TCP is called reliable because it sends a response confirming transmission.
UDP is called unreliable because it is asynchronous. This is not synonymous with not useful however verification of transmission can be done at the application layer and in some cases with repetitive data or otherwise verification of transmission is not as critical as speed or other factors.
| echo | 7 | TCP/UDP |
| discard | 9 | TCP/UDP |
| daytime | 13 | TCP/UDP |
| FTP data | 20 | TCP |
| FTP | 21 | TCP |
| SSH | 22 | TCP |
| Telnet | 23 | TCP |
| SMTP | 25 | TCP |
| time | 37 | TCP/UDP |
| whois | 43 | TCP |
| finger | 79 | TCP |
| HTTP | 80 | TCP |
| POP3 | 110 | TCP |
| NNTP | 119 | TCP |
| IMAP | 143 | TCP |
| RMI Registry | 1099 | TCP |
