Subject: [quicwg/base-drafts] Packet Number Echo in Public Header (#269)

From: martinduke <[email-masked]>
Date: Wed, 08 Feb 2017 11:24:36 -0800

In Mirja's Management draft:
[https://tools.ietf.org/html/draft-kuehlewind-quic-appman-00#section-3](url)

> [EDITOR'S NOTE: the addition of a simple packet number echo would
   allow passive RTT measurement and partial passive downstream loss/
   reordering measurement.  Packet number echo can be sampled at the
   echo-side (i.e. one-in-N packets or 1/p packets can carry an echo)
   for efficiency tradeoff, if necessary.]

I endorse this idea. Service providers are very interesting in measuring their users' quality of experience and this is one key thing we can expose to enable that. On the face of it, it seems that the privacy hit is marginal.

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From: Brian Trammell <[email-masked]>
Date: Tue, 28 Feb 2017 13:50:01 -0800

Adding packet-number echo is, IMO, much easier with Jana's alternate header format proposal. This could be done with just two additional codepoints. Propose:

`35 and b5: 1-RTT packet (4-byte packet number + 4-byte packet echo)`

```
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|      Type     |             
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Packet Number (4)                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Packet Number Echo (4)                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Payload                          ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```

`3d and bd: 1-RTT packet with connection ID(4-byte packet number + 4-byte packet echo)`

```
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|      Type     |             
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                        Connection ID                          +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Packet Number (4)                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Packet Number Echo (4)                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Payload                          ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```

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From: Martin Thomson <[email-masked]>
Date: Tue, 28 Feb 2017 16:51:19 -0800

@britram, Who would be able to decide to use this?  Does it require mutual agreement (to use or to disable)?  Or, could be unilateral?

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From: Brian Trammell <[email-masked]>
Date: Tue, 28 Feb 2017 16:56:43 -0800

Right now, packet number echo is only intended for the use of devices on path, to support passive RTT/loss measurement -- QUIC uses ACK frames for the same. So the decision whether to echo packet numbers, and how often, is unilateral. 

Whether this would be an application-level or system-level configuration option is IMO implementation-specific.

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