tcp-ip : tcp接收数据

在ip层进行初始化时，对每种协议都会定义一个结构，并将其注册到全局列表。在IP层收到数据，要将其提交到上一层，这时，根据上层协议的类型从全局列表中找到对应的结构，然后调用其中的函数对数据进行处理：

[ net/ipv4/af_inet.c ]

static const struct net_protocol tcp_protocol = {.early_demux=tcp_v4_early_demux,.handler=tcp_v4_rcv,.err_handler=tcp_v4_err,.no_policy=1,.netns_ok=1,.icmp_strict_tag_validation = 1,};

其中tcp_v4_rcv就是在TCP层用来接收数据的函数。

当收到一个包后，先要对这个包的头部进行一些检测，首先要检查头部的长度是否正确，这时会用到下面的函数：

[ include/linux/skbuff.h ]

static inline int pskb_may_pull(struct sk_buff *skb, unsigned int len){/* len <= skb主buffer的长度，大多数情况满足这一条件 */if (likely(len <= skb_headlen(skb)))return 1;/* len > skb的长度，这种情况不太容易出现 */if (unlikely(len > skb->len))return 0;/* 将分片中的数据拷贝到skb主buffer的尾部 * 如果主buffer中空间不足，会重新分配内存 * 所以此函数调用后所有指向skb内部数据的指针都会失效 */return __pskb_pull_tail(skb, len - skb_headlen(skb)) != NULL;}

然后是检测校验和，有几个函数要用到：

[ include/linux/skbuff.h ]

static inline int skb_csum_unnecessary(const struct sk_buff *skb){return skb->ip_summed & CHECKSUM_UNNECESSARY;}

CHECKSUM_UNNECESSARY:

The hardware you're dealing with doesn't calculate the full checksum  (as in CHECKSUM_COMPLETE), but it does parse headers and verify checksums for specific protocols e.g. TCP/UDP/SCTP, then, for such packets it will set CHECKSUM_UNNECESSARY if their checksums are okay. skb->csum is still undefined in this case though. It is a bad option, but, unfortunately, nowadays most vendors do this.

CHECKSUM_COMPLETE:

This is the most generic way. The device supplied checksum of the _whole_packet as seen by netif_rx() and fills out in skb->csum. Meaning, the hardware doesn't need to parse L3/L4 headers to implement this.

Note: Even if device supports only some protocols, but is able to produce skb->csum, it MUST use CHECKSUM_COMPLETE, not CHECKSUM_UNNECESSARY.

在TCP头部前面有一个12字节的pseudo header，分别为32位源地址，32位目的地址，8位保留位，8位协议类型，16位长度（头部和数据，不包含pseudo header）。计算校验和时要考虑pseudo header。有关联的函数为：

[ arch/x86/include/asm/checksum_32.h ]

/** * csum_tcpup_nofold - Compute an IPv4 pseudo header checksum. * @saddr: source address * @daddr: destination address * @len: length of packet * @proto: ip protocol of packet * @sum: initial sum to be added in (32bit unfolded) * * Returns the pseudo header checksum the input data. Result is * 32bit unfolded. */static inline __wsumcsum_tcpudp_nofold(__be32 saddr, __be32 daddr, unsigned short len,   unsigned short proto, __wsum sum){/* 初始校验和 ＋ 目的地址 ＋ 源地址 ＋ （（长度 ＋ 协议）<< 8） * 最后的<<8是左移8位，成为16位。因为计算校验和是以16位为单位的 */asm("  addl %1, %0\n"    "  adcl %2, %0\n"    "  adcl %3, %0\n"    "  adcl $0, %0\n": "=r" (sum)    : "g" (daddr), "g" (saddr), "g" ((len + proto) << 8), "0" (sum));return sum;} /** * csum_fold - Fold and invert a 32bit checksum. * sum: 32bit unfolded sum * * Fold a 32bit running checksum to 16bit and invert it. This is usually * the last step before putting a checksum into a packet. * Make sure not to mix with 64bit checksums. */static inline __sum16 csum_fold(__wsum sum){    /* 32位的sum的高16位和低16位相加，对和取反     */    __asm__(        "  addl %1,%0\n"        "  adcl $0xffff,%0"        : "=r" (sum)        : "r" ((__force u32)sum << 16),          "0" ((__force u32)sum & 0xffff0000)    );    return (__force __sum16)(~(__force u32)sum >> 16);}/* * computes the checksum of the TCP/UDP pseudo-header * returns a 16-bit checksum, already complemented */static inline __sum16 csum_tcpudp_magic(__be32 saddr, __be32 daddr,                       unsigned short len,                       unsigned short proto,                       __wsum sum){    return csum_fold(csum_tcpudp_nofold(saddr,daddr,len,proto,sum));}/* * Calculate(/check) TCP checksum */static inline __sum16 tcp_v4_check(int len, __be32 saddr,                   __be32 daddr, __wsum base){    /* IPPROTO_TCP = 6     */    return csum_tcpudp_magic(saddr,daddr,len,IPPROTO_TCP,base);}

以上函数会在接收下面函数中调用：
[ net/ipv4/tcp_ipv4.c ]

static __sum16 tcp_v4_checksum_init(struct sk_buff *skb){const struct iphdr *iph = ip_hdr(skb);if (skb->ip_summed == CHECKSUM_OMPLETE) {/* 此时校验和己经计算过并保存在skb->csum * 对TCP头部前面的12字节的pseudo header计算校验和 * 结果0说明校验和正确，设置校验和状态为CHECKSUM_UNNECESSARY */if (!tcp_v4_check(skb->len, iph->saddr,  iph->daddr, skb->csum)) {skb->ip_summed = CHECKSUM_UNNECESSARY;return 0;}}/* 初始化校验和 * 只是将pseudo header各位相加 */skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr,       skb->len, IPPROTO_TCP, 0);if (skb->len <= 76) {/* 包的长度不超过76字节，对整个包计算校验和 */return __skb_checksum_complete(skb);}return 0;}