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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+Identifier Locator Addressing (ILA)
+===================================
+
+
+Introduction
+============
+
+Identifier-locator addressing (ILA) is a technique used with IPv6 that
+differentiates between location and identity of a network node. Part of an
+address expresses the immutable identity of the node, and another part
+indicates the location of the node which can be dynamic. Identifier-locator
+addressing can be used to efficiently implement overlay networks for
+network virtualization as well as solutions for use cases in mobility.
+
+ILA can be thought of as means to implement an overlay network without
+encapsulation. This is accomplished by performing network address
+translation on destination addresses as a packet traverses a network. To
+the network, an ILA translated packet appears to be no different than any
+other IPv6 packet. For instance, if the transport protocol is TCP then an
+ILA translated packet looks like just another TCP/IPv6 packet. The
+advantage of this is that ILA is transparent to the network so that
+optimizations in the network, such as ECMP, RSS, GRO, GSO, etc., just work.
+
+The ILA protocol is described in Internet-Draft draft-herbert-intarea-ila.
+
+
+ILA terminology
+===============
+
+  - Identifier
+		A number that identifies an addressable node in the network
+		independent of its location. ILA identifiers are sixty-four
+		bit values.
+
+  - Locator
+		A network prefix that routes to a physical host. Locators
+		provide the topological location of an addressed node. ILA
+		locators are sixty-four bit prefixes.
+
+  - ILA mapping
+		A mapping of an ILA identifier to a locator (or to a
+		locator and meta data). An ILA domain maintains a database
+		that contains mappings for all destinations in the domain.
+
+  - SIR address
+		An IPv6 address composed of a SIR prefix (upper sixty-
+		four bits) and an identifier (lower sixty-four bits).
+		SIR addresses are visible to applications and provide a
+		means for them to address nodes independent of their
+		location.
+
+  - ILA address
+		An IPv6 address composed of a locator (upper sixty-four
+		bits) and an identifier (low order sixty-four bits). ILA
+		addresses are never visible to an application.
+
+  - ILA host
+		An end host that is capable of performing ILA translations
+		on transmit or receive.
+
+  - ILA router
+		A network node that performs ILA translation and forwarding
+		of translated packets.
+
+  - ILA forwarding cache
+		A type of ILA router that only maintains a working set
+		cache of mappings.
+
+  - ILA node
+		A network node capable of performing ILA translations. This
+		can be an ILA router, ILA forwarding cache, or ILA host.
+
+
+Operation
+=========
+
+There are two fundamental operations with ILA:
+
+  - Translate a SIR address to an ILA address. This is performed on ingress
+    to an ILA overlay.
+
+  - Translate an ILA address to a SIR address. This is performed on egress
+    from the ILA overlay.
+
+ILA can be deployed either on end hosts or intermediate devices in the
+network; these are provided by "ILA hosts" and "ILA routers" respectively.
+Configuration and datapath for these two points of deployment is somewhat
+different.
+
+The diagram below illustrates the flow of packets through ILA as well
+as showing ILA hosts and routers::
+
+    +--------+                                                +--------+
+    | Host A +-+                                         +--->| Host B |
+    |        | |              (2) ILA                   (')   |        |
+    +--------+ |            ...addressed....           (   )  +--------+
+	       V  +---+--+  .  packet      .  +---+--+  (_)
+   (1) SIR     |  | ILA  |----->-------->---->| ILA  |   |   (3) SIR
+    addressed  +->|router|  .              .  |router|->-+    addressed
+    packet        +---+--+  .     IPv6     .  +---+--+        packet
+		   /        .    Network   .
+		  /         .              .   +--+-++--------+
+    +--------+   /          .              .   |ILA ||  Host  |
+    |  Host  +--+           .              .- -|host||        |
+    |        |              .              .   +--+-++--------+
+    +--------+              ................
+
+
+Transport checksum handling
+===========================
+
+When an address is translated by ILA, an encapsulated transport checksum
+that includes the translated address in a pseudo header may be rendered
+incorrect on the wire. This is a problem for intermediate devices,
+including checksum offload in NICs, that process the checksum. There are
+three options to deal with this:
+
+- no action	Allow the checksum to be incorrect on the wire. Before
+		a receiver verifies a checksum the ILA to SIR address
+		translation must be done.
+
+- adjust transport checksum
+		When ILA translation is performed the packet is parsed
+		and if a transport layer checksum is found then it is
+		adjusted to reflect the correct checksum per the
+		translated address.
+
+- checksum neutral mapping
+		When an address is translated the difference can be offset
+		elsewhere in a part of the packet that is covered by
+		the checksum. The low order sixteen bits of the identifier
+		are used. This method is preferred since it doesn't require
+		parsing a packet beyond the IP header and in most cases the
+		adjustment can be precomputed and saved with the mapping.
+
+Note that the checksum neutral adjustment affects the low order sixteen
+bits of the identifier. When ILA to SIR address translation is done on
+egress the low order bits are restored to the original value which
+restores the identifier as it was originally sent.
+
+
+Identifier types
+================
+
+ILA defines different types of identifiers for different use cases.
+
+The defined types are:
+
+      0: interface identifier
+
+      1: locally unique identifier
+
+      2: virtual networking identifier for IPv4 address
+
+      3: virtual networking identifier for IPv6 unicast address
+
+      4: virtual networking identifier for IPv6 multicast address
+
+      5: non-local address identifier
+
+In the current implementation of kernel ILA only locally unique identifiers
+(LUID) are supported. LUID allows for a generic, unformatted 64 bit
+identifier.
+
+
+Identifier formats
+==================
+
+Kernel ILA supports two optional fields in an identifier for formatting:
+"C-bit" and "identifier type". The presence of these fields is determined
+by configuration as demonstrated below.
+
+If the identifier type is present it occupies the three highest order
+bits of an identifier. The possible values are given in the above list.
+
+If the C-bit is present,  this is used as an indication that checksum
+neutral mapping has been done. The C-bit can only be set in an
+ILA address, never a SIR address.
+
+In the simplest format the identifier types, C-bit, and checksum
+adjustment value are not present so an identifier is considered an
+unstructured sixty-four bit value::
+
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                            Identifier                         |
+     +                                                               +
+     |                                                               |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+The checksum neutral adjustment may be configured to always be
+present using neutral-map-auto. In this case there is no C-bit, but the
+checksum adjustment is in the low order 16 bits. The identifier is
+still sixty-four bits::
+
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                            Identifier                         |
+     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                               |  Checksum-neutral adjustment  |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+The C-bit may used to explicitly indicate that checksum neutral
+mapping has been applied to an ILA address. The format is::
+
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |     |C|                    Identifier                         |
+     |     +-+                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                               |  Checksum-neutral adjustment  |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+The identifier type field may be present to indicate the identifier
+type. If it is not present then the type is inferred based on mapping
+configuration. The checksum neutral adjustment may automatically
+used with the identifier type as illustrated below::
+
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     | Type|                      Identifier                         |
+     +-+-+-+                         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                               |  Checksum-neutral adjustment  |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+If the identifier type and the C-bit can be present simultaneously so
+the identifier format would be::
+
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     | Type|C|                    Identifier                         |
+     +-+-+-+-+                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                               |  Checksum-neutral adjustment  |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+
+Configuration
+=============
+
+There are two methods to configure ILA mappings. One is by using LWT routes
+and the other is ila_xlat (called from NFHOOK PREROUTING hook). ila_xlat
+is intended to be used in the receive path for ILA hosts .
+
+An ILA router has also been implemented in XDP. Description of that is
+outside the scope of this document.
+
+The usage of for ILA LWT routes is:
+
+ip route add DEST/128 encap ila LOC csum-mode MODE ident-type TYPE via ADDR
+
+Destination (DEST) can either be a SIR address (for an ILA host or ingress
+ILA router) or an ILA address (egress ILA router). LOC is the sixty-four
+bit locator (with format W:X:Y:Z) that overwrites the upper sixty-four
+bits of the destination address.  Checksum MODE is one of "no-action",
+"adj-transport", "neutral-map", and "neutral-map-auto". If neutral-map is
+set then the C-bit will be present. Identifier TYPE one of "luid" or
+"use-format." In the case of use-format, the identifier type field is
+present and the effective type is taken from that.
+
+The usage of ila_xlat is:
+
+ip ila add loc_match MATCH loc LOC csum-mode MODE ident-type TYPE
+
+MATCH indicates the incoming locator that must be matched to apply
+a the translaiton. LOC is the locator that overwrites the upper
+sixty-four bits of the destination address. MODE and TYPE have the
+same meanings as described above.
+
+
+Some examples
+=============
+
+::
+
+     # Configure an ILA route that uses checksum neutral mapping as well
+     # as type field. Note that the type field is set in the SIR address
+     # (the 2000 implies type is 1 which is LUID).
+     ip route add 3333:0:0:1:2000:0:1:87/128 encap ila 2001:0:87:0 \
+	  csum-mode neutral-map ident-type use-format
+
+     # Configure an ILA LWT route that uses auto checksum neutral mapping
+     # (no C-bit) and configure identifier type to be LUID so that the
+     # identifier type field will not be present.
+     ip route add 3333:0:0:1:2000:0:2:87/128 encap ila 2001:0:87:1 \
+	  csum-mode neutral-map-auto ident-type luid
+
+     ila_xlat configuration
+
+     # Configure an ILA to SIR mapping that matches a locator and overwrites
+     # it with a SIR address (3333:0:0:1 in this example). The C-bit and
+     # identifier field are used.
+     ip ila add loc_match 2001:0:119:0 loc 3333:0:0:1 \
+	 csum-mode neutral-map-auto ident-type use-format
+
+     # Configure an ILA to SIR mapping where checksum neutral is automatically
+     # set without the C-bit and the identifier type is configured to be LUID
+     # so that the identifier type field is not present.
+     ip ila add loc_match 2001:0:119:0 loc 3333:0:0:1 \
+	 csum-mode neutral-map-auto ident-type use-format