Delay-Tolerant Networks (DTNs)A tutorial

http://www.ipnsig.org/reports/DTN_Tutorial11.pdf

Eun Kyoung Kim

Today’s Internet

• Successful at interconnecting communication devices across the globe

• Based on TCP/IP protocol suite and wired links• Connected in end-to-end, low-delay paths

between sources and destinations• Low error rates and relatively symmetric

bidirectional data rates

Evolving Wireless Networks Outside the Internet

• Independent networks, each supporting specialized communication requirements and adapted to a particular homogeneous communication region

• Support long and variable delays, arbitrarily long periods of link disconnection, high error rates, and large bidirectional data-rate asymmetries

Evolving Wireless Networks Outside the Internet

• Examples– Terrestrial civilian networks connecting mobile

wireless devices– Wireless military battlefield networks connecting

troops, aircraft, satellites, and sensors– Outer-space networks, such as the InterPlaNetary

(IPN) Internet project• Require the intervention of an agent that can

translate between incompatible networks characteristics and act as a buffer for mismatched network delays

The Concept of a Delay-Tolerant Networks (DTN)

• A network of regional networks supporting interoperability among them

• An overlay on top of regional networks, including the Internet

• accommodate long delays between and within regional networks, and translate between regional network communication characteristics

Why a Delay-Tolerant Network (DTN)?

• The Internet’s underlying assumptions– Continuous, bidirectional end-to-end path– Short round-trips– Symmetric data rates– Low error rates

• The characteristics of evolving and potential networks– Intermittent connectivity– Long or variable delay– Asymmetric data rates– High error rates

• New architectural concept is needed!

Store-And-Forward Message Switching

• The problems of DTNs can be overcome by store-and-forward massage switching

• DTN routers need persistent storage for their queues because– A communication link may not be available for a long

time– One node may send or receive data much faster or

more reliably than the other node– A message, once transmitted, may need to be

retransmitted for some reasons

Intermittent Connectivity

• Assume communicating devices (nodes) in motion and/or operation with limited power

• When nodes must conserve power or preserve secrecy, links are shut down -> intermittent connectivity, network partition

• On the Internet, intermittent connectivity causes loss of data, while DTNs isolate delay with a store-and-forward technique

Opportunistic Contacts

• Network nodes may need to communicate during opportunistic contacts, in which a sender and receiver make contact at an unscheduled time

Scheduled Contacts

• If potentially communicating nodes move along predictable paths, they can predict or receive time schedules of their future positions and thereby arrange their future communication sessions

• Require time-synchronization

The Bundle Layer

• A new protocol layer overlaid on top of heterogeneous region-specific lower layers, with which application programs can communicate across multiple regions

Bundles and Bundle Encapsulation

• Bundles (messages) consist of– A source-application’s user data– Control information, provided by the source

application for the destination application– A bundle header, inserted by the bundle layer

A Non-Conversational Protocol

• DTN bundle layers communicate between themselves using simple sessions with minimal or no round-trips

• Any acknowledgement from the receiving node is optional, depending on the class of service selected

DTN Nodes

• An entity with a bundle layer– Host – sends and/or receives bundles, but does

not forward them. Optionally supports custody transfers.

– Router – forwards bundles within a single DTN region. Optionally supports custody transfers.

– Gateway – forwards bundles between tow or more DTN regions. Must support custody transfers.

Delay Isolation via Transport-Layer Termination

• DTN routers and gateways terminate transport protocols at the bundle layer

Custody Transfers

• The bundle layer supports node-to-node retransmission by means of custody transfers

• If no ACK is returned before the sender’s time-to-ACK expires, the sender retransmits the bundle

• A bundle custodian must store a bundle until– Another node accepts custody, or– Expiration of the bundle’s time-to-live

• Do not guarantee end-to-end reliability

Moving Points of Retransmission Forward

• The bundle layer uses reliable transport-layer protocols together with custody transfers to move points of retransmission progressively forward toward the destination

Internet vs. DTN Routing

• The protocol stacks of all nodes include both bundle and transport layers

• DTN gateways can run different lower-layer protocols (below the bundle layer) on each side of their double stack, which allows gateways to span two regions that use different lower-layer protocols

Classes of Bundle Services

• Custody Transfer• Return Receipt• Custody-Transfer Notification• Bundle-Forwarding Notification• Priority of Delivery• Authentication

DTN Regions

• A region composing a DTN, in which communication characteristics are homogeneous

• Has a unique region ID

Names and Addresses

• Each DTN node has a two-part name, consisting of a region ID and an entity ID

Security

• Forwarding nodes are authenticated as well as user identities and the integrity of messages

• Sender information is authenticated by forwarding nodes

Security

• Both users and forwarding nodes have private and public key-pairs and certificates

• Senders can sign their bundles with their private key, producing a bundle-specific digital signature

• Receivers can confirm the authenticity of the sender, the integrity of message, and the sender’s CoS rights using the signature and the sender’s public key