Title: Ch 13 WAN Technologies and Routing
1Ch 13 WAN Technologies and Routing
2Motivation
- How to build a packet switching system that can
span a large area - Building blocks
- Point-to-point long-distance connections
- Packet switches
- Key issue that separate MAN/WAN from LAN is
scalability - Provide sufficient capacity to permit computers
to communicate simultaneously
3Motivation
4Packet Switches
- A hardware device used to connects to
- Other packet switches
- Computers
5Illustration of a Packet Switch
- Special-purpose computer system
- CPU
- Memory
- I/O interfaces
- Firmware
6Illustration of a WAN
7Forming a WAN
- Place one or more packet switches at each site
- Interconnect switches
- LAN technology for local connections
- Leased digital circuits for long-distance
connections - Interconnections depend on
- Estimated traffic (e.g., T1)
- Reliability needed
8Store and Forward
- Packets
- Sent from source computer
- Travels switch-to-switch to destination
- Switch
- Stores packet in buffer
- Examines packets destination address
- Forwards packet toward destination
- Output-port buffer
9Physical Addressing in a WAN
- Each WAN technology defines the exact frame
format - Each device connected to a WAN is assigned a
unique physical address - Many WANs use a hierarchical addressing scheme
for efficient forwarding - Packet switch number
- Port number
10Illustration of WAN Addressing
- An address is encoded as a single binary value
- Higher-order bits for switch number
- Low-order bits for computer number
11Next-Hop Forwarding
- The process of forwarding a packet to its next
hop is known as routing - Routing table for switch 2
12Forwarding Table Abbreviations
- Many entries point to same next hop can be
condensed (default) - Only examines the first part of the address
- Reduces the size of routing table (scalability)
- Improves lookup efficiency
13Source Independency
- Next-hop forwarding does not depends on
- Packets source address
- The path the packet has taken
- Next-hop forwarding does depends on
- Packets destination
14Relationship of Routing to Graph
edge
15Default Route
- Used to eliminate the case of duplication routing
16Source of Routing Table Information
- Static routing (manual)
- Table created by hand
- Useful in small networks (simplicity and low
network overhead) - Useful if routes never change
- Dynamic routing (automatic)
- Software creates / updates table
- Needed in large networks
- Changes routes when failures occur
17Dynamic Routing
- Distance Vector (DV)
- Switches exchange information in their routing
tables - Link-state
- Switches exchange link status information
- Can have a global view
- Both used in practice
18Distance Vector
- Periodic, two-way exchange between neighbors
- During exchange, switch sends
- List of pairs
- Each pair gives (destination, distance)
- Receiver
- Compares each item in list to local routes
- Changes routes if better path exists
19Distance Vector Intuition
- If no local route to V or local routed has cost
greater than C, install a route with next hop N
and cost C - Else ignore pair
Dest. Next Dist.
V P 2
N
Cost2
A
Dest. Next Dist.
V X 6
Dest. Next Dist.
V N 4
20Link-State Routing
- Overcomes instabilities in DV
- Pair of switches periodically
- test link between them
- broadcast link status message
- Switch
- Receives status message
- Computes new routes
- Uses Dijkstras algorithm
21Link-State Routing
22Shortest Path Computation in a Graph
- Possible distance metric
- Geographic distance
- Economic cost
- Inverse of capacity
- Darkened path is minimum for node 4 to node 5
23Shortest Path Computation in a Graph
- Dijkstras Algorithms
- Routing table for switch 4 is constructed during
the computation of the paths - A switch
- Only communicates with directly attached
neighbors - Must learn route to each destination
24Early WAN Technologies
- ARPANET
- Historically important in packet switching
- Fast when invented, slow by current standards
- X.25
- Early commercial service
- Still Used
- More popular in Europe
25Recent WAN Technologies
- Frame Relay
- Offered by phone companies
- Widely used commercial service
- SMDS (Switched Multi-megabit Data Service)
- Offered by phone companies
- Not as popular as Frame Relay
- ATM
26Summary
- Wide Area Networks (WANs)
- Span long distances
- Connect many computers
- Built from packet switches
- Use store-and-forward
- WAN addressing
- Two-part address
- Switch/computer
27Summary (continued)
- Routing
- Each switch contains routing table
- Table gives next-hop for destination
- Routing tables created
- Manually
- Automatically
- Two basic routing algorithms
- Distance vector
- Link state
28Summary (continued)
- Example WAN technologies
- ARPANET
- X.25
- SMDS
- Frame Relay
- ATM
29Example of Distance Vector Routing
- Consider transmission of one DV message
- Node 2 send to 3, 5, and 6
- Node 6 installs cost 8 route to 2
- Later 3 sends update to 6
- 6 changes route to make 3 the next hop for
destination 2
30Dijkstras Shortest Path Algorithm
- Input
- Graph with weighted edges
- Node, n
- Output
- Set of shortest paths from n to each node
- Cost of each path
- Called Shortest Path First (SPF) algorithm
31Dijkstras Algorithm
Ru
32Algorithm Intuition
- Start with self as source node
- Move outward
- At each step
- Find node u such that it
- Has not been considered
- Is closest to source
- Compute
- Distance from u to each neighbor v
- If distance shorter, make path from u go through v
33Result of Dijkstras Algorithm
- Example routes from node 6
- To 3, next hop 3, cost 2
- To 2, next hop 3, cost 5
- To 5, next hop 3, cost 11
- To 4, next hop 7, cost 8
1 1 2 2 3 3 4 4 5 5 7 7
D R D R D R D R D R D R
S1,2,3,4,5,7 ? 0 8 2 2 3 ? 0 ? 0 5 7
S1,2,4,5,7 ? 0 5 3 13 3 ? 0 5 7
S1,4,5,7 ? 0 13 3 11 3 5 7
S1,4,5 ? 0 8 7 11 3
S1,5 ? 0 11 3
S1 20 3
S