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Linux 内核参数全解析--TCP内核参数篇(Doing)

2020/8/11 20:18:27 文章标签:

 /proc 接口概述
       所有系统级的同产品参数设置都保存在  /proc/sys/net/ipv4/ 目录下.  另外家需要注意的是,很多对于 ip 的内核参数设置也会对 tcp 的行为产生影响.具体请参考linux 对于 ip 参数的帮助手册. 参数通常使用一个整形值来代表布尔型,设置为 0 时通常表示 "false",对应的一个非零值则表示 ("true").

tcp_abc (Integer; default: 0; Linux 2.6.15 to Linux 3.8)
     控制在 RFC 3465 上定义的 Appropriate Byte Count (ABC), 该值是一个用于在接收到部分 ACK 情况下 缓慢地增长拥塞窗口(cwnd),RFC3465给出了ABC算法,在ABC中,使用被ACK的字节数而不是ACK的数量来作为增窗的反馈信号.可用的值有:

              0  每个 ACK 增加一个 cwnd (不开启 ABC);

              1  每当被ACK的数据字节数达到了一窗的大小即增加一个 cwnd ;

              2  允许补偿延迟发送的 ACK,当接收到2窗大小字节数的 ack 时, 增加 2个 cwnd .

参考:https://blog.csdn.net/dog250/article/details/51348568


tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
    当一个服务器进入瓶颈响应太慢或者无法进行accept时,重置连接. 这意味着如果应为突发流量导致overflow,该链接可以被恢复. 仅在你已经确定启用 listen 的服务已经无法针对接受连接的速率做进一步调优的情况下启用该配置.因为启用该配置后配置可能会误伤客户端.


tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
    如果 tcp_adv_win_scale 大于 0,则使用 bytes/2^tcp_adv_win_scale 方式计算缓存开销,否则(tcp_adv_win_scale <=0>)则使用 bytes-bytes/2^(-tcp_adv_win_scale) 的公式计算.

    tcp socket 的接收缓存空间是应用与内核共享的, 其中归于 tcp 维护的部分即是 TCP 的 window, 该 window 作为可接收的 window ,tcp 将其通告给对端. 剩余部分用作应用缓存,用于将网络与 调度和应用的延迟 隔离开. tcp_adv_win_scale 默认值为 2, 标识 1/4 大小的用于应用的缓存.

tcp_allowed_congestion_control (String; default: 见文本说明; since Linux 2.4.20)
    该选项用于查看和设置非特权进程的可用的拥塞控制算法(参考 TCP_CONGESTION socket 选项). 使用空格键分割列表中个元素,使用换行结束. 这里配置的列表必须是参数 tcp_available_congestion_control 中列表的子集.  该选项值列表的默认值是 "reno" 加上 tcp_congestion_control 参数中的配置值.

tcp_available_congestion_control (String; read-only; since Linux 2.4.20)

        展示已经注册的可用的拥塞控制算法列表.使用空格键分割列表中个元素,用换行结束.这是限制在 tcp_allowed_congestion_control 中可用的配置的算法集合.更多的拥塞控制算法可以通过 内核模块的方式提供和加载,这里没有提及.

tcp_congestion_control (String; default: see text; since Linux 2.4.13)

        设置用于新建连接的默认拥塞控制算法.  "reno" 总是可用的, 额外的选择的则取决于对该配置. 该选项将被作为系统内核配置的一部分.

[root@CQDC ~]# cat /proc/sys/net/ipv4/tcp_allowed_congestion_control
cubic reno
[root@CQDC ~]# cat /proc/sys/net/ipv4/tcp_available_congestion_control
cubic reno
[root@CQDC ~]# cat /proc/sys/net/ipv4/tcp_congestion_control
cubic

tcp_autocorking (Boolean; default: enabled; since Linux 3.14)
    启用时,内核将尽可能多的合并小块的写操作用来减少发包的数量 (from consecutive write(2) and sendmsg(2) calls) .  当至少有一个以前的包在队列规则(Qdisc)或者设备的传送队列中时,合并生效. 应用程序依然可以自行设置socket的 TCP_CORK 选项来优化其行为使其在适当的情况下不要执行合并操作.

tcp_app_win (integer; default: 31; since Linux 2.4)
    该变量定义了 TCP 窗口中多少字节用于缓存开销.设置为非零整数时,最大 (window/2^tcp_app_win, mss) 字节将被保留为应用的缓存开销,  设置为0则表示不预留.

tcp_base_mss (Integer; default: 512; since Linux 2.6.17)
    打包分组层路径 MTU 探索(MTU probing) 机制中 search_low 参数的起始值.如果启用了MTU probing,这是连接使用的起始 MSS.

tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
    启用 BIC-TCP 拥塞控制算法.  BIC-TCP 是一个仅发送方变更的算法,该算法在提供与tcp优化集成与扩展性的情况下确保大窗口下的RTT线性公平.该协议包含2套方案分别是 叠加增长(additive increase) 和 二分查找增长(binary search increase).当拥塞窗口很大时,叠加增长大幅增加确保了线性 RTT 公平性和可扩展性. 在小窗口拥塞的情况下, 二分查找增长提供了tcp的友好性.

tcp_bic (integer; default: 14; Linux 2.4.27/2.6.6 to 2.6.13)
    设置一个BIC TCP 算法生效的窗口最低阈值(单位是 packet数量). 低于该阈值时 BIC TCP和默认的 TCP Reno 表现一致.

tcp_bic_fast_convergence (Boolean; default: enabled; Linux 2.4.27/2.6.6 to 2.6.13)
    强制 BIC TCP 在拥塞窗口发生变化时更快地做出响应. 允许共享一个连接的两个流可以更快收敛.

tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
    如果系统和内核配置中已经存在 CONFIG_NET_DMA 选项的配置值,该配置项提供一个以 字节为单位的下限值配置,指示socket的读操作可以被卸载到 DMA复制引擎 的大小. 


tcp_dsack (Boolean; default: enabled; since Linux 2.4)
    启用 RFC 2883 中对于 TCP Duplicate SACK 机制的支持.

tcp_ecn (Integer; default: see below; since Linux 2.4)
    启用 RFC 3168 中 显式拥塞通告 Explicit Congestion Notification (ECN)机制.

    允许的配置值及含义如下:

              0      不启用 ECN. 不会发起或接受 ECN.  这是 Linux 2.6.30及以前版本的默认配置.

              1      当被动建立连接对端要求开启时启用 ECN,当主动发起连接时,尝试要求对端开启ECN.

              2      当被动建立连接,对端要求开启时启用 ECN,当主动发起连接时,不尝试要求对端开启ECN. 这是自 Linux 2.6.31
                     开始支持的特性,且被设置为默认值

    当该项功能开启时, 与某些目标端的连接可能被之前的连接所影响,一些沿途路径上的设备的误操作可能导致连接丢失.但是为了鼓励利用模式 1 并处理这项漏洞, linux 又引入了 tcp_ecn_fallback 选项

tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
    启用 RFC 3168 中 6.1.1.1.章节的 fallback 特性.  该特性启用时,出站的 ECN-setup SYN 数据包如果超出了标准的 SYN 重传超时时间,将会在重传时清除 CWR and ECE.
    这个参数设置为非0时,如果内核侦测到ECN的错误行为,就会关闭ECN功能。 这个参数实际上是控制后向兼容的一个参数,TCP建立连接的时候需要进行ECN协商过程,SYN报文中需要同时设置CWR和ECE标志位,如果tcp_ecn_fallback设置为非0,那么重传SYN报文的时候就会取消CWR和ECE标志的设置

参考:https://www.cnblogs.com/lshs/p/6038839.html

tcp_fack (Boolean; default: enabled; since Linux 2.2)
     启用 TCP Forward Acknowledgement 支持.

tcp_fin_timeout (integer; default: 60; since Linux 2.2)
    该选项描述了系统将在等待最后的 FIN 报文多久后强制关闭该 socket.可以用来防止DOS攻击.在Linux 2.2中, 默认值是180s.

tcp_frto (integer; default: 见下文; since Linux 2.4.21/2.6)
    启用 F-RTO, 一个用于计算 TCP 重传超时时间(RTO)的加强算法.该算法在面对无线连接中信号干扰造成的丢包时尤为有效, 参考 RFC 4138 查看更多细节.

    允许的配置值及含义如下:

              0  关闭.  Linux 2.6.23及以前的版本,这是默认值.

              1  开启 F-RTO 基础版本算法.

              2  如果流使用SACK,那么开启使用 SACK 增强的 F-RTO算法. 当启用SACK时,也可以使用基础版本,但这种情况下,启用了
                 SACK的TCP流中会存在一些 F-RTO 交互计算包时表现不佳的情形. 从Linux 2.6.24版本以后,该值为默认配置.

    注意 Linux 2.6.22及以前的版本, 该参数仅支持 Boolean 值(0和1).

tcp_frto_response (integer; default: 0; since Linux 2.6.22)
     当 F-RTO 检测到一个tcp重传超时时间不太准确(虚假)时,(例如, the timeout would have been avoided had TCP set a longer retransmission timeout), TCP可以根据选项决定接下来的处理办法:

              0  减半; 平滑而保守的反应,将拥塞窗口(cwnd)和慢启动阈值(ssthresh)在一个 RTT后设置为原来的一般.

              1  非常保守的处理方式; 虽然不推荐但依然可用, 和剩余的不良的Linux TCP交互, 立即将 cwnd and ssthresh 减半.

              2  较为进取的处理方式; 不做已知无用的拥塞控制测量 (忽悠可能存在的重传丢失); cwnd and ssthresh将被恢复到之前的值.

tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
    发出 TCP keep-alive 探测报文的间隔时间(秒).

tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
    指定最大数量的 TCP keep-alive 探测包发送后依然没有收到对端回复的连接将被放弃并杀掉.

tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
    tcp连接处于 idle状态多长时间(秒)后发送 keep-alive 探测报文. 只有在 socket上设置了SO_KEEPALIVE 选项的才会发送 Keep-alive. 默认值是 7200 秒(2 小时). 在 keep-alive 开启时一个空闲连接在该选项时间加上 11 分钟(9次间隔 75 秒的探测).

    注意,连接状态跟踪分析和应用的超时可能远远短于该时间.

tcp_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6;obsolete since Linux 4.14)
    启用时,TCP 协议栈将更倾向于低延迟的处理方式. 如果禁用该选项,则更倾向与高吞吐量.修改该配置的一个例子是一个 Beowulf 计算集群. Linux 4.14版本以后, 该配置文件虽然还在,但是其值不会发生任何作用.

tcp_max_orphans (integer; default: see below; since Linux 2.4)
    系统中可以存在的最大数量的孤儿 TCP socket 数量orphaned (没有与任何用户文件关联) .当超出此数量时,孤儿连接将被 RESET并且打印一条警告.  该限制用于防止 DOS 攻击. 不建议降低该配置的值.实际网络条件可能会迫使你增加该配置的值, 但是注意每一个孤儿socket可能会占用最高 ~64 kB不可交换的内存空间.  起始默认值被设置为等于内核参数的 NR_FILE. 可以根据系统的内存大小调节该值.

tcp_orphan_retries (integer; default: 8; since Linux 2.4)

         尝试探测由本端主动关闭的连接的对端的最大次数.


tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)

              The maximum number of queued connection requests which have
              still not received an acknowledgement from the connecting
              client.  If this number is exceeded, the kernel will begin
              dropping requests.  The default value of 256 is increased to
              1024 when the memory present in the system is adequate or
              greater (>= 128 MB), and reduced to 128 for those systems with
              very low memory (<= 32 MB).

              Prior to Linux 2.6.20, it was recommended that if this needed
              to be increased above 1024, the size of the SYNACK hash table
              (TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to
              keep

                  TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog

              and the kernel should be recompiled.  In Linux 2.6.20, the
              fixed sized TCP_SYNQ_HSIZE was removed in favor of dynamic
              sizing.

       tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
              The maximum number of sockets in TIME_WAIT state allowed in
              the system.  This limit exists only to prevent simple denial-
              of-service attacks.  The default value of NR_FILE*2 is
              adjusted depending on the memory in the system.  If this
              number is exceeded, the socket is closed and a warning is
              printed.

       tcp_moderate_rcvbuf (Boolean; default: enabled; since Linux
       2.4.17/2.6.7)
              If enabled, TCP performs receive buffer auto-tuning,
              attempting to automatically size the buffer (no greater than
              tcp_rmem[2]) to match the size required by the path for full
              throughput.

       tcp_mem (since Linux 2.4)
              This is a vector of 3 integers: [low, pressure, high].  These
              bounds, measured in units of the system page size, are used by
              TCP to track its memory usage.  The defaults are calculated at
              boot time from the amount of available memory.  (TCP can only
              use low memory for this, which is limited to around 900
              megabytes on 32-bit systems.  64-bit systems do not suffer
              this limitation.)

              low    TCP doesn't regulate its memory allocation when the
                     number of pages it has allocated globally is below this
                     number.

              pressure
                     When the amount of memory allocated by TCP exceeds this
                     number of pages, TCP moderates its memory consumption.
                     This memory pressure state is exited once the number of
                     pages allocated falls below the low mark.

              high   The maximum number of pages, globally, that TCP will
                     allocate.  This value overrides any other limits
                     imposed by the kernel.

       tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
              This parameter controls TCP Packetization-Layer Path MTU
              Discovery.  The following values may be assigned to the file:

              0  Disabled

              1  Disabled by default, enabled when an ICMP black hole
                 detected

              2  Always enabled, use initial MSS of tcp_base_mss.

       tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
              By default, TCP saves various connection metrics in the route
              cache when the connection closes, so that connections
              established in the near future can use these to set initial
              conditions.  Usually, this increases overall performance, but
              it may sometimes cause performance degradation.  If
              tcp_no_metrics_save is enabled, TCP will not cache metrics on
              closing connections.



       tcp_reordering (integer; default: 3; since Linux 2.4)
              The maximum a packet can be reordered in a TCP packet stream
              without TCP assuming packet loss and going into slow start.
              It is not advisable to change this number.  This is a packet
              reordering detection metric designed to minimize unnecessary
              back off and retransmits provoked by reordering of packets on
              a connection.

       tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
              Try to send full-sized packets during retransmit.

       tcp_retries1 (integer; default: 3; since Linux 2.2)
              The number of times TCP will attempt to retransmit a packet on
              an established connection normally, without the extra effort
              of getting the network layers involved.  Once we exceed this
              number of retransmits, we first have the network layer update
              the route if possible before each new retransmit.  The default
              is the RFC specified minimum of 3.

       tcp_retries2 (integer; default: 15; since Linux 2.2)
              The maximum number of times a TCP packet is retransmitted in
              established state before giving up.  The default value is 15,
              which corresponds to a duration of approximately between 13 to
              30 minutes, depending on the retransmission timeout.  The
              RFC 1122 specified minimum limit of 100 seconds is typically
              deemed too short.

       tcp_rfc1337 (Boolean; default: disabled; since Linux 2.2)
              Enable TCP behavior conformant with RFC 1337.  When disabled,
              if a RST is received in TIME_WAIT state, we close the socket
              immediately without waiting for the end of the TIME_WAIT
              period.

       tcp_rmem (since Linux 2.4)
              This is a vector of 3 integers: [min, default, max].  These
              parameters are used by TCP to regulate receive buffer sizes.
              TCP dynamically adjusts the size of the receive buffer from
              the defaults listed below, in the range of these values,
              depending on memory available in the system.

              min    minimum size of the receive buffer used by each TCP
                     socket.  The default value is the system page size.
                     (On Linux 2.4, the default value is 4 kB, lowered to
                     PAGE_SIZE bytes in low-memory systems.)  This value is
                     used to ensure that in memory pressure mode,
                     allocations below this size will still succeed.  This
                     is not used to bound the size of the receive buffer
                     declared using SO_RCVBUF on a socket.

              default
                     the default size of the receive buffer for a TCP
                     socket.  This value overwrites the initial default
                     buffer size from the generic global
                     net.core.rmem_default defined for all protocols.  The
                     default value is 87380 bytes.  (On Linux 2.4, this will
                     be lowered to 43689 in low-memory systems.)  If larger
                     receive buffer sizes are desired, this value should be
                     increased (to affect all sockets).  To employ large TCP
                     windows, the net.ipv4.tcp_window_scaling must be
                     enabled (default).

              max    the maximum size of the receive buffer used by each TCP
                     socket.  This value does not override the global
                     net.core.rmem_max.  This is not used to limit the size
                     of the receive buffer declared using SO_RCVBUF on a
                     socket.  The default value is calculated using the
                     formula

                         max(87380, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))

                     (On Linux 2.4, the default is 87380*2 bytes, lowered to
                     87380 in low-memory systems).

       tcp_sack (Boolean; default: enabled; since Linux 2.2)
              Enable RFC 2018 TCP Selective Acknowledgements.

       tcp_slow_start_after_idle (Boolean; default: enabled; since Linux
       2.6.18)
              If enabled, provide RFC 2861 behavior and time out the
              congestion window after an idle period.  An idle period is
              defined as the current RTO (retransmission timeout).  If
              disabled, the congestion window will not be timed out after an
              idle period.

       tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
              If this option is enabled, then use the RFC 1122
              interpretation of the TCP urgent-pointer field.  According to
              this interpretation, the urgent pointer points to the last
              byte of urgent data.  If this option is disabled, then use the
              BSD-compatible interpretation of the urgent pointer: the
              urgent pointer points to the first byte after the urgent data.
              Enabling this option may lead to interoperability problems.

       tcp_syn_retries (integer; default: 6; since Linux 2.2)
              The maximum number of times initial SYNs for an active TCP
              connection attempt will be retransmitted.  This value should
              not be higher than 255.  The default value is 6, which
              corresponds to retrying for up to approximately 127 seconds.
              Before Linux 3.7, the default value was 5, which (in
              conjunction with calculation based on other kernel parameters)
              corresponded to approximately 180 seconds.

       tcp_synack_retries (integer; default: 5; since Linux 2.2)
              The maximum number of times a SYN/ACK segment for a passive
              TCP connection will be retransmitted.  This number should not
              be higher than 255.

       tcp_syncookies (Boolean; since Linux 2.2)
              Enable TCP syncookies.  The kernel must be compiled with
              CONFIG_SYN_COOKIES.  Send out syncookies when the syn backlog
              queue of a socket overflows.  The syncookies feature attempts
              to protect a socket from a SYN flood attack.  This should be
              used as a last resort, if at all.  This is a violation of the
              TCP protocol, and conflicts with other areas of TCP such as
              TCP extensions.  It can cause problems for clients and relays.
              It is not recommended as a tuning mechanism for heavily loaded
              servers to help with overloaded or misconfigured conditions.
              For recommended alternatives see tcp_max_syn_backlog,
              tcp_synack_retries, and tcp_abort_on_overflow.

       tcp_timestamps (integer; default: 1; since Linux 2.2)
              Set to one of the following values to enable or disable
              RFC 1323 TCP timestamps:

              0  Disable timestamps.

              1  Enable timestamps as defined in RFC1323 and use random
                 offset for each connection rather than only using the
                 current time.

              2  As for the value 1, but without random offsets.  Setting
                 tcp_timestamps to this value is meaningful since Linux
                 4.10.

       tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
              This parameter controls what percentage of the congestion
              window can be consumed by a single TCP Segmentation Offload
              (TSO) frame.  The setting of this parameter is a tradeoff
              between burstiness and building larger TSO frames.

       tcp_tw_recycle (Boolean; default: disabled; Linux 2.4 to 4.11)
              Enable fast recycling of TIME_WAIT sockets.  Enabling this
              option is not recommended as the remote IP may not use
              monotonically increasing timestamps (devices behind NAT,
              devices with per-connection timestamp offsets).  See RFC 1323
              (PAWS) and RFC 6191.

       tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
              Allow to reuse TIME_WAIT sockets for new connections when it
              is safe from protocol viewpoint.  It should not be changed
              without advice/request of technical experts.

       tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to
       2.6.13)
              Enable TCP Vegas congestion avoidance algorithm.  TCP Vegas is
              a sender-side-only change to TCP that anticipates the onset of
              congestion by estimating the bandwidth.  TCP Vegas adjusts the
              sending rate by modifying the congestion window.  TCP Vegas
              should provide less packet loss, but it is not as aggressive
              as TCP Reno.

       tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to
       2.6.13)
              Enable TCP Westwood+ congestion control algorithm.  TCP
              Westwood+ is a sender-side-only modification of the TCP Reno
              protocol stack that optimizes the performance of TCP
              congestion control.  It is based on end-to-end bandwidth
              estimation to set congestion window and slow start threshold
              after a congestion episode.  Using this estimation, TCP
              Westwood+ adaptively sets a slow start threshold and a
              congestion window which takes into account the bandwidth used
              at the time congestion is experienced.  TCP Westwood+
              significantly increases fairness with respect to TCP Reno in
              wired networks and throughput over wireless links.

       tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
              Enable RFC 1323 TCP window scaling.  This feature allows the
              use of a large window (> 64 kB) on a TCP connection, should
              the other end support it.  Normally, the 16 bit window length
              field in the TCP header limits the window size to less than
              64 kB.  If larger windows are desired, applications can
              increase the size of their socket buffers and the window
              scaling option will be employed.  If tcp_window_scaling is
              disabled, TCP will not negotiate the use of window scaling
              with the other end during connection setup.

       tcp_wmem (since Linux 2.4)
              This is a vector of 3 integers: [min, default, max].  These
              parameters are used by TCP to regulate send buffer sizes.  TCP
              dynamically adjusts the size of the send buffer from the
              default values listed below, in the range of these values,
              depending on memory available.

              min    Minimum size of the send buffer used by each TCP
                     socket.  The default value is the system page size.
                     (On Linux 2.4, the default value is 4 kB.)  This value
                     is used to ensure that in memory pressure mode,
                     allocations below this size will still succeed.  This
                     is not used to bound the size of the send buffer
                     declared using SO_SNDBUF on a socket.

              default
                     The default size of the send buffer for a TCP socket.
                     This value overwrites the initial default buffer size
                     from the generic global /proc/sys/net/core/wmem_default
                     defined for all protocols.  The default value is 16 kB.
                     If larger send buffer sizes are desired, this value
                     should be increased (to affect all sockets).  To employ
                     large TCP windows, the
                     /proc/sys/net/ipv4/tcp_window_scaling must be set to a
                     nonzero value (default).

              max    The maximum size of the send buffer used by each TCP
                     socket.  This value does not override the value in
                     /proc/sys/net/core/wmem_max.  This is not used to limit
                     the size of the send buffer declared using SO_SNDBUF on
                     a socket.  The default value is calculated using the
                     formula

                         max(65536, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))

                     (On Linux 2.4, the default value is 128 kB, lowered
                     64 kB depending on low-memory systems.)

       tcp_workaround_signed_windows (Boolean; default: disabled; since
       Linux 2.6.26)
              If enabled, assume that no receipt of a window-scaling option
              means that the remote TCP is broken and treats the window as a
              signed quantity.  If disabled, assume that the remote TCP is
              not broken even if we do not receive a window scaling option
              from it.

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