Bundle Protocol
draft-ietf-dtn-bpbis-02
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| Document | Type | Active Internet-Draft (dtn WG) | |
|---|---|---|---|
| Authors | Scott C. Burleigh , Kevin Fall , Edward J. Birrane | ||
| Last updated | 2016-01-11 | ||
| Replaces | draft-dtnwg-bp | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-dtn-bpbis-02
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
January 11, 2016
Bundle Protocol
draft-ietf-dtn-bpbis-02.txt
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Copyright Notice
Copyright (c) 2016 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
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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...................................................3
2. Conventions used in this document..............................5
3. Service Description............................................6
3.1. Definitions...............................................6
3.2. Discussion of BP concepts.................................9
3.3. Services Offered by Bundle Protocol Agents...............14
4. Bundle Format.................................................14
4.1. Bundle Processing Control Flags..........................14
4.2. Block Processing Control Flags...........................16
4.3. Identifiers..............................................16
4.3.1. Endpoint ID.........................................16
4.3.2. Node ID.............................................17
4.4. Contents of Bundle Blocks................................17
4.4.1. Primary Bundle Block................................17
4.4.2. Canonical Bundle Block Format.......................19
4.5. Extension Blocks.........................................20
4.5.1. Current Custodian...................................20
4.5.2. Flow Label..........................................20
4.5.3. Previous Node ID....................................21
4.5.4. Bundle Age..........................................21
4.5.5. Hop Count...........................................21
5. Bundle Processing.............................................21
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5.1. Generation of Administrative Records.....................22
5.2. Bundle Transmission......................................23
5.3. Bundle Dispatching.......................................23
5.4. Bundle Forwarding........................................23
5.4.1. Forwarding Contraindicated..........................25
5.4.2. Forwarding Failed...................................26
5.5. Bundle Expiration........................................26
5.6. Bundle Reception.........................................27
5.7. Local Bundle Delivery....................................28
5.8. Bundle Fragmentation.....................................28
5.9. Application Data Unit Reassembly.........................30
5.10. Custody Transfer........................................30
5.10.1. Custody Acceptance.................................30
5.10.2. Custody Release....................................31
5.11. Custody Transfer Success................................31
5.12. Custody Transfer Failure................................31
5.13. Bundle Deletion.........................................32
5.14. Discarding a Bundle.....................................32
5.15. Canceling a Transmission................................32
6. Administrative Record Processing..............................33
6.1. Administrative Records...................................33
6.1.1. Bundle Status Reports...............................33
6.1.2. Custody Signals.....................................36
6.2. Generation of Administrative Records.....................37
6.3. Reception of Custody Signals.............................37
7. Services Required of the Convergence Layer....................38
7.1. The Convergence Layer....................................38
7.2. Summary of Convergence Layer Services....................38
8. Security Considerations.......................................38
9. IANA Considerations...........................................40
10. References...................................................40
10.1. Normative References....................................40
10.2. Informative References..................................40
11. Acknowledgments..............................................41
12. Significant Changes from RFC 5050............................41
13. Open Issues..................................................42
13.1. Application Agent.......................................42
13.2. Primary block CRC type..................................42
Appendix A. For More Information.................................43
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
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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 may be viewed as sitting 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, including cases
where the sender and receiver are not concurrently present in
the network
. Ability to take advantage of scheduled, predicted, and
opportunistic connectivity, whether bidirectional or
unidirectional, in addition to continuous connectivity
. Late binding of overlay network endpoint identifiers to
underlying constituent network addresses
For descriptions of these capabilities and the rationale for the DTN
architecture, see [ARCH] and [SIGC]. [TUT] contains a tutorial-
level overview of DTN concepts.
BP's location within the standard protocol stack is as shown in
Figure 1. BP uses underlying "native" transport and/or 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).
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+-----------+ +-----------+
| 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 in 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.
. 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.
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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.
Block - A bundle protocol block is one of the protocol data
structures that together constitute a well-formed bundle.
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; it is the content of
the bundle's payload block. The terms "bundle content", "bundle
payload", and "payload" are used interchangeably in this document.
Partial payload - A partial payload is a payload that comprises
either the first N bytes or the last N bytes of some other payload
of length M, such that 0 < N < M.
Fragment - A fragment is a bundle whose payload block contains a
partial payload.
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.
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.
Convergence layer adapter - A convergence layer adapter (CLA) is a
node component that 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.
Application agent - The application agent (AA) of a node is the node
component that utilizes the BP services to effect communication for
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some user purpose. The application agent in turn has two elements,
an administrative element and an application-specific element.
Application-specific element - The application-specific element of
an AA is the node component that constructs, requests transmission
of, accepts delivery of, and processes units of user application
data.
Administrative element - The administrative element of an AA is the
node component that constructs and requests transmission of
administrative records (defined below), including status reports and
custody signals, and accepts delivery of and processes any custody
signals that the node receives.
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 4.3.1 below).
Singleton endpoint - A singleton endpoint is an endpoint that always
contains exactly one member.
Registration - A registration is the state machine characterizing a
given node's membership in a given endpoint. Any single
registration has an associated delivery failure action as defined
below and must at any time be in one of two states: Active or
Passive.
Delivery - 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 any relevant metadata
(an implementation matter), has been presented to the node's
application agent in a manner consistent with the state of that
registration.
Deliverability - 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
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(c) the bundle has not yet been "abandoned" (as defined below)
subject to this registration.
Abandonment - To abandon a bundle subject to some registration is to
assert that the bundle is not deliverable subject to that
registration.
Delivery failure action - The delivery failure action of a
registration is the action that is to be taken when a bundle that is
"deliverable" subject to that registration is received at a time
when the registration is in the Passive state.
Destination - The destination of a bundle is the endpoint comprising
the node(s) at which the bundle is to be delivered (as defined
below).
Minimum transmission group - The minimum transmission group of an
endpoint is the minimum number of members of the endpoint (nodes) at
which the bundle must have been delivered in order for the bundle to
be considered delivered to the 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.
Forwarding - To forward a bundle to a node is to invoke the services
of a CLA in a sustained effort to cause a copy of the bundle to be
received by that node.
Discarding - To discard a bundle is to cease all operations on the
bundle and functionally erase all references to it. The specific
procedures by which this is accomplished are an implementation
matter.
Retention constraint - A retention constraint is an element of the
state of a bundle that prevents the bundle from being discarded.
That is, a bundle cannot be discarded while it has any retention
constraints.
Deletion - To delete a bundle is to remove unconditionally all of
the bundle's retention constraints, enabling the bundle to be
discarded.
Custodian - A custodian of a bundle is a node that has determined
that it will retain a copy of that bundle for an indefinite period
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of time, forwarding and possibly re-forwarding the bundle as
appropriate, until it detects one of the conditions under which it
may cease being a custodian of that bundle (discussed later).
Taking custody - To take custody of a bundle is to become a
custodian of that bundle.
Accepting custody - To accept custody of a bundle is to take custody
of the bundle, mark the bundle in such a way as to indicate this
custodianship to nodes that subsequently receive copies of the
bundle, and announce this custodianship to all current custodians of
the bundle.
Refusing custody - 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.
Releasing custody - To release custody of a bundle is to cease to be
a custodian of the bundle.
3.2. Discussion of BP concepts
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 (copy), in that node's local memory,
of some bundle that is in the network.
The 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 payload for a bundle
forwarded in response to reception of a bundle is the payload of the
received bundle.
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.
The manner in which the functions of the BPA are performed 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
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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.
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, again, wholly
an implementation matter. The definitions of CLAs and CL protocols
are beyond the scope of this specification.
Note that the administrative element of a node's application agent
may itself, in some cases, function as a convergence-layer adapter.
That is, outgoing bundles may be "tunneled" through encapsulating
bundles:
. An outgoing bundle that has been encoded in some documented
representation constitutes a byte array. This byte array may,
like any other, be presented to the bundle protocol agent as an
application data unit that is to be transmitted to some
endpoint.
. The original encoded bundle thus forms the payload of an
encapsulating bundle that is forwarded using some other
convergence-layer protocol(s).
. When the encapsulating bundle is received, its payload is
delivered to the peer application agent administrative element,
which then instructs the bundle protocol agent to dispatch that
original encoded bundle in the usual way.
The purposes for which this technique may be useful (such as cross-
domain security) are beyond the scope of this specification.
The only interface between the BPA and the application-specific
element of the AA is the BP service interface. But between the BPA
and the administrative element of the AA there is a (conceptual)
private control interface in addition to the BP service interface.
This private control interface enables the BPA and the
administrative element of the AA to direct each 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
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the BPA, the manner in which the AA performs its functions is wholly
an implementation matter.
Singletons are the most familiar sort of endpoint, but in general
the endpoint notion is meant to be broader. For example, the nodes
in a sensor network might constitute a set of bundle nodes that
identify themselves by a single common endpoint ID and thus form a
single bundle endpoint. *Note* too that a given bundle node might
identify itself by multiple endpoint IDs and thus be a member of
multiple bundle endpoints.
The destination of every bundle is an endpoint, which may or may not
be singleton. The source of every bundle is a node, identified by
the endpoint ID for some singleton endpoint that contains that node.
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 newly 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.
The minimum reception group of an endpoint may be any one of the
following: (a) ALL of the nodes registered in an endpoint that is
permitted to contain multiple nodes (in which case forwarding to the
endpoint is functionally similar to "multicast" operations in the
Internet, though possibly very different in implementation); (b) ANY
N of the nodes registered in an endpoint that is permitted to
contain multiple nodes, where N is in the range from zero to the
cardinality of the endpoint; 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
typically be determined from the endpoint's ID. 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.
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When the bundle protocol agent of a node determines that a bundle
must be forwarded to a node (either to a node that is a member of
the bundle's destination endpoint or to 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.
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 newly 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.
Whenever, for some implementation-specific reason, a node's BPA
finds it impossible to immediately deliver a bundle that is
deliverable, delivery of the bundle has failed. In this event, the
delivery failure action associated with the applicable registration
must be taken. Delivery failure action MUST be one of the following:
. defer delivery 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 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 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.
While the state of a registration is Active, reception of a bundle
that is deliverable subject to this registration MUST cause the
bundle to be delivered automatically as soon as it is the next
bundle that is due for delivery according to the BPA's bundle
delivery scheduling policy, an implementation matter.
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Normally only registrations' registered delivery failure actions
cause deliveries to be abandoned.
Custody of a bundle MAY be taken only if the destination of the
bundle is a singleton endpoint. Custody MAY be released only 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.
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 must be forwarded, but forwarding is
impossible, BP must recover from this error. BP can "return" the
bundle back toward some node for forwarding along some other 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 or
refusal 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.
However, because custody transfer's only remedy for loss of any part
of a bundle is 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 invoking retransmission protocols at the
convergence layer.
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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.
4. Bundle Format
NOTE that only the abstract structures of blocks are defined here.
The specific bitstream that is emitted when a CLA sends a bundle
will be dictated by the applicable bundle representation
specification to which the sending and receiving nodes conform, an
implementation matter. It is important to note that not all BP
implementations are required to implement all bundle representation
specifications. 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. Bundle
representation specifications are beyond the scope of this document.
Each bundle SHALL be a concatenated sequence of at least two blocks.
The first block in the sequence MUST be a primary bundle block, and
the bundle MUST have exactly 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 [BPSEC]. Exactly one of the blocks in the sequence
MUST be a payload block, and the payload block MUST be the last
block in the sequence.
4.1. Bundle Processing Control Flags
Bundle processing control flags assert properties of the bundle as a
whole rather than of any particular block of the bundle. They are
conveyed in the primary block of the bundle.
The following properties are asserted by the bundle processing
control flags:
. The bundle is a fragment. (Boolean)
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. The bundle's payload is an administrative record. (Boolean)
. The bundle must not be fragmented. (Boolean)
. Custody transfer is requested for this bundle. (Boolean)
. The bundle's destination endpoint is a singleton. (Boolean)
. Acknowledgment by the user application is requested. (Boolean)
. Status time is requested in all status reports. (Boolean)
. Type of CRC that is present in the bundle's primary block. (An
unsigned integer CRC type code; 0 indicates that the block
contains no CRC, other valid values TBD)
. Flags requesting types of status reports (all Boolean):
o Request reporting of bundle reception.
o Request reporting of custody acceptance.
o Request reporting of bundle forwarding.
o Request reporting of bundle delivery.
o Request reporting of bundle deletion.
If the bundle processing control flags indicate that the bundle's
application data unit is an administrative record, then the custody
transfer requested flag value must be zero and all status report
request flag values must be zero.
If the custody transfer requested flag is 1, then the source node is
requesting that every receiving node accept custody of the bundle.
If the bundle's source node is omitted (i.e., the source endpoint ID
is the null endpoint, which has no members as discussed 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 value must be zero, the
"Bundle must not be fragmented" flag value must be 1, and all status
report request flag values must be zero.
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4.2. Block Processing Control Flags
The block processing control flags assert properties of individual
bundle blocks other than the primary block. They are conveyed in
the header of the block to which they pertain.
The following properties are asserted by the block processing
control flags:
. This block must be replicated in every fragment. (Boolean)
. Status report must be transmitted if this block can't be
processed. (Boolean)
. Block must be removed from the bundle if it can't be processed.
(Boolean)
. Bundle must be deleted if this block can't be processed.
(Boolean)
For each bundle whose bundle processing control flags indicate that
the bundle's application data unit is an administrative record, the
value of the "Transmit status report if block can't be processed"
flag in every block of the bundle other than the primary block must
be zero.
4.3. Identifiers
4.3.1. 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.
Note that, although endpoint IDs are URIs, implementations of the BP
service interface may support expression of endpoint IDs in some
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internationalized manner (e.g., Internationalized Resource
Identifiers (IRIs); see [RFC3987]).
Note also that the representation of an EID in the bitstream that is
emitted when a CLA sends a bundle will be dictated by the applicable
bundle representation specification to which the sending and
receiving nodes conform, an implementation matter.
The endpoint ID "dtn:none" identifies the "null endpoint", the
endpoint that by definition never has any members.
4.3.2. Node ID
For many purposes of the Bundle Protocol it is important to identify
the node that is operative in some context.
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
using that endpoint's ID.
4.4. Contents of Bundle Blocks
This section describes the contents of the primary block in detail
and the contents of all non-primary blocks in general. 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 [BPSEC]) may require
that BP implementations conforming to those protocols construct and
process additional blocks.
4.4.1. 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:
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Version: An unsigned integer value indicating the version of the
bundle protocol that constructed this block. The present document
describes version 7 of the bundle protocol.
Bundle Processing Control Flags: The Bundle Processing Control Flags
are discussed in Section 4.1 above.
Destination EID: The Destination EID field identifies the bundle
endpoint that is the bundle's destination, i.e., the endpoint that
contains the node(s) at which the bundle is to be delivered.
Source node ID: The Source node ID field identifies the bundle node
at which the bundle was initially transmitted, except that Source
node ID may be the null endpoint ID in the event that the bundle's
source chooses to remain anonymous.
Report-to EID: The Report-to EID field identifies the bundle
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 unsigned
integers 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 of these integers 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 any 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 unsigned integer 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 a
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bundle's age exceeds its lifetime, bundle nodes need no longer
retain or forward the bundle; the bundle SHOULD be deleted from the
network.
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.
Fragment Offset: If and only if the Bundle Processing Control Flags
of this Primary block indicate that the bundle is a fragment, then
the Fragment Offset field SHALL be an unsigned integer 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 SHALL be omitted from the
block.
Total Application Data Unit Length: If and only 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 SHALL be an unsigned integer 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
SHALL be omitted from the block.
If and only if the CRC type in the Bundle Processing Control Flags
of this Primary block is non-zero, a CRC SHALL be present the
primary block. The length and nature of the CRC SHALL be as
indicated by the CRC type. The CRC SHALL be 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.
4.4.2. 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, an unsigned integer. Bundle block type code 1
indicates that the block is a bundle payload block. Block type
codes 2 through 9 are explicitly reserved as noted later in
this specification. Block type codes 192 through 255 are not
reserved and are available for private and/or experimental use.
All other block type code values are reserved for future use.
. Block number, an unsigned integer. 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
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block number sequence in the bundle. The block number of the
payload block is always zero.
. Block processing control flags as discussed above.
. Block data length, an unsigned integer. 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 nature and order are
type-specific and whose aggregate length in octets is the value
of the block data length field. For the Payload Block in
particular (block type 1), there SHALL be exactly one block-
type-specific data field, the "payload", i.e., the application
data carried by this bundle.
4.5. 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.
The values of the block processing control flags indicate the action
to be taken by the bundle protocol agent when this is the case.
The extension blocks of the Bundle Security Protocol (block types 2
and 3) are defined separately in the Bundle Security Protocol
specification (work in progress).
The following extension blocks are defined in the current document.
4.5.1. 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 node ID of a custodian. The
bundle MAY contain one or more occurrences of this type of block.
4.5.2. 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.
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4.5.3. 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 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.
4.5.4. Bundle Age
The Bundle Age block, block type 8, 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 unsigned integer 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.
4.5.5. Hop Count
The Hop Count block, block type 9, contains two unsigned integers,
hop limit and hop count. 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.
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. Supplementary DTN protocol
specifications (including, but not restricted to, the Bundle
Security Protocol [BPSEC]) may augment, override, or supersede the
mandates of this document.
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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.
. 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.
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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 null endpoint ID,
indicating that the source of the bundle is anonymous, or else the
EID of a singleton endpoint whose only member is the node of which
the BPA is a component.
Step 3: Processing proceeds from Step 1 of Section 5.4.
5.3. Bundle Dispatching
The steps in dispatching a bundle are:
Step 1: If the bundle's destination endpoint is an endpoint of which
the node is a member, the bundle delivery procedure defined in
Section 5.7 MUST be followed.
Step 2: Processing proceeds from Step 1 of Section 5.4.
5.4. Bundle Forwarding
The steps in forwarding a bundle are:
Step 1: The retention constraint "Forward pending" MUST be added to
the bundle, and the bundle's "Dispatch pending" retention constraint
MUST be removed.
Step 2: The bundle protocol agent MUST determine whether or not
forwarding is contraindicated for any of the reasons listed in
Figure 12. In particular:
. The bundle protocol agent MUST determine which 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
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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 on 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
but finds it impossible to select any node(s) to forward the
bundle to, then forwarding is contraindicated.
o
o
. 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. 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 any appropriate convergence layer
adapter(s) to use in forwarding this bundle, then forwarding is
contraindicated.
. Provided the bundle protocol agent succeeded in selecting the
node(s) to forward the bundle to and additionally succeeded in
selecting the appropriate convergence layer adapter(s), the
bundle protocol agent MUST determine the applicable bundle
representation by which the bundle must be encoded when sent to
each such node so that the bundle will be intelligible when
received by that node. The manner in which applicable bundle
representations are selected is beyond the scope of this
document. If the agent finds that there are no applicable
bundle representations for any of the nodes to which the bundle
is to be sent, then forwarding is contraindicated.
Step 3: If forwarding of the bundle is determined to be
contraindicated for any of the reasons listed in Figure 12, then the
Forwarding Contraindicated procedure defined in Section 5.4.1 MUST
be followed; the remaining steps of Section 5 are skipped at this
time.
Step 4: If the bundle's custody transfer requested flag (in the
bundle processing flags field) is set to 1, then the custody
transfer procedure defined in Section 5.10.2 MUST be followed.
Step 5: For each node selected for forwarding, the bundle protocol
agent MUST encode the bundle in the selected applicable
representation(s) and then 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
protocol agent invokes convergence layer adapter services is a BPA
implementation matter. Determining the time at which each
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convergence layer adapter subsequently responds to this service
invocation by sending the bundle is a convergence-layer adapter
implementation matter. Note that:
. If the bundle contains a flow label extension block then that
flow label value MAY identify 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
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 are:
Step 1: The bundle protocol agent MUST determine whether or not to
declare failure in forwarding the bundle. 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.
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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 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
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.
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5.6. Bundle Reception
The steps in processing a bundle that has been received from another
node and decoded from its serialized representation are:
Step 1: The retention constraint "Dispatch pending" MUST be added to
the bundle.
Step 2: If the "request reporting of bundle reception" flag in the
bundle's status report request field is set to 1, then a bundle
reception status report with reason code "No additional information"
SHOULD be generated, destined for the bundle's report-to endpoint
ID.
Step 3: For each block in the bundle that is an extension block that
the bundle protocol agent cannot process:
. If the block processing flags in that block indicate that a
status report is requested in this event, then a bundle
reception status report with reason code "Block unintelligible"
SHOULD be generated, destined for the bundle's report-to
endpoint ID.
. If the block processing flags in that block indicate that the
bundle must be deleted in this event, then the bundle protocol
agent MUST delete the bundle for the reason "Block
unintelligible"; the bundle deletion procedure defined in
Section 5.13 MUST be followed and all remaining steps of the
bundle reception procedure MUST be skipped.
. If the block processing flags in that block do NOT indicate
that the bundle must be deleted in this event but do indicate
that the block must be discarded, then the bundle protocol
agent MUST remove this block from the bundle.
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.
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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).
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.
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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
fragmented bundle (that is, the bundle that is being
fragmented). Note that the payloads of fragments resulting from
different fragmentation episodes, in different parts of the
network, may be overlapping subsets of the fragmented bundle's
payload.
. The primary block of each fragment MUST differ from that of the
fragmented bundle, in that the bundle processing flags of the
fragment MUST indicate that the bundle is a fragment and both
fragment offset and total application data unit length must be
provided. Additionally, the CRC of the fragmented bundle, if
any, MUST be replaced in each fragment by a new CRC computed
for the primary block of that fragment.
. The payload blocks of fragments will differ from that of the
fragmented bundle as noted above.
. If the fragmented bundle is not a fragment or is the fragment
with offset zero, then all extension blocks of the fragmented
bundle MUST be replicated in the fragment whose offset is zero.
. Each of the fragmented bundle's extension blocks whose "Block
must be replicated in every fragment" flag 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 is a custodian of the fragmented bundle, then
the BPA MUST release custody of the fragmented bundle before
fragmentation occurs and MUST take custody of every fragment.
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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.
. The BPA MUST take custody of 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 then release custody of every fragment whose
payload is a subset of the reassembled application data unit,
for which it has taken custody.
. 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
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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)
but has not yet been transmitted, then this report SHOULD be
generated by simply turning on the "Reporting node accepted custody
of bundle" flag in that earlier report.
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
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.
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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:
. 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.
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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:
. Record type code (an unsigned integer for which valid values
are as defined below).
. Record content in type-specific format.
Valid administrative record type codes are defined as follows:
+---------+--------------------------------------------+
| Value | Meaning |
+=========+============================================+
| 1 | Bundle status report. |
+---------+--------------------------------------------+
| 2 | Custody signal. |
+---------+--------------------------------------------+
| (other) | Reserved for future use. |
+---------+--------------------------------------------+
Figure 2: Administrative Record Type Codes
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
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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.
Every bundle status report comprises the following fields, in this
order:
. Status flags. The following conditions are asserted by the
bundle status report status flags (all Boolean):
o Reporting node received bundle.
o Reporting node accepted custody of bundle.
o Reporting node forwarded the bundle.
o Reporting node delivered the bundle.
o Reporting node deleted the bundle.
. Reason code, an unsigned integer explaining the values of the
status flags. Status report reason codes are as defined below,
but 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 [BPSEC]) may define additional reason codes.
. Status times, one unsigned integer for each condition asserted
by any status flag, indicating the time (as reported by the
local system clock, an implementation matter) at which the
indicated condition became true for this bundle. These fields
are included in the status report if and only if the "Report
status time" flag was set to 1 in the subject bundle's bundle
processing flags. Status time is expressed in seconds since
the start of the year 2000, on the Coordinated Universal Time
(UTC) scale [UTC].
. Source node, the node ID of the source of the bundle whose
status is being reported.
. Creation timestamp, the creation timestamp of the bundle whose
status is being reported.
. Fragment offset, the fragment offset of the bundle whose status
is being reported (omitted if omitted from the subject bundle's
primary block).
. Fragment length, the length of the payload of the bundle whose
status is being reported (omitted if fragment offset is omitted
from the subject bundle's primary block).
Valid status report reason codes are defined as follows:
+---------+--------------------------------------------+
| Value | Meaning |
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+=========+============================================+
| 0 | No additional information. |
+---------+--------------------------------------------+
| 1 | Lifetime expired. |
+---------+--------------------------------------------+
| 2 | Forwarded over unidirectional link. |
+---------+--------------------------------------------+
| 3 | Transmission canceled. |
+---------+--------------------------------------------+
| 4 | Depleted storage. |
+---------+--------------------------------------------+
| 5 | Destination endpoint ID unintelligible. |
+---------+--------------------------------------------+
| 6 | No known route to destination from here. |
+---------+--------------------------------------------+
| 7 | No timely contact with next node on route. |
+---------+--------------------------------------------+
| 8 | Block unintelligible. |
+---------+--------------------------------------------+
| (other) | Reserved for future use. |
+---------+--------------------------------------------+
Figure 3: Status Report Reason Codes
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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.
Every custody signal comprises the following fields, in this order:
. "Custody transfer succeeded" flag (Boolean).
. Reason code, an unsigned integer explaining the value of the
"Custody transfer succeeded" flag. Custody signal reason codes
are as defined below.
. Source node, the node ID of the source of the bundle for which
custodial activity is being reported.
. Creation timestamp, the creation timestamp of the bundle for
which custodial activity is being reported.
. Fragment offset, the fragment offset of the bundle for which
custodial activity is being reported (omitted if omitted from
the subject bundle's primary block).
. Fragment length, the length of the payload of the bundle for
which custodial activity is being reported (omitted if fragment
offset is omitted from the subject bundle's primary block).
Valid custody signal reason codes are defined as follows:
+---------+--------------------------------------------+
| Value | Meaning |
+=========+============================================+
| 0 | No additional information. |
+---------+--------------------------------------------+
| 1 | Reserved for future use. |
+---------+--------------------------------------------+
| 2 | Reserved for future use. |
+---------+--------------------------------------------+
| 3 | Redundant (reception by a node that is a |
| | custodial node for this bundle). |
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+---------+--------------------------------------------+
| 4 | Depleted storage. |
+---------+--------------------------------------------+
| 5 | Destination endpoint ID unintelligible. |
+---------+--------------------------------------------+
| 6 | No known route destination from here. |
+---------+--------------------------------------------+
| 7 | No timely contact with next node on route. |
+---------+--------------------------------------------+
| 8 | Block unintelligible. |
+---------+--------------------------------------------+
| (other) | Reserved for future use. |
+---------+--------------------------------------------+
Figure 4: Custody Signal Reason Codes
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
contain the fragment offset and fragment length.
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
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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 [BPSEC]) may expect convergence layer adapters
that 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
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Protocol specification [BPSEC]; an informative overview of this
architecture is provided in [SECO].
The bundle protocol has been designed with the notion that it may 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.
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.
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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.
[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.
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[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.
[BPSEC] Birrane, E., "Bundle Security Protocol Specification", Work
In Progress, October 2015.
[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,
Michael Demmer of the University of California at Berkeley, Robert
Durst, Keith Scott, and Susan Symington of The MITRE Corporation,
Kevin Fall of Carnegie Mellon University, 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:
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. 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.
. Restructure primary block, making it immutable. Add optional
CRC.
. 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 conflict between fragmentation and custody transfer
that Ed Birrane pointed out.
. Remove representation specifications from the document, making
the protocol specification representation-neutral.
13. Open Issues
13.1. Application Agent
Need to add a diagram explaining how the various components of the
BPA interact.
13.2. 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.?
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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) 2016 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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