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Spin bit is path-bound #3984

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Spin bit is path-bound #3984

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Spin bit is path-bound
martinthomson Aug 5, 2020
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First packet with a given connection ID
martinthomson Aug 13, 2020
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29 changes: 15 additions & 14 deletions draft-ietf-quic-transport.md
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Expand Up @@ -4904,6 +4904,7 @@ version-independent. The remaining fields are specific to the selected QUIC
version. See {{QUIC-INVARIANTS}} for details on how packets from different
versions of QUIC are interpreted.


### Latency Spin Bit {#spin-bit}

The latency spin bit enables passive latency monitoring from observation points
Expand Down Expand Up @@ -4932,29 +4933,29 @@ bit to a random value either chosen independently for each packet or chosen
independently for each connection ID.

If the spin bit is enabled for the connection, the endpoint maintains a spin
value and sets the spin bit in the short header to the currently stored
value when a packet with a short header is sent out. The spin value is
initialized to 0 in the endpoint at connection start. Each endpoint also
remembers the highest packet number seen from its peer on the connection.
value for each network path and sets the spin bit in the short header to the
currently stored value when a packet with a short header is sent on that path.
The spin value is initialized to 0 in the endpoint for each network path. Each
endpoint also remembers the highest packet number seen from its peer on each
path.

When a server receives a short header packet that increments the highest
packet number seen by the server from the client, it sets the spin value to be
equal to the spin bit in the received packet.
When a server receives a short header packet that increases the highest packet
number seen by the server from the client on a given network path, it sets the
spin value for that path to be equal to the spin bit in the received packet.

When a client receives a short header packet that increments the highest
packet number seen by the client from the server, it sets the spin value to the
inverse of the spin bit in the received packet.
When a client receives a short header packet that increases the highest packet
number seen by the client from the server on a given network path, it sets the
spin value for that path to the inverse of the spin bit in the received packet.

An endpoint resets its spin value to zero when sending the first packet of a
given connection with a new connection ID. This reduces the risk that transient
spin bit state can be used to link flows across connection migration or ID
change.
An endpoint resets the spin value for a network path to zero when changing the
connection ID being used on that network path.

With this mechanism, the server reflects the spin value received, while the
client 'spins' it after one RTT. On-path observers can measure the time
between two spin bit toggle events to estimate the end-to-end RTT of a
connection.


# Transport Parameter Encoding {#transport-parameter-encoding}

The extension_data field of the quic_transport_parameters extension defined in
Expand Down