TCP Timestamp選項

<a href="http://blog.csdn.net/zhangskd/article/details/7195795">TCP Timestamp選項</a>

Description

Protocol suite: TCP/IP.

Protocol type: Transport layer protocol.

Option length: 10 bytes.

The TCP Timestamp option obsoletes the TCP Echo request and Echo reply options.

RFC 1323:

The timestamps are used for two distinct mechanisms: RTTM (Round Trip Time Measurement) and PAWS (Protected Against Wrapped Squences).

結構圖

Kind. 8 bits. Set to 8.

Length. 8 bits. Set to 10.

Timestamp Value (TSval). 32 bits.

This field contains the current value of the timestamp clock of the TCP sending the option.

TImestamp Echo Reply (TSecr). 32 bits.

This field only valid if the ACK bit is set in the TCP header. If it is valid, it echos a timestamp value that was sent by the remote TCP in the TSval field of a Timestamps option. When TSecr is not valid, its value must be zero. The TSecr value will generally be from the most recent Timestamp option that was received; however, there are exceptions that are explained below. A TCP may send the Timestamp option in an initial SYN segment(i.e., segment containing a SYN bit and no ACK bit), and may send a TSopt in other segments only if it received a TSopt in the initial SYN segment for the connection.

Linux核心中的使用

如果支援Timestamp選項，那麼可以用此選項來計算RTT。

static void tcp_ack_saw_tstamp(struct sock *sk , int flag)

{

/* RTTM Rule: A TSecr value received in a segment is used to

* update the averaged RTT measurement only if the segment

* acknowledges some new data, i.e., only if it advances the

* left edge of the send window.

*

* Changed: reset backoff as soon as we see the first valid

* sample. If we do not, we get strongly overestimated rto.

* With timestamps samples are accepted even from very

* old segments: f.e., when rtt=1 increases to 8, we retransmit

* 5 times and after 8 seconds delayed answer arrives rto

* becomes 120 seconds!

*/

struct tcp_sock *tp = tcp_sk(sk);

tcp_valid_rtt_meas(sk, tcp_time_stamp - tp-&gt;rx_opt.rcv_tsecr);

}

rtt即等于現在的時間tcp_time_stamp減去Timestamp Echo Reply，即tp-&gt;rx_opt.rcv_tsecr。

TCP timestamp option的作用：

1)allow more accurate round-trip time measurements for deriving the retransmission timeout estimator.

2)protect against old segments from the previous incarnations of the TCP connection.

3)allow detection of unnecessary retransmissions.

tcp_tw_recycle/tcp_timestamps都開啟的條件下，60s内同一源ip主機的socket connect請求中的timestamp必須是遞增的。

原文

http://blog.csdn.net/zhangskd/article/details/7195795

tcp_tw_recycle和tcp_timestamps導緻connect失敗問題

http://blog.sina.com.cn/s/blog_781b0c850100znjd.html