The appearance of large public IM services based on XMPP Core  and XMPP IM  makes it desirable to implement protocols that discourage the sending of large quantities of instant messaging spam (a.k.a. "spim"). Spim could be generated by XMPP clients connected to legitimate servers or by XMPP servers with virtual clients, where the malicious entities are hosted on networks of "zombie" machines. Spim is defined here as any type of unsolicited XMPP stanza sent by a "robot" and delivered to a human, including messages and subscription requests. Spim has the potential to disrupt people even more than spam, because each message interrupts the receiver (humans typically filter SPAM in batch mode).
Spim blocking is more efficiently performed on the receiving server for several reasons:
However, no automated spim recognition techniques work perfectly all the time. This document is designed to give users control over the spim recognition their servers perform on their behalf.
Clients should be able to:
The stanza blocking protocol defined in XEP-0016 enables a client to control explicitly which senders its server must block stanzas from. Such explicit blocking is suitable for privacy control, but not for filtering spim.
This document contradicts an assumption expressed in the standard blocking protocol in order to extend client control to spim blocking. More specifically, it simply defines a spim recognition privacy-list fall-through action that is different from the 'allow' default assumed in XEP-0016. 
The various spim recognition procedures that may be employed by the server are beyond the scope of this document. No single measure can differentiate all spim perfectly. It is RECOMMENDED that servers implement a combination of complementary spim recognition techniques (and other anti-spim techniques ).
For example, a server could employ traffic and reputation analysis to filter the majority of spim, and use CAPTCHA Forms to identify the remainder (feeding what it learns back to the traffic and reputation analysis).
A client MAY confirm that its server supports Spim-Blocking Control using Service Discovery (XEP-0030) .
This section specifies server functionality not defined in XEP-0016. This document will not reach Draft status until Server-Based Privacy Rules has been modified to permit this functionality.
A server that supports this protocol MAY maintain a separate  list of each user's correspondents. Each list contains all the bare JIDs the user has either sent a stanza to or received a stanza from (over the past few weeks or months).
Note: When it blocks a stanza, the server MUST NOT add the 'from' attribute of the stanza to the correspondents list.
Note: The lists of correspondents have a very different function from rosters. Edits are initiated by the server not the client, allowing the lists to be completely transparent to clients.
The server SHOULD perform the following procedures whenever it receives a stanza that falls through the active privacy list of the user it is addressed to without being either allowed or denied:
When a spim recognition procedure delays delivery of a stanza the server SHOULD store it temporarily.
While delivery is being delayed:
Once delivery of a stanza has been delayed for an implementation-specific length of time, or an implementation-specific number of stanzas from the same sender (or same sending server) are being delayed, the server SHOULD deny delivery of the stanza without informing the sender.
A good example of a delayed spim recognition procedure is when servers use the CAPTCHA Forms protocol to confirm whether or not a client is a spim robot before denying or allowing the delivery of a stanza from a new correspondent.  
This informative section requires no client or server functionality beyond that defined in XEP-0016.
The client SHOULD use the 'subscription' type to exclude all JIDs on the user's roster from spim blocking (see the items with order 20, 30 and 40 in the example below).
At least in the medium term, clients that use non-XMPP protocols cannot be expected to support interactive spim recognition techniques (like CAPTCHA Forms). So, if its server uses interactive techniques, the client MAY use the 'jid' type to ensure its server does not block stanzas arriving from the transports the user has registered with (see the item with order 50 in the example below).
If a user believes spim will not be sent by users of a particular server (e.g. the user's own corporate server), then the client MAY use the 'jid' type to exclude all these users from spim blocking (see the item with order 60 in the example below).
No spim recognition techniques are perfect. Senders are sometimes falsely identified as spim bots. (For example, when a server sends CAPTCHA Forms, but the client does not support that protocol.)
In these cases the user MAY ask out-of-band the person he is trying to communicate with to allow communications in one of the following ways:
If stanzas from a spim robot running on a zombie domain somehow manage to get past the server's spim recognition then the client MAY use the 'jid' type to block one or all JIDs from the domain. (XEP-0016 already enables this functionality.)
A client MAY disable server-side spim blocking by ensuring the default fall-through action is never applied. It does this simply by including an explicit fall-through item in its active privacy list.
Note: Before a server with existing accounts deploys this protocol, it MAY ensure all users' privacy lists have an explicit fall-through item. As a result, spim recognition would be disabled until users choose to switch it on.
If a server implements this protocol and its security is compromised, then the attacker may be able to access the list of all previous correspondants for every user. People are unable to delete their own lists and they may not even be aware that they exist. So servers MUST ensure they protect this sensitive information very carefully.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
Until this specification advances to a status of Draft, its associated service discovery node shall be "http://www.xmpp.org/extensions/xep-00158.html#node"; upon advancement of this specification, the XMPP Registrar  shall issue a permanent identifier in accordance with the process defined in Section 4 of XMPP Registrar Function (XEP-0053) .
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
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The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
5. The language used in XEP-0016 appears to be deliberately vague (informative rather than normative) in order to permit other default fall-through actions. The protocol defined in this document is therefore (arguably) compatible with XEP-0016.
6. Other examples of anti-spim policies and protocols include: requiring a user to pass a robot challenge before registering a new account, invite-only and/or out-of-band user account registration, providing a standard protocol for reporting spim to both the servers involved, server-to-server connection dialback, karma (client-to-server and server-to-server), legal agreements not to send spim during user account registration, and IP blocking.
8. If the server were to maintain a single unified list of the correspondents of all its users, then spimmers would only need to pass a single spim recognition test before being allowed to send spim to all the server's users.
9. For example, the first time a stanza is delivered the correspondents list will change, typically triggering the immediate delivery of any other delayed stanzas.
10. The very occasional inconvenience of responding to a challenge is small and perfectly acceptable -- especially when compared to the countless robot-generated interruptions people might otherwise have to filter every day.
11. If a human user fails such a robot challenge then his client SHOULD give him the option to resend the stanza immediately.
12. The subscription type SHOULD never be 'none' because XEP-0016 specifies that, for the purposes of blocking, all JIDs not on the roster also have the subscription type 'none'.
13. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
14. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Updated to reflect move of privacy rules to XEP-0016; modified provisional namespaces to adhere to XMPP Registrar policies; completed editorial review.
Added reference to Server-Based Privacy Rules.
Initial published version.