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Network Working Group                                            V. CerfRequest for Comments: 4838              Google/Jet Propulsion LaboratoryCategory: Informational                                      S. Burleigh                                                                A. Hooke                                                            L. Torgerson                                          NASA/Jet Propulsion Laboratory                                                                R. Durst                                                                K. Scott                                                   The MITRE Corporation                                                                 K. Fall                                                       Intel Corporation                                                                H. Weiss                                                            SPARTA, Inc.                                                              April 2007Delay-Tolerant Networking ArchitectureStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The IETF Trust (2007).IESG Note   This RFC is a product of the Internet Research Task Force and is not   a candidate for any level of Internet Standard.  The IRTF publishes   the results of Internet-related research and development activities.   These results might not be suitable for deployment on the public   Internet.Abstract   This document describes an architecture for delay-tolerant and   disruption-tolerant networks, and is an evolution of the architecture   originally designed for the Interplanetary Internet, a communication   system envisioned to provide Internet-like services across   interplanetary distances in support of deep space exploration.  This   document describes an architecture that addresses a variety of   problems with internetworks having operational and performance   characteristics that make conventional (Internet-like) networking   approaches either unworkable or impractical.  We define a message-   oriented overlay that exists above the transport (or other) layers ofCerf, et al.                 Informational                      [Page 1]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   the networks it interconnects.  The document presents a motivation   for the architecture, an architectural overview, review of state   management required for its operation, and a discussion of   application design issues.  This document represents the consensus of   the IRTF DTN research group and has been widely reviewed by that   group.Table of Contents1. Introduction ....................................................32. Why an Architecture for Delay-Tolerant Networking? ..............43. DTN Architectural Description ...................................5      3.1. Virtual Message Switching Using Store-and-Forward           Operation ..................................................53.2. Nodes and Endpoints ........................................73.3. Endpoint Identifiers (EIDs) and Registrations ..............83.4. Anycast and Multicast .....................................103.5. Priority Classes ..........................................103.6. Postal-Style Delivery Options and Administrative Records ..113.7. Primary Bundle Fields .....................................153.8. Routing and Forwarding ....................................163.9. Fragmentation and Reassembly ..............................183.10. Reliability and Custody Transfer .........................193.11. DTN Support for Proxies and Application Layer Gateways ...213.12. Timestamps and Time Synchronization ......................223.13. Congestion and Flow Control at the Bundle Layer ..........223.14. Security .................................................234. State Management Considerations ................................254.1. Application Registration State ............................254.2. Custody Transfer State ....................................264.3. Bundle Routing and Forwarding State .......................264.4. Security-Related State ....................................274.5. Policy and Configuration State ............................275. Application Structuring Issues .................................28   6. Convergence Layer Considerations for Use of Underlying      Protocols ......................................................287. Summary ........................................................298. Security Considerations ........................................299. IANA Considerations ............................................3010. Normative References ..........................................3011. Informative References ........................................3012. Acknowledgments ...............................................32Cerf, et al.                 Informational                      [Page 2]

RFC 4838         Delay-Tolerant Networking Architecture       April 20071.  Introduction   This document describes an architecture for delay and disruption-   tolerant interoperable networking (DTN).  The architecture embraces   the concepts of occasionally-connected networks that may suffer from   frequent partitions and that may be comprised of more than one   divergent set of protocols or protocol families.  The basis for this   architecture lies with that of the Interplanetary Internet, which   focused primarily on the issue of deep space communication in high-   delay environments.  We expect the DTN architecture described here to   be utilized in various operational environments, including those   subject to disruption and disconnection and those with high-delay;   the case of deep space is one specialized example of these, and is   being pursued as a specialization of this architecture (See [IPN01]   and [SB03] for more details).   Other networks to which we believe this architecture applies include   sensor-based networks using scheduled intermittent connectivity,   terrestrial wireless networks that cannot ordinarily maintain end-to-   end connectivity, satellite networks with moderate delays and   periodic connectivity, and underwater acoustic networks with moderate   delays and frequent interruptions due to environmental factors.  A   DTN tutorial [FW03], aimed at introducing DTN and the types of   networks for which it is designed, is available to introduce new   readers to the fundamental concepts and motivation.  More technical   descriptions may be found in [KF03], [JFP04], [JDPF05], and [WJMF05].   We define an end-to-end message-oriented overlay called the "bundle   layer" that exists at a layer above the transport (or other) layers   of the networks on which it is hosted and below applications.   Devices implementing the bundle layer are called DTN nodes.  The   bundle layer forms an overlay that employs persistent storage to help   combat network interruption.  It includes a hop-by-hop transfer of   reliable delivery responsibility and optional end-to-end   acknowledgement.  It also includes a number of diagnostic and   management features.  For interoperability, it uses a flexible naming   scheme (based on Uniform Resource Identifiers [RFC3986]) capable of   encapsulating different naming and addressing schemes in the same   overall naming syntax.  It also has a basic security model,   optionally enabled, aimed at protecting infrastructure from   unauthorized use.   The bundle layer provides functionality similar to the internet layer   of gateways described in the original ARPANET/Internet designs   [CK74].  It differs from ARPANET gateways, however, because it is   layer-agnostic and is focused on virtual message forwarding rather   than packet switching.  However, both generally provide   interoperability between underlying protocols specific to oneCerf, et al.                 Informational                      [Page 3]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   environment and those protocols specific to another, and both provide   a store-and-forward forwarding service (with the bundle layer   employing persistent storage for its store and forward function).   In a sense, the DTN architecture provides a common method for   interconnecting heterogeneous gateways or proxies that employ store-   and-forward message routing to overcome communication disruptions.   It provides services similar to electronic mail, but with enhanced   naming, routing, and security capabilities.  Nodes unable to support   the full capabilities required by this architecture may be supported   by application-layer proxies acting as DTN applications.2.  Why an Architecture for Delay-Tolerant Networking?   Our motivation for pursuing an architecture for delay tolerant   networking stems from several factors.  These factors are summarized   below; much more detail on their rationale can be explored in [SB03],   [KF03], and [DFS02].   The existing Internet protocols do not work well for some   environments, due to some fundamental assumptions built into the   Internet architecture:   - that an end-to-end path between source and destination exists for     the duration of a communication session   - (for reliable communication) that retransmissions based on timely     and stable feedback from data receivers is an effective means for     repairing errors   - that end-to-end loss is relatively small   - that all routers and end stations support the TCP/IP protocols   - that applications need not worry about communication performance   - that endpoint-based security mechanisms are sufficient for meeting     most security concerns   - that packet switching is the most appropriate abstraction for     interoperability and performance   - that selecting a single route between sender and receiver is     sufficient for achieving acceptable communication performance   The DTN architecture is conceived to relax most of these assumptions,   based on a number of design principles that are summarized here (and   further discussed in [KF03]):Cerf, et al.                 Informational                      [Page 4]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   - Use variable-length (possibly long) messages (not streams or     limited-sized packets) as the communication abstraction to help     enhance the ability of the network to make good scheduling/path     selection decisions when possible.   - Use a naming syntax that supports a wide range of naming and     addressing conventions to enhance interoperability.   - Use storage within the network to support store-and-forward     operation over multiple paths, and over potentially long timescales     (i.e., to support operation in environments where many and/or no     end-to-end paths may ever exist); do not require end-to-end     reliability.   - Provide security mechanisms that protect the infrastructure from     unauthorized use by discarding traffic as quickly as possible.   - Provide coarse-grained classes of service, delivery options, and a     way to express the useful lifetime of data to allow the network to     better deliver data in serving the needs of applications.   The use of the bundle layer is guided not only by its own design   principles, but also by a few application design principles:   - Applications should minimize the number of round-trip exchanges.   - Applications should cope with restarts after failure while network     transactions remain pending.   - Applications should inform the network of the useful life and     relative importance of data to be delivered.   These issues are discussed in further detail inSection 5.3.  DTN Architectural Description   The previous section summarized the design principles that guide the   definition of the DTN architecture.  This section presents a   description of the major features of the architecture resulting from   design decisions guided by the aforementioned design principles.3.1.  Virtual Message Switching Using Store-and-Forward Operation   A DTN-enabled application sends messages of arbitrary length, also   called Application Data Units or ADUs [CT90], which are subject to   any implementation limitations.  The relative order of ADUs might not   be preserved.  ADUs are typically sent by and delivered toCerf, et al.                 Informational                      [Page 5]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   applications in complete units, although a system interface that   behaves differently is not precluded.   ADUs are transformed by the bundle layer into one or more protocol   data units called "bundles", which are forwarded by DTN nodes.   Bundles have a defined format containing two or more "blocks" of   data.  Each block may contain either application data or other   information used to deliver the containing bundle to its   destination(s).  Blocks serve the purpose of holding information   typically found in the header or payload portion of protocol data   units in other protocol architectures.  The term "block" is used   instead of "header" because blocks may not appear at the beginning of   a bundle due to particular processing requirements (e.g., digital   signatures).   Bundles may be split up ("fragmented") into multiple constituent   bundles (also called "fragments" or "bundle fragments") during   transmission.  Fragments are themselves bundles, and may be further   fragmented.  Two or more fragments may be reassembled anywhere in the   network, forming a new bundle.   Bundle sources and destinations are identified by (variable-length)   Endpoint Identifiers (EIDs, described below), which identify the   original sender and final destination(s) of bundles, respectively.   Bundles also contain a "report-to" EID used when special operations   are requested to direct diagnostic output to an arbitrary entity   (e.g., other than the source).  An EID may refer to one or more DTN   nodes (i.e., for multicast destinations or "report-to" destinations).   While IP networks are based on "store-and-forward" operation, there   is an assumption that the "storing" will not persist for more than a   modest amount of time, on the order of the queuing and transmission   delay.  In contrast, the DTN architecture does not expect that   network links are always available or reliable, and instead expects   that nodes may choose to store bundles for some time.  We anticipate   that most DTN nodes will use some form of persistent storage for this   -- disk, flash memory, etc. -- and that stored bundles will survive   system restarts.   Bundles contain an originating timestamp, useful life indicator, a   class of service designator, and a length.  This information provides   bundle-layer routing with a priori knowledge of the size and   performance requirements of requested data transfers.  When there is   a significant amount of queuing that can occur in the network (as is   the case in the DTN version of store-and-forward), the advantage   provided by knowing this information may be significant for making   scheduling and path selection decisions [JFP04].  An alternative   abstraction (i.e., of stream-based delivery based on packets) wouldCerf, et al.                 Informational                      [Page 6]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   make such scheduling much more difficult.  Although packets provide   some of the same benefits as bundles, larger aggregates provide a way   for the network to apply scheduling and buffer management to units of   data that are more useful to applications.   An essential element of the bundle-based style of forwarding is that   bundles have a place to wait in a queue until a communication   opportunity ("contact") is available.  This highlights the following   assumptions:   1. that storage is available and well-distributed throughout the      network,   2. that storage is sufficiently persistent and robust to store      bundles until forwarding can occur, and   3. (implicitly) that this "store-and-forward" model is a better      choice than attempting to effect continuous connectivity or other      alternatives.   For a network to effectively support the DTN architecture, these   assumptions must be considered and must be found to hold.  Even so,   the inclusion of long-term storage as a fundamental aspect of the DTN   architecture poses new problems, especially with respect to   congestion management and denial-of-service mitigation.  Node storage   in essence represents a new resource that must be managed and   protected.  Much of the research in DTN revolves around exploring   these issues.  Congestion is discussed inSection 3.13, and security   mechanisms, including methods for DTN nodes to protect themselves   from handling unauthorized traffic from other nodes, are discussed in   [DTNSEC] and [DTNSOV].3.2.  Nodes and Endpoints   A DTN node (or simply "node" in this document) is an engine for   sending and receiving bundles -- an implementation of the bundle   layer.  Applications utilize DTN nodes to send or receive ADUs   carried in bundles (applications also use DTN nodes when acting as   report-to destinations for diagnostic information carried in   bundles).  Nodes may be members of groups called "DTN endpoints".  A   DTN endpoint is therefore a set of DTN nodes.  A bundle is considered   to have been successfully delivered to a DTN endpoint when some   minimum subset of the nodes in the endpoint has received the bundle   without error.  This subset is called the "minimum reception group"   (MRG) of the endpoint.  The MRG of an endpoint may refer to one node   (unicast), one of a group of nodes (anycast), or all of a group of   nodes (multicast and broadcast).  A single node may be in the MRG of   multiple endpoints.Cerf, et al.                 Informational                      [Page 7]

RFC 4838         Delay-Tolerant Networking Architecture       April 20073.3.  Endpoint Identifiers (EIDs) and Registrations   An Endpoint Identifier (EID) is a name, expressed using the general   syntax of URIs (see below), that identifies a DTN endpoint.  Using an   EID, a node is able to determine the MRG of the DTN endpoint named by   the EID.  Each node is also required to have at least one EID that   uniquely identifies it.   Applications send ADUs destined for an EID, and may arrange for ADUs   sent to a particular EID to be delivered to them.  Depending on the   construction of the EID being used (see below), there may be a   provision for wildcarding some portion of an EID, which is often   useful for diagnostic and routing purposes.   An application's desire to receive ADUs destined for a particular EID   is called a "registration", and in general is maintained persistently   by a DTN node.  This allows application registration information to   survive application and operating system restarts.   An application's attempt to establish a registration is not   guaranteed to succeed.  For example, an application could request to   register itself to receive ADUs by specifying an Endpoint ID that is   uninterpretable or unavailable to the DTN node servicing the request.   Such requests are likely to fail.3.3.1.  URI Schemes   Each Endpoint ID is expressed syntactically as a Uniform Resource   Identifier (URI) [RFC3986].  The URI syntax has been designed as a   way to express names or addresses for a wide range of purposes, and   is therefore useful for constructing names for DTN endpoints.   In URI terminology, each URI begins with a scheme name.  The scheme   name is an element of the set of globally-managed scheme names   maintained by IANA [ISCHEMES].  Lexically following the scheme name   in a URI is a series of characters constrained by the syntax defined   by the scheme.  This portion of the URI is called the scheme-specific   part (SSP), and can be quite general.  (See, as one example, the URI   scheme for SNMP [RFC4088]).  Note that scheme-specific syntactical   and semantic restrictions may be more constraining than the basic   rules ofRFC 3986.Section 3.1 of RFC 3986 provides guidance on the   syntax of scheme names.   URI schemes are a key concept in the DTN architecture, and evolved   from an earlier concept called regions, which were tied more closely   to assumptions of the network topology.  Using URIs, significant   flexibility is attained in the structuring of EIDs.  They might, for   example, be constructed based on DNS names, or might look likeCerf, et al.                 Informational                      [Page 8]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   "expressions of interest" or forms of database-like queries as in a   directed diffusion-routed network [IGE00] or in intentional naming   [WSBL99].  As names, EIDs are not required to be related to routing   or topological organization.  Such a relationship is not prohibited,   however, and in some environments using EIDs this way may be   advantageous.   A single EID may refer to an endpoint containing more than one DTN   node, as suggested above.  It is the responsibility of a scheme   designer to define how to interpret the SSP of an EID so as to   determine whether it refers to a unicast, multicast, or anycast set   of nodes.  SeeSection 3.4 for more details.   URIs are constructed based on rules specified inRFC 3986, using the   US-ASCII character set.  However, note this excerpt fromRFC 3986,   Section 1.2.1, on dealing with characters that cannot be represented   by US-ASCII:  "Percent-encoded octets (Section 2.1) may be used   within a URI to represent characters outside the range of the US-   ASCII coded character set if this representation is allowed by the   scheme or by the protocol element in which the URI is referenced.   Such a definition should specify the character encoding used to map   those characters to octets prior to being percent-encoded for the   URI".3.3.2.  Late Binding   Binding means interpreting the SSP of an EID for the purpose of   carrying an associated message towards a destination.  For example,   binding might require mapping an EID to a next-hop EID or to a lower-   layer address for transmission.  "Late binding" means that the   binding of a bundle's destination to a particular set of destination   identifiers or addresses does not necessarily happen at the bundle   source.  Because the destination EID is potentially re-interpreted at   each hop, the binding may occur at the source, during transit, or   possibly at the destination(s).  This contrasts with the name-to-   address binding of Internet communications where a DNS lookup at the   source fixes the IP address of the destination node before data is   sent.  Such a circumstance would be considered "early binding"   because the name-to-address translation is performed prior to data   being sent into the network.   In a frequently-disconnected network, late binding may be   advantageous because the transit time of a message may exceed the   validity time of a binding, making binding at the source impossible   or invalid.  Furthermore, use of name-based routing with late binding   may reduce the amount of administrative (mapping) information thatCerf, et al.                 Informational                      [Page 9]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   must propagate through the network, and may also limit the scope of   mapping synchronization requirements to a local topological   neighborhood where changes are made.3.4.  Anycast and Multicast   As mentioned above, an EID may refer to an endpoint containing one or   more DTN nodes.  When referring to a group of size greater than one,   the delivery semantics may be of either the anycast or multicast   variety (broadcast is considered to be of the multicast variety).   For anycast group delivery, a bundle is delivered to one node among a   group of potentially many nodes, and for multicast delivery it is   intended to be delivered to all of them, subject to the normal DTN   class of service and maximum useful lifetime semantics.   Multicast group delivery in a DTN presents an unfamiliar issue with   respect to group membership.  In relatively low-delay networks, such   as the Internet, nodes may be considered to be part of the group if   they have expressed interest to join it "recently".  In a DTN,   however, nodes may wish to receive data sent to a group during an   interval of time earlier than when they are actually able to receive   it [ZAZ05].  More precisely, an application expresses its desire to   receive data sent to EID e at time t.  Prior to this, during the   interval [t0, t1], t > t1, data may have been generated for group e.   For the application to receive any of this data, the data must be   available a potentially long time after senders have ceased sending   to the group.  Thus, the data may need to be stored within the   network in order to support temporal group semantics of this kind.   How to design and implement this remains a research issue, as it is   likely to be at least as hard as problems related to reliable   multicast.3.5.  Priority Classes   The DTN architecture offers *relative* measures of priority (low,   medium, high) for delivering ADUs.  These priorities differentiate   traffic based upon an application's desire to affect the delivery   urgency for ADUs, and are carried in bundle blocks generated by the   bundle layer based on information specified by the application.   The (U.S. or similar) Postal Service provides a strong metaphor for   the priority classes offered by the forwarding abstraction offered by   the DTN architecture.  Traffic is generally not interactive and is   often one-way.  There are generally no strong guarantees of timely   delivery, yet there are some forms of class of service, reliability,   and security.Cerf, et al.                 Informational                     [Page 10]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   We have defined three relative priority classes to date.  These   priority classes typically imply some relative scheduling   prioritization among bundles in queue at a sender:   - Bulk - Bulk bundles are shipped on a "least effort" basis.  No     bundles of this class will be shipped until all bundles of other     classes bound for the same destination and originating from the     same source have been shipped.   - Normal - Normal-class bundles are shipped prior to any bulk-class     bundles and are otherwise the same as bulk bundles.   - Expedited - Expedited bundles, in general, are shipped prior to     bundles of other classes and are otherwise the same.   Applications specify their requested priority class and data lifetime   (see below) for each ADU they send.  This information, coupled with   policy applied at DTN nodes that select how messages are forwarded   and which routing algorithms are in use, affects the overall   likelihood and timeliness of ADU delivery.   The priority class of a bundle is only required to relate to other   bundles from the same source.  This means that a high priority bundle   from one source may not be delivered faster (or with some other   superior quality of service) than a medium priority bundle from a   different source.  It does mean that a high priority bundle from one   source will be handled preferentially to a lower priority bundle sent   from the same source.   Depending on a particular DTN node's forwarding/scheduling policy,   priority may or may not be enforced across different sources.  That   is, in some DTN nodes, expedited bundles might always be sent prior   to any bulk bundles, irrespective of source.  Many variations are   possible.3.6.  Postal-Style Delivery Options and Administrative Records   Continuing with the postal analogy, the DTN architecture supports   several delivery options that may be selected by an application when   it requests the transmission of an ADU.  In addition, the   architecture defines two types of administrative records: "status   reports" and "signals".  These records are bundles that provide   information about the delivery of other bundles, and are used in   conjunction with the delivery options.Cerf, et al.                 Informational                     [Page 11]

RFC 4838         Delay-Tolerant Networking Architecture       April 20073.6.1.  Delivery Options   We have defined eight delivery options.  Applications sending an ADU   (the "subject ADU") may request any combination of the following,   which are carried in each of the bundles produced ("sent bundles") by   the bundle layer resulting from the application's request to send the   subject ADU:   - Custody Transfer Requested - requests sent bundles be delivered     with enhanced reliability using custody transfer procedures.  Sent     bundles will be transmitted by the bundle layer using reliable     transfer protocols (if available), and the responsibility for     reliable delivery of the bundle to its destination(s) may move     among one or more "custodians" in the network.  This capability is     described in more detail inSection 3.10.   - Source Node Custody Acceptance Required - requires the source DTN     node to provide custody transfer for the sent bundles.  If custody     transfer is not available at the source when this delivery option     is requested, the requested transmission fails.  This provides a     means for applications to insist that the source DTN node take     custody of the sent bundles (e.g., by storing them in persistent     storage).   - Report When Bundle Delivered - requests a (single) Bundle Delivery     Status Report be generated when the subject ADU is delivered to its     intended recipient(s).  This request is also known as "return-     receipt".   - Report When Bundle Acknowledged by Application - requests an     Acknowledgement Status Report be generated when the subject ADU is     acknowledged by a receiving application.  This only happens by     action of the receiving application, and differs from the Bundle     Delivery Status Report.  It is intended for cases where the     application may be acting as a form of application layer gateway     and wishes to indicate the status of a protocol operation external     to DTN back to the requesting source.  SeeSection 11 for more     details.   - Report When Bundle Received - requests a Bundle Reception Status     Report be generated when each sent bundle arrives at a DTN node.     This is designed primarily for diagnostic purposes.   - Report When Bundle Custody Accepted  - requests a Custody     Acceptance Status Report be generated when each sent bundle has     been accepted using custody transfer.  This is designed primarily     for diagnostic purposes.Cerf, et al.                 Informational                     [Page 12]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   - Report When Bundle Forwarded - requests a Bundle Forwarding Status     Report be generated when each sent bundle departs a DTN node after     forwarding.  This is designed primarily for diagnostic purposes.   - Report When Bundle Deleted - requests a Bundle Deletion Status     Report be generated when each sent bundle is deleted at a DTN node.     This is designed primarily for diagnostic purposes.   The first four delivery options are designed for ordinary use by   applications.  The last four are designed primarily for diagnostic   purposes and their use may be restricted or limited in environments   subject to congestion or attack.   If the security procedures defined in [DTNSEC] are also enabled, then   three additional delivery options become available:   - Confidentiality Required - requires the subject ADU be made secret     from parties other than the source and the members of the     destination EID.   - Authentication Required - requires all non-mutable fields in the     bundle blocks of the sent bundles (i.e., those which do not change     as the bundle is forwarded) be made strongly verifiable (i.e.,     cryptographically strong).  This protects several fields, including     the source and destination EIDs and the bundle's data.  SeeSection3.7 and [BSPEC] for more details.   - Error Detection Required - requires modifications to the non-     mutable fields of each sent bundle be made detectable with high     probability at each destination.3.6.2.  Administrative Records: Bundle Status Reports and Custody        Signals   Administrative records are used to report status information or error   conditions related to the bundle layer.  There are two types of   administrative records defined:  bundle status reports (BSRs) and   custody signals.  Administrative records correspond (approximately)   to messages in the ICMP protocol in IP [RFC792].  In ICMP, however,   messages are returned to the source.  In DTN, they are instead   directed to the report-to EID for BSRs and the EID of the current   custodian for custody signals, which might differ from the source's   EID.  Administrative records are sent as bundles with a source EID   set to one of the EIDs associated with the DTN node generating the   administrative record.  In some cases, arrival of a single bundle or   bundle fragment may elicit multiple administrative records (e.g., in   the case where a bundle is replicated for multicast forwarding).Cerf, et al.                 Informational                     [Page 13]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   The following BSRs are currently defined (also see [BSPEC] for more   details):   - Bundle Reception - sent when a bundle arrives at a DTN node.     Generation of this message may be limited by local policy.   - Custody Acceptance - sent when a node has accepted custody of a     bundle with the Custody Transfer Requested option set.  Generation     of this message may be limited by local policy.   - Bundle Forwarded - sent when a bundle containing a Report When     Bundle Forwarded option departs from a DTN node after having been     forwarded.  Generation of this message may be limited by local     policy.   - Bundle Deletion - sent from a DTN node when a bundle containing a     Report When Bundle Deleted option is discarded.  This can happen     for several reasons, such as expiration.  Generation of this     message may be limited by local policy but is required in cases     where the deletion is performed by a bundle's current custodian.   - Bundle Delivery - sent from a final recipient's (destination) node     when a complete ADU comprising sent bundles containing Report When     Bundle Delivered options is consumed by an application.   - Acknowledged by application - sent from a final recipient's     (destination) node when a complete ADU comprising sent bundles     containing Application Acknowledgment options has been processed by     an application.  This generally involves specific action on the     receiving application's part.   In addition to the status reports, the custody signal is currently   defined to indicate the status of a custody transfer.  These are sent   to the current-custodian EID contained in an arriving bundle:   - Custody Signal - indicates that custody has been successfully     transferred.  This signal appears as a Boolean indicator, and may     therefore indicate either a successful or a failed custody transfer     attempt.   Administrative records must reference a received bundle.  This is   accomplished by a method for uniquely identifying bundles based on a   transmission timestamp and sequence number discussed inSection 3.12.Cerf, et al.                 Informational                     [Page 14]

RFC 4838         Delay-Tolerant Networking Architecture       April 20073.7.  Primary Bundle Fields   The bundles carried between and among DTN nodes obey a standard   bundle protocol specified in [BSPEC].  Here we provide an overview of   most of the fields carried with every bundle.  The protocol is   designed with a mandatory primary block, an optional payload block   (which contains the ADU data itself), and a set of optional extension   blocks.  Blocks may be cascaded in a way similar to extension headers   in IPv6.  The following selected fields are all present in the   primary block, and therefore are present for every bundle and   fragment:   - Creation Timestamp - a concatenation of the bundle's creation time     and a monotonically increasing sequence number such that the     creation timestamp is guaranteed to be unique for each ADU     originating from the same source.  The creation timestamp is based     on the time-of-day an application requested an ADU to be sent (not     when the corresponding bundle(s) are sent into the network).  DTN     nodes are assumed to have a basic time synchronization capability     (seeSection 3.12).   - Lifespan - the time-of-day at which the message is no longer     useful.  If a bundle is stored in the network (including the     source's DTN node) when its lifespan is reached, it may be     discarded.  The lifespan of a bundle is expressed as an offset     relative to its creation time.   - Class of Service Flags - indicates the delivery options and     priority class for the bundle.  Priority classes may be one of     bulk, normal, or expedited.  SeeSection 3.6.1.   - Source EID - EID of the source (the first sender).   - Destination EID - EID of the destination (the final intended     recipient(s)).   - Report-To Endpoint ID - an EID identifying where reports (return-     receipt, route-tracing functions) should be sent.  This may or may     not identify the same endpoint as the Source EID.   - Custodian EID - EID of the current custodian of a bundle (if any).   The payload block indicates information about the contained payload   (e.g., its length) and the payload itself.  In addition to the fields   found in the primary and payload blocks, each bundle may have fields   in additional blocks carried with each bundle.  See [BSPEC] for   additional details.Cerf, et al.                 Informational                     [Page 15]

RFC 4838         Delay-Tolerant Networking Architecture       April 20073.8.  Routing and Forwarding   The DTN architecture provides a framework for routing and forwarding   at the bundle layer for unicast, anycast, and multicast messages.   Because nodes in a DTN network might be interconnected using more   than one type of underlying network technology, a DTN network is best   described abstractly using a *multigraph* (a graph where vertices may   be interconnected with more than one edge).  Edges in this graph are,   in general, time-varying with respect to their delay and capacity and   directional because of the possibility of one-way connectivity.  When   an edge has zero capacity, it is considered to not be connected.   Because edges in a DTN graph may have significant delay, it is   important to distinguish where time is measured when expressing an   edge's capacity or delay.  We adopt the convention of expressing   capacity and delay as functions of time where time is measured at the   point where data is inserted into a network edge.  For example,   consider an edge having capacity C(t) and delay D(t) at time t.  If B   bits are placed in this edge at time t, they completely arrive by   time t + D(t) + (1/C(t))*B.  We assume C(t) and D(t) do not change   significantly during the interval [t, t+D(t)+(1/C(t))*B].   Because edges may vary between positive and zero capacity, it is   possible to describe a period of time (interval) during which the   capacity is strictly positive, and the delay and capacity can be   considered to be constant [AF03].  This period of time is called a   "contact".  In addition, the product of the capacity and the interval   is known as a contact's "volume".  If contacts and their volumes are   known ahead of time, intelligent routing and forwarding decisions can   be made (optimally for small networks) [JFP04].  Optimally using a   contact's volume, however, requires the ability to divide large ADUs   and bundles into smaller routable units.  This is provided by DTN   fragmentation (seeSection 3.9).   When delivery paths through a DTN graph are lossy or contact   intervals and volumes are not known precisely ahead of time, routing   computations become especially challenging.  How to handle these   situations is an active area of work in the (emerging) research area   of delay tolerant networking.3.8.1.  Types of Contacts   Contacts typically fall into one of several categories, based largely   on the predictability of their performance characteristics and   whether some action is required to bring them into existence.  To   date, the following major types of contacts have been defined:Cerf, et al.                 Informational                     [Page 16]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   Persistent Contacts      Persistent contacts are always available (i.e., no connection-      initiation action is required to instantiate a persistent      contact).  An 'always-on' Internet connection such as a DSL or      Cable Modem connection would be a representative of this class.   On-Demand Contacts      On-Demand contacts require some action in order to instantiate,      but then function as persistent contacts until terminated.  A      dial-up connection is an example of an On-Demand contact (at      least, from the viewpoint of the dialer; it may be viewed as an      Opportunistic Contact, below, from the viewpoint of the dial-up      service provider).   Intermittent - Scheduled Contacts      A scheduled contact is an agreement to establish a contact at a      particular time, for a particular duration.  An example of a      scheduled contact is a link with a low-earth orbiting satellite.      A node's list of contacts with the satellite can be constructed      from the satellite's schedule of view times, capacities, and      latencies.  Note that for networks with substantial delays, the      notion of the "particular time" is delay-dependent.  For example,      a single scheduled contact between Earth and Mars would not be at      the same instant in each location, but would instead be offset by      the (non-negligible) propagation delay.   Intermittent - Opportunistic Contacts      Opportunistic contacts are not scheduled, but rather present      themselves unexpectedly.  For example, an unscheduled aircraft      flying overhead and beaconing, advertising its availability for      communication, would present an opportunistic contact.  Another      type of opportunistic contact might be via an infrared or      Bluetooth communication link between a personal digital assistant      (PDA) and a kiosk in an airport concourse.  The opportunistic      contact begins as the PDA is brought near the kiosk, lasting an      undetermined amount of time (i.e., until the link is lost or      terminated).   Intermittent - Predicted Contacts      Predicted contacts are based on no fixed schedule, but rather are      predictions of likely contact times and durations based on a      history of previously observed contacts or some other information.      Given a great enough confidence in a predicted contact, routes mayCerf, et al.                 Informational                     [Page 17]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007      be chosen based on this information.  This is an active research      area, and a few approaches having been proposed [LFC05].3.9.  Fragmentation and Reassembly   DTN fragmentation and reassembly are designed to improve the   efficiency of bundle transfers by ensuring that contact volumes are   fully utilized and by avoiding retransmission of partially-forwarded   bundles.  There are two forms of DTN fragmentation/reassembly:   Proactive Fragmentation      A DTN node may divide a block of application data into multiple      smaller blocks and transmit each such block as an independent      bundle.  In this case, the *final destination(s)* are responsible      for extracting the smaller blocks from incoming bundles and      reassembling them into the original larger bundle and, ultimately,      ADU.  This approach is called proactive fragmentation because it      is used primarily when contact volumes are known (or predicted) in      advance.   Reactive Fragmentation      DTN nodes sharing an edge in the DTN graph may fragment a bundle      cooperatively when a bundle is only partially transferred.  In      this case, the receiving bundle layer modifies the incoming bundle      to indicate it is a fragment, and forwards it normally.  The      previous- hop sender may learn (via convergence-layer protocols,      seeSection 6) that only a portion of the bundle was delivered to      the next hop, and send the remaining portion(s) when subsequent      contacts become available (possibly to different next-hops if      routing changes).  This is called reactive fragmentation because      the fragmentation process occurs after an attempted transmission      has taken place.      As an example, consider a ground station G, and two store-and-      forward satellites S1 and S2, in opposite low-earth orbit.  While      G is transmitting a large bundle to S1, a reliable transport layer      protocol below the bundle layer at each indicates the transmission      has terminated, but that half the transfer has completed      successfully.  In this case, G can form a smaller bundle fragment      consisting of the second half of the original bundle and forward      it to S2 when available.  In addition, S1 (now out of range of G)      can form a new bundle consisting of the first half of the original      bundle and forward it to whatever next hop(s) it deems      appropriate.Cerf, et al.                 Informational                     [Page 18]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   The reactive fragmentation capability is not required to be available   in every DTN implementation, as it requires a certain level of   support from underlying protocols that may not be present, and   presents significant challenges with respect to handling digital   signatures and authentication codes on messages.  When a signed   message is only partially received, most message authentication codes   will fail.  When DTN security is present and enabled, it may   therefore be necessary to proactively fragment large bundles into   smaller units that are more convenient for digital signatures.   Even if reactive fragmentation is not present in an implementation,   the ability to reassemble fragments at a destination is required in   order to support DTN fragmentation.  Furthermore, for contacts with   volumes that are small compared to typical bundle sizes, some   incremental delivery approach must be used (e.g., checkpoint/restart)   to prevent data delivery livelock.  Reactive fragmentation is one   such approach, but other protocol layers could potentially handle   this issue as well.3.10.  Reliability and Custody Transfer   The most basic service provided by the bundle layer is   unacknowledged, prioritized (but not guaranteed) unicast message   delivery.  It also provides two options for enhancing delivery   reliability:  end-to-end acknowledgments and custody transfer.   Applications wishing to implement their own end-to-end message   reliability mechanisms are free to utilize the acknowledgment.  The   custody transfer feature of the DTN architecture only specifies a   coarse-grained retransmission capability, described next.   Transmission of bundles with the Custody Transfer Requested option   specified generally involves moving the responsibility for reliable   delivery of an ADU's bundles among different DTN nodes in the   network.  For unicast delivery, this will typically involve moving   bundles "closer" (in terms of some routing metric) to their ultimate   destination(s), and retransmitting when necessary.  The nodes   receiving these bundles along the way (and agreeing to accept the   reliable delivery responsibility) are called "custodians".  The   movement of a bundle (and its delivery responsibility) from one node   to another is called a "custody transfer".  It is analogous to a   database commit transaction [FHM03].  The exact meaning and design of   custody transfer for multicast and anycast delivery remains to be   fully explored.   Custody transfer allows the source to delegate retransmission   responsibility and recover its retransmission-related resources   relatively soon after sending a bundle (on the order of the minimum   round-trip time to the first bundle hop(s)).  Not all nodes in a DTNCerf, et al.                 Informational                     [Page 19]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   are required by the DTN architecture to accept custody transfers, so   it is not a true 'hop-by-hop' mechanism.  For example, some nodes may   have sufficient storage resources to sometimes act as custodians, but   may elect to not offer such services when congested or running low on   power.   The existence of custodians can alter the way DTN routing is   performed.  In some circumstances, it may be beneficial to move a   bundle to a custodian as quickly as possible even if the custodian is   further away (in terms of distance, time or some routing metric) from   the bundle's final destination(s) than some other reachable node.   Designing a system with this capability involves constructing more   than one routing graph, and is an area of continued research.   Custody transfer in DTN not only provides a method for tracking   bundles that require special handling and identifying DTN nodes that   participate in custody transfer, it also provides a (weak) mechanism   for enhancing the reliability of message delivery.  Generally   speaking, custody transfer relies on underlying reliable delivery   protocols of the networks that it operates over to provide the   primary means of reliable transfer from one bundle node to the next   (set).  However, when custody transfer is requested, the bundle layer   provides an additional coarse-grained timeout and retransmission   mechanism and an accompanying (bundle-layer) custodian-to-custodian   acknowledgment signaling mechanism.  When an application does *not*   request custody transfer, this bundle layer timeout and   retransmission mechanism is typically not employed, and successful   bundle layer delivery depends solely on the reliability mechanisms of   the underlying protocols.   When a node accepts custody for a bundle that contains the Custody   Transfer Requested option, a Custody Transfer Accepted Signal is sent   by the bundle layer to the Current Custodian EID contained in the   primary bundle block.  In addition, the Current Custodian EID is   updated to contain one of the forwarding node's (unicast) EIDs before   the bundle is forwarded.   When an application requests an ADU to be delivered with custody   transfer, the request is advisory.  In some circumstances, a source   of a bundle for which custody transfer has been requested may not be   able to provide this service.  In such circumstances, the subject   bundle may traverse multiple DTN nodes before it obtains a custodian.   Bundles in this condition are specially marked with their Current   Custodian EID field set to a null endpoint.  In cases where   applications wish to require the source to take custody of the   bundle, they may supply the Source Node Custody Acceptance RequiredCerf, et al.                 Informational                     [Page 20]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   delivery option.  This may be useful to applications that desire a   continuous "chain" of custody or that wish to exit after being   ensured their data is safely held in a custodian.   In a DTN network where one or more custodian-to-custodian hops are   strictly one directional (and cannot be reversed), the DTN custody   transfer mechanism will be affected over such hops due to the lack of   any way to receive a custody signal (or any other information) back   across the path, resulting in the expiration of the bundle at the   ingress to the one-way hop.  This situation does not necessarily mean   the bundle has been lost; nodes on the other side of the hop may   continue to transfer custody, and the bundle may be delivered   successfully to its destination(s).  However, in this circumstance a   source that has requested to receive expiration BSRs for this bundle   will receive an expiration report for the bundle, and possibly   conclude (incorrectly) that the bundle has been discarded and not   delivered.  Although this problem cannot be fully solved in this   situation, a mechanism is provided to help ameliorate the seemingly   incorrect information that may be reported when the bundle expires   after having been transferred over a one-way hop.  This is   accomplished by the node at the ingress to the one-way hop reporting   the existence of a known one-way path using a variant of a bundle   status report.  These types of reports are provided if the subject   bundle requests the report using the 'Report When Bundle Forwarded'   delivery option.3.11.  DTN Support for Proxies and Application Layer Gateways   One of the aims of DTN is to provide a common method for   interconnecting application layer gateways and proxies.  In cases   where existing Internet applications can be made to tolerate delays,   local proxies can be constructed to benefit from the existing   communication capabilities provided by DTN [S05,T02].  Making such   proxies compatible with DTN reduces the burden on the proxy author   from being concerned with how to implement routing and reliability   management and allows existing TCP/IP-based applications to operate   unmodified over a DTN-based network.   When DTN is used to provide a form of tunnel encapsulation for other   protocols, it can be used in constructing overlay networks comprised   of application layer gateways.  The application acknowledgment   capability is designed for such circumstances.  This provides a   common way for remote application layer gateways to signal the   success or failure of non-DTN protocol operations initiated as a   result of receiving DTN ADUs.  Without this capability, such   indicators would have to be implemented by applications themselves in   non-standard ways.Cerf, et al.                 Informational                     [Page 21]

RFC 4838         Delay-Tolerant Networking Architecture       April 20073.12.  Timestamps and Time Synchronization   The DTN architecture depends on time synchronization among DTN nodes   (supported by external, non-DTN protocols) for four primary purposes:   bundle and fragment identification, routing with scheduled or   predicted contacts, bundle expiration time computations, and   application registration expiration.   Bundle identification and expiration are supported by placing a   creation timestamp and an explicit expiration field (expressed in   seconds after the source timestamp) in each bundle.  The origination   timestamps on arriving bundles are made available to consuming   applications in ADUs they receive by some system interface function.   Each set of bundles corresponding to an ADU is required to contain a   timestamp unique to the sender's EID.  The EID, timestamp, and data   offset/length information together uniquely identify a bundle.   Unique bundle identification is used for a number of purposes,   including custody transfer and reassembly of bundle fragments.   Time is also used in conjunction with application registrations.   When an application expresses its desire to receive ADUs destined for   a particular EID, this registration is only maintained for a finite   period of time, and may be specified by the application.  For   multicast registrations, an application may also specify a time range   or "interest interval" for its registration.  In this case, traffic   sent to the specified EID any time during the specified interval will   eventually be delivered to the application (unless such traffic has   expired due to the expiration time provided by the application at the   source or some other reason prevents such delivery).3.13.  Congestion and Flow Control at the Bundle Layer   The subject of congestion control and flow control at the bundle   layer is one on which the authors of this document have not yet   reached complete consensus.  We have unresolved concerns about the   efficiency and efficacy of congestion and flow control schemes   implemented across long and/or highly variable delay environments,   especially with the custody transfer mechanism that may require nodes   to retain bundles for long periods of time.   For the purposes of this document, we define "flow control" as a   means of assuring that the average rate at which a sending node   transmits data to a receiving node does not exceed the average rate   at which the receiving node is prepared to receive data from that   sender. (Note that this is a generalized notion of flow control,   rather than one that applies only to end-to-end communication.)  We   define "congestion control" as a means of assuring that the aggregate   rate at which all traffic sources inject data into a network does notCerf, et al.                 Informational                     [Page 22]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   exceed the maximum aggregate rate at which the network can deliver   data to destination nodes over time.  If flow control is propagated   backward from congested nodes toward traffic sources, then the flow   control mechanism can be used as at least a partial solution to the   problem of congestion as well.   DTN flow control decisions must be made within the bundle layer   itself based on information about resources (in this case, primarily   persistent storage) available within the bundle node.  When storage   resources become scarce, a DTN node has only a certain degree of   freedom in handling the situation.  It can always discard bundles   which have expired -- an activity DTN nodes should perform regularly   in any case.  If it ordinarily is willing to accept custody for   bundles, it can cease doing so.  If storage resources are available   elsewhere in the network, it may be able to make use of them in some   way for bundle storage.  It can also discard bundles which have not   expired but for which it has not accepted custody.  A node must avoid   discarding bundles for which it has accepted custody, and do so only   as a last resort.  Determining when a node should engage in or cease   to engage in custody transfers is a resource allocation and   scheduling problem of current research interest.   In addition to the bundle layer mechanisms described above, a DTN   node may be able to avail itself of support from lower-layer   protocols in affecting its own resource utilization.  For example, a   DTN node receiving a bundle using TCP/IP might intentionally slow   down its receiving rate by performing read operations less frequently   in order to reduce its offered load.  This is possible because TCP   provides its own flow control, so reducing the application data   consumption rate could effectively implement a form of hop-by-hop   flow control.  Unfortunately, it may also lead to head-of-line   blocking issues, depending on the nature of bundle multiplexing   within a TCP connection.  A protocol with more relaxed ordering   constraints (e.g. SCTP [RFC2960]) might be preferable in such   circumstances.   Congestion control is an ongoing research topic.3.14.  Security   The possibility of severe resource scarcity in some delay-tolerant   networks dictates that some form of authentication and access control   to the network itself is required in many circumstances.  It is not   acceptable for an unauthorized user to flood the network with traffic   easily, possibly denying service to authorized users.  In many cases   it is also not acceptable for unauthorized traffic to be forwarded   over certain network links at all.  This is especially true for   exotic, mission-critical links.  In light of these considerations,Cerf, et al.                 Informational                     [Page 23]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   several goals are established for the security component of the DTN   architecture:   - Promptly prevent unauthorized applications from having their data     carried through or stored in the DTN.   - Prevent unauthorized applications from asserting control over the     DTN infrastructure.   - Prevent otherwise authorized applications from sending bundles at a     rate or class of service for which they lack permission.   - Promptly discard bundles that are damaged or improperly modified in     transit.   - Promptly detect and de-authorize compromised entities.   Many existing authentication and access control protocols designed   for operation in low-delay, connected environments may not perform   well in DTNs.  In particular, updating access control lists and   revoking ("blacklisting") credentials may be especially difficult.   Also, approaches that require frequent access to centralized servers   to complete an authentication or authorization transaction are not   attractive.  The consequences of these difficulties include delays in   the onset of communication, delays in detecting and recovering from   system compromise, and delays in completing transactions due to   inappropriate access control or authentication settings.   To help satisfy these security requirements in light of the   challenges, the DTN architecture adopts a standard but optionally   deployed security architecture [DTNSEC] that utilizes hop-by-hop and   end-to-end authentication and integrity mechanisms.  The purpose of   using both approaches is to be able to handle access control for data   forwarding and storage separately from application-layer data   integrity.  While the end-to-end mechanism provides authentication   for a principal such as a user (of which there may be many), the hop-   by-hop mechanism is intended to authenticate DTN nodes as legitimate   transceivers of bundles to each-other.  Note that it is conceivable   to construct a DTN in which only a subset of the nodes participate in   the security mechanisms, resulting in a secure DTN overlay existing   atop an insecure DTN overlay.  This idea is relatively new and is   still being explored.   In accordance with the goals listed above, DTN nodes discard traffic   as early as possible if authentication or access control checks fail.   This approach meets the goals of removing unwanted traffic from being   forwarded over specific high-value links, but also has the associated   benefit of making denial-of-service attacks considerably harder toCerf, et al.                 Informational                     [Page 24]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   mount more generally, as compared with conventional Internet routers.   However, the obvious cost for this capability is potentially larger   computation and credential storage overhead required at DTN nodes.   For more detailed information on DTN security provisions, refer to   [DTNSEC] and [DTNSOV].4.  State Management Considerations   An important aspect of any networking architecture is its management   of state.  This section describes the state managed at the bundle   layer and discusses how it is established and removed.4.1.  Application Registration State   In long/variable delay environments, an asynchronous application   interface seems most appropriate.  Such interfaces typically include   methods for applications to register callback actions when certain   triggering events occur (e.g., when ADUs arrive).  These   registrations create state information called application   registration state.   Application registration state is typically created by explicit   request of the application, and is removed by a separate explicit   request, but may also be removed by an application-specified timer   (it is thus "firm" state).  In most cases, there must be a provision   for retaining this state across application and operating system   termination/restart conditions because a client/server bundle round-   trip time may exceed the requesting application's execution time (or   hosting system's uptime).  In cases where applications are not   automatically restarted but application registration state remains   persistent, a method must be provided to indicate to the system what   action to perform when the triggering event occurs (e.g., restarting   some application, ignoring the event, etc.).   To initiate a registration and thereby establish application   registration state, an application specifies an Endpoint ID for which   it wishes to receive ADUs, along with an optional time value   indicating how long the registration should remain active.  This   operation is somewhat analogous to the bind() operation in the common   sockets API.   For registrations to groups (i.e., joins), a time interval may also   be specified.  The time interval refers to the range of origination   times of ADUs sent to the specified EID.  SeeSection 3.4 above for   more details.Cerf, et al.                 Informational                     [Page 25]

RFC 4838         Delay-Tolerant Networking Architecture       April 20074.2.  Custody Transfer State   Custody transfer state includes information required to keep account   of bundles for which a node has taken custody, as well as the   protocol state related to transferring custody for one or more of   them.  The accounting-related state is created when a bundle is   received.  Custody transfer retransmission state is created when a   transfer of custody is initiated by forwarding a bundle with the   custody transfer requested delivery option specified.  Retransmission   state and accounting state may be released upon receipt of one or   more Custody Transfer Succeeded signals, indicating custody has been   moved.  In addition, the bundle's expiration time (possibly mitigated   by local policy) provides an upper bound on the time when this state   is purged from the system in the event that it is not purged   explicitly due to receipt of a signal.4.3.  Bundle Routing and Forwarding State   As with the Internet architecture, we distinguish between routing and   forwarding.  Routing refers to the execution of a (possibly   distributed) algorithm for computing routing paths according to some   objective function (see [JFP04], for example).  Forwarding refers to   the act of moving a bundle from one DTN node to another.  Routing   makes use of routing state (the RIB, or routing information base),   while forwarding makes use of state derived from routing, and is   maintained as forwarding state (the FIB, or forwarding information   base).  The structure of the FIB and the rules for maintaining it are   implementation choices.  In some DTNs, exchange of information used   to update state in the RIB may take place on network paths distinct   from those where exchange of application data takes place.   The maintenance of state in the RIB is dependent on the type of   routing algorithm being used.  A routing algorithm may consider   requested class of service and the location of potential custodians   (for custody transfer, seesection 3.10), and this information will   tend to increase the size of the RIB.  The separation between FIB and   RIB is not required by this document, as these are implementation   details to be decided by system implementers.  The choice of routing   algorithms is still under study.   Bundles may occupy queues in nodes for a considerable amount of time.   For unicast or anycast delivery, the amount of time is likely to be   the interval between when a bundle arrives at a node and when it can   be forwarded to its next hop.  For multicast delivery of bundles,   this could be significantly longer, up to a bundle's expiration time.   This situation occurs when multicast delivery is utilized in such a   way that nodes joining a group can obtain information previously sent   to the group.  In such cases, some nodes may act as "archivers" thatCerf, et al.                 Informational                     [Page 26]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   provide copies of bundles to new participants that have already been   delivered to other participants.4.4.  Security-Related State   The DTN security approach described in [DTNSEC], when used, requires   maintenance of state in all DTN nodes that use it.  All such nodes   are required to store their own private information (including their   own policy and authentication material) and a block of information   used to verify credentials.  Furthermore, in most cases, DTN nodes   will cache some public information (and possibly the credentials) of   their next-hop (bundle) neighbors.  All cached information has   expiration times, and nodes are responsible for acquiring and   distributing updates of public information and credentials prior to   the expiration of the old set (in order to avoid a disruption in   network service).   In addition to basic end-to-end and hop-by-hop authentication, access   control may be used in a DTN by one or more mechanisms such as   capabilities or access control lists (ACLs).  ACLs would represent   another block of state present in any node that wishes to enforce   security policy.  ACLs are typically initialized at node   configuration time and may be updated dynamically by DTN bundles or   by some out of band technique.  Capabilities or credentials may be   revoked, requiring the maintenance of a revocation list ("black   list", another form of state) to check for invalid authentication   material that has already been distributed.   Some DTNs may implement security boundaries enforced by selected   nodes in the network, where end-to-end credentials may be checked in   addition to checking the hop-by-hop credentials.  (Doing so may   require routing to be adjusted to ensure all bundles comprising each   ADU pass through these points.)  Public information used to verify   end-to-end authentication will typically be cached at these points.4.5.  Policy and Configuration State   DTN nodes will contain some amount of configuration and policy   information.  Such information may alter the behavior of bundle   forwarding.  Examples of policy state include the types of   cryptographic algorithms and access control procedures to use if DTN   security is employed, whether nodes may become custodians, what types   of convergence layer (seeSection 6) and routing protocols are in   use, how bundles of differing priorities should be scheduled, where   and for how long bundles and other data is stored, what status   reports may be generated or at what rate, etc.Cerf, et al.                 Informational                     [Page 27]

RFC 4838         Delay-Tolerant Networking Architecture       April 20075.  Application Structuring Issues   DTN bundle delivery is intended to operate in a delay-tolerant   fashion over a broad range of network types.  This does not mean   there *must* be large delays in the network; it means there *may* be   very significant delays (including extended periods of disconnection   between sender and intended recipient(s)).  The DTN protocols are   delay tolerant, so applications using them must also be delay   tolerant in order to operate effectively in environments subject to   significant delay or disruption.   The communication primitives provided by the DTN architecture are   based on asynchronous, message-oriented communication which differs   from conversational request/response communication.  In general,   applications should attempt to include enough information in an ADU   so that it may be treated as an independent unit of work by the   network and receiver(s).  The goal is to minimize synchronous   interchanges between applications that are separated by a network   characterized by long and possibly highly variable delays.  A single   file transfer request message, for example, might include   authentication information, file location information, and requested   file operation (thus "bundling" this information together).   Comparing this style of operation to a classic FTP transfer, one sees   that the bundled model can complete in one round trip, whereas an FTP   file "put" operation can take as many as eight round trips to get to   a point where file data can flow [DFS02].   Delay-tolerant applications must consider additional factors beyond   the conversational implications of long delay paths.  For example, an   application may terminate (voluntarily or not) between the time it   sends a message and the time it expects a response.  If this   possibility has been anticipated, the application can be "re-   instantiated" with state information saved in persistent storage.   This is an implementation issue, but also an application design   consideration.   Some consideration of delay-tolerant application design can result in   applications that work reasonably well in low-delay environments, and   that do not suffer extraordinarily in high or highly-variable delay   environments.6.  Convergence Layer Considerations for Use of Underlying Protocols   Implementation experience with the DTN architecture has revealed an   important architectural construct and interface for DTN nodes   [DBFJHP04].  Not all underlying protocols in different protocol   families provide the same exact functionality, so some additional   adaptation or augmentation on a per-protocol or per-protocol-familyCerf, et al.                 Informational                     [Page 28]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   basis may be required.  This adaptation is accomplished by a set of   convergence layers placed between the bundle layer and underlying   protocols.  The convergence layers manage the protocol-specific   details of interfacing with particular underlying protocols and   present a consistent interface to the bundle layer.   The complexity of one convergence layer may vary substantially from   another, depending on the type of underlying protocol it adapts.  For   example, a TCP/IP convergence layer for use in the Internet might   only have to add message boundaries to TCP streams, whereas a   convergence layer for some network where no reliable transport   protocol exists might be considerably more complex (e.g., it might   have to implement reliability, fragmentation, flow-control, etc.) if   reliable delivery is to be offered to the bundle layer.   As convergence layers implement protocols above and beyond the basic   bundle protocol specified in [BSPEC], they will be defined in their   own documents (in a fashion similar to the way encapsulations for IP   datagrams are specified on a per-underlying-protocol basis, such as   inRFC 894 [RFC894]).7.  Summary   The DTN architecture addresses many of the problems of heterogeneous   networks that must operate in environments subject to long delays and   discontinuous end-to-end connectivity.  It is based on asynchronous   messaging and uses postal mail as a model of service classes and   delivery semantics.  It accommodates many different forms of   connectivity, including scheduled, predicted, and opportunistically   connected delivery paths.  It introduces a novel approach to end-to-   end reliability across frequently partitioned and unreliable   networks.  It also proposes a model for securing the network   infrastructure against unauthorized access.   It is our belief that this architecture is applicable to many   different types of challenged environments.8.  Security Considerations   Security is an integral concern for the design of the Delay Tolerant   Network Architecture, but its use is optional.  Sections3.6.1,3.14,   and 4.4 of this document present some factors to consider for   securing the DTN architecture, but separate documents [DTNSOV] and   [DTNSEC] define the security architecture in much more detail.Cerf, et al.                 Informational                     [Page 29]

RFC 4838         Delay-Tolerant Networking Architecture       April 20079.  IANA Considerations   This document specifies the architecture for Delay Tolerant   Networking, which uses Internet-standard URIs for its Endpoint   Identifiers.  URIs intended for use with DTN should be compliant with   the guidelines given in [RFC3986].10.  Normative References   [RFC3986]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform               Resource Identifier (URI): Generic Syntax", STD 66,RFC3986, January 2005.11.  Informative References   [IPN01]     InterPlaNetary Internet Project, Internet Society IPN               Special Interest Group,http://www.ipnsig.org.   [SB03]      S. Burleigh, et al., "Delay-Tolerant Networking - An               Approach to Interplanetary Internet", IEEE Communications               Magazine, July 2003.   [FW03]      F. Warthman, "Delay-Tolerant Networks (DTNs): A Tutorial               v1.1", Wartham Associates, 2003.  Available fromhttp://www.dtnrg.org.   [KF03]      K. Fall, "A Delay-Tolerant Network Architecture for               Challenged Internets", Proceedings SIGCOMM, Aug 2003.   [JFP04]     S. Jain, K. Fall, R. Patra, "Routing in a Delay Tolerant               Network", Proceedings SIGCOMM, Aug/Sep 2004.   [DFS02]     R. Durst, P. Feighery, K. Scott, "Why not use the               Standard Internet Suite for the Interplanetary               Internet?", MITRE White Paper, 2002.  Available fromhttp://www.ipnsig.org/reports/TCP_IP.pdf.   [CK74]      V. Cerf, R. Kahn, "A  Protocol for Packet Network               Intercommunication", IEEE Trans. on Comm., COM-22(5), May               1974.   [IGE00]     C. Intanagonwiwat, R. Govindan, D. Estrin, "Directed               Diffusion: A Scalable and Robust Communication Paradigm               for Sensor Networks", Proceedings MobiCOM, Aug 2000.Cerf, et al.                 Informational                     [Page 30]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   [WSBL99]    W. Adjie-Winoto, E. Schwartz, H. Balakrishnan, J. Lilley,               "The Design and Implementation of an Intentional Naming               System", Proc. 17th ACM SOSP, Kiawah Island, SC, Dec.               1999.   [CT90]      D. Clark, D. Tennenhouse, "Architectural Considerations               for a New Generation of Protocols", Proceedings SIGCOMM,               1990.   [ISCHEMES]  IANA, Uniform Resource Identifer (URI) Schemes,http://www.iana.org/assignments/uri-schemes.html.   [JDPF05]    S. Jain, M. Demmer, R. Patra, K. Fall, "Using Redundancy               to Cope with Failures in a Delay Tolerant Network",               Proceedings SIGCOMM, 2005.   [WJMF05]    Y. Wang, S. Jain, M. Martonosi, K. Fall, "Erasure Coding               Based Routing in Opportunistic Networks", Proceedings               SIGCOMM Workshop on Delay Tolerant Networks, 2005.   [ZAZ05]     W. Zhao, M. Ammar, E. Zegura, "Multicast in Delay               Tolerant Networks", Proceedings SIGCOMM Workshop on Delay               Tolerant Networks, 2005.   [LFC05]     J. Leguay, T. Friedman, V. Conan, "DTN Routing in a               Mobility Pattern Space", Proceedings SIGCOMM Workshop on               Delay Tolerant Networks, 2005.   [AF03]      J. Alonso, K. Fall, "A Linear Programming Formulation of               Flows over Time with Piecewise Constant Capacity and               Transit Times", Intel Research Technical Report IRB-TR-               03-007, June 2003.   [FHM03]     K. Fall, W. Hong, S. Madden, "Custody Transfer for               Reliable Delivery in Delay Tolerant Networks", Intel               Research Technical Report IRB-TR-03-030, July 2003.   [BSPEC]     K. Scott, S. Burleigh,"Bundle Protocol Specification",               Work in Progress, December 2006.   [DTNSEC]    S. Symington, S. Farrell, H. Weiss, "Bundle Security               Protocol Specification", Work in Progress, October 2006.   [DTNSOV]    S. Farrell, S. Symington, H. Weiss, "Delay-Tolerant               Networking Security Overview", Work in Progress, October               2006.Cerf, et al.                 Informational                     [Page 31]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   [DBFJHP04]  M. Demmer, E. Brewer, K. Fall, S. Jain, M. Ho, R. Patra,               "Implementing Delay Tolerant Networking", Intel Research               Technical Report IRB-TR-04-020, Dec. 2004.   [RFC792]    Postel, J., "Internet Control Message Protocol", STD 5,RFC 792, September 1981.   [RFC894]    Hornig, C., "A Standard for the Transmission of IP               Datagrams over Ethernet Networks", STD 41,RFC 894, April               1 1984.   [RFC2960]   Stewart, R., Xie, Q., Morneault, K., Sharp, C.,               Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,               Zhang, L., and V. Paxson, "Stream Control Transmission               Protocol",RFC 2960, October 2000.   [RFC4088]   Black, D., McCloghrie, K., and J. Schoenwaelder, "Uniform               Resource Identifier (URI) Scheme for the Simple Network               Management Protocol (SNMP)",RFC 4088, June 2005.   [S05]       K. Scott, "Disruption Tolerant Networking Proxies for               On-the-Move Tactical Networks", Proc. MILCOM 2005               (unclassified track), Oct. 2005.   [T02]       W. Thies, et al., "Searching the World Wide Web in Low-               Connectivity Communities", Proc. WWW Conference (Global               Community track), May 2002.12.  Acknowledgments   John Wroclawski, David Mills, Greg Miller, James P. G. Sterbenz, Joe   Touch, Steven Low, Lloyd Wood, Robert Braden, Deborah Estrin, Stephen   Farrell, Melissa Ho, Ting Liu, Mike Demmer, Jakob Ericsson, Susan   Symington, Andrei Gurtov, Avri Doria, Tom Henderson, Mark Allman,   Michael Welzl, and Craig Partridge all contributed useful thoughts   and criticisms to versions of this document.  We are grateful for   their time and participation.   This work was performed in part under DOD Contract DAA-B07-00-CC201,   DARPA AO H912; JPL Task Plan No. 80-5045, DARPA AO H870; and NASA   Contract NAS7-1407.Cerf, et al.                 Informational                     [Page 32]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007Authors' Addresses   Dr. Vinton G. Cerf   Google Corporation   Suite 384   13800 Coppermine Rd.   Herndon, VA 20171   Phone: +1 (703) 234-1823   Fax:   +1 (703) 848-0727   EMail: vint@google.com   Scott C. Burleigh   Jet Propulsion Laboratory   4800 Oak Grove Drive   M/S: 179-206   Pasadena, CA 91109-8099   Phone: +1 (818) 393-3353   Fax:   +1 (818) 354-1075   EMail: Scott.Burleigh@jpl.nasa.gov   Robert C. Durst   The MITRE Corporation   7515 Colshire Blvd., M/S H440   McLean, VA 22102   Phone: +1 (703) 983-7535   Fax:   +1 (703) 983-7142   EMail: durst@mitre.org   Dr. Kevin Fall   Intel Research, Berkeley   2150 Shattuck Ave., #1300   Berkeley, CA 94704   Phone: +1 (510) 495-3014   Fax:   +1 (510) 495-3049   EMail: kfall@intel.com   Adrian J. Hooke   Jet Propulsion Laboratory   4800 Oak Grove Drive   M/S: 303-400   Pasadena, CA 91109-8099   Phone: +1 (818) 354-3063   Fax:   +1 (818) 393-3575   EMail: Adrian.Hooke@jpl.nasa.govCerf, et al.                 Informational                     [Page 33]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007   Dr. Keith L. Scott   The MITRE Corporation   7515 Colshire Blvd., M/S H440   McLean, VA 22102   Phone: +1 (703) 983-6547   Fax:   +1 (703) 983-7142   EMail: kscott@mitre.org   Leigh Torgerson   Jet Propulsion Laboratory   4800 Oak Grove Drive   M/S: 238-412   Pasadena, CA 91109-8099   Phone: +1 (818) 393-0695   Fax:   +1 (818) 354-6825   EMail: ltorgerson@jpl.nasa.gov   Howard S. Weiss   SPARTA, Inc.   7075 Samuel Morse Drive   Columbia, MD 21046   Phone: +1 (410) 872-1515 x201   Fax:   +1 (410) 872-8079   EMail: howard.weiss@sparta.com   Please refer comments to dtn-interest@mailman.dtnrg.org.  The Delay   Tolerant Networking Research Group (DTNRG) web site is located athttp://www.dtnrg.org.Cerf, et al.                 Informational                     [Page 34]

RFC 4838         Delay-Tolerant Networking Architecture       April 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Cerf, et al.                 Informational                     [Page 35]