Outline: Ethernet Technologies

• Ethernet
• Addresses
• ARP/RARP
• Hubs
• Switches
• Routers

Ethernet

• Layer 2 (Data Link) technology
• Originally shared medium
• Now point-to-point
• Tree topology, with hubs as nodes
• CSMA/CD
• 48-bit addresses

Data Communication

• Computer gives a packet to its ethernet interface:
  • Source Address
  • Destination Address
  • Data
• Interface puts packet on wire connecting to hub
• Hub (Layer 1/Physical device) puts packet on all its outgoing wires
• Interface on destination computer(s) accept the packet
• All computers accept packets sent to the broadcast address

CSMA/CD

• Carrier Sense Multiple Access/Collision Detection
• If an interface is receiving a packet, it waits to send (CSMA)
• If a hub receives multiple packets at once, it jams all its wires so the senders (who are listening to their own transmission) abort and the receivers ignore the packet.
• CSMA/CD imposes a maximum length of an ethernet (a hundred meters per link).

Addresses

• 48-bit Ethernet addresses are globally unique, but have no intrinsic meaning.
• Network addresses (32 bits for IPv4) tell the packet how to reach the destination, but must be assigned.
• Domain Name System addresses (ASCII strings, e.g. "www.hawaii.edu", not discussed today) are used by humans to name computers.
• Network addresses are used by programs, but the ethernet packet must show the destination ethernet address. Converting a network address to a "hardware" address is called address resolution.

Simple Address Resolution

• Assign network addresses to each computer
• Read hardware addresses on each computer
• Create a table of network ==> hardware mappings
• Distribute the table to each computer

Address Resolution Protocol
ARP

1. arp-found: If the translation is found in the table, use that
2. If the translation is not found, broadcast a request stating the network address
3. The machine with the given network address replies specifying the hardware address.
4. If there is a reply, add the new translation to the table and return to step "arp-found".
5. Every so often, clear out "old" translations to keep table size small.

Reverse ARP
RARP

• Diskless node at boot time knows its hardware address, needs its network address.
• Broadcast a request with the hardware address.
• Server is configured with the table, replies with the network address.
• Diskless node can now use ARP to connect to other machines.

Hubs, Switches, Routers

• Hubs are level 1 (Physical) devices
• Switches are level 2 (Data Link/Ethernet) devices
• Routers are level 3 (Network) devices

Ethernet Switches

• connect multiple ethernets on different interfaces
• can buffer a whole packet before forwarding it (if the destination interface is busy)
• keep a table of known destination ==> interface mappings
• if a packet is to a known destination, only forward it on that interface
• otherwise, forward it on all interfaces
• forward broadcast packets on all interfaces
• when a packet from S is received on interface I, record S ==> I.
• Every so often, clear out "old" translations.

Switch topology

• Switches are self-configuring, optimizing
• Switches connect many physical ethernets into one logical ethernet (e.g CMU-CS).
• Broadcasting algorithm does not work in the presence of loops
• Switches can compute a spanning tree, only forward along interfaces belonging to the spanning tree (note textbook p. 20 is inaccurate).

Switch Example

Routers

• Level 3 devices: use network address
• Connect multiple ethernets into an Internet
• Switches do not modify the ethernet packet; routers change the ethernet address.
• Unlike switches, routers can support arbitrary topologies, e.g. multiple links to one destination.

How Routers work

• A router has an ARP table on each interface and a global {\em routing table} mapping network addresses ==> (interface, next hop).
• Once an interface and next hop have been selected, ARP is used on that interface to resolve the next hop.

Router Example

Limitations

• Size of a hub-based ethernet limited by collision domain size.
• Size of a switch-based ethernet limited by tree topology (max. bandwidth), number of broadcast messages (host load), and switch table size.
• Size of a router-based network limited only by routing table size and network address size.

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