Presenter: Anika Aziz National Institute of Informatics (NII), The Graduate University for Advanced Studies, Tokyo, Japan

In a Delay Tolerant Network (DTN), a custodian node which is storage constrained, can mitigate the congestion by reserving buffer space and applying queuing algorithm as a function of priority classes that is Class Of Services(COS). These COS can be mapped to a shared Priority Queuing (PQ) approach or to the separate PQs with different thresholds of the buffer.

The DTN architecture, the Bundle protocol Custody transfer in DTN Congestion in DTN The Storage Routing (SR) Proposed algorithms Analysis & comparison of the algorithms Conclusion Q & A

An acknowledged transfer of data to persistent, reliable storage A msg B ack (store the msg) (delete the msg) custody transfer complete (assumes custody transfer complete) - To use hop-by-hop reliability to improve end-to-end reliability - To free retransmission buffers at a sender as soon as possible - When faced with persistent demand…….congestion occurs

Rate based or window based congestion control techniques-nearly ineffective A region under the administrative control of a single entity can apply some form of admission control as a proactive means If its insufficient or unavailable, reactive means ( involving direct flow control) must be used Different solutions…cease accepting bundles with custody transfer & move bundles somewhere else

int -Message selection -Node selection push -Retrieval selectionpull alternate custody transfer source custody transferint dest -No custody refusal - Messages are dropped under the worst case situation Networks containing traffic sources which continuously generate application data (e.g. sensor networks) can not be flow controlled instead Storage routing

Apply some standard queue management or scheduling algorithm Priority classes that is Classes Of Service (COS), bulk, normal and expediated priorities, Applications specify the COS & data lifetime (TTL) for each message they send, based upon its desire to affect the delivery urgency for messages Push Lowest Priority mechanism as the push policy Expanding Ring Search (ERS) method

Enqueue ׀m׀ at n Priority0 ? ? Find alternative custodian x such that a x (t)>׀m׀ Is x Found? (ERS) Drop ׀m׀ Push ׀m׀ to x F n (t) is increased Y N Y N Compare with others in the queue Is it the lowest one? Y N Push the lowest one in the queue If all are of equal priority, push the message with the highest TTL Does ׀m׀ has the highest TTL? Y N Is still | m | > F n (t)? N Y Message ׀m׀ arrives at node n ׀m׀>F n (t) ? Y N Figure 4. The SR algorithm: COS mapped to the shared PQ

׀m׀> F n (t)? Priority0 ? Priority1 ? N Y YY N N Priority 2 ABM2 ABM1 Compute avg queue length of F n (t) avg<minth0 ? ׀m׀ is selected for push policy Y avg>max0 ? Drop Y Look into residual TTL value N High residual TTL? Y N Active Buffer Management (ABM0) technique Enqueue ׀m׀ At node n Message ׀m׀ arrives at node n N Figure 5. SR algorithm: COS mapped to PQ and thresholds of Active Buffer Management

Both the algorithm ensures to push the lowest priority message Messages are dropped in the worst case situation the 2 nd approach works more dynamically and precisely No Priority Inversion & Head-Of-Line blocking

The value of Custody Transfer and DTN congestion remains to be fully understood… until the DTN architecture is widely deployed and carries significant traffic loads Similar story as TCP protocol specification included no management of congestion… more than 10 years after the 1 st experiments with Internet technology were performed.