Lecture #14

OSSF

• file data

  • contiguous allocation of blocks
  • linked-list allocation of blocks
  • allocation tables (FAT)
  • direct & indirect index structure
    • fast for small files but also supports larger files

• journaling filesystem

• free space management

  • bit vector
  • linked list

• RAID

  • RAID0: striping
  • RAID1: mirroring
  • RAID4, RAID5, RAID6: Reed Solomon error correction

• Hamming error correction (7, 4)

  • single bit error correction
  • double bit error detection

Virtual Filesystem

• file ops implemented in kernel via function pointers
  • C-style OO

Unix Filesytem

TODO: diagram

• in-memory inode is same data structure as on-disk inode

Distrubuted Systems

• i.e. networking

• RS232

  • used to be ubiquitous
    • basically the teletype (tty) interface
  • pretty much superceeded by USB (much more complex protocol)
  • simple
    • bits per second
    • bits per byte (7 or 8)
    • parity bit
    • flow control

TODO: diagram

• asymmetrical

  • DTE: Data Terminal Equipment
  • DCE: Data Communication Equipement

• null modem: 2 computers talk, each thinking the other is a terminal

  • crossover pins 2 & 3

• dialup network

  • store & forward
  • uucp (Unix-to-Unix Copy)
    • bang paths
  • applications on top of UUCP
    • email
    • usenet news (BBS)

• /dev/tty*

  • use ioctl(2) to set parameters`

• but for every problem, there is a precipitate...

  • line noise
  • intern tripping over cable
  • hunters shooting out lines

• the network is not reliable

  • see: "Fallacies of Distributed Computing"

Fallacies of Distributed Computing

1. The network is reliable
2. Latency is zero
3. Bandwidth is infinite
4. The network is secure
   • IOT
5. Topology doesn't change
6. There's a single administrator
7. Transport cost is zero
8. The network is homogeneous

Network Topologies

TODO: Diagram

• multi-way communication
  • common bus
    • ethernet

TODO: Diagram

• ethernet

  • each computer "broadcasts" message
    • message is data structure (struct): "packet"
    • packet includes address of sender & destination
      • MAC (Media Access Control) address: 48 bits
  • other computers ignore messages not for them
  • if two computers try to broadcast at the same time, they backoff & retry after delay
    • exponential backoff: successive backoffs increases delay exponentially
  • works well if utilization is below bandwidth capacity
    • kinda like hashing
  • security? what security?
    • "trusted network"
    • or, higher-level protocol for security
      • assume your messages are snooped
      • encryption
        • bad actor can still track "metadata"

• LAN: Local-Area Network

Data Link Layer

• think: ethernet

TODO: diagram

• CSMH/CD: Carrier Sense Multipe Access/Collision Detection

  • listen/transmit/detect collisions
    • exponential backoff on collisions
      • why exponential? retry increases traffic, which increases likelihood of collision

• MAC (Media Access Control) address: unique address for each NIC (Network Interface Card)

• packet (frame) TODO: diagram

• max frame size: 1518 bytes (excluding preamble, inter-frame ap)

Non-Local (Wide Area) Network

• local-area network (LAN): single campus (notes are dated: need more current #s)

  • 100 Mb - 10 Gb (higher speeds)
  • all nodes visible
    • "broadcast" messaging
  • nodes are computers or peripheral devices
    • e.g. LAN-connected printer

• wide-area network (WAN): geographically separated sites

  • point-to-point (P2P) "long-haul lines"
  • lower bandwidth than LAN
    • up to 1 Gb/s with fiber
    • from telephony standards
      • T1: 24 channels (phone lines): 1.544 Mb/s
      • T3: 45 Mb/s

• bandwidth in bits per second (divide by 10 to get approx. bytes per second)

Network of Networks

• "an internet" vs. "the Internet"

TODO: diagram

• compex protocols

Routing

• fixed vs dynamic

• session-based (virtual circuits)

• connection-oriented vs. connectionless

OSI (Open-System Interconnection)

• 7 layers of protocol

TODO: diagram

OSI vs. Internet

• while the academics in Europe were trying to get it "right" with OSI, Americans were busy inventing TCP/IP & UDP/IP

• TCP/IP

  • 2-layer protocol: Transmission Control Protocol & Internet Protocol
  • developed by people who wanted to get stuff done
  • standards docs: RFC ("Request for Comments")

OSI (cont.)

• layer 1: physical
  • electromechanical
  • wiring: twisted pair, coaxial cable (coax), fiber
• layer 2: data link
  • framing
  • physical error detection & recovery
• layer 3: network
  • connections
  • routing
  • load balancing
  • routers: operate at layer 3
• layer 4: transport
  • message transfer (end-to-end)
  • packeting
    • ordering/error recovery
  • flow control
  • physical address layer 5: session
  • process-process communication protol
    • e.g. remote login, mail, file transfer
• layer 6: presentation
  • data format conversions
    • e.g. UTF-8, encryption
• layer 7: application
  • end-to-end interaction

Network Layer

• IP Address

TODO: diagram

• IP packet: payload inside ethernet frame

IPv4 Datagam

TODO: diagram