C# 4 and .NET 4

1344 ❘ ChaPTer 45 peer-tO-peer netwOrkinG
PnrP iDs
PNRP IDs are 256-bit identifiers. The low-order 128 bits are used to uniquely identify a particular peer,
and the high-order 128 bits identify a peer name. The high-order 128 bits are a hashed combination
of a hashed public key from the publishing peer and a string of up to 149 characters that identifies the
peer name. The hashed public key (known as the authority) combined with this string (the classifier)
are together referred to as the P2P ID. It is also possible to use a value of 0 instead of a hashed public
key, in which case the peer name is said to be unsecured (as opposed to secured peer names, which use a
public key).
The structure of a PNRP ID is illustrated in Figure 45-4.
PNRP ID
128-bit hashed P2P ID
128-bit service location
P2P ID
Authority (hashed public key)
Classifier (peer name identifier)
figure 45-4
The PNRP service on a peer is responsible for maintaining a list of PNRP IDs, including the ones
that it publishes as well as a cached list of those it has obtained by PNRP service instances elsewhere
in the cloud. When a peer attempts to resolve a PNRP ID, the PNRP service either uses a cached
copy of the endpoint to resolve the peer that published the PNRP or it asks its neighbors if they can
resolve it. Eventually a connection to the publishing peer is made and the PNRP service can resolve
the PNRP ID.
Note that all this happens without you having to intervene in any way. All you have to do is ensure
that peers know what to do with peer names after they have resolved them using their local
PNRP service.
Peers can use PNRP to locate PNRP IDs that match a particular P2P ID. You can use this to
implement a very basic form of discovery for unsecured peer names. This is because if several peers
expose an unsecured peer name that uses the same classifier, the P2P ID will be the same. Of course,
because any peer can use an unsecured peer name you have no guarantee that the endpoint you connect
to will be the sort of endpoint you expect, so this is only really a viable solution for discovery over a
local network.
PnrP Clouds
In the preceding discussion you learned how PNRP registers and resolves peer names in clouds. A cloud is
maintained by a seed server, which can be any server running the PNRP service that maintains a record of
at least one peer. Two types of clouds are available to the PNRP service:
➤
➤
Link local — These clouds consist of the computers attached to a local network. A PC may be connected
to more than one link local cloud if it has multiple network adapters.
Global — This cloud consists of computers connected to the Internet by default, although it is also
possible to define a private global cloud. The difference is that Microsoft maintains the seed server
for the global Internet cloud, whereas if you define a private global cloud you must use your own seed
server. If you use your own seed server you must ensure that all peers connect to it by configuring
policy settings.
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