'-n' 付けてみましたが・・・

# tcpdump -n -i em0 port 21
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on em0, link-type EN10MB (Ethernet), capture size 96 bytes
01:13:41.860704 IP 192.168.1.55.3776 > 192.168.1.1.21: R 1047316341:1047316341(0) ack 631648578 win 0
01:13:41.879697 IP 192.168.1.55.3780 > 192.168.1.1.21: S 2602868932:2602868932(0) win 65535 <mss 1414,nop,wscale 2,nop,nop,sackOK>
01:13:41.879781 IP 192.168.1.1.21 > 192.168.1.55.3780: S 3258941258:3258941258(0) ack 2602868933 win 65535 <mss 1414,nop,wscale 3,sackOK,eol>
01:13:41.879936 IP 192.168.1.55.3780 > 192.168.1.1.21: . ack 1 win 65044
01:13:41.902322 IP 192.168.1.1.21 > 192.168.1.55.3780: F 1:1(0) ack 1 win 8307
01:13:41.902555 IP 192.168.1.55.3780 > 192.168.1.1.21: . ack 2 win 65044
01:13:41.902806 IP 192.168.1.55.3780 > 192.168.1.1.21: R 1:1(0) ack 2 win 0