Instead of having to do an arp hash table lookup for each
outgoing ip packet, forward the Routehint pointer to the
medium's bwrite() function and let it cache the arp entry
pointer.
This avoids route and arp hash table lookups for tcp, il
and connection oriented udp.
It also allows us to avoid multiple route and arp table
lookups for the retransmits once an arp/neighbour solicitation
response arrives.
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