直接上upd_input

void
udp_input(struct pbuf *p, struct netif *inp)
{
  struct udp_hdr *udphdr;
  struct udp_pcb *pcb, *prev;
  struct udp_pcb *uncon_pcb;
  struct ip_hdr *iphdr;
  u16_t src, dest;
  u8_t local_match;

  PERF_START;   //看到這個是不是想起來perf.h裡的內容了呀

  UDP_STATS_INC(udp.recv);

  iphdr = p->payload;

  /* Check minimum length (IP header + UDP header)
   * and move payload pointer to UDP header */
  if (p->tot_len < (IPH_HL(iphdr) * 4 + UDP_HLEN) || pbuf_header(p, -(s16_t)(IPH_HL(iphdr) * 4))) {  //判斷接收的資料符不符合規範, 且將p->payload移到udp頭部

    /* drop short packets */
    LWIP_DEBUGF(UDP_DEBUG,
                ("udp_input: short UDP datagram (%"U16_F" bytes) discarded\n", p->tot_len));
    UDP_STATS_INC(udp.lenerr);
    UDP_STATS_INC(udp.drop);
    snmp_inc_udpinerrors();
    pbuf_free(p);
    goto end;
  }

  udphdr = (struct udp_hdr *)p->payload;

  LWIP_DEBUGF(UDP_DEBUG, ("udp_input: received datagram of length %"U16_F"\n", p->tot_len));

  /* convert src and dest ports to host byte order */
  src = ntohs(udphdr->src);
  dest = ntohs(udphdr->dest);

  udp_debug_print(udphdr);

  /* print the UDP source and destination */
  LWIP_DEBUGF(UDP_DEBUG,
              ("udp (%"U16_F".%"U16_F".%"U16_F".%"U16_F", %"U16_F") <-- "
               "(%"U16_F".%"U16_F".%"U16_F".%"U16_F", %"U16_F")\n",
               ip4_addr1(&iphdr->dest), ip4_addr2(&iphdr->dest),
               ip4_addr3(&iphdr->dest), ip4_addr4(&iphdr->dest), ntohs(udphdr->dest),
               ip4_addr1(&iphdr->src), ip4_addr2(&iphdr->src),
               ip4_addr3(&iphdr->src), ip4_addr4(&iphdr->src), ntohs(udphdr->src)));

#if LWIP_DHCP
  pcb = NULL;
  /* when LWIP_DHCP is active, packets to DHCP_CLIENT_PORT may only be processed by
     the dhcp module, no other UDP pcb may use the local UDP port DHCP_CLIENT_PORT */
  if (dest == DHCP_CLIENT_PORT) {
    /* all packets for DHCP_CLIENT_PORT not coming from DHCP_SERVER_PORT are dropped! */
    if (src == DHCP_SERVER_PORT) {
      if ((inp->dhcp != NULL) && (inp->dhcp->pcb != NULL)) {
        /* accept the packe if
           (- broadcast or directed to us) -> DHCP is link-layer-addressed, local ip is always ANY!
           - inp->dhcp->pcb->remote == ANY or iphdr->src */
        if ((ip_addr_isany(&inp->dhcp->pcb->remote_ip) ||
           ip_addr_cmp(&(inp->dhcp->pcb->remote_ip), &(iphdr->src)))) {
          pcb = inp->dhcp->pcb;
        }
      }
    }
  } else
#endif /* LWIP_DHCP */
  {
    prev = NULL;
    local_match = 0;
    uncon_pcb = NULL;
    /* Iterate through the UDP pcb list for a matching pcb.
     * 'Perfect match' pcbs (connected to the remote port & ip address) are
     * preferred. If no perfect match is found, the first unconnected pcb that
     * matches the local port and ip address gets the datagram. */
    for (pcb = udp_pcbs; pcb != NULL; pcb = pcb->next) {   //從udp_pcbs裡找尋符合目標的udp控制塊
      local_match = 0;
      /* print the PCB local and remote address */
      LWIP_DEBUGF(UDP_DEBUG,
                  ("pcb (%"U16_F".%"U16_F".%"U16_F".%"U16_F", %"U16_F") --- "
                   "(%"U16_F".%"U16_F".%"U16_F".%"U16_F", %"U16_F")\n",
                   ip4_addr1(&pcb->local_ip), ip4_addr2(&pcb->local_ip),
                   ip4_addr3(&pcb->local_ip), ip4_addr4(&pcb->local_ip), pcb->local_port,
                   ip4_addr1(&pcb->remote_ip), ip4_addr2(&pcb->remote_ip),
                   ip4_addr3(&pcb->remote_ip), ip4_addr4(&pcb->remote_ip), pcb->remote_port));

      /* compare PCB local addr+port to UDP destination addr+port */
      if ((pcb->local_port == dest) &&
          (ip_addr_isany(&pcb->local_ip) ||
           ip_addr_cmp(&(pcb->local_ip), &(iphdr->dest)) ||
#if LWIP_IGMP
           ip_addr_ismulticast(&(iphdr->dest)) ||
#endif /* LWIP_IGMP */
           ip_addr_isbroadcast(&(iphdr->dest), inp))) { //判斷接收的地址對不對，包括埠
        local_match = 1;
        if ((uncon_pcb == NULL) &&
            ((pcb->flags & UDP_FLAGS_CONNECTED) == 0)) {
          /* the first unconnected matching PCB */     
          uncon_pcb = pcb;
        }
      }
      /* compare PCB remote addr+port to UDP source addr+port */
      if ((local_match != 0) &&
          (pcb->remote_port == src) &&
          (ip_addr_isany(&pcb->remote_ip) ||
           ip_addr_cmp(&(pcb->remote_ip), &(iphdr->src)))) {
        /* the first fully matching PCB */
        if (prev != NULL) {
          /* move the pcb to the front of udp_pcbs so that is
             found faster next time */
          prev->next = pcb->next;
          pcb->next = udp_pcbs;
          udp_pcbs = pcb;
        } else {
          UDP_STATS_INC(udp.cachehit);
        }
        break;
      }
      prev = pcb;
    }
    /* no fully matching pcb found? then look for an unconnected pcb */
    if (pcb == NULL) {
      pcb = uncon_pcb;
    }
  }

  /* Check checksum if this is a match or if it was directed at us. */
  if (pcb != NULL || ip_addr_cmp(&inp->ip_addr, &iphdr->dest)) {
    LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, ("udp_input: calculating checksum\n"));
#if LWIP_UDPLITE
    if (IPH_PROTO(iphdr) == IP_PROTO_UDPLITE) {
      /* Do the UDP Lite checksum */
#if CHECKSUM_CHECK_UDP
      u16_t chklen = ntohs(udphdr->len);
      if (chklen < sizeof(struct udp_hdr)) {
        if (chklen == 0) {
          /* For UDP-Lite, checksum length of 0 means checksum
             over the complete packet (See RFC 3828 chap. 3.1) */
          chklen = p->tot_len;
        } else {
          /* At least the UDP-Lite header must be covered by the
             checksum! (Again, see RFC 3828 chap. 3.1) */
          UDP_STATS_INC(udp.chkerr);
          UDP_STATS_INC(udp.drop);
          snmp_inc_udpinerrors();
          pbuf_free(p);
          goto end;
        }
      }
      if (inet_chksum_pseudo_partial(p, (struct ip_addr *)&(iphdr->src),
                             (struct ip_addr *)&(iphdr->dest),
                             IP_PROTO_UDPLITE, p->tot_len, chklen) != 0) {
        LWIP_DEBUGF(UDP_DEBUG | 2,
                    ("udp_input: UDP Lite datagram discarded due to failing checksum\n"));
        UDP_STATS_INC(udp.chkerr);
        UDP_STATS_INC(udp.drop);
        snmp_inc_udpinerrors();
        pbuf_free(p);
        goto end;
      }
#endif /* CHECKSUM_CHECK_UDP */
    } else
#endif /* LWIP_UDPLITE */
    {
#if CHECKSUM_CHECK_UDP
      if (udphdr->chksum != 0) {
        if (inet_chksum_pseudo(p, (struct ip_addr *)&(iphdr->src),
                               (struct ip_addr *)&(iphdr->dest),
                               IP_PROTO_UDP, p->tot_len) != 0) {
          LWIP_DEBUGF(UDP_DEBUG | 2,
                      ("udp_input: UDP datagram discarded due to failing checksum\n"));
          UDP_STATS_INC(udp.chkerr);
          UDP_STATS_INC(udp.drop);
          snmp_inc_udpinerrors();
          pbuf_free(p);
          goto end;
        }
      }
#endif /* CHECKSUM_CHECK_UDP */
    }
    if(pbuf_header(p, -UDP_HLEN)) {   //將p->payload移到資料區域
      /* Can we cope with this failing? Just assert for now */
      LWIP_ASSERT("pbuf_header failed\n", 0);
      UDP_STATS_INC(udp.drop);
      snmp_inc_udpinerrors();
      pbuf_free(p);
      goto end;
    }
    if (pcb != NULL) {
      snmp_inc_udpindatagrams();
      /* callback */
      if (pcb->recv != NULL) {
        /* now the recv function is responsible for freeing p */
        pcb->recv(pcb->recv_arg, pcb, p, &(iphdr->src), src);   //這裡呼叫udp回撥函式，通過udp_accept函式實現
      } else {
        /* no recv function registered? then we have to free the pbuf! */
        pbuf_free(p);
        goto end;
      }
    } else {  
      LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, ("udp_input: not for us.\n"));

#if LWIP_ICMP
      /* No match was found, send ICMP destination port unreachable unless
         destination address was broadcast/multicast. */
      if (!ip_addr_isbroadcast(&iphdr->dest, inp) &&
          !ip_addr_ismulticast(&iphdr->dest)) {
        /* move payload pointer back to ip header */
        pbuf_header(p, (IPH_HL(iphdr) * 4) + UDP_HLEN);
        LWIP_ASSERT("p->payload == iphdr", (p->payload == iphdr));
        icmp_dest_unreach(p, ICMP_DUR_PORT);  //如果不是，傳送ICMP報文
      }
#endif /* LWIP_ICMP */
      UDP_STATS_INC(udp.proterr);
      UDP_STATS_INC(udp.drop);
      snmp_inc_udpnoports();
      pbuf_free(p);
    }
  } else {
    pbuf_free(p);
  }
end:
  PERF_STOP("udp_input");
}