Re: [MirageOS-devel] Query about mirage-tcpip and checksums

[Richard Mortier <richard.mortier@xxxxxxxxxxxx>]

Ah, thanks!

Any objections if I paste this into an issue (and perhaps make
reference to it in a comment in the code)?

On 20 November 2015 at 21:38, Balraj Singh <balrajsingh@xxxxxxxx> wrote:
> The short answer is that the checksum is assumed to have been verified
> before the packet is handed to the Mirage stack.
>
> The problem with doing an additional redundant verification in Mirage is
> that when two local VMs are talking then the Xen driver, which has offered
> checksum offload, does not do any actual checksum computation.  So all
> packets arrive with invalid checksums.
>
> The particular call to the null function is there just as a marker that this
> is where it should be checked.
>
> A proper solution would be to correctly process the offload offers, which
> mirage doesn't, AFAIK.  Clearly if the driver offers offload then checksum
> computation should not be done on the way out or verification on the way in,
> else it should.  There is a complication that checksum offload can be turned
> on or off midstream so it may have to change per packet.
>
> A good project for someone interested.
>
>
> On Fri, Nov 20, 2015 at 7:31 PM, Richard Mortier
> <richard.mortier@xxxxxxxxxxxx> wrote:
>>
>> One of my undergraduate students who's doing a project with MirageOS
>> and the TCP/IP stack in particular asked a question that I couldn't
>> immediately answer -- wondered if someone else could :)
>>
>> Specifically,
>> https://github.com/mirage/mirage-tcpip/blob/master/tcp/pcb.ml#L99
>> appears to define `verify_checksum` to always return `true`; but it is
>> then called at
>> https://github.com/mirage/mirage-tcpip/blob/master/tcp/pcb.ml#L510
>> in the `input` function in a way that probably expects checksums to
>> have actually been verified.
>>
>> Which seems rather odd -- am I missing something?
>>
>> --
>> Richard Mortier
>> richard.mortier@xxxxxxxxxxxx
>>
>> _______________________________________________
>> MirageOS-devel mailing list
>> MirageOS-devel@xxxxxxxxxxxxxxxxxxxx
>> http://lists.xenproject.org/cgi-bin/mailman/listinfo/mirageos-devel
>
>



-- 
Richard Mortier
richard.mortier@xxxxxxxxxxxx

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