Default gateway consideration for netlmm
RA sent to inform MN about the network prefix and gateway existed.But within a netlmm domain,MN should think that it's home network is kept unchanged when it is roaming and it's default gateway is unchanged is the best.But current the netlmm draft only can make the MN know the home network prefix.It can't give a convience way to make the gateway keep unchanged when the MN is roaming within a netlmm domain.