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+## IOAM and SRv6 for M-Anycast service selection    {#ioam_manycast_doc}
+
+Anycast is often used to have a client choose one out of multiple servers.
+This might be due to performance, scale, or availability reasons.
+If a client initiates a TCP connection in an anycast scenario, 
+the TCP session is usually established with the server which answers the quickest.
+
+There are cases where it is desirable to:
+- allow choosing the destination server not based on "fastest response time",
+but based on the delay between server and client (e.g. for a streaming application).
+- allow choosing the destination server based on other parameters, 
+such as server load information.
+- ensure that all TCP connections of a particular client are hooked up to the same 
+server, i.e. that all TCP sessions following the first one are connected to the same server as the first session.
+
+M-anycast combines IOAM and Segment Routing v6 (SRv6) to provide for a solution:
+- IOAM information is added to the initial TCP SYN packet to understand the transmit delay, as well as to the SYN-ACK packet to understand the return delay.
+- SRv6 is used to steer traffic to the set of servers, rather than rely on anycast procedures. 
+Client and Servers can be left unchanged. 
+SRv6 and iOAM information is added and removed "in transit"
+
+Introduce M-Anycast Server as a solution component to leverage Segment Routing to steer 
+traffic, IOAM for optimized service selection.
+M-Anycast Server:
+- Hosts the Anycast address of the services
+- Intercepts TCP-SYN, replicates the SYN and sends to a selected subset of all services using SRv6 spray policy
+- Chooses an appropriate SYN-ACK using embedded in-band OAM data and forwards that SYN-ACK to the client with SRv6 header intact. The SRv6 header in the SYN-ACK received by the client is used to reach the selected server for subsequent packet exchange.
+
+VPP can function as an M-Anycast server. VPP can also be used as a IOAM and SRv6 decapsulating node at the application server edge. This allows for caching of IOAM and reattaching it to correlate the path performance across request-response (SYN/SYN-ACK) forwarding path. 
+
+## Configuration
+Example: Highly redundant video-caches as micro-services hosted in multiple public clouds. All Services have an IPv6 address allocated from anycast IPv6 prefix (db06::/64).
+Here configuration to enable VPP as an M-Anycast server and IOAM caching/reattach node is provided.
+
+### M-Anycast Server
+- Enable M-Anycast service selection using:
+    
+
+    set ioam ip6 sr-tunnel-select [disable] [oneway|rtt] [wait_for_responses <n|default 3>] sr_localsid <IPv6 address>
+    
+Example:
+
+
+    set ioam ip6 sr-tunnel-select oneway sr_localsid db0a::2
+    
+- Enable IOAM tracing. Override node for selected traffic processing
+Example:
+To enable M-Anycast service selection with IOAM tracing enabled for db06::/64 prefix and on the return path to process service selection for SRv6 localsid db0a::2:
+
+
+    classify table acl-miss-next ip6-node ip6-lookup mask hex 000000000000ffffffffffffffff0000 buckets 2 skip 2 match 1
+    
+    classify session acl-hit-next ip6-node ip6-add-syn-hop-by-hop table-index 0 match hex 0000000000000000000000000000000000000000000000000000000000000000000000000000db060000000000000000 ioam-encap anycast
+    
+    classify session acl-hit-next ip6-node ip6-lookup table-index 0 match hex 0000000000000000000000000000000000000000000000000000000000000000000000000000db0a0000000000000000 ioam-decap anycast
+    
+    set int input acl intfc GigabitEthernet0/4/0 ip6-table 0
+    set int input acl intfc GigabitEthernet0/5/0 ip6-table 0
+    set ioam-trace profile trace-type 0x09 trace-elts 3 trace-tsp 1 node-id 0x1
+    set ioam rewrite trace
+
+
+- Enable SRv6 spray policy for steering traffic towards M-Anycast prefix.
+Example:
+To steer anycast prefix db06::/64 towards servers with address db07::1, db08::1, db09::1:
+
+
+    sr policy add bsid db11::2 next db07::1 insert spray
+    sr policy mod add sl bsid db11::2 next db08::1  
+    sr policy mod add sl bsid db11::2 next db09::1  
+    sr steer l3 db06::/64 via sr policy bsid db11::2
+    sr localsid address db0a::2 behavior end
+    
+    
+### IOAM Caching/reattach at application service edge
+- Enable IOAM data caching
+
+
+    set ioam ip6 cache sr_localsid <ip6 address> [disable]
+    
+Example: 
+    
+    
+    set ioam ip6 cache sr_localsid db07::1 
+
+- Enable IOAM decap
+Example: To decap IOAM and cache the data towards db06::/64 and reinsert the data towards db04::/64:
+
+
+    classify table acl-miss-next ip6-node ip6-lookup mask hex     000000000000ffffffffffffffff0000 buckets 2 skip 2 match 1
+   
+    classify session acl-hit-next ip6-node ip6-lookup table-index 0 match hex 0000000000000000000000000000000000000000000000000000000000000000000000000000db060000000000000000 ioam-decap anycast
+    
+    classify session acl-hit-next ip6-node ip6-lookup table-index 0 match hex 0000000000000000000000000000000000000000000000000000000000000000000000000000db070000000000000000 ioam-decap anycast
+    
+    classify session acl-hit-next ip6-node ip6-add-from-cache-hop-by-hop table-index 0 match hex 0000000000000000000000000000000000000000000000000000000000000000000000000000db040000000000000000 ioam-encap anycast-response
+    
+    set int input acl intfc GigabitEthernet0/4/0 ip6-table 0   
+    
+    set ioam-trace profile trace-type 0x1f trace-elts 4 trace-tsp 1 node-id 0x3 app-data 0x1234   
+    
+- Enable SRv6 localsid processing to strip SRv6 header before forwarding towards application service
+
+
+    sr localsid address db07::1 behavior end psp
+