TCP/IP详解V2之UDP协议

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UDP

UDP是一个面向数据报的简单运输层协议。

数据结构

struct udphdr {
    u_short uh_sport;       //源端口
    u_short uh_dport;       //目的端口
    short   uh_ulen;        //UDP数据报中的数据长度
    u_short uh_sum;     //检验和,包括数据
};
struct udpiphdr {
    struct ipovly ui_i;     //模仿IP的实现,会有很多便利
    struct udphdr ui_u;     /* udp header */
};
struct ipovly {
    caddr_t ih_next, ih_prev;   /* for protocol sequence q‘s */
    u_char  ih_x1;          /* (unused) */
    u_char  ih_pr;                  //协议域
    short   ih_len;                 //这个相当于IP头部,len = data Len + udp HeaderLen + ip header
    struct  in_addr ih_src;     //源地址
    struct  in_addr ih_dst;     //目标地址
};

udp_init

void
udp_init()
{
    udb.inp_next = udb.inp_prev = &udb;    //将头部PCB的指针指向自己,形成一个双向链表
}

udp_output

int
udp_output(inp, m, addr, control)
    register struct inpcb *inp;    //输出的Internet PCB
    register struct mbuf *m;    //数据mbuf
    struct mbuf *addr, *control;    //地址与控制信息mbuf
{
    register struct udpiphdr *ui;
    register int len = m->m_pkthdr.len;    //获取发送数据的长度
    struct in_addr laddr;
    int s, error = 0;

    if (control)    //丢弃控制信息。UDP不适用任何控制信息
        m_freem(control);       /* XXX */

    if (addr) {
        laddr = inp->inp_laddr;    //获取本地信息
        if (inp->inp_faddr.s_addr != INADDR_ANY) {    //如果这个PCB已经被绑定(是UDP啊),返回错误
            error = EISCONN;
            goto release;
        }
        /*
         * Must block input while temporarily connected.
         */
        s = splnet();    //通过调整优先级来达到锁的目的
        error = in_pcbconnect(inp, addr);    //暂时的连接,填充远程地址与端口
        if (error) {
            splx(s);    //如果在绑定远程地址的过程中出现错误,释放数据
            goto release;
        }
    } else {
        if (inp->inp_faddr.s_addr == INADDR_ANY) {    //显式的关联远程地址之后仍然没有地址的话,放弃数据mbuf
            error = ENOTCONN;
            goto release;
        }
    }
    /*
     * Calculate data length and get a mbuf
     * for UDP and IP headers.
     */
    M_PREPEND(m, sizeof(struct udpiphdr), M_DONTWAIT);    //在数据mbuf前面分配空间以存储udp/ip header
    if (m == 0) {    //分配失败的话,释放资源
        error = ENOBUFS;
        goto release;
    }

    /*
     * Fill in mbuf with extended UDP header
     * and addresses and length put into network format.
     */
    ui = mtod(m, struct udpiphdr *);    //已经在mbuf的首部为udp/ip header分配好了资源,填充这些数据
    ui->ui_next = ui->ui_prev = 0;
    ui->ui_x1 = 0;
    ui->ui_pr = IPPROTO_UDP;
    ui->ui_len = htons((u_short)len + sizeof (struct udphdr));
    ui->ui_src = inp->inp_laddr;
    ui->ui_dst = inp->inp_faddr;
    ui->ui_sport = inp->inp_lport;
    ui->ui_dport = inp->inp_fport;
    ui->ui_ulen = ui->ui_len;    //数据长度

    /*
     * Stuff checksum and output datagram.
     */
    ui->ui_sum = 0;    //计算校验和
    if (udpcksum) {
        if ((ui->ui_sum = in_cksum(m, sizeof (struct udpiphdr) + len)) == 0)
        ui->ui_sum = 0xffff;
    }
    ((struct ip *)ui)->ip_len = sizeof (struct udpiphdr) + len;    //IP数据报中的len = IP header + udp header + data 
    ((struct ip *)ui)->ip_ttl = inp->inp_ip.ip_ttl; /* XXX */
    ((struct ip *)ui)->ip_tos = inp->inp_ip.ip_tos; /* XXX */
    udpstat.udps_opackets++;
    error = ip_output(m, inp->inp_options, &inp->inp_route,
        inp->inp_socket->so_options & (SO_DONTROUTE | SO_BROADCAST),
        inp->inp_moptions);    //计算结束之后,将数据包交由ip层进行处理

    if (addr) {    //如果提供了addr,以为着在发送前调用connect将PCB与远程地址关联起来了
        in_pcbdisconnect(inp);
        inp->inp_laddr = laddr;
        splx(s);
    }
    return (error);

release:
    m_freem(m);    //释放数据资源
    return (error);
}

udp_input

  • 功能A:将UDP数据报放置到合适的插口缓存内,唤醒该插口上因输入阻塞的所有进程。不重点关注多播与广播的情况。

    void
    udp_input(m, iphlen)
    register struct mbuf *m;    //数据mbuf
    int iphlen;    //ip首部的长度
    {
    register struct ip *ip;
    register struct udphdr *uh;
    register struct inpcb *inp;
    struct mbuf *opts = 0;
    int len;
    struct ip save_ip;
    
    udpstat.udps_ipackets++;    //更新UDP的全局统计量
    
    /*
     * Strip IP options, if any; should skip this,
     * make available to user, and use on returned packets,
     * but we don‘t yet have a way to check the checksum
     * with options still present.
     */
    if (iphlen > sizeof (struct ip)) {    //如果存在IP选项,丢弃IP选项并更改iphlen
        ip_stripoptions(m, (struct mbuf *)0);
        iphlen = sizeof(struct ip);
    }
    
    /*
     * Get IP and UDP header together in first mbuf.
     */
    ip = mtod(m, struct ip *);        //从mbuf中获取IP首部
    if (m->m_len < iphlen + sizeof(struct udphdr)) {    //如果mbuf中的数据长度小于ip header + udp header
        if ((m = m_pullup(m, iphlen + sizeof(struct udphdr))) == 0) {    //使用m_pullup将外部簇中的数据提取到mbuf中
            udpstat.udps_hdrops++;    //如果失败,增加UDP的全局计数
            return;
        }
        ip = mtod(m, struct ip *);    //否则的话,ip指向正确的ip首部位置
    }
    uh = (struct udphdr *)((caddr_t)ip + iphlen);    //udp指向UDP正确的位置
    
    /*
     * Make mbuf data length reflect UDP length.
     * If not enough data to reflect UDP length, drop.
     */
    len = ntohs((u_short)uh->uh_ulen);    //将UDP中的关于数据报的长度转换为主机字节序
    if (ip->ip_len != len) {
        if (len > ip->ip_len) {    //如果数据的长度大于IP header + udp header + data,就丢弃数据包
            udpstat.udps_badlen++;
            goto bad;
        }
        m_adj(m, len - ip->ip_len);    //调整ip数据报中的长度为data len
        /* ip->ip_len = len; */
    }
    /*
     * Save a copy of the IP header in case we want restore it
     * for sending an ICMP error message in response.
     */
    save_ip = *ip;    //使用局部变量保存IP变量
    
    /*
     * Checksum extended UDP header and data.
     */
    if (udpcksum && uh->uh_sum) {    //检查UDP的校验和,如果验证失败,在全局变量中记录后直接丢弃
        ((struct ipovly *)ip)->ih_next = 0;
        ((struct ipovly *)ip)->ih_prev = 0;
        ((struct ipovly *)ip)->ih_x1 = 0;
        ((struct ipovly *)ip)->ih_len = uh->uh_ulen;
        if (uh->uh_sum = in_cksum(m, len + sizeof (struct ip))) {
            udpstat.udps_badsum++;
            m_freem(m);
            return;
        }
    }
    
    if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
        in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) {    //处理多播的情况,这些数据被提交给所有匹配的插口
        struct socket *last;
        /*
         * Deliver a multicast or broadcast datagram to *all* sockets
         * for which the local and remote addresses and ports match
         * those of the incoming datagram.  This allows more than
         * one process to receive multi/broadcasts on the same port.
         * (This really ought to be done for unicast datagrams as
         * well, but that would cause problems with existing
         * applications that open both address-specific sockets and
         * a wildcard socket listening to the same port -- they would
         * end up receiving duplicates of every unicast datagram.
         * Those applications open the multiple sockets to overcome an
         * inadequacy of the UDP socket interface, but for backwards
         * compatibility we avoid the problem here rather than
         * fixing the interface.  Maybe 4.5BSD will remedy this?)
         */
    
        /*
         * Construct sockaddr format source address.
         */
        udp_in.sin_port = uh->uh_sport;    //更新获得数据的全局变量
        udp_in.sin_addr = ip->ip_src;
        m->m_len -= sizeof (struct udpiphdr);    //调整mbuf中的打他data pointer与data length
        m->m_data += sizeof (struct udpiphdr);
        /*
         * Locate pcb(s) for datagram.
         * (Algorithm copied from raw_intr().)
         */
        last = NULL;
        for (inp = udb.inp_next; inp != &udb; inp = inp->inp_next) {    //遍历所有的PCB
            if (inp->inp_lport != uh->uh_dport)    //如果端口不相等,再次遍历
                continue;
            if (inp->inp_laddr.s_addr != INADDR_ANY) {    //如果地址不匹配,再次遍历
                if (inp->inp_laddr.s_addr !=
                    ip->ip_dst.s_addr)
                    continue;
            }
            if (inp->inp_faddr.s_addr != INADDR_ANY) {    //端口不匹配,也需要再次遍历
                if (inp->inp_faddr.s_addr !=
                    ip->ip_src.s_addr ||
                    inp->inp_fport != uh->uh_sport)
                    continue;
            }
    
            if (last != NULL) {    //
                struct mbuf *n;
    
                if ((n = m_copy(m, 0, M_COPYALL)) != NULL) {    //将数据copy到合适的端口的发送缓存中
                    if (sbappendaddr(&last->so_rcv,
                        (struct sockaddr *)&udp_in,
                        n, (struct mbuf *)0) == 0) {
                        m_freem(n);
                        udpstat.udps_fullsock++;
                    } else
                        sorwakeup(last);
                }
            }
            last = inp->inp_socket;
            /*
             * Don‘t look for additional matches if this one does
             * not have either the SO_REUSEPORT or SO_REUSEADDR
             * socket options set.  This heuristic avoids searching
             * through all pcbs in the common case of a non-shared
             * port.  It * assumes that an application will never
             * clear these options after setting them.
             */
            if ((last->so_options&(SO_REUSEPORT|SO_REUSEADDR) == 0))    //如果没有设置REUSE选项,直接退出循环
                break;
        }
    
        if (last == NULL) {    //如果没有找到合适的发送socket结构
            /*
             * No matching pcb found; discard datagram.
             * (No need to send an ICMP Port Unreachable
             * for a broadcast or multicast datgram.)
             */
            udpstat.udps_noportbcast++;    //退出循环
            goto bad;
        }
        if (sbappendaddr(&last->so_rcv, (struct sockaddr *)&udp_in,
             m, (struct mbuf *)0) == 0) {    //将数据copy进接收缓存中,然后唤醒左右在接收缓存上等待的进程
            udpstat.udps_fullsock++;
            goto bad;
        }
        sorwakeup(last);
        return;
    }
    /*
     * Locate pcb for datagram.
     */
    inp = udp_last_inpcb;    //单播地址,如果从缓存中获取的PCB中的四元组与数据报中的四元组不同的话,从PCBs中寻找合适的四元组,如果找到,顺便更新缓存中的PCB
    if (inp->inp_lport != uh->uh_dport ||
        inp->inp_fport != uh->uh_sport ||
        inp->inp_faddr.s_addr != ip->ip_src.s_addr ||
        inp->inp_laddr.s_addr != ip->ip_dst.s_addr) {
        inp = in_pcblookup(&udb, ip->ip_src, uh->uh_sport,
            ip->ip_dst, uh->uh_dport, INPLOOKUP_WILDCARD);
        if (inp)
            udp_last_inpcb = inp;
        udpstat.udpps_pcbcachemiss++;
    }
    if (inp == 0) {    //如果没有找到
        udpstat.udps_noport++;    //更新全局变量,并判断是否是多播地址OR广播地址
        if (m->m_flags & (M_BCAST | M_MCAST)) {
            udpstat.udps_noportbcast++;
            goto bad;
        }
        *ip = save_ip;    //修改IP数据报的长度,并发送ICMP端口不可达报文
        ip->ip_len += iphlen;
        icmp_error(m, ICMP_UNREACH, ICMP_UNREACH_PORT, 0, 0);
        return;
    }
    
    /*
     * Construct sockaddr format source address.
     * Stuff source address and datagram in user buffer.
     */
    udp_in.sin_port = uh->uh_sport;    //将收到数据报的IP与Port保存在全局的端口中
    udp_in.sin_addr = ip->ip_src;
    if (inp->inp_flags & INP_CONTROLOPTS) { //如果存在UDP选项,将UDP选项保存在合适的mbuf上
        struct mbuf **mp = &opts;
    
        if (inp->inp_flags & INP_RECVDSTADDR) {
            *mp = udp_saveopt((caddr_t) &ip->ip_dst,
                sizeof(struct in_addr), IP_RECVDSTADDR);
            if (*mp)
                mp = &(*mp)->m_next;
        }
    }
    iphlen += sizeof(struct udphdr);    //调整data mbuf中的data pointer与data length
    m->m_len -= iphlen;
    m->m_pkthdr.len -= iphlen;
    m->m_data += iphlen;
    if (sbappendaddr(&inp->inp_socket->so_rcv, (struct sockaddr *)&udp_in,
        m, opts) == 0) {    //将准备好的数据放到socket的缓存中
        udpstat.udps_fullsock++;    //失败的话,返回插口缓存已满的错误
        goto bad;
    }
    sorwakeup(inp->inp_socket);    //唤醒所有等待在插口上的进程
    return;
    bad:
    m_freem(m);    //释放数据与控制mbuf
    if (opts)
        m_freem(opts);
    }

udp_detach

static void
udp_detach(inp)        //将PCB从PCB链表中进行分离
    struct inpcb *inp;
{
    int s = splnet();

    if (inp == udp_last_inpcb)
        udp_last_inpcb = &udb;
    in_pcbdetach(inp);
    splx(s);
}

udp_usrrep

int
udp_usrreq(so, req, m, addr, control)
    struct socket *so;
    int req;
    struct mbuf *m, *addr, *control;
{
    struct inpcb *inp = sotoinpcb(so);    //从socket中获取PCB
    int error = 0;
    int s;

    if (req == PRU_CONTROL)    //如果是控制选项,转接调用in_control函数进行处理
        return (in_control(so, (int)m, (caddr_t)addr,
            (struct ifnet *)control));
    if (inp == NULL && req != PRU_ATTACH) {    //如果参数不正确,直接返回
        error = EINVAL;
        goto release;
    }
    /*
     * Note: need to block udp_input while changing
     * the udp pcb queue and/or pcb addresses.
     */
    switch (req) {

    case PRU_ATTACH:        //这是来自socket的系统调用
        if (inp != NULL) {
            error = EINVAL;
            break;
        }
        s = splnet();
        error = in_pcballoc(so, &udb);    //为UDP SOCKET分配一个PCB
        splx(s);
        if (error)
            break;
        error = soreserve(so, udp_sendspace, udp_recvspace);    //为UDP SOCKET分配缓存空间。默认情况下,SendSpace=9216,RecvSpace=41600
        if (error)
            break;
        ((struct inpcb *) so->so_pcb)->inp_ip.ip_ttl = ip_defttl;    //设置默认的TTL
        break;

    case PRU_DETACH:        //close系统调用
        udp_detach(inp);    //稍后观察
        break;

    case PRU_BIND:        //bind系统调用,关联本地地址与本地端口
        s = splnet();
        error = in_pcbbind(inp, addr);
        splx(s);
        break;

    case PRU_LISTEN:        //listen系统调用
        error = EOPNOTSUPP;    //UDP SOCKET没有listen操作
        break;

    case PRU_CONNECT:        //connect系统调用
        if (inp->inp_faddr.s_addr != INADDR_ANY) {        //关联远程地址,如果初始化部位INADDR_ANY,那么就返回错误
            error = EISCONN;
            break;
        }
        s = splnet();
        error = in_pcbconnect(inp, addr);
        splx(s);
        if (error == 0)
            soisconnected(so);        //将socket标记为已连接
        break;

    case PRU_CONNECT2:        //socketpair系统调用,仅用于UNIX域协议
        error = EOPNOTSUPP;
        break;

    case PRU_ACCEPT:    //accept系统调用,仅用于TCP协议
        error = EOPNOTSUPP;
        break;

    case PRU_DISCONNECT:        //销毁与远程地址之间的关联,并将远程地址设置为INADDR_ANY
        if (inp->inp_faddr.s_addr == INADDR_ANY) {
            error = ENOTCONN;
            break;
        }
        s = splnet();
        in_pcbdisconnect(inp);
        inp->inp_laddr.s_addr = INADDR_ANY;
        splx(s);
        so->so_state &= ~SS_ISCONNECTED;        //将socket标记为未连接
        break;

    case PRU_SHUTDOWN:        //shutdown系统调用,UDP很少使用
        socantsendmore(so);
        break;

    case PRU_SEND:    //发送数据请求
        return (udp_output(inp, m, addr, control));

    case PRU_ABORT:    //异常请求,UDP从不使用
        soisdisconnected(so);        //先将UDP SOCKET标记为未连接
        udp_detach(inp);    //然后销毁PCB
        break;

    case PRU_SOCKADDR:    //设置本地地址
        in_setsockaddr(inp, addr);
        break;

    case PRU_PEERADDR:    //设置远程地址
        in_setpeeraddr(inp, addr);
        break;

    case PRU_SENSE:
        /*
         * stat: don‘t bother with a blocksize.
         */
        return (0);

    case PRU_SENDOOB:
    case PRU_FASTTIMO:
    case PRU_SLOWTIMO:
    case PRU_PROTORCV:
    case PRU_PROTOSEND:
        error =  EOPNOTSUPP;
        break;

    case PRU_RCVD:
    case PRU_RCVOOB:
        return (EOPNOTSUPP);    /* do not free mbuf‘s */

    default:
        panic("udp_usrreq");
    }

release:
    if (control) {        //释放控制mbuf
        printf("udp control data unexpectedly retained\n");
        m_freem(control);
    }
    if (m)    //释放数据mbuf
        m_freem(m);
    return (error);
}

总结:

  • 问题1:IP数据报中和UDP数据报中length的表达意义?
    • IP数据报:len = IP header length + UDP header length + data length
    • UDP数据报:len = data length
  • 问题2:UDP的校验和
    UDP数据报计算UDP + data的校验和,IP仅仅计算IP头部的校验和
  • 问题3:UDP的优化措施
    • 在copy数据的时候顺便计算校验和
    • 使用其他高级数据结构进行PCB的查找

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