Networking Services and Ports
There are two general types of network services, which are connection less and connection oriented. Connection oriented service performs connection establishment, data transfer, and connection termination.
Ping
The "ping" program uses ICMP echo message requests and listens for ICMP echo message reply messages from its intended host. Using the -R option with ping enables the record route feature. If this option is used ping will set the record route (RR) in the outgoing ICMP IP datagram
Traceroute
The "traceroute" program uses ICMP messaging and the time to live (TTL) field in the IP header. It works by sending a packet to the intended host with a TTL value of 1. The first router will send back the ICMP "time exceeded" message to the sending host. Then the traceroute program will send a message with a TTL of 2, then 3, etc. This way it will get information about each router using the information received in the ICMP packets. To get information about the receiving host, the message is sent to a port that is not likely to be serviced by that host. A ICMP "port unreachable" error message is generated and sent back.
Telnet
Some telnet command codes and their meanings| Command Code | Description |
| 236 | EOF |
| 237 | SUSP - Suspend the current process |
| 238 | ABORT - Abort process |
| 239 | EOR - End of record |
| 240 | SE - Suboption end |
| 241 | NOP - No operation |
| 242 | DM - Data Mark |
| 243 | BRK - Break |
| 244 | IP - Interrupt process |
| 245 | AO - Abort output |
| 246 | AYT - Are you there |
| 247 | EC - Escape character |
| 248 | EL - Erase Line |
| 249 | GA - Go ahead |
| 250 | SB - Suboption begin |
| 251 | WILL - Sender wants to enable option / Receiver says OK |
| 252 | WONT - Sender wants to disable option / Receiver says not OK |
| 253 | DO - Sender wants receiver to enable option / Receiver says OK |
| 254 | DONT - Sender wants receiver to disable option / Receiver says not OK |
On items 251 through 254 above, a third byte specifies options as follows:
| ID | Name | RFC |
| 1 | Echo | 857 |
| 3 | Supress go ahead | 858 |
| 5 | Status | 859 |
| 6 | Timing Mark | 860 |
| 24 | Terminal type | 1091 |
| 31 | Window size | 1073 |
| 32 | Terminal speed | 1079 |
| 33 | Remote flow control | 1372 |
| 34 | Line mode | 1184 |
| 36 | Environment variables | 1408 |
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