RT-1.21: BGP TCP MSS and PMTUD

feature/experimental/bgp/otg_tests/bgp_tcp_mss_path_mtu/README.md

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+# RT-1.21: BGP TCP MSS and PMTUD 
+
+## Summary
+
+*   Validate changes in TCP MSS value is allowed and takes effect.
+*   Validate DUT's PMTUD compliance.
+
+## Topology
+
+*   ATE:port1 <-> port1:DUT1:Port2 <-> port1:DUT2
+
+## Procedure
+
+*   Establish BGP sessions as follows:
+    *   ATE:port1 --- eBGP-IPv4/IPv6 ---- DUT1:port1.
+    *   ATE:port1 ---- iBGP IPv4 ---- DUT2:port1.
+*   Verify that the default TCP MSS value is set below the default interface MTU value.
+*   Change the Interface MTU on the DUT1:port1 port as well as ATE:port1 to 5040B
+*   Configure IP TCP MSS value of 4096 bytes on the DUT1:port1.
+*   Re-establish the EBGP sessions by tcp reset.
+*   Verify that the TCP MSS value is set to 4096 bytes for the IPv4 and IPv6 EBGP sessions.
+*   Establish iBGP session with MD5 enabled between ATE:port1 and DUT2:port2.
+*   Ensure that the MTU on the DUT1:port1 towards ATE1:port1 is left at default (1500B) while the ATE1:port1 interface towards DUT1:port1 is set at 5040B. Please also make sure that the DUT2:port2 MTU is set at 5040B as well.
+*   Enable PMTUD on DUT2:port2. 
+*   Re-establish the IBGP sessions by tcp reset.
+*   Validate that the min MSS value has been adjusted to be below 1500 bytes on the tcp session.
+
+## Config Parameter coverage
+
+*   /neighbors/neighbor/transport/config/tcp-mss 
+*   /neighbors/neighbor/transport/config/mtu-discovery 
+
+## Telemetry Parameter coverage
+
+*   /neighbors/neighbor/transport/state/tcp-mss 
+*   /neighbors/neighbor/transport/state/mtu-discovery 
+
+## Protocol/RPC Parameter coverage
+
+N/A
+
+## Minimum DUT platform requirement
+
+N/A