Resolution of Some Cases of Bgp Inter-Domain Oscillations with the Spvpoc Algorithm.

ISSN (e): 2250 – 3005 || Volume, 05 || Issue, 05 || May – 2015 ||
International Journal of Computational Engineering Research (IJCER)
www.ijceronline.com Open Access Journal Page 35
Resolution of Some Cases of Bgp Inter-Domain Oscillations with
the Spvpoc Algorithm.
Alphonse Binele Abana1,
Emmanuel Tonye2
1&2
(Department of Electrical and Telecommunication Engineering, Polytechnic National Advanced
School of Engineering, University of Yaoundé 1, Cameroon)
I. Introduction
A theoretical model is the representation of a complex system following a theoretical analysis; the
construction of a theoretical model formalizes a process according to the theory. Internet is a set of autonomous
systems or AS (set of routers under a single network administration) interconnected each other and applying
their own external routing policy. Thus, Internet can be seen as a set of nodes, each with its particular properties
to elapse traffic. Each AS decide its internal routing protocol (RIP, OSPF …) and its external routing policy.
The BGP (Border Gateway Protocol) routing protocol [01] is used to interconnect the various Internet operators.
It is thus possible to model web as a graph where each autonomous system is represented by a node and the
edges model the communication links (physical) between the AS. Consequently, any operation on BGP protocol
directly influences the Internet. BGP has been designed for two purposes: to best meet the requirements imposed
by routing policies of operators and respect the principle of confidentiality of such policies. Inconsistencies of
these policies are responsible of path oscillations phenomenon (unstable routes to a destination at an AS). In this
article, we study how to avoid this phenomenon. In other to fulfill this task, we have to first understand the
operation of the BGP protocol and then understand the conflict detection method proposed by Griffin [09]
before seeing how the SPVPOC algorithm proposed by Binele and Tonye [03] resolves inter-domain
oscillations in some cases.
II. General View
2.1 Modeling of an AS
As described in [19], Each AS does not necessarily contain a single BGP router. All these routers apply
the same external routing policy defined by the AS. From the outside, all BGP routers in the AS will be seen as
a single router. Thus, a network of ASes is modeled by a graph where the vertices and edges respectively
represent the ASes and BGP links. Rather, it is a multi-graph since there may be multiple BGP connections
between two vertices. Figure.1 represents the network and Figure 2 shows the modeling.
Abstract:
BGP was introduced in order to allow autonomous systems to exchange information across the Internet.
An autonomous system is a set of routers under a single network administration. Each AS decide its
internal routing protocol (RIP, OSPF, ...) and its external routing policy. The BGP routing protocol is
used to interconnect the various Internet operators together and is designed for the following two
purposes: to best meet the requirements imposed by the routing policies of an operator and respect the
principle of confidentiality of these policies. The inconsistencies of these policies cause path
oscillations. In this paper, after modeling the operation of the bgp protocol, we present the algorithm
SPVPOC and we show how it resolves bgp oscillations in some cases
Keywords: Internet, bgp, Autonomous Systems, path, inter-domain oscillations

Resolution Of Some Cases Of Bgp…
Figure 1-a : example of a Network of ASes Figure 1-b : Modeling of the network of
ASes of figure 1-a
2.2 BGP route attributes
Each router learns from its neighbors a path to a destination. A path P received by a router R on an AS
v contains the following attributes [01] :
- LOCAL_PREF: preference value indicating the classification of the choice of the path P in the local
routing policy of the AS v
- AS path: AS sequence along the path to reach the destination d of the current AS v
- MED: to discriminate links when two ASes are interconnected with multiple links by associating a
degree of preference to each link. A small value of MED indicates a greater preference of the link. This attribute
is the cause of internal oscillations of BGP [04] [05].
- Next_hop: The IP address of the border router "next hop" along the path P. If the traffic of R along
the path P traverses other routers before leaving AS v, then next_hop is the IP address of the border router that is
the exit point of the AS v. If the traffic of R along the path P goes directly from R to a neighboring router in
another AS, then next_hop is the IP address of the neighboring router.
For each host AS, a router receives a path (potentially empty) to reach the destination. From this set of
paths, the router must choose the best path and adopt it as its own way. The best path is selected according to the
algorithm implemented. If a router adopts a new path, if the best path is not the path chosen before, the router
informs all of his peers on the newly chosen path.
2.3 Unstable network detection tool: directed conflict graph
In this section we will present the approach of T.G. Griffin [09] who has done an important work in the
study of oscillations between AS.
Griffin calculates a conflict graph of path assignments [09] [18] as follows:
Each node of the conflict graph shows a possible path in the network.
Let Q be a path allowed at node v and P a path admitted at node u with v as the first node crossed (P =
(u, v) P [v, 0]); It exists two types of arcs between different paths:
- Transmission arcs (dashed line)
- Conflict arcs (solid line)
There is a transmission arc from vP to (u, v)P if u and v are neighbors. In other words, if at node u we
have the path 130 and the path 30 at node v, then we put a transmission arc from 30 to 130.
There is a conflict arc from Q to P if the node v may increase the rank of its best path by abandoning
P[v,0] in favor of Q which has the effect of forcing u to abandon P. In other words, there is a conflict if a path at
node v can force the abandonment of a path at a neighboring node u.
The main property of this graph is that if there is no cycle in the conflict graph, then the network is stable.

2.4 The SPVPOC algorithm (Stable Path Vector Path Occurrences)
The SPVPOC algorithm gives a solution to solve the SPP (Stable Path Problem) of BGP which was
introduced by Griffin in [06]. Let’s resume the SPVPOC algorithm that Binele and Tonye proposed in [03] as
follows:
Let G = (V, E) be a graph, such as the elements V and E respectively represent ASes and BGP links.
Each AS determines a list of paths ordered by order of preference. ASes will try to achieve and maintain the
path having a preference value as high as possible. An AS can choose a path only if all the ASes it traverses
have chosen the corresponding sub-path. Each AS thus defines a set “choice” containing all possible paths to a
destination. The function “Max” will return the route with the highest preference possible at node u while the
function “best” will return the route with the highest preference possible at node u if it appearance number is
less than the threshold value:
(1)
Where u is an AS, “ ” is an historic of all the possible paths to a destination at an AS
with their appearance numbers (occurrences), is the appearance number of the
route (path) with the highest preference “ ” at AS u; the function check returns 1 if the this
number is lower than the threshold value “Max_path_occurence” and 0 if not. Thus,
(2)
From (1) and (2) we have:
(3)
if best(u)= , the AS will not change it best path thus, it will retain its current best path at that step.
Let’s take the example of the Bad Gadget with 5 ASes of figure 3. The table 1 shows the historic
“Hist_path_occurence” which gives the appearance number of all the paths for all ASes at each step. For
example for AS2 at step 6, we have the historic [(210,1);(20,1)]: thus, Max(choice(AS2))=20 and
Hist_path_occurence(20)=1. If the threshold value of appeareance number is fixed to 2,
check[Hist_path_occurence(20)]=check(1)=1. So best(AS2)=20.
III. Application To Some Cases
3.1 Bad Gadget with 5 ASes
Let be the Bad Gadget with 5 ASes below
Figure 3: Bad Gadget with 5 ASes Figure 4: Conflict graph of the Bad Gadget
with 5 ASes
Considering the figure 4 above, we can see that there is a conflict on this network as explained on the
section 2.3. We will apply the SPVPOC protocol to this network to solve this conflict. Each AS only retains the
state changes of its path (change of the number of occurrences of its paths). In this way, we obtain a local
management of the path states. In Table 1, at the transition from step 8 to step 9, if we consider that the
Max_path_occurrence value is 2, AS 2 detects an oscillation on the path 210 as its number of occurrences is 2;
therefore it is considered as a bad path.
30
130 210
3420 420
430
10
20

Etape
AS1 AS2 AS3 AS4
130 10 rib-in 210 20 rib-in 3420 30 rib-in 420 430 rib-in
0 * * 10 * * 20 * * 3420 * * 420
1 * * 10 1 * 210 * * 3420 * * 420
2 * * 10 1 * 210 * * 3420 * * Ø
3 * * 10 1 * 210 * 1 30 * * Ø
4 1 * 130 1 * 210 * 1 30 * 1 430
5 1 * 130 1 1 20 * 1 30 * 1 430
6 1 * 130 1 1 20 * 1 30 1 1 420
7 1 * 130 1 1 20 1 1 3420 1 1 420
8 1 1 10 1 1 20 1 1 3420 1 1 420
9 1 1 10 2 1 210 1 1 3420 1 1 420
Table 1: example of path occurrences counting in the Bad Gadget with 5 ASes.
AS 2 will retain its best path 20 to the destination 0 obtained in step 8 and the system will then
converge.
3.2 Bad Gadget with 4 ASes
Let be the Bad Gadget with 4 ASes below
Figure 5: Bad Gadget with 4 ASes Figure 6: Conflict graph of the Bad Gadget with 4 ASes
As seen in the previous section we can see that there is a conflict in the network (cycle in the conflict graph), each
AS only retains the state changes of its paths (change of the number of occurrences of its paths). In Table 2, at the transition
from step 6 to step 7, if we consider that the Max_path_occurrence value is 2, AS 2 detects an oscillation on the path 210 as
its number of occurrences is 2; therefore it is considered as a bad path.
Etape
AS1 AS2 AS3
130 10 rib-in 210 20 rib-in 320 30 rib-in
0 * * 10 * * 20 * * 320
1 * * 10 1 * 210 * * 320
2 * * 10 1 * 210 * 1 30
3 1 * 130 1 * 210 * 1 30
4 1 * 130 1 1 20 * 1 30
5 1 * 130 1 1 20 1 1 320
6 1 1 10 1 1 20 1 1 30
7 1 1 130 2 1 210 1 1 30
Table 2: example of path occurrences counting in the Bad Gadget with 4 ASes.

Thus, AS 2 will retain its best path 20 to the destination 0 obtained in step 6 and the system will then
converge.
3.3 Bad Backup
Let be the Bad Backup with 5 ASes below
Figure 7: Bad Backup with 5 ASes Figure 8:Conflict graph of the Bad Backup
with 5 ASes
As seen in the previous section we can observe a cycle on the conflict graph. By applying the SPVPOC
algorithm, each AS only retains the state changes of its paths (change of the number of occurrences of its paths).
In Table 3, at the transition from step 9 to step 10, if we consider that the threshold (Max_path_occurrence)
value is 2, AS 2 detects an oscillation on the path 210 as its number of occurrences is 2; therefore it is
considered as a bad path.
Etape
AS1 AS2 AS3 AS4
130 10 rib-in 210 20 rib-in 3420 30 rib-in 420 430 40 rib-in
0 * * 10 * * 20 * * 3420 * * * 420
1 * * 10 1 * 210 * * 3420 * * * 420
2 * * 10 1 * 210 * * 3420 * * 1 40
3 * * 10 1 * 210 * 1 30 * * 1 40
4 1 * 130 1 * 210 * 1 30 * * 1 40
5 1 * 130 1 * 210 * 1 30 * 1 1 430
6 1 * 130 1 1 20 * 1 30 * 1 1 430
7 1 * 130 1 1 20 * 1 30 1 1 1 420
8 1 * 130 1 1 20 1 1 3420 1 1 1 420
9 1 1 10 1 1 20 1 1 3420 1 1 1 420
10 1 1 10 2 1 210 1 1 3420 1 1 1 420
Table 3: Bad Backup with 5 ASes
Thus, AS 2 will retain its best path 20 to the destination 0 obtained in step 9 and the system will then
converge.
IV. Conclusion
BGP is an inter-AS routing protocol and presents many oscillation problems which must be resolved
quickly because the operation of the Internet depends on it. In this paper, we have studied and characterized
cases of oscillations caused by the overall inconsistency of policies adopted by the ASes. Then we presented the
new SPVPOC algorithm proposed by Binele and Tonye [03]. This solution inherits all the properties of the
current BGP and is more stable. Finally, we applied this algorithm to few cases of inter-domain oscillations like
the Bad Gadget with 4, 5 ASes and the Bad Backup with 5 ASes. A perspective would be to study the behavior
of the protocol for intra-domain oscillations (iBGP).
30
130 210
3420 420
430
10
20
40

References
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Network Protocols (ICNP’99), pages pp. 21–30,1999.
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