Introduction¶

Identifier-locator addressing (ILA) is a technique used with IPv6 thatdifferentiates between location and identity of a network node. Part of anaddress expresses the immutable identity of the node, and another partindicates the location of the node which can be dynamic. Identifier-locatoraddressing can be used to efficiently implement overlay networks fornetwork virtualization as well as solutions for use cases in mobility.

ILA can be thought of as means to implement an overlay network withoutencapsulation. This is accomplished by performing network addresstranslation on destination addresses as a packet traverses a network. Tothe network, an ILA translated packet appears to be no different than anyother IPv6 packet. For instance, if the transport protocol is TCP then anILA translated packet looks like just another TCP/IPv6 packet. Theadvantage of this is that ILA is transparent to the network so thatoptimizations 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 networkindependent of its location. ILA identifiers are sixty-fourbit values.

• Locator

  A network prefix that routes to a physical host. Locatorsprovide the topological location of an addressed node. ILAlocators are sixty-four bit prefixes.

• ILA mapping

  A mapping of an ILA identifier to a locator (or to alocator and meta data). An ILA domain maintains a databasethat 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 ameans for them to address nodes independent of theirlocation.

• ILA address

  An IPv6 address composed of a locator (upper sixty-fourbits) and an identifier (low order sixty-four bits). ILAaddresses are never visible to an application.

• ILA host

  An end host that is capable of performing ILA translationson transmit or receive.

• ILA router

  A network node that performs ILA translation and forwardingof translated packets.

• ILA forwarding cache

  A type of ILA router that only maintains a working setcache of mappings.

• ILA node

  A network node capable of performing ILA translations. Thiscan 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 ingressto an ILA overlay.
• Translate an ILA address to a SIR address. This is performed on egressfrom the ILA overlay.

ILA can be deployed either on end hosts or intermediate devices in thenetwork; these are provided by “ILA hosts” and “ILA routers” respectively.Configuration and datapath for these two points of deployment is somewhatdifferent.

The diagram below illustrates the flow of packets through ILA as wellas 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 checksumthat includes the translated address in a pseudo header may be renderedincorrect on the wire. This is a problem for intermediate devices,including checksum offload in NICs, that process the checksum. There arethree 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 addresstranslation must be done.

• adjust transport checksum

  When ILA translation is performed the packet is parsedand if a transport layer checksum is found then it isadjusted to reflect the correct checksum per thetranslated address.

• checksum neutral mapping

  When an address is translated the difference can be offsetelsewhere in a part of the packet that is covered bythe checksum. The low order sixteen bits of the identifierare used. This method is preferred since it doesn’t requireparsing a packet beyond the IP header and in most cases theadjustment can be precomputed and saved with the mapping.

Note that the checksum neutral adjustment affects the low order sixteenbits of the identifier. When ILA to SIR address translation is done onegress the low order bits are restored to the original value whichrestores 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 bitidentifier.

Identifier formats¶

Kernel ILA supports two optional fields in an identifier for formatting:“C-bit” and “identifier type”. The presence of these fields is determinedby configuration as demonstrated below.

If the identifier type is present it occupies the three highest orderbits 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 checksumneutral mapping has been done. The C-bit can only be set in anILA address, never a SIR address.

In the simplest format the identifier types, C-bit, and checksumadjustment value are not present so an identifier is considered anunstructured sixty-four bit value:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                            Identifier                         |+                                                               +|                                                               |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The checksum neutral adjustment may be configured to always bepresent using neutral-map-auto. In this case there is no C-bit, but thechecksum adjustment is in the low order 16 bits. The identifier isstill sixty-four bits:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                            Identifier                         ||                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                               |  Checksum-neutral adjustment  |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The C-bit may used to explicitly indicate that checksum neutralmapping 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 identifiertype. If it is not present then the type is inferred based on mappingconfiguration. The checksum neutral adjustment may automaticallyused with the identifier type as illustrated below:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Type|                      Identifier                         |+-+-+-+                         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                               |  Checksum-neutral adjustment  |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If the identifier type and the C-bit can be present simultaneously sothe identifier format would be:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Type|C|                    Identifier                         |+-+-+-+-+                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                               |  Checksum-neutral adjustment  |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Configuration¶

There are two methods to configure ILA mappings. One is by using LWT routesand the other is ila_xlat (called from NFHOOK PREROUTING hook). ila_xlatis intended to be used in the receive path for ILA hosts .

An ILA router has also been implemented in XDP. Description of that isoutside 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 ingressILA router) or an ILA address (egress ILA router). LOC is the sixty-fourbit locator (with format W:X:Y:Z) that overwrites the upper sixty-fourbits of the destination address. Checksum MODE is one of “no-action”,“adj-transport”, “neutral-map”, and “neutral-map-auto”. If neutral-map isset 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 ispresent 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 applya the translaiton. LOC is the locator that overwrites the uppersixty-four bits of the destination address. MODE and TYPE have thesame 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 luidila_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