diff --git a/lib/irc.js b/lib/irc.js
index 16fbee6..b540051 100644
--- a/lib/irc.js
+++ b/lib/irc.js
@@ -320,6 +320,7 @@ export function parseCTCP(msg) {
 }
 
 export function parseISUPPORT(tokens, params) {
+	var changed = [];
 	tokens.forEach((tok) => {
 		if (tok.startsWith("-")) {
 			var k = tok.slice(1);
@@ -333,6 +334,168 @@ export function parseISUPPORT(tokens, params) {
 			k = tok.slice(0, i);
 			v = tok.slice(i + 1);
 		}
-		params.set(k.toUpperCase(), v);
+
+		k = k.toUpperCase();
+
+		params.set(k, v);
+		changed.push(k);
+	});
+	return changed;
+}
+
+export const CaseMapping = {
+	ASCII(str) {
+		var out = "";
+		for (var i = 0; i < str.length; i++) {
+			var ch = str[i];
+			if ("A" <= ch && ch <= "Z") {
+				ch = ch.toLowerCase();
+			}
+			out += ch;
+		}
+		return out;
+	},
+
+	RFC1459(str) {
+		var out = "";
+		for (var i = 0; i < str.length; i++) {
+			var ch = str[i];
+			if ("A" <= ch && ch <= "Z") {
+				ch = ch.toLowerCase();
+			} else if (ch == "{") {
+				ch = "[";
+			} else if (ch == "}") {
+				ch = "]";
+			} else if (ch == "\\") {
+				ch = "|";
+			} else if (ch == "~") {
+				ch = "^";
+			}
+			out += ch;
+		}
+		return out;
+	},
+
+	RFC1459Strict(str) {
+		var out = "";
+		for (var i = 0; i < str.length; i++) {
+			var ch = str[i];
+			if ("A" <= ch && ch <= "Z") {
+				ch = ch.toLowerCase();
+			} else if (ch == "{") {
+				ch = "[";
+			} else if (ch == "}") {
+				ch = "]";
+			} else if (ch == "\\") {
+				ch = "|";
+			}
+			out += ch;
+		}
+		return out;
+	},
+
+	byName(name) {
+		switch (name) {
+		case "ascii":
+			return CaseMapping.ASCII;
+		case "rfc1459":
+			return CaseMapping.RFC1459;
+		case "rfc1459-strict":
+			return CaseMapping.RFC1459Strict;
+		}
+		return null;
+	},
+};
+
+function createIterator(next) {
+	var it = { next };
+	// Not defining this can lead to surprises when feeding the iterator
+	// to e.g. Array.from
+	it[Symbol.iterator] = () => it;
+	return it;
+}
+
+function mapIterator(it, f) {
+	return createIterator(() => {
+		var { value, done } = it.next();
+		if (done) {
+			return { done: true };
+		}
+		return { value: f(value), done: false };
 	});
 }
+
+export class CaseMapMap {
+	caseMap = null;
+	map = null;
+
+	constructor(iterable, cm) {
+		if ((iterable instanceof CaseMapMap) && (iterable.caseMap === cm || !cm)) {
+			// Fast-path if we're just cloning another CaseMapMap
+			this.caseMap = iterable.caseMap;
+			this.map = new Map(iterable.map);
+		} else {
+			if (!cm) {
+				throw new Error("Missing case-mapping when creating CaseMapMap");
+			}
+
+			this.caseMap = cm;
+			this.map = new Map();
+
+			if (iterable) {
+				for (var [key, value] of iterable) {
+					this.set(key, value);
+				}
+			}
+		}
+	}
+
+	get size() {
+		return this.map.size;
+	}
+
+	has(key) {
+		return this.map.has(this.caseMap(key));
+	}
+
+	get(key) {
+		var kv = this.map.get(this.caseMap(key));
+		if (kv) {
+			return kv.value;
+		}
+		return undefined;
+	}
+
+	set(key, value) {
+		this.map.set(this.caseMap(key), { key, value });
+	}
+
+	delete(key) {
+		this.map.delete(this.caseMap(key));
+	}
+
+	entries() {
+		var it = this.map.values();
+		return mapIterator(it, (kv) => {
+			return [kv.key, kv.value];
+		});
+	}
+
+	keys() {
+		var it = this.map.values();
+		return mapIterator(it, (kv) => {
+			return kv.key;
+		});
+	}
+
+	values() {
+		var it = this.map.values();
+		return mapIterator(it, (kv) => {
+			return kv.value;
+		});
+	}
+
+	[Symbol.iterator]() {
+		return this.entries();
+	}
+}