.. _srv6_ad_plugin_doc:

SRv6 dynamic proxy
==================

SRv6 endpoint to SR-unaware appliance via dynamic proxy (End.AD)
----------------------------------------------------------------

Overview
~~~~~~~~

The dynamic proxy is an improvement over the static proxy (@ref
srv6_as_plugin_doc) that dynamically learns the SR information before
removing it from the incoming traffic. The same information can then be
re-attached to the traffic returning from the SF. As opposed to the
static SR proxy, no CACHE information needs to be configured. Instead,
the dynamic SR proxy relies on a local caching mechanism on the node
instantiating this segment. Therefore, a dynamic proxy segment cannot be
the last segment in an SR SC policy. A different SR behavior should thus
be used if the SF is meant to be the final destination of an SR SC
policy.

Upon receiving a packet whose active segment matches a dynamic SR proxy
function, the proxy node pops the top MPLS label or applies the SRv6 End
behavior, then compares the updated SR information with the cache entry
for the current segment. If the cache is empty or different, it is
updated with the new SR information. The SR information is then removed
and the inner packet is sent towards the SF.

The cache entry is not mapped to any particular packet, but instead to
an SR SC policy identified by the receiving interface (IFACE-IN). Any
non-link-local IP packet or non-local Ethernet frame received on that
interface will be re-encapsulated with the cached headers as described
in @ref srv6_as_plugin_doc. The SF may thus drop, modify or generate new
packets without affecting the proxy.

For more information, please see
`draft-xuclad-spring-sr-service-chaining <https://datatracker.ietf.org/doc/draft-xuclad-spring-sr-service-chaining/>`__.

CLI configuration
~~~~~~~~~~~~~~~~~

The following command instantiates a new End.AD segment that sends the
inner packets on interface ``IFACE-OUT`` towards an appliance at address
``S-ADDR`` and restores the encapsulation headers of the packets coming
back on interface ``IFACE-IN``.

::

   sr localsid address SID behavior end.ad nh S-ADDR oif IFACE-OUT iif IFACE-IN

For example, the below command configures the SID ``1::A1`` with an
End.AD function for sending traffic on interface
``GigabitEthernet0/8/0`` to the appliance at address ``A1::``, and
receiving it back on interface ``GigabitEthernet0/9/0``.

::

   sr localsid address 1::A1 behavior end.ad nh A1:: oif GigabitEthernet0/8/0 iif GigabitEthernet0/9/0

Pseudocode
~~~~~~~~~~

The dynamic proxy SRv6 pseudocode is obtained by inserting the following
instructions between lines 1 and 2 of the static proxy SRv6 pseudocode.

::

   IF NH=SRH & SL > 0 THEN
       Decrement SL and update the IPv6 DA with SRH[SL]
       IF C(IFACE-IN) different from IPv6 encaps THEN              ;; Ref1
           Copy the IPv6 encaps into C(IFACE-IN)                   ;; Ref2
   ELSE
       Drop the packet

**Ref1:** “IPv6 encaps” represents the IPv6 header and any attached
extension header.

**Ref2:** C(IFACE-IN) represents the cache entry associated to the
dynamic SR proxy segment. It is identified with IFACE-IN in order to
efficiently retrieve the right SR information when a packet arrives on
this interface.

In addition, the inbound policy should check that C(IFACE-IN) has been
defined before attempting to restore the IPv6 encapsulation, and drop
the packet otherwise.
