A: Yes, but that information is not allowed to change, because that would imply changing the identifier. The old identifier should remain valid, so that changing it would imply that an entity has two identifiers, violating the second property of identifiers.

A: Such identifiers can be generated locally in the following way. Take the network address of the machine where the identifier is generated, append the local time to that address, along with a generated pseudo-random number.

Although, in theory, it is possible that another machine in the world can generate

the same number, chances that this happens are negligible.

A: Assuming a process knows it is dealing with a URL, it first extracts the scheme identifier from the URL, such as the string ftp:. It can subsequently look up this string in a table to find an interface to a local implementation of the FTP protocol. The next part of the closure mechanism consists of extracting the host name from the URL, such as and passing that to

the local DNS name server. Knowing where and how to contact the DNS server is an important part of the closure mechanism. It is often hard-coded into the URL name resolver that the process is executing. Finally, the last part of the URL, which refers to a file that is to be looked up, is passed to the identified host. The latter uses its own local closure mechanism to start the name resolution of the file name.

A: A hard link is a named entry in a directory file pointing to the same file descriptor as another named entry (in possibly a different directory). A symbolic link is a file containing the (character string) name of another file.

A: Probably not: because the high-level name servers constitute the global layer of the DNS name space, it can be expected that changes to that part of the name space do not occur often. Consequently, caching will be highly effective, and much long-haul communication will be avoided anyway. Note that recursive name resolution for low-level name servers is important, because in that case, name resolution can be kept local at the lower-level domain in which the resolution is taking place.

A: The DNS name of a mobile host would be used as (rather poor) identifier for that host. Each time the name is resolved, it should return the current IP address of the host. This implies that the DNS server responsible for providing that IP address will act as the host’s name server. Each time the host moves, it contacts this home server and provides it with its current address.

Note that a mechanism should be available to avoid caching of the address. In other words, other name servers should be told not to cache the address found.

A: Each service has a unique identifier associated with it. Any server implementing that service, inserts its network-level address into the location service at the directory node of the leaf domain in which the server resides. Lookup requests use the identifier of the service, and will automatically return the nearest server implementing that service.

A: The root is formed jointly by all home locations, but is partitioned in such a way that each mobile entity has its own root server.

A: Simply encode the domain D in the identifier for the entity that is used for the lookup operation. The operation can then be immediately forwarded to the directory node dir(D), from where the search continues.

A: Changing location can be described as the combination of an insert and a delete operation. An insert operation requires that at worst k +1 location records are to be changed. Likewise, a delete operation also requires changing k +1 records, where the record in the root is shared between the two operations.

This leads to a total of 2k +1 records.

A: Combine the hierarchical location service with forwarding pointers. When the entity starts to move, it leaves behind a forwarding pointer at A to its next (intermediate) location. Each time it moves again, a forwarding pointer is left behind. Upon arrival in B, the entity inserts its new address into the hierarchical location service. The chain of pointers is subsequently cleaned up, and the address in A is deleted.

A: No, because in that case, process P2 may decide to immediately remove its reference before P1 has a chance to increment the counter at the object. Consequently, when P2 sends its decrement message, the counter may drop to zero and the object may be incorrectly removed.

A: Creation or removal of reference can be done with only a single message, as is also the case with simple reference counting. Passing a reference is much cheaper, as it can be done with only one message containing the copied reference with its partial weight set to half of the weight of the original reference. There is thus no need to contact the object, in contrast to simple reference

counting.

A: No. Whenever a reference of generation k is removed (so that possibly the associated entry in G[k ] becomes zero), the object is always informed about any outstanding references of generation k +1. If that generation was not yet known to the object, it will be at the time a reference of generation k is removed.

A: Yes. Suppose a reference of generation k is copied, leading to a reference of generation k +1. If the latter is removed before the reference from which it was copied, G[k +1] will be decremented by one. If no reference of generation k had yet been removed, G[k +1] will then drop below zero.

A: No. It may be possible that the process is temporarily unreachable due to a network partition, for example, caused by a failing router or gateway. In that case, the reference is lost and the skeleton may falsely decide that the object can be removed if the list becomes empty.

A: Assume that each process can decide whether its local garbage collector has changed any of the markings on proxies or skeletons during the local propagation step. A very simple, nonscalable solution, is to report whether or not any changes have been made to a central coordinator. As soon as that coordinator has recorded that no more changes have occurred, it tells each process

to start reclaiming garbage.