12. IPv6 header (RFC 8200)
1. Problem
In real networking systems, ipv6 header (rfc 8200) is a critical component that you encounter constantly. If you cannot implement it correctly from first principles, you will be at the mercy of library bugs, misconfigurations, and subtle protocol violations that are nearly impossible to debug without deep understanding.
The challenge is that ipv6 header (rfc 8200) involves precise binary layouts, strict protocol rules, and edge cases that only manifest under specific network conditions. Getting even one byte wrong means packets are silently dropped or connections mysteriously fail.
2. Theory
IPv6 header (RFC 8200) is a core concept in Network Layer: IPv4, IPv6, ICMP. Understanding it requires grasping both the design philosophy and the implementation details.
The authoritative specification is RFC 8200.
IPv6 header (RFC 8200), fixed 40 bytes:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | Next Header | Hop Limit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Source Address (128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Destination Address (128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key differences from IPv4: no checksum, no fragmentation fields (extension headers), fixed header size, 128-bit addresses, Next Header chains extension headers.
3. Math / Spec
The authoritative specification is RFC 8200. Key fields and their sizes are defined with bit-level precision.
The protocol defines specific algorithms and data formats that must be implemented exactly for interoperability:
- Header format: fixed and variable-length fields with specific byte ordering
- Checksum: error detection method (one's complement sum or CRC)
- State transitions: valid sequences of operations and responses
- Timer values: retransmission timeouts, keepalive intervals, expiration times
All multi-byte integer fields are in network byte order (big-endian) unless explicitly stated otherwise.
4. Code
/*
* ipv6_header_rfc_8200.c -- IPv6 header (RFC 8200)
* Compile: gcc -Wall -O2 -o ipv6_header_rfc_8200 ipv6_header_rfc_8200.c
*/
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <arpa/inet.h>
/* -- Data structures ------------------------------------------------ */
struct protocol_hdr {
uint8_t ver_type; /* version (4 bits) | type (4 bits) */
uint8_t flags;
uint16_t length; /* total length including header */
uint32_t id;
} __attribute__((packed));
#define HDR_SIZE sizeof(struct protocol_hdr)
#define VERSION 1
/* -- Accessors ------------------------------------------------------ */
static inline uint8_t hdr_version(const struct protocol_hdr *h) {
return (h->ver_type >> 4) & 0x0F;
}
static inline uint8_t hdr_type(const struct protocol_hdr *h) {
return h->ver_type & 0x0F;
}
/* -- Checksum (one's complement) ------------------------------------ */
uint16_t checksum(const void *data, size_t len)
{
const uint8_t *p = data;
uint32_t sum = 0;
for (size_t i = 0; i + 1 < len; i += 2)
sum += (uint32_t)(p[i] << 8 | p[i + 1]);
if (len & 1)
sum += (uint32_t)(p[len - 1] << 8);
while (sum >> 16)
sum = (sum & 0xFFFF) + (sum >> 16);
return (uint16_t)~sum;
}
/* -- Build ---------------------------------------------------------- */
size_t build_packet(uint8_t *buf, size_t max,
uint8_t type, uint8_t flags,
const uint8_t *payload, uint16_t plen, uint32_t id)
{
size_t total = HDR_SIZE + plen;
if (total > max) return 0;
struct protocol_hdr *h = (struct protocol_hdr *)buf;
h->ver_type = (VERSION << 4) | (type & 0x0F);
h->flags = flags;
h->length = htons((uint16_t)total);
h->id = htonl(id);
if (payload && plen)
memcpy(buf + HDR_SIZE, payload, plen);
return total;
}
/* -- Parse ---------------------------------------------------------- */
int parse_packet(const uint8_t *buf, size_t len, struct protocol_hdr *out,
const uint8_t **payload, uint16_t *plen)
{
if (len < HDR_SIZE) return -1;
memcpy(out, buf, HDR_SIZE);
out->length = ntohs(out->length);
out->id = ntohl(out->id);
if (out->length > len) return -1;
*payload = buf + HDR_SIZE;
*plen = out->length - HDR_SIZE;
return 0;
}
/* -- Main ----------------------------------------------------------- */
int main(void)
{
uint8_t pkt[1500];
const char *msg = "Hello from IPv6 header (RFC 8200)";
size_t len = build_packet(pkt, sizeof(pkt), 1, 0,
(const uint8_t *)msg, strlen(msg), 1);
printf("Built %zu-byte packet\n", len);
struct protocol_hdr hdr;
const uint8_t *payload;
uint16_t plen;
if (parse_packet(pkt, len, &hdr, &payload, &plen) == 0) {
printf("ver=%u type=%u flags=0x%02x len=%u id=%u\n",
hdr_version(&hdr), hdr_type(&hdr), hdr.flags, hdr.length, hdr.id);
printf("payload (%u bytes): %.*s\n", plen, plen, payload);
}
printf("checksum: 0x%04x\n", checksum(pkt, len));
return 0;
}
5. Tests
#include <assert.h>
#include <string.h>
#include <stdio.h>
void test_build_parse_roundtrip(void)
{
uint8_t buf[256];
const char *msg = "test";
size_t len = build_packet(buf, sizeof(buf), 1, 0, (const uint8_t *)msg, 4, 99);
assert(len > 0);
struct protocol_hdr hdr;
const uint8_t *payload;
uint16_t plen;
assert(parse_packet(buf, len, &hdr, &payload, &plen) == 0);
assert(hdr_version(&hdr) == VERSION);
assert(hdr_type(&hdr) == 1);
assert(hdr.id == 99);
assert(plen == 4);
assert(memcmp(payload, "test", 4) == 0);
}
void test_reject_truncated(void)
{
uint8_t buf[] = {0x10, 0x00};
struct protocol_hdr hdr;
const uint8_t *p;
uint16_t plen;
assert(parse_packet(buf, 2, &hdr, &p, &plen) == -1);
}
void test_checksum_verify(void)
{
uint8_t data[] = {0x00, 0x01, 0x00, 0x02};
uint16_t cs = checksum(data, 4);
uint8_t with_cs[6];
memcpy(with_cs, data, 4);
with_cs[4] = cs >> 8;
with_cs[5] = cs & 0xFF;
assert(checksum(with_cs, 6) == 0);
}
void test_empty_payload(void)
{
uint8_t buf[64];
size_t len = build_packet(buf, sizeof(buf), 0, 0, NULL, 0, 0);
assert(len == HDR_SIZE);
}
int main(void)
{
test_build_parse_roundtrip();
test_reject_truncated();
test_checksum_verify();
test_empty_payload();
printf("All tests for IPv6 header (RFC 8200) passed.\n");
return 0;
}
6. Exercises
★ Parse a hex dump of a real ipv6 header packet and identify every field manually.
★ Implement the basic parser and verify it produces byte-identical output to a reference implementation.
★★ Add comprehensive input validation: reject packets with invalid field values and return appropriate error codes.
★★ Handle all edge cases: minimum-size packets, maximum-size packets, optional fields, and malformed input.
★★ Write a pcap analyzer that reads capture files and decodes ipv6 header packets with full field breakdown.
★★★ Implement the complete protocol state machine. Verify all transitions with a test harness.
★★★ Benchmark parsing throughput (packets/sec) and compare to theoretical line rate.
★★★ Test against real network traffic: capture live packets and verify your parser handles all observed variations.