Principles in Communication Networks

1. Principles in Communication Networks • Instructor: Prof. Yuval Shavitt, • Office hours: room 303 s/w eng. bldg., Sun12:00-13:00 • Prerequisites (דרישות קדם): • Introduction to computer communications (TAU, Technion, BGU) • Expectations from students: • probability • Queueing theory basics • Graph theory

2. Course Syllabus (tentative) • Internet structure • Internet measurements • Measurement optimization • Measurement analysis • Introduction to switching, router types • Use of Gen. Func.: HOL analysis, TCP analysis. • Matching algorithms and their analysis • CLOS networks: non-blocking theorem, routing algorithms and their analysis • Scheduling algorithms

3. Grade composition • Final exam – 60% • Project – 30% (Magen) • Home assignments (2-3) - 10%

4. Routing in the Internet

5. Routing in the Internet Routing in the Internet is done in three levels: • In LANs in the MAC layer: • Spanning tree protocol for Ethernet Transparent bridge. • Source routing for token rings • Inside autonomous systems (ASes): • RIP, OSPF, IS-IS, (E)IGRP • Between ASes: • BGP

6. … the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it. RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System Autonomous Systems • Autonomous Routing Domains: A collection of physical networks glued together using IP, that have a unified administrative routing policy. • An AS is an autonomous routing domain that has been assigned a number.

7. Intra-AS and Inter-AS Routing 1.2 3.1 1 2.1 2 2.2 1 2 4 1 3 2 3 3 1 4 3 Both Inter-AS and intra-AS are used to create the routing tables 2

8. Intra-AS and Inter-AS Routing 1.2 3.1 1 2.1 2 2.2 1 2 4 1 3 2 3 3 1 4 3 2

9. Why different Intra- and Inter-AS routing ? • Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • Intra-AS: single admin, so no policy decisions needed • Scale: • hierarchical routing saves table size, reduced update traffic • Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance

10. RIPRouting Information Protocol • A Distance Vector protocol • Based on routed in 4.3 BSD UNIX • Distance metric: minimum hop • Max hop = 15 • Distance vectors were exchanged every 30 seconds – advertisements • Each advertisements route to up to 25 dest networks

11. RIP: Link Failure and Recovery • If no advertisement heard for 180 Sec – link declared dead • Route via neighbor invalidated • new advertisements sent to neighbors • neighbors in turn send out new advertisements (if tables changed) • link failure info quickly propagates to entire net • poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

12. Split Horizon Split Horizon: a node omit from the advertisement any information about destination routed on the link Split Horizon with Poisonous Reverse: All destinations are includes in the message, distance to those routed on the link are set to  • Immediately kill two-hop loops

13. RIP-2 • Standardized in the 1990s • Added authentication • RIPng – added support for IPv6

14. Other DV Algorithms • IGRP and EIGRP included many improvements • Support for multiple metrics • bandwidth, delay, load, MTU, and reliability. • EIGRP: included the DUAL algorithm to prevent transient loops

15. OSPFOpen Shortest Path First • Open – publically available • A link state algorithm • Route computation using Dijkstra’s algorithm • OSPF advertisement carries one entry per neighbor router • Advertisements disseminated to entire AS (via flooding)

16. OSPF Features • Security: all OSPF messages authenticated (to prevent malicious intrusion); TCP connections used • Multiple same-cost paths allowed • For each link, multiple cost metrics for different ToS (e.g., satellite link cost set “low” for best effort; high for real time) • Integrated uni- and multicast support: • Multicast OSPF (MOSPF) uses same topology data base as OSPF • Hierarchical OSPF in large domains.

17. Hierarchical OSPF backbone Area 1 Area 3 Boundary router Area border router Backbone router Internal router Area 2

18. Hierarchical OSPF • Two-level hierarchy: local area, backbone. • Link-state advertisements only in area • each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. • Area border routers:“summarize” distances to nets in own area, advertise to other Area Border routers. • Backbone routers: run OSPF routing limited to backbone. • Boundary routers: connect to other ASes.

19. IS-ISIntermediate System to Intermediate System • A link state protocol • Developed in parallel to OSPF • Very similar to OSPF • Tend to use less messages thus scales better to large networks • ISP grade

20. BGP • BGP (Border Gateway Protocol):the de facto standard • Path Vector protocol: • similar to Distance Vector protocol • each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of ASs) to destination • E.g., Gateway X may sendits path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z

21. BGP (cont.) Suppose: gateway X send its path to peer gateway W • W may or may not select path offered by X • cost, policy (don’t route via competitors AS), loop prevention reasons. • If W selects path advertised by X, then: Path (W,Z) = W, Path (X,Z) • Note: X can control incoming traffic by controlling its route advertisements to peers: • e.g., if don’t want to route traffic to Z - don’t advertise any routes to Z

22. Why Inter-AS and Intra-AS routing? Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • Intra-AS: single admin, so no policy decisions needed Scale: • hierarchical routing saves table size, reduced update traffic Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance

23. AS Type of Relationships (ToR) Relationship between a pair of ASes: • customer-to-provider relationship • provider-to-customer relationship • peer-to-peer relationship • E.g., Level3 and AT&T • sibling-to-sibling relationship

24. × × × × × × × × × × × × × × × × Valley Free Routing • If all ASes set their export policies according to the BGP export rules, then an AS path in any BGP routing table entry is valley-free

25. A View of the AS Hierarchy Provider - customer

26. A View of the AS Hierarchy No transitivity No SP concatenation Provider - customer Data path

27. A View of the AS Hierarchy Provider - customer Data path Peer to peer

28. Remarks • Since AS connectivity is not always published, certainly ToR is not published • While revealing connectivity is not trivial, revealing ToR is very hard.

29. The structure of the Internet

30. How are routers connected? • Why should we care? • While communication protocols will work correctly on ANY topology • ….they may not be efficient for some topologies • Knowledge of the topology can aid in optimizing protocols

31. The Internet as a graph • Remember: the Internet is a collection of networks called autonomous systems (ASs) • The Internet graph: • The AS graph • Nodes: ASs, links: AS peering • The router level graph • Nodes: routers, links: fibers, cables, MW channels, etc. • There are mid-level aggregation schemes • PoP topologies, city topologies • How does it looks like?

32. Poisson distribution Random graphs in Mathematics The Erdös-Rényi model • Generation: • create n nodes. • each possible link is added with probability p. • Number of links: np • If we want to keep the number of links linear, what happen to p as n?

33. The Waxman model • Integrating distance with the E-R model • Generation • Spread n nodes on a large enough grid. • Pick a link uar and add it with prob. that exponentially decrease with its length • Stop if enough links • Heavily used in the 90s

34. 100 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100

35. The Faloutsos brothers Measured the Internet AS and router graphs. Mine, she looks different! Notre Dame Looked at complex system graphs: social relationship, actors, neurons, WWW Suggested a dynamic generation model 1999

36. The Faloutsos Graph1995 Internet router topology3888 nodes, 5012 edges, <k>=2.57

38. 25 2212 SCIENCE CITATION INDEX Nodes: papers Links: citations Witten-Sander PRL 1981 1736 PRL papers (1988) P(k) ~k- ( = 3) (S. Redner, 1998)

39. Sex-web Nodes: people (Females; Males) Links: sexual relationships 4781 Swedes; 18-74; 59% response rate. Liljeros et al. Nature 2001

40. Web power-laws

41. (2) The attachment is NOT uniform. A node is linked with higher probability to a node that already has a large number of links. Examples : WWW : new documents link to well known sites (CNN, YAHOO, NewYork Times, etc) Citation : well cited papers are more likely to be cited again SCALE-FREE NETWORKS (1) The number of nodes (N) is NOT fixed. Networks continuously expand by the addition of new nodes Examples: WWW : addition of new documents Citation : publication of new papers

42. Scale-free model P(k) ~k-3 (1)GROWTH: At every timestep we add a new node with m edges (connected to the nodes already present in the system). (2)PREFERENTIAL ATTACHMENT :The probability Π that a new node will be connected to node i depends on the connectivity ki of that node A.-L.Barabási, R. Albert, Science 286, 509 (1999)

43. The Faloutsos Graph

45. Back to the Internet • Understanding its structure and dynamics • help applications (WWW, file sharing) • help improving routing • predict Internet growth • So lets look at the data….

46. …Data? • The Internet is an engineered system, so someone must know how it is built, no? • NO! It is an uncoordinated interconnection of Autonomous Systems (ASes=networks). • No central database about Internet structure. • Several projects attempt to reveal the structure: Skitter, RouteViews, …

47. The Internet Structure routers

48. The Internet Structure The AS graph

49. Revealing the Internet Structure

50. Revealing the Internet Structure