--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/util/rfc6724.lua	Tue Apr 30 18:34:03 2013 +0200
@@ -0,0 +1,142 @@
+-- Prosody IM
+-- Copyright (C) 2011-2013 Florian Zeitz
+--
+-- This project is MIT/X11 licensed. Please see the
+-- COPYING file in the source package for more information.
+--
+
+-- This is used to sort destination addresses by preference
+-- during S2S connections.
+-- We can't hand this off to getaddrinfo, since it blocks
+
+local ip_commonPrefixLength = require"util.ip".commonPrefixLength
+local new_ip = require"util.ip".new_ip;
+
+local function commonPrefixLength(ipA, ipB)
+	local len = ip_commonPrefixLength(ipA, ipB);
+	return len < 64 and len or 64;
+end
+
+local function t_sort(t, comp)
+	for i = 1, (#t - 1) do
+		for j = (i + 1), #t do
+			local a, b = t[i], t[j];
+			if not comp(a,b) then
+				t[i], t[j] = b, a;
+			end
+		end
+	end
+end
+
+local function source(dest, candidates)
+	local function comp(ipA, ipB)
+		-- Rule 1: Prefer same address
+		if dest == ipA then
+			return true;
+		elseif dest == ipB then
+			return false;
+		end
+
+		-- Rule 2: Prefer appropriate scope
+		if ipA.scope < ipB.scope then
+			if ipA.scope < dest.scope then
+				return false;
+			else
+				return true;
+			end
+		elseif ipA.scope > ipB.scope then
+			if ipB.scope < dest.scope then
+				return true;
+			else
+				return false;
+			end
+		end
+
+		-- Rule 3: Avoid deprecated addresses
+		-- XXX: No way to determine this
+		-- Rule 4: Prefer home addresses
+		-- XXX: Mobility Address related, no way to determine this
+		-- Rule 5: Prefer outgoing interface
+		-- XXX: Interface to address relation. No way to determine this
+		-- Rule 6: Prefer matching label
+		if ipA.label == dest.label and ipB.label ~= dest.label then
+			return true;
+		elseif ipB.label == dest.label and ipA.label ~= dest.label then
+			return false;
+		end
+
+		-- Rule 7: Prefer temporary addresses (over public ones)
+		-- XXX: No way to determine this
+		-- Rule 8: Use longest matching prefix
+		if commonPrefixLength(ipA, dest) > commonPrefixLength(ipB, dest) then
+			return true;
+		else
+			return false;
+		end
+	end
+
+	t_sort(candidates, comp);
+	return candidates[1];
+end
+
+local function destination(candidates, sources)
+	local sourceAddrs = {};
+	local function comp(ipA, ipB)
+		local ipAsource = sourceAddrs[ipA];
+		local ipBsource = sourceAddrs[ipB];
+		-- Rule 1: Avoid unusable destinations
+		-- XXX: No such information
+		-- Rule 2: Prefer matching scope
+		if ipA.scope == ipAsource.scope and ipB.scope ~= ipBsource.scope then
+			return true;
+		elseif ipA.scope ~= ipAsource.scope and ipB.scope == ipBsource.scope then
+			return false;
+		end
+
+		-- Rule 3: Avoid deprecated addresses
+		-- XXX: No way to determine this
+		-- Rule 4: Prefer home addresses
+		-- XXX: Mobility Address related, no way to determine this
+		-- Rule 5: Prefer matching label
+		if ipAsource.label == ipA.label and ipBsource.label ~= ipB.label then
+			return true;
+		elseif ipBsource.label == ipB.label and ipAsource.label ~= ipA.label then
+			return false;
+		end
+
+		-- Rule 6: Prefer higher precedence
+		if ipA.precedence > ipB.precedence then
+			return true;
+		elseif ipA.precedence < ipB.precedence then
+			return false;
+		end
+
+		-- Rule 7: Prefer native transport
+		-- XXX: No way to determine this
+		-- Rule 8: Prefer smaller scope
+		if ipA.scope < ipB.scope then
+			return true;
+		elseif ipA.scope > ipB.scope then
+			return false;
+		end
+
+		-- Rule 9: Use longest matching prefix
+		if commonPrefixLength(ipA, ipAsource) > commonPrefixLength(ipB, ipBsource) then
+			return true;
+		elseif commonPrefixLength(ipA, ipAsource) < commonPrefixLength(ipB, ipBsource) then
+			return false;
+		end
+
+		-- Rule 10: Otherwise, leave order unchanged
+		return true;
+	end
+	for _, ip in ipairs(candidates) do
+		sourceAddrs[ip] = source(ip, sources);
+	end
+
+	t_sort(candidates, comp);
+	return candidates;
+end
+
+return {source = source,
+	destination = destination};