Network Working Group V. Cerf Request for Comments: 4838 Google/Jet Propulsion Laboratory Category: 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 2007 Delay-Tolerant Networking Architecture Status 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.
AbstractThis 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 of
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. 1. Introduction ....................................................3 2. Why an Architecture for Delay-Tolerant Networking? ..............4 3. DTN Architectural Description ...................................5 3.1. Virtual Message Switching Using Store-and-Forward Operation ..................................................5 3.2. Nodes and Endpoints ........................................7 3.3. Endpoint Identifiers (EIDs) and Registrations ..............8 3.4. Anycast and Multicast .....................................10 3.5. Priority Classes ..........................................10 3.6. Postal-Style Delivery Options and Administrative Records ..11 3.7. Primary Bundle Fields .....................................15 3.8. Routing and Forwarding ....................................16 3.9. Fragmentation and Reassembly ..............................18 3.10. Reliability and Custody Transfer .........................19 3.11. DTN Support for Proxies and Application Layer Gateways ...21 3.12. Timestamps and Time Synchronization ......................22 3.13. Congestion and Flow Control at the Bundle Layer ..........22 3.14. Security .................................................23 4. State Management Considerations ................................25 4.1. Application Registration State ............................25 4.2. Custody Transfer State ....................................26 4.3. Bundle Routing and Forwarding State .......................26 4.4. Security-Related State ....................................27 4.5. Policy and Configuration State ............................27 5. Application Structuring Issues .................................28 6. Convergence Layer Considerations for Use of Underlying Protocols ......................................................28 7. Summary ........................................................29 8. Security Considerations ........................................29 9. IANA Considerations ............................................30 10. Normative References ..........................................30 11. Informative References ........................................30 12. Acknowledgments ...............................................32
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 one
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. 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]):
- 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 in Section 5. CT90], which are subject to any implementation limitations. The relative order of ADUs might not be preserved. ADUs are typically sent by and delivered to
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) would
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 in Section 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].
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 of RFC 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 like
"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. See Section 3.4 for more details. URIs are constructed based on rules specified in RFC 3986, using the US-ASCII character set. However, note this excerpt from RFC 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".
must propagate through the network, and may also limit the scope of mapping synchronization requirements to a local topological neighborhood where changes are made. 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.
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.
Section 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. See Section 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.
- 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. See Section 3.7 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. 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).
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 in Section 3.12.
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 (see Section 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. See Section 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.
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 (see Section 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.
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 may
be chosen based on this information. This is an active research area, and a few approaches having been proposed [LFC05]. Section 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.
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. 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 DTN
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 Required
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. 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.
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.
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 to
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]. Section 3.4 above for more details.
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, see section 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" that
provide copies of bundles to new participants that have already been delivered to other participants. 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. Section 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.
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. 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-family
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 in RFC 894 [RFC894]). Sections 3.6.1, 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.
RFC3986]. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [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 from http://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 from http://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.
[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.
[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.
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: firstname.lastname@example.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: email@example.com Howard S. Weiss SPARTA, Inc. 7075 Samuel Morse Drive Columbia, MD 21046 Phone: +1 (410) 872-1515 x201 Fax: +1 (410) 872-8079 EMail: firstname.lastname@example.org Please refer comments to email@example.com. The Delay Tolerant Networking Research Group (DTNRG) web site is located at http://www.dtnrg.org.
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