On the Impact of Layer-2 on Node Degree Distribution

Transcription

1 On the Impact of Layer-2 on Node Degree Distribution Pascal Mérindol, Benoit Donnet, Jean-Jacques Pansiot, Olivier Bonaventure Melbourne - 2 th November 2010 Internet Measurement Conference 2010

2 Agenda Topology Discovery Background Data Collection : mrinfo-rec Problem Statement Revisiting the node degree distribution Domain Specificities Model key distributions

3 Topology Discovery Internet seen as a dynamic graph of IP interfaces traceroute, route_record of routers alias resolution : ally, iffinder,... Goals of Autonomous Systems IP to AS mapping (routeview project), router to AS mappping? IP network models & simulations ground truth input for topology generation

4 Topology mrinfo Discovery Topology discovery using mrinfo Uses IGMP messages ASK_NEIGHBORS NEIGHBORS_REPLY Output All multicast interfaces of a given router All multicast neighbors/links mrinfo applied recursively mrinfo-rec R R R 0 switch R 1 R probe all neighbors daily based [version 12.4] [1/0/pim/querier] [1/0/pim/querier] [1/0/pim/querier] [1/0/pim/leaf]

5 mrinfo-rec Limitations multicast scope IGMP filtering non compliant routers Advantages Sprint network friendly probing : 1 probe injected per router aliasing : no need to gather IP interfaces forwarding independent : backup links visible [IMC2009] layer-2 vision : distinguish the IP logical layer over MAC Pa while (--ncount >= 0 && p < ep ) { /*NEIGHBOR ADDRESS*/ register u_int32 neighbor = *(u_int32*)p; p += 4;

6 The dataset More than 4 years of daily collected data ~10000 routers ~ IPs ~ AS Raw data towards IP network graphs Cleaning process (tunnels, private IP, disabled IP,...) Router to AS mapping [PAM2010] Layer-2 Inference Graph selection : one per AS per month dimensions ( V=R+S, E ) connectivity influence of L2 hardware... V = set of vertex/nodes E = set of edges R = set of routers S = set of layer-2 nodes

7 Topology Discovery Problem : MAC vs. IP layer Ethernet Switches & Shared Risk Link Groups R (SRLG) 4 R 1 R 2 IP logical view R 4 R 1 R 2 R 3 R 3 mrinfo, layer-2 view: L2 graph traceroute, layer-3 view: IP graph Number of nodes with k links Many nodes with a few links A few hubs with large number of links The node degree distribution follows a heavy tailed power law? What is the impact of L2 on scale free & preferential attachment models? Number of links (k)

8 Problem : Global Overview WAN L2 switch point-to-point connection L3 router IP-only layer vision MAN point-to-multipoint connection L1 hub LAN L1 bus

9 L2 inference validation Using our dataset, we can infer several kinds of L2 device Focusing on Ethernet Broadcast networks, we check our inference validity using three rules : symmetry, querier election and subnet mask. Three states: coherent - incoherente - incomplete Most of L2 inferred devices seems to reveal Ethernet broadcast devices such as switches minimal covering prefix for p2p and p2mp connections proportion of Ethernet switches

10 Revisiting the node degree distribution We construct two graphs of the same network the L2 inferred graph (L2-aware) and the L3 graph (L2-agnostic) The power law distribution is much less heavy tailed considering an L2 view! What impact on other graph/routing characteritics? R 1 R 4 R 2 t(1)=4, t(4)=1 R 3 Overall networks analysis R 1 t(x): degree distribution of L2 and L3 nodes in the L2 graph r (x): degree distribution of L3 nodes in the L3 graph R 4 R 2 r (3)=4 R 3

11 Revisiting the node degree distribution ~10-30 % of L2 nodes... α : proportion of routers connected to at least one L2 device β - L2 : proportion of p2p connections going through an L2 device in the L2 aware network β - L3 : proportion of p2p connections going through an L2 device in the L2 agnostic network...but resulting in two strongly different graphs : The number of edges/connections in the L3 graph is much more larger than in the L2 graph, Between 40% and 75% of routers are connected to an L2 device, Most of the L3 connections rely on L2 Ethernet switches......even in the L2 graph, almost half of router-to-router connections goes through an L2 node!

12 Per AS analysis: a uniform observation? Sprint R 1 IUNET R 4 R 2 R 3 t(x): degree distribution of L2 and L3 nodes in the L2 graph r (x): degree distribution of L3 nodes in the L3 graph Level 3 R 1 Telecom Italia R 4 R 2 R 3

13 Why such a great tail shift? How a small (L2-aware) degree router can become a large L3 degree router? L2 aware degree distribution L3 degree distribution R R R 3 7 multi-graph problem Must look at specific distributions on the L2-aware graph r : router degree distribution s : switch degree distribution b : L2 degree distribution of routers (#L2 neighbors) p : degree distribution of routers connected to L2 devices («subdistribution» of r)

14 A closer look at key distributions Sprint IUNET r - L3 node degree Level 3 Telecom Italia b - #L2 neighbors p - routers connected to L2 neighbors s- L2 node degree

15 Practical IP networks design MAN access distributes IP connectivity at low cost quite typical accross our dataset... Redundancy may exacerbate this phenomenon VLAN can introduce an additional MAC logical layer (lower-bound analysis) Access/core networks? ASBR Level 3 - AS3356 FRANKFURT MAN ACCESS SWITCH ROUTER NEIGHBOR AS VLAN?

16 A large degree router has more L2 neighbors IUNET Sprint p: degree distribution of L3 nodes connected to b switches in the L2 graph b: number of L2 neighbors Level 3 Telecom Italia

17 Towards a model? R L2 aware degree distribution L3 degree distribution R How to model this shift? What are key factors? «Replay» the collected data using key distributions......and one of their possible correlations r (router degree distribution), s (switch degree distribution) b (L2 degree distribution of routers = #L2 neighbors) p b : correlation between p and b distributions Is this sufficient? What about AS specificities?

18 Towards a model? Sprint IUNET data: degree distribution of L3 nodes in the L2-agnostic graph model: degree distribution of L3 nodes in the generated graph Level 3 Telecom Italia

19 Conclusion mrinfo-rec is a useful tool for three reasons describe a connected multicast topology at the router level (no need for alias resolution) can discover backup links (no forwarding dependence) able to natively infer L2 devices (hybrid bipartite graph) Layer-2 switches strongly impact the node degree distribution most of IP router-to-router connections go through Broadcast Ethernet switches! such switches are generally connected to a large number of routers correlation between the L2-aware degree of a router and its number of L2 neighbors

20 Ongoing Work Improve Active Topology Discovery using the best of mrinfo, traceroute and alias resolution techniques MERLIN : A new measurement platform to MEasure the Router Level of the INternet Various Analysis on other graph/routing metrics clustering coefficient/assortativity : understand the dynamic of attachement preferences path diversity : measure the physical resilience of IP networks

21 Questions? Pascal Mérindol, Benoit Donnet, Jean-Jacques Pansiot, Matthew Luckie, Young Huyn. MERLIN: MEasure the Router Level of the INternet Université catholique de Louvain, Technical Report , September Jean-Jacques Pansiot, Pascal Mérindol, Benoit Donnet, and Olivier Bonaventure. Extracting Intra-Domain Topology from mrinfo Probing In Proc. Passive and Active Measurement Conference (PAM), April Pascal Mérindol, Virginie Van den Schrieck, Benoit Donnet, Olivier Bonaventure and Jean-Jacques Pansiot. Quantifying ASes Multiconnectivity using Multicast Information In Proc. ACM/USENIX Internet Measurement Conference (IMC), November 2009.