The peer-to-peer model assumes that participating nodes will continuously join and leave the network. CUP must be able to handle both node arrivals and departures seamlessly.
Arrivals. When a new node N enters a structured peer-to-peer network, it becomes responsible for a portion of another node M's share of the global index and becomes the authority node for those index entries mapped into that portion. N, M, and all surrounding affected nodes (old neighbors of M) update the bookkeeping structures they maintain for indexing and routing purposes. This is a necessary part of maintaining the connectivity of any structured peer-to-peer network when the set of nodes in the network changes.
For CUP, the issues at hand are updating the interest bit vectors of the affected nodes and deciding what to do with the index entries stored at M. This may require bit vector translation. For example, if a node that previously had M as its neighbor now has N as its neighbor, the node must make the bit ID that pointed to M now point to N.
To deal with its stored index entries, M could simply not hand over any of its entries to N. This would cause entries at some of M's previous neighbors to expire and subsequent queries from those nodes would establish new update propagations from N. Alternatively, M could give a copy of its stored index entries to N. Both N and M would then go through each entry and patch their bit vectors. Both solutions are viable. The first solution requires no bit translation but temporarily loses the CUP update benefits and behaves like PCX for the untransferred entries. The second solution gets the CUP benefits for the transferred entries, at the expense of transferring them and performing the bit vector patching. The metadata and bit vectors for thousands of index entries can be compressed into a few kilobytes and can be piggybacked onto messages that are already being exchanged to reconfigure the topology. Once the transfer occurs, the bit vector patching is an in-memory, local operation that with today's CPU and memory capacities takes only a few seconds for a few million entries.
Departures. Node departures can be either graceful (planned) or ungraceful (due to sudden failure of a node). In either case the peer-to-peer index mechanism dictates that a neighboring node M take over the departing node N's portion of the global index. To support CUP, the interest bit vectors of all affected nodes must be patched to reflect N's departure.
If N leaves gracefully, N can choose not to hand over to M its index entries. Any entries at surrounding nodes that were dependent on N to be updated will simply expire and subsequent queries will establish new update propagations. Again, alternatively N may give M its set of entries. M must then merge its own set of index entries with N's, by eliminating duplicate entries and patching the interest bit vectors as necessary. If N's departure is due to a failure, there can be no hand-over of entries and all entries in the affected neighboring nodes will expire as in PCX.