1. WAN Technologies

2. 2 Large Spans and Wide Area Networks MAN networks: Have not been commercially successful

3. 3 Key issues in WAN technologies Long distance: Span sites in multiple cities Scalability: Grow as needed to connect many sites Deliver reasonable performance for a large scale network

4. Forming a WAN: Scalability A packet switch provides local connections for computers at the site and connections to other sites. – A small computer system with a processor, memory, and I/O devices used to send and receive packets. – Earlier than LAN technologies and the switch used in LAN

5. Forming a WAN: Long distance WANs use leased data circuits from phone companies

6. 6 Modern WAN (Internet) Architecture Most WANs separate a packet switch into two parts: – A Layer 2 switch that connects local computers – Local connections – A router that connects to other sites – Global connections Communication with local computers can be separated from transmission across a WAN.

7. WAN Store and Forward Paradigm To allow as many computers as possible to send packets simultaneously, WANs can store and forward packets

8. WAN Store and Forward Paradigm In order to correctly forward the packets, each packet switch needs to know the MAC address of every computer on the network Not feasible. Billions of devices are connected to the Internet today  need better routing mechanism

9. 9 Addressing in a WAN WANs addresses follow a hierarchical addressing mechanism: (site, computer at the site) In practice, instead of identifying a site, each packet switch is assigned a unique number First part of an address identifies a packet switch Second part identifies a specific computer

10. 10 Addressing in a WAN When a packet arrives, a packet switch P examines the destination address in the packet and extracts the packet switch number Q of the targeted computer. – Packets targeted for a local computer (P = Q) The switch sends the packet directly to the computer. – Otherwise (P ≠ Q) The packet is forwarded over to another switch Using only one part of a two-part hierarchical address  Much faster forwarding

11. 11 Next-Hop Forwarding More generally, a WAN can be modeled as a graph. – Packet switches as nodes and connections between them as links or edges. (k, j): a link from node k to node j

12. 12 Next-Hop Forwarding A packet switch only needs to compute the next hop for a packet. Forwarding table

13. 13 Forwarding Table Computation Forwarding table must guarantee the following: Universal communication: Contain a valid next-hop route for each possible destination address Optimal routes Point to the shortest path to the destination Example: Computing the forwarding table for node 3: 100m 700m 300m 100m 500m 300m (3,4)

14. Exercise 12 34 5 100m 240m 200m 120m 100m 300m Computing the forwarding table for node 1: In a network with millions or even billions of computers, efficient algorithms are needed After class reading: Section 18.13

15. What are “Shortest” paths anyhow? 100m 500m 300m

16. 16 Default Routes To eliminate duplicate entries, a single entry is used for a long list of entries that have the same next-hop value Only one default entry is allowed The entry has lower priority

17. Discussion Consider a WAN with two packet switches. Assume each switch uses a default entry that points to the other switch. Under what circumstances will the scheme fail? To reachNext hop 1-- *(1,2) To reachNext hop 2-- *(2,1)

18. Summary: WAN Technologies Hardware 1.Scalability: packet switches 2.Long distance: leased circuits Software / Efficient forwarding scheme 1.Hierarchical address 2.Next-hop forwarding 3.Default routes

19. One More Problem Suppose computer [1,2] wants to send a message to [3,5]. From previous chapters we know that NIC of computer [3,5] uses its MAC address (say, 78:DF:4C:84:BB:F2) to do addressing filtering. But if we use the new hierarchical address [3,5] to forward the packet, the packet would fail the addressing filtering.

20. We Need Both Address Layering: – Layer 3 (hierarchical) address: for routing – Layer 2 (MAC) address: for address filtering Compose a frame 54:DD:91 11:CA:3B E8:25:32 46:C2:8F I like you[3,5][1,2] Layer 3 header Layer 3 payload Layer 2 header

21. From LAN/WAN to Internet When there are several LANs or WANs wanting to connect with each other, we will get an internetwork. The largest internetwork is the Internet. – Router: A hardware system dedicated to interconnecting networks

22. From LAN/WAN to Internet Problem: there may be several computer [1,1] on the Internet, one for each WAN Solution: for every router and computer on the internet, we need a unique hierarchical address for routing Next class: IP Address