Home' RTCA Documents for Review : C2 Link Systems MASPS_Draft Contents Appendix E
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C2 Link System Connections and Switchovers
The C2 Link System Connection between UA and CS may be established over a terrestrial
link that provides coverage while the UA is within RLOS of the GRS. The C2 Link System
Connection between UA and CS may alternatively be established over a SATCOM link
where the UAS operator uses satellite transponder capacity and a SATCOM ground e arth
station (GES) at or near the CS to control a UA that is operating beyond RLOS (BRLOS)
of the terrestrial links. The C2 Link System may also make use of mobile air/ground
networks that provide service over a wide geographic continental area such as a UAS
specific Terrestrial Wireless Network or a SATCOM Network that provides global service
over oceanic, continental and remote areas.
Although the CS and UA C2 Link Management Systems may initiate transmission of User
Data to each other without prior coordination (defined as “connectionless data transfer”)
and this mode of data transmission can be useful, it is not flexible and vulnerable to
exploitation and is consequentially not considered in this MASPS. When flexible data
transfer requiring (1) negotiation of certain link parameters such as message sizes, and/or
(2) secure data transfer where authentication of the parties involved prior to exchange of
User Data is desired or required, a “connection-oriented data transfer” protocol is more
appropriate and is the basis of the requirements in this MASPS..
When using a “connection-oriented data transfer,” a logical association, otherwise referred
to as a “Connection,” must first be established by the two parties, in this case, the CS and
the UA. The establishment, maintenance and termination of a Connection occurs in three
User Data transfer
During the Connection establishment phase, the CS and UA agree to exchange data and
perform authentication of their identities to make sure that the two parties are whom they
say they are. In its simplest form, one party, e.g., the UA, will send a “Connection Request”
Control Message to the CS. The CS will authenticate the identity of the sen der (the UA)
and accept the request if the sender address matches the UA address it has on record. The
response accepting the request, called the “Connection Confirm” Control Message may
include some link parameters that need to be used. For example, these would include packet
size and data rate if it is a Point-to-Point link. Secure Connections require more complex
exchanges to negotiate the means to be used to generate “authentication tags” to verify that
the UA and CS are whom they say they are. These latter exchanges are called a
“handshake,” and the messages exchanged during the hand-shake are Control Messages.
After Connection establishment, the User Data transfer phase is entered. User Data
messages are exchanged using the authenticated sender and destination addresses. In the
case of secure CS-UA Connections, each message will also include an authentication tag
computed using a keyed-Hash Message Authentication Code (HMAC) or Cypher-based
Message Authentication Code (CMAC).
The Connection termination phase is entered when one side or the other wishes to terminate
and does so by sending a “Disconnect Request” Control Message. After the other side has
responded by sending a “Disconnect Confirm” Control Message the Connection is
terminated in an orderly manner.
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