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+Classes
+-------
+
+ "Class" is a complete routing table in common sense.
+ I.e. it is tree of nodes (destination prefix, tos, metric)
+ with attached information: gateway, device etc.
+ This tree is looked up as specified in RFC1812 5.2.4.3
+ 1. Basic match
+ 2. Longest match
+ 3. Weak TOS.
+ 4. Metric. (should not be in kernel space, but they are)
+ 5. Additional pruning rules. (not in kernel space).
+
+ We have two special type of nodes:
+ REJECT - abort route lookup and return an error value.
+ THROW - abort route lookup in this class.
+
+
+ Currently the number of classes is limited to 255
+ (0 is reserved for "not specified class")
+
+ Three classes are builtin:
+
+ RT_CLASS_LOCAL=255 - local interface addresses,
+ broadcasts, nat addresses.
+
+ RT_CLASS_MAIN=254 - all normal routes are put there
+ by default.
+
+ RT_CLASS_DEFAULT=253 - if ip_fib_model==1, then
+ normal default routes are put there, if ip_fib_model==2
+ all gateway routes are put there.
+
+
+Rules
+-----
+ Rule is a record of (src prefix, src interface, tos, dst prefix)
+ with attached information.
+
+ Rule types:
+ RTP_ROUTE - lookup in attached class
+ RTP_NAT - lookup in attached class and if a match is found,
+ translate packet source address.
+ RTP_MASQUERADE - lookup in attached class and if a match is found,
+ masquerade packet as sourced by us.
+ RTP_DROP - silently drop the packet.
+ RTP_REJECT - drop the packet and send ICMP NET UNREACHABLE.
+ RTP_PROHIBIT - drop the packet and send ICMP COMM. ADM. PROHIBITED.
+
+ Rule flags:
+ RTRF_LOG - log route creations.
+ RTRF_VALVE - One way route (used with masquerading)
+
+Default setup:
+
+root@amber:/pub/ip-routing # iproute -r
+Kernel routing policy rules
+Pref Source Destination TOS Iface Cl
+ 0 default default 00 * 255
+ 254 default default 00 * 254
+ 255 default default 00 * 253
+
+
+Lookup algorithm
+----------------
+
+ We scan rules list, and if a rule is matched, apply it.
+ If a route is found, return it.
+ If it is not found or a THROW node was matched, continue
+ to scan rules.
+
+Applications
+------------
+
+1. Just ignore classes. All the routes are put into MAIN class
+ (and/or into DEFAULT class).
+
+ HOWTO: iproute add PREFIX [ tos TOS ] [ gw GW ] [ dev DEV ]
+ [ metric METRIC ] [ reject ] ... (look at iproute utility)
+
+ or use route utility from current net-tools.
+
+2. Opposite case. Just forget all that you know about routing
+ tables. Every rule is supplied with its own gateway, device
+ info. record. This approach is not appropriate for automated
+ route maintenance, but it is ideal for manual configuration.
+
+ HOWTO: iproute addrule [ from PREFIX ] [ to PREFIX ] [ tos TOS ]
+ [ dev INPUTDEV] [ pref PREFERENCE ] route [ gw GATEWAY ]
+ [ dev OUTDEV ] .....
+
+ Warning: As of now the size of the routing table in this
+ approach is limited to 256. If someone likes this model, I'll
+ relax this limitation.
+
+3. OSPF classes (see RFC1583, RFC1812 E.3.3)
+ Very clean, stable and robust algorithm for OSPF routing
+ domains. Unfortunately, it is not widely used in the Internet.
+
+ Proposed setup:
+ 255 local addresses
+ 254 interface routes
+ 253 ASE routes with external metric
+ 252 ASE routes with internal metric
+ 251 inter-area routes
+ 250 intra-area routes for 1st area
+ 249 intra-area routes for 2nd area
+ etc.
+
+ Rules:
+ iproute addrule class 253
+ iproute addrule class 252
+ iproute addrule class 251
+ iproute addrule to a-prefix-for-1st-area class 250
+ iproute addrule to another-prefix-for-1st-area class 250
+ ...
+ iproute addrule to a-prefix-for-2nd-area class 249
+ ...
+
+ Area classes must be terminated with reject record.
+ iproute add default reject class 250
+ iproute add default reject class 249
+ ...
+
+4. The Variant Router Requirements Algorithm (RFC1812 E.3.2)
+ Create 16 classes for different TOS values.
+ It is a funny, but pretty useless algorithm.
+ I listed it just to show the power of new routing code.
+
+5. All the variety of combinations......
+
+
+GATED
+-----
+
+ Gated does not understand classes, but it will work
+ happily in MAIN+DEFAULT. All policy routes can be set
+ and maintained manually.
+
+IMPORTANT NOTE
+--------------
+ route.c has a compilation time switch CONFIG_IP_LOCAL_RT_POLICY.
+ If it is set, locally originated packets are routed
+ using all the policy list. This is not very convenient and
+ pretty ambiguous when used with NAT and masquerading.
+ I set it to FALSE by default.
+
+
+Alexey Kuznetov
+kuznet@ms2.inr.ac.ru