How To Read The Book

* Skim it first
* Separate concepts from details
* Use other sources (eg Wikipedia, RFCs, tutorials, other
  stds docs)
  * RFCs use specific language
* Develop specific questions
  * Skim it again
  * Answer them

All about Ethernet

* One of the earlier LAN protocols
  * "LAN" vs "MAN" vs "WAN"
* Ideas taken from earlier radio networks e.g. ALOHA
* Evolution of Ethernet
  * Ethernet classic w. vampire taps 10Mbps
  * Ethernet 10BASE-2 still a bus 10Mbps
  * Ethernet 10BASE-T PTP hub architecture 10Mbps
  * Ethernet switch/bridge architecture 100Mbps
  * Ethernet 1Gbps
  * Ethernet NG 40Gps
* Laboratory of basic networking ideas

Ethernet Requirements

* Connect computers "locally"
* Zero configuration
* High bandwidth / low latency
* Cheap HW
* Anything fast in HW
* High-ish reliability

Terminology and Basic Concepts

* IEEE 802.3x
* Protocol Data Unit ("PDU") - "Frame"
* Maximum Transmission Unit ("MTU")
* IEEE 802.11 (WiFi), 802.16 (WiMAX)
* Giant Table of IEEE Standards (p. 82)

"Manchester Encoding"

* Encode with bit transitions rather than bits
* Ensures minimum as well as maximum frequency
* Ensures clock recovery
* Burns half your bits
* HW matters

"CSMA/CD"

* Carrier Sense Multiple Access / Collision Detect
* Problem: time delay on wires
  * Two stations find the wire clear at once
  * Both transmit
  * Neither is received
* Solution: be able to sense
  * Whether a packet is being transmitted
  * Whether your transmission is colliding
  * Both colliding transmitters "back off"
    * Random exponential backoff
* Requires: Transmitters close together relative to their
  block size.
* cf Collision Avoidance ("CSMA/CA")

Ethernet frame

* Diagram p. 85
  o Inter-Packet Gap (12B)
  o Preamble (7B)
  o Start Frame Delimiter (1B)
  ---
  x Destination MAC address (6B)
  x Source MAC (6B)
  x Length (if < 1536) or Type thingy (2B)
  * Optional envelope (<= 482B) e.g. Q Tag (4B)
  ---
  * Payload - encapsulated PDU (<= 1500B, <= 1982B)
  ---
  * Pad (frame must be >= 64B)
  x CRC (FCS) (4B)
  * Carrier extension (maybe)
* Sum of xs is 18B, sum of os is 20B

About "MAC addresses"

* http://en.wikipedia.org/wiki/MAC_address
* IEEE allocation
* Six bytes, usually colon-separated hex, e.g.
    00:22:19:1f:de:a9
* First three bytes are vendor-specific
  Organizationally Unique Identifier ("OUI")
  * Vendors get three-byte blocks
  * Can get 12-bit blocks if desired
* Some vendor codes correspond to
  * Broadcast/Multicast (byte 1 bit 0)
  * Locally-configured (byte 1 bit 1)
* Ideally, bound to HW; can almost always
  override as desired

Ethernet Environments

* Data Center
* Home / Office
* Institutional (eg PSU)


Wackiness

* "Jumbo Frames" (9KB) and "GigE"
* "VLAN"
* "Bonding" / "Link Aggregation"
* "Autonegotiation"
  * "Full Duplex"
  * Crossover
* Wake-on-LAN, power-saving
* MAC "PAUSE"
* Ethernet over Power
* Power over Ethernet
Hubs, Bridges, Switches

* "Hub": acts like a piece of wire
  (typically no digital functionality)
* "Switch": Sends packet to all connected
  wires except receive wire (digital)
* "Learning": Switch routes packet only
  to wire of destination MAC
  * Unlearn by (~5m) timeout, MAC from
    new wire
* "Bridge": Learning switch, but
  usually w/ additional functions for
  dealing with non-tree topologies,
  tunneling, etc.


Spanning Tree Protocol 

* "STP", "RSTP" p. 102
* Discover virtual spanning tree
  * Prevents routing loops
* Bridges communicate with "BPDUs"
  * Bridges "elect" root bridge
  * Root bridge finds least-cost path
    to each other bridge
  * Root asks other bridges to turn off
    transmit on extra bridge interfaces
  * BPDU diagram p. 106

802.11 requirements

* Work on actual "ether" (2.4GHz / 5GHz)
* Act enough like Ethernet to save most
  of the SW stack
* Cheap HW
* Address security / privacy issues
* High throughput / low latency

802.11 solutions

* Revised LLC CSMA/CA protocol
  * exchanges RTS, CTS, ACK
    for flow control and CA
* Uses 802.3-ish MAC
  * Adds "BSSID", "ESSID"
* Cheap radios
  * Direct-Sequence Spread Spectrum ("DSSS") is cheap cheap
  * Orthogonal Frequency Division Multiplexing ("OFDM")
    is not too bad on modern "Software-Defined Radios" (SDRs)
  * 1Mbps / 11Mbps / 40-70Mbps (in 20MHz band!)
* Provides standards for link-level encryption
  * So-called "Wired Equivalent Privacy" ("WEP")
    * Up to 80-bit RC4
    * Symmetric-key setup
    * Full of holes and troubles
  * WiFi Protected Access ("WPA"), WPA2
    * Secure block ciphers
    * Lots of different authentication schemes
    * Not perfect, but pretty good

Point-to-Point Protocol

* "PPP" is intended for tunneling
* Original target is serial lines: no framing
  or other link layer there
  * Serial lines were running Serial Line over IP ("SLIP")
    directly: meant IP-only
  * Adds framing, PDU types, etc
  * Basically HDLC/SDLC, borrows SLIP framing
* Wacky LCP establishment
* CCP for compression
* Header compression (VJ et al)

* "Let's overengineer! If we thereby make things worse, we
  will add engineering until the situation improves!"

Tunneling

* Problem: Sometimes you want an "interLAN"
  * Link layer is missing functionality you need
  * Physical network goes where you want to go, but
    LL doesn't
  * Etc
* Idea: Transmit link-level frame as PDU in a frame
  * Sender packs it up as if transmitted normally
  * Receiver unpacks it and sends it on as if transmitted
    normally
* This works about as well as you might expect.

Outstanding questions

* Link-layer reliability?
  * Retransmission
  * ECC, FEC
  * Flow control

* Amount of layering? Layer independence?