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Versions: 00 draft-ietf-dtn-bpbis

Delay-Tolerant Networking Working Group                     S. Burleigh
Internet Draft                          JPL, Calif. Inst. Of Technology
Intended status: Standards Track                                K. Fall
Expires: December 2015                 Carnegie Mellon University / SEI
                                                             E. Birrane
                                          APL, Johns Hopkins University
                                                          June 21, 2015

                              Bundle Protocol
                           draft-dtnwg-bp-00.txt


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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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

Abstract

   This Internet Draft presents a specification for Bundle Protocol,
   adapted from the experimental Bundle Protocol specification
   developed by the Delay-Tolerant Networking Research group of the
   Internet Research Task Force and documented in RFC 5050.

Table of Contents


   1. Introduction...................................................4
   2. Conventions used in this document..............................6
   3. Service Description............................................6
      3.1. Definitions...............................................6
      3.2. Implementation Architectures.............................12
         3.2.1. Bundle protocol application server..................12
         3.2.2. Peer application nodes..............................13
         3.2.3. Sensor network nodes................................13
         3.2.4. Dedicated bundle router.............................13
      3.3. Services Offered by Bundle Protocol Agents...............13
   4. Bundle Format.................................................14
      4.1. Self-Delimiting Numeric Values (SDNVs)...................14
      4.2. Bundle Processing Control Flags..........................16
      4.3. Block Processing Control Flags...........................18
      4.4. Identifiers..............................................19
         4.4.1. Endpoint ID.........................................19
         4.4.2. Node ID.............................................20
      4.5. Formats of Bundle Blocks.................................21
         4.5.1. Primary Bundle Block................................23
         4.5.2. Canonical Bundle Block Format.......................25
         4.5.3. Bundle Payload Block................................26
      4.6. Extension Blocks.........................................27
         4.6.1. Current Custodian...................................27
         4.6.2. Flow Label..........................................28
         4.6.3. Previous Node ID....................................28


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         4.6.4. Bundle Age..........................................28
         4.6.5. Hop Count...........................................28
   5. Bundle Processing.............................................29
      5.1. Generation of Administrative Records.....................29
      5.2. Bundle Transmission......................................30
      5.3. Bundle Dispatching.......................................30
      5.4. Bundle Forwarding........................................31
         5.4.1. Forwarding Contraindicated..........................33
         5.4.2. Forwarding Failed...................................33
      5.5. Bundle Expiration........................................34
      5.6. Bundle Reception.........................................34
      5.7. Local Bundle Delivery....................................35
      5.8. Bundle Fragmentation.....................................36
      5.9. Application Data Unit Reassembly.........................37
      5.10. Custody Transfer........................................38
         5.10.1. Custody Acceptance.................................38
         5.10.2. Custody Release....................................39
      5.11. Custody Transfer Success................................39
      5.12. Custody Transfer Failure................................39
      5.13. Bundle Deletion.........................................39
      5.14. Discarding a Bundle.....................................40
      5.15. Canceling a Transmission................................40
   6. Administrative Record Processing..............................40
      6.1. Administrative Records...................................40
         6.1.1. Bundle Status Reports...............................41
         6.1.2. Custody Signals.....................................45
      6.2. Generation of Administrative Records.....................47
      6.3. Reception of Custody Signals.............................48
   7. Services Required of the Convergence Layer....................48
      7.1. The Convergence Layer....................................48
      7.2. Summary of Convergence Layer Services....................48
   8. Security Considerations.......................................49
   9. IANA Considerations...........................................50
   10. References...................................................50
      10.1. Normative References....................................50
      10.2. Informative References..................................51
   11. Acknowledgments..............................................51
   12. Significant Changes From RFC 5050............................52
   13. Open Issues..................................................52
      13.1. Definitions section structure...........................52
      13.2. Payload nomenclature....................................53
      13.3. Application Agent.......................................53
      13.4. Bundle Endpoint definition..............................53
      13.5. Alignment with ICN......................................53
      13.6. Implementation Architectures............................53
      13.7. Security protocol name..................................54
      13.8. Bundle format...........................................54


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      13.9. SDNVs...................................................54
      13.10. Bundle Processing Control Flags........................54
      13.11. Extended class of service features.....................54
      13.12. Primary block CRC type.................................54
      13.13. Inventory..............................................54
      13.14. Block numbers..........................................55
      13.15. Clearing flag..........................................55
      13.16. Overriding BP spec.....................................55
      13.17. Time of forwarding.....................................55
      13.18. Block multiplicity.....................................55
   Appendix A. For More Information.................................56

1. Introduction

   Since the publication of the Bundle Protocol Specification
   (Experimental RFC 5050[RFC5050]) in 2007, the Delay-Tolerant
   Networking Bundle Protocol has been implemented in multiple
   programming languages and deployed to a wide variety of computing
   platforms for a wide range of successful exercises.  This
   implementation and deployment experience has demonstrated the
   general utility of the protocol for challenged network operations.

   It has also, as expected, identified opportunities for making the
   protocol simpler, more capable, and easier to use.  The present
   document, standardizing the Bundle Protocol (BP), is adapted from
   RFC 5050 in that context.

   This document describes version 7 of BP.

   Delay Tolerant Networking is a network 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 networks, forming a store-carry-forward
   overlay network.  Key capabilities of BP include:

     . Custodial forwarding
     . Ability to cope with intermittent connectivity
     . Ability to take advantage of scheduled, predicted, and
        opportunistic connectivity (in addition to continuous
        connectivity)
     . Late binding of overlay network endpoint identifiers to
        underlying constituent network addresses





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   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.

   BP's location within the standard protocol stack is as shown in
   Figure 1.  BP uses underlying "native" network protocols for
   communications within a given constituent network.

   The interface between the bundle protocol and a specific underlying
   protocol 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-+   +-^---------+
   |         >>>>>>>>^         >>>>>>>>>>^         >>>>>>>>^         |
   +-----------+   +-------------+     +-------------+   +-----------+
   |                     |                     |                     |
   |<---- A network ---->|                     |<---- A network ---->|
   |                     |                     |                     |

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

     . 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.)
     . The bundle route computation algorithm.
     . Mechanisms for populating the routing or forwarding information
        bases of bundle nodes.
     . The mechanisms for securing bundles en-route.


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     . The mechanisms for managing bundle nodes.

2. Conventions used in this document

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

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

3. Service Description

3.1. Definitions

   Bundle - A bundle is a protocol data unit of BP, so named because
   negotiation of the parameters of a data exchange may be impractical
   in a delay-tolerant network: it is often better practice to "bundle"
   with a unit of data all metadata that might be needed in order to
   make the data immediately usable when delivered to applications.
   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 and/or differing in some blocks -- 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.



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   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' protocol stack that is supported in one of the
   networks within which the node is functionally located. As such,
   every CLA implements its own thin layer of protocol, interposed
   between BP and the (usually "top") protocol(s) of the underlying
   native protocol stack; this "CL protocol" may only serve to
   multiplex and de-multiplex bundles to and from the underlying native
   protocol, or it may offer additional CL-specific functionality.  The
   manner in which a CLA sends and receives bundles is wholly an
   implementation matter, exactly as described for the BPA.  The
   definitions of CLAs and CL protocols are beyond the scope of this
   specification.

   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 (including status
   reports and custody signals), and it accepts delivery of and


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   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 BP
   "router", 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.

   Administrative record - A BP administrative record is an application
   data unit that is exchanged between the administrative elements of
   nodes' application agents for some BP administrative purpose.  The
   formats of some fundamental administrative records (and of no other
   application data units) are defined in this specification.

   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 common identifier, called a "bundle endpoint ID"
   (or, in this document, simply "endpoint ID"; endpoint IDs are
   described in detail in Section 4.4.4 below). The special case of an
   endpoint that contains exactly one node is termed a "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. For a bundle to be considered
   "delivered" to an endpoint, a minimum number of receiving nodes may
   be required to receive it successfully.  This lower limit is called
   the minimum reception group, and is defined in the Transmission
   discussion below.  *Note* too that a given bundle node might
   identify itself by multiple endpoint IDs and thus be a member of
   multiple bundle endpoints.  The destination of every bundle is an
   endpoint, which may or may not be singleton.  The source of every
   bundle is a singleton endpoint.

   Transmission - A transmission is an attempt by a node's BPA to cause
   copies of a bundle to be delivered at all nodes in the minimum
   reception group of some endpoint (the bundle's destination) in
   response to a transmission request issued by the node's application
   agent. The minimum reception group of an endpoint may be any one of
   the following: (a) ALL of the nodes registered (see definition
   below) 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


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   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; 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.  Any
   number of transmissions may be concurrently undertaken by the bundle
   protocol agent of a given node.

   Forwarding - When the bundle protocol agent of a node determines
   that a bundle must be "forwarded" to a node (either a node that is a
   member of the bundle's destination endpoint or some intermediate
   forwarding node) in the course of completing the successful
   transmission of that bundle, it invokes the services of a CLA in a
   sustained effort to cause a copy of the bundle to be received by
   that node.

   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


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   delivered automatically as soon as it is the next bundle that is due
   for delivery according to the BPA's bundle delivery scheduling
   policy, an implementation matter. 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
   received by 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 node


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   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.

   Custody - A node "takes custody" of a bundle when it determines that
   it will retain a copy of the bundle for some period, forwarding and
   possibly re-forwarding the bundle as appropriate and destroying that
   retained copy only when custody of that bundle is formally
   "released". Custody of a bundle may only be taken if the destination
   of the bundle is a singleton endpoint. A "custodial node" (or
   "custodian") of a bundle is a node that has taken custody of the
   bundle and has not yet released that custody. To "accept custody"
   upon receiving a bundle is to take custody of the bundle, mark the
   bundle in such a way as to indicate to nodes that subsequently
   receive the bundle that it has taken custody, and notify all current
   custodians of the bundle that it has taken custody. Custody may only
   be 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; (c) the current
   custodian chooses to fragment the bundle, releasing custody of the
   original bundle and taking custody of the fragments instead, or (d)
   the bundle is explicitly deleted for some reason, such as lifetime
   expiration. To "refuse custody" of a bundle is to notify all current
   custodians of that bundle that an opportunity to take custody of the
   bundle has been declined.

   The custody transfer mechanism in BP is primarily intended as a
   means of recovering from forwarding failures.  When a bundle arrives
   at a node from which it cannot be forwarded, BP must recover from
   this error. BP can "return" the bundle back toward some node for
   forwarding along some different path in the network, or else it can
   instead send a small "signal" bundle back to such a node, in the
   event that this node has retained a copy of the bundle ("taken
   custody") and is therefore able to re-forward the bundle without
   receiving a copy.  Custody transfer sharply reduces the network
   traffic required for recovery from forwarding failures, at the cost
   of increased buffer occupancy and state management at the custodial
   nodes.

   Note that custodial re-forwarding can also be initiated by
   expiration of a timer prior to reception of a custody acceptance
   signal.  Since the absence of a custody acceptance signal might be
   caused by failure to receive the bundle, rather than only a
   disinclination to take custody, custody transfer can additionally
   serve as an automated retransmission mechanism.  Because custody
   transfer's only remedy for loss of any part of a bundle is


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   retransmission of the entire bundle (not just the lost portion),
   custody transfer is a less efficient automated retransmission
   mechanism than the reliable transport protocols that are typically
   available at the convergence layer; configuring BPAs to use reliable
   convergence-layer protocols between nodes is generally the best
   means of ensuring bundle delivery at the destination node(s).  But
   there are some use cases (typically involving unidirectional links)
   in which custody transfer in BP may be a more cost-effective
   solution for reliable transmission between two BP agents than
   operating retransmission protocols at the convergence layer.

   Embargo - Forwarding failures are not just operational anomalies;
   they may also convey information about the network, i.e., a
   forwarding failure may indicate a sustained lapse in forwarding
   capability.  Since forwarding a bundle to a dead end wastes time and
   bandwidth, the bundle protocol agent may choose to manage such a
   lapse by imposing a temporary "embargo" on subsequent forwarding
   activity that is similar to the forwarding attempt that has been
   seen to fail.  Mechanisms for motivating, imposing, enforcing, and
   lifting embargoes are beyond the scope of this document.

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.

3.2.1. 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.




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3.2.2. Peer application nodes

   Any number of bundle protocol application processes, each one
   constituting a single bundle node, run 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.

3.2.3. 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
   hardware, 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.

3.2.4. 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
   specialized hardware, 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 BPA of each node is expected to provide the following services
   to the node's application agent:

     . commencing a registration (registering the node in an
        endpoint);
     . terminating a registration;
     . switching a registration between Active and Passive states;
     . transmitting a bundle to an identified bundle endpoint;
     . canceling a transmission;
     . polling a registration that is in the passive state;
     . delivering a received bundle.



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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]. Exactly one of the blocks in the
   sequence must 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:

     . extensibility to address requirements not yet identified, and
     . 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 sub-
   identifiers 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


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         {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 larger than (2^64 -
   1).




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   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

           7 6 5 4 3 2 1 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.|CRC|        General        |

          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 3: Bundle Processing Control Flags Bit Layout

   The bits in positions 0 through 13 of the value of the bundle
   processing control flags SDNV are flags that characterize the bundle
   as follows:

   0 -- Bundle is a fragment.

   1 -- Payload is an administrative record.

   2 -- Bundle must not be fragmented.

   3 -- Custody transfer is requested.



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   4 -- Destination endpoint is a singleton.

   5 -- Acknowledgement by application is requested.

   6 -- Bundle is critical.

   7 -- Best-efforts forwarding is requested.

   8 -- Reliable forwarding is requested.

   9-11 -- Reserved for future use.

   The bits in positions 12 through 13 are used to indicate the type of
   CRC that is present at the end of the primary block.  The options
   are:

   0 -- No CRC.

   1 -- CRC-8.

   2 -- CRC-16.

   3 -- CRC-32.

   The bits in positions 14 through 20 are used to indicate the
   bundle's class of service. They constitute a seven-bit priority
   field indicating the bundle's priority, a value from 0 to 127, with
   higher values being of higher priority (greater urgency). Within
   this field, bit 20 is the most significant bit.

   The bits in positions 21 through 27 are status report request flags.
   These flags are used to request status reports as follows:

   21 -- Request reporting of bundle reception.

   22 -- Request reporting of custody acceptance.

   23 -- Request reporting of bundle forwarding.

   24 -- Request reporting of bundle delivery.

   25 -- Request reporting of bundle deletion.

   26 -- Reserved for future use.

   27 -- Reserved for future use.



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   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 source node requests that every receiving node accept
   custody of the bundle. If the bundle's source endpoint is the null
   endpoint (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 - Reserved for future use.


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   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. Identifiers

4.4.1. Endpoint ID

   The destinations of bundles are bundle endpoints, identified by text
   strings termed "endpoint IDs" (see Section 3.1). Each endpoint ID
   (EID) 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" >

   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.

   As used for the purposes of the bundle protocol, the length of an
   SSP must not exceed 1023 bytes.

   Note that, although endpoint IDs are 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]).

   The endpoint ID "dtn:none" identifies the "null endpoint", the
   endpoint that by definition never has any members.

   Whenever an endpoint ID appears in a bundle block, it is encoded not
   in its native URI representation but rather in an encoded
   representation that reduces consumption of transmission bandwidth.
   The encoded representation of an endpoint ID is as follows:

     . An SDNV identifying the scheme of the EID (as discussed below),
        followed by
     . the encoded representation of the EID's scheme-specific part.


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   The encoded representation of the null endpoint ID is scheme
   identifier zero, followed by zero octets of scheme-specific part.

   Every URI scheme used for forming any other EID is classified as
   either "numeric", meaning that all information conveyed in the
   scheme-specific part is to be encoded as a sequence of one or more
   unsigned integers in SDNV representation, or else "non-numeric"
   (otherwise).  The scheme identifier numbers used in the encoded
   representations of EIDs are assigned as follows:

     . Scheme identifier zero is reserved for the null endpoint ID.
     . Scheme identifier numbers in the range 1-63 are used
        exclusively for numeric EID schemes.
     . All other scheme identifier numbers are used exclusively for
        non-numeric EID schemes.

   Note that scheme of the EID is numeric if and only if the scheme
   identifier is non-zero and the two high-order bits of the first
   octet of the scheme identifier are both zero.

   For each numeric EID scheme, the encoded representation of the EID's
   scheme-specific part shall be a sequence of from 1 to 100 SDNVs as
   mandated by the definition of the scheme.

   For each non-numeric EID scheme, the encoded representation of the
   EID's scheme-specific part shall comprise:

     . a single SDNV indicating the length of the remainder of the
        encoded representation of the scheme-specific part of the EID,
        followed by
     . the remainder of the encoded representation of the scheme-
        specific part of the EID, formed according to the definition of
        the scheme. If the scheme's definition does not include a
        specification for encoded representation, then the EID's native
        scheme-specific part appears here without alteration.

   It is important to note that not all BP implementations are required
   to implement the definitions of all EID schemes.  The BP
   implementations used to instantiate nodes in a given network must be
   chosen with care in order for every node to be able to exchange
   bundles with every other node.

4.4.2. Node ID

   For many purposes of the Bundle Protocol it is important to identify
   the node that is operative in some context.



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   As discussed in 3.1 above, nodes are distinct from endpoints;
   specifically, an endpoint is a set of zero or more nodes.  But
   rather than define a separate namespace for node identifiers, we
   instead use endpoint identifiers to identify nodes, subject to the
   following restrictions:

      . Every node must be a member of at least one singleton endpoint.
      . The EID of any singleton endpoint of which a node is a member
        may be used to identify that node. A "node ID" is an EID that
        is used in this way.
      . A node's membership in a given singleton endpoint must be
        sustained at least until the nominal operation of the Bundle
        Protocol no longer depends on the identification of that node
        by that endpoint's ID.

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 these two basic BP blocks is shown in Figure 5 below.

   Primary Bundle Block

   +---------+-----------------------+----------------+---------------+

   | Version |      Block length     |    Bundle Processing flags (*) |

   +---------+-----------------------+----------------+---------------+

   |       Destination EID (*)       |        Source Node ID (*)      |

   +----------------+----------------+----------------+---------------+

   |       Report-to EID (*)         |   Creation timestamp time (*)  |

   +----------------+----------------+----------------+---------------+

   |      Creation Timestamp sequence number (*)      |  Lifetime (*) |

   +----------------+----------------+----------------+---------------+


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   | Inventory len. |  Inventory (*) |      [Fragment offset (*)]     |

   +----------------+----------------+----------------+---------------+

   |     [Total application data unit length (*)]     |   [CRC (*)]   |

   +----------------+----------------+----------------+---------------+

   Bundle Payload Block

   +----------------+----------------+----------------+---------------+

   |   Block type   |Block number (*)| Proc. flags (*)| Blk length(*) |

   +----------------+----------------+----------------+---------------+

   / Bundle payload (variable) /

   +------------------------------------------------------------------+

                   Figure 5: Basic Bundle Block Formats

   (*) Notes:

   The bundle processing control flags field in the Primary Bundle
   Block is an SDNV and is therefore of variable length. A two-octet
   SDNV is shown here for convenience in representation.

   The destination EID, source node ID, and report-to EID in the
   Primary Bundle Block are EIDs in encoded representation and are
   therefore of variable length.  Two-octet fields are shown here for
   convenience in representation.

   The creation timestamp time in the Primary Bundle Block is an SDNV
   and is therefore of variable length. A two-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 of variable length. A three-octet
   SDNV is shown here for convenience in representation.

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

   The inventory field in the Primary Bundle Block is an array of block
   types (one octet each) whose length is given by the value of the


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   Inventory Length field and is therefore variable.  A one-octet
   inventory array 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 field is
   set to 1. It is an SDNV and is therefore of variable length; a two-
   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 field is set to 1. It is an SDNV and is therefore of variable
   length; a three-octet SDNV is shown here for convenience in
   representation.

   The CRC field of the Primary Bundle Block is present only if the CRC
   type field in the block's processing flags field is non-zero.  Its
   actual length depends on the CRC type; a one-octet CRC is shown here
   for convenience in representation.

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

   The block length ("Blk length") field of the Payload Block is an
   SDNV and is therefore of variable length. A one-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
   forward bundles to their destinations. The fields of the primary
   bundle block are:

   Version: A 4-bit field indicating the version of the bundle protocol
   that constructed this block. The present document describes version
   0x07 of the bundle protocol.

   Block Length: a 12-bit field that contains the aggregate length (in
   bytes) of all remaining fields of the primary block.  Note that,
   although many fields of the primary bundle block are variable-length
   SDNVs, the lengths of all of these SDNVs are in practice limited;
   the lengths of the scheme-specific parts of non-numeric EIDs are
   likewise limited. These limitations make it reasonable to limit the
   total length of the primary block to 4095 octets.





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   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.

   Destination EID: The Destination EID field contains the encoded
   representation 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.

   Source node ID: The Source node ID field contains the encoded
   representation of an endpoint ID that identifies the node from which
   the bundle was initially transmitted, except that it may contain the
   null endpoint ID in the event that the bundle's source chooses to
   remain anonymous.

   Report-to EID: The Report-to EID field contains the encoded
   representation of the ID of the endpoint to which status reports
   pertaining to the forwarding and delivery of this bundle are to be
   transmitted.

   Creation Timestamp: The creation timestamp is a pair of SDNVs that,
   together with the source node 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. For nodes
   that lack accurate clocks (that is, nodes that are not at all
   moments able to determine the current UTC time to within 30
   seconds), bundle creation time MUST be set to zero and the counter
   used as the source of the bundle sequence count MUST NEVER be reset
   to zero. In either case, a source Bundle Protocol Agent must never
   create two distinct bundles with the same source node ID and bundle
   creation timestamp. The combination of source node ID and bundle
   creation timestamp serves to identify a single transmission request,
   enabling it to be acknowledged by the receiving application
   (provided the source node ID is not the null endpoint ID).

   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 bundle's age exceeds


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   its lifetime, bundle nodes need no longer retain or forward the
   bundle; the bundle SHOULD be deleted from the network.

   Inventory: The Primary block may contain an accounting of all blocks
   that were in the bundle at the time it was transmitted from the
   source node.  This accounting comprises an inventory list length (an
   SDNV) followed by an inventory list (an array of N octets, where N
   is the value of the inventory list length).  This feature is
   optional: if the inventory is to be omitted, the inventory length
   must be set to zero.  Otherwise the values of the octets in the
   inventory list must be the block types of all of the non-primary
   blocks in the bundle as originally transmitted, exactly one list
   element per block.  Since a bundle may contain multiple instances of
   a given block type, multiple elements of the inventory list may have
   the same value.  The order of block types appearing in the inventory
   list is undefined.

   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.

   CRC: If and only if the CRC type in the Bundle Processing Control
   Flags of this Primary block is non-zero, a CRC is appended to the
   primary block.  The length of the CRC is 8 bits, 16 bits, or 32 bits
   as indicated by the CRC type.  The CRC is computed over the
   concatenation of all bytes of the primary block including the CRC
   field itself, which for this purpose is temporarily populated with
   the value zero.

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:

     . 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 2 through 10 are defined as


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        noted later in this specification.  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.
     . Block number, an unsigned integer expressed as an SDNV. The
        block number uniquely identifies the block within the bundle,
        enabling blocks (notably bundle security protocol blocks) to
        explicitly reference other blocks in the same bundle. Block
        numbers need not be in continuous sequence, and blocks need not
        appear in block number sequence in the bundle. The block number
        of the payload block is always zero.
     . 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.
     . 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.
     . 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 number (*)| Proc. Flags (*)| Blk length(*) |

   +----------------+----------------+----------------+---------------+

   / Block body data (variable) /

   +------------------------------------------------------------------+

                          Figure 6: Block Layout

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 Number: The Block Number field is an SDNV that contains the
   unique identifying number of the block.  The block number of the
   bundle payload block is always zero.



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

   Block-type-specific Data: The Block-type-specific Data field of the
   Payload Block contains the "payload", i.e., the application data
   carried by this bundle.

   That is, bundle payload blocks conform to the canonical format
   described in the previous section.

4.6. Extension Blocks

   "Extension blocks" are all blocks other than the primary and payload
   blocks. Because not all extension blocks are 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.

   The extension blocks of the Bundle Security Protocol (block types 2,
   3, and 4) are defined separately in the Bundle Security Protocol
   specification (work in progress).

   The following extension blocks are defined in the current document.

4.6.1. Current Custodian

   The Current Custodian block, block type 5, identifies a node that is
   known to have accepted custody of the bundle.  The block-type-
   specific data of this block is the encoded representation of the


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   node ID of a custodian.  The bundle MAY contain one or more
   occurrences of this type of block.

4.6.2. Flow Label

   The Flow Label block, block type 6, indicates the flow label that is
   intended to govern transmission of the bundle by convergence-layer
   adapters.  The syntax and semantics of BP flow labels are beyond the
   scope of this document.

4.6.3. Previous Node ID

   The Previous Node ID block, block type 7, identifies the node that
   forwarded this bundle to the local node; its block-type-specific
   data is the encoded representation of the node ID of that node.  If
   the local node is the source of the bundle, then the bundle MUST NOT
   contain any Previous Node ID block.  Otherwise the bundle MUST
   contain one (1) occurrence of this type of block.  If present, the
   Previous Node ID block MUST be the FIRST block following the primary
   block, as the processing of other extension blocks may depend on its
   value.

4.6.4. Bundle Age

   The Bundle Age block, block type 9, contains the number of seconds
   that have elapsed between the time the bundle was created and time
   at which it was most recently forwarded.  It is intended for use by
   nodes lacking access to an accurate clock, to aid in determining the
   time at which a bundle's lifetime expires. The block-type-specific
   data of this block is an SDNV containing the age of the bundle (the
   sum of all known intervals of the bundle's residence at forwarding
   nodes, up to the time at which the bundle was most recently
   forwarded) in seconds. If the bundle's creation time is zero, then
   the bundle MUST contain exactly one (1) occurrence of this type of
   block; otherwise, the bundle MAY contain at most one (1) occurrence
   of this type of block.

4.6.5. Hop Count

   The Hop Count block, block type 10, contains two SDNVs, hop limit
   and hop count, in that order.  It is mainly intended as a safety
   mechanism, a means of identifying bundles for removal from the
   network that can never be delivered due to a persistent forwarding
   error: a bundle may be deleted when its hop count exceeds its hop
   limit.  Procedures for determining the appropriate hop limit for a
   block are beyond the scope of this specification.  A bundle MAY
   contain at most one (1) occurrence of this type of block.


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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 transmission of bundles is in response to bundle transmission
   requests presented by nodes' application agents. When required to
   "generate" an administrative record (such as 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:

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



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     . A "custody acceptance status report" is a bundle status report
        with the "reporting node accepted custody of bundle" flag set
        to 1.
     . A "bundle forwarding status report" is a bundle status report
        with the "reporting node forwarded the bundle" flag set to 1.
     . A "bundle delivery status report" is a bundle status report
        with the "reporting node delivered the bundle" flag set to 1.
     . A "bundle deletion status report" is a bundle status report
        with the "reporting node deleted the bundle" flag set to 1.
     . A "Succeeded" custody signal is a custody signal with the
        "custody transfer succeeded" flag set to 1.
     . A "Failed" custody signal is a custody signal with the "custody
        transfer succeeded" flag set to zero.
     . A "current custodian" of a bundle is a node identified in a
        Current Custodian extension block of that bundle.

5.2. Bundle Transmission

   The steps in processing a bundle transmission request are:

   Step 1: If custody transfer is requested for this bundle
   transmission then the destination must be a singleton endpoint.  If,
   moreover, custody acceptance by the source node is required but the
   conditions under which custody of the bundle may be accepted are not
   satisfied, then the request cannot be honored and all remaining
   steps of this procedure must be skipped.

   Step 2: Transmission of the bundle is initiated. An outbound bundle
   must be created per the parameters of the bundle transmission
   request, with the retention constraint "Dispatch pending". The
   source node ID of the bundle must be either the EID of a singleton
   endpoint whose only member is the node of which the BPA is a
   component or else the null endpoint ID, indicating that the source
   of the bundle is anonymous.

   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.



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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 node(s) to
        forward the bundle to.  The bundle protocol agent may choose
        either to forward the bundle directly to its destination
        node(s) (if possible) or to forward the bundle to some other
        node(s) for further forwarding. The manner in which this
        decision is made may depend on the scheme name in the
        destination endpoint ID and/or other state but in any case is
        beyond the scope of this document. If the BPA elects to forward
        the bundle to some other node(s) for further forwarding:
          o  If the "Bundle is critical" flag (in the bundle processing
             flags) is set to 1, then ALL nodes that have some
             plausible prospect of forwarding the bundle to its
             destination node(s) SHOULD be selected for this purpose.
          o  If the agent finds it impossible to select any node(s) to
             forward the bundle to, then forwarding is contraindicated.
     . Provided the bundle protocol agent succeeded in selecting the
        node(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 those nodes.  If
        both the "Best-efforts forwarding requested" and the "Reliable
        forwarding is requested" bundle processing flags are set to 1,
        then all selected CLAs MUST be for bundle streaming CL
        protocols such as the proposed Bundle Streaming Service
        Protocol. Otherwise, if only the "Reliable forwarding is
        requested" bundle processing flag is set to 1, then all
        selected CLAs MUST be for reliable protocols such as TCP/IP.
        Otherwise, if only the "Best-efforts forwarding requested"
        bundle processing flag is set to 1, then all selected CLAs MUST
        be for best-efforts protocols such as UDP/IP. Otherwise, any
        available CLAs may be selected.  The manner in which specific
        appropriate convergence layer adapters are selected is beyond
        the scope of this document. If the agent finds it impossible to
        select appropriate convergence layer adapters to use in
        forwarding this bundle, then forwarding is contraindicated.



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   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 node 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 that
   node. Determining the time at which the bundle is to be sent by each
   convergence layer adapter is an implementation matter.  Note that:

     . The order in which convergence layer adapters send bundles
        SHOULD normally conform to the priority indicated in each
        bundle's bundle processing control flags field: all bundles of
        priority 255 sent from any single source should be sent before
        all bundles of priority 254 sent from the same source and so
        on.
     . But if the bundle contains a flow label extension block then
        that flow label value may identify overriding procedures for
        determining the order in which convergence layer adapters must
        send bundles, e.g., considering bundle source when determining
        the order in which bundles are sent.  The definition of such
        procedures is beyond the scope of this specification.
     . If the bundle has a bundle age block, then at the last possible
        moment before the CLA initiates conveyance of the bundle node
        via the CL protocol the bundle age value MUST be increased by
        the difference between the current time and the time at which
        the bundle was received (or, if the local node is the source of
        the bundle, created).

   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 to which the
        bundle was forwarded 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


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        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. 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(s); 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
   each current custodian still has a copy.)

   If the bundle's custody transfer requested flag (in the bundle
   processing flags field) is set to 0, then the bundle protocol agent
   MAY forward the bundle back to the node that sent it, as identified
   by the Previous Node ID block.

   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


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   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 bundle's age exceeds its lifetime as
   specified in the primary bundle block. Bundle age MAY be determined
   by subtracting the bundle's creation timestamp time from the current
   time if (a) that timestamp time is not zero and (b) the local node's
   clock is known to be accurate (as discussed in section 4.5.1 above);
   otherwise bundle age MUST be obtained from the Bundle Age extension
   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


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        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 node 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. 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 must be 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).



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   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. The bundle protocol agent must report custodial
        delivery by generating a "Succeeded" custody signal for the
        bundle, destined for the bundle's current custodian(s).

5.8. Bundle Fragmentation

   It may at times be advantageous for bundle protocol agents to reduce
   the sizes of bundles in order to forward them. This might be the
   case, for example, if a node 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 node 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 that has any Current Custodian extension block citing any
   node other than the local node MUST NOT be fragmented.  This
   restriction aside, 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:

     . 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



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        parts of the network, may be overlapping subsets of the
        original bundle's payload.
     . The bundle processing flags in the primary block of each
        fragment must differ from those of the bundle that is being
        fragmented, in that they must 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.  Additionally, the CRC of the bundle that
        is being fragmented, if any, must be replaced in each fragment
        by a new CRC computed for the primary block of that fragment.
     . The primary blocks of the fragments will differ from that of
        the fragmented bundle as noted above.
     . The payload blocks of fragments will differ from that of the
        fragmented bundle as noted above.
     . If the bundle being fragmented is not a fragment or is the
        fragment with offset zero, then all extension blocks of the
        bundle being fragmented MUST be replicated in the fragment
        whose offset is zero.
     . Each extension block whose "Block must be replicated in every
        fragment" flag, in the block processing flags, is set to 1 MUST
        be replicated in every fragment.
     . Beyond these rules, replication of extension blocks in the
        fragments is an implementation matter.
     . If the local node had taken custody of the fragmented bundle,
        then the BPA MUST release custody of the fragmented bundle
        before fragmentation occurs and MUST take custody of every
        fragment.

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 node 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:

     . This byte array -- the reassembled application data unit --
        must replace the payload of this fragment.
     . For each fragmentary bundle whose payload is a subset of the
        reassembled application data unit, for which custody transfer
        is requested but the BPA has not yet taken custody, the BPA
        must take custody of that bundle.
     . The BPA must then release custody of all fragments whose
        payload is a subset of the reassembled application data unit,
        for which it has taken custody.



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     . 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
   the nodes that are members of destination endpoints as necessary; an
   application data unit may also be reassembled at some other node on
   the path to the destination.

5.10. Custody Transfer

   The decision as to whether or not to accept custody of a bundle is
   an implementation matter that may involve both resource and policy
   considerations.

   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 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(s).

   The bundle protocol agent must assert the new current custodian for
   the bundle. It does so by inserting a new Current Custodian
   extension block whose value is the node ID of the local node or by
   changing the value of an existing Current Custodian extension block
   to the local node ID.

   The bundle protocol agent may set a custody transfer countdown timer
   for this bundle; upon expiration of this timer prior to expiration


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   of the bundle itself and prior to custody transfer success for this
   bundle, the custody transfer failure procedure detailed in Section
   5.12 may 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

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

5.11. Custody Transfer Success

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

5.12. Custody Transfer Failure

   Custody transfer 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, then:



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     . 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.

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.  Note that additional types of
   administrative records may be defined by supplementary DTN protocol
   specification documents.

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

   +---------+--------------------------------------------+

   |  Value  |                   Meaning                  |

   +=========+============================================+



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   | 00001   | Bundle status report.                      |

   +---------+--------------------------------------------+

   | 00010   | 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

   The contents of the two types of administrative records defined in
   the present document 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.

   +----------------+----------------+----------------+---------------+



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   | Status Flags   | Reason code    | Fragment offset (*) (if

   +----------------+----------------+----------------+---------------+

     present)       | Fragment length (*) (if present)                |

   +----------------+----------------+----------------+---------------+

   | Source node ID of bundle X (*)                                   |

   +----------------+----------------+----------------+---------------+

   | Copy of bundle X's Creation Timestamp time (*)                   |

   +----------------+----------------+----------------+---------------+

   | Copy of bundle X's Creation Timestamp sequence number (*)        |

   +----------------+----------------+----------------+---------------+

   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 Source Node ID and Creation Timestamp fields replicate the
   Source Node ID and Creation Timestamp fields in the primary block of
   the subject bundle. As such they are of variable length. Four-octet
   values are 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                    |

   +==========+============================================+



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   | 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                   |

   +=========+============================================+



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   | 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.



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   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.

   Source Node ID of Subject Bundle: The source node ID of the bundle
   that caused the status report to be generated.

   Creation Timestamp of Subject Bundle: A copy of the creation
   timestamp 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 nodes 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)                                 |

   +----------------+----------------+----------------+---------------+

   | Source node ID of bundle X (*)                                   |

   +----------------+----------------+----------------+---------------+

   | Copy of bundle X's Creation Timestamp time (*)                   |

   +----------------+----------------+----------------+---------------+

   | Copy of bundle X's Creation Timestamp sequence number (*)        |

   +----------------+----------------+----------------+---------------+

                     Figure 13: Custody Signal Format

   (*) Notes:



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   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 Source Node ID and Creation Timestamp fields replicate the
   Source Node ID and Creation Timestamp fields in the primary block of
   the subject bundle. As such they are of variable length. Four-octet
   values are 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 by a node that is a   |

   |         | custodial node for this bundle).           |

   +---------+--------------------------------------------+

   | 0x04    | Depleted storage.                          |

   +---------+--------------------------------------------+



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   | 0x05    | Destination endpoint ID unintelligible.    |

   +---------+--------------------------------------------+

   | 0x06    | No known route 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 custody signal 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.

   Source Node ID of Subject Bundle: The source node ID of the bundle
   that caused the custody signal to be generated.

   Creation Timestamp of Subject Bundle: A copy of the creation
   timestamp 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


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   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:

     . sending a bundle to a bundle node that is reachable via the
        convergence layer protocol;
     . delivering to the bundle protocol agent a bundle that was sent
        by a 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


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   serve BP implementations conforming to those protocols to provide
   additional services such as retransmitting data that were lost in
   transit, discarding bundle-conveying data units that the convergence
   layer protocol determines are corrupt or inauthentic, or reporting
   on the integrity and/or authenticity of delivered bundles.

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 as soon as practicable.

   Likewise, because of the potentially high 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 two 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.


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   To provide end-to-end bundle authenticity and integrity, the Block
   Integrity Block (BIB) is used. The BIB allows any security-enabled
   entity along the delivery path to ensure the integrity of the
   bundle's payload or any other block other than a Block
   Confidentiality Block.

   To provide payload confidentiality, the use of the Block
   Confidentiality Block (BCB) is available. The bundle payload, or any
   other block aside from the primary block and the Bundle Security
   Protocol blocks, may be encrypted to provide end-to-end payload
   confidentiality/privacy.

   Additionally, convergence-layer protocols that ensure authenticity
   of communication between adjacent nodes in BP network topology
   SHOULD be used where available, to minimize the ability of
   unauthenticated nodes to introduce inauthentic traffic into the
   network.

   Bundle security must not be invalidated by forwarding nodes even
   though they themselves might not use the Bundle Security Protocol.

   In particular, while blocks may be added to bundles transiting
   intermediate nodes, removal of blocks with the 'Discard block if it
   can't be processed' flag unset in the block processing control flags
   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" and "ipn" URI schemes have been provisionally registered
   by IANA. See http://www.iana.org/assignments/uri-schemes.html for
   the latest details.

   Registries of scheme type numbers, extension block type numbers, and
   administrative record type numbers will be required.

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.




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

   [URIREG] Thaler, D., Hansen, T., and T. Hardie, "Guidelines and
   Registration Procedures for URI Schemes", RFC 7595, BCP 35, June
   2015.

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.

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

   [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.

11. Acknowledgments

   This work is freely adapted from [RFC5050], which was an effort of
   the Delay Tolerant Networking Research Group. The following DTNRG
   participants contributed significant technical material and/or
   inputs to that document: Dr. Vinton Cerf of Google, Scott Burleigh,
   Adrian Hooke, and Leigh Torgerson of the Jet Propulsion Laboratory,


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   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, and Howard Weiss of SPARTA, Inc.

   This document was prepared using 2-Word-v2.0.template.dot.

12. Significant Changes From RFC 5050

   Points on which this draft significantly differs from RFC 5050
   include the following:

     . Clarify the difference between transmission and forwarding.
     . Amplify discussion of custody transfer.  Move current custodian
        to an extension block, of which there can be multiple
        occurrences (possible support for the MITRE idea of multiple
        concurrent custodians, from several years ago); define that
        block in this specification.
     . Introduce the concept of "node ID" as functionally distinct
        from endpoint ID, while having the same syntax.
     . Introduce a new method of encoding endpoint IDs (including node
        IDs) in a transmitted bundle, replacing both the "dictionary"
        and the CBHE compression mechanism.
     . Add ECOS features to primary block.
     . Restrict the scope of bundle prioritization to all bundles from
        the same source.
     . Restructure primary block, making it immutable.  Add optional
        CRC and inventory.
     . Add optional CRCs to non-primary blocks.
     . Add block ID number to canonical block format (to support
        streamlined BSP).
     . Add bundle age extension block, defined in this specification.
     . Add previous node ID extension block, defined in this
        specification.
     . Add flow label block, *not* defined in this specification.
     . Add hop count extension block, defined in this specification.
     . Clean up a disconnect between fragmentation and custody
        transfer that Ed pointed out.
     . Remove "DTN time" values from admin records.

13. Open Issues

13.1. Definitions section structure

   Would it be better to restrict the Definitions section to
   definitions only, and move description, conceptual operation,


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   potential implementation, justification, and commentary to one or
   more additional sections?  Or would fractionating this information
   across multiple sections would make it harder to grasp?

13.2. Payload nomenclature

   It has been proposed that (a) there is no need to define "nominal
   payload" and (b) "partial" payload would be better than
   "fragmentary" payload.  (The term "nominal payload" is used in the
   definition of fragmentation.)

13.3. Application Agent

   Does the discussion of Application Agent functionality need to be in
   the BP spec?  If so, should there be a diagram explaining how the
   various components of the BPA interact?

13.4. Bundle Endpoint definition

   Is the source of a bundle always a node, or do we want to define a
   way in which a set of nodes (an endpoint) can collectively transmit
   a bundle?  Does the latter trace back to a use case we need to
   support?

   Is a bundle custodian always a node, or do we want to define a way
   in which a set of nodes (an endpoint) can collectively take custody
   of a bundle?  Does the latter trace back to a use case we need to
   support?.

13.5. Alignment with ICN

   Is it necessary to modify the bundle transmission procedure to
   enable BP to be used for information-centric networking, i.e.,
   delivering data to a node who requests that data after it has
   already been transmitted?  Specifically, would a DTN ICN cache point
   "transmit" data to a client (i.e., source a new bundle) or would it
   merely "forward" a previously transmitted bundle of which it has
   retained a copy?

13.6. Implementation Architectures

   Should the BP spec be divided into two documents? One to talk about
   conops and context and one that focuses specifically on the
   protocol?





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13.7. Security protocol name

   Will the name of the DTN security protocol be Bundle Security
   Protocol or Streamlined Bundle Security Protocol?

13.8. Bundle format

   Should the rules for defining block structure be presented at the
   start of section 4 or in the discussion of the payload block and the
   "last block" flag?  Should we require that the payload block always
   be the last block of the bundle, so that the "last block" flag is no
   longer needed?  (This would make reactive fragmentation easier.)

13.9. SDNVs

   Should the SDNV discussion in 4.1 be deleted?

13.10. Bundle Processing Control Flags

   Should the bit numbering convention described in section 4.2 be
   moved to another location in the document?

13.11. Extended class of service features

   Should these features (critical bundle, best-efforts forwarding
   requested, reliable forwarding requested) be omitted from the
   primary block?  If they are omitted, should these application-
   selected CoS markings be supported in some other way?  If the
   "critical" CoS feature is retained, should it have a different
   name?

   Note: a node selection (route computation) procedure might consider
   the availability of CLAs that match the bundle's CoS when selecting
   a node to forward to, and that is entirely the business of the route
   computation procedure.  (Not all route computation procedures will
   do so.)

13.12. Primary block CRC type

   What are the best CRC options to support here?  CRC-16-ARINC, CRC-
   16-CCITT, CRC-16-CDMA2000, CRC-16-DECT, etc.? Are there more than 4?

13.13. Inventory

   Is a list of the block types of all blocks in the bundle as
   forwarded by the source node a good implementation of the requested
   "inventory" feature?  If not, what would be better?


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13.14. Block numbers

   Should the payload always have block number zero?

13.15. Clearing flag

   Should an node that is able to process a given extension block be
   permitted to clear block's "Block was forwarded without being
   processed" flag?



13.16. Overriding BP spec

   Is a supplementary DTN protocol specification allowed to override or
   supersede the BP specification (other than making some BP procedures
   moot by requiring that the processing of a bundle be terminated
   under fault conditions recognized by that protocol)?

13.17. Time of forwarding

   Should the BPA control the time at which a bundle is to be forwarded
   to another node, or should that determination be left to the
   selected convergence-layer protocol adapter(s)?

13.18. Block multiplicity

   Would it be good to restrict BP extensions to one extension block
   per extension per bundle?  That is, should we require that all
   information needed to implement a given BP extension for a given
   bundle be contained in a single extension block?

   This would entail encapsulating any necessary multiplicity for a
   given extension (for example, multiple Metadata records) within a
   single block.

   Among the advantages: no need for block numbers (block type would
   always be sufficient to identify the block), therefore no need for a
   block number generation mechanism; shorter and simpler inventory;
   simpler extension implementation (all information is in one block,
   no need to search through extension blocks for additional relevant
   information).

   Among the disadvantages: very different from RFC 5050; would in some
   cases require that security blocks operate on data structures that
   are internal to extension blocks rather than always operate on
   entire extension blocks.


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Appendix A.                 For More Information

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

   Copyright (c) 2015 IETF Trust and the persons identified as authors
   of the code. All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, is permitted pursuant to, and subject to the license
   terms contained in, the Simplified BSD License set forth in Section
   4.c of the IETF Trust's Legal Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info).

Authors' Addresses

   Scott Burleigh
   Jet Propulsion Laboratory, California Institute of Technology
   4800 Oak Grove Dr.
   Pasadena, CA 91109-8099
   US
   Phone: +1 818 393 3353
   EMail: Scott.Burleigh@jpl.nasa.gov

   Kevin Fall
   Carnegie Mellon University / Software Engineering Institute
   4500 Fifth Avenue
   Pittsburgh, PA 15213
   US
   Phone: +1 412 268 3304
   Email: kfall@cmu.edu

   Edward J. Birrane
   Johns Hopkins University Applied Physics Laboratory
   11100 Johns Hopkins Rd
   Laurel, MD 20723
   US
   Phone: +1 443 778 7423
   Email: Edward.Birrane@jhuapl.edu










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