doc: update README

README.md

@@ -1,13 +1,17 @@
 # libicmp
 
-Non-blocking ICMPv4 library for ping, traceroute, and network diagnostics.
+Non-blocking ICMPv4 library for building ping, traceroute, and network
+diagnostic tools.
 
 ## Dependencies
 
 - libc
-- GNU Make
+- GNU make
 - clang
-- Criterion (optional, for tests)
+- POSIX-compliant OS
+
+For tests only:
+- criterion
 
 ## Build
 
@@ -18,6 +22,93 @@ make install PREFIX=/usr/local
 make test
 ```
 
+## API
+
+### Handle lifecycle
+
+```c
+icmp_handle_t *icmp_create(void);
+void           icmp_destroy(icmp_handle_t *h);
+int            icmp_get_fd(const icmp_handle_t *h);
+```
+
+`icmp_create()` opens a raw ICMP socket and returns an opaque handle.
+`icmp_get_fd()` exposes the underlying file descriptor for use with
+`select()` or `poll()`. Errors are retrieved via `icmp_strerror()`.
+
+### Send
+
+```c
+int icmp_send_echo(icmp_handle_t *h, struct in_addr dest, uint16_t id,
+                   uint16_t seq, uint8_t ttl,
+                   const void *payload, size_t payload_len);
+
+int icmp_send_raw(icmp_handle_t *h, uint8_t type, uint8_t code,
+                  uint32_t header_rest,
+                  const void *payload, size_t len, struct in_addr dest,
+                  uint8_t ttl);
+```
+
+`icmp_send_echo()` builds and sends an ICMP ECHO_REQUEST with the given
+id, seq, and payload. `icmp_send_raw()` sends an arbitrary ICMP packet
+by type and code.
+
+### Receive
+
+```c
+typedef void (*icmp_callback_t)(const icmp_reply_t *reply, void *userdata);
+
+int icmp_process(icmp_handle_t *h, icmp_callback_t cb, void *userdata,
+                 size_t max_packets);
+```
+
+`icmp_process()` drains up to `max_packets` from the socket in a
+non-blocking loop, invoking `cb` for each one. The `icmp_reply_t` passed
+to the callback contains the ICMP type/code, source address, TTL,
+timestamp, and payload. Returns the number of packets processed, or -1
+on error.
+
+## Modules
+
+### handle/
+Handle lifecycle: `icmp_create()`, `icmp_destroy()`, `icmp_get_fd()`.
+Owns the opaque `icmp_handle_t` struct and the error state attached to it.
+
+### socket/
+Raw socket creation, non-blocking configuration, and the DF (Don't
+Fragment) flag via `IP_MTU_DISCOVER`.
+
+### send/
+Packet building and transmission. Assembles the ICMP header and payload,
+computes the checksum, and writes to the socket.
+
+### recv/
+Non-blocking receive loop. Reads datagrams, timestamps each one on
+arrival, and dispatches them through the caller-supplied callback.
+
+### packet/
+ICMP and IP packet parsing and building. Extracts fields from raw
+datagrams, including nested packets inside ICMP error messages
+(`icmp_offending_packet_t`).
+
+### error/
+Error state storage and formatting. Wraps `errno` and library-specific
+error codes into a single slot on the handle, exposed via
+`icmp_strerror()` and `icmp_should_retry()`.
+
+### utils/
+RFC 1071 one's-complement checksum and nanosecond time helpers
+(`icmp_get_time()`, `icmp_time_diff_ns()`).
+
 ## License
 
-GPL v3
+This project is licensed under the GNU General Public License v3.0 - see the [LICENSE](../LICENSE) file for details.
+
+## Authors
+
+- **lohhiiccc** - [GitHub](https://github.com/lohhiiccc)
+
+## See Also
+
+- [RFC 792 - Internet Control Message Protocol](https://datatracker.ietf.org/doc/html/rfc792)
+- [RFC 1071 - Computing the Internet Checksum](https://datatracker.ietf.org/doc/html/rfc1071)