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Re: [Qemu-devel] [PATCH] slirp: support dynamic block size for TFTP tran
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From: |
Thomas Huth |
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Subject: |
Re: [Qemu-devel] [PATCH] slirp: support dynamic block size for TFTP transfers |
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Date: |
Tue, 22 Nov 2016 09:19:05 +0100 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 |
On 21.11.2016 20:51, H. Peter Anvin wrote:
> On 11/21/16 11:45, Hervé Poussineau wrote:
>> The blocksize option is defined in RFC 1783.
>> We now support block sizes between 1 and 1432 bytes, instead of 512 only.
>
> It ought to be 1476: Ethernet MTU = 1500, minus a minimum of 20 bytes
> for an IPv4 header and 4 for a TFTP header.
Don't forget the size of the UDP header - so the maximum value is 1468.
However, if you want to play safe, you should also consider that
somebody is putting additional options into the IPv4 header, so the IPv4
header can be up to 60 bytes instead of only 20 bytes. So a real safe
value for the maximum TFTP block size is:
1500 - 60 - 8 - 4 = 1428
This is also what is mentioned in RFC2348 which obsoletes RFC1783 (that
mentions 1432).
Thomas