根据dhcp协议规定，默认情况下client需要在T1/T2这两个时间向server发送DHCPREQUEST来续租ip（T1 = 50% * 租期， T2 = 87.5% * 租期）。当然，client在T1时刻发送DHCPREQUEST后没有收到对应的DHCPACK，才会在T2时刻再次尝试发送DHCPREQUEST。对server来说，如果在整个租期内都没有收到client的DHCPREQUEST续租请求，则需要重置lease的状态，使其可以被分配给其他client。

   以下就根据isc-dhcp的代码，主要从两个方面来简单分析server端是如何维护地址池租期的。

（注：在lease结构体中关键的成员starts/ends/sort_time，分别表示lease的开始/结束/下次排序时间）

1. 续租成功，更新lease

   在server收到client发来的DHCPREQUEST报文后，构建DHCPACK回复时，同时更新对应lease的开始和结束时间。可以看看isc-dhcp中的代码：

/* At this point, we have a lease that we can offer the client.       Now we construct a lease structure that contains what we want,       and call supersede_lease to do the right thing with it. */    lt = (struct lease *)0;    result = lease_allocate (<, MDL);    if (result != ISC_R_SUCCESS) {        log_info ("%s: can't allocate temporary lease structure: %s",              msg, isc_result_totext (result));        free_lease_state (state, MDL);        if (host)            host_dereference (&host, MDL);        return;    }                                                                    /* Use the ip address of the lease that we finally found in       the database. */    lt -> ip_addr = lease -> ip_addr;    /* Start now. */    lt -> starts = cur_time;    /* Figure out how long a lease to assign.    If this is a       dynamic BOOTP lease, its duration must be infinite. */    if (offer) {        ……    } else {        lt->flags |= BOOTP_LEASE;        ……        lt -> ends = state -> offered_expiry = cur_time + lease_time;        lt -> next_binding_state = FTS_ACTIVE;    }

2. 租约到期，释放lease

   定时器是个很好的工具，但此处不对isc的timer做介绍。主要阅读dhcp的定时更新地址池内leases的代码。

/* Timer called when a lease in a particular pool expires. */void pool_timer (vpool)    void *vpool;{    struct pool *pool;    struct lease *next = (struct lease *)0;    struct lease *lease = (struct lease *)0;#define FREE_LEASES 0#define ACTIVE_LEASES 1#define EXPIRED_LEASES 2#define ABANDONED_LEASES 3#define BACKUP_LEASES 4#define RESERVED_LEASES 5    struct lease **lptr[RESERVED_LEASES+1];    TIME next_expiry = MAX_TIME;    int i;    struct timeval tv;  // 定时器时间间隔    pool = (struct pool *)vpool;    lptr [FREE_LEASES] = &pool -> free;    lptr [ACTIVE_LEASES] = &pool -> active;    lptr [EXPIRED_LEASES] = &pool -> expired;    lptr [ABANDONED_LEASES] = &pool -> abandoned;    lptr [BACKUP_LEASES] = &pool -> backup;    lptr[RESERVED_LEASES] = &pool->reserved;/* 遍历所有状态下的lease */    for (i = FREE_LEASES; i <= RESERVED_LEASES; i++) {        /* If there's nothing on the queue, skip it. */        if (!*(lptr [i]))            continue;#if defined (FAILOVER_PROTOCOL)        ……#endif             lease_reference (&lease, *(lptr [i]), MDL);        while (lease) {            /* Remember the next lease in the list. */            if (next)                lease_dereference (&next, MDL);            if (lease -> next)                lease_reference (&next, lease -> next, MDL);            /* If we've run out of things to expire on this list, stop. */            /* 由于一开始就维护着链表按时间来排序，当sort_time               在cur_time之后，则可以终止本次遍历。因为接下来的               所有lease的sort_time均大于cur_time */            if (lease -> sort_time > cur_time) {                if (lease -> sort_time < next_expiry)                    next_expiry = lease -> sort_time;                break;            }            /* If there is a pending state change, and               this lease has gotten to the time when the               state change should happen, just call               supersede_lease on it to make the change               happen. */            if (lease->next_binding_state != lease->binding_state)            {#if defined(FAILOVER_PROTOCOL)        ……#endif                /* 更改lease状态 */                supersede_lease(lease, NULL, 1, 1, 1);            }            lease_dereference (&lease, MDL);            if (next)                lease_reference (&lease, next, MDL);        }        if (next)            lease_dereference (&next, MDL);        if (lease)            lease_dereference (&lease, MDL);    }    if (next_expiry != MAX_TIME) {        pool -> next_event_time = next_expiry;        tv . tv_sec = pool -> next_event_time;        tv . tv_usec = 0;        /* 设置下次pool_timer执行时间 */        add_timeout (&tv, pool_timer, pool,                 (tvref_t)pool_reference,                 (tvunref_t)pool_dereference);    } else        pool -> next_event_time = MIN_TIME;}