| Decimal | Keyword | Protocol | References |
|---|---|---|---|
| 0 | HOPOPT | IPv6 Hop-by-Hop Option | [RFC1883] |
| 1 | ICMP | Internet Control Message | RFC792] |
| 2 | IGMP | Internet Group Management | [RFC1112] |
| 3 | GGP | Gateway-to-Gateway | [RFC823] |
| 4 | IP | IP in IP (encapsulation) | [RFC2003] |
| 5 | ST | Stream | [RFC1190,IEN119] |
| 6 | TCP | Transmission Control | [RFC793] |
| 7 | CBT | CBT | [Ballardie] |
| 8 | EGP | Exterior Gateway Protocol | [RFC888,DLM1] |
| 9 | IGP | Any private interior gateway(used by Cisco for their IGRP) | [IANA] |
| 10 | BBN-RCC-MON | BBN RCC Monitoring | [SGC] |
| 11 | NVP-II | Network Voice Protocol | [RFC741,SC3] |
| 12 | PUP | PUP | [PUP,XEROX] |
| 13 | ARGUS | ARGUS | [RWS4] |
| 14 | EMCON | EMCON | [BN7] |
| 15 | XNET | Cross Net Debugger | [IEN158,JFH2] |
| 16 | CHAOS | Chaos | [NC3] |
| 17 | UDP | User Datagram | [RFC768,JBP] |
| 18 | MUX | Multiplexing | [IEN90,JBP] ... |
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