added rough mavlink 2.1 proposal

text/mavlink2_1.md

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+  * Start date: 2022-10-12
+  * Contributors: Andrew Tridgell
+
+# Summary
+
+Rough proposal for MAVLink 2.1
+
+# Motivation
+
+Key features wanted:
+
+ * support for more than 256 system IDs
+ * support for checking CRCs in routers that don't have all xml
+ * better target_system/target_component support
+ * maximum compatibility with existing mavlink2 systems
+
+# Detailed Design
+
+The design takes advantage of the incompat_flags and compat_flags
+fields in the MAVLink2 header.
+
+New incompat_flags flags would be:
+
+ * MAVLINK_IFLAG_SYSID16 : system ID in header is 16 bits wide
+ * MAVLINK_IFLAG_TARGETTED : header has target_system and target_component appended
+ * MAVLINK_IFLAG_SPLIT_CRC : 16 bit crc is as described below
+
+New compat_flags flags would be:
+
+ * MAVLINK_FLAG_21_CAPABLE : sender is capable of handling MAVLink 2.1
+
+Note that for the SYSID16 and TARGETTED flags the sender should only
+use the flag if is it needed. So if the systemID in the message is
+<=255 then the flag should not be set and the existing 8 bit field
+should be used. For the TARGETTED bit this should only be used if the
+target systemID is >255 (needing 16 bits) or if the message that needs
+targetting doesn't have a target_system or target_component field.
+
+Following these rules maximises compatibility with existing MAVLink2
+capable systems.
+
+Note that when SYSID16 flag is set then the target_system would be 16
+bits when TARGETTED is set.
+
+For messages with existing target_system/target_component fields, if
+the TARGETTED bit is set then the existing fields are ignored by the
+recipient. The sender should set the old fields to zero.
+
+The 16 bit size of the expanded systemID could perhaps be 32 bits,
+allowing for an IPv4 address to be used, which could be useful for
+many use cases. The target audience is drone light shows which often
+use WiFi.
+
+## Split CRC
+
+The idea of the split CRC is to address a concern from people making
+MAVLink routing tools that they can't CRC new messages that they
+didn't have the XML for at the time they built the router firmware.
+
+The solution is to do what we should have done for MAVLink 1.0, which
+is to make 8 bits of the 16 bit CRC depend on the structural seed, and
+8 bits not depend on the seed.