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NTCP2: Add clock skew guidance

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2022-01-26 07:51:46 -05:00
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i2p2www/spec/ntcp2.rst
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@ -3,8 +3,8 @@ NTCP 2
======
.. meta::
:category: Transports
:lastupdated: 2021-03
:accuratefor: 0.9.50
:lastupdated: 2022-01
:accuratefor: 0.9.53
.. contents::
@ -1991,6 +1991,36 @@ Variants, Fallbacks, and General Issues
Alice may not retry using NTCP 1.
Clock Skew Guidelines
======================
Peer timestamps are included in the first two handshake messages, Session Request and Session Created.
A clock skew between two peers of greater than +/- 60 seconds is generally fatal.
If Bob thinks that his local clock is bad, he may adjust her clock using the
calculated skew, or some external source.
Otherwise, Bob should reply with a Session Created even if the maximum skew is exceeded,
rather than simply closing the connection. This allows Alice to get Bob's timestamp and
calculate the skew, and take action if necessary.
Bob does not have Alice's router identity at this point, but
to conserve resources,
it may be desirable for Bob to ban incoming connections from Alice's IP for some period of time,
or after repeated connection attempts with an excessive skew.
Alice should adjust the calculated clock skew by subtracting half the RTT.
If Alice thinks that her local clock is bad, she may adjust her clock using the
calculated skew, or some external source.
If Alice thinks that Bob's clock is bad, she may ban Bob for some period of time.
In either case, Alice should close the connection.
If Alice does reply with Session Confirmed
(probably because the skew is very close to the 60s limit,
and the Alice and Bob calculations are not exactly the same due to RTT),
Bob should adjust the calculated clock skew by subtracting half the RTT.
If the adjusted clock skew exceeds the maximum, Bob should then reply with
a Disconnect message containing a clock skew reason code, and close
the connection. At this point, Bob has Alice's router identity,
and may ban Alice for some period of time.
References
==========