- Paper By Yunfeng Lin, Baochun Li, Ben Liang

Outline Motivation Constraints in DTN Network coding Vs Replication Binary Spraying Vs Epidemic routing Protocol Principle Assumptions Description Example Performance Average transmission / relay Transmission delay Relay buffer size Optimal Maximal Spray count Vs Identical Maximal spray count

Motivation – Constraints in DTN Opportunistic connections between nodes Nodes have limited transmission capabilities Buffer space limitations Battery power limited Nodes might be mobile Delay in packet delivery will be large Node density is sparse

Motivation – NC Vs Replication Replication – Transmitting copies of a packet. Network coding – combining packets and transmitting the resultant packet Example: K = 3, K – Number of source packets, namely, A, B and C. Number of encoded packets, K’ = 3 Buffer Space of each relay, BS = 2 Number of nodes = 4, which includes one source, one destination. Number of relays is two.

Motivation – Replication Example A B C A B A B R1 R2 DESTINATION

Motivation – Replication Example R1 R2 A B C A C Replaces B A Here also C replaces B DESTINATION ONLY A AND C ARE RETRIEVED. FOR, B MORE DELAY IS INCURRED

Motivation – NC Example A + B + C A + 2B + 3C 2A + 2B + C A + B + C A + 2B + 3C A + B + C A + 2B + 3C R1 R2 DESTINATION

Motivation – NC Example R1 R2 A + B + C A + 2B + 3C 2A + 2B + C 3A +3B +2C, x = 1 3A + 4B + 4C, x = 1 5A + 5B + 3C, x = 2 5A + 6B + 5C, x = 2 DESTINATION ANY THREE PACKETS CAN HELP RETRIEVE A, B AND C

Motivation – Binary Spraying Vs ER S, K = 2 R1 R2 R3 R4 D Epidemic routing Binary spraying S, K = 2, L = 2 R1, L = 1 R2, L = 1 R3 D R4 R5

Protocol - Principle This protocol called the E-NCR, is a combination of Network coding and Binary spraying. NCERBinary Spraying E-NCR ER NCER – Network Coding based Epidemic routing ER – Epidemic Routing E-NCR - Efficient Network coding based routing

Protocol - Assumptions There is once source with ‘K’ packets to be transmitted, ‘n’ relay nodes and a destination For every opportunistic contact, only one packet can be transmitted. The source ‘S’ and the Destination ‘D’ have enough buffer space to hold all ‘K’ packets. But relay nodes have buffer space B, defined as 1 <= B <= K While a packet transmission is made, no other back-ground traffic exists. A packet in the buffer in an node is purged as soon as a ACK is received from ‘D’ or Time-to-live field is zero.

Protocol - Description SOURCE-RELAY: K' = K + some more encoded packets L = c * log k, where c is some constant i = 0; S = K'; do { if(detect any node and not already there with that node) { send an encoded packet i++; } }while(S != i);

Protocol - Description RELAY-RELAY, SENDER SIDE: do { if(detect any node X) { get spray list of X; //list element is a tuple,where i is index of packet, ‘l’ is the //remaining spray count do { compare this->spraylist with x->spraylist; if(any this->spraylist-> such that l >=0 and i does not exist in x->spraylist) { send encoded packet to node x; update tuple to ; } }while(end of x->spraylist); } }while(true);

Protocol - Description DESTINATION: do { if(got a packet) { add to packet list try to decode list of packets; if(decode possible) { exit loop; } }while(true); RELAY-RELAY, RECEIVER SIDE: if(packet received) { if(buffer size == max_buffer_size) { encode incoming packet with all packets in list; } else { place packet in free slot; } add of incoming packet to spray list; }

Protocol - Example In the following example, There are 7 nodes. One Source, One destination and 5 relays The buffer space is 1 for the relay nodes and 3 for the source and destination Number of packets to be transmitted from source is k = 3 L, the maximal spray count is set to log 2 (k) which is approximately 2 and the threshold is T = 1. In the first level, l, the spray count reduces to L/2 which is 1. Since, this is the threshold, all future transmissions are one per packet.

Protocol - Example E1, E2, E3 E2, L = 2 E2 E3 E1, L = 2 E1 E1 + (X) E2 L = 1 E3 + (Y) E1 L = 1 E1 + (X) E2E3 + (Y) E1 E3 + (Z) E2 E3, L = 2

Performance - Avg transmission per relay

Performance - Transmission delay

Performance - Relay buffer size

Performance – Optimal Vs Identical Maximal spray count

Some Limitations Destination has to wait till minimum of ‘K’ encoded packets have to be received Some packets which have linear dependence could arise during encoding at relays.

Acknowledgements My Audience for your comments, time and patience. Prof. Chan Mun Choon Dr. A. V. Babu Manjunath Chen Bin Bin The Authors of the paper