Authors:Kevin Fall, Wei Hong, Samuel Madden
Year:2003
What is custody transfer?
—Accepting a message with custody transfer amounts to promising not to delete it until it can be reliably delivered to another node providing custody(or to the message’s destination)
What is a custodian?
—Nodes holding a message with custody are called custodians.
Sole custody and joint custody
Ordinarily there is a single custodian for a message,but in some cuircumstances more than one custodian owns a message or message fragment
In some circumstances a node that had previously been accepting messages and custody for them may cease accepting custody (while continuing to accept messages ) if local node resources become substantially consumed and it may resume accepting custody when resources become more plentiful.
Joint custody occurs when the acknowledgement and custody acceptance message is not received by the previous custodian.At this point, the previous custodian has not recevied notice of the custody transfer,and the current custodian has already asserted custody.Therefore,both nodes believed each is the custodiano of the message and attempts to forwards the message at its custodian.
Joint custodian leads to message duplication.DTN destination nodes, or intermediate nodes, must be able to deal with duplication by recording the receipt of unique messages to indentify duplicate messages and eliminate them from the DTN, which requires future work.
Whether to accept a particular message based on message size, security, maximum message lifetime and local policy constraints.
Fragmentation
Messages may be fragmented or aggregated as they traverse the network.Fragmentation may be employed by a sending node in circumstances where a contact may not be of suficient volume to move an entire message to its next hop.Another form of fragmentation, called reactive fragmentation, may be employed by a receiving next-hop node if a link should become inoperative while a message is transfered.
Fragments can be reassembled when multiple fragments for the same destination together await an outgoing communication opportunity.If reassembly of fragments does not occur in the network, it is accomplished at the end(receiver).
Advantages of custody transfer:
Free storage resources comparatively quickly ( especially important to sensor networks.)
Address of head-of-line blocking
Three strategies(at least):
- reservation of persistent storage to non-custody transfer messages.(how much storage to reserve?based on dynamics of topology,link bandwidth,maximum available storage and expected size of messages.)
- in-memory store-and-forward.(limited to the amount of available volatile memory and is subject to loss upon node outage)
- cut-through in-memory routing(minimizing overhead at the router)
Semantics of the transfer operation
Messages indicate their current custodian in the DTN protocol header and this field is updated as custody of the message is moved from custodian to custodian.The ability to transfer fragments complicates this situation somewhat.