Re: [Aodvimpl] Sequence number doubt

[Wang R. <wan...@ue...>]

HI,
	The answer is in RFC3561.
	According to "6.1. Maintaining Sequence Numbers",

   When the destination increments its sequence number, it MUST do so by
   treating the sequence number value as if it were an UNSIGNED NUMBER.
   To accomplish SEQUENCE NUMBER ROLLOVER, if the sequence number has
   already been assigned to be the largest possible number representable
   as a 32-bit unsigned integer (i.e., 4294967295), then when it is
   incremented it will then have a value of zero (0).  On the other
   hand, if the sequence number currently has the value 2147483647,
   which is the largest possible positive integer if 2's complement
   arithmetic is in use with 32-bit integers, the next value will be
   2147483648, which is the most negative possible integer in the same
   numbering system.  The representation of negative numbers is not
   relevant to the increment of AODV sequence numbers.  This is in
   contrast to the manner in which the result of comparing two AODV
   sequence numbers is to be treated (see below).

   In order to ascertain that information about a destination is not
   stale, the node compares its current numerical value for the sequence
   number with that obtained from the incoming AODV message.  This
   comparison MUST be done using signed 32-bit arithmetic, this is
   necessary to accomplish sequence number rollover.  If the result of
   subtracting the currently stored sequence number from the value of
   the incoming sequence number is less than zero, then the information
   related to that destination in the AODV message MUST be discarded,
   since that information is stale compared to the node's currently
   stored information.

When a node's seq number rolls over to 0, its previous seq numbers are
negative, when we get a positive number when we subtract a negative number
from 0 which do not satisfy "If the result of  subtracting the currently
stored sequence number from the value of  the incoming sequence number is
less than zero, then the information related to that destination in the AODV
message MUST be discarded.". No extra operations are required.

Regards,
Wang Rui

From: Deepak Gopalakrishnan [mailto:Dee...@Ln...] 
Sent: 2009-7-7 15:14
To: aod...@li...
Subject: [Aodvimpl] Sequence number doubt


Hi 
Im trying to implement AODV algorithm and i have this doubt about sequence
number. 
What happens when there is a roll over in the sequence number? I am not
getting a clear idea of how to implement a algorithm according to what is
given in the aodv RFC3561. 
For example, 
i have allocated a 8bit value for sequence number.. 

suppose in a intermediate node the sequence number is 254.....but at the
destination, the sequence has incremented twice due to some reason....then
the present sequence wud be 1... 
now if we compare the sequence numbers....the fresh packet will be discarded
by the intermediate node.... 

also..if the intermediate node has updated the sequence number to 2(the case
after a roll over has occured)...but receives a packet which was created
before the roll over...then the node wud accept that node thinking that it
has a "fresher" packet.... 

im totally confused.... 

Can anyone help me with the algorithm to tackle this roll over problem..... 
thanks in advance 

Regards,
Deepak Gopalakrishnan