See discussion in
HDFS-2617 (Replaced Kerberized SSL for image transfer and fsck with SPNEGO-based solution).
To handle the transition from Hadoop1.0 systems running KSSL authentication to Hadoop systems running SPNEGO, it would be good to fix the client in both 1 and 2 to try SPNEGO and then fall back to try KSSL.
This will allow organizations that are running a lot of Hadoop 1.0 to gradually transition over, without needing to convert all clusters at the same time. They would first need to update their 1.0 HFTP clients (and 2.0/0.23 if they are already running those) and then they could copy data between clusters without needing to move all clusters to SPNEGO in a big bang.