Author:John Burgess Brian Gallagher David Jensen Brian Neil Levine
Publication:InfoCom
Year::2006
Abstract::MaxProp is based on prioritizing both the schedule of packets transmitted to other peers and the schedule of packets to be dropped.These priorities are based on the path likelihoods to peers according to historical data and also on several complementary mechanisms,including acknowledgements,a head-start for new packets, and lists of previous intermediaries.[Real DTN network UMassDieselNet]
Introduction:
MaxProp addresses scenarios in which either transfer duration or storage is a limited resource in the network.
MaxProp uses hop counts in packets as a measure of network resource fairness.
MaxProp uses acknowledgments that are propagated network wide,and not just to the source.
MaxProp stores a list of previous intermediaries to prevent data from propagating twice to the same node.
Model:
- Each peer has an effectively unlimited buffer for messages that they originate,but a fixed-size buffer for carrying messages originated by others.
- Transfer opportunities are limited both in duration and bandwidth.
- Peers have no priori knowledge of network connectivity,no control over their movement,no knowledge of geographic locaiton.
- There are no always-on stationary peers in the environment.
In a real network, DTN operations proceeds roughly in three stages.
- Neighbor Discovery
- Data Transfer.(Peers do not know the duration of each opportunity)
- Storage management.
Each peer carries all messages until the next meeting occurs.A peer will continue to forward a message to any number of other peers until its copy of the messages times out,it is notified of delivery by an ack, or the message is dropped due to a full buffer.
Protocol Definition:
At the core of the MaxProp protocol is a ranked list of the peer’s stored packets based on a cost assigned to each destination.The cost is an estimate of delivery likelihood.
In addition, MaxProp used acks sent to all peers to notify them of packet deliveries.
MaxProp assigns a higher priority to new packets, and it also attempts to prevent reception of the same packet twice.
Estimating Delivery Likelihood:incremental averaging.
Complementary Mechanism:
When two peers discover each other, MaxProp exchanges packets in a specific priority order:
- all messages destined to the neighbor peer are transferred.
- second,routing information is passed between peers;Specifically, a vector listing estimations of the probability of meeting every other node, as explained above.
- Acks of delivered data are transfered,regardless of source and destination.An ack consists of the cryptographic hash of the content,source and destination of each message, and is therefore about 128bits.[to clear out buffers]
- Packets that have not traversed far in the network are given priority.[Head-start:Giving new packet higher priority so that they have the chance to be delivered.]MaxProp logically splits the buffer in two according to whether the packets have a hop count less than a threshold t hops.Packets above are sorted by the scoring mechanism described above.MaxProp takes an adaptive approach to setting the threshold.
- The remaining,untransmitted packets are sent in an order basedon the scores described in aforthmentioned.
- A hop list in each packet stores peers that the packet has already traversed,including peers to which the current node has sent the packet(A similar algorithm is used in Usenet/NNTP to limit flooding)
Managing Buffers:
There are only three reasons a peer p can drop a packet m,without reducing the overall delivery rate of the network unnecessarily.
Criteria 1:A copy of msg m has already been delivered to its destination
Criteria 2:No route with suffcient bandwith will exist between p and m’s destination during the lifetime of message m.
Criteria 3:No copy of m has been delivered,but some copy of m will be delivered even if peer p drops its copy.