Add fastdp-crypto docs

docs/fastdp-crypto.md

@@ -4,14 +4,15 @@ This document describes implementation details of the fast datapath encryption.
 
 At the high level, we use the ESP protocol ([RFC 2406][esp]) in the Transport
 mode. Each packet is encrypted with AES in GCM mode ([RFC 4106][aesgcm]), with
-32byte key and 4byte salt. This combo provides the following security
+32 byte key and 4 byte salt. This combo provides the following security
 properties:
 
 * Data confidentiality.
 * Data origin authentication.
 * Integrity.
-* Anti-replay service.
-* Limited traffic flow confidentiality.
+* Anti-replay.
+* Limited traffic flow confidentiality as fast datapath VXLAN packets are fully
+  encrypted.
 
 ## Notation
 

docs/fastdp.md

@@ -52,9 +52,9 @@ overlay, which has a more sophisticated dynamic mechanism for coping
 with low path MTUs.
 
 To avoid triggering this fallback in typical deployments, the datapath
-interface is statically configured with an MTU of 1410 bytes allowing
+interface is statically configured with an MTU of 1376 bytes allowing
 it to work with most underlay network provider MTUs, including GCE at
-1460 bytes (the fifty byte difference accommodates the vxlan overhead).
+1460 bytes (the eighty four byte difference accommodates the encrypted vxlan overhead).
 This value can be overridden by setting `WEAVE_MTU` at launch if
 necessary.