Networking

UDP carries discrete datagrams between transport endpoints. Its eight-byte header contains source port, destination port, length, and checksum fields. The protocol does not add TCP’s reliable ordered byte stream or its congestion-control behavior. This gives applications room to choose their own delivery policy, but it does not promise lower latency for every workload. A missing datagram can be more costly than a handshake when the application needs its contents. ...

Traceroute sends probes with increasing TTL values to discover where packets expire and which systems report that expiration. An IPv4 router forwarding a packet decrements its TTL; when it expires, an ICMP Time Exceeded response normally identifies a responding hop. A traditional UDP trace finishes when the destination reports an unused port. The output is a series of observations about probes and replies, not a guaranteed inventory of every forwarding device. ...

Discovering A and AAAA records provides possible destinations. It does not select a successful connection by itself. Address ordering, source-address selection, routing, and the application’s connection algorithm all contribute. RFC 6724 describes default source and destination selection rules, while applications can impose their own choices. A program requesting only IPv4 has already narrowed the result before any dual-stack connection strategy can act. ...

A hostname lookup starts with the program making it. On Linux, an application using glibc name-service interfaces can consult the services listed for hosts in /etc/nsswitch.conf. A program with its own DNS client may take a different path. There is no universal sequence in which every application checks a cache, then a hosts file, then the internet. ...