W4140 Network Laboratory Lecture 6 Oct 16 - Fall 2006 Shlomo Hershkop Columbia University

Announcements  midterm evaluations will be going up soon please fill them out, you will be getting credit for class participation for this  Project proposals are due this week groups of 2 or 3 students need to designate project lead  PL meet with me this week Wednesday who will be part of the group what you will be doing for each phase who will do what what background references are you using (if appropriate) due in three weeks will provide any software/hardware required for this have fun, if you will be working with PHD, can get a paper publication out of this most probably

Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF & BGP

Dynamic Routing Protocols Link State Routing

The Gang of Four Link StateVectoring EGP IGP BGP RIP IS-IS OSPF

Link State Routing  Based on Dijkstra’ s Shortest-Path-First algorithm.  Each router starts by knowing: Prefixes of its attached networks. Links to its neighbors.  Each router advertises to the entire network (flooding): Prefixes of its directly connected networks. Active links to its neighbors.  Each router learns: A complete topology of the network (routers, links).  Each router computes shortest path to each destination.  In a stable situation, all routers have the same graph, and compute the same paths.

Dijkstra’s Shortest Path Algorithm for a Graph Input: Graph (N,E) with N the set of nodes and E the set of edges c vw link cost (c vw = 1 if (v,w)  E, c vv = 0) s source node. Output : D n cost of the least-cost path from node s to node n M = {s}; for each n  M D n = c sn ; while (M  all nodes) do Find w  M for which D w = min{D j ; j  M}; Add w to M; for each neighbor n of w and n  M D n = min[ D n, D w + c wn ]; Update route; end for end while end for

Link state routing: graphical illustration a b cd a 3 6 b c a’s view: a b c 3 1 b’s view: cd 2 d’s view: Collecting all views yield a global & complete view of the network! Global view: a b cd 1 6 c’s view: 2

Operation of a Link State Routing protocol Received LSAs IP Routing Table Dijkstra’s Algorithm Link State Database LSAs are flooded to other interfaces

Link State Routing: Properties  Each node requires complete topology information  Link state information must be flooded to all nodes  Guaranteed to converge

Distance Vector vs. Link State Routing  With distance vector routing, each node has information only about the next hop:  Node A: to reach F go to B  Node B: to reach F go to D  Node D: to reach F go to E  Node E: go directly to F  Distance vector routing makes poor routing decisions if directions are not completely correct (e.g., because a node is down).  If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged. A A B B C C D D E E F F

Distance Vector vs. Link State Routing  In link state routing, each node has a complete map of the topology  If a node fails, each node can calculate the new route  Difficulty: All nodes need to have a consistent view of the network A A B B C C D D E E F F ABC DE F ABC DE F ABC DE F ABC DE F ABC DE F ABC DE F

Topology information is flooded within the routing domain Best end-to-end paths are computed locally at each router. Best end-to-end paths determine next-hops. Based on minimizing some notion of distance Works only if policy is shared and uniform Examples: OSPF, IS-IS Distance Vector vs. Link State Routing Each router knows little about network topology Only best next-hops are chosen by each router for each destination network. Best end-to-end paths result from composition of all next- hop choices Does not require any notion of distance Does not require uniform policies at all routers Examples: RIP, BGP Link StateVectoring

Dynamic Routing Protocols Open Shortest Path First

 OSPF = Open Shortest Path First  The OSPF routing protocol is the most important link state routing protocol on the Internet (another link state routing protocol is IS-IS (intermediate system to intermediate system)  The complexity of OSPF is significant RIP (RFC 2453 ~ 40 pages) OSPF (RFC 2328 ~ 250 pages)  History: 1989: RFC 1131 OSPF Version : RFC1247 OSPF Version : RFC 1583 OSPF Version 2 (revised) 1997: RFC 2178 OSPF Version 2 (revised) 1998: RFC 2328 OSPF Version 2 (current version) OSPF

Features of OSPF  Provides authentication of routing messages  Enables load balancing by allowing traffic to be split evenly across routes with equal cost  Type-of-Service routing allows to setup different routes dependent on the TOS field  Supports subnetting  Supports multicasting  Allows hierarchical routing

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.

Example Network Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address Link costs are called Metric Metric is in the range [0, 2 16 ] Metric can be asymmetric / / / / / / / /

Link State Advertisement (LSA)  The LSA of router is as follows:  Link State ID: = Router ID  Advertising Router: = Router ID  Number of links: 3 = 2 links plus router itself  Description of Link 1: Link ID = , Metric = 4  Description of Link 2: Link ID = , Metric = 3  Description of Link 3: Link ID = , Metric = / / / / / / / /

Network and Link State Database Each router has a database which contains the LSAs from all other routers LS TypeLinkStateIDAdv. RouterChecksumLSSeqNoLS Age Router-LSA x9b470x Router-LSA x219e0x Router-LSA x6b530x Router-LSA xe39a0x a20 Router-LSA xd2a60x Router-LSA x05c30x / / / / / / / /

Link State Database  The collection of all LSAs is called the link-state database  Each router has an identical link-state database  Useful for debugging: Each router has a complete description of the network  If neighboring routers discover each other for the first time, they will exchange their link-state databases  The link-state databases are synchronized using reliable flooding

OSPF Packet Format Destination IP: neighbor’s IP address or (ALLSPFRouters) or (AllDRouters) TTL: set to 1 (in most cases) OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89

OSPF Packet Format 2: current version is OSPF V2 Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID of the Area from which the packet originated Standard IP checksum taken over entire packet 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) Prevents replay attacks

OSPF LSA Format LSA Header Link 1 Link 2

Discovery of Neighbors  Routers multicasts OSPF Hello packets on all OSPF- enabled interfaces.  If two routers share a link, they can become neighbors, and establish an adjacency  After becoming a neighbor, routers exchange their link state databases Scenario: Router restarts

Neighbor discovery and database synchronization Sends empty database description Scenario: Router restarts Discovery of adjacency Sends database description. (description only contains LSA headers) Database description of Acknowledges receipt of description After neighbors are discovered the nodes exchange their databases

Regular LSA exchanges explicitly requests each LSA from sends requested LSAs Link State Request packets, LSAs = Router-LSA, , Router-LSA, , Router-LSA, , Router-LSA, , Router-LSA, , Router-LSA, , Link State Update Packet, LSAs = Router-LSA, ,0x Router-LSA, , 0x Router-LSA, , 0x Router-LSA, , 0x a Router-LSA, , 0x Router-LSA, , 0x

Dissemination of LSA-Update  A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet)  Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes.  Acknowledgements of LSA-updates:  explicit ACK, or  implicit via reception of an LSA-Update  Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?

Dynamic Routing Protocols (Inter-domain) Border Gateway Protocol

BGP Quick View  BGP = Border Gateway Protocol. Currently in version 4, specified in RFC (~ 60 pages)  Note: In the context of BGP, a gateway is nothing else but an IP router that connects autonomous systems.  Interdomain routing protocol for routing between autonomous systems  Uses TCP to establish a BGP session and to send routing messages over the BGP session  BGP is a path vector protocol. Routing messages in BGP contain complete routes.  Network administrators can specify routing policies

BGP Policy-based Routing  Each node is assigned an AS number (ASN)  BGP’s goal is to find any AS-path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible.  Network administrator sets BGP’s policies to determine the best path to reach a destination network.

How Many ASNs are there today? Thanks to Geoff Huston. on October 9, ,570 14,588 origin only (no transit)

today's data

Autonomous Routing Domains Don’t Always Need BGP or an ASN Qwest Yale University Nail up default routes /0 pointing to Qwest Nail up routes /16 pointing to Yale /16 Static routing is the most common way of connecting an autonomous routing domain to the Internet. This helps explain why BGP is a mystery to many … ARDs versus ASes

ASNs Can Be “Shared” (RFC 2270) AS 701 UUNet ASN 7046 is assigned to UUNet. It is used by Customers single homed to UUNet, but needing BGP for some reason (load balancing, etc..) [RFC 2270] AS 7046 Crestar Bank AS 7046 NJIT AS 7046 Hood College /16

ARDs and ASes: Summary  Most ARDs have no ASN (statically routed at Internet edge)  Some unrelated ARDs share the same ASN (RFC 2270)  Some ARDs are implemented with multiple ASNs (example: Worldcom) ASes are just an implementation detail of Inter-domain routing

How many prefixes today? Thanks to Geoff Huston. on October 9, , %23% IPv4 Address space covered

Policy-Based vs. Distance-Based Routing? ISP1 ISP2 ISP3 Cust1 Cust2 Cust3 Host 1 Host 2 Minimizing “hop count” can violate commercial relationships that constrain inter- domain routing. YES NO Thanks to Tim Griffin

Customer versus Provider Customer pays provider for access to the Internet provider customer IP traffic provider customer

Regional ISP1 Regional ISP2 Regional ISP3 Cust1 Cust3 Cust2 National ISP1 National ISP2 YES NO Shortest path routing is not compatible with commercial relations Why not minimize “AS hop Count”?

peer customerprovider Peers provide transit between their respective customers Peers do not provide transit between peers Peers (often) do not exchange $$$ traffic allowed traffic NOT allowed The “Peering” Relationship

Peering also allows connectivity between the customers of “Tier 1” providers. peer customerprovider Peering Provides Shortcuts

Peering Wars  Reduces upstream transit costs  Can increase end-to-end performance  May be the only way to connect your customers to some part of the Internet (“Tier 1”)  You would rather have customers  Peers are usually your competition  Peering relationships may require periodic renegotiation Peering struggles are by far the most contentious issues in the ISP world! Peering agreements are often confidential. PeerDon’t Peer

BGP = RFC “optional” extensions RFC 1997 (communities) RFC 2439 (damping) RFC 2796 (reflection) RFC3065 (confederation) … + routing policy configuration languages (vendor-specific) + Current Best Practices in management of Interdomain Routing BGP was not DESIGNED. It EVOLVED. The Border Gateway Protocol (BGP)

BGP Route Processing Best Route Selection Apply Import Policies Best Route Table Apply Export Policies Install forwarding Entries for best Routes. Receive BGP Updates Best Routes Transmit BGP Updates Apply Policy = filter routes & tweak attributes Based on Attribute Values IP Forwarding Table Apply Policy = filter routes & tweak attributes Open ended programming. Constrained only by vendor configuration language

BGP Attributes Value Code Reference ORIGIN [RFC1771] 2 AS_PATH [RFC1771] 3 NEXT_HOP [RFC1771] 4 MULTI_EXIT_DISC [RFC1771] 5 LOCAL_PREF [RFC1771] 6 ATOMIC_AGGREGATE [RFC1771] 7 AGGREGATOR [RFC1771] 8 COMMUNITY [RFC1997] 9 ORIGINATOR_ID [RFC2796] 10 CLUSTER_LIST [RFC2796] 11 DPA [Chen] 12 ADVERTISER [RFC1863] 13 RCID_PATH / CLUSTER_ID [RFC1863] 14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen] reserved for development From IANA: Most important attributes Not all attributes need to be present in every announcement

AS /16 AS Path = 6341 AS 1239 Sprint AS 1755 Ebon e AT&T AS 3549 Global Crossing /16 AS Path = /16 AS Path = AS /16 AT&T Research Prefix Originated AS RIPE NCC RIS project AS 1129 Global Access /16 AS Path = /16 AS Path = /16 AS Path = /16 AS Path = ASPATH Attribute

Next up  if you missed the intro on projects, please take a look online  need to form groups of 2 or 3 students  need to have an idea of what you are doing and get approval by wed/Thursday  due in 3 weeks – Nov 17 make sure to let me know if you need more time – am flexible for short extensions ONLY if you start early