list. When an Internet Standard is updated, its STD number stays the same, now referring to a new RFC or set of RFCs. A given Internet Standard, STD n, may be RFCs x and y at a given time, but later the same standard may be updated to be RFC z instead. For example, in 2007 RFC 3700 was an Internet Standard—STD 1—and in May 2008 it was replaced with RFC 5000, so RFC 3700 changed to Historic, RFC 5000 became an Internet Standard, and STD 1 is RFC 5000. RFC 5000 is replaced by RFC 7100, updating RFC 2026 to no longer use STD 1. (Best Current Practices work in a similar fashion; BCP n refers to a certain RFC or set of RFCs, but which RFC or RFCs may change over time). Informational An informational RFC can be nearly anything from April 1 jokes to widely recognized essential RFCs like Domain Name System Structure and Delegation (RFC 1591). Some informational RFCs formed the FYI sub-series. Experimental An experimental RFC can be an IETF document or an individual submission to the RFC Editor. A draft is designated experimental if it is unclear the proposal will work as intended or unclear if the proposal will be widely adopted. An experimental RFC may be promoted to standards track if it becomes popular and works well. Best Current Practice The Best Current Practice subseries collects administrative documents and other texts which are considered as official rules and not only informational, but which do not affect over the wire data. The border between standards track and BCP is often unclear. If a document only affects the Internet Standards Process, like BCP 9, or IETF administration, it is clearly a BCP. If it only defines rules and regulations for Internet Assigned Numbers Authority (IANA) registries it is less clear; most of these documents are BCPs, but some are on the standards track. The BCP series also covers technical recommendations for how to practice Internet standards; for instance, the recommendation to use source filtering to make DoS attacks more difficult (RFC 2827: "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing") is BCP 38. Historic A historic RFC is one that the technology defined by the RFC is no longer recommended for use, which differs from "Obsoletes" header in a replacement RFC. For example, RFC 821 (SMTP) itself is obsoleted