RE: UDP Out 0f Sequence

RE: UDP Out 0f Sequence

[Majumder, Rajib]

Does this mean if we have 2 hosts connected back to back (there's no network device in between), sequence is guaranteed even in UDP? 

-----Original Message-----
From: Rick Jones [mailto:rick.jones2@hp.com]
Sent: 21 September 2006 00:47
To: Majumder, Rajib
Cc: 'netdev@vger.kernel.org'
Subject: Re: UDP Out 0f Sequence


Majumder, Rajib wrote:
> Hi,
> 
> If I write UDP datagrams 1,2 and 3 to network and if the receiver
> receives in order 2,1, and 3, where can the sequence get changed? Is it
> at the source stack, network transit or destination stack?

Yes. :)

Although network transit is by far the most likely case.  Destination 
stack is a distant second and source stack an even more distant third. 
Generally stack writers try to avoid having places in their stacks where 
things can reorder, but it isn't completely unknown.

rick jones


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Re: UDP Out 0f Sequence

[David Miller]

From: "Majumder, Rajib" <rajib.majumder@credit-suisse.com>
Date: Thu, 21 Sep 2006 10:50:17 +0800

> Does this mean if we have 2 hosts connected back to back (there's no
> network device in between), sequence is guaranteed even in UDP?

Not true.  Even for back to back systems SMP can cause packets
to be delivered out of order even locally within the system
on receive.

Re: UDP Out 0f Sequence

[Ian McDonald]

On 9/21/06, Majumder, Rajib <rajib.majumder@credit-suisse.com> wrote:
> Does this mean if we have 2 hosts connected back to back (there's no network device in between), sequence is guaranteed even in UDP?
>
I think if you're trying to make the packets appear in order you need
to untie the Gordian knot http://en.wikipedia.org/wiki/Gordian_Knot

In other words you should fix the application rather than the near
impossible task of trying to make the packets in order...

Ian
-- 
Ian McDonald
Web: http://wand.net.nz/~iam4
Blog: http://imcdnzl.blogspot.com
WAND Network Research Group
Department of Computer Science
University of Waikato
New Zealand

RE: UDP Out 0f Sequence

[Majumder, Rajib]

Let's say we have 2 uniprocessor hosts connected back to back. Is there any possibility of an out-of-order scenario on recv? Is this same for all kernel (linux/solaris)?  

-----Original Message-----
From: David Miller [mailto:davem@davemloft.net]
Sent: 21 September 2006 11:51
To: rajib.majumder@credit-suisse.com
Cc: rick.jones2@hp.com; netdev@vger.kernel.org
Subject: Re: UDP Out 0f Sequence


From: "Majumder, Rajib" <rajib.majumder@credit-suisse.com>
Date: Thu, 21 Sep 2006 10:50:17 +0800

> Does this mean if we have 2 hosts connected back to back (there's no
> network device in between), sequence is guaranteed even in UDP?

Not true.  Even for back to back systems SMP can cause packets
to be delivered out of order even locally within the system
on receive.

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Re: UDP Out 0f Sequence

[Rick Jones]

Majumder, Rajib wrote:
> Let's say we have 2 uniprocessor hosts connected back to back. Is
> there any possibility of an out-of-order scenario on recv? 

Your application should be written on the assumption that it is 
possible, regardless of the specifics of the hosts involved, however 
unlikely they may be to reorder traffic.

 > Is this same for all kernel (linux/solaris)?

Your application should be written on the assumtion that it is possible, 
regardless of the specifics of the OSes involved, however unlikely they 
may be to reorder traffic.

rick jones

Re: UDP Out 0f Sequence

[Randy Macleod]

Rick Jones wrote:
> Majumder, Rajib wrote:
>> Let's say we have 2 uniprocessor hosts connected back to back. Is
>> there any possibility of an out-of-order scenario on recv? 
> 
> Your application should be written on the assumption that it is 
> possible, regardless of the specifics of the hosts involved, however 
> unlikely they may be to reorder traffic.
> 
>  > Is this same for all kernel (linux/solaris)?
> 
> Your application should be written on the assumtion that it is possible, 
> regardless of the specifics of the OSes involved, however unlikely they 
> may be to reorder traffic.

Or you should use a different protocol stack that guarantees order.
Have you considered TCP, DCCP or even TIPC?
The solution depends on your application requirements and your
projects tolerance for risk.

// Randy

UDP Out 0f Sequence

[Majumder, Rajib]

Hi,

If I write UDP datagrams 1,2 and 3 to network and if the receiver receives in order 2,1, and 3, where can the sequence get changed? Is it at the source stack, network transit or destination stack? 

Any reply is highly appreciated. 

Thanks

Rajib 

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Re: UDP Out 0f Sequence

[kc]

network transit. different datagrams might go through different
routes, hence the out-of-sequence arrival.

On 9/20/06, Majumder, Rajib <rajib.majumder@credit-suisse.com> wrote:
> Hi,
>
> If I write UDP datagrams 1,2 and 3 to network and if the receiver receives in order 2,1, and 3, where can the sequence get changed? Is it at the source stack, network transit or destination stack?
>
> Any reply is highly appreciated.
>
> Thanks
>
> Rajib
>
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> Please access the attached hyperlink for an important electronic communications disclaimer:
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> http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html
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Re: UDP Out 0f Sequence

[Rick Jones]

Majumder, Rajib wrote:
> Hi,
> 
> If I write UDP datagrams 1,2 and 3 to network and if the receiver
> receives in order 2,1, and 3, where can the sequence get changed? Is it
> at the source stack, network transit or destination stack?

Yes. :)

Although network transit is by far the most likely case.  Destination 
stack is a distant second and source stack an even more distant third. 
Generally stack writers try to avoid having places in their stacks where 
things can reorder, but it isn't completely unknown.

rick jones