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4.2.2 Timestamp Clock Connected: An Internet Encyclopedia
4.2.2 Timestamp Clock

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Up: RFC 1323
Up: 4. PAWS: PROTECT AGAINST WRAPPED SEQUENCE NUMBERS
Up: 4.2 The PAWS Mechanism
Prev: 4.2.1 Basic PAWS Algorithm
Next: 4.2.3 Outdated Timestamps

4.2.2 Timestamp Clock

4.2.2 Timestamp Clock

It is important to understand that the PAWS algorithm does not require clock synchronization between sender and receiver. The sender's timestamp clock is used to stamp the segments, and the sender uses the echoed timestamp to measure RTT's. However, the receiver treats the timestamp as simply a monotone- increasing serial number, without any necessary connection to its clock. From the receiver's viewpoint, the timestamp is acting as a logical extension of the high-order bits of the sequence number.

The receiver algorithm does place some requirements on the frequency of the timestamp clock.

  1. The timestamp clock must not be "too slow".

    It must tick at least once for each 2**31 bytes sent. In fact, in order to be useful to the sender for round trip timing, the clock should tick at least once per window's worth of data, and even with the RFC-1072 window extension, 2**31 bytes must be at least two windows.

    To make this more quantitative, any clock faster than 1 tick/sec will reject old duplicate segments for link speeds of ~8 Gbps. A 1ms timestamp clock will work at link speeds up to 8 Tbps (8*10**12) bps!

  2. The timestamp clock must not be "too fast".

    Its recycling time must be greater than MSL seconds. Since the clock (timestamp) is 32 bits and the worst-case MSL is 255 seconds, the maximum acceptable clock frequency is one tick every 59 ns.

    However, it is desirable to establish a much longer recycle period, in order to handle outdated timestamps on idle connections (see Section 4.2.3), and to relax the MSL requirement for preventing sequence number wrap-around. With a 1 ms timestamp clock, the 32-bit timestamp will wrap its sign bit in 24.8 days. Thus, it will reject old duplicates on the same connection if MSL is 24.8 days or less. This appears to be a very safe figure; an MSL of 24.8 days or longer can probably be assumed by the gateway system without requiring precise MSL enforcement by the TTL value in the IP layer.

Based upon these considerations, we choose a timestamp clock frequency in the range 1 ms to 1 sec per tick. This range also matches the requirements of the RTTM mechanism, which does not need much more resolution than the granularity of the retransmit timer, e.g., tens or hundreds of milliseconds.

The PAWS mechanism also puts a strong monotonicity requirement on the sender's timestamp clock. The method of implementation of the timestamp clock to meet this requirement depends upon the system hardware and software.

  • Some hosts have a hardware clock that is guaranteed to be monotonic between hardware resets.

  • A clock interrupt may be used to simply increment a binary integer by 1 periodically.

  • The timestamp clock may be derived from a system clock that is subject to being abruptly changed, by adding a variable offset value. This offset is initialized to zero. When a new timestamp clock value is needed, the offset can be adjusted as necessary to make the new value equal to or larger than the previous value (which was saved for this purpose).


Next: 4.2.3 Outdated Timestamps

Connected: An Internet Encyclopedia
4.2.2 Timestamp Clock

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