Is there a error in Ipv4Header?

[NanerLee]

Hello everyone !

In the ipv4-header.h:241, there are some codes about flags related to IP fragmentation

  enum FlagsE {

    DONT_FRAGMENT = (1<<0),

    MORE_FRAGMENTS = (1<<1)

  };

But I think their positions are wrong, I think it should be:

  enum FlagsE {

    DONT_FRAGMENT = (1<<1),

    MORE_FRAGMENTS = (1<<0)

  };

is is right about my understanding ? If I am wrong, please tell me. Thanks!

[Natale Patriciello]

Hi!

Please remember that the order in which they are declared is not the
order in which they are then serialized. If you want to look to that
order, please take a look in the Serialize () method.

Nat

[NanerLee]

Thanks for your reply, But the order I said is not the order  in which they are declared. I means the positions left-shifted.

In the source code, DONT_FRAGMENT left-shifted by 0 = 001; MORE_FRAGMENTS left-shifted by 1 = 010.

As we know, DONT_FRAGMENT should set 1 at the second position(010), and MORE_FRAGMENTS should set 1 at the third position(001). So I think the positions is wrong.

在 2016年11月29日星期二 UTC+8下午11:50:13，Nat P写道：

[Natale Patriciello]

On 29/11/16 at 06:05pm, NanerLee wrote:
> Thanks for your reply, But the order I said is not the order in which they

> are declared. I means the positions *left-shifted*.

>
> In the source code, DONT_FRAGMENT left-shifted by 0 = 001; MORE_FRAGMENTS
> left-shifted by 1 = 010.
>
> As we know, DONT_FRAGMENT should set 1 at the second position(010), and
> MORE_FRAGMENTS should set 1 at the third position(001). So I think the
> positions is wrong.

Hi,

I'm absolutely not an expert of the Ipv4 code, but I suggest you to not
focus on the declaration only, take also a look on the Serialize ()
method and the various Set*Fragments () methods.

Print the values, and then look what is written in the Serialize ()
and various Set*Fragments() methods. Just for reference, this is what goes on
the network:

uint32_t fragmentOffset = m_fragmentOffset / 8;
uint8_t flagsFrag = (fragmentOffset >> 8) & 0x1f;
if (m_flags & DONT_FRAGMENT)
{
flagsFrag |= (1<<6);
}
if (m_flags & MORE_FRAGMENTS)
{
flagsFrag |= (1<<5);
}
i.WriteU8 (flagsFrag);

In which the values DONT_FRAGMENT and MORE_FRAGMENTS are not written directly
into the buffer.


Nat

[NanerLee]

Thanks for your detailed explanation，and I apologize for my lazy that I don't see the Serialize ()

And, now I understand the problem. Thank you again !