Re: [PATCHv2] dccp: Do not send Dccp-Sync after received sequence-invalid

[Eddie Kohler <kohler@cs.ucla.edu>]

Gerrit Renker wrote:
>> 7.5.4.  Handling Sequence-Invalid Packets
>>
>> o  A sequence-invalid DCCP-Reset packet MUST elicit a DCCP-Sync
>>   packet in response (subject to a possible rate limit).  This
>>   response packet MUST use a new Sequence Number, and thus will
>>   increase GSS; GSR will not change, however, since the received
>>   packet was sequence-invalid.  The response packet's
>>   Acknowledgement Number MUST equal GSR.
>>
>> But reponse to a sequence-invalid DCCP-Reset with acknowledgement
>> number equal to GSR will help to attack for sequence number. ...
 >
> The requiremement of using GSR here is related to fixing another bug which
> leads to a flood of Sync/Reset packets. A description of that bug is on
> http://www.mail-archive.com/dccp@vger.kernel.org/msg01594.html

Furthermore, Yongjun, I don't see how this is an "attack."  DCCP is not robust 
against an attacker who can receive packets in the relevant connection, such 
as the two DCCP-Syncs in your example.  Your attack is out of the threat model.

Eddie