Syntax:    keepalive_timeout timeout [header_timeout];
Default:    
keepalive_timeout 75s;
Context:    http, server, location

Syntax:    keepalive_requests number;
Default:    
keepalive_requests 1000;
Context:    http, server, location

void
ngx_http_update_location_config(ngx_http_request_t *r)
{
    ngx_http_core_loc_conf_t  *clcf;
    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
    if (r->method & clcf->limit_except) {
        r->loc_conf = clcf->limit_except_loc_conf;
        clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
    }
    if (r == r->main) {
        ngx_set_connection_log(r->connection, clcf->error_log);
    }
    if ((ngx_io.flags & NGX_IO_SENDFILE) && clcf->sendfile) {
        r->connection->sendfile = 1;
    } else {
        r->connection->sendfile = 0;
    }
    if (clcf->client_body_in_file_only) {
        r->request_body_in_file_only = 1;
        r->request_body_in_persistent_file = 1;
        r->request_body_in_clean_file =
            clcf->client_body_in_file_only == NGX_HTTP_REQUEST_BODY_FILE_CLEAN;
        r->request_body_file_log_level = NGX_LOG_NOTICE;
    } else {
        r->request_body_file_log_level = NGX_LOG_WARN;
    }
    r->request_body_in_single_buf = clcf->client_body_in_single_buffer;
    if (r->keepalive) {
        if (clcf->keepalive_timeout == 0) {
            r->keepalive = 0;
        } else if (r->connection->requests >= clcf->keepalive_requests) {
            r->keepalive = 0;
        } else if (ngx_current_msec - r->connection->start_time
                   > clcf->keepalive_time)
        {
            r->keepalive = 0;
        } else if (r->headers_in.msie6
                   && r->method == NGX_HTTP_POST
                   && (clcf->keepalive_disable
                       & NGX_HTTP_KEEPALIVE_DISABLE_MSIE6))
        {
            /*
             * MSIE may wait for some time if an response for
             * a POST request was sent over a keepalive connection
             */
            r->keepalive = 0;
        } else if (r->headers_in.safari
                   && (clcf->keepalive_disable
                       & NGX_HTTP_KEEPALIVE_DISABLE_SAFARI))
        {
            /*
             * Safari may send a POST request to a closed keepalive
             * connection and may stall for some time, see
             *     https://bugs.webkit.org/show_bug.cgi?id=5760
             */
            r->keepalive = 0;
        }
    }
    if (!clcf->tcp_nopush) {
        /* disable TCP_NOPUSH/TCP_CORK use */
        r->connection->tcp_nopush = NGX_TCP_NOPUSH_DISABLED;
    }
    if (clcf->handler) {
        r->content_handler = clcf->handler;
    }
}

static ngx_int_t
ngx_http_header_filter(ngx_http_request_t *r)
{
    u_char                    *p;
    size_t                     len;
    ngx_str_t                  host, *status_line;
    ngx_buf_t                 *b;
    ngx_uint_t                 status, i, port;
    ngx_chain_t                out;
    ngx_list_part_t           *part;
    ngx_table_elt_t           *header;
    ngx_connection_t          *c;
    ngx_http_core_loc_conf_t  *clcf;
    ngx_http_core_srv_conf_t  *cscf;
    u_char                     addr[NGX_SOCKADDR_STRLEN];
    if (r->header_sent) {
        return NGX_OK;
    }
    r->header_sent = 1;
    if (r != r->main) {
        return NGX_OK;
    }
    //......
    if (r->keepalive && (ngx_terminate || ngx_exiting)) {
        r->keepalive = 0;
    }
    //......
    if (r->headers_out.status == NGX_HTTP_SWITCHING_PROTOCOLS) {
        len += sizeof("Connection: upgrade" CRLF) - 1;
    } else if (r->keepalive) {
        len += sizeof("Connection: keep-alive" CRLF) - 1;
        /*
         * MSIE and Opera ignore the "Keep-Alive: timeout=<N>" header.
         * MSIE keeps the connection alive for about 60-65 seconds.
         * Opera keeps the connection alive very long.
         * Mozilla keeps the connection alive for N plus about 1-10 seconds.
         * Konqueror keeps the connection alive for about N seconds.
         */
        if (clcf->keepalive_header) {
            len += sizeof("Keep-Alive: timeout=") - 1 + NGX_TIME_T_LEN + 2;
        }
    } else {
        len += sizeof("Connection: close" CRLF) - 1;
    }
    //......    
    if (r->headers_out.status == NGX_HTTP_SWITCHING_PROTOCOLS) {
        b->last = ngx_cpymem(b->last, "Connection: upgrade" CRLF,
                             sizeof("Connection: upgrade" CRLF) - 1);
    } else if (r->keepalive) {
        b->last = ngx_cpymem(b->last, "Connection: keep-alive" CRLF,
                             sizeof("Connection: keep-alive" CRLF) - 1);
        if (clcf->keepalive_header) {
            b->last = ngx_sprintf(b->last, "Keep-Alive: timeout=%T" CRLF,
                                  clcf->keepalive_header);
        }
    } else {
        b->last = ngx_cpymem(b->last, "Connection: close" CRLF,
                             sizeof("Connection: close" CRLF) - 1);
    }    
}