Re: DISCUSS's on rfc1981bis - MK Comments

["Mirja Kuehlewind (IETF)" <[e-mail]>]

See below.


> Am 20.05.2017 um 17:11 schrieb Gorry Fairhurst <[e-mail]>:
> 
> On 19/05/2017, 15:51, Mirja Kuehlewind (IETF) wrote:
>> Hi Gorry, hi all,
>> 
>> thanks for the work and the update. All comments from Gorry seemed fine to me and I will try to review the changes in the updated doc soon. One more minor comment on this:
>> 
>>> Am 09.05.2017 um 08:07 schrieb Gorry Fairhurst<[e-mail]>:
>>> 
>>>> I don't understand the following paragraph. Can this be removed?
>>>> "Note: A packetization layer must not retransmit in response to
>>>> every Packet Too Big message, since a burst of several oversized
>>>> segments will give rise to several such messages and hence several
>>>> retransmissions of the same data. If the new estimated PMTU is
>>>> still wrong, the process repeats, and there is an exponential
>>>> growth in the number of superfluous segments sent."
>>>> 
>>> GF: I think the example is important. I'm not sure why that would be removed.
>> The problem is I don’t understand this example. Is there an assumption that all mtu probe packets have been ‚generated‘ on the same packet? If so that makes sense but must the spelled out somewhere!
>> 
>> Mirja
>> 
>> 
> I think the wording is maybe jumbled, but the point is OK. Perhaps something like this would be clearer?
> 
> "Note: A packetization layer ought to avoid retransmitting the data in a probe packet
> using the size reported in the last Packet Too Big message.
> This is to avoid an exponential growth in the number of superfluous segments
> that would be sent when a path encounters several successive smaller PMTU limits.
> (Each new estimated PMTU would result in retransmission of the data in a smaller
> packet that may itself fail if it encounters a still smaller PMTU in a device further
> along the same path)."
> 
> Gorry
> 
I still don’t get it. Each time a packet is dropped, you have to retransmit it (if your transport is reliable) because otherwise the other end will not have it. I also still don’t see how there can be an exponential grows. Sorry if I miss something but maybe you can explain again this case using different words…?

Mirja