1. CPS-356- Computer Networks Class 7: Switching Continued+ Network Layer
Theophilus Benson
Based partly on lecture notes by Rodrigo Fonseca, David Mazières, Phil Levis, John Jannotti

2. Today’s Lecture Switching (Take II)
Ethernet (datagram)
Spanning-Tree
ATM (Virtual Circuits)
Network layer: Internet Protocol (v4)
Forwarding
Addressing
Fragmentation
ARP
DHCP
NATs

3. Ethernet Switching Hosts come preconfigured with IDs
Each host has a MAC-address
Network automatically determines routes
Flood to discover who is connected.

4. Drawbacks of Flooding B1 B4 B3 Alice B5 Brige1 LAN 3 A B Brige4 Brige3
Bob B5

5. Drawbacks of Flooding B1 Bob A B3 B4 Alice LAN 2 LAN 3 LAN 4 Brige1

6. Drawbacks of Flooding B1 Bob A B3 B4 Alice LAN 2 LAN 3 LAN 4 Brige1

7. Drawbacks of Flooding B1 Bob A B3 B4 Alice LAN 2 LAN 3 LAN 4 Brige1

8. Drawbacks of Flooding B1 Bob A Alice B B3 Alice B B4 Alice A Alice
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3
Brige4 A B
Bob B5
Bob A

9. Drawbacks of Flooding B1 Bob A Alice B B3 Alice B Bob A B4 Alice A Bob
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3
Brige4 A B
Bob B5
Bob A

10. Drawbacks of Flooding Can not deal with loops
Can not scale to a large number of devices.

11. Drawbacks of Flooding Can not deal with loops
Solution: Spanning Tree
Can not scale to a large number of devices.
Solution: VLANs

12. Drawbacks of Flooding B1 Bob A Alice B B3 Alice B Bob A B4 Alice A Bob
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3
Brige4 A B
Bob B5
Bob A

13. Drawbacks of Flooding B1 Bob A Alice B B3 Alice B Bob A B4 Alice A Bob
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3
Brige4 A B
Bob B5
Bob A

14. Drawbacks of Flooding B1 Bob A Alice B B3 Alice B Bob A B4 Alice A Bob
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3
Brige4 A B
Bob B5
Bob A

15. Drawbacks of Flooding B1 Bob A Alice B B3 Alice B Bob A B4 Alice A Bob
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3
Brige4 A B
Bob B5
Bob A

16. Drawbacks of Flooding Can not deal with loops
Solution: Spanning Tree
Can not scale to a large number of devices.
Solution: VLANs

17. Spanning-Tree Exchange BPDU messages
BPDU = Bridge Protocol Data Unit
Discover a routing topology free of loop
Eliminates redundancy: wastes extra links
root ID
cost
bridge ID
port ID
root bridge (what the sender thinks it is) root path cost for sending bridge Identifies sending bridge Identifies the sending port

18. Building a Spanning Tree: Time 0: Everyone thinks they are ‘root’
B3 think B3 is Root B3
B1 think B1 is Root B1
B4 think B4 is Root B4
Alice
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3
Brige4 A B
Bob
B5 think B5 is Root B5

19. Building a Spanning Tree: Time 1: Everyone heard from B1
Building a Spanning Tree: Time 1: Everyone heard from B1. B1 has a lower bridge B1 .. Must be root
B3 think B1 is Root B3
B1 think B1 is Root B1
B4 think B1 is Root B4
Alice
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3
Brige4 A B R R R
Bob
B5 think B1 is Root B5

20. Building a Spanning Tree: Time 2: Tell each other B1 is root
B3 think B1 is Root B3 1 B
B1 think B1 is Root B1 1
B4 think B1 is Root B4 1 A
Alice
Brige1
LAN 3 A B
Brige4
Brige3 R R A
LAN 2 B B R A
Bob B A
Brige5
LAN 4
B5 think B1 is Root B5 1 A

21. Building a Spanning Tree: Time 3: Discover Duplication Turn off ports.
B3 think B1 is Root B3 B1 1 B
B1 think B1 is Root B1 1
B4 think B1 is Root B4 B1 1 A
Alice
Brige1
LAN 3 A B
Brige4
Brige3 R R A
LAN 2 B D B R A
Bob B A
Brige5
LAN 4
B5 think B1 is Root B5 B1 1 A
Professors

22. The Spanning Tree
Brige1
Brige5
Brige3
Brige4 B

23. The Spanning Tree Brige1 Port Type Rules R
Accept & forward flood traffic. Don’t forward BDPU D
Accept & Forward flood traffic. Forward BDPU D D
LAN 3
Bob
LAN 2 R R
Brige4
Brige3
Brige5 R B B D
TWO WASTED LINKS
IN THIS TOPOLOGY
LAN 4
Alice
Sent packets
Packets not sent

24. Drawbacks of Flooding Can not deal with loops
Solution: Spanning Tree
Can not scale to a large number of devices.
Solution: VLANs

25. Virtual LANs Assign switch ports to a VLAN ID (color)
Alice
Brige1
LAN 3 A B
Brige4
Brige3 A
LAN 2 B B A
Bob B A
Brige5
LAN 4
Assign switch ports to a VLAN ID (color)
Isolate traffic: only same color
Trunk links may belong to multiple VLANs
Encapsulate packets: add 12-bit VLAN ID
Easy to change, no need to rewire

26. Virtual LANs Assign switch ports to a VLAN ID (color)
Alice
Brige1
LAN 3 A B
Brige4
Brige3 A
LAN 2 B B A
Bob B A
Brige5
LAN 4
Assign switch ports to a VLAN ID (color)
Isolate traffic: only same color
Trunk links may belong to multiple VLANs
Encapsulate packets: add 12-bit VLAN ID
Easy to change, no need to rewire

27. Virtual LANs Assign switch ports to a VLAN ID (color)
Alice
Brige1
LAN 3 A B
Brige4
Brige3 A
LAN 2 B B A
Bob B A
Brige5
LAN 4
Assign switch ports to a VLAN ID (color)
Isolate traffic: only same color
Trunk links may belong to multiple VLANs
Encapsulate packets: add 12-bit VLAN ID
Easy to change, no need to rewire

28. Other Uses for VLANs (Virtual LANs)
Finance: 1
Brige1
LAN 3 A B
Brige4
Brige3 A
LAN 2 B B A B
Finance: 1 A
Professors
Brige5
LAN 4
Company network, A and B departments
May not want traffic between the two departments
Topology has to mirror physical locations
What if employees move between offices?

29. What Do Switches Look Like?

30. Generic Switch Architecture
Goal: deliver packets from input to output ports
Potential performance concerns:
Throughput in bytes/second
Throughput in packets/second
Latency
Different if packets are variable size

31. Shared Memory Switch 1st Generation – like a regular PC
NIC DMAs packet to memory over I/O bus
CPU examines header, sends to destination NIC
I/O bus is serious bottleneck
For small packets, CPU may be limited too
Typically < 0.5 Gbps

32. Shared Bus Switch 2st Generation
NIC has own processor, cache of forwarding table
Shared bus, doesn’t have to go to main memory
Typically limited to bus bandwidth
(Cisco 5600 has a 32Gbps bus)

33. Point to Point Switch 3rd Generation: overcomes single-bus bottleneck
Example: Cross-bar switch
Any input-output permutation
Multiple inputs to same output requires trickery
Cisco series: 60Gbps
Layer 3 routers are similar

34. Cut through vs. Store and Forward
Two approaches to forwarding a packet
Receive a full packet, then send to output port
Start retransmitting as soon as you know output port, before full packet
Cut-through routing can greatly decrease latency
Disadvantage
Can waste transmission (classic optimistic approach)
CRC may be bad
If Ethernet collision, may have to send runt packet on output link

35. Cut through
Store and forward

36. Buffering
Buffering of packets can happen at input ports, fabric, and/or output ports
Queuing discipline is very important
Consider FIFO + input port buffering
Only one packet per output port at any time
If multiple packets arrive for port 2, they may block packets to other ports that are free
Head-of-line blocking: can limit throughput to ~ 58% under some reasonable conditions*
Also allows QoS, Fairness, Prioritization
For independent and uniform traffic 2
Port 1 1 2
Port 2
* For independent, uniform traffic, with same-size frames

37. Head-of-Line Blocking2 1
Port 1
Port 2
Solution: Virtual Output Queueing
Each input port has n FIFO queues, one for each output
Switch using matching in a bipartite graph
Shown to achieve 100% throughput*
*MCKEOWN et al.: ACHIEVING 100% THROUGHPUT IN AN INPUT-QUEUED SWITCH, 1999

38. Today’s Lecture Switching (Take II)
Ethernet (datagram)
Spanning-Tree
ATM (Virtual Circuits)
Network layer: Internet Protocol (v4)
Forwarding
Addressing
Fragmentation
ARP
DHCP
NATs

39. ATM Cells Fixed-size packets If payload smaller than 48B, uses padding
5 bytes header
48 bytes payload
If payload smaller than 48B, uses padding
If greater than 48B, breaks it

40. Why small, fixed-length packets?
Cons: maximum efficiency 48/53=90.6%
Pros:
Suitable for high-speed hardware implementation
Many switching elements doing the same thing in parallel
Reducing priority packet latency
Good for QoS
Reducing transmission latency

41. Reduce queuing latency Reducing preemption latency
Transmission + propagation + queuing
Reducing preemption latency

42. Why 48 bytes It’s from the telephone technology
Thought data would be mostly voice
A compromise
US: 64 bytes
Europe: 32 bytes
64+32 = 48 bytes

43. Virtual paths 24-bit virtual circuit identifiers (VCIs)
Discussed in our previous lecture
Two-levels of VCIs
8-bit virtual path, 16-bit VCI
Virtual paths shared by multiple connections

44. Today’s Lecture Switching (Take II)
Ethernet (datagram)
Spanning-Tree
ATM (Virtual Circuits)
Network layer: Internet Protocol (v4)
Forwarding
Addressing
Fragmentation
ARP
DHCP
NATs

45. Internet Protocol Goal
How to connect everybody?
New global network or connect existing networks?
Glue lower-level networks together:
allow packets to be sent between any pair or hosts
Wasn’t this the goal of switching?
Le Theo Net
(ATM)
Le Duke Net
(Token Ring)

46. Internetworking Challenges
Heterogeneity
Different addresses
Different service models
Different allowable packet sizes
Scaling
Congestion control

47. How would you design such a protocol?
Circuits or packets (datagram)?
Predictability
Service model
Reliability, timing, bandwidth guarantees
Any-to-any
Finding nodes: naming, routing
Maintenance (join, leave, add/remove links,…)
Forwarding: message formats

48. How would you design such a protocol?
Circuits or packets (datagram)?
Predictability
Service model
Reliability, timing, bandwidth guarantees
Any-to-any
Finding nodes: naming, routing
Maintenance (join, leave, add/remove links,…)
Forwarding: message formats
What happens when people join/leave
What should the format of a message be?

49. IP’s Decisions Packet switched Service model Any-to-any
Unpredictability, statistical multiplexing
Service model
Lowest common denominator: best effort, connectionless datagram
Any-to-any
Common message format
Separated routing from forwarding
Naming: uniform addresses, hierarchical organization
Routing: hierarchical, prefix-based (longest prefix matching)
Maintenance: delegated, hierarchical

50. A Bit of History Packet switched networks: Arpanet’s IMPs
Late 1960’s
RFC 1, 1969!
Segmentation, framing, routing, reliability, reassembly, primitive flow control
Network Control Program (NCP)
Provided connections, flow control
Assumed reliable network: IMPs
Used by programs like telnet, mail, file transfer
Wanted to connect multiple networks
Not all reliable, different formats, etc…
Interface Message Processors

51. TCP/IP Introduced Vint Cerf, Robert Kahn Replace NCP
Initial design: single protocol providing a unified reliable pipe
Could support any application
Different requirements soon emerged, and the two were separated
IP: basic datagram service among hosts
TCP: reliable transport
UDP: unreliable multiplexed datagram service

52. An excellent read
David D. Clark, “The design Philosophy of the DARPA Internet Protocols”, 1988
Primary goal: multiplexed utilization of existing interconnected networks
Other goals (works and works):
Communication continues despite loss of networks or gateways
Support a variety of communication services
Accommodate a variety of networks
Permit distributed management of its resources
Be cost effective
Low effort for host attachment
Resources must be accountable
In order.
Missing: security

53. Still An excellent read
David D. Clark, “The design Philosophy of the DARPA Internet Protocols”, 1988
Primary goal: multiplexed utilization of existing interconnected networks
None-Other goals (other real world issues):
Security
Privacy
Flow money
In order.
Missing: security

54. Internet Protocol IP Protocol running on all hosts and routers
Routers are present in all networks they join
Uniform addressing
Forwarding/Fragmentation
Complementary:
Routing, Error Reporting, Address Translation
Routing
Switch
(diff framing)

55. IP Protocol Provides addressing and forwarding
Addressing is a set of conventions for naming nodes in an IP network
e.g. your name: Theo
Forwarding is a local action by a router: passing a packet from input to output port
e.g. how to get to theo
IP forwarding finds output port based on destination address (based on mapping)
Also defines certain conventions on how to handle packets (e.g., fragmentation, time to live)
Contrast with routing (defines mapping)
Routing is the process of determining how to map packets to output ports (topic of next two lectures)

56. Service Model Connectionless (datagram-based)
Best-effort delivery (unreliable service)
packets may be lost
packets may be delivered out of order
duplicate copies of packets may be delivered
packets may be delayed for a long time
It’s the lowest common denominator
A network that delivers no packets fits the bill!
All these can be dealt with above IP (if probability of delivery is non-zero…)

57. Format of IP addresses Globally unique (or made seem that way)
32-bit integers, read in groups of 8-bits:
Hierarchical: network + host
Originally, routing prefix embedded in address
Class A (8-bit prefix), B (16-bit), C (24-bit)
Routers need only know route for each network

58. 128.*.*.*.. Class A
*.* .. Class B
*.* .. Class B
*.*
*.*

59. Forwarding Tables
Exploit hierarchical structure of addresses: need to know how to reach networks, not hosts
Keyed by network portion, not entire address
Next address should be local: router knows how to reach it directly* (we’ll see how soon)
Network
Next Address *
18.*.*.*
*.*
Default

60. Classed Addresses Hierarchical: network + host
Saves memory in backbone routers (no default routes)
Originally, routing prefix embedded in address
Routers in same network must share network part
Inefficient use of address space
Class C with 2 hosts (2/255 = 0.78% efficient)
Class B with 256 hosts (256/65535 = 0.39% efficient)
Shortage of IP addresses
Makes address authorities reluctant to give out class B’s
Still too many networks
Routing tables do not scale
Routing protocols do not scale

61. Subnetting Add another level to address/routing hierarchy
Subnet mask defines variable portion of host part
Subnets visible only within site
Better use of address space
Much better efficiency: can break large networks without increasing global routing table size

62. Scaling: Supernetting
Problem: routing table growth
Idea: assign blocks of contiguous networks to nearby networks
Called CIDR: Classless Inter-Domain Routing
Represent blocks with a single pair
(first network address, count)
Restrict block sizes to powers of 2
Use a bit mask (CIDR mask) to identify block size
Address aggregation: reduce routing tables

63. CIDR Forwarding Table Network Next Address 212.31.32/24 0.0.0.0 18/8
/16
/17
0/0

64. Example H1-> H2: H2.ip & H1.mask != H1.subnet => no direct path=
Mask:
H1-> H2: H2.ip & H1.mask != H1.subnet => no direct path

65. R1’s Forwarding Table Network Subnet Mask Next Address 128.96.34.0

66. IP v4 packet format

67. IP header details Forwarding based on destination address
TTL (time-to-live) decremented at each hop
Originally was in seconds (no longer)
Mostly prevents forwarding loops
Other cool uses…
Fragmentation possible for large packets
Fragmented in network if crossing link w/ small frame
MF: more fragments for this IP packet
DF: don’t fragment (returns error to sender)
Following IP header is “payload” data
Typically beginning with TCP or UDP header

68. Other fields Version: 4 (IPv4) for most packets, there’s also 6
Header length: in 32-bit units (>5 implies options)
Type of service (won’t go into this)
Protocol identifier (TCP: 6, UDP: 17, ICMP: 1, …)
Checksum over the header

69. Fragmentation & Reassembly
Each network has maximum transmission unit (MTU)
Strategy
Fragment when necessary (MTU < size of datagram)
Source tries to avoid fragmentation (why?)
Re-fragmentation is possible
Fragments are self-contained datagrams
Delay reassembly until destination host
No recovery of lost fragments
Fragmentation: loss of one fragment implies all fragments discarded
Delay of entire packet is worst case delay over all fragments

70. Fragmentation Example
Ethernet MTU is 1,500 bytes
PPP MTU is 576 bytes
R2 must fragment IP packets to forward them

71. Fragmentation Example (cont)
IP addresses plus ident field identify fragments of same packet
MF (more fragments bit) is 1 in all but last fragment
Fragment offset multiple of 8 bytes
Multiply offset by 8 for fragment position original packet

72. Today’s Lecture Switching (Take II)
Ethernet (datagram)
Spanning-Tree
ATM (Virtual Circuits)
Network layer: Internet Protocol (v4)
Forwarding
Addressing
Fragmentation
ARP
DHCP
NATs

73. Translating IP to lower level addresses or… How to reach these local addresses?
Map IP addresses into physical addresses
E.g., Ethernet address of destination host
or Ethernet address of next hop router
Techniques
Encode physical address in host part of IP address (IPv6)
Each network node maintains lookup table (IP->phys)

74. ARP – address resolution protocol
Dynamically builds table of IP to physical address bindings for a local network
Broadcast request if IP address not in table
All learn IP address of requesting node (broadcast)
Target machine responds with its physical address
Table entries are discarded if not refreshed

75. ARP Ethernet frame format
Why include source hardware address?

76. Obtaining Host IP Addresses - DHCP
Networks are free to assign addresses within block to hosts
Tedious and error-prone: e.g., laptop going from CIT to library to coffee shop
Solution: Dynamic Host Configuration Protocol
Client: DHCP Discover to (broadcast)
Server(s): DHCP Offer to (why broadcast?)
Client: choose offer, DHCP Request (broadcast, why?)
Server: DHCP ACK (again broadcast)
Result: address, gateway, netmask, DNS server

77. Obtaining IP Addresses
Blocks of IP addresses allocated hierarchically
ISP obtains an address block, may subdivide
ISP: /
Client 1: /
Client 2: /
Client 3: /
Global allocation: ICANN, /8’s (ran out!)
Regional registries: ARIN, RIPE, APNIC, LACNIC, AFRINIC

78. Network Address Translation (NAT)
Despite CIDR, it’s still difficult to allocate addresses (232 is only 4 billion)
We’ll talk about IPv6 later
NAT “hides” entire network behind one address
Hosts are given private addresses
Routers map outgoing packets to a free address/port
Router reverse maps incoming packets
Problems?
Incoming flows
Shared ports
Breaks assumptions

79. Internet Control Message Protocol (ICMP)
Echo (ping)
Redirect
Destination unreachable (protocol, port, or host)
TTL exceeded
Checksum failed
Reassembly failed
Can’t fragment
Many ICMP messages include part of packet that triggered them
See

80. ICMP message format

81. Example: Time Exceeded
Code usually 0 (TTL exceeded in transit)
Discussion: traceroute

82. Example: Can’t Fragment
Sent if DF=1 and packet length > MTU
What can you use this for?
Path MTU Discovery
Can do binary search on packet sizes
But better: base algorithm on most common MTUs
Path MTU discovery