Layered end-to-end communication
The TCP/IP architecture separates application behavior, transport, internet delivery and link access. IP offers best-effort forwarding; TCP adds reliable ordered byte-stream delivery between endpoints. UDP is a datagram transport without TCP’s stream reliability mechanisms. An application selects a transport appropriate to its own design.
Handshake and sequence reasoning
A TCP client sends SYN with an initial sequence number; server replies SYN-ACK acknowledging the next expected number; client ACK completes establishment. Data advances sequence space by byte count. Retransmission responds to loss, while acknowledgements report reception and flow-control windows protect receiver capacity.
Closing and errors
FIN closes one direction after queued data; the peer acknowledges and can finish its own direction. RST abruptly aborts a connection. A timeout is not equivalent to refusal: policy, loss or route issues may silence a SYN, while a reset can indicate an actively rejected connection.
Separating services
DNS name resolution, ARP next-hop discovery, IP forwarding and TCP state each answer a different question. Test them individually. TCP success proves a transport exchange, not user authorization or valid application payload. The current topology engine provides selected diagnostics, not full packet-level TCP emulation.
Signal / protocol flow
- 1DNS answer
- 2Local next-hop ARP
- 3IP forwarding
- 4TCP SYN/SYN-ACK/ACK
- 5Application exchange
Technical terminology
- Sequence number: byte-stream position
- ACK: next expected sequence
- Window: receiver flow-control allowance
- Retransmission: resend missing data
Worked example
Client SYN sequence 100 consumes one position; server acknowledges 101. Later client sends 200 bytes starting at 101, so the next expected sequence becomes 301 if all arrive.
Configuration reference
# Reference diagnostics, not an arbitrary host shell nslookup intranet.local ping 192.168.10.5 # ITAV supported service check: see help for test-port syntax.
Troubleshooting scenario
Name resolves and ping succeeds but TCP port test fails. Check listening service and permitted protocol/port, then return-path state; do not alter a correct DNS record.
Knowledge check
After receiving 200 bytes starting at sequence 101, what ACK is expected?
Related practice
References
Educational model — not real hardware, traffic or official certification.