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