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.

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.