destinations of bundles are bundle endpoints, identified by text
   strings termed "endpoint IDs" (see Section 3.1).  Each endpoint ID
   conveyed in any bundle block takes the form of a Uniform Resource
   Identifier (URI; [URI]).  As such, each endpoint ID can be
   characterized as having this general structure:

   < scheme name > : < scheme-specific part, or "SSP" >

   As used for the purposes of the bundle protocol, neither the length
   of a scheme name nor the length of an SSP may exceed 1023 bytes.

   Bundle blocks cite a number of endpoint IDs for various purposes of
   the bundle protocol.  Many, though not necessarily all, of the
   endpoint IDs referred to in the blocks of a given bundle are conveyed
   in the "dictionary" byte array in the bundle's primary block.  This
   array is simply the concatenation of any number of null-terminated
   scheme names and SSPs.

   "Endpoint ID references" are used to cite endpoint IDs that are
   contained in the dictionary; all endpoint ID citations in the primary
   bundle block are endpoint ID references, and other bundle blocks may
   contain endpoint ID references as well.  Each endpoint ID reference
   is an ordered pair of SDNVs:

   o  The first SDNV contains the offset within the dictionary of the
      first character of the referenced endpoint ID's scheme name.

   o  The second SDNV contains the offset within the dictionary of the
      first character of the referenced endpoint ID's SSP.

   This encoding enables a degree of block compression: when the source
   and report-to of a bundle are the same endpoint, for example, the
   text of that endpoint's ID may be cited twice yet appear only once in
   the dictionary.

   The scheme identified by the < scheme name > in an endpoint ID is a
   set of syntactic and semantic rules that fully explain how to parse
   and interpret the SSP.  The set of allowable schemes is effectively
   unlimited.  Any scheme conforming to [URIREG] may be used in a bundle
   protocol endpoint ID.  In addition, a single additional scheme is
   defined by the present document:

   o  The "dtn" scheme, which is used at minimum in the representation
      of the null endpoint ID "dtn:none".  The forwarding of a bundle to
      the null endpoint is never contraindicated, and the minimum
      reception group for the null endpoint is the empty set.

   Note that, although the endpoint IDs conveyed in bundle blocks are
   expressed as URIs, implementations of the BP service interface may
   support expression of endpoint IDs in some internationalized manner
   (e.g., Internationalized Resource Identifiers (IRIs); see [RFC3987]).

4.5.  Formats of Bundle Blocks

   This section describes the formats of the primary block and payload
   block.  Rules for processing these blocks appear in Section 5 of this
   document.

   Note that supplementary DTN protocol specifications (including, but
   not restricted to, the Bundle Security Protocol [BSP]) may require
   that BP implementations conforming to those protocols construct and
   process additional blocks.

   The format of the two basic BP blocks is shown in Figure 5 below.

   Primary Bundle Block
   +----------------+----------------+----------------+----------------+
   |    Version     |                  Proc. Flags (*)                 |
   +----------------+----------------+----------------+----------------+
   |                          Block length (*)                         |
   +----------------+----------------+---------------------------------+
   |   Destination scheme offset (*) |     Destination SSP offset (*)  |
   +----------------+----------------+----------------+----------------+
   |      Source scheme offset (*)   |        Source SSP offset (*)    |
   +----------------+----------------+----------------+----------------+
   |    Report-to scheme offset (*)  |      Report-to SSP offset (*)   |
   +----------------+----------------+----------------+----------------+
   |    Custodian scheme offset (*)  |      Custodian SSP offset (*)   |
   +----------------+----------------+----------------+----------------+
   |                    Creation Timestamp time (*)                    |
   +---------------------------------+---------------------------------+
   |             Creation Timestamp sequence number (*)                |
   +---------------------------------+---------------------------------+
   |                           Lifetime (*)                            |
   +----------------+----------------+----------------+----------------+
   |                        Dictionary length (*)                      |
   +----------------+----------------+----------------+----------------+
   |                  Dictionary byte array (variable)                 |
   +----------------+----------------+---------------------------------+
   |                      [Fragment offset (*)]                        |
   +----------------+----------------+---------------------------------+
   |              [Total application data unit length (*)]             |
   +----------------+----------------+---------------------------------+

   Bundle Payload Block
   +----------------+----------------+----------------+----------------+
   |  Block type    | Proc. Flags (*)|        Block length(*)          |
   +----------------+----------------+----------------+----------------+
   /                     Bundle Payload (variable)                     /
   +-------------------------------------------------------------------+

                      Figure 5: Bundle Block Formats

   (*) Notes:

   The bundle processing control ("Proc.") flags field in the Primary
   Bundle Block is an SDNV and is therefore variable length.  A three-
   octet SDNV is shown here for convenience in representation.

   The block length field of the Primary Bundle Block is an SDNV and is
   therefore variable length.  A four-octet SDNV is shown here for
   convenience in representation.

   Each of the eight offset fields in the Primary Bundle Block is an
   SDNV and is therefore variable length.  Two-octet SDNVs are shown
   here for convenience in representation.

   The Creation Timestamp time field in the Primary Bundle Block is an
   SDNV and is therefore variable length.  A four-octet SDNV is shown
   here for convenience in representation.

   The Creation Timestamp sequence number field in the Primary Bundle
   Block is an SDNV and is therefore variable length.  A four-octet SDNV
   is shown here for convenience in representation.

   The Lifetime field in the Primary Bundle Block is an SDNV and is
   therefore variable length.  A four-octet SDNV is shown here for
   convenience in representation.

   The dictionary length field of the Primary Bundle Block is an SDNV
   and is therefore variable length.  A four-octet SDNV is shown here
   for convenience in representation.

   The fragment offset field of the Primary Bundle Block is present only
   if the Fragment flag in the block's processing flags byte is set to
   1.  It is an SDNV and is therefore variable length; a four-octet SDNV
   is shown here for convenience in representation.

   The total application data unit length field of the Primary Bundle
   Block is present only if the Fragment flag in the block's processing
   flags byte is set to 1.  It is an SDNV and is therefore variable
   length; a four-octet SDNV is shown here for convenience in
   representation.

   The block processing control ("Proc.") flags field of the Payload
   Block is an SDNV and is therefore variable length.  A one-octet SDNV
   is shown here for convenience in representation.

   The block length field of the Payload Block is an SDNV and is
   therefore variable length.  A two-octet SDNV is shown here for
   convenience in representation.

4.5.1.  Primary Bundle Block

   The primary bundle block contains the basic information needed to
   route bundles to their destinations.  The fields of the primary
   bundle block are:

   Version:   A 1-byte field indicating the version of the bundle
      protocol that constructed this block.  The present document
      describes version 0x06 of the bundle protocol.

   Bundle Processing Control Flags:   The Bundle Processing Control
      Flags field is an SDNV that contains the bundle processing control
      flags discussed in Section 4.2 above.

   Block Length:   The Block Length field is an SDNV that contains the
      aggregate length of all remaining fields of the block.

   Destination Scheme Offset:   The Destination Scheme Offset field
      contains the offset within the dictionary byte array of the scheme
      name of the endpoint ID of the bundle's destination, i.e., the
      endpoint containing the node(s) at which the bundle is to be
      delivered.

   Destination SSP Offset:   The Destination SSP Offset field contains
      the offset within the dictionary byte array of the scheme-specific
      part of the endpoint ID of the bundle's destination.

   Source Scheme Offset:   The Source Scheme Offset field contains the
      offset within the dictionary byte array of the scheme name of the
      endpoint ID of the bundle's nominal source, i.e., the endpoint
      nominally containing the node from which the bundle was initially
      transmitted.

   Source SSP Offset:   The Source SSP Offset field contains the offset
      within the dictionary byte array of the scheme-specific part of
      the endpoint ID of the bundle's nominal source.

   Report-to Scheme Offset:   The Report-to Scheme Offset field contains
      the offset within the dictionary byte array of the scheme name of
      the ID of the endpoint to which status reports pertaining to the
      forwarding and delivery of this bundle are to be transmitted.

   Report-to SSP Offset:   The Report-to SSP Offset field contains the
      offset within the dictionary byte array of the scheme-specific
      part of the ID of the endpoint to which status reports pertaining
      to the forwarding and delivery of this bundle are to be
      transmitted.

   Custodian Scheme Offset:   The "current custodian endpoint ID" of a
      primary bundle block identifies an endpoint whose membership
      includes the node that most recently accepted custody of the
      bundle upon forwarding this bundle.  The Custodian Scheme Offset
      field contains the offset within the dictionary byte array of the
      scheme name of the current custodian endpoint ID.

   Custodian SSP Offset:   The Custodian SSP Offset field contains the
      offset within the dictionary byte array of the scheme-specific
      part of the current custodian endpoint ID.

   Creation Timestamp:   The creation timestamp is a pair of SDNVs that,
      together with the source endpoint ID and (if the bundle is a
      fragment) the fragment offset and payload length, serve to
      identify the bundle.  The first SDNV of the timestamp is the
      bundle's creation time, while the second is the bundle's creation
      timestamp sequence number.  Bundle creation time is the time --
      expressed in seconds since the start of the year 2000, on the
      Coordinated Universal Time (UTC) scale [UTC] -- at which the
      transmission request was received that resulted in the creation of
      the bundle.  Sequence count is the latest value (as of the time at
      which that transmission request was received) of a monotonically
      increasing positive integer counter managed by the source node's
      bundle protocol agent that may be reset to zero whenever the
      current time advances by one second.  A source Bundle Protocol
      Agent must never create two distinct bundles with the same source
      endpoint ID and bundle creation timestamp.  The combination of
      source endpoint ID and bundle creation timestamp therefore serves
      to identify a single transmission request, enabling it to be
      acknowledged by the receiving application (provided the source
      endpoint ID is not "dtn:none").

   Lifetime:   The lifetime field is an SDNV that indicates the time at
      which the bundle's payload will no longer be useful, encoded as a
      number of seconds past the creation time.  When the current time
      is greater than the creation time plus the lifetime, bundle nodes
      need no longer retain or forward the bundle; the bundle may be
      deleted from the network.

   Dictionary Length:   The Dictionary Length field is an SDNV that
      contains the length of the dictionary byte array.

   Dictionary:   The Dictionary field is an array of bytes formed by
      concatenating the null-terminated scheme names and SSPs of all
      endpoint IDs referenced by any fields in this Primary Block
      together with, potentially, other endpoint IDs referenced by
      fields in other TBD DTN protocol blocks.  Its length is given by
      the value of the Dictionary Length field.

   Fragment Offset:   If the Bundle Processing Control Flags of this
      Primary block indicate that the bundle is a fragment, then the
      Fragment Offset field is an SDNV indicating the offset from the
      start of the original application data unit at which the bytes
      comprising the payload of this bundle were located.  If not, then
      the Fragment Offset field is omitted from the block.

   Total Application Data Unit Length:   If the Bundle Processing
      Control Flags of this Primary block indicate that the bundle is a
      fragment, then the Total Application Data Unit Length field is an
      SDNV indicating the total length of the original application data
      unit of which this bundle's payload is a part.  If not, then the
      Total Application Data Unit Length field is omitted from the
      block.

4.5.2.  Canonical Bundle Block Format

   Every bundle block of every type other than the primary bundle block
   comprises the following fields, in this order:

   o  Block type code, expressed as an 8-bit unsigned binary integer.
      Bundle block type code 1 indicates that the block is a bundle
      payload block.  Block type codes 192 through 255 are not defined
      in this specification and are available for private and/or
      experimental use.  All other values of the block type code are
      reserved for future use.

   o  Block processing control flags, an unsigned integer expressed as
      an SDNV.  The individual bits of this integer are used to invoke
      selected block processing control features.

   o  Block EID reference count and EID references (optional).  If and
      only if the block references EID elements in the primary block's
      dictionary, the 'block contains an EID-reference field' flag in
      the block processing control flags is set to 1 and the block
      includes an EID reference field consisting of a count of EID
      references expressed as an SDNV followed by the EID references
      themselves.  Each EID reference is a pair of SDNVs.  The first
      SDNV of each EID reference contains the offset of a scheme name in
      the primary block's dictionary, and the second SDNV of each
      reference contains the offset of a scheme-specific part in the
      dictionary.

   o  Block data length, an unsigned integer expressed as an SDNV.  The
      Block data length field contains the aggregate length of all
      remaining fields of the block, i.e., the block-type-specific data
      fields.

   o  Block-type-specific data fields, whose format and order are type-
      specific and whose aggregate length in octets is the value of the
      block data length field.  All multi-byte block-type-specific data
      fields are represented in network byte order.

          +-----------+-----------+-----------+-----------+
          |Block type | Block processing ctrl flags (SDNV)|
          +-----------+-----------+-----------+-----------+
          |            Block length  (SDNV)               |
          +-----------+-----------+-----------+-----------+
          /          Block body data (variable)           /
          +-----------+-----------+-----------+-----------+

             Figure 6: Block Layout without EID Reference List

          +-----------+-----------+-----------+-----------+
          |Block Type | Block processing ctrl flags (SDNV)|
          +-----------+-----------+-----------+-----------+
          |        EID Reference Count  (SDNV)            |
          +-----------+-----------+-----------+-----------+
          |  Ref_scheme_1 (SDNV)  |    Ref_ssp_1 (SDNV)   |
          +-----------+-----------+-----------+-----------+
          |  Ref_scheme_2 (SDNV)  |    Ref_ssp_2 (SDNV)   |
          +-----------+-----------+-----------+-----------+
          |            Block length  (SDNV)               |
          +-----------+-----------+-----------+-----------+
          /          Block body data (variable)           /
          +-----------+-----------+-----------+-----------+

             Figure 7: Block Layout Showing Two EID References

4.5.3.  Bundle Payload Block

   The fields of the bundle payload block are:

   Block Type:   The Block Type field is a 1-byte field that indicates
      the type of the block.  For the bundle payload block, this field
      contains the value 1.

   Block Processing Control Flags:   The Block Processing Control Flags
      field is an SDNV that contains the block processing control flags
      discussed in Section 4.3 above.

   Block Length:   The Block Length field is an SDNV that contains the
      aggregate length of all remaining fields of the block - which is
      to say, the length of the bundle's payload.

   Payload:   The Payload field contains the application data carried by
      this bundle.

   That is, bundle payload blocks follow the canonical format of the
   previous section with the restriction that the 'block contains an

   EID-reference field' bit of the block processing control flags is
   never set.  The block body data for payload blocks is the application
   data carried by the bundle.

4.6.  Extension Blocks

   "Extension blocks" are all blocks other than the primary and payload
   blocks.  Because extension blocks are not defined in the Bundle
   Protocol specification (the present document), not all nodes
   conforming to this specification will necessarily instantiate Bundle
   Protocol implementations that include procedures for processing (that
   is, recognizing, parsing, acting on, and/or producing) all extension
   blocks.  It is therefore possible for a node to receive a bundle that
   includes extension blocks that the node cannot process.

   Whenever a bundle is forwarded that contains one or more extension
   blocks that could not be processed, the "Block was forwarded without
   being processed" flag must be set to 1 within the block processing
   flags of each such block.  For each block flagged in this way, the
   flag may optionally be cleared (i.e., set to zero) by another node
   that subsequently receives the bundle and is able to process that
   block; the specifications defining the various extension blocks are
   expected to define the circumstances under which this flag may be
   cleared, if any.

4.7.  Dictionary Revision

   Any strings (scheme names and SSPs) in a bundle's dictionary that are
   referenced neither from the bundle's primary block nor from the block
   EID reference field of any extension block may be removed from the
   dictionary at the time the bundle is forwarded.

   Whenever removal of a string from the dictionary causes the offsets
   (within the dictionary byte array) of any other strings to change,
   all endpoint ID references that refer to those strings must be
   adjusted at the same time.  Note that these references may be in the
   primary block and/or in the block EID reference fields of extension
   blocks.

5.  Bundle Processing

   The bundle processing procedures mandated in this section and in
   Section 6 govern the operation of the Bundle Protocol Agent and the
   Application Agent administrative element of each bundle node.  They
   are neither exhaustive nor exclusive.  That is, supplementary DTN
   protocol specifications (including, but not restricted to, the Bundle
   Security Protocol [BSP]) may require that additional measures be
   taken at specified junctures in these procedures.  Such additional

   measures shall not override or supersede the mandated bundle protocol
   procedures, except that they may in some cases make these procedures
   moot by requiring, for example, that implementations conforming to
   the supplementary protocol terminate the processing of a given
   incoming or outgoing bundle due to a fault condition recognized by
   that protocol.

5.1.  Generation of Administrative Records

   All initial transmission of bundles is in response to bundle
   transmission requests presented by nodes' application agents.  When
   required to "generate" an administrative record (a bundle status
   report or a custody signal), the bundle protocol agent itself is
   responsible for causing a new bundle to be transmitted, conveying
   that record.  In concept, the bundle protocol agent discharges this
   responsibility by directing the administrative element of the node's
   application agent to construct the record and request its
   transmission as detailed in Section 6 below.  In practice, the manner
   in which administrative record generation is accomplished is an
   implementation matter, provided the constraints noted in Section 6
   are observed.

   Under some circumstances, the requesting of status reports could
   result in an unacceptable increase in the bundle traffic in the
   network.  For this reason, the generation of status reports is
   mandatory only in one case, the deletion of a bundle for which
   custody transfer is requested.  In all other cases, the decision on
   whether or not to generate a requested status report is left to the
   discretion of the bundle protocol agent.  Mechanisms that could
   assist in making such decisions, such as pre-placed agreements
   authorizing the generation of status reports under specified
   circumstances, are beyond the scope of this specification.

   Notes on administrative record terminology:

   o  A "bundle reception status report" is a bundle status report with
      the "reporting node received bundle" flag set to 1.

   o  A "custody acceptance status report" is a bundle status report
      with the "reporting node accepted custody of bundle" flag set to
      1.

   o  A "bundle forwarding status report" is a bundle status report with
      the "reporting node forwarded the bundle" flag set to 1.

   o  A "bundle delivery status report" is a bundle status report with
      the "reporting node delivered the bundle" flag set to 1.

   o  A "bundle deletion status report" is a bundle status report with
      the "reporting node deleted the bundle" flag set to 1.

   o  A "Succeeded" custody signal is a custody signal with the "custody
      transfer succeeded" flag set to 1.

   o  A "Failed" custody signal is a custody signal with the "custody
      transfer succeeded" flag set to zero.

   o  The "current custodian" of a bundle is the endpoint identified by
      the current custodian endpoint ID in the bundle's primary block.

5.2.  Bundle Transmission

   The steps in processing a bundle transmission request are:

   Step 1:   If custody transfer is requested for this bundle
      transmission and, moreover, custody acceptance by the source node
      is required, then either the bundle protocol agent must commit to
      accepting custody of the bundle -- in which case processing
      proceeds from Step 2 -- or the request cannot be honored and all
      remaining steps of this procedure must be skipped.  The bundle
      protocol agent must not commit to accepting custody of a bundle if
      the conditions under which custody of the bundle may be accepted
      are not satisfied.  The conditions under which a node may accept
      custody of a bundle whose destination is not a singleton endpoint
      are not defined in this specification.

   Step 2:   Transmission of the bundle is initiated.  An outbound
      bundle must be created per the parameters of the bundle
      transmission request, with current custodian endpoint ID set to
      the null endpoint ID "dtn:none" and with the retention constraint
      "Dispatch pending".  The source endpoint ID of the bundle must be
      either the ID of an endpoint of which the node is a member or the
      null endpoint ID "dtn:none".

   Step 3:   Processing proceeds from Step 1 of Section 5.4.

5.3.  Bundle Dispatching

   The steps in dispatching a bundle are:

   Step 1:   If the bundle's destination endpoint is an endpoint of
      which the node is a member, the bundle delivery procedure defined
      in Section 5.7 must be followed.

   Step 2:   Processing proceeds from Step 1 of Section 5.4.

5.4.  Bundle Forwarding

   The steps in forwarding a bundle are:

   Step 1:   The retention constraint "Forward pending" must be added to
      the bundle, and the bundle's "Dispatch pending" retention
      constraint must be removed.

   Step 2:   The bundle protocol agent must determine whether or not
      forwarding is contraindicated for any of the reasons listed in
      Figure 12.  In particular:

      *  The bundle protocol agent must determine which endpoint(s) to
         forward the bundle to.  The bundle protocol agent may choose
         either to forward the bundle directly to its destination
         endpoint (if possible) or to forward the bundle to some other
         endpoint(s) for further forwarding.  The manner in which this
         decision is made may depend on the scheme name in the
         destination endpoint ID but in any case is beyond the scope of
         this document.  If the agent finds it impossible to select any
         endpoint(s) to forward the bundle to, then forwarding is
         contraindicated.

      *  Provided the bundle protocol agent succeeded in selecting the
         endpoint(s) to forward the bundle to, the bundle protocol agent
         must select the convergence layer adapter(s) whose services
         will enable the node to send the bundle to the nodes of the
         minimum reception group of each selected endpoint.  The manner
         in which the appropriate convergence layer adapters are
         selected may depend on the scheme name in the destination
         endpoint ID but in any case is beyond the scope of this
         document.  If the agent finds it impossible to select
         convergence layer adapters to use in forwarding this bundle,
         then forwarding is contraindicated.

   Step 3:   If forwarding of the bundle is determined to be
      contraindicated for any of the reasons listed in Figure 12, then
      the Forwarding Contraindicated procedure defined in Section 5.4.1
      must be followed; the remaining steps of Section 5 are skipped at
      this time.

   Step 4:   If the bundle's custody transfer requested flag (in the
      bundle processing flags field) is set to 1, then the custody
      transfer procedure defined in Section 5.10.2 must be followed.

   Step 5:   For each endpoint selected for forwarding, the bundle
      protocol agent must invoke the services of the selected
      convergence layer adapter(s) in order to effect the sending of the
      bundle to the nodes constituting the minimum reception group of
      that endpoint.  Determining the time at which the bundle is to be
      sent by each convergence layer adapter is an implementation
      matter.

      To keep from possibly invalidating bundle security, the sequencing
      of the blocks in a forwarded bundle must not be changed as it
      transits a node; received blocks must be transmitted in the same
      relative order as that in which they were received.  While blocks
      may be added to bundles as they transit intermediate nodes,
      removal of blocks that do not have their 'Discard block if it
      can't be processed' flag in the block processing control flags set
      to 1 may cause security to fail.

   Step 6:   When all selected convergence layer adapters have informed
      the bundle protocol agent that they have concluded their data
      sending procedures with regard to this bundle:

      *  If the "request reporting of bundle forwarding" flag in the
         bundle's status report request field is set to 1, then a bundle
         forwarding status report should be generated, destined for the
         bundle's report-to endpoint ID.  If the bundle has the
         retention constraint "custody accepted" and all of the nodes in
         the minimum reception group of the endpoint selected for
         forwarding are known to be unable to send bundles back to this
         node, then the reason code on this bundle forwarding status
         report must be "forwarded over unidirectional link"; otherwise,
         the reason code must be "no additional information".

      *  The bundle's "Forward pending" retention constraint must be
         removed.

5.4.1.  Forwarding Contraindicated

   The steps in responding to contraindication of forwarding for some
   reason are:

   Step 1:   The bundle protocol agent must determine whether or not to
      declare failure in forwarding the bundle for this reason.  Note:
      this decision is likely to be influenced by the reason for which
      forwarding is contraindicated.

   Step 2:   If forwarding failure is declared, then the Forwarding
      Failed procedure defined in Section 5.4.2 must be followed.
      Otherwise, (a) if the bundle's custody transfer requested flag (in
      the bundle processing flags field) is set to 1, then the custody
      transfer procedure defined in Section 5.10 must be followed; (b)
      when -- at some future time - the forwarding of this bundle ceases
      to be contraindicated, processing proceeds from Step 5 of
      Section 5.4.

5.4.2.  Forwarding Failed

   The steps in responding to a declaration of forwarding failure for
   some reason are:

   Step 1:   If the bundle's custody transfer requested flag (in the
      bundle processing flags field) is set to 1, custody transfer
      failure must be handled.  Procedures for handling failure of
      custody transfer for a bundle whose destination is not a singleton
      endpoint are not defined in this specification.  For a bundle
      whose destination is a singleton endpoint, the bundle protocol
      agent must handle the custody transfer failure by generating a
      "Failed" custody signal for the bundle, destined for the bundle's
      current custodian; the custody signal must contain a reason code
      corresponding to the reason for which forwarding was determined to
      be contraindicated.  (Note that discarding the bundle will not
      delete it from the network, since the current custodian still has
      a copy.)

   Step 2:   If the bundle's destination endpoint is an endpoint of
      which the node is a member, then the bundle's "Forward pending"
      retention constraint must be removed.  Otherwise, the bundle must
      be deleted: the bundle deletion procedure defined in Section 5.13
      must be followed, citing the reason for which forwarding was
      determined to be contraindicated.

5.5.  Bundle Expiration

   A bundle expires when the current time is greater than the bundle's
   creation time plus its lifetime as specified in the primary bundle
   block.  Bundle expiration may occur at any point in the processing of
   a bundle.  When a bundle expires, the bundle protocol agent must
   delete the bundle for the reason "lifetime expired": the bundle
   deletion procedure defined in Section 5.13 must be followed.

5.6.  Bundle Reception

   The steps in processing a bundle received from another node are:

   Step 1:   The retention constraint "Dispatch pending" must be added
      to the bundle.

   Step 2:   If the "request reporting of bundle reception" flag in the
      bundle's status report request field is set to 1, then a bundle
      reception status report with reason code "No additional
      information" should be generated, destined for the bundle's
      report-to endpoint ID.

   Step 3:   For each block in the bundle that is an extension block
      that the bundle protocol agent cannot process:

      *  If the block processing flags in that block indicate that a
         status report is requested in this event, then a bundle
         reception status report with reason code "Block unintelligible"
         should be generated, destined for the bundle's report-to
         endpoint ID.

      *  If the block processing flags in that block indicate that the
         bundle must be deleted in this event, then the bundle protocol
         agent must delete the bundle for the reason "Block
         unintelligible"; the bundle deletion procedure defined in
         Section 5.13 must be followed and all remaining steps of the
         bundle reception procedure must be skipped.

      *  If the block processing flags in that block do NOT indicate
         that the bundle must be deleted in this event but do indicate
         that the block must be discarded, then the bundle protocol
         agent must remove this block from the bundle.

      *  If the block processing flags in that block indicate NEITHER
         that the bundle must be deleted NOR that the block must be
         discarded, then the bundle protocol agent must set to 1 the
         "Block was forwarded without being processed" flag in the block
         processing flags of the block.

   Step 4:   If the bundle's custody transfer requested flag (in the
      bundle processing flags field) is set to 1 and the bundle has the
      same source endpoint ID, creation timestamp, and (if the bundle is
      a fragment) fragment offset and payload length as another bundle
      that (a) has not been discarded and (b) currently has the
      retention constraint "Custody accepted", custody transfer
      redundancy must be handled.  Otherwise, processing proceeds from
      Step 5.  Procedures for handling redundancy in custody transfer

      for a bundle whose destination is not a singleton endpoint are not
      defined in this specification.  For a bundle whose destination is
      a singleton endpoint, the bundle protocol agent must handle
      custody transfer redundancy by generating a "Failed" custody
      signal for this bundle with reason code "Redundant reception",
      destined for this bundle's current custodian, and removing this
      bundle's "Dispatch pending" retention constraint.

   Step 5:   Processing proceeds from Step 1 of Section 5.3.

5.7.  Local Bundle Delivery

   The steps in processing a bundle that is destined for an endpoint of
   which this node is a member are:

   Step 1:   If the received bundle is a fragment, the application data
      unit reassembly procedure described in Section 5.9 must be
      followed.  If this procedure results in reassembly of the entire
      original application data unit, processing of this bundle (whose
      fragmentary payload has been replaced by the reassembled
      application data unit) proceeds from Step 2; otherwise, the
      retention constraint "Reassembly pending" must be added to the
      bundle and all remaining steps of this procedure are skipped.

   Step 2:   Delivery depends on the state of the registration whose
      endpoint ID matches that of the destination of the bundle:

      *  If the registration is in the Active state, then the bundle
         must be delivered subject to this registration (see Section 3.1
         above) as soon as all previously received bundles that are
         deliverable subject to this registration have been delivered.

      *  If the registration is in the Passive state, then the
         registration's delivery failure action must be taken (see
         Section 3.1 above).

   Step 3:   As soon as the bundle has been delivered:

      *  If the "request reporting of bundle delivery" flag in the
         bundle's status report request field is set to 1, then a bundle
         delivery status report should be generated, destined for the
         bundle's report-to endpoint ID.  Note that this status report
         only states that the payload has been delivered to the
         application agent, not that the application agent has processed
         that payload.

      *  If the bundle's custody transfer requested flag (in the bundle
         processing flags field) is set to 1, custodial delivery must be
         reported.  Procedures for reporting custodial delivery for a
         bundle whose destination is not a singleton endpoint are not
         defined in this specification.  For a bundle whose destination
         is a singleton endpoint, the bundle protocol agent must report
         custodial delivery by generating a "Succeeded" custody signal
         for the bundle, destined for the bundle's current custodian.

5.8.  Bundle Fragmentation

   It may at times be necessary for bundle protocol agents to reduce the
   sizes of bundles in order to forward them.  This might be the case,
   for example, if the endpoint to which a bundle is to be forwarded is
   accessible only via intermittent contacts and no upcoming contact is
   long enough to enable the forwarding of the entire bundle.

   The size of a bundle can be reduced by "fragmenting" the bundle.  To
   fragment a bundle whose payload is of size M is to replace it with
   two "fragments" -- new bundles with the same source endpoint ID and
   creation timestamp as the original bundle -- whose payloads are the
   first N and the last (M - N) bytes of the original bundle's payload,
   where 0 < N < M.  Note that fragments may themselves be fragmented,
   so fragmentation may in effect replace the original bundle with more
   than two fragments.  (However, there is only one 'level' of
   fragmentation, as in IP fragmentation.)

   Any bundle whose primary block's bundle processing flags do NOT
   indicate that it must not be fragmented may be fragmented at any
   time, for any purpose, at the discretion of the bundle protocol
   agent.

   Fragmentation shall be constrained as follows:

   o  The concatenation of the payloads of all fragments produced by
      fragmentation must always be identical to the payload of the
      bundle that was fragmented.  Note that the payloads of fragments
      resulting from different fragmentation episodes, in different
      parts of the network, may be overlapping subsets of the original
      bundle's payload.

   o  The bundle processing flags in the primary block of each fragment
      must be modified to indicate that the bundle is a fragment, and
      both fragment offset and total application data unit length must
      be provided at the end of each fragment's primary bundle block.

   o  The primary blocks of the fragments will differ from that of the
      fragmented bundle as noted above.

   o  The payload blocks of fragments will differ from that of the
      fragmented bundle as noted above.

   o  All blocks that precede the payload block at the time of
      fragmentation must be replicated in the fragment with the lowest
      offset.

   o  All blocks that follow the payload block at the time of
      fragmentation must be replicated in the fragment with the highest
      offset.

   o  If the 'Block must be replicated in every fragment' bit is set to
      1, then the block must be replicated in every fragment.

   o  If the 'Block must be replicated in every fragment' bit is set to
      zero, the block should be replicated in only one fragment.

   o  The relative order of all blocks that are present in a fragment
      must be the same as in the bundle prior to fragmentation.

5.9.  Application Data Unit Reassembly

   If the concatenation -- as informed by fragment offsets and payload
   lengths -- of the payloads of all previously received fragments with
   the same source endpoint ID and creation timestamp as this fragment,
   together with the payload of this fragment, forms a byte array whose
   length is equal to the total application data unit length in the
   fragment's primary block, then:

   o  This byte array -- the reassembled application data unit -- must
      replace the payload of this fragment.

   o  The "Reassembly pending" retention constraint must be removed from
      every other fragment whose payload is a subset of the reassembled
      application data unit.

   Note: reassembly of application data units from fragments occurs at
   destination endpoints as necessary; an application data unit may also
   be reassembled at some other endpoint on the route to the
   destination.

5.10.  Custody Transfer

   The conditions under which a node may accept custody of a bundle
   whose destination is not a singleton endpoint are not defined in this
   specification.

   The decision as to whether or not to accept custody of a bundle whose
   destination is a singleton endpoint is an implementation matter that
   may involve both resource and policy considerations; however, if the
   bundle protocol agent has committed to accepting custody of the
   bundle (as described in Step 1 of Section 5.2), then custody must be
   accepted.

   If the bundle protocol agent elects to accept custody of the bundle,
   then it must follow the custody acceptance procedure defined in
   Section 5.10.1.

5.10.1.  Custody Acceptance

   Procedures for acceptance of custody of a bundle whose destination is
   not a singleton endpoint are not defined in this specification.

   Procedures for acceptance of custody of a bundle whose destination is
   a singleton endpoint are defined as follows.

   The retention constraint "Custody accepted" must be added to the
   bundle.

   If the "request reporting of custody acceptance" flag in the bundle's
   status report request field is set to 1, a custody acceptance status
   report should be generated, destined for the report-to endpoint ID of
   the bundle.  However, if a bundle reception status report was
   generated for this bundle (Step 1 of Section 5.6), then this report
   should be generated by simply turning on the "Reporting node accepted
   custody of bundle" flag in that earlier report's status flags byte.

   The bundle protocol agent must generate a "Succeeded" custody signal
   for the bundle, destined for the bundle's current custodian.

   The bundle protocol agent must assert the new current custodian for
   the bundle.  It does so by changing the current custodian endpoint ID
   in the bundle's primary block to the endpoint ID of one of the
   singleton endpoints in which the node is registered.  This may entail
   appending that endpoint ID's null-terminated scheme name and SSP to
   the dictionary byte array in the bundle's primary block, and in some
   case it may also enable the (optional) removal of the current
   custodian endpoint ID's scheme name and/or SSP from the dictionary.

   The bundle protocol agent may set a custody transfer countdown timer
   for this bundle; upon expiration of this timer prior to expiration of
   the bundle itself and prior to custody transfer success for this
   bundle, the custody transfer failure procedure detailed in
   Section 5.12 must be followed.  The manner in which the countdown
   interval for such a timer is determined is an implementation matter.

   The bundle should be retained in persistent storage if possible.

5.10.2.  Custody Release

   Procedures for release of custody of a bundle whose destination is
   not a singleton endpoint are not defined in this specification.

   When custody of a bundle is released, where the destination of the
   bundle is a singleton endpoint, the "Custody accepted" retention
   constraint must be removed from the bundle and any custody transfer
   timer that has been established for this bundle must be destroyed.

5.11.  Custody Transfer Success

   Procedures for determining custody transfer success for a bundle
   whose destination is not a singleton endpoint are not defined in this
   specification.

   Upon receipt of a "Succeeded" custody signal at a node that is a
   custodial node of the bundle identified in the custody signal, where
   the destination of the bundle is a singleton endpoint, custody of the
   bundle must be released as described in Section 5.10.2.

5.12.  Custody Transfer Failure

   Procedures for determining custody transfer failure for a bundle
   whose destination is not a singleton endpoint are not defined in this
   specification.  Custody transfer for a bundle whose destination is a
   singleton endpoint is determined to have failed at a custodial node
   for that bundle when either (a) that node's custody transfer timer
   for that bundle (if any) expires or (b) a "Failed" custody signal for
   that bundle is received at that node.

   Upon determination of custody transfer failure, the action taken by
   the bundle protocol agent is implementation-specific and may depend
   on the nature of the failure.  For example, if custody transfer
   failure was inferred from expiration of a custody transfer timer or
   was asserted by a "Failed" custody signal with the "Depleted storage"
   reason code, the bundle protocol agent might choose to re-forward the
   bundle, possibly on a different route (Section 5.4).  Receipt of a
   "Failed" custody signal with the "Redundant reception" reason code,

   on the other hand, might cause the bundle protocol agent to release
   custody of the bundle and to revise its algorithm for computing
   countdown intervals for custody transfer timers.

5.13.  Bundle Deletion

   The steps in deleting a bundle are:

   Step 1:   If the retention constraint "Custody accepted" currently
      prevents this bundle from being discarded, and the destination of
      the bundle is a singleton endpoint, then:

      *  Custody of the node is released as described in Section 5.10.2.

      *  A bundle deletion status report citing the reason for deletion
         must be generated, destined for the bundle's report-to endpoint
         ID.

      Otherwise, if the "request reporting of bundle deletion" flag in
      the bundle's status report request field is set to 1, then a
      bundle deletion status report citing the reason for deletion
      should be generated, destined for the bundle's report-to endpoint
      ID.

   Step 2:   All of the bundle's retention constraints must be removed.

5.14.  Discarding a Bundle

   As soon as a bundle has no remaining retention constraints it may be
   discarded.

5.15.  Canceling a Transmission

   When requested to cancel a specified transmission, where the bundle
   created upon initiation of the indicated transmission has not yet
   been discarded, the bundle protocol agent must delete that bundle for
   the reason "transmission cancelled".  For this purpose, the procedure
   defined in Section 5.13 must be followed.

5.16.  Polling

   When requested to poll a specified registration that is in the
   Passive state, the bundle protocol agent must immediately deliver the
   least recently received bundle that is deliverable subject to the
   indicated registration, if any.

6.  Administrative Record Processing

6.1.  Administrative Records

   Administrative records are standard application data units that are
   used in providing some of the features of the Bundle Protocol.  Two
   types of administrative records have been defined to date: bundle
   status reports and custody signals.

   Every administrative record consists of a four-bit record type code
   followed by four bits of administrative record flags, followed by
   record content in type-specific format.  Record type codes are
   defined as follows:

           +---------+--------------------------------------------+
           |  Value  |                  Meaning                   |
           +=========+============================================+
           |  0001   |  Bundle status report.                     |
           +---------+--------------------------------------------+
           |  0010   |  Custody signal.                           |
           +---------+--------------------------------------------+
           | (other) |  Reserved for future use.                  |
           +---------+--------------------------------------------+

                Figure 8: Administrative Record Type Codes

           +---------+--------------------------------------------+
           |  Value  |                  Meaning                   |
           +=========+============================================+
           |  0001   |  Record is for a fragment; fragment        |
           |         |  offset and length fields are present.     |
           +---------+--------------------------------------------+
           | (other) |  Reserved for future use.                  |
           +---------+--------------------------------------------+

                   Figure 9: Administrative Record Flags

   All time values in administrative records are UTC times expressed in
   "DTN time" representation.  A DTN time consists of an SDNV indicating
   the number of seconds since the start of the year 2000, followed by
   an SDNV indicating the number of nanoseconds since the start of the
   indicated second.

   The contents of the various types of administrative records are
   described below.

6.1.1.  Bundle Status Reports

   The transmission of 'bundle status reports' under specified
   conditions is an option that can be invoked when transmission of a
   bundle is requested.  These reports are intended to provide
   information about how bundles are progressing through the system,
   including notices of receipt, custody transfer, forwarding, final
   delivery, and deletion.  They are transmitted to the Report-to
   endpoints of bundles.

   +----------------+----------------+----------------+----------------+
   |  Status Flags  |  Reason code   |      Fragment offset (*) (if
   +----------------+----------------+----------------+----------------+
       present)     |      Fragment length (*) (if present)            |
   +----------------+----------------+----------------+----------------+
   |       Time of receipt of bundle X (a DTN time, if present)        |
   +----------------+----------------+----------------+----------------+
   |  Time of custody acceptance of bundle X (a DTN time, if present)  |
   +----------------+----------------+----------------+----------------+
   |     Time of forwarding of bundle X (a DTN time, if present)       |
   +----------------+----------------+----------------+----------------+
   |      Time of delivery of bundle X (a DTN time, if present)        |
   +----------------+----------------+----------------+----------------+
   |      Time of deletion of bundle X (a DTN time, if present)        |
   +----------------+----------------+----------------+----------------+
   |          Copy of bundle X's Creation Timestamp time (*)           |
   +----------------+----------------+----------------+----------------+
   |     Copy of bundle X's Creation Timestamp sequence number (*)     |
   +----------------+----------------+----------------+----------------+
   |      Length of X's source endpoint ID (*)        |   Source
   +----------------+---------------------------------+                +
                        endpoint ID of bundle X (variable)             |
   +----------------+----------------+----------------+----------------+

                  Figure 10: Bundle Status Report Format

   (*) Notes:

   The Fragment Offset field, if present, is an SDNV and is therefore
   variable length.  A three-octet SDNV is shown here for convenience in
   representation.

   The Fragment Length field, if present, is an SDNV and is therefore
   variable length.  A three-octet SDNV is shown here for convenience in
   representation.

   The Creation Timestamp fields replicate the Creation Timestamp fields
   in the primary block of the subject bundle.  As such they are SDNVs
   (see Section 4.5.1 above) and are therefore variable length.  Four-
   octet SDNVs are shown here for convenience in representation.

   The source endpoint ID length field is an SDNV and is therefore
   variable length.  A three-octet SDNV is shown here for convenience in
   representation.

   The fields in a bundle status report are:

   Status Flags:   A 1-byte field containing the following flags:

           +----------+--------------------------------------------+
           |  Value   |                  Meaning                   |
           +==========+============================================+
           | 00000001 |  Reporting node received bundle.           |
           +----------+--------------------------------------------+
           | 00000010 |  Reporting node accepted custody of bundle.|
           +----------+--------------------------------------------+
           | 00000100 |  Reporting node forwarded the bundle.      |
           +----------+--------------------------------------------+
           | 00001000 |  Reporting node delivered the bundle.      |
           +----------+--------------------------------------------+
           | 00010000 |  Reporting node deleted the bundle.        |
           +----------+--------------------------------------------+
           | 00100000 |  Unused.                                   |
           +----------+--------------------------------------------+
           | 01000000 |  Unused.                                   |
           +----------+--------------------------------------------+
           | 10000000 |  Unused.                                   |
           +----------+--------------------------------------------+

              Figure 11: Status Flags for Bundle Status Reports

   Reason Code:   A 1-byte field explaining the value of the flags in
      the status flags byte.  The list of status report reason codes
      provided here is neither exhaustive nor exclusive; supplementary
      DTN protocol specifications (including, but not restricted to, the
      Bundle Security Protocol [BSP]) may define additional reason
      codes.  Status report reason codes are defined as follows:

           +---------+--------------------------------------------+
           |  Value  |                  Meaning                   |
           +=========+============================================+
           |  0x00   |  No additional information.                |
           +---------+--------------------------------------------+
           |  0x01   |  Lifetime expired.                         |
           +---------+--------------------------------------------+
           |  0x02   |  Forwarded over unidirectional link.       |
           +---------+--------------------------------------------+
           |  0x03   |  Transmission canceled.                    |
           +---------+--------------------------------------------+
           |  0x04   |  Depleted storage.                         |
           +---------+--------------------------------------------+
           |  0x05   |  Destination endpoint ID unintelligible.   |
           +---------+--------------------------------------------+
           |  0x06   |  No known route to destination from here.  |
           +---------+--------------------------------------------+
           |  0x07   |  No timely contact with next node on route.|
           +---------+--------------------------------------------+
           |  0x08   |  Block unintelligible.                     |
           +---------+--------------------------------------------+
           | (other) |  Reserved for future use.                  |
           +---------+--------------------------------------------+

                    Figure 12: Status Report Reason Codes

   Fragment Offset:   If the bundle fragment bit is set in the status
      flags, then the offset (within the original application data unit)
      of the payload of the bundle that caused the status report to be
      generated is included here.

   Fragment length:   If the bundle fragment bit is set in the status
      flags, then the length of the payload of the subject bundle is
      included here.

   Time of Receipt (if present):   If the bundle-received bit is set in
      the status flags, then a DTN time indicating the time at which the
      bundle was received at the reporting node is included here.

   Time of Custody Acceptance (if present):   If the custody-accepted
      bit is set in the status flags, then a DTN time indicating the
      time at which custody was accepted at the reporting node is
      included here.

   Time of Forward (if present):   If the bundle-forwarded bit is set in
      the status flags, then a DTN time indicating the time at which the
      bundle was first forwarded at the reporting node is included here.

   Time of Delivery (if present):   If the bundle-delivered bit is set
      in the status flags, then a DTN time indicating the time at which
      the bundle was delivered at the reporting node is included here.

   Time of Deletion (if present):   If the bundle-deleted bit is set in
      the status flags, then a DTN time indicating the time at which the
      bundle was deleted at the reporting node is included here.

   Creation Timestamp of Subject Bundle:  A copy of the creation
      timestamp of the bundle that caused the status report to be
      generated.

   Length of Source Endpoint ID:   The length in bytes of the source
      endpoint ID of the bundle that caused the status report to be
      generated.

   Source Endpoint ID text:   The text of the source endpoint ID of the
      bundle that caused the status report to be generated.

6.1.2.  Custody Signals

   Custody signals are administrative records that effect custody
   transfer operations.  They are transmitted to the endpoints that are
   the current custodians of bundles.

   Custody signals have the following format.

   Custody signal regarding bundle 'X':

   +----------------+----------------+----------------+----------------+
   |     Status     |      Fragment offset (*) (if present)            |
   +----------------+----------------+----------------+----------------+
   |                   Fragment length (*) (if present)                |
   +----------------+----------------+----------------+----------------+
   |                   Time of signal (a DTN time)                     |
   +----------------+----------------+----------------+----------------+
   |          Copy of bundle X's Creation Timestamp time (*)           |
   +----------------+----------------+----------------+----------------+
   |     Copy of bundle X's Creation Timestamp sequence number (*)     |
   +----------------+----------------+----------------+----------------+
   |      Length of X's source endpoint ID (*)        |   Source
   +----------------+---------------------------------+                +
                        endpoint ID of bundle X (variable)             |
   +----------------+----------------+----------------+----------------+

                     Figure 13: Custody Signal Format

   (*) Notes:

   The Fragment Offset field, if present, is an SDNV and is therefore
   variable length.  A three-octet SDNV is shown here for convenience in
   representation.

   The Fragment Length field, if present, is an SDNV and is therefore
   variable length.  A four-octet SDNV is shown here for convenience in
   representation.

   The Creation Timestamp fields replicate the Creation Timestamp fields
   in the primary block of the subject bundle.  As such they are SDNVs
   (see Section 4.5.1 above) and are therefore variable length.  Four-
   octet SDNVs are shown here for convenience in representation.

   The source endpoint ID length field is an SDNV and is therefore
   variable length.  A three-octet SDNV is shown here for convenience in
   representation.

   The fields in a custody signal are:

   Status:   A 1-byte field containing a 1-bit "custody transfer
      succeeded" flag followed by a 7-bit reason code explaining the
      value of that flag.  Custody signal reason codes are defined as
      follows:

           +---------+--------------------------------------------+
           |  Value  |                  Meaning                   |
           +=========+============================================+
           |  0x00   |  No additional information.                |
           +---------+--------------------------------------------+
           |  0x01   |  Reserved for future use.                  |
           +---------+--------------------------------------------+
           |  0x02   |  Reserved for future use.                  |
           +---------+--------------------------------------------+
           |  0x03   |  Redundant reception (reception by a node  |
           |         |  that is a custodial node for this bundle).|
           +---------+--------------------------------------------+
           |  0x04   |  Depleted storage.                         |
           +---------+--------------------------------------------+
           |  0x05   |  Destination endpoint ID unintelligible.   |
           +---------+--------------------------------------------+
           |  0x06   |  No known route to destination from here.  |
           +---------+--------------------------------------------+
           |  0x07   |  No timely contact with next node on route.|
           +---------+--------------------------------------------+
           |  0x08   |  Block unintelligible.                     |
           +---------+--------------------------------------------+
           | (other) |  Reserved for future use.                  |
           +---------+--------------------------------------------+

                    Figure 14: Custody Signal Reason Codes

   Fragment offset:   If the bundle fragment bit is set in the status
      flags, then the offset (within the original application data unit)
      of the payload of the bundle that caused the status report to be
      generated is included here.

   Fragment length:   If the bundle fragment bit is set in the status
      flags, then the length of the payload of the subject bundle is
      included here.

   Time of Signal:   A DTN time indicating the time at which the signal
      was generated.

   Creation Timestamp of Subject Bundle:   A copy of the creation
      timestamp of the bundle to which the signal applies.

   Length of Source Endpoint ID:   The length in bytes of the source
      endpoint ID of the bundle to which the signal applied.

   Source Endpoint ID text:   The text of the source endpoint ID of the
      bundle to which the signal applies.

6.2.  Generation of Administrative Records

   Whenever the application agent's administrative element is directed
   by the bundle protocol agent to generate an administrative record
   with reference to some bundle, the following procedure must be
   followed:

   Step 1:   The administrative record must be constructed.  If the
      referenced bundle is a fragment, the administrative record must
      have the Fragment flag set and must contain the fragment offset
      and fragment length fields.  The value of the fragment offset
      field must be the value of the referenced bundle's fragment
      offset, and the value of the fragment length field must be the
      length of the referenced bundle's payload.

   Step 2:   A request for transmission of a bundle whose payload is
      this administrative record must be presented to the bundle
      protocol agent.

6.3.  Reception of Custody Signals

   For each received custody signal that has the "custody transfer
   succeeded" flag set to 1, the administrative element of the
   application agent must direct the bundle protocol agent to follow the
   custody transfer success procedure in Section 5.11.

   For each received custody signal that has the "custody transfer
   succeeded" flag set to 0, the administrative element of the
   application agent must direct the bundle protocol agent to follow the
   custody transfer failure procedure in Section 5.12.

7.  Services Required of the Convergence Layer

7.1.  The Convergence Layer

   The successful operation of the end-to-end bundle protocol depends on
   the operation of underlying protocols at what is termed the
   "convergence layer"; these protocols accomplish communication between
   nodes.  A wide variety of protocols may serve this purpose, so long
   as each convergence layer protocol adapter provides a defined minimal
   set of services to the bundle protocol agent.  This convergence layer
   service specification enumerates those services.

7.2.  Summary of Convergence Layer Services

   Each convergence layer protocol adapter is expected to provide the
   following services to the bundle protocol agent:

   o  sending a bundle to all bundle nodes in the minimum reception
      group of the endpoint identified by a specified endpoint ID that
      are reachable via the convergence layer protocol; and

   o  delivering to the bundle protocol agent a bundle that was sent by
      a remote bundle node via the convergence layer protocol.

   The convergence layer service interface specified here is neither
   exhaustive nor exclusive.  That is, supplementary DTN protocol
   specifications (including, but not restricted to, the Bundle Security
   Protocol [BSP]) may expect convergence layer adapters that serve BP
   implementations conforming to those protocols to provide additional
   services.

8.  Security Considerations

   The bundle protocol has taken security into concern from the outset
   of its design.  It was always assumed that security services would be
   needed in the use of the bundle protocol.  As a result, the bundle
   protocol security architecture and the available security services
   are specified in an accompanying document, the Bundle Security
   Protocol specification [BSP]; an informative overview of this
   architecture is provided in [SECO].

   The bundle protocol has been designed with the notion that it will be
   run over networks with scarce resources.  For example, the networks
   might have limited bandwidth, limited connectivity, constrained
   storage in relay nodes, etc.  Therefore, the bundle protocol must
   ensure that only those entities authorized to send bundles over such
   constrained environments are actually allowed to do so.  All
   unauthorized entities should be prevented from consuming valuable
   resources.

   Likewise, because of the potentially long latencies and delays
   involved in the networks that make use of the bundle protocol, data
   sources should be concerned with the integrity of the data received
   at the intended destination(s) and may also be concerned with
   ensuring confidentiality of the data as it traverses the network.
   Without integrity, the bundle payload data might be corrupted while
   in transit without the destination able to detect it.  Similarly, the
   data source can be concerned with ensuring that the data can only be
   used by those authorized, hence the need for confidentiality.

   Internal to the bundle-aware overlay network, the bundle nodes should
   be concerned with the authenticity of other bundle nodes as well as
   the preservation of bundle payload data integrity as it is forwarded
   between bundle nodes.

   As a result, bundle security is concerned with the authenticity,
   integrity, and confidentiality of bundles conveyed among bundle
   nodes.  This is accomplished via the use of three independent
   security-specific bundle blocks, which may be used together to
   provide multiple bundle security services or independently of one
   another, depending on perceived security threats, mandated security
   requirements, and security policies that must be enforced.

   The Bundle Authentication Block (BAB) ensures the authenticity and
   integrity of bundles on a hop-by-hop basis between bundle nodes.  The
   BAB allows each bundle node to verify a bundle's authenticity before
   processing or forwarding the bundle.  In this way, entities that are
   not authorized to send bundles will have unauthorized transmissions
   blocked by security-aware bundle nodes.

   Additionally, to provide "security-source" to "security-destination"
   bundle authenticity and integrity, the Payload Security Block (PSB)
   is used.  A "security-source" may not actually be the origination
   point of the bundle but instead may be the first point along the path
   that is security-aware and is able to apply security services.  For
   example, an enclave of networked systems may generate bundles but
   only their gateway may be required and/or able to apply security
   services.  The PSB allows any security-enabled entity along the
   delivery path, in addition to the "security-destination" (the
   recipient counterpart to the "security-source"), to ensure the
   bundle's authenticity.

   Finally, to provide payload confidentiality, the use of the
   Confidentiality Block (CB) is available.  The bundle payload may be
   encrypted to provide "security-source" to "security-destination"
   payload confidentiality/privacy.  The CB indicates the cryptographic
   algorithm and key IDs that were used to encrypt the payload.

   Note that removal of strings from the dictionary at a given point in
   a bundle's end-to-end path, and attendant adjustment of endpoint ID
   references in the blocks of that bundle, may make it necessary to re-
   compute values in one or more of the bundle's security blocks.

   Bundle security must not be invalidated by forwarding nodes even
   though they themselves might not use the Bundle Security Protocol.
   In particular, the sequencing of the blocks in a forwarded bundle
   must not be changed as it transits a node; received blocks must be
   transmitted in the same relative order as that in which they were

   received.  While blocks may be added to bundles as they transit
   intermediate nodes, removal of blocks that do not have their 'Discard
   block if it can't be processed' flag in the block processing control
   flags set to 1 may cause security to fail.

   Inclusion of the Bundle Security Protocol in any Bundle Protocol
   implementation is RECOMMENDED.  Use of the Bundle Security Protocol
   in Bundle Protocol operations is OPTIONAL.

9.  IANA Considerations

   The "dtn:" URI scheme has been provisionally registered by IANA.  See
   http://www.iana.org/assignments/uri-schemes.html for the latest
   details.

10.  References

10.1.  Normative References

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

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", RFC 3986,
              STD 66, January 2005.

   [URIREG]   Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", RFC 4395,
              BCP 115, February 2006.

10.2.  Informative References

   [ARCH]     V. Cerf et. al., "Delay-Tolerant Network Architecture",
              RFC 4838, April 2007.

   [ASN1]     "Abstract Syntax Notation One (ASN.1), "ASN.1 Encoding
              Rules: Specification of Basic Encoding Rules (BER),
              Canonical Encoding Rules (CER) and Distinguished Encoding
              Rules (DER)," ITU-T Rec. X.690 (2002) | ISO/IEC 8825-
              1:2002", 2003.

   [BSP]      Symington, S., "Bundle Security Protocol Specification",
              Work Progress, October 2007.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [SECO]     Farrell, S., Symington, S., Weiss, H., and P. Lovell,
              "Delay-Tolerant Networking Security Overview",
              Work Progress, July 2007.

   [SIGC]     Fall, K., "A Delay-Tolerant Network Architecture for
              Challenged Internets", SIGCOMM 2003 .

   [TUT]      Warthman, F., "Delay-Tolerant Networks (DTNs): A
              Tutorial", <http://www.dtnrg.org>.

   [UTC]      Arias, E. and B. Guinot, ""Coordinated universal time UTC:
              historical background and perspectives" in Journees
              systemes de reference spatio-temporels", 2004.

Appendix A.  Contributors

   This was an effort of the Delay Tolerant Networking Research Group.
   The following DTNRG participants contributed significant technical
   material and/or inputs: Dr. Vinton Cerf of Google, Scott Burleigh,
   Adrian Hooke, and Leigh Torgerson of the Jet Propulsion Laboratory,
   Michael Demmer of the University of California at Berkeley, Robert
   Durst, Keith Scott, and Susan Symington of The MITRE Corporation,
   Kevin Fall of Intel Research, Stephen Farrell of Trinity College
   Dublin, Peter Lovell of SPARTA, Inc., Manikantan Ramadas of Ohio
   University (most of Section 4.1), and Howard Weiss of SPARTA, Inc.
   (text of Section 8).

Appendix B.  Comments

   Please refer comments to dtn-interest@mailman.dtnrg.org.  The Delay
   Tolerant Networking Research Group (DTNRG) Web site is located at
   http://www.dtnrg.org.

Authors' Addresses

   Keith L. Scott
   The MITRE Corporation
   7515 Colshire Drive
   McLean, VA  21102
   US

   Phone: +1 703 983 6547
   Fax:   +1 703 983 7142
   EMail: kscott@mitre.org

   Scott Burleigh
   NASA Jet Propulsion Laboratory
   4800 Oak Grove Dr.
   Pasadena, CA  91109-8099
   US

   Phone: +1 818 393 3353
   Fax:   +1 818 354 1075
   EMail: Scott.Burleigh@jpl.nasa.gov

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