[网络协议]lwIp源码解析--udp

lwIp源码解析–udp

一、简介
?? 1.1 lwip版本
?? lwip 2.1.2
?? 1.2 代码路径

lwip-2.1.2\src\core\udp.c

?? 代码行数1.3K，UDP是无连接、无状态的传输层协议，在实现上比TCP要简单很多。

二、源码解析
?? 源码分为四部分，分别是UDP头部定义、UDP初始化函数、UDP发送函数、UDP接收函数。

?? 2.1 udp 头部

struct udp_hdr {
  PACK_STRUCT_FIELD(u16_t src);
  PACK_STRUCT_FIELD(u16_t dest);  /* src/dest UDP ports */
  PACK_STRUCT_FIELD(u16_t len);
  PACK_STRUCT_FIELD(u16_t chksum);
} PACK_STRUCT_STRUCT;

?? udp的头部有八个字节，分别是源/目的端口号、UDP头部加上数据的总长度len（此时的总长度和IP头的负载长度的一致）、以及校验和 chksum（此处的校验和要加上12字节的伪头部，包括源/目的IP地址、协议号、UDP长度）

?? 2.2 初始化函数

struct udp_pcb * udp_new        (void);
struct udp_pcb * udp_new_ip_type(u8_t type);
void             udp_remove     (struct udp_pcb *pcb);
err_t            udp_bind       (struct udp_pcb *pcb, const ip_addr_t *ipaddr,
                                 u16_t port);
void             udp_bind_netif (struct udp_pcb *pcb, const struct netif* netif);
err_t            udp_connect    (struct udp_pcb *pcb, const ip_addr_t *ipaddr,
                                 u16_t port);
void             udp_disconnect (struct udp_pcb *pcb);
void             udp_recv       (struct udp_pcb *pcb, udp_recv_fn recv,
                                 void *recv_arg);

?? 2.2.1 创建udp_pcb协议控制块

/**
 * @ingroup udp_raw
 * Creates a new UDP pcb which can be used for UDP communication. The
 * pcb is not active until it has either been bound to a local address
 * or connected to a remote address.
 *
 * @return The UDP PCB which was created. NULL if the PCB data structure
 * could not be allocated.
 *
 * @see udp_remove()
 */
struct udp_pcb *
udp_new(void)
{
  struct udp_pcb *pcb;

  LWIP_ASSERT_CORE_LOCKED();

  pcb = (struct udp_pcb *)memp_malloc(MEMP_UDP_PCB);
  /* could allocate UDP PCB? */
  if (pcb != NULL) {
    /* UDP Lite: by initializing to all zeroes, chksum_len is set to 0
     * which means checksum is generated over the whole datagram per default
     * (recommended as default by RFC 3828). */
    /* initialize PCB to all zeroes */
    memset(pcb, 0, sizeof(struct udp_pcb));
    pcb->ttl = UDP_TTL;
#if LWIP_MULTICAST_TX_OPTIONS
    udp_set_multicast_ttl(pcb, UDP_TTL);
#endif /* LWIP_MULTICAST_TX_OPTIONS */
  }
  return pcb;
}

/**
 * @ingroup udp_raw
 * Create a UDP PCB for specific IP type.
 * The pcb is not active until it has either been bound to a local address
 * or connected to a remote address.
 * 
 * @param type IP address type, see @ref lwip_ip_addr_type definitions.
 * If you want to listen to IPv4 and IPv6 (dual-stack) packets,
 * supply @ref IPADDR_TYPE_ANY as argument and bind to @ref IP_ANY_TYPE.
 * @return The UDP PCB which was created. NULL if the PCB data structure
 * could not be allocated.
 *
 * @see udp_remove()
 */
struct udp_pcb *
udp_new_ip_type(u8_t type)
{
  struct udp_pcb *pcb;

  LWIP_ASSERT_CORE_LOCKED();

  pcb = udp_new();
#if LWIP_IPV4 && LWIP_IPV6
  if (pcb != NULL) {
    IP_SET_TYPE_VAL(pcb->local_ip,  type);
    IP_SET_TYPE_VAL(pcb->remote_ip, type);
  }
#else
  LWIP_UNUSED_ARG(type);
#endif /* LWIP_IPV4 && LWIP_IPV6 */
  return pcb;
}

?? 以上两个函数都是创建udp_pcb协议控制块，差别在于udp_new_ip_type() 可以传入type指定pcb对应的IP类型为IPv6、IPv4或IPv6/v4。而 udp_new() 默认创建的pcb对应IP类型为IPv4,因为IPv4的IP TYPE值是0，如下所示:

enum lwip_ip_addr_type {
  /** IPv4 */
  IPADDR_TYPE_V4 =   0U,
  /** IPv6 */
  IPADDR_TYPE_V6 =   6U,
  /** IPv4+IPv6 ("dual-stack") */
  IPADDR_TYPE_ANY = 46U
};

?? 2.2.2 绑定和连接
?? 创建了udp_pcb之后，必须通过bind()或者connect()操作，将pcb加入到pcb链表struct udp_pcb *udp_pcbs中，才能由内核管理接收和发送数据。其中插入链表采用的是头插法。

/**
 * @ingroup udp_raw
 * Bind an UDP PCB.
 * 
 * @param pcb UDP PCB to be bound with a local address ipaddr and port.
 * @param ipaddr local IP address to bind with. Use IP_ANY_TYPE to
 * bind to all local interfaces.
 * @param port local UDP port to bind with. Use 0 to automatically bind
 * to a random port between UDP_LOCAL_PORT_RANGE_START and
 * UDP_LOCAL_PORT_RANGE_END.
 *
 * ipaddr & port are expected to be in the same byte order as in the pcb.
 *
 * @return lwIP error code.
 * - ERR_OK. Successful. No error occurred.
 * - ERR_USE. The specified ipaddr and port are already bound to by
 * another UDP PCB.
 *
 * @see udp_disconnect()
 */
err_t
udp_bind(struct udp_pcb *pcb, const ip_addr_t *ipaddr, u16_t port)
{
  struct udp_pcb *ipcb;
  u8_t rebind;
#if LWIP_IPV6 && LWIP_IPV6_SCOPES
  ip_addr_t zoned_ipaddr;
#endif /* LWIP_IPV6 && LWIP_IPV6_SCOPES */

  LWIP_ASSERT_CORE_LOCKED();

#if LWIP_IPV4
  /* Don't propagate NULL pointer (IPv4 ANY) to subsequent functions */
  if (ipaddr == NULL) {
    ipaddr = IP4_ADDR_ANY;
  }
#else /* LWIP_IPV4 */
  LWIP_ERROR("udp_bind: invalid ipaddr", ipaddr != NULL, return ERR_ARG);
#endif /* LWIP_IPV4 */

  LWIP_ERROR("udp_bind: invalid pcb", pcb != NULL, return ERR_ARG);

  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, ("udp_bind(ipaddr = "));
  ip_addr_debug_print(UDP_DEBUG | LWIP_DBG_TRACE, ipaddr);
  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, (", port = %"U16_F")\n", port));

  rebind = 0;
  /* Check for double bind and rebind of the same pcb */
  for (ipcb = udp_pcbs; ipcb != NULL; ipcb = ipcb->next) {
    /* is this UDP PCB already on active list? */
    if (pcb == ipcb) {
      rebind = 1;
      break;
    }
  }

#if LWIP_IPV6 && LWIP_IPV6_SCOPES
  /* If the given IP address should have a zone but doesn't, assign one now.
   * This is legacy support: scope-aware callers should always provide properly
   * zoned source addresses. Do the zone selection before the address-in-use
   * check below; as such we have to make a temporary copy of the address. */
  if (IP_IS_V6(ipaddr) && ip6_addr_lacks_zone(ip_2_ip6(ipaddr), IP6_UNKNOWN)) {
    ip_addr_copy(zoned_ipaddr, *ipaddr);
    ip6_addr_select_zone(ip_2_ip6(&zoned_ipaddr), ip_2_ip6(&zoned_ipaddr));
    ipaddr = &zoned_ipaddr;
  }
#endif /* LWIP_IPV6 && LWIP_IPV6_SCOPES */

  /* no port specified? */
  if (port == 0) {
    port = udp_new_port();
    if (port == 0) {
      /* no more ports available in local range */
      LWIP_DEBUGF(UDP_DEBUG, ("udp_bind: out of free UDP ports\n"));
      return ERR_USE;
    }
  } else {
    for (ipcb = udp_pcbs; ipcb != NULL; ipcb = ipcb->next) {
      if (pcb != ipcb) {
        /* By default, we don't allow to bind to a port that any other udp
           PCB is already bound to, unless *all* PCBs with that port have tha
           REUSEADDR flag set. */
#if SO_REUSE
        if (!ip_get_option(pcb, SOF_REUSEADDR) ||
            !ip_get_option(ipcb, SOF_REUSEADDR))
#endif /* SO_REUSE */
        {
          /* port matches that of PCB in list and REUSEADDR not set -> reject */
          if ((ipcb->local_port == port) &&
              /* IP address matches or any IP used? */
              (ip_addr_cmp(&ipcb->local_ip, ipaddr) || ip_addr_isany(ipaddr) ||
              ip_addr_isany(&ipcb->local_ip))) {
            /* other PCB already binds to this local IP and port */
            LWIP_DEBUGF(UDP_DEBUG,
                        ("udp_bind: local port %"U16_F" already bound by another pcb\n", port));
            return ERR_USE;
          }
        }
      }
    }
  }

  ip_addr_set_ipaddr(&pcb->local_ip, ipaddr);

  pcb->local_port = port;
  mib2_udp_bind(pcb);
  /* pcb not active yet? */
  if (rebind == 0) {
    /* place the PCB on the active list if not already there */
    pcb->next = udp_pcbs;
    udp_pcbs = pcb;
  }
  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("udp_bind: bound to "));
  ip_addr_debug_print_val(UDP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, pcb->local_ip);
  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, (", port %"U16_F")\n", pcb->local_port));
  return ERR_OK;
}

?? bind() 函数将传入的IP地址和端口号绑定到pcb本地IP（pcb->local_ip）和端口号（pcb->local_port）中，并且加入到udp_pcbs链表。同时也做了机制防止一个pcb重复绑定以及不同pcb绑定同一个IP和端口。

/**
 * @ingroup udp_raw
 * Sets the remote end of the pcb. This function does not generate any
 * network traffic, but only sets the remote address of the pcb.
 *
 * @param pcb UDP PCB to be connected with remote address ipaddr and port.
 * @param ipaddr remote IP address to connect with.
 * @param port remote UDP port to connect with.
 *
 * @return lwIP error code
 *
 * ipaddr & port are expected to be in the same byte order as in the pcb.
 *
 * The udp pcb is bound to a random local port if not already bound.
 *
 * @see udp_disconnect()
 */
err_t
udp_connect(struct udp_pcb *pcb, const ip_addr_t *ipaddr, u16_t port)
{
  struct udp_pcb *ipcb;

  LWIP_ASSERT_CORE_LOCKED();

  LWIP_ERROR("udp_connect: invalid pcb", pcb != NULL, return ERR_ARG);
  LWIP_ERROR("udp_connect: invalid ipaddr", ipaddr != NULL, return ERR_ARG);

  if (pcb->local_port == 0) {
    err_t err = udp_bind(pcb, &pcb->local_ip, pcb->local_port);
    if (err != ERR_OK) {
      return err;
    }
  }

  ip_addr_set_ipaddr(&pcb->remote_ip, ipaddr);
#if LWIP_IPV6 && LWIP_IPV6_SCOPES
  /* If the given IP address should have a zone but doesn't, assign one now,
   * using the bound address to make a more informed decision when possible. */
  if (IP_IS_V6(&pcb->remote_ip) &&
      ip6_addr_lacks_zone(ip_2_ip6(&pcb->remote_ip), IP6_UNKNOWN)) {
    ip6_addr_select_zone(ip_2_ip6(&pcb->remote_ip), ip_2_ip6(&pcb->local_ip));
  }
#endif /* LWIP_IPV6 && LWIP_IPV6_SCOPES */

  pcb->remote_port = port;
  pcb->flags |= UDP_FLAGS_CONNECTED;

  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("udp_connect: connected to "));
  ip_addr_debug_print_val(UDP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE,
                          pcb->remote_ip);
  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, (", port %"U16_F")\n", pcb->remote_port));

  /* Insert UDP PCB into the list of active UDP PCBs. */
  for (ipcb = udp_pcbs; ipcb != NULL; ipcb = ipcb->next) {
    if (pcb == ipcb) {
      /* already on the list, just return */
      return ERR_OK;
    }
  }
  /* PCB not yet on the list, add PCB now */
  pcb->next = udp_pcbs;
  udp_pcbs = pcb;
  return ERR_OK;
}

?? 由client调用udp_connect() 函数，绑定remote ip和端口，置为UDP_FLAGS_CONNECTED，同时添加到udp_pcb中。

?? 2.2.3 绑定接受处理函数

/**
 * @ingroup udp_raw
 * Set a receive callback for a UDP PCB.
 * This callback will be called when receiving a datagram for the pcb.
 *
 * @param pcb the pcb for which to set the recv callback
 * @param recv function pointer of the callback function
 * @param recv_arg additional argument to pass to the callback function
 */
void
udp_recv(struct udp_pcb *pcb, udp_recv_fn recv, void *recv_arg)
{
  LWIP_ASSERT_CORE_LOCKED();

  LWIP_ERROR("udp_recv: invalid pcb", pcb != NULL, return);

  /* remember recv() callback and user data */
  pcb->recv = recv;
  pcb->recv_arg = recv_arg;
}

?? pcb->recv() 由udp_input() 函数调用，处理对端发来的udp数据。udp_recv_fn recv回调函数自行实现。无论是客户端还是服务端，都需要绑定此回调函数。

?? 2.2.4 绑定网卡

/**
 * @ingroup udp_raw
 * Bind an UDP PCB to a specific netif.
 * After calling this function, all packets received via this PCB
 * are guaranteed to have come in via the specified netif, and all
 * outgoing packets will go out via the specified netif.
 *
 * @param pcb UDP PCB to be bound.
 * @param netif netif to bind udp pcb to. Can be NULL.
 *
 * @see udp_disconnect()
 */
void
udp_bind_netif(struct udp_pcb *pcb, const struct netif *netif)
{
  LWIP_ASSERT_CORE_LOCKED();

  if (netif != NULL) {
    pcb->netif_idx = netif_get_index(netif);
  } else {
    pcb->netif_idx = NETIF_NO_INDEX;
  }
}

?? 将pcb->netif_idx = netif->num + 1; 从而与netif的编号绑定，绑定后此pcb的输入输出都指定一个网卡。

?? 2.3 发送函数

err_t            udp_sendto_if  (struct udp_pcb *pcb, struct pbuf *p,
                                 const ip_addr_t *dst_ip, u16_t dst_port,
                                 struct netif *netif);
err_t            udp_sendto_if_src(struct udp_pcb *pcb, struct pbuf *p,
                                 const ip_addr_t *dst_ip, u16_t dst_port,
                                 struct netif *netif, const ip_addr_t *src_ip);
err_t            udp_sendto     (struct udp_pcb *pcb, struct pbuf *p,
                                 const ip_addr_t *dst_ip, u16_t dst_port);
err_t            udp_send       (struct udp_pcb *pcb, struct pbuf *p);

#if LWIP_CHECKSUM_ON_COPY && CHECKSUM_GEN_UDP
err_t            udp_sendto_if_chksum(struct udp_pcb *pcb, struct pbuf *p,
                                 const ip_addr_t *dst_ip, u16_t dst_port,
                                 struct netif *netif, u8_t have_chksum,
                                 u16_t chksum);
err_t            udp_sendto_chksum(struct udp_pcb *pcb, struct pbuf *p,
                                 const ip_addr_t *dst_ip, u16_t dst_port,
                                 u8_t have_chksum, u16_t chksum);
err_t            udp_send_chksum(struct udp_pcb *pcb, struct pbuf *p,
                                 u8_t have_chksum, u16_t chksum);
err_t            udp_sendto_if_src_chksum(struct udp_pcb *pcb, struct pbuf *p,
                                 const ip_addr_t *dst_ip, u16_t dst_port, struct netif *netif,
                                 u8_t have_chksum, u16_t chksum, const ip_addr_t *src_ip);
#endif /* LWIP_CHECKSUM_ON_COPY && CHECKSUM_GEN_UDP */

?? 发送函数的调用顺序依次为: udp_send() -> udp_sendto() -> udp_sendto_chksum() -> udp_sendto_if() -> udp_sendto_if_chksum() -> udp_sendto_if_src() -> udp_sendto_if_src_chksum();

?? 函数的参数也是逐渐增多，udp_sendto() 比udp_send() 多了目的IP和端口，udp_sendto_if() 比udp_sendto() 多了选择的netif, udp_sendto_if_src() 比udp_sendto_if() 多了源IP。最后调用ip_output_if_src() 发送函数传到下一层进行封包。

?? udp_sendto_if_src_chksum() 时加入了伪头部计算，如果校验和为0， 设置为0xffff.

?? 2.4 接收处理函数
??接收函数udp_input() 由ip_put() 函数调用， 目的就是经过IP选择之后将UDP数据报传递到udp_pcb的回调函数recv中处理。

/**
 * Process an incoming UDP datagram.
 *
 * Given an incoming UDP datagram (as a chain of pbufs) this function
 * finds a corresponding UDP PCB and hands over the pbuf to the pcbs
 * recv function. If no pcb is found or the datagram is incorrect, the
 * pbuf is freed.
 *
 * @param p pbuf to be demultiplexed to a UDP PCB (p->payload pointing to the UDP header)
 * @param inp network interface on which the datagram was received.
 *
 */
void
udp_input(struct pbuf *p, struct netif *inp)
{
  struct udp_hdr *udphdr;
  struct udp_pcb *pcb, *prev;
  struct udp_pcb *uncon_pcb;
  u16_t src, dest;
  u8_t broadcast;
  u8_t for_us = 0;

  LWIP_UNUSED_ARG(inp);

  LWIP_ASSERT_CORE_LOCKED();

  LWIP_ASSERT("udp_input: invalid pbuf", p != NULL);
  LWIP_ASSERT("udp_input: invalid netif", inp != NULL);

  PERF_START;

  UDP_STATS_INC(udp.recv);

  /* Check minimum length (UDP header) */
  if (p->len < UDP_HLEN) {
    /* 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);
    MIB2_STATS_INC(mib2.udpinerrors);
    pbuf_free(p);
    goto end;
  }

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

  /* is broadcast packet ? */
  broadcast = ip_addr_isbroadcast(ip_current_dest_addr(), ip_current_netif());

  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 = lwip_ntohs(udphdr->src);
  dest = lwip_ntohs(udphdr->dest);

  udp_debug_print(udphdr);

  /* print the UDP source and destination */
  LWIP_DEBUGF(UDP_DEBUG, ("udp ("));
  ip_addr_debug_print_val(UDP_DEBUG, *ip_current_dest_addr());
  LWIP_DEBUGF(UDP_DEBUG, (", %"U16_F") <-- (", lwip_ntohs(udphdr->dest)));
  ip_addr_debug_print_val(UDP_DEBUG, *ip_current_src_addr());
  LWIP_DEBUGF(UDP_DEBUG, (", %"U16_F")\n", lwip_ntohs(udphdr->src)));

  pcb = NULL;
  prev = NULL;
  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) {
    /* print the PCB local and remote address */
    LWIP_DEBUGF(UDP_DEBUG, ("pcb ("));
    ip_addr_debug_print_val(UDP_DEBUG, pcb->local_ip);
    LWIP_DEBUGF(UDP_DEBUG, (", %"U16_F") <-- (", pcb->local_port));
    ip_addr_debug_print_val(UDP_DEBUG, pcb->remote_ip);
    LWIP_DEBUGF(UDP_DEBUG, (", %"U16_F")\n", pcb->remote_port));

    /* compare PCB local addr+port to UDP destination addr+port */
    if ((pcb->local_port == dest) &&
        (udp_input_local_match(pcb, inp, broadcast) != 0)) {
      if ((pcb->flags & UDP_FLAGS_CONNECTED) == 0) {
        if (uncon_pcb == NULL) {
          /* the first unconnected matching PCB */
          uncon_pcb = pcb;
#if LWIP_IPV4
        } else if (broadcast && ip4_current_dest_addr()->addr == IPADDR_BROADCAST) {
          /* global broadcast address (only valid for IPv4; match was checked before) */
          if (!IP_IS_V4_VAL(uncon_pcb->local_ip) || !ip4_addr_cmp(ip_2_ip4(&uncon_pcb->local_ip), netif_ip4_addr(inp))) {
            /* uncon_pcb does not match the input netif, check this pcb */
            if (IP_IS_V4_VAL(pcb->local_ip) && ip4_addr_cmp(ip_2_ip4(&pcb->local_ip), netif_ip4_addr(inp))) {
              /* better match */
              uncon_pcb = pcb;
            }
          }
#endif /* LWIP_IPV4 */
        }
#if SO_REUSE
        else if (!ip_addr_isany(&pcb->local_ip)) {
          /* prefer specific IPs over catch-all */
          uncon_pcb = pcb;
        }
#endif /* SO_REUSE */
      }

      /* compare PCB remote addr+port to UDP source addr+port */
      if ((pcb->remote_port == src) &&
          (ip_addr_isany_val(pcb->remote_ip) ||
           ip_addr_cmp(&pcb->remote_ip, ip_current_src_addr()))) {
        /* 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) {
    for_us = 1;
  } else {
#if LWIP_IPV6
    if (ip_current_is_v6()) {
      for_us = netif_get_ip6_addr_match(inp, ip6_current_dest_addr()) >= 0;
    }
#endif /* LWIP_IPV6 */
#if LWIP_IPV4
    if (!ip_current_is_v6()) {
      for_us = ip4_addr_cmp(netif_ip4_addr(inp), ip4_current_dest_addr());
    }
#endif /* LWIP_IPV4 */
  }

  if (for_us) {
    LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, ("udp_input: calculating checksum\n"));
#if CHECKSUM_CHECK_UDP
    IF__NETIF_CHECKSUM_ENABLED(inp, NETIF_CHECKSUM_CHECK_UDP) {
#if LWIP_UDPLITE
      if (ip_current_header_proto() == IP_PROTO_UDPLITE) {
        /* Do the UDP Lite checksum */
        u16_t chklen = lwip_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) */
            goto chkerr;
          }
        }
        if (ip_chksum_pseudo_partial(p, IP_PROTO_UDPLITE,
                                     p->tot_len, chklen,
                                     ip_current_src_addr(), ip_current_dest_addr()) != 0) {
          goto chkerr;
        }
      } else
#endif /* LWIP_UDPLITE */
      {
        if (udphdr->chksum != 0) {
          if (ip_chksum_pseudo(p, IP_PROTO_UDP, p->tot_len,
                               ip_current_src_addr(),
                               ip_current_dest_addr()) != 0) {
            goto chkerr;
          }
        }
      }
    }
#endif /* CHECKSUM_CHECK_UDP */
    if (pbuf_remove_header(p, UDP_HLEN)) {
      /* Can we cope with this failing? Just assert for now */
      LWIP_ASSERT("pbuf_remove_header failed\n", 0);
      UDP_STATS_INC(udp.drop);
      MIB2_STATS_INC(mib2.udpinerrors);
      pbuf_free(p);
      goto end;
    }

    if (pcb != NULL) {
      MIB2_STATS_INC(mib2.udpindatagrams);
#if SO_REUSE && SO_REUSE_RXTOALL
      if (ip_get_option(pcb, SOF_REUSEADDR) &&
          (broadcast || ip_addr_ismulticast(ip_current_dest_addr()))) {
        /* pass broadcast- or multicast packets to all multicast pcbs
           if SOF_REUSEADDR is set on the first match */
        struct udp_pcb *mpcb;
        for (mpcb = udp_pcbs; mpcb != NULL; mpcb = mpcb->next) {
          if (mpcb != pcb) {
            /* compare PCB local addr+port to UDP destination addr+port */
            if ((mpcb->local_port == dest) &&
                (udp_input_local_match(mpcb, inp, broadcast) != 0)) {
              /* pass a copy of the packet to all local matches */
              if (mpcb->recv != NULL) {
                struct pbuf *q;
                q = pbuf_clone(PBUF_RAW, PBUF_POOL, p);
                if (q != NULL) {
                  mpcb->recv(mpcb->recv_arg, mpcb, q, ip_current_src_addr(), src);
                }
              }
            }
          }
        }
      }
#endif /* SO_REUSE && SO_REUSE_RXTOALL */
      /* callback */
      if (pcb->recv != NULL) {
        /* now the recv function is responsible for freeing p */
        pcb->recv(pcb->recv_arg, pcb, p, ip_current_src_addr(), src);
      } 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 || LWIP_ICMP6
      /* No match was found, send ICMP destination port unreachable unless
         destination address was broadcast/multicast. */
      if (!broadcast && !ip_addr_ismulticast(ip_current_dest_addr())) {
        /* move payload pointer back to ip header */
        pbuf_header_force(p, (s16_t)(ip_current_header_tot_len() + UDP_HLEN));
        icmp_port_unreach(ip_current_is_v6(), p);
      }
#endif /* LWIP_ICMP || LWIP_ICMP6 */
      UDP_STATS_INC(udp.proterr);
      UDP_STATS_INC(udp.drop);
      MIB2_STATS_INC(mib2.udpnoports);
      pbuf_free(p);
    }
  } else {
    pbuf_free(p);
  }
end:
  PERF_STOP("udp_input");
  return;
#if CHECKSUM_CHECK_UDP
chkerr:
  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
              ("udp_input: UDP (or UDP Lite) datagram discarded due to failing checksum\n"));
  UDP_STATS_INC(udp.chkerr);
  UDP_STATS_INC(udp.drop);
  MIB2_STATS_INC(mib2.udpinerrors);
  pbuf_free(p);
  PERF_STOP("udp_input");
#endif /* CHECKSUM_CHECK_UDP */
}

?? 先进行校验和校验，然后从udp_pcbs链表中找对应的udp_pcb，主要是匹配本地IP和端口，同时考虑广播的情况。找到udp_pcb之后将负载数据传入接口中，回调用户的接收函数。