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# SRv6 endpoint to SR-unaware appliance via static proxy (End.AS) {#srv6_as_plugin_doc}
## Overview
The static proxy is an SR endpoint behavior for processing SR-MPLS or SRv6
encapsulated traffic on behalf of an SR-unaware SF. This proxy thus receives SR
traffic that is formed of an MPLS label stack or an IPv6 header on top of an
inner packet, which can be Ethernet, IPv4 or IPv6.
A static SR proxy segment is associated with the following mandatory parameters:
- INNER-TYPE: Inner packet type
- S-ADDR: Ethernet or IP address of the SF (only for inner type IPv4 and IPv6)
- IFACE-OUT: Local interface for sending traffic towards the SF
- IFACE-IN: Local interface receiving the traffic coming back from the SF
- CACHE: SR information to be attached on the traffic coming back from the SF,
including at least
* CACHE.SA: IPv6 source address (SRv6 only)
* CACHE.LIST: Segment list expressed as MPLS labels or IPv6 address
A static SR proxy segment is thus defined for a specific SF, inner packet type
and cached SR information. It is also bound to a pair of directed interfaces on
the proxy. These may be both directions of a single interface, or opposite
directions of two different interfaces. The latter is recommended in case the SF
is to be used as part of a bi-directional SR SC policy. If the proxy and the SF
both support 802.1Q, IFACE-OUT and IFACE-IN can also represent sub-interfaces.
The first part of this behavior is triggered when the proxy node receives a
packet whose active segment matches a segment associated with the static proxy
behavior. It removes the SR information from the packet then sends it on a
specific interface towards the associated SF. This SR information corresponds to
the full label stack for SR-MPLS or to the encapsulation IPv6 header with any
attached extension header in the case of SRv6.
The second part is an inbound policy attached to the proxy interface receiving
the traffic returning from the SF, IFACE-IN. This policy attaches to the
incoming traffic the cached SR information associated with the SR proxy segment.
If the proxy segment uses the SR-MPLS data plane, CACHE contains a stack of
labels to be pushed on top the packets. With the SRv6 data plane, CACHE is
defined as a source address, an active segment and an optional SRH (tag,
segments left, segment list and metadata). The proxy encapsulates the packets
with an IPv6 header that has the source address, the active segment as
destination address and the SRH as a routing extension header. After the SR
information has been attached, the packets are forwarded according to the active
segment, which is represented by the top MPLS label or the IPv6 Destination
Address.
In this scenario, there are no restrictions on the operations that can be
performed by the SF on the stream of packets. It may operate at all protocol
layers, terminate transport layer connections, generate new packets and initiate
transport layer connections. This behavior may also be used to integrate an
IPv4-only SF into an SRv6 policy. However, a static SR proxy segment can be used
in only one service chain at a time. As opposed to most other segment types, a
static SR proxy segment is bound to a unique list of segments, which represents
a directed SR SC policy. This is due to the cached SR information being defined
in the segment configuration. This limitation only prevents multiple segment
lists from using the same static SR proxy segment at the same time, but a single
segment list can be shared by any number of traffic flows. Besides, since the
returning traffic from the SF is re-classified based on the incoming interface,
an interface can be used as receiving interface (IFACE-IN) only for a single SR
proxy segment at a time. In the case of a bi-directional SR SC policy, a
different SR proxy segment and receiving interface are required for the return
direction.
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.AS segment that sends the inner
packets on interface `IFACE-OUT` towards an appliance at address `S-ADDR` and
restores the segment list ``<S1, S2, S3>`` with a source address `SRC-ADDR` on
the packets coming back on interface `IFACE-IN`.
```
sr localsid address SID behavior end.as nh S-ADDR oif IFACE-OUT iif IFACE-IN src SRC-ADDR next S1 next S2 next S3
```
For example, the below command configures the SID `1::A1` with an End.AS
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.as nh A1:: oif GigabitEthernet0/8/0 iif GigabitEthernet0/9/0 src 1:: next 2::20 next 3::30 next 4::40
```
## Pseudocode
### Static proxy for inner type IPv4
Upon receiving an IPv6 packet destined for S, where S is an IPv6 static proxy
segment for IPv4 traffic, a node N does:
```
IF ENH == 4 THEN ;; Ref1
Remove the (outer) IPv6 header and its extension headers
Forward the exposed packet on IFACE-OUT towards S-ADDR
ELSE
Drop the packet
```
**Ref1:** 4 refers to IPv4 encapsulation as defined by IANA allocation for Internet
Protocol Numbers.
Upon receiving a non link-local IPv4 packet on IFACE-IN, a node N does:
```
Decrement TTL and update checksum
IF CACHE.SRH THEN ;; Ref2
Push CACHE.SRH on top of the existing IPv4 header
Set NH value of the pushed SRH to 4
Push outer IPv6 header with SA, DA and traffic class from CACHE
Set outer payload length and flow label
Set NH value to 43 if an SRH was added, or 4 otherwise
Lookup outer DA in appropriate table and proceed accordingly
```
**Ref2:** CACHE.SRH represents the SRH defined in CACHE, if any, for the static SR
proxy segment associated with IFACE-IN.
### Static proxy for inner type IPv6
Upon receiving an IPv6 packet destined for S, where S is an IPv6 static proxy
segment for IPv6 traffic, a node N does:
```
IF ENH == 41 THEN ;; Ref1
Remove the (outer) IPv6 header and its extension headers
Forward the exposed packet on IFACE-OUT towards S-ADDR
ELSE
Drop the packet
```
**Ref1:** 41 refers to IPv6 encapsulation as defined by IANA allocation for Internet
Protocol Numbers.
Upon receiving a non-link-local IPv6 packet on IFACE-IN, a node N does:
```
Decrement Hop Limit
IF CACHE.SRH THEN ;; Ref2
Push CACHE.SRH on top of the existing IPv6 header
Set NH value of the pushed SRH to 41
Push outer IPv6 header with SA, DA and traffic class from CACHE
Set outer payload length and flow label
Set NH value to 43 if an SRH was added, or 41 otherwise
Lookup outer DA in appropriate table and proceed accordingly
```
**Ref2:** CACHE.SRH represents the SRH defined in CACHE, if any, for the static SR
proxy segment associated with IFACE-IN.
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