Re: Clarify SCTP max packet size.

On Jul 7, 2009, at 8:50 AM, <[email protected]> <[email protected] 
 > wrote:

> Dear all,
>
> Just to add with the SCTP bundling feature. If the "SCTP MTU" is  
> 1500, and the system is bundling messages at the SCTP level, you  
> should have the risk to segment at the IP level (because of the IP  
> header). So it is not efficient
The MTU SCTP uses is the link layer MTU minus the headers for IP and  
SCTP.
I have not seen an implementation which bundles at the SCTP layer and  
than
needs to fragment at the IP layer. I would consider this broken...
> to bundle sctp messages and to segment them at the IP level. Some  
> vendors limits "SCTP MTU" to avoid that. This is only the Maximum  
> transmission unit. If one SCTP message to transmit is greater, they  
> will segment it.
>
> Regards.
>
> Pierre
>
> -----Message d'origine-----
> De : [email protected] [mailto:[email protected]] De  
> la part de Michael Tüxen
> Envoyé : lundi 6 juillet 2009 23:24
> À : [email protected]; [email protected] 
> ; _VF-PT=22?=
> Cc : [email protected]; [email protected]
> Objet : Re: [Sigtran] Clarify SCTP max packet size.
>
> On Jul 3, 2009, at 9:38 PM, Gomes, Rui Filipe Efigénio, VF-PT wrote:
>
>> Dear Sirs,
>>
>> A major equipment vendor (A) states on their documentation that:
>> "The max packet size usable by SCTP is 1416 bytes."
> What does this mean?
> 1500 (MTU) - 20 (IP header) - 12 (SCTP common header) - 16 (SACK
> chunk) - 16 (SATA chunk header) - 20 (whatever)
>
> This would mean that the SCTP stack might only be able to send user  
> messages of the length 1416. Depending on the upper layer and your  
> application, this might be OK.
>>
>> According to RFC2960 and RFC 4960:
>> "An SCTP receiver MUST be able to receive a minimum of 1500 bytes in
>> one SCTP packet.  This means that an SCTP endpoint MUST NOT indicate
>> less than 1500 bytes in its initial a_rwnd sent in the INIT or INIT"
>>
>> Even if we took the "full" 1500 Ethernet IPv4 packet SCTP will have a
>> max of 1466 bytes.
> A full ethernet frame would have an IP packet of length 1500 byte,  
> so typically an SCTP packet of size 1480, which means 1468 for chunks.
>
> However, the limitation of max. use message sizes is different from  
> limiting the a_rwnd.
>
> The statement in the RFC is that the a_rwnd must be 1500 or larger  
> on the INIT and INIT-ACK.
> The receiver should handle full sized frames for 1500 MTU links.
>
> If the a_rwnd is smaller than 1500, you will have serious interop  
> and performance problems. So you might want to clearify first if you  
> are talking about the maximum user message size or the maximum  
> advertised receiver window and also about the maximum size of a  
> received user message. Reassembly is optional...
>>
>> I tested and it seems that SCTP packet with longer size is being
>> discarded. Node never acknowledges its reception.
>>
>> This means that major vendor A is violating the RFC?
>> I'm having an IOT issue between two major vendors and I think that
>> vendor A is not following SCTP RFCs (old and new).
>>
>> Please confirm.
>>
>>
>> Thanks in advance and best regards,
>> Rui Gomes
>>
>> Source:
>> http://tools.ietf.org/html/rfc2960#page-67 6. User Data Transfer
>> paragraph 4, http://tools.ietf.org/html/rfc4960#page-73 6. User Data
>> Transfer paragraph 4) _______________________________________________
>> Sigtran mailing list
>> [email protected]
>> https://www.ietf.org/mailman/listinfo/sigtran
>>
>
> _______________________________________________
> Sigtran mailing list
> [email protected]
> https://www.ietf.org/mailman/listinfo/sigtran
>