我在学习uip，PC通过交叉网线连接DM9000A，串口打印DM9000A的输入输出数据，发现DM9000A发出去的ARP请求的帧长是42字节，PC发送给路由的ARP请求是64字节，TCP/IP书上说了，以太网的最小帧长是64字节。
查看uip源码，ARP请求和回应的最后数据帧长度确实就一个ARP头部长度，没有填充0，没有CRC校验，接下来立即将这个包提交给DM9000A发出。
查看lwip的ARP请求，情况也是一样的。
ARP请求
if (i == UIP_ARPTAB_SIZE) {
/* The destination address was not in our ARP table, so we
overwrite the IP packet with an ARP request. */
memset(BUF->ethhdr.dest.addr, 0xff, 6);
memset(BUF->dhwaddr.addr, 0x00, 6);
memcpy(BUF->ethhdr.src.addr, uip_ethaddr.addr, 6);
memcpy(BUF->shwaddr.addr, uip_ethaddr.addr, 6);
uip_ipaddr_copy(BUF->dipaddr, ipaddr);
uip_ipaddr_copy(BUF->sipaddr, uip_hostaddr);
BUF->opcode = HTONS(ARP_REQUEST); /* ARP request. */
BUF->hwtype = HTONS(ARP_HWTYPE_ETH);
BUF->protocol = HTONS(UIP_ETHTYPE_IP);
BUF->hwlen = 6;
BUF->protolen = 4;
BUF->ethhdr.type = HTONS(UIP_ETHTYPE_ARP);
uip_appdata = &uip_buf[UIP_TCPIP_HLEN + UIP_LLH_LEN];
uip_len = sizeof(struct arp_hdr);
return;
}
ARP回应
/* ARP request. If it asked for our address, we send out a
reply. */
if (uip_ipaddr_cmp(BUF->dipaddr, uip_hostaddr)) {
/* First, we register the one who made the request in our ARP
table, since it is likely that we will do more communication
with this host in the future. */
uip_arp_update(BUF->sipaddr, &BUF->shwaddr);
/* The reply opcode is 2. */
BUF->opcode = HTONS(2);
memcpy(BUF->dhwaddr.addr, BUF->shwaddr.addr, 6);
memcpy(BUF->shwaddr.addr, uip_ethaddr.addr, 6);
memcpy(BUF->ethhdr.src.addr, uip_ethaddr.addr, 6);
memcpy(BUF->ethhdr.dest.addr, BUF->dhwaddr.addr, 6);
BUF->dipaddr[0] = BUF->sipaddr[0];
BUF->dipaddr[1] = BUF->sipaddr[1];
BUF->sipaddr[0] = uip_hostaddr[0];
BUF->sipaddr[1] = uip_hostaddr[1];
BUF->ethhdr.type = HTONS(UIP_ETHTYPE_ARP);
uip_len = sizeof(struct arp_hdr);
}
DM9000A发出的ARP请求以太网数据帧：
[0]tx: length: 42
tx: 00: ff ff ff ff ff ff 12 34
tx: 08: 56 78 9a bc 08 06 00 01
tx: 10: 08 00 06 04 00 01 12 34
tx: 18: 56 78 9a bc c0 a8 02 20
tx: 20: 00 00 00 00 00 00 c0 a8
tx: 28: 02 20
PC发送给路由的ARP请求以太网数据帧：
[0]rx: length: 64
rx: 00: ff ff ff ff ff ff 00 23
rx: 08: 8b 94 8b 31 08 06 00 01
rx: 10: 08 00 06 04 00 01 00 23
rx: 18: 8b 94 8b 31 c0 a8 02 cf
rx: 20: 00 00 00 00 00 00 c0 a8
rx: 28: 02 01 00 00 00 00 00 00
rx: 30: 00 00 00 00 00 00 00 00
rx: 38: 00 00 00 00 6a bc be 18
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