1. Chapter 14 1 Unicast Routing Protocols There isn’t a person anywhere that isn’t capable of doing more than he thinks he can. - Henry Ford

2. Chapter 14 2 Objectives List advantages and disadvantages of routing protocols Describe how routing tables are dynamically built up in a router Explain how routing protocols can be categorized as interior and exterior routing protocols Describe how routing loops can occur and techniques used for minimizing them Describe basic features of RIP, OSPF and BGP

3. Chapter 14 3 Advantages & Disadvantages of Routing Protocols Compared to manual configuration, much easier to maintain in large networks Represent a point of failure that attackers can exploit Can take some time for a router on one side of a large network to learn about a topology change on the other side of the network Advanced routing protocols can be very complex Inherent lack of control. For eg: if there are multiple paths to a destination network, routing protocol will decide which one to use. While metrics can be manually tweaked to make one path preferred, one needs to understand the consequences of the manual changes

4. Chapter 14 4 Routing Domains and Autonomous Systems A routing domain is a group of routers under the control of a single administrative entity, running a common interior routing protocol. An autonomous system consists of a collection of routers under the control of a single administrative entity - for example, all the routers belonging to a particular ISP, corporation or university. An autonomous system can choose one or more routing protocols to run within the autonomous system, but typically may use only a single interior routing protocol.

5. Chapter 14 5 Grouping Routing Protocols Routed or Routable protocols (in contrast to Routing protocols) are Layer 3 protocols used to carry application data through an internetwork (Eg: IP). Routed protocols use information in routing tables built up by routing protocols for forwarding packets to their destinations Intra- and Inter-domain routing protocols –Intra-domain (or, interior) routing protocols are used inside an autonomous system. They are also called “interior gateway protocols”. E.g: RIP, OSPF –Inter-domain (or, exterior) routing protocols are used between autonomous systems. They are also called “exterior gateway protocols”. E.g: BGP (Border Gateway Protocol)

6. Chapter 14 6 Figure 14.1 Autonomous systems

7. Chapter 14 7 Figure 14.2 Grouping routing protocols

8. Chapter 14 8 Distance Vector Routing Protocols In distance vector routing, the least cost route between any two routers is the route with minimum distance. Each router maintains a vector (table) of minimum distances to other nodes. The minimum distance (called the “routing metric”) is a “measure of goodness” of a path to another router. Each node shares its routing table with its immediate neighbors periodically (by sending Periodic Updates) and whenever there is a change in network topology (by sending Triggered Updates).

9. Chapter 14 9 Figure 14.3 Distance vector routing tables

10. Chapter 14 10 How the Routing Table is built up... Initially, each router knows only about its immediate neighbors - those directly connected to it. Using the configuration data, a router builds up the distance vector info for each directly connected link. Each router periodically sends its routing table (distance vector information) on all directly connected links. When a router receives a routing table from a neighbor, it updates its own routing table based on the information in the neighbor’s routing table. After some time, if there is no change in the network (such as a link failure), all routers will have stable routing tables. Routers are then called to be in a “converged” state.

11. Chapter 14 11 Instability due to Routing Loops An example of a routing loop: When a router X believes the best path to a network N is via a second router Y, and at the same time, the second router Y believes the best path to network N is through the first router X X & Y will forward a packet to each other until the packet is finally discarded due to TTL expiry With each routing update received from the other, X & Y will update its distance metric until “infinity”. This is called “Counting to Infinity” problem. Implementations of distance-vector routing protocols define the “infinity” as a smaller number (E.g. 16) and then the route is marked as “unreachable”.

12. Chapter 14 12 Figure 14.6 Two-node instability

13. Chapter 14 13 Techniques to minimize routing loops Split Horizon –Essence of this technique: a router won’t advertise a particular route to a neighbor, if that route was originally learned from that neighbor Split Horizon with poisoned reverse –Routes learned from a neighbor are marked with a metric of “infinity” (poisoning the routes) when the routing table is sent to the neighbor. It tells the neighbor “I have learnt about these routes from you. My paths to these networks are via you”.

14. Chapter 14 14 RIP - Routing Information Protocol A distance-vector intra-domain routing protocol Two versions: V1 (RFC 1058); V2 (RFC 2453) UDP port 520 used for RIP messages “Distance” (or, the metric) used is “hop count” - the number of links that have to be crossed to reach the destination network “Infinity” is defined as 16.

15. Chapter 14 15 RIP V1 When RIPv1 routers first come up, they send a RIP announcement about their directly connected links Next, the router sends a RIP request, asking neighbors to send their routing tables These two steps are used to build the routing table Figure 14.9 RIP message format

16. Chapter 14 16 RIP V1 Command (1 byte) –RIP Request (1) or RIP Response (2) Version (1 byte) –Indicates the RIP version Address Family Identifier (2 bytes) –Defines the protocol family that is using RIP (2 for TCP/IP) Network Address (4 bytes) –IP address of the destination network being advertised Distance (or, Metric) (4 bytes) –Hop count from the advertising router to the destination network

17. Chapter 14 17 RIP V2 Enhancements in RIP V2 are: support of subnet masks, basic authentication, and multicasting for routing updates (instead of broadcasting) Figure 14.13 RIP version 2 format

18. Chapter 14 18 RIP V2 No new fields are added First entry of the message contains authentication info. “Protocol Family” field is set to 0xFFFF to indicate that the entry contains authentication info. Figure 14.14 Authentication

19. Chapter 14 19 RIP V2 Authentication Type (2 bytes) –Currently defined Type value = 2 Authentication Information (16 bytes) –Contains a plain text password –If the password is shorter than 16 bytes, it is left-justified and padded with Hex.00’s on the right Route Tag (2 bytes) –Indicates whether the route information that follows is an internal route entry (received from within this routing area), or an external routing entry (learned through another IGP or EGP outside this routing area)

20. Chapter 14 20 RIPv2 Network Address –IP address of the destination network being advertised Subnet Mask –Subnet mask associated with the destination network address being advertised Next Hop –IP address to which packets to the destination specified by this route entry should be forwarded. –0.0.0.0 indicates that routing should be via the originator of the RIP advertisement.

21. Chapter 14 21 Advantages and Drawbacks of RIP Simpler to understand and implement (compared to OSPF, for example) and is suitable for small networks Not suitable for medium- to large-sized networks - can take a long time to converge Use of “hop count” as metric, creates two problems: –Limits the diameter of a RIP network to 15 router hops –Administrators cannot not use such factors as bandwidth and/or delay as the routing metric Slow to flush unreachable destinations from the network, going through the “counting to infinity” procedure Anyone can bring up a bogus RIP router, advertising bogus routes to disrupt routing (even though there is basic authentication in RIP v2)

22. Chapter 14 22 Link-State Routing Protocols Routers do not broadcast their entire routing tables. Each router “floods” its domain with Link State Packets (LSPs) containing information about its directly connected links: –Periodically, with the period being much longer (in the range of 1 - 2 hours) compared to distance-vector routing protocols (period in the range of 30 sec.) –When there is a change in topology of the domain Network convergence time is relatively shorter compared to distance-vector routing protocols and therefore scalable for large networks

23. Chapter 14 23 How flooding of LSPs work After a router prepares an LSP (containing info such as: router identity, list of directly connected links, sequence number & age of LSP), it is sent out of each interface When a router receives an LSP, if it is older than a copy it already has, the newly received copy is discarded If the received LSP is newer: –Discards any old copy and keeps the new one –Sends a copy out of each interface except the one from which the LSP was received After receiving LSPs from all other routers in the domain, each router compiles the whole topology of the domain

24. Chapter 14 24 How the Routing Table is built From the topology information, using Dijkstra’s algorithm, each router creates a shortest path tree, with itself as the root. Shortest path tree of a router contains the shortest path from itself to every other router in the routing domain. Each router then constructs its routing table using the info in shortest path tree.

25. Chapter 14 25 Figure 14.15 Concept of link state routing

26. Chapter 14 26 Figure 14.18 Formation of shortest path tree by A

27. Chapter 14 27 Table 14.1 Routing table for Router A

28. Chapter 14 28 Open Shortest Path First (OSPF) OSPF (Version 2, RFC 2328), is the most commonly used intra-domain, link-state routing protocol in TCP/IP networks. OSPF runs directly over IP without using TCP or UDP. OSPF routing is based on configurable metrics based on network bandwidth, delay or monetary cost. By default, the metric used for route determination is based on network bandwidth. OSPF routers send Hello packets on the directly connected links to learn about their neighbors. By default, hello packets are sent every 10 sec. This interval is configurable. A router learns about neighbors when it receives neighbors’ Hello packets in turn.

29. Chapter 14 29 Simplified version of how OSPF works Each router in the OSPF routing domain is responsible for sending out Link State Advertisements (LSAs) to all other routers using “flooding”. LSAs describe the sending router’s local part of the routing domain. There are different types of LSAs. “Router-LSAs” (LS Type = 1) includes the sending router’s set of directly connected links, neighbors and the metrics. Taken together, the collection of LSAs generated by all of the routers in a routing domain is called the “Link-State Database”. Once all the routers have received other routers’ LSAs (i.e., when the routers are in a converged state), they all have the same identical link-state database.

30. Chapter 14 30 Simplified version of how OSPF works Using link-state database as input, applying Dijkstra’s algorithm, each router computes its routing table. When the network is in a steady state (i.e., no routers or links are going in or out of service), the only OSPF routing traffic is periodic Hello packets between neighboring OSPF routers and the occasional refresh of pieces of the link-state database. Every 30 min, a router refloods the pieces of the link-state database that it is responsible for, just in case those pieces have been lost from or corrupted in one of the other routers’ databases. If an LSA has not been updated after an hour, the LSA is assumed to be no longer valid and is removed from the database.

31. Chapter 14 31 Areas and Area Border Routers OSPF divides an autonomous system (AS) into areas, with each area given an Area ID An area is a collection of networks, hosts & routers contained within an AS Each AS has a backbone area with an Area ID of 0 (can be written as 0.0.0.0). Routers inside the backbone area are called backbone routers. All other areas in the AS must be connected to the backbone area and exchange routing info through Area 0. Ass are connected together using AS boundary routers.

32. Chapter 14 32 Figure 14.19 Areas in an autonomous system

33. Chapter 14 33 Path Vector Routing One (or, more) router(s) in an AS (called the speaker node) acts on behalf of the entire AS in creating a routing table and advertising it to speaker nodes in the neighboring Ass. Similar to distance vector routing except that only speaker nodes (i.e., routers) in each AS exchange routing info with each other What is advertised is also different compared to distance vector routing. A speaker node advertises the path, and not the metric. At the beginning, each speaker node knows only the reachability of nodes inside its AS.

34. Chapter 14 34 Figure 14.48 Initial routing tables in path vector routing

35. Chapter 14 35 Path Vector Routing Similar to distance vector routing, a speaker in an AS shares its routing table with immediate neighbors in other ASs When a speaker node receives a table from a neighbor, it updates its own table by adding the nodes that are not in its routing table After a while, each speaker node knows how to reach each node in other Ass.

36. Chapter 14 36 Figure 14.49 Stabilized tables for four ASs

37. Chapter 14 37 Border Gateway Protocol (BGP) BGP is an inter-domain routing protocol using path vector routing Current version is BGP 4 (RFC 1771) BGP uses TCP as its transport protocol. A BGP session is established between two BGP routers for exchanging routing info. As a TCP connection created for BGP can last for a long time, BGP sessions are sometimes referred to as semi-permanent connections.

38. Chapter 14 38 External & Internal BGP BGP can have external or internal sessions External BGP (E-BGP) session is used to exchange routing info between two speaker nodes in two different ASs Internal BGP (I-BGP) session is used to exchange routing info between two routers inside an AS

39. Chapter 14 39 Figure 14.50 Internal and external BGP sessions

40. Chapter 14 40 Figure 14.51 Types of BGP messages To create a neighborhood relationship, a BGP router opens a TCP connection with a neighbor and sends an Open message

41. Chapter 14 41 BGP Messages If the neighbor accepts the neighborhood relationship, it responds with a keepalive message Update messages are used to announce new routes to a destination or removing routes Notification messages are sent by a router when an error condition is detected or to close a BGP session.