1.. SPDX-License-Identifier: GPL-2.0
2
3===================================
4Identifier Locator Addressing (ILA)
5===================================
6
7
8Introduction
9============
10
11Identifier-locator addressing (ILA) is a technique used with IPv6 that
12differentiates between location and identity of a network node. Part of an
13address expresses the immutable identity of the node, and another part
14indicates the location of the node which can be dynamic. Identifier-locator
15addressing can be used to efficiently implement overlay networks for
16network virtualization as well as solutions for use cases in mobility.
17
18ILA can be thought of as means to implement an overlay network without
19encapsulation. This is accomplished by performing network address
20translation on destination addresses as a packet traverses a network. To
21the network, an ILA translated packet appears to be no different than any
22other IPv6 packet. For instance, if the transport protocol is TCP then an
23ILA translated packet looks like just another TCP/IPv6 packet. The
24advantage of this is that ILA is transparent to the network so that
25optimizations in the network, such as ECMP, RSS, GRO, GSO, etc., just work.
26
27The ILA protocol is described in Internet-Draft draft-herbert-intarea-ila.
28
29
30ILA terminology
31===============
32
33  - Identifier
34		A number that identifies an addressable node in the network
35		independent of its location. ILA identifiers are sixty-four
36		bit values.
37
38  - Locator
39		A network prefix that routes to a physical host. Locators
40		provide the topological location of an addressed node. ILA
41		locators are sixty-four bit prefixes.
42
43  - ILA mapping
44		A mapping of an ILA identifier to a locator (or to a
45		locator and meta data). An ILA domain maintains a database
46		that contains mappings for all destinations in the domain.
47
48  - SIR address
49		An IPv6 address composed of a SIR prefix (upper sixty-
50		four bits) and an identifier (lower sixty-four bits).
51		SIR addresses are visible to applications and provide a
52		means for them to address nodes independent of their
53		location.
54
55  - ILA address
56		An IPv6 address composed of a locator (upper sixty-four
57		bits) and an identifier (low order sixty-four bits). ILA
58		addresses are never visible to an application.
59
60  - ILA host
61		An end host that is capable of performing ILA translations
62		on transmit or receive.
63
64  - ILA router
65		A network node that performs ILA translation and forwarding
66		of translated packets.
67
68  - ILA forwarding cache
69		A type of ILA router that only maintains a working set
70		cache of mappings.
71
72  - ILA node
73		A network node capable of performing ILA translations. This
74		can be an ILA router, ILA forwarding cache, or ILA host.
75
76
77Operation
78=========
79
80There are two fundamental operations with ILA:
81
82  - Translate a SIR address to an ILA address. This is performed on ingress
83    to an ILA overlay.
84
85  - Translate an ILA address to a SIR address. This is performed on egress
86    from the ILA overlay.
87
88ILA can be deployed either on end hosts or intermediate devices in the
89network; these are provided by "ILA hosts" and "ILA routers" respectively.
90Configuration and datapath for these two points of deployment is somewhat
91different.
92
93The diagram below illustrates the flow of packets through ILA as well
94as showing ILA hosts and routers::
95
96    +--------+                                                +--------+
97    | Host A +-+                                         +--->| Host B |
98    |        | |              (2) ILA                   (')   |        |
99    +--------+ |            ...addressed....           (   )  +--------+
100	       V  +---+--+  .  packet      .  +---+--+  (_)
101   (1) SIR     |  | ILA  |----->-------->---->| ILA  |   |   (3) SIR
102    addressed  +->|router|  .              .  |router|->-+    addressed
103    packet        +---+--+  .     IPv6     .  +---+--+        packet
104		   /        .    Network   .
105		  /         .              .   +--+-++--------+
106    +--------+   /          .              .   |ILA ||  Host  |
107    |  Host  +--+           .              .- -|host||        |
108    |        |              .              .   +--+-++--------+
109    +--------+              ................
110
111
112Transport checksum handling
113===========================
114
115When an address is translated by ILA, an encapsulated transport checksum
116that includes the translated address in a pseudo header may be rendered
117incorrect on the wire. This is a problem for intermediate devices,
118including checksum offload in NICs, that process the checksum. There are
119three options to deal with this:
120
121- no action	Allow the checksum to be incorrect on the wire. Before
122		a receiver verifies a checksum the ILA to SIR address
123		translation must be done.
124
125- adjust transport checksum
126		When ILA translation is performed the packet is parsed
127		and if a transport layer checksum is found then it is
128		adjusted to reflect the correct checksum per the
129		translated address.
130
131- checksum neutral mapping
132		When an address is translated the difference can be offset
133		elsewhere in a part of the packet that is covered by
134		the checksum. The low order sixteen bits of the identifier
135		are used. This method is preferred since it doesn't require
136		parsing a packet beyond the IP header and in most cases the
137		adjustment can be precomputed and saved with the mapping.
138
139Note that the checksum neutral adjustment affects the low order sixteen
140bits of the identifier. When ILA to SIR address translation is done on
141egress the low order bits are restored to the original value which
142restores the identifier as it was originally sent.
143
144
145Identifier types
146================
147
148ILA defines different types of identifiers for different use cases.
149
150The defined types are:
151
152      0: interface identifier
153
154      1: locally unique identifier
155
156      2: virtual networking identifier for IPv4 address
157
158      3: virtual networking identifier for IPv6 unicast address
159
160      4: virtual networking identifier for IPv6 multicast address
161
162      5: non-local address identifier
163
164In the current implementation of kernel ILA only locally unique identifiers
165(LUID) are supported. LUID allows for a generic, unformatted 64 bit
166identifier.
167
168
169Identifier formats
170==================
171
172Kernel ILA supports two optional fields in an identifier for formatting:
173"C-bit" and "identifier type". The presence of these fields is determined
174by configuration as demonstrated below.
175
176If the identifier type is present it occupies the three highest order
177bits of an identifier. The possible values are given in the above list.
178
179If the C-bit is present,  this is used as an indication that checksum
180neutral mapping has been done. The C-bit can only be set in an
181ILA address, never a SIR address.
182
183In the simplest format the identifier types, C-bit, and checksum
184adjustment value are not present so an identifier is considered an
185unstructured sixty-four bit value::
186
187     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
188     |                            Identifier                         |
189     +                                                               +
190     |                                                               |
191     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
192
193The checksum neutral adjustment may be configured to always be
194present using neutral-map-auto. In this case there is no C-bit, but the
195checksum adjustment is in the low order 16 bits. The identifier is
196still sixty-four bits::
197
198     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
199     |                            Identifier                         |
200     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
201     |                               |  Checksum-neutral adjustment  |
202     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
203
204The C-bit may used to explicitly indicate that checksum neutral
205mapping has been applied to an ILA address. The format is::
206
207     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
208     |     |C|                    Identifier                         |
209     |     +-+                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
210     |                               |  Checksum-neutral adjustment  |
211     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
212
213The identifier type field may be present to indicate the identifier
214type. If it is not present then the type is inferred based on mapping
215configuration. The checksum neutral adjustment may automatically
216used with the identifier type as illustrated below::
217
218     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
219     | Type|                      Identifier                         |
220     +-+-+-+                         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
221     |                               |  Checksum-neutral adjustment  |
222     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
223
224If the identifier type and the C-bit can be present simultaneously so
225the identifier format would be::
226
227     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
228     | Type|C|                    Identifier                         |
229     +-+-+-+-+                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
230     |                               |  Checksum-neutral adjustment  |
231     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
232
233
234Configuration
235=============
236
237There are two methods to configure ILA mappings. One is by using LWT routes
238and the other is ila_xlat (called from NFHOOK PREROUTING hook). ila_xlat
239is intended to be used in the receive path for ILA hosts .
240
241An ILA router has also been implemented in XDP. Description of that is
242outside the scope of this document.
243
244The usage of for ILA LWT routes is:
245
246ip route add DEST/128 encap ila LOC csum-mode MODE ident-type TYPE via ADDR
247
248Destination (DEST) can either be a SIR address (for an ILA host or ingress
249ILA router) or an ILA address (egress ILA router). LOC is the sixty-four
250bit locator (with format W:X:Y:Z) that overwrites the upper sixty-four
251bits of the destination address.  Checksum MODE is one of "no-action",
252"adj-transport", "neutral-map", and "neutral-map-auto". If neutral-map is
253set then the C-bit will be present. Identifier TYPE one of "luid" or
254"use-format." In the case of use-format, the identifier type field is
255present and the effective type is taken from that.
256
257The usage of ila_xlat is:
258
259ip ila add loc_match MATCH loc LOC csum-mode MODE ident-type TYPE
260
261MATCH indicates the incoming locator that must be matched to apply
262a the translaiton. LOC is the locator that overwrites the upper
263sixty-four bits of the destination address. MODE and TYPE have the
264same meanings as described above.
265
266
267Some examples
268=============
269
270::
271
272     # Configure an ILA route that uses checksum neutral mapping as well
273     # as type field. Note that the type field is set in the SIR address
274     # (the 2000 implies type is 1 which is LUID).
275     ip route add 3333:0:0:1:2000:0:1:87/128 encap ila 2001:0:87:0 \
276	  csum-mode neutral-map ident-type use-format
277
278     # Configure an ILA LWT route that uses auto checksum neutral mapping
279     # (no C-bit) and configure identifier type to be LUID so that the
280     # identifier type field will not be present.
281     ip route add 3333:0:0:1:2000:0:2:87/128 encap ila 2001:0:87:1 \
282	  csum-mode neutral-map-auto ident-type luid
283
284     ila_xlat configuration
285
286     # Configure an ILA to SIR mapping that matches a locator and overwrites
287     # it with a SIR address (3333:0:0:1 in this example). The C-bit and
288     # identifier field are used.
289     ip ila add loc_match 2001:0:119:0 loc 3333:0:0:1 \
290	 csum-mode neutral-map-auto ident-type use-format
291
292     # Configure an ILA to SIR mapping where checksum neutral is automatically
293     # set without the C-bit and the identifier type is configured to be LUID
294     # so that the identifier type field is not present.
295     ip ila add loc_match 2001:0:119:0 loc 3333:0:0:1 \
296	 csum-mode neutral-map-auto ident-type use-format
297