linux-kernel - Re: [PATCH 01/41] rxrpc: Fix RTT determination to use PING ACKs as a source

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Date:   Fri, 10 Nov 2023 16:54:46 -0500
From:   Jeffrey E Altman <jaltman@...istor.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Marc Dionne <marc.dionne@...istor.com>,
        linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/41] rxrpc: Fix RTT determination to use PING ACKs as a
 source

On 11/10/2023 12:25 PM, David Howells wrote:
> Jeffrey E Altman<jaltman@...istor.com>  wrote:
>
>>> I do ignore ack.serial == 0 for this purpose.
>> Zero has the special meaning - this ACK is not explicitly in response to a
>> received packet.
>>
>> However, as mentioned, the serial number counter wraps frequently and most
>> RxRPC implementations
>> do not transition from serial 0xffffffff -> 0x00000001 when wrapping.
> I don't skip zero serial numbers either.  I'm not sure whether it would be
> better to do so.

If a DATA packet is sent with serial number zero and an ACK packet is 
sent in response to it
with the ack.serial field set to the DATA packet serial number (zero), 
then the receiver of the
ACK will be unable to compute an RTT from that DATA packet.   It will 
happen rarely but it
will happen.

Jeffrey


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