extract icmp into its own static library; replace all __attribute__((constructor)) self-registrations with explicit init() calls in dispatch_init

icmp/icmp.h

@@ -0,0 +1,41 @@
+#pragma once
+#include <cstdint>
+#include <span>
+#include "eth.h"
+#include "ipv4.h"
+#include "span_writer.h"
+
+namespace icmp {
+
+struct __attribute__((packed)) echo {
+    uint8_t type;
+    uint8_t code;
+    uint16_t checksum;
+    uint16_t id;
+    uint16_t seq;
+};
+static_assert(sizeof(echo) == 8);
+
+void init();
+void handle(std::span<const uint8_t> frame, span_writer& tx);
+
+template <typename Buf>
+void prepend_echo_request(Buf& buf,
+                          eth::mac_addr src_mac, ipv4::ip4_addr src_ip,
+                          eth::mac_addr dst_mac, ipv4::ip4_addr dst_ip,
+                          uint16_t id, uint16_t seq,
+                          size_t payload_len = 0) {
+    auto* e = buf.template prepend<echo>();
+    e->type = 8;
+    e->code = 0;
+    e->checksum = 0;
+    e->id = id;
+    e->seq = seq;
+    size_t icmp_len = sizeof(echo) + payload_len;
+    e->checksum = ipv4::checksum(e, icmp_len);
+    ipv4::prepend(buf, dst_mac, src_mac, src_ip, dst_ip, 1, icmp_len);
+}
+
+bool parse_echo_reply(std::span<const uint8_t> frame, ipv4::ip4_addr& src_ip, uint16_t expected_id);
+
+} // namespace icmp