Network Layer Lecture 15 Imran Ahmed University of Management & Technology

1. Network Layer Lecture 15 Imran Ahmed University of Management & Technology

2. Agenda • Introduction & Network layer functions • Routing principles • Hierarchical routing • The Internet protocol (IP) • Routing in the Internet

3. OSPF • It’s a Link-State protocol • Used for Intra-AS routing • Publicly available protocol • Classless routing protocol • Administrative distance = 110 • OSPF v2 is defined in RFC 2328

4. OSPF: How It Works • "Hello" packets sent periodically on all OSPF-enabled interfaces. • Adjacencies (virtual point-to-point links) formed between some neighbors. • Once an adjacency is established, trade information with your neighbor. • Topology information is packaged in a "link state announcement“. • Announcements are sent ONCE, and only updated if there's a change (or every 30 minutes).

5. OSPF: How It Works • Each router sends Link State Announcements (LSAs) over all adjacencies. • LSAs describe router's links, interfaces and state • Each router receives LSAs, adds them into its database, and passes the information along to its neighbors. • Each router builds identical link-state database. • Runs SPF algorithm on the database to build SPF tree. • Forwarding table built from SPF tree.

6. OSPF: How It Works • When change occurs: • All routers run SPF algorithm. • Install output into forwarding table.

7. HELLO Packets • Broadcast* HELLO on network segment. • Receive ACK. • Repeat periodically: • Default: HELLO sent every 10 seconds. • Default: if no HELLO heard for 40 seconds, link is assumed to be dead. • Now establish adjacencies. * Actually uses Multicast addresses (224.0.0.9, 224.0.0.10) so that non-OSPF devices can ignore the packets

8. The HELLO Packet • Router priority • Hello interval • Router dead interval • Network mask • List of neighbors HELLO HELLO HELLO These must match

9. Neighbors • Bi-directional communication. • Result of OSPF hello packets. • Need not exchange routing information.

10. Who is Adjacent? • "Adjacent" neighbors exchange routing information. • Not all neighbors are adjacent. • On a point-to-point link: • everyone • On broadcast medium: • not everyone • why?

11. Broadcast Medium • Select a neighbor: Designated Router (DR): • It is said to be adjacent to all the others routers and exchanges information among them. • All routers become adjacent to DR. • Exchange routing information with the DR. • DR updates all the other neighbors. • Backup Designated Router (BDR).

12. LSAs propagate along adjacencies DR BDR

13. Broadcast Medium • Assigning of DR & BDR are as follows: • Checks the router’s priority (by default, 1). • Router with highest priority will be assigned DR. • Router with lowest priority will be assigned BDR. • If priority same then decision will be made on the basis of router id. • Router id – the highest IP address on router’s ports.

14. Some Other Features of OSPF • Authentication (optional). • Equal-cost multi-path: • more than one "best" path - share traffic. • Proper classless support (CIDR). • Multiple areas: • For very large networks (>150 routers). • Aggregate routes across area boundaries. • Keep route flaps within an area. • Proper use of areas reduce bandwidth and CPU utilization. • Backbone is Area 0.

15. Hierarchical Structure of 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. • A hierarchical structure of OSPF network defines four types of OSPF routers: • Internal routers – these routers are in non-backbone areas and perform only intra-AS routing. • Area border routers (ABR) – these routers belong to both an area and the backbone. It “summarizes” distances to nets in own area, advertise to other Area Border routers. • Backbone routers (non-border routers) – these routers perform routing within the backbone but themselves are not area border routers. • Boundary routers – a boundary router exchanges routing information with routers belonging to other AS. This router might, for example, use BGP to perform inter-AS routing.

16. Hierarchical OSPF

17. Internet inter-AS routing: BGP • BGP (Border Gateway Protocol):The de facto standard • BGP provides each AS a means to: • Obtain subnet reachability information from neighboring ASs. • Propagate the reachability information to all routers internal to the AS. • Determine “good” routes to subnets based on reachability information and policy. • Allows a subnet to advertise its existence to rest of the Internet: “I am here”

18. Inter-AS routing in the Internet: BGP

19. 3a 3b 2a AS3 AS2 1a 2c AS1 2b eBGP session 3c 1b 1d 1c iBGP session BGP basics • Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions • Note that BGP sessions do not correspond to physical links. • When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix. • AS2 can aggregate prefixes in its advertisement

20. 3a 3b 2a AS3 AS2 1a 2c AS1 2b eBGP session 3c 1b 1d 1c iBGP session Distributing reachability info • With eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. • 1c can then use iBGP do distribute this new prefix reach info to all routers in AS1 • 1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP session • When router learns about a new prefix, it creates an entry for the prefix in its forwarding table.

21. Path attributes & BGP routes • When advertising a prefix, advert includes BGP attributes. • prefix + attributes = “route” • Two important attributes: • AS-PATH: contains the ASs through which the advert for the prefix passed: AS 67 AS 17 • NEXT-HOP: Indicates the specific internal-AS router to next-hop AS. (There may be multiple links from current AS to next-hop-AS.) • When gateway router receives route advert, uses import policy to accept/decline.

22. BGP route selection • Router may learn about more than 1 route to some prefix. Router must select route. • Elimination rules: • Local preference value attribute: policy decision • Shortest AS-PATH • Closest NEXT-HOP router: hot potato routing • Additional criteria

23. BGP messages • BGP messages exchanged using TCP. • BGP messages: • OPEN: opens TCP connection to peer and authenticates sender • UPDATE: advertises new path (or withdraws old) • KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous msg; also used to close connection

24. BGP routing policy • A,B,C are provider networks • X,W,Y are customer (of provider networks) • X is dual-homed: attached to two networks • X does not want to route from B via X to C • .. so X will not advertise to B a route to C

25. BGP routing policy (2) • A advertises to B the path AW • B advertises to X the path BAW • Should B advertise to C the path BAW? • No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers • B wants to force C to route to w via A • B wants to route only to/from its customers!

26. 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