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Networking 101

Lecture 5

lukepeters, kashun

(slide credits abizer, longlian, night)

Course Resources

• Your facilitators!
• Ed, Gradescope
• OCF Slack (ocf.io/slack) or Discord (ocf.io/discord) #decal-general
• All materials available at decal.ocf.io
• Ask questions / work on lab with us during lab sessions! (Tuesday 8-9pm in OCF Lab)

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Outline

• What makes up a network system?
• Addressing & Routing
• Protocols
• SysAdmin Tools

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What makes up a network system?

Big Question: Why do we need networks?

• We want easy communication!

What do we need for easy communication?

• Low latency
• High bandwidth
• Ability to easily identify a computer
• Fault-tolerance

Conceptual Model: TCP/IP

• 4-layer model
• Loosely defined (functions can be in-between levels)
• Higher layers abstract lower layers

Application

Transport

Internet

Link

Link Layer

• Communication between physically connected nodes
• MAC Addresses
• ARP/NDP
• Underlying hardware implementations are abstracted outside of TCP/IP model
• “Who can I talk to without going through a router?”

Internet Layer

• Transmitting data to different networks
• Global Routing (IP Addresses)
• No guarantees on reliability (just like real life!)

Transport Layer

• Defines protocols for communication between computers
• Dealing with unreliability of sending packets
• TCP/UDP

Application Layer

• Application-specific, actually deals with interpreting the data being sent
• Most “user-facing” functions live here
• File sharing, message passing, database access
• Examples: HTTP/FTP/DNS

Addressing & Routing

Media Access Control (MAC) Addresses

• Looks like this: 00:14:22:01:2a:c5
• 48 bits, divided into 6 octets
• First 3 octets identify the interface manufacturer
• Unique to each device
• Used to identify devices on local network
• How do we talk to devices outside the local network?

IP Addresses (IPv4)

• IP addresses uniquely identify a host
• IP addresses can be changed
• 32 bits long, divided into 4 octets, look like: 123.123.123.123
• IPv4 octets are commonly expressed in decimal notation, although alternative formats, though rarely employed, are also permissible.

A note on bits, decimal and hexadecimal

• Different ways to write a number!
• Decimal: uses 0 through 9
• Add another digit after reaching 9
• Hex: 0 through 9, a through f
• Add another digit after reaching f
• Binary: uses bits (0 or 1)
• Add another digit… every two numbers… :(
• . or : in addresses are just separators. Make it easy to read

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Example: Sixty

Decimal:

60

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Used in IPv4

Hex:

3c

(often written 0x3c)

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Used in MAC addresses, IPv6

Binary:

0b111100

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It’s more convenient to use hex instead!

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Subnets

• Remember that IP addresses are represented with binary
• Sometimes, you’ll see 192.168.1.0/24
• The /24 is a subnet mask
• Subnet: a range of IP addresses (e.g. 192.168.1.0-255)
• The first 24 bits of of the address: network prefix
• The last 8 bits: host identifier

IPv6

• There are only 4.3 billion IPv4 addresses, but almost 8 billion people in the world - what do we do?
• IPv6 addresses contain 128 bits
• Formatted as follows: 1234:5678:89:0:ab:cd:ef:beef
• We have a million billion IPv6 addresses for each cell in every human on the planet (enough?)

Address Resolution Protocol (ARP)

• Translates IP address to a specific MAC address
• ARP Request: asks devices on local network who an IP address belongs to
• Kernel caches values in an “ARP table”

Routing

• How do we decide where to jump to?
• Routers maintain routing tables, which tell us where to hop to next

OCF router routing table

Domain Name System (DNS)

• Translates domains (as found in URLs) to IP addresses
• Can manually set domain -> IP mappings, or can use external DNS resolver

Protocols

Transmission Control Protocol (TCP)

• Ensures reliable transmission
• Connection-oriented: need to initiate a connection before sending data
• Used when reliability is more important than speed
• Examples: website loading, file transfer

User Datagram Protocol (UDP)

• No reliability guarantees
• Does not establish a connection
• Good for real-time applications where some loss is acceptable
• Examples: (lossy) streaming, gaming

Internet Control Message Protocol (ICMP)

• Not used to transmit data
• Technically not a “transport protocol”
• Used to transmit error messages and status info
• Used by diagnostic tools

Internet Control Message Protocol (ICMP)

SysAdmin Tools

ping

ping

traceroute

traceroute

arp

dig

dig

ip

Can do many things, including displaying/modifying ip addresses and routing info. Example: ip addr