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IP协议是一种无连接且不可靠的协议，只提供尽力而为的服务。

IP报头结构

IP报头结构如下：

字段 长度（字节）
版本/头部长度 1
服务类型 1
总长度 2
标识 2
片偏移字段 2
生存时间（TTL） 1
协议 1
校验和 2
源IP地址 4
目的IP地址 4

字段

校验和

IP协议的校验和原理同UDP协议，都是采用原码与反码之和作为校验判断的。为了能够使用16位字保存校验和，计算校验和时采取了循环进位的方式。

如果数据二进制反码的求和大于16位，那么让超出16位的部分继续与低16位相加，直到校验和为16位。

IPv4

IPv6

分片

IP转发

lwip实现

结构体

lwip中，IP首部结构体定义如下：

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struct ip_hdr {
  /* version / header length */
  PACK_STRUCT_FLD_8(u8_t _v_hl);
  /* type of service */
  PACK_STRUCT_FLD_8(u8_t _tos);
  /* total length */
  PACK_STRUCT_FIELD(u16_t _len);
  /* identification */
  PACK_STRUCT_FIELD(u16_t _id);
  /* fragment offset field */
  PACK_STRUCT_FIELD(u16_t _offset);
#define IP_RF 0x8000U        /* reserved fragment flag */
#define IP_DF 0x4000U        /* don't fragment flag */
#define IP_MF 0x2000U        /* more fragments flag */
#define IP_OFFMASK 0x1fffU   /* mask for fragmenting bits */
  /* time to live */
  PACK_STRUCT_FLD_8(u8_t _ttl);
  /* protocol*/
  PACK_STRUCT_FLD_8(u8_t _proto);
  /* checksum */
  PACK_STRUCT_FIELD(u16_t _chksum);
  /* source and destination IP addresses */
  PACK_STRUCT_FLD_S(ip4_addr_p_t src);
  PACK_STRUCT_FLD_S(ip4_addr_p_t dest);
} PACK_STRUCT_STRUCT;

发送（IPv4）

发送源码如下：

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/**
 * Same as ip_output_if() but 'src' address is not replaced by netif address
 * when it is 'any'.
 */
err_t
ip4_output_if_src(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *dest,
                  u8_t ttl, u8_t tos,
                  u8_t proto, struct netif *netif)
{
#if IP_OPTIONS_SEND
  return ip4_output_if_opt_src(p, src, dest, ttl, tos, proto, netif, NULL, 0);
}

/**
 * Same as ip_output_if_opt() but 'src' address is not replaced by netif address
 * when it is 'any'.
 */
err_t
ip4_output_if_opt_src(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *dest,
                      u8_t ttl, u8_t tos, u8_t proto, struct netif *netif, void *ip_options,
                      u16_t optlen)
{
#endif /* IP_OPTIONS_SEND */
  struct ip_hdr *iphdr;
  ip4_addr_t dest_addr;
#if CHECKSUM_GEN_IP_INLINE
  u32_t chk_sum = 0;
#endif /* CHECKSUM_GEN_IP_INLINE */

  LWIP_ASSERT_CORE_LOCKED();
  LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);

  MIB2_STATS_INC(mib2.ipoutrequests);

  /* Should the IP header be generated or is it already included in p? */
  if (dest != LWIP_IP_HDRINCL) {
    u16_t ip_hlen = IP_HLEN;
#if IP_OPTIONS_SEND
    u16_t optlen_aligned = 0;
    if (optlen != 0) {
#if CHECKSUM_GEN_IP_INLINE
      int i;
#endif /* CHECKSUM_GEN_IP_INLINE */
      if (optlen > (IP_HLEN_MAX - IP_HLEN)) {
        /* optlen too long */
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_output_if_opt: optlen too long\n"));
        IP_STATS_INC(ip.err);
        MIB2_STATS_INC(mib2.ipoutdiscards);
        return ERR_VAL;
      }
      /* round up to a multiple of 4 */
      optlen_aligned = (u16_t)((optlen + 3) & ~3);
      ip_hlen = (u16_t)(ip_hlen + optlen_aligned);
      /* First write in the IP options */
      if (pbuf_add_header(p, optlen_aligned)) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_output_if_opt: not enough room for IP options in pbuf\n"));
        IP_STATS_INC(ip.err);
        MIB2_STATS_INC(mib2.ipoutdiscards);
        return ERR_BUF;
      }
      MEMCPY(p->payload, ip_options, optlen);
      if (optlen < optlen_aligned) {
        /* zero the remaining bytes */
        memset(((char *)p->payload) + optlen, 0, (size_t)(optlen_aligned - optlen));
      }
#if CHECKSUM_GEN_IP_INLINE
      for (i = 0; i < optlen_aligned / 2; i++) {
        chk_sum += ((u16_t *)p->payload)[i];
      }
#endif /* CHECKSUM_GEN_IP_INLINE */
    }
#endif /* IP_OPTIONS_SEND */
    /* generate IP header */
    if (pbuf_add_header(p, IP_HLEN)) {
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_output: not enough room for IP header in pbuf\n"));

      IP_STATS_INC(ip.err);
      MIB2_STATS_INC(mib2.ipoutdiscards);
      return ERR_BUF;
    }

    iphdr = (struct ip_hdr *)p->payload;
    LWIP_ASSERT("check that first pbuf can hold struct ip_hdr",
                (p->len >= sizeof(struct ip_hdr)));

    IPH_TTL_SET(iphdr, ttl);
    IPH_PROTO_SET(iphdr, proto);
#if CHECKSUM_GEN_IP_INLINE
    chk_sum += PP_NTOHS(proto | (ttl << 8));
#endif /* CHECKSUM_GEN_IP_INLINE */

    /* dest cannot be NULL here */
    ip4_addr_copy(iphdr->dest, *dest);
#if CHECKSUM_GEN_IP_INLINE
    chk_sum += ip4_addr_get_u32(&iphdr->dest) & 0xFFFF;
    chk_sum += ip4_addr_get_u32(&iphdr->dest) >> 16;
#endif /* CHECKSUM_GEN_IP_INLINE */

    IPH_VHL_SET(iphdr, 4, ip_hlen / 4);
    IPH_TOS_SET(iphdr, tos);
#if CHECKSUM_GEN_IP_INLINE
    chk_sum += PP_NTOHS(tos | (iphdr->_v_hl << 8));
#endif /* CHECKSUM_GEN_IP_INLINE */
    IPH_LEN_SET(iphdr, lwip_htons(p->tot_len));
#if CHECKSUM_GEN_IP_INLINE
    chk_sum += iphdr->_len;
#endif /* CHECKSUM_GEN_IP_INLINE */
    IPH_OFFSET_SET(iphdr, 0);
    IPH_ID_SET(iphdr, lwip_htons(ip_id));
#if CHECKSUM_GEN_IP_INLINE
    chk_sum += iphdr->_id;
#endif /* CHECKSUM_GEN_IP_INLINE */
    ++ip_id;

    if (src == NULL) {
      ip4_addr_copy(iphdr->src, *IP4_ADDR_ANY4);
    } else {
      /* src cannot be NULL here */
      ip4_addr_copy(iphdr->src, *src);
    }

#if CHECKSUM_GEN_IP_INLINE
    chk_sum += ip4_addr_get_u32(&iphdr->src) & 0xFFFF;
    chk_sum += ip4_addr_get_u32(&iphdr->src) >> 16;
    chk_sum = (chk_sum >> 16) + (chk_sum & 0xFFFF);
    chk_sum = (chk_sum >> 16) + chk_sum;
    chk_sum = ~chk_sum;
    IF__NETIF_CHECKSUM_ENABLED(netif, NETIF_CHECKSUM_GEN_IP) {
      iphdr->_chksum = (u16_t)chk_sum; /* network order */
    }
#if LWIP_CHECKSUM_CTRL_PER_NETIF
    else {
      IPH_CHKSUM_SET(iphdr, 0);
    }
#endif /* LWIP_CHECKSUM_CTRL_PER_NETIF*/
#else /* CHECKSUM_GEN_IP_INLINE */
    IPH_CHKSUM_SET(iphdr, 0);
#if CHECKSUM_GEN_IP
    IF__NETIF_CHECKSUM_ENABLED(netif, NETIF_CHECKSUM_GEN_IP) {
      IPH_CHKSUM_SET(iphdr, inet_chksum(iphdr, ip_hlen));
    }
#endif /* CHECKSUM_GEN_IP */
#endif /* CHECKSUM_GEN_IP_INLINE */
  } else {
    /* IP header already included in p */
    if (p->len < IP_HLEN) {
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_output: LWIP_IP_HDRINCL but pbuf is too short\n"));
      IP_STATS_INC(ip.err);
      MIB2_STATS_INC(mib2.ipoutdiscards);
      return ERR_BUF;
    }
    iphdr = (struct ip_hdr *)p->payload;
    ip4_addr_copy(dest_addr, iphdr->dest);
    dest = &dest_addr;
  }

  IP_STATS_INC(ip.xmit);

  LWIP_DEBUGF(IP_DEBUG, ("ip4_output_if: %c%c%"U16_F"\n", netif->name[0], netif->name[1], (u16_t)netif->num));
  ip4_debug_print(p);

#if ENABLE_LOOPBACK
  if (ip4_addr_eq(dest, netif_ip4_addr(netif))
#if !LWIP_HAVE_LOOPIF
      || ip4_addr_isloopback(dest)
#endif /* !LWIP_HAVE_LOOPIF */
     ) {
    /* Packet to self, enqueue it for loopback */
    LWIP_DEBUGF(IP_DEBUG, ("netif_loop_output()\n"));
    return netif_loop_output(netif, p);
  }
#if LWIP_MULTICAST_TX_OPTIONS
  if ((p->flags & PBUF_FLAG_MCASTLOOP) != 0) {
    netif_loop_output(netif, p);
  }
#endif /* LWIP_MULTICAST_TX_OPTIONS */
#endif /* ENABLE_LOOPBACK */
#if IP_FRAG
  /* don't fragment if interface has mtu set to 0 [loopif] */
  if (netif->mtu && (p->tot_len > netif->mtu)) {
    return ip4_frag(p, netif, dest);
  }
#endif /* IP_FRAG */

  LWIP_DEBUGF(IP_DEBUG, ("ip4_output_if: call netif->output()\n"));
  return netif->output(netif, p, dest);
}

BSD实现