WAN Technologies

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Large Spans and Wide Area Networks

• Consider a company that uses a satellite bridge to connect LANs at two sites (one PC and one printer at each site). Should the network be classified as a WAN or as an extended LAN?

MAN networks: Have not been commercially successful

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Large Spans and Wide Area Networks

• The key issue that separates WAN technologies from LAN technologies is scalability. A WAN must be able to• Grow as needed to connect many sites• Deliver reasonable performance for a large scale network

• Thus, a satellite bridge that connects a pair of PCs and printers is merely an extended LAN

Forming a WAN

• 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

Forming a WAN

• WANs use leased data circuits from phone companies

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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.

WAN Store and Forward Paradigm • To allow as many computers as possible to

send packets simultaneously, WANs can store and forward packets

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

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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

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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

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Next-Hop Forwarding • More generally, a WAN can be modeled as a graph.– Packet switches as nodes and connections between them as

links.• (k, j): a link from node k to node j

• Why a graph?– Abstraction– An old theory with rich algorithms (e.g., computing the shortest

path)

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

Forwarding table

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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:

100m700m

300m

300m

100m500m

300m

300m

(3,4)

Exercise

1 2

3 4

5

100m100m

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

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Dynamic Routing Updates in a WAN

• Network changes/failures further complicate forwarding• A network manager cannot merely configure a forwarding

table to contain static values that do not change

• Most WANs use dynamic routing • A program builds an initial forwarding table when a switch

boots• Program then alters the table as conditions in the network

change

• Question: How can a switch know the network condition of some location 2000 miles away?

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Distributed Route Computation • In practice, WANs need to perform distributed route computation

– Each packet switch must compute its own forwarding table locally– All packet switches must participate in distributed route computation

• Example: Link-State Routing (LSR)1. Each packet switches periodically broadcast messages that

carry the status of a link between two packet switches• E.g., switch 3 says: “the link between 3 and 5 is up”

2. Each switch collects incoming status messages and build a graph of the network

3. Each switch compute its own forwarding table based on the graph

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.

We Need Both Address

• Layering:– Layer 3 (hierarchical) address: for routing– Layer 2 (MAC) address: for address filtering

• Compose a frame54:DD:9111:CA:3B

E8:25:3246:C2:8F I like you[3,5] [1,2]

Layer 3 header Layer 3 payload

Layer 2 header Layer 2 header

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

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