On Fri, 6 May 2016, Jonathan Morton wrote:

>> On 6 May, 2016, at 22:14, David Lang <david@lang.hm> wrote:
>>
>> On Fri, 6 May 2016, Jonathan Morton wrote:
>>
>>>> On 6 May, 2016, at 21:50, David Lang <david@lang.hm> wrote:
>>>>
>>>> what IP id are you referring to? I don't remember any such field in the packet header.
>>>
>>> It’s the third halfword.
>>
>> half a word is hardly enough to be unique across the Internet, anything that small would lead to lots of attackes that inserted garbage data into threads.
>
> It doesn’t need to be globally unique.  It merely identifies, in conjunction with src/dst address pair (so 80 bits in total), a particular sequence of fragments to be reassembled into the original packet.  If the fourth halfword is zero (or has only the Don’t Fragment bit set), the IP ID field has no meaning.  Hence the entire second word can be considered fragmentation related.
>
> I agree that it’s not a very robust mechanism; it breaks under extensive packet reordering at high packet rates (circumstances which are probably showing up in iperf tests against flow-isolating AQMs).  It would be better not to have fragmentation at the IP layer at all.  But it’s not as bad as you say; it does work for low packet rates, which is all it was intended for.
>
> Here’s my preferred reference diagram:  https://nmap.org/book/tcpip-ref.html

rfc-6864 shows that this field is not used the way you think it is in practice 
(if it was, nobody would have been able to exceed 6.4Mbps)

Given all the things that can cause fragmentation on virtually every packet 
(tunnels/vpns), and the fact that having this be unique would restrict all 
traffice between a given source and destination to 6.4Mbps, I am extremely 
doubtful that it is used the way that rfc-6864 suggests (after all it's a recent 
RFC, 2013)

I know that I've looked at packet dumps that have shown fragmented data and 
seen the port numbers in the fragment headers.

I'd bet that in practice firewalls/etc ignore the IP ID field.

David Lang