07. The TCP header
1. Problem
In real networking systems, the tcp header 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 the tcp header 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
The TCP header is a core concept in Transport: UDP, TCP, QUIC. Understanding it requires grasping both the design philosophy and the implementation details.
TCP header (RFC 9293), 20 bytes minimum:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset| Res |C|E|U|A|P|R|S|F| Window |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (variable) ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: CWR, ECE, URG, ACK, PSH, RST, SYN, FIN Data offset: header length in 32-bit words (min 5, max 15 = 60 bytes)
3. Math / Spec
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
/*
* the_tcp_header.c -- The TCP header
* Compile: gcc -Wall -O2 -o the_tcp_header the_tcp_header.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 The TCP header";
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 The TCP header passed.\n");
return 0;
}
6. Exercises
★ Parse a hex dump of a real the tcp 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 the tcp 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.