Every nation in the world has intrenationally recognized borders, including Israel. This is what constitutes a border. Not some indefinite and variable moving invisible line which must be "negotiated" periodically. The only "core issue" involved is moving Israel back behind it's own borders to allow creation of a Palestinisn nation. This will be discussed in direct negotiations.
Thousands gather in Kuala Lumpur for rally urging Malaysian leader to quit (AP)
from the article: EU: Abbas close to accepting direct Mideast peace talks