Re: DISCUSS's on rfc1981bis - MK Comments

[Gorry Fairhurst <[e-mail]>]

On 22/05/2017 10:49, Mirja Kuehlewind (IETF) wrote:
> 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.
 >
Sure. this says nothing about how you find out what segments need to
be retransmit, only about the *SIZE* of the packets you use to perform 
that transmission.
>
 >
> 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
>


Before I try different words, we should make sure the topic is understood.

Suppose we wish to send a probe for 8KB, and suppose the path supports
3 router MTU sizes: a hop with 4KB MTU and then a later hop with 2KB 
MTU, and finally one of just 1400B.

Largest probe size = 8KB

The first probe sends 8KB as the Segment Size
->------------------------------X
Dropped by first router along path.
In example case PTB indicates next hop =  4KB.
Data not reliably sent.

Largest probe size = 4KB

Retransmit 2 packet for same segment of data, each size 4KB
------------->------------------------------X
------------->------------------------------X
Dropped by router later along path.
In example case PTB indicates next hop =  2KB.
Data still not reliably sent.

Largest probe size = 2KB

Retransmit 4 packets for same segment of data.
In example case PTB indicates next hop =  1400B.
Data still not reliably sent.
------------->------------------------------>---X
------------->------------------------------>---X
------------->------------------------------>---X
------------->------------------------------>---X

Largest probe size = 1400B

Data finally transmitted in 6 packets all less than PMTU.
------------->------------------------------>-------------->
------------->------------------------------>-------------->
------------->------------------------------>-------------->
------------->------------------------------>-------------->
------------->------------------------------>-------------->
------------->------------------------------>-------------->

All data delivered. PMTU=1400B is confirmed.

Total sent packets = 13 packets
No of re-transmissions = 7 (6 failed re-transmissions in 3 
re-transmission attempts)

Now let's try a method that heeds the warning in the text and 
re-transmits the probe data using a "safe: MTU:

The first probe sends 8KB as the Segment Size
->------------------------------X

Dropped by first router along path.
In example case PTB indicates next hop =  4KB.
Data not reliably sent.

Largest probe size = 4KB

Sender resends the 8KB segment using a known workable PMTU (in the above 
1500B, if that were cached, or even 1280B.

------------->------------------------------>-------------->
------------->------------------------------>-------------->
------------->------------------------------>-------------->
------------->------------------------------>-------------->
------------->------------------------------>-------------->
------------->------------------------------>-------------->

Total sent packets = 7 packets
No of re-transmissions = 6 (all successful, 1 re-transmission)

And afterwards separately resend a single probe packet to detect a the 
largest PMTU using the 4KB probe.

Obviously, there are many combinations of paths, and not all cases lead
to exponential growth, but all cases lead to multiple probes, and
multiple re-transmissions.

Gorry