Alternative Custodians for Congestion Control in DTNs
Author:Matthew Seligman, Kevin Fall, Padma Mundur
Publication:SIGCOMM
Year:2006
Storage Routing(SR) is a set of algorithms invoked by a DTN custodian when congestion becomes evident and when it abates.
Main Idea:
With the onset of congestion,the algorightms provide a selection of messages to migrate and a set of alternative custodian(s) to which the selected messages are to be migrated. The custodian then invokes the conventional DTN custody transfer mechanism to transfer the selected messages to alternative storage locations using the push operation. When a custodian has sufficient storage resources to take custody for messages, it invokes a retrieval algorithm to migrate, or pull, messages for which it may have had custody earlier toward their destination(s).
Three Steps:
- message selection.
- node selection
- retrieval selection
Message selection:
Three selection factor types considered are temporal, size, and priority.
Table 1.Push Policy Description and Usage
The DTN architecture suggestes the idea that when a custodian becomes congested it may cease accepting custody for subsequent messages(a form of flow congrol).But it’t not suitable in networks where sources produce data continously.
In some sense, this approach could be characterized as a form of congestion management implemented using distributed storage load balancing.
Message should be deleted as a last resort,only when there is no available storage at any neighboring node.
Node Selection:
The simplest way to select alternative custodians is to select targets in the topological (hop count) sense.
aggregate migration cost(minimus value is 0 and maximum is 1):
storage cost:
transmission cost:
To find targes, a form of expanding ring search(ERS) may be employed from a current custodian c.
Message Retrieval:
The decision of which messages to migrate at this point mirror the push decision described earlier,except in this case the decision is made by the receiver rather than the sender.
Simulation :
The msg selection policy is pushTail and the node selection uses the procedure described above.The msg retrieval algorithm selects a message from all sufficiently small messages available in Nc(1).
Assumptions:
- No separate flow control
- Node must take custody when forwarding messages
- Custodian nodes drop as a last resort
- Nodes are symmetric with respect to storage
- Each link has the same bandwidth in both directions
- Each link is either up or down.
Simulator:A custom simulation enviroment was developed based on the generic YAC-SIM discrete event simulation engine and augmented with custom simulation software supporting intermittent links, custody transfer, and the other DTN mechanisms.
Simulation results:
Three SR variations are compared:No SR,SR with k=1 and SR with k=2
