Implicit Withdrawal - BGP Update Message

Dear Ado,

    the term "implicit withdrawal" applies to a BGP update U_2 that 
comes to a BGP speaker from the same peer P that sent a previous update 
U_1. When the peer P sends the second update U_2, the meaning is that 
the previous update U_1 is implicitly withdrawn by P and replaced with 
U_2. In other words, the sequence U_1, U_2 has the same effect of U_1, 
Withdrawal, U_2.
    The four BGP announcements that you list in your email do not come 
all from the same peer of router 192.65.185.40. However, the third 
announcement (FROM: 192.65.185.144, AS_PATH: 6893 3561 209 297) is an 
implicit withdrawal of the first (FROM: 192.65.185.144, AS_PATH: 6893 
8938 1 297). There is only one implicit withdrawal in the sequence.
    After all four announcements are received by router 192.65.185.40, 
router 192.65.185.40 has to choose its best route to prefix 
192.152.102.0/24. The choice is among the 2nd, 3rd and 4th route, since 
the 1st announcement is implicitly withdrawn by the 3rd.

Best,
Maurizio

On 10/11/2015 12:26 PM, Ado Maja wrote:
> Dear all,
> I am trying to account for all implicit withdrawal messages within a 
> certain period of time looking at BGP update messages collected at 
> rrc, Ripe.
> Implicit Withdrawal Definition: If the sender announces a route to a 
> currently reachable address and the new route is identical to the 
> current route, this is a duplicate announcement. Otherwise, the sender 
> is replacing the current route with a new route and this is an 
> implicit withdrawal. (Wang et al. in the "Observation and Analysis of 
> BGP iBehavior under Stress")
> What I am trying to understand is how do I count implicit withdrawals? 
> If for example, I have 4 BGP messages shown below that have same IP 
> address and different AS-PATH attribute, how many implicit withdrawals 
> do I have? Is it 3? If I look at consecutive messages that have same 
> IP address and different AS-PATH attribute the result would be 3 (I 
> compare first to the second, second to the third and third to the 
> fourth – assuming they have same IP address and different AS-PATH I 
> increment implicit withdrawals). But I have come across a solution 
> that takes a first message and compares it to the rest of the 
> messages. Than second message compares to the rest of the messages. 
> And lastly compares the third to the fourth. In this case number of 
> implicit withdrawals is 6. Please advise.
> Thanks for your help.
> TIME: 2001-9-16 00:00:06
> TYPE: BGP4MP/BGP4MP_MESSAGE AFI_IP
> FROM: 192.65.185.144
> TO: 192.65.185.40
> BGP PACKET TYPE: UPDATE
> ORIGIN: IGP
> AS_PATH: 6893 8938 1 297
> NEXT_HOP: 192.65.185.144
> ANNOUNCED: 192.152.102.0/24
> TIME: 2001-9-16 00:00:18
> TYPE: BGP4MP/BGP4MP_MESSAGE AFI_IP
> FROM: 192.65.184.3
> TO: 192.65.185.40
> BGP PACKET TYPE: UPDATE
> ORIGIN: IGP
> AS_PATH: 513 10764 6509 297
> NEXT_HOP: 192.65.185.9
> ANNOUNCED: 192.152.102.0/24
> TIME: 2001-9-16 00:00:36
> TYPE: BGP4MP/BGP4MP_MESSAGE AFI_IP
> FROM: 192.65.185.144
> TO: 192.65.185.40
> BGP PACKET TYPE: UPDATE
> ORIGIN: IGP
> AS_PATH: 6893 3561 209 297
> NEXT_HOP: 192.65.185.144
> ANNOUNCED: 192.152.102.0/24
> TIME: 2001-9-16 00:00:44
> TYPE: BGP4MP/BGP4MP_MESSAGE AFI_IP
> FROM: 192.65.185.130
> TO: 192.65.185.40
> BGP PACKET TYPE: UPDATE
> ORIGIN: IGP
> AS_PATH: 559 8933 6509 297
> NEXT_HOP: 192.65.185.130
> ANNOUNCED: 192.152.102.0/24


-- 
Maurizio "Titto" Patrignani
Dipartimento di Ingegneria, Universita' Roma Tre
Tel +39.06.57333233, Fax +39.06.57333612
http://www.dia.uniroma3.it/~patrigna

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