Networking background

Internet components and structure

The Internet is:

• roughly hierarchical
• composed of networks
• composed of routers, links, a wide variety of edge devices

• networks connect using customer-provider relationships (usually in a contract!)
• tier-1 ISPs provide international coverage, treat each other as peers

• Lumen’s network map shows what a Tier 1 ISP looks like

• the Internet is only roughly hierarchical, there may be many connections among peers and across levels

Internet architecture

• layering helps to build complex systems

  • split large system into smaller pieces
  • identify each layer’s functionality and interfaces
  • can change a layer’s implementation as long as interfaces remain the same

• encapsulation is used when sending packets, decapsulation is used when receiving packets

Link layer

• MAC addresses

  • used to get frame from one interface to another physically-connected interface (on the same network)
  • most are 48 bits long, depends on link-layer protocol
  • address burned into the adapter ROM
  • broadcast address usually all ones (FF-FF-FF-FF-FF-FF)

• address assignment administered by IEEE

  • manufacturer buys portion of MAC address space (prefix)
  • uses that prefix for all MAC addresses and ensures it does not reuse the suffix
  • uniqueness provides address portability

Network Layer

• IP — Internet Protocol
• simplest definition of the Internet: all the devices that speak IP

IPv4

• common protocol for all networks
• very simple
• has lasted nearly years (since ~1978!)

• source IP address

• destination IP address

• TTL (hop count)

• protocol: typically UDP or TCP

• IP addresses

  • 32 bits
  • dotted decimal notation
  • byu.edu is 128.187.16.184

• forwarding process: check the destination address

  • is this one of my addresses? send to next protocol specified in IP header
  • is this one of my subnets? send to link layer to forward to the destination
  • do I have a route? send to link layer to forward to next IP hop
  • destination unknown! send an ICMP error to the source of the IP packet

IPv6

• simplified

• result of negotiation

• flow label and traffic class: used to map traffic into classes of service

• Next Header: upper layer protocol or option

• IPv6 addresses: 128 bits

Routing Protocols

• routing protocols decide which route should a packet at A take to reach B
• different protocols for intra-network and inter-network routing — see CS 460

Transport Layer

UDP

• best-effort service
  • connectionless: no state setup
  • unreliable: lost packets are not re-sent • no flow control
  • no congestion control (won’t slow down, you can send as much and as fast as you want)

• defined in RFC 768, from 1980

  • port numbers (16 bits each)
  • length of UDP segment, including header (bytes)
  • checksum: detect some bit errors in transmitted segment
  • application data (sometimes called ADU)

• well-known ports: FTP (21), SSH (22), SMTP (25), HTTP (80), HTTPS (443)

• binary protocol

TCP

• pipelining: multiple data segments outstanding at once

  • VERY important to use the high bandwidth available on the Internet

• TCP has two primary functions

  • reliability
  • congestion control

• reliability

  • make sure every packet gets through
  • every packet gets a sequence number
  • resend any packets that are dropped

• congestion control

  • control the rate at which packets are sent

• basic algorithm

  • gradually increase rate
  • any dropped packet means you sent too fast — slow down
  • then ramp back up again
  • continually probe for more bandwidth, but slow down when loss occurs
  • converges over time to a fair allocation of bandwidth among competing flows

• sequence and ACK number: count in terms of bytes
• flags
  • A: ACK number is valid
  • R (RST): reset connection
  • S (SYN): establish connection
  • F (FIN): close connection
  • U (URG): urgent data, typically not used
  • P (PSH): push (send) data immediately, used for TELNET
• receive window: number of bytes receiver can accept (flow control)

Why is the Internet so vulnerable to attacks?

The Design Philosophy of the DARPA Internet Protocols

• fundamental goal: develop an internetwork for existing networks
• second-level goals (in order of importance)
  • survivability
  • multiple types of service (delay vs bandwidth, reliable vs datagram)
  • variety of networks
  • distributed management
  • cost effective
  • host attachment with low effort
  • accountable resources

→ security was not considered Brief History of the Internet