[Iccrg] Looking for reviewer for draft-ietf-ledbat-congestion-06
Lisong Xu
xu at cse.unl.edu
Sat Jun 18 01:46:11 BST 2011
Hi Mirja,
A scientific paper is meant for a researcher, but an RFC is for a
programmer who wants to implement the protocol. This is why I want to
specify the data type of acknowledgement.delay (possible others too).
OK, here is an example to illustrate the problem of unsigned variables.
assume all variables are unsigned short.
at some time (low queueing delay):
local_timestamp = 3, and remote_timestamp = 5
then acknowledgement.delay = 65534
then base_delay = 65534
at some time later (high queueing delay):
local_timestamp = 7, and remote_timestamp = 6
then acknowledgement.delay = 1
then base_delay = 1
then queueing_delay = 0
Thanks
Lisong
On 6/17/2011 2:33 AM, Mirja Kuehlewind wrote:
> Hi Lisong,
>
> you are right, if it is an unsigned this might cause errors but its actually
> not a problem. If local_timestamp-remote_timestamp is negative but
> acknowledgement.delay is signed you will get a wrong result (but this doesn't
> matter as this delay does not show a real delay value anyway) but you will
> get a result because there simply will be an overflow.
> If you now calculate queuing_delay = current_delay - base_delay. Also this
> error will cancel out and you get an valid result.
>
> Anyway, its in the responsibility of the implementor to get the type and range
> right; its not part of the algorithm itself, from my point of view. Do you
> have a different opinion here?
>
> Mirja
>
>
> On Thursday 16 June 2011 18:36:24 Lisong Xu wrote:
>> Mirja,
>>
>> Thanks for your reply. I have only one question now.
>>
>> "it doesn't matter if the delay is positive or negative because its
>> canceled out in the calculation of the queuing delay and even an
>> overflow would work here."
>>
>> You are right that it does not matter whether acknowledgement.delay is
>> positive or negative. But variable acknowledgement.delay MUST be a
>> signed int. If it is implemented as an unsigned int, then sometimes
>> there will be a problem.
>>
>> Thanks
>> Lisong
>>
>> On 6/15/2011 6:07 AM, Mirja Kühlewind wrote:
>>> Hi Lisong
>>>
>>> thank you very much for your review! Please see comments inline.
>>>
>>> Mirja
>>>
>>> On Sunday 12 June 2011 08:53:10 Lisong Xu wrote:
>>>> Hi Michael,
>>>>
>>>> Below are my comments after reading the draft.
>>>> Thanks
>>>> Lisong
>>>>
>>>>
>>>> **** Section 3.1 ****
>>>> First sentence: "a loss occurs [RFC5681], which, in the absence of any
>>>> Active Queue Management (AQM) in the network, occurs only when the queue
>>>> at the bottleneck link on the end-to-end path overflows "
>>>>
>>>> Since a loss may occur due to link errors, such as fiber errors and
>>>> wireless errors, we may change the first sentence to
>>>> "... (AQM) and link errors in the network,...."
>>>
>>> Done.
>>>
>>>> **** Section 3.3 ****
>>>> The data type of each variable is not specified. Since some variables
>>>> must be signed variables, it is important to specify the type of each
>>>> variable in order to correctly implement the algorithm.
>>>>
>>>> For example, tcp_time_stamp is an unsigned int, so local_timestamp and
>>>> remote_timestamp are also unsigned int. But variable
>>>> acknowledgement.delay should be a signed int, since
>>>> local_timestamp-remote_timestamp may be negative.
>>>
>>> I don't want to specify any data type here because that's only pseudo
>>> code of the algorithm and the range and resolution of a variable are
>>> implementation specific and might depend on the framing protocol used and
>>> of course as well on the programming language.
>>>
>>> Your example is a quite special case as one would assume that delay is
>>> always positive but you might get negative values because of the clock
>>> offset. Issues like clock offset and skew, which can come up in a real
>>> system, are not regarded as part of the algorithm. Anyway there is some
>>> discussion in the appendix. Moreover, in fact in this special example it
>>> doesn't matter if the delay is positive or negative because its canceled
>>> out in the calculation of the queuing delay and even an overflow would
>>> work here.
>>>
>>> The only case where it is important that a value can/must get negative is
>>> the off_target and this is mentioned explicit in 3.4.1.
>>>
>>>> **** Section 3.4.2 ****
>>>> "a LEDBAT sender stores BASE_HISTORY+1 separate minima- one each for the
>>>> last BASE_HISTORY minutes, and one for the running current minute."
>>>>
>>>> But the update_current_delay(delay) code maintains only BASE_HISTORY
>>>> minima. So the above sentence should be changed to
>>>> "a LEDBAT sender stores BASE_HISTORY separate minima- one each for the
>>>> last BASE_HISTORY-1 minutes, and one for the running current minute."
>>>
>>> Thanks. You are right! Done.
>>>
>>>> **** Section 3.5 ****
>>>> "we recommend that the sender SHOULD use at least 4 samples in each RTT.
>>>> Thus, CURRENT_FILTER SHOULD be at least 4, and limited such that no
>>>> samples in list are older than an RTT in the
>>>> past."
>>>>
>>>> But what if cwnd is less than 4? In this case, there are less than 4
>>>> samples in each RTT.
>>>
>>> That's why there is a SHOULD and not a MUST. If you have less samples,
>>> you can, of course, only use whatever you get. But the results might be
>>> very noisy and if you permanently have less then 4 samples, LEDBAT might
>>> not work correctly (because of noise in real systems).
>>>
>>> Should we make this more explicit?
>>>
>>>> **** General *****
>>>>
>>>> It is not clear what the relation between cwnd and flightsize is. Is
>>>> flightsize always the same as cwnd, or could be less than cwnd? When is
>>>> flightsize less than cwnd?
>>>
>>> flightsize can be less if your transfer is application-limited (and
>>> ALLOWED_INCREASE is large enough). Thus you don't have enough data to
>>> fill the cwnd.
>>>
>>> In the code you find
>>> " on acknowledgment:
>>> # flightsize is the amount of data outstanding before this ack
>>> # was received and is updated later by update_flightsize();
>>> # bytes_newly_acked is the number of bytes that this ack
>>> # newly acknowledges, and it MAY be set to MSS; and
>>> # cwnd is in bytes."
>>>
>>> I've added the following to 'on initialization:' (eventhough you find the
>>> same explanation in 3.2.)
>>> " # cwnd is the amount of data that is allowed to send in one RTT and
>>> # is defined in bytes."
>>>
>>> Does this help?
>>>
>>>> On 6/5/2011 12:38 AM, Michael Welzl wrote:
>>>>> Hi,
>>>>>
>>>>> We're urgently looking for a volunteer to review
>>>>> draft-ietf-ledbat-congestion-06:
>>>>> http://tools.ietf.org/html/draft-ietf-ledbat-congestion-06
>>>>> which is currently in Last Call, ending in less than two weeks (15
>>>>> June).
>>>>>
>>>>> If anyone here is willing to do a review, please drop Murari and me a
>>>>> note ASAP!
>>>>>
>>>>> Thanks,
>>>>> Michael
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Iccrg mailing list
>>>>> Iccrg at cs.ucl.ac.uk
>>>>> http://oakham.cs.ucl.ac.uk/mailman/listinfo/iccrg
>
>
>
--
Lisong Xu, Associate Professor
Computer Science & Engineering
University of Nebraska-Lincoln
http://cse.unl.edu/~xu
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