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The DTN Reference Implementation

This document provides an overview of the DTN Reference Implementation, including its components, event queue, bundle actions, and convergence layers.

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The DTN Reference Implementation

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  1. The DTN Reference Implementation Michael Demmer IETF DTNRG Meeting - March 9, 2005

  2. Implementation Overview • Written primarily in C++ • ~22,000 non-comment lines of C++ (~5,000 in C) • 10K of which in generic system support classes (oasys) • 189 individual classes • Multithreaded (pthreads), system mutex or faster spin lock • STL for data structures (string, list, hashtable, …) • Emphasis on clarity, cleanliness, flexibility • Ported to Linux, Solaris, Win32 (Cygwin), Linux on PDA (ARM), FreeBSD, Mac OSX (Source line statistics generated using David A. Wheeler's 'SLOCCount’) DTN Reference Implementation

  3. Persistent Store Bundle Daemon (dtnd) Components API Server Event Queue Bundle Actions BundleDaemon BundleRouter ContactManager UDP TCP FragmentationManager Sneakernet Convergence Layers Tcl Console /Configuration = Thread dtnd DTN Reference Implementation

  4. Bundle Daemon Components • Bundle Daemon: core event queue and dispatcher • Bundle Router: all “smarts” in the system, effects policies via a the Bundle Actions interface • API Server: handler for application IPC interface • Convergence Layers: transport-specific mechanism for transmitting bundles to a next-hop • Contact Manager: service for detecting connection opportunities with neighbors • Fragmentation Manager: proactive and reactive fragmentation, maintains reassembly buffers • Persistent Store: object database abstraction for durable storage, uses Berkeley DB or SQL backends DTN Reference Implementation

  5. Persistent Store Bundle Daemon Events API Server Event Queue Bundle Actions BundleDaemon BundleRouter ContactManager UDP TCP FragmentationManager Sneakernet All events (e.g. bundle arrival, contact disruption, reassembly completed, etc.) posted to the daemon event queue. Convergence Layers Tcl Console /Configuration = Thread dtnd DTN Reference Implementation

  6. Persistent Store Bundle Router Actions API Server Event Queue Bundle Actions BundleDaemon BundleRouter ContactManager UDP TCP FragmentationManager Sneakernet Events are then forwarded to the router, which responds with a set of actions that are dispatched to components. Convergence Layers Tcl Console /Configuration = Thread dtnd DTN Reference Implementation

  7. Bundle Router • Not just traditional routing – responsible for all “policy” decisions in the daemon • e.g: where / when to send bundles, whether to accept custody, apply fragmentation, store bundles, etc… • Insulated by event/action interface • Support for multiple implementations, selected by config • Why this design? • Believe routing is the hardest problem, may need all events • Easier to change policies if consolidated in one place DTN Reference Implementation

  8. Evaluation HOP E2E user kernel user kernel user kernel user kernel • Approaches compared (both e2e and hop): • SFTP (simple ftp-like TCP app) – kernel IP forwarding or ‘plug proxy’ • MAIL (Sendmail, no ckpoint/restart) (both end to end and • DTN (store-and-forward with restart) hop configurations) • Compare robustness to interruption / link errors • Link up/down patterns: aligned, shifted, sequential, random DTN Reference Implementation

  9. DTN Overhead – No Disruptions DTN does best of any for small msgs, modest overhead overall DTN Reference Implementation

  10. Interruption Tolerance Zero throughput for e2e Note zero throughput for all E2E cases Up/down 1m/3min; 40KB messages; shift: 10s DTN Reference Implementation

  11. Simulator Framework • Implementation also includes basics of a discrete event simulator for prototyping and testing • TCL based simulation script to generate contact schedules, test traffic patterns, link bandwidth / delay, etc.1 • Uses the same events and abstractions • Implemented via a SimConvergenceLayer, Bundle Payload option for no data, modified event queue, and SimBundleActions • Other core classes and architecture are unmodified • Allows testing of the whole code base except convergence layers • Key characteristic is that to the router, there is no distinction between simulation or deployment 1 not all planned simulation features are yet implemented DTN Reference Implementation

  12. Current Status • Basic system operational and (fairly) stable • Primitives for in-memory and on-disk Bundle management • Extensible naming and addressing framework • Next-hop taxonomy of peers, links, contacts • TCP and UDP convergence layers (with pending ID specs) • Implemented (mostly) Bundling protocol v3 • Berkeley db and sql storage implementations • “Sockets-like” application interface • Sample applications: dtnping, dtnsend, dtnrecv, dtncp • Proactive and reactive fragmentation and reassembly DTN Reference Implementation

  13. Major ToDo Items DTN Reference Implementation

  14. Backup: Implementation Details • ISO Stack View • Example Event / Action Flow • Naming / Address Families • Bundle Class Representation • Bundle Consumers (next hop) • Bundle Lists • Convergence Layer API • Persistent Storage Interface DTN Reference Implementation

  15. Questions DTN Reference Implementation

  16. ISO Stack View DTN Application DTN2 socket-like API Application XDR IPC (plan to change to TCP) UDP Embedded Application Bundle Daemon Application can also run as a thread within the daemon Bundle Embedded Application Bundle Protocol Presentation Sneakernet TCP UDP Transport (in progress) to/from next hop DTN Reference Implementation

  17. action: OPEN_LINK event: BUNDLE_TRANSMITTED action: STORE_ADD Example Event / Action Flow User configures a new ONDEMAND TCP link and a route for the link. Router checks bundle mappings and finds no other destinations Database stores the bundle. event: LINK_CREATED TCPCL resolves the destination admin string and establishes a connection to the next hop. action: STORE_DEL event: LINK_AVAILABLE Database removes the bundle. Router adds link to route table. event: CONTACT_UP Router checks the link, finds a pending bundle, moves the bundle to the contact. TCPCL interface accept()’s a new connection and constructs a new bundle from the wire. Disruption occurs on the connection, TCPCL times out on a keepalive message action: ENQUEUE_BUNDLE event: BUNDLE_ARRIVED event: CONTACT_DOWN event: LINK_UNAVAILABLE TCP Connection thread picks bundle off contact queue, sends it to the next hop. Router matches the link in the route table as the only next hop for the bundle. Router checks queues for any bundles that need to be re-routed to other links. DTN Reference Implementation

  18. Naming / Address Families URI (scheme:scheme-specific-part) bundles://<region>/<admin> addressfamily (AF) “dns-style” string 1 smtp addressing has not actually been implemented (yet) DTN Reference Implementation

  19. Naming / Address Families (2) • AF abstraction allows essentially any format for admin strings • BundlePattern class implements AF-specific wildcard matching DTN Reference Implementation

  20. Bundle Class Representation • Bundle Metadata always in memory • Reference-counted C++ object for safety • Various fields “parsed” from the wire format into member variables • BundleProtocol class to convert to and from • Object serialization layer for synchronizing with the database • BundlePayload abstraction • Manages on-disk representation of payload • Methods for read_data(), write_data() • For small bundles, keeps in-memory copy DTN Reference Implementation

  21. Bundle Consumers • Routing can return any consumer as a valid next-hop • virtual override of enqueue_bundle to implement different abstractions • Allows router to use most-natural expression of an algorithm • e.g. enqueue bundle on peer implies send on next contact on any link to the peer link0: Scheduled, UDP link1: Opportunistic, SneakerNet peer0 link2: OnDemand, TCP peer1 DTN Reference Implementation

  22. Bundle Lists / Bundle Mappings Pending Bundles Contact 1 Contact 2 Bundle Lists bundle_list: Pending Bundles list_position: 1op: NONE bundle_list: Contact 1 list_position: 2op: FORWARD_ONCEgroup id: 0x10timeout: 1097563442.125245 bundle_list: Contact 2 list_position: 1op: FORWARD_ONCEgroup id: 0x10timeout: 1097553431.324565 Bundle 1 Bundle 2 Bundle 3 BundleMappings DTN Reference Implementation

  23. Convergence Layer API • Though the abstraction is intended to be general, to date only two functional implementations (TCP and UDP) • API may (will likely) need to be extended to account for other transports • Other things of note: • May add an MTU (e.g. UDP limited to 16K max bundle) • Likely need abstractions for neighbor discovery, peer identification, etc. DTN Reference Implementation

  24. Persistent Storage Interface • Several layers of abstraction for implementation transparency • Easy to extend with new storage types (e.g. routing state) or storage implementations DTN Reference Implementation

  25. Persistent Storage Interface (2) • SerializableObject interface allows a single function to define marshal / unmarshal for all DB types int Bundle::Serialize(SerializeAction* a) { a->process(“bundleid”, &bundleid_); a->process(“source”, &source_); a->process(“dest”, &dest_); a->process(“priority”, &priority_); a->process(“custreq”, &cust_req_); a->process(“return_rcpt”, &return_rcpt_); // ... } • Adding a new DB implementation requires implementing a handful of functions for different basic types (int, string, etc) • Berkeley DB implementation packs fields into a byte array • SQL implementations generate a command to update the database, e.g. update bundles set custreq = true where bundleid = 10 DTN Reference Implementation

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