LWIP(Chapter 5) UDP协议和源码

U D P是一个简单的面向数据报的运输层协议：进程的每个输出操作都正好产生一个 U D P数据报，并组装成一份待发送的 I P数据报。U D P不提供可靠性：它把应用程序传给 I P层的数据发送出去，但是并不保证它们能到达目的地。

UDP直接在IP数据报的数据部分。
相比之前的数据报，UDP有一个16位的端口号，用于区分数据来源和接收的应用程序。
应用程序为了标识自己发送的数据需要用一个序列来标识自己，这个就是端口号。

代码

UDP数据报的来源是IP层
UDP主要使用端口号来区分应用程序

udp_input

/**
 * 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.
 * @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;
  struct ip_hdr *iphdr;
  u16_t src, dest;
  u8_t local_match;
  u8_t broadcast;

  PERF_START;

  UDP_STATS_INC(udp.recv);

  iphdr = p->payload;

  /* Check minimum length (IP header + UDP header)
   * and move payload pointer to UDP header */
   /*收到的数据报长度检查，丢弃过短的数据报，把数据指针移动到UDP*/
  if (p->tot_len < (IPH_HL(iphdr) * 4 + UDP_HLEN) || pbuf_header(p, -(s16_t)(IPH_HL(iphdr) * 4))) {
    /* 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;
  }
  /*UDP 首部*/
  udphdr = (struct udp_hdr *)p->payload;

  /* is broadcast packet ? */
  broadcast = ip_addr_isbroadcast(&(iphdr->dest), inp);

  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 */
  /*获取 源端口号和目标端口号*/
  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. */
     /*UDP 对应的是应用程序，所以数据报需要提交给应用程序，所以去扫描已经在PCB中注册的端口*/
    for (pcb = udp_pcbs; pcb != NULL; pcb = pcb->next) {
      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 */
      /*比较当前的PCB是否匹配UDP的目的地址和端口，*/
      if ((pcb->local_port == dest) &&
          ((!broadcast && 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 */
#if IP_SOF_BROADCAST_RECV
           (broadcast && (pcb->so_options & SOF_BROADCAST)))) {
#else  /* IP_SOF_BROADCAST_RECV */
           (broadcast))) {
#endif /* IP_SOF_BROADCAST_RECV */
        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 */
      /*比较PCB 的远端 IP地址和对口号和UDP数据包的源地址和端口*/
      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 */
        /*完全匹配*/
        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;
      }
/* This part of code has been modified by ST's MCD Application Team */
/* To use the UPnP responder for device discovery */
#if LWIP_UPNP
      if((local_match != 0) && (dest == 1900)) {
        break;
      }
#endif /* LWIP_UPNP */
      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 | LWIP_DBG_LEVEL_SERIOUS,
                   ("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 | LWIP_DBG_LEVEL_SERIOUS,
                      ("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)) {
      /* 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);
      } 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 (!broadcast &&
          !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);
      }
#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");
}

UDP部分特别的地方是RAW PCB，通过PCB上交数据。

output

UDP input数据的归宿是 RAW PCB
UDP output数据的开始是 RAW PCB

RAW PCB是UDP的应用层接口
输出的传参 是PCB 和 pbuf
UDP 层 PCB是应用层下的数据传输 源和目的地，也就是数据从哪儿来，到哪里去的问题
IP层 是数据怎么从来的地方到目的地的线路问题
MAC层以太网帧，是千里之行始于足下，每一步的脚印靠以太网层走出来。
发送部分
1.应用层接口，参数PCB，pbuf

}

/**
 * Send data using UDP.
 *
 * @param pcb UDP PCB used to send the data.
 * @param p chain of pbuf's to be sent.
 *
 * The datagram will be sent to the current remote_ip & remote_port
 * stored in pcb. If the pcb is not bound to a port, it will
 * automatically be bound to a random port.
 *
 * @return lwIP error code.
 * - ERR_OK. Successful. No error occured.
 * - ERR_MEM. Out of memory.
 * - ERR_RTE. Could not find route to destination address.
 * - More errors could be returned by lower protocol layers.
 *
 * @see udp_disconnect() udp_sendto()
 */
err_t
udp_send(struct udp_pcb *pcb, struct pbuf *p)
{
  /* send to the packet using remote ip and port stored in the pcb */
  return udp_sendto(pcb, p, &pcb->remote_ip, pcb->remote_port);
}

2.UDP层

/**
 * Send data to a specified address using UDP.
 *
 * @param pcb UDP PCB used to send the data.
 * @param p chain of pbuf's to be sent.
 * @param dst_ip Destination IP address.
 * @param dst_port Destination UDP port.
 *
 * dst_ip & dst_port are expected to be in the same byte order as in the pcb.
 *
 * If the PCB already has a remote address association, it will
 * be restored after the data is sent.
 * 
 * @return lwIP error code (@see udp_send for possible error codes)
 *
 * @see udp_disconnect() udp_send()
 */
err_t
udp_sendto(struct udp_pcb *pcb, struct pbuf *p,
  struct ip_addr *dst_ip, u16_t dst_port)
{
  struct netif *netif;

  LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, ("udp_send\n"));

  /* find the outgoing network interface for this packet */
#if LWIP_IGMP
  netif = ip_route((ip_addr_ismulticast(dst_ip))?(&(pcb->multicast_ip)):(dst_ip));
#else
  netif = ip_route(dst_ip);
#endif /* LWIP_IGMP */

  /* no outgoing network interface could be found? */
  if (netif == NULL) {
    LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("udp_send: No route to 0x%"X32_F"\n", dst_ip->addr));
    UDP_STATS_INC(udp.rterr);
    return ERR_RTE;
  }
  return udp_sendto_if(pcb, p, dst_ip, dst_port, netif);
}

2.UDP层需要关系把数据发给IP层，需要处理IP层输入参数 dst_ip, dst_port, netif

/**
 * Send data to a specified address using UDP.
 * The netif used for sending can be specified.
 *
 * This function exists mainly for DHCP, to be able to send UDP packets
 * on a netif that is still down.
 *
 * @param pcb UDP PCB used to send the data.
 * @param p chain of pbuf's to be sent.
 * @param dst_ip Destination IP address.
 * @param dst_port Destination UDP port.
 * @param netif the netif used for sending.
 *
 * dst_ip & dst_port are expected to be in the same byte order as in the pcb.
 *
 * @return lwIP error code (@see udp_send for possible error codes)
 *
 * @see udp_disconnect() udp_send()
 */
err_t
udp_sendto_if(struct udp_pcb *pcb, struct pbuf *p,
  struct ip_addr *dst_ip, u16_t dst_port, struct netif *netif)

3.IP层接口
组合IP数据包 发送 err = ip_output_if(q, src_ip, dst_ip, pcb->ttl, pcb->tos, IP_PROTO_UDP, netif);