mininet + xterm

mininet> r1 xterm
Warning: This program is an suid-root program or is being run by the root user.
The full text of the error or warning message cannot be safely formatted
in this environment. You may get a more descriptive message by running the
program as a non-root user or by removing the suid bit on the executable.
xterm: Xt error: Can‘t open display: %s
xterm: DISPLAY is not set
mininet> h1 echo $DISPLAY

mininet> xterm h1
Error: Cannot connect to display
mininet> h1 xterm &
mininet> 

root@ubuntu:/data1/root/ryu/SDN-IP-Ryu# whereis xterm
xterm: /usr/bin/xterm /usr/share/man/man1/xterm.1.gz
root@ubuntu:/data1/root/ryu/SDN-IP-Ryu# 

# xterm
Warning: This program is an suid-root program or is being run by the root user.
The full text of the error or warning message cannot be safely formatted
in this environment. You may get a more descriptive message by running the
program as a non-root user or by removing the suid bit on the executable.
xterm: Xt error: Can‘t open display: %s
xterm: DISPLAY is not set

root@ubuntu:/data1/root/ryu/SDN-IP-Ryu# mn -x
*** No default OpenFlow controller found for default switch!
*** Falling back to OVS Bridge
*** Creating network
*** Adding controller
*** Adding hosts:
h1 h2 
*** Adding switches:
s1 
*** Adding links:
(h1, s1) (h2, s1) 
*** Configuring hosts
h1 h2 
Error starting terms: Cannot connect to display
*** Starting controller

*** Starting 1 switches
s1 
*** Starting CLI:
mininet> quit
*** Stopping 0 controllers

*** Stopping 1 switches
s1 ..
*** Stopping 2 links

*** Stopping 2 hosts
h1 h2 
*** Done
completed in 21.936 seconds