If the arp table (i.e., arp cache) does not contain an entry for a given ip address, arp_ip_eth() returns NW_SUSPEND and the outgoing packet is placed in the ip port's de_arp_head/de_arp_tail queue. Before being placed in this queue, the packet is encapsulated in a "xmit" header (as opposed to an ethernet header).
xmit_hdr is declared in generic/ip_eth.c:
typedef struct xmit_hdr
where xh_time is the time at which the packet is placed in the de_arp_head/de_arp_tail queue and xh_ipaddr is the destination ip address of the packet.