Even a single-bit error causes both SFV’s CRC and md5sum’s cryptographic hash to fail, typically requiring the entire file to be re-fetched from scratch. For this reason, the Parchive and rsync utilities are often preferred for verifying that a file has not been accidentally corrupted in transmission, since they can correct common small errors with a much shorter download.Despite above-mentioned weaknesses possessed by the SFV format, it is still a popular data verification technique. This is due to the relatively small amount of time taken by SFV utilities to calculate the CRC32 checksums, especially when compared to the time taken to calculate equivalent cryptographic hashes such as MD5 or SHA-1.One of the first programs to use the SFV format was WinSFV.SFV uses a plain text file containing one line for each file and its checksum in the format FILENAMECHECKSUM. Any line starting with a semicolon ‘;’ is considered to be a comment and is ignored for the purposes of file verification. The delimiter between the filename and checksum is always one or several spaces; tabs are never used.

* In opennet network, peer nodes are discovered automatically. There is no configuration required but little control available over which nodes become peers.
* In a darknet network, users manually establish connections with nodes run by people they know. Darknet typically needs more effort to set up but a node only has trusted nodes as peers.

Some networks like Freenet support both network types simultaneously (a node can have 5 manually added darknet peer nodes and 5 automatically selected opennet peers) .

In a friend-to-friend (or F2F) network, users only make direct connections with people they know. Many F2F networks support indirect anonymous or pseudonymous communication between users who do not know or trust one another. For example, a node in a friend-to-friend overlay can automatically forward a file (or a request for a file) anonymously between two friends, without telling either of them the other’s name or IP address. These friends can in turn forward the same file (or request) to their own friends, and so on. Users in a friend-to-friend network cannot find out who else is participating beyond their own circle of friends, so F2F networks can grow in size without compromising their users’ anonymity.

Some friend-to-friend networks allow the user to control what kind of files can be exchanged with friends within the node, in order to stop them from exchanging files that user disapproves of.

Advantages and disadvantages of opennet compared to darknet are disputed, see Friend to Friend article for summary.

P2P users who desire anonymity usually do so as they do not wish to be identified as a publisher (sender), or reader (receiver), of information. Common reasons include:

* The material or its distribution is illegal or incriminating
* Material is legal but socially deplored, embarrassing or problematic in the individual’s social world (for example, anonymity is seen as a key requirement for organizations like Alcoholics Anonymous)
* Fear of retribution (whistleblowers, unofficial leaks, and activists who do not believe in restrictions on information or knowledge)
* Censorship at the local, organizational, or national level
* Personal privacy preferences such as preventing tracking or datamining activities

A particularly open view on legal and illegal content is given in The Philosophy Behind Freenet.

Governments are also interested in anonymous P2P technology. The United States Navy funded the original onion routing research that led to the development of the Tor network, which is now funded by the Electronic Frontier Foundation.

Examples include:

* poisoning attacks (e.g. providing files whose contents are different from the description)
* polluting attacks (e.g. inserting “bad” chunks/packets into an otherwise valid file on the network)
* freeloaders (Sometimes known as ‘Leechers’) (users or software that make use of the network without contributing resources to it)
* insertion of viruses to carried data (e.g. downloaded or carried files may be infected with viruses or other malware)
* malware in the peer-to-peer network software itself (e.g. distributed software may contain spyware)
* denial of service attacks (attacks that may make the network run very slowly or break completely)
* filtering (network operators may attempt to prevent peer-to-peer network data from being carried)
* identity attacks (e.g. tracking down the users of the network and harassing or legally attacking them)
* spamming (e.g. sending unsolicited information across the network- not necessarily as a denial of service attack)

Most attacks can be defeated or controlled by careful design of the peer-to-peer network and through the use of encryption. P2P network defense is in fact closely related to the “Byzantine Generals Problem”. However, almost any network will fail when the majority of the peers are trying to damage it, and many protocols may be rendered impotent by far fewer numbers.