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.

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.

1 sr localsid address SID behavior end.ad 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.

1 sr localsid address 1::A1 behavior end.ad 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.