[strongSwan] AES GCM question

[Guru Shetty]

Hello All,
 In the test results page of the wiki for ikev2, the following
configuration is being used.

conn %default
	ikelifetime=60m
	keylife=20m
	rekeymargin=3m
	keyingtries=1
	keyexchange=ikev2
	ike=aes256gcm16-aesxcbc-modp2048!
	esp=aes256gcm16-modp2048!

Is there a reason for using "aesxcbc" for ike? Doesn't aes-gcm provide
both authenticity and confidentiality?

Also, if I want to use the transport mode for the above configuration,
do I need to worry about anything?

Thanks,
Guru