10. Switching tables and MAC learning
1. Problem
In real networking systems, switching tables and mac learning 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 switching tables and mac learning 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
Switching tables and MAC learning is a core concept in Link Layer. Understanding it requires grasping both the design philosophy and the implementation details.
MAC learning and switching table:
Frame arrives on Port 1 from MAC aa:bb:cc:dd:ee:01
-> Learn: aa:bb:cc:dd:ee:01 is on Port 1 (add to FDB)
-> Lookup destination MAC in FDB:
- Found on Port 3? Forward only to Port 3 (unicast)
- Not found? Flood to all ports except Port 1 (unknown unicast)
- Broadcast MAC? Flood to all ports except Port 1
FDB (Forwarding Database):
+---------------------+------+---------+
| MAC Address | Port | Age |
+---------------------+------+---------+
| aa:bb:cc:dd:ee:01 | 1 | 0 sec |
| aa:bb:cc:dd:ee:02 | 3 | 15 sec |
| ff:ff:ff:ff:ff:ff | ALL | static |
+---------------------+------+---------+
Aging: entries expire after ~300 seconds (default) if no traffic seen.
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
/*
* switching_tables_and_mac_learning.c -- Switching tables and MAC learning
* Compile: gcc -Wall -O2 -o switching_tables_and_mac_learning switching_tables_and_mac_learning.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 Switching tables and MAC learning";
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 Switching tables and MAC learning passed.\n");
return 0;
}
6. Exercises
★ Parse a hex dump of a real switching tables and mac learning 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 switching tables and mac learning 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.