Re: [quicwg/base-drafts] DPLPMTU merge tweaks (#3702)

Jana Iyengar <notifications@github.com> Thu, 28 May 2020 01:19 UTC

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Date: Wed, 27 May 2020 18:19:25 -0700
From: Jana Iyengar <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] DPLPMTU merge tweaks (#3702)
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@janaiyengar requested changes on this pull request.

A high order point. This could use a clean up in terms of noun consistency:
- PMTU, DPLPMPTU
- PMTUD/DPLPMTUD/PMTU/DPLPMTU probes / probe packets. I would simply use "probes" and define this away.

My suggestion would be to add a `Terminology` subsection at the top of this section, so that we can simply define away the ambiguities.

> @@ -3795,37 +3795,48 @@ later time in the connection.
 # Packet Size {#packet-size}
 
 The QUIC packet size includes the QUIC header and protected payload, but not the
-UDP or IP header.
+UDP or IP headers.
+
+QUIC depends upon a minimum packet size of at least 1280 bytes.  This is the
+IPv6 minimum size {{?RFC8200}} and is also supported by most modern IPv4
+networks.  Assuming the minimum IP header size, this results in a QUIC maximum
+packet size of 1232 bytes for IPv6 and 1252 bytes for IPv4.
+
+The QUIC maximum packet size is the largest size of QUIC packet that can be sent
+across a network path using a single packet. Any maximum packet size larger than
+1200 bytes is discovered using PMTUD/DPLPMTUD.

```suggestion
1200 bytes can be discovered using PMTUD/DPLPMTUD.
```

> @@ -3900,42 +3917,82 @@ determines that the quoted packet has actually been lost.
 
 ## Datagram Packetization Layer PMTU Discovery
 
-Section 6.3 of {{!DPLPMTUD}} provides considerations for implementing Datagram
-Packetization Layer PMTUD (DPLPMTUD) with QUIC.
+When implementing the algorithm in Section 5 of {{!DPLPMTUD}}, the initial value
+of BASE_PMTU SHOULD be consistent with the minimum QUIC packet size. The
+MIN_PLPMTU is the same as the BASE_PMTU.
+
+QUIC endpoints implementing DPLPMTUD maintain a maximum packet size (DPLPMTUD
+MPS) for each combination of local and remote IP addresses.
+
+If a QUIC endpoint determines that the PLPMTU between any pair of local and
+remote IP addresses has fallen below the size needed to support the minimum QUIC
+packet size (BASE_PLPMTU), it MUST immediately cease sending QUIC packets,
+except for DPLPMTUD probe packets, on the affected path. An endpoint MAY

This clause does not make sense to me. What does a DPLPMTUD probe packet look like if it's not a QUIC packet? I think this clause should be removed.

> +
+QUIC endpoints implementing DPLPMTUD maintain a maximum packet size (DPLPMTUD
+MPS) for each combination of local and remote IP addresses.
+
+If a QUIC endpoint determines that the PLPMTU between any pair of local and
+remote IP addresses has fallen below the size needed to support the minimum QUIC
+packet size (BASE_PLPMTU), it MUST immediately cease sending QUIC packets,
+except for DPLPMTUD probe packets, on the affected path. An endpoint MAY
+terminate the connection if an alternative path cannot be found.
+
+
+### DPLPMTUD and Initial Connectivity
+
+From the perspective of DPLPMTUD, QUIC transport is an acknowledged
+packetization layer (PL). A sender can therefore enter the DPLPMTUD BASE state
+when the QUIC connection handshake has been completed and the endpoint has

This should use "handshake confirmed". That said, is there a reason to disallow this during the handshake?

> +packet size (BASE_PLPMTU), it MUST immediately cease sending QUIC packets,
+except for DPLPMTUD probe packets, on the affected path. An endpoint MAY
+terminate the connection if an alternative path cannot be found.
+
+
+### DPLPMTUD and Initial Connectivity
+
+From the perspective of DPLPMTUD, QUIC transport is an acknowledged
+packetization layer (PL). A sender can therefore enter the DPLPMTUD BASE state
+when the QUIC connection handshake has been completed and the endpoint has
+established a 1-RTT key.
+
+
+### Sending QUIC DPLPMTUD Probe Packets
+
+DPLPMTU probe packets are ack-eliciting packets.  Probe packets that use the

This sounds like we are using a term without defining it. Perhaps "DPLPMTU probe packets require acknowledgements (Section 3 of {{!DPLPMTUD}}), and are therefore ack-eliciting packets"?

> +except for DPLPMTUD probe packets, on the affected path. An endpoint MAY
+terminate the connection if an alternative path cannot be found.
+
+
+### DPLPMTUD and Initial Connectivity
+
+From the perspective of DPLPMTUD, QUIC transport is an acknowledged
+packetization layer (PL). A sender can therefore enter the DPLPMTUD BASE state
+when the QUIC connection handshake has been completed and the endpoint has
+established a 1-RTT key.
+
+
+### Sending QUIC DPLPMTUD Probe Packets
+
+DPLPMTU probe packets are ack-eliciting packets.  Probe packets that use the
+PADDING frame therefore implement "Probing using padding data", as defined in

```suggestion
PADDING frame implement "Probing using padding data", as defined in
```

>  
-A PING frame can be included in a PMTU probe to ensure that a valid probe is
-acknowledged.
+QUIC provides an acknowledged PL, therefore a sender does not implement the
+DPLPMTUD CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.

```suggestion
DPLPMTUD CONFIRMATION_TIMER while in the SEARCH_COMPLETE state; see Section
5.2 of {{!DPLPMTUD}}.
```

>  
-The considerations for processing ICMP messages in the previous section also
+From the perspective of DPLPMTUD, QUIC transport is an acknowledged
+packetization layer (PL). A sender can therefore enter the DPLPMTUD BASE state
+when the QUIC connection handshake has been completed and the endpoint has
+established a 1-RTT key.
+
+
+### Sending QUIC DPLPMTUD Probe Packets
+
+DPLPMTU probe packets are ack-eliciting packets.  Probe packets that use the
+PADDING frame therefore implement "Probing using padding data", as defined in
+Section 4.1 of {{!DPLPMTUD}}.  Endpoints could limit the content of probe
+packets to PING and PADDING frames as packets that are larger than the current

Yes, this was starting to leak badly. I think there's some cleaning up that needs to be done in terms of bringing consistency overall, see meta comment above.

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