Routing: Exterior Gateway Protocols and Autonomous Systems

1. Routing: Exterior Gateway Protocols and Autonomous Systems Chapter 15

2. Adding Complexity to an internet • We learned last time that we cannot continue to add routers to an already full backbone and continue to process efficiently. Why? • It takes a lot of bandwidth for many routers to communicate routing information • In a large internet, networks and routers may be owned or managed by different groups • “…it is impractical for all routers in an arbitrarily large internet to particpate in a single routing update protocol.”

3. Issues on size of an internet • Delay • How long does it take to notify all routers of changes in the internet? • Overhead • How much of the total traffic on an internet is made up of routing data? • What about combinations of: • low delay and high capacity? • high delay and low capacity?

4. Issues on size of an internet • As the number of hosts on a network grows over time, more of the network traffic is consumed by the increased traffic • Network managers usually implement a network monitoring scheme • a monitor listens passively to a network and records statistics about the traffic, determining: • network utilization - bandwidth used • percentage of packets containing routing messages

5. The Extra Hop Problem • Usually, we have several routers connecting to a backbone and those routers agree on a single routing protocol • Another router may also be connected to that backbone, but is considered a non-participating router, as in Figure 15.1 • If the non-participating router chooses one of the particpating routers as its default router, routes chosen may be suboptimal • Router 3 wants to send to Router 2, but has Router 1 as its default; an extra hop is taken

6. Hidden Networks • Local networks may be hidden from participating routers, as local network 4 in Figure 15.2 • Information must flow in two directions • Routing information flows from participating routers to nonparticipating routers • Nonparticipating routers pass information about hidden networks to the participating group of routers • Which router should do this? R3 which is one hop from R1, or R4 which knows local network 4?

7. Autonomous Systems • A group of networks and routers controlled by a single administrative authority is called an autonomous system (AS) • One router apprises outside world of the networks inside this group • R3 might be chosen to serve that purpose • The system is free to choose internal routing architectures and protocols • discovering, propagating, validating and checking consistency of routes

8. From Core to Autonomous Systems • The natural evolution from the core system is shown in Figure 15.3 • Advertisement of local information is made available to other AS’s through a designated router • Each AS is given an AS number (ASN) to distinguish among the AS’s

9. An Exterior Gateway Protocol • EGP is a general term for protocols used in passing routing information between AS’s • TCP/IP uses the Border Gateway Protocol version 4 (BGP) • When AS’s agree to exchange routing information, each designates a router to speak BGP on its behalf • the two routers are BGP peers of each other • routers chosen are “near the edge”, thus Border, as in Figure 15.4

10. BGP Characteristics • Allows AS’s to communicate with each other • Coordinates among multiple BGP speakers if there is more than one • Advertises reachable destinations • Supplies next hop information (as distance vector) • Allows a router’s configuration to adapt to various policies • Uses TCP for reliable transport

11. BGP Characteristics • Advertises path information so receiver can learn a series of AS’s along path to destination • Exchanges full information once, then updates with incremental changes • Supports CIDR addressing and sending masks • Aggregates route information • Allows a receiver to verify the identity of a sender (authentication)

12. BGP Functionality and Message Types • BGP performs 3 functions • Initial peer acquisition and authentication • Two peers establish a TCP connection • Each side sends positive or negative reachability information • sender can advertise reachable destinations and next hop • or sender can declare that previously available sites are no longer accessible • Continual verification that peers and network connections are functioning correctly

13. BGP Functionality and Message Types • BGP defines 4 message types • Open - initialize communication • Update - advertise or withdraw routes • Notification - response to an incorrect message • Keepalive - continually check peer connectivity

14. BGP Message Header • Marker field 16-octets - a value that both sides agree to use to mark the beginning of a message • initial message consists of all ones • since TCP does not preserve message boundaries, this is necessary • Length field 16-bits - total message length in octets, minimum is 19 • Type field 8 bits - indicates message type

15. OPEN Message • Two BGP peers establish a TCP connection • They send each other an OPEN with • their ASN • a value for a hold timer for the maximum time to wait • a BGP identifier IP address (a router must choose one of its IP addresses to use with all BGP peers • See Figure 15.7

16. UPDATE Message • Indicates: • Destinations to be withdrawn (variable, not required if there are none) • Advertisement of new destinations (also variable) • Lengths for both of the above - size zero if none • See Figure 15.8 • Routers need to advertise a next hop that is optimal from the outsider’s perspective, See Figure 15.12

17. Compressed Mask-Address Pairs • To accommodate classless addressing, for each of the addresses in the UPDATE message, an IP address and a mask are compressed • See Figure 15.9 • Mask information is encoded into 8 bits, which represents the number of bits in the mask (0-32) • The address is also compressed • If the value of the mask is < 8, one octet is covered and that is what follows • If the value is between 9 and 16, two octets follow, etc.

18. Path Attributes • BGP is not a pure distance vector protocol because it advertises more than a next hop • Additional information is in the Path Attributes field of the UPDATE message • Allows the receiver to: • check for routing loops and sender tospecify an exact path through AS’s to destination • implement policy constraints and accept/reject routes unsafe • know the source of all routes • Path attributes are factored, so that the attributes apply to all destinations advertised in one message

19. Path Attributes • The Path Attributes field consists of a triple: (type, length, value) • The two octets shown in Figure 15.10 precede each item in the Path Attributes list of Figure 15.8

20. KEEPALIVE Message • Verifies that two peers are continually functioning • Consists of the header and no data (19 octets) • Why? • BGP uses TCP for transport and TCP does not have a mechanism to continue to check for endpoint reachability • Saves bandwith as opposed to continually sending routing information (which generally changes infrequently) • Standards recommend keepalive timer = 1/3 hold timer

21. A Restriction of Exterior Gateway Protocols • Exterior Gateway Protocols do not communicate or interpret distance metrics • BGP can only specify whether a destination is reachable, it cannot determine a best path • BGP does not know the cost of routes across intermediate AS’s • We say that BGP is a reachability protocol rather than a routing protocol • See consequences on pages 286 and 287

22. Internet Routing Arbiter System • For an internet to work, routing information must be globally consistent • The Routing Arbiter (RA) system consists of a replicated, authenticated database of reachability information • updates are authenticated • generally, only the AS that owns a network is allowed to adverise reachability

23. Internet Routing Arbiter System • Major ISPs interconnect at Network Access Points (NAPs) • Thus, an NAP represents the boundary between multiple AS’s • Each NAP has a computer called a route server (RS) that maintains a copy of the database and runs BGP • Each ISP designates one router near a NAP to be a BGP border router • This router maintains a connection to the route server • The ISP advertises reachability to its networks and networks of its customers

24. NOTIFICATION Message • This message type is used when errors occur • Once an error is detected, the TCP connection is closed • Error codes are indicated in Figure 15.14 • Subcodes for each of the error codes are shown in Figure 15.15

25. Unanswered Questions • How can we move from a centralized router system? • Can we have trust between Autonomous Systems?

26. Summary • Routers must be partitioned into groups or the amount of routing information exchanged is too large • The Internet consists of many Autonomous Systems • consisting of routers and networks under one administrative authority • the AS’s use an EGP to advertise reachability of its networks from outside • TCP’s EGP is BGP

27. Summary • Border Gateway Protocol is the most widely used EGP • BGP message types • initiate communication (OPEN) • send reachability information (UPDATE) • report errors (NOTIFICATION) • ensure that peers are connected (KEEPALIVE) • Multiple ISP’s connect at NAP’s and each NAP includes a route server, which uses BGP

28. For Next Time • Read Chapter 16