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Class02

Class02

as slideshow

Protocols

Part 1

1/21/2009

Questions?

• Do you have any questions from the previous class?
• If I don't ask, feel free to chime in!

OSI Model

• wikipedia has examples
• more layers

Standards

• The nice thing about standards is that there are so many of them to choose from. (wikiquote; models too!)
• IETF
  • RFC, Request For Comments
• IEEE
  • standards: get 802.1, 802.3 references
• ITU
  • ITU recommendations
• What is a standard?

Physical

• coax

• thicknet, 10Base5

• thinnet, 10Base2
• more specs ranging from 10Base2 to 1000BaseT, very helpful

Physical

• fiber optic, 10BaseF:

• fiber is tougher than you think!

• single-mode:

• multimode:

Physical

• fiber connectors
  • SC (LC)
  • ST
  • MT-RJ

Fiber

Physical

• twisted-pair, 10(100,1000)BaseT: not

• (copper) Ethernet cables: straight-through or crossover

• Cat1 and 2 are telephone, Cat 3 is 4-conductor 10BaseT only, Cat4 Token Ring
• Cat5 is defined in EIA/TIA 568B ("regular" and crossover) for wiring order, pair twisting, install requirements like cable bends and length, and signal characteristics for attenuation and crosstalk
• Category 5e (enhanced for higher data rate), Category 6 (Gigabit)

What's a Base?

• baseband: the signal is the information
• broadband: information (what would be baseband) rides on a carrier signal

Ethernet II packet

• preamble (frame) of 8 octets of alternating ones and zeroes

• 6 octets for destination MAC

• vendor code in first 3 octets

• 6 octets for source MAC

Ethernet II packet

• 2 octets for frame type
  • Ethernet frame types
• 46 to 1500 octets of data

• 4 octets of CRC

MTU

• All fragments must arrive before re-assembly.
• In TCP, a single dropped fragment means transmitting the whole segment again.
• But larger packets are more efficient.
• Path MTU is very important!

MTU

More Ethernet

• Ethernet II has frame type (positions C and D) > 0x05EE

• SNAP, Simple Network Attachment Protocol
  • DSAP/SSAP, Destination/Source Service Access Point, match; for SNAP, both are 0xAA
  • bytes E and F (start of "data") are 0xAAAA
• 802.3 raw (Netware)
  • bytes E and F are 0xFFFF because header checksum not implemented
• 802.2
  • the rest
• 802.3 raw and 802.2 are not well-behaved, not welcome

More Ethernet

• SNAP
• 802.3 raw / Netware (bad netizen)
• 802.2 (bad netizen)

IP packet

IP packet

• 4 bits - version 4
• 4 bits - header length
• 8 bits - type of service
  • 3 bits - precedence
  • 4 bits - service (at most one can be set)
    • minimum delay, maximum throughput, most reliable, least cost
  • 1 bit - 0/unused

IP packet

• 16 bits - total length of packet
• 16 bits - identification
• 3 bits - IP flags
  • 0 - reserved
  • DF - don't fragment
  • MF - more fragments (0 for last fragment)

IP packet

• 13 bits - fragment offset
• 8 bits - TTL
• 8 bits - protocol
• 16 bits - header checksum

IP packet

• 32 bits - source IP address
• 32 bits - destination IP address
• 24 bits - IP options, if any
• 8 bits - padding (if needed)
• minimum and most common IP header is 20 octets

IP Addresses

• 32 bits, dotted decimal
• Class A (0-127), 8 bits network, 24 bits host
• Class B (128-191), 16 bits each
• Class C (192-223), 24 bits network, 8 bits host
• Class D (224-239), multicast
• Class E (240-255), reserved

Netmask

• Class A 255.0.0.0, Class B 255.255.0.0, Class C 255.255.255.0
• CIDR
• private addresses from RFC 1918
  • 10.0.0.0/8 or 10.0.0.0 to 10.255.255.255
  • 172.16.0.0/12 or 172.16.0.0 to 172.31.255.255
  • 192.168.0.0/16 or 192.168.0.0 to 192.168.255.255

Netmask

• You apply your netmask to your IP address, and the result tells you what addresses are local to you.

• How to mask an IP address

ARP

• local delivery on LAN needs MAC address
• ARP maps Layer 3 address to Layer 2 address, usually IP to MAC
• RARP goes the other way
• RFC 826

ARP packet

ARP packet header

• 6 octets - Ethernet destination address, all 1s, broadcast (reply is unicast)
• 6 octets - Ethernet source address
• 2 octets - frame type, 0x0806 for ARP

ARP packet, request or reply

• 2 octets - hardware type, 1 for Ethernet
• 2 octets - protocol type, 0x0800 for IP, same as Ethernet header
• 1 octet - hardware address size (6 octets)
• 1 octet - protocol address size (4 octets)
• 2 octets - op field
  • 1: ARP request
  • 2: ARP reply
  • 3: RARP request
  • 4: RARP reply

More about ARP

• When a station hears an ARP for itself, it stores the sender's MAC in its ARP table because it assumes communication may follow.
• proxy ARP - a router answers on behalf of one of its networks; promiscuous ARP, ARP hack
• gratuitous ARP - ARP for self to make sure IP address is available, usually when configuring NIC at boot
• RARP, manually configured, is required for a diskless end station to get an IP when it boots (RFC 903). The Ethernet frame type is 0x8035 but otherwise what you expect. The RARP server must be in the same broadcast domain because routers won't forward L2 broadcasts.

Transmission Process: Before

Pre-Transmission Process: Local

Pre-Transmission Process: Wide Area

Reading

• Steinberg, Steve G. "Netheads vs. Bellheads," Wired. Issue 4.10. October 1996. link
  • The updated version is CellHeads and AirHeads. PPT or PDF
• Boggs, David R., Jeffrey C. Mogul, and Christopher A. Kent. "Measured Capacity of an Ethernet: Myths and Reality," Proceedings of the SIGCOMM 88 Symposium on Communications Architectures and Protocols, ACM SIGCOMM, Stanford, California, August 1988. Available via citeseer: link
• Leland, Will E., Murad S. Taqqu, Walter Willinger, Daniel V. Wilson. "On the Self-Similar Nature of Ethernet Traffic," Proc. SIGCOM93, 1993, San Francisco, California, p. 183-193. Available via citeseer: link

Light Reading

• Browse Charles Spurgeon's Ethernet site to supplement class coverage of Ethernet if you want.

Topical Books

• Barabási, Albert-László. Linked: The New Science of Networks. Cambridge, MA: Perseus Publishing, 2002.
  • Very enjoyable reading.
• Comer, Douglas E. Internetworking with TCP/IP: Principles, Protocols, and Architecture. Upper Saddle River, NJ: Prentice Hall, 1995.
  • Classic networking text.
• Perlman, Radia. Interconnections: Bridges, Routers, Switches, and Internetworking Protocols. Reading, MA: Addison-Wesley, 2000.
  • Another classic networking text.
• Stevens, W. Richard. TCP/IP Illustrated, Volume 1. Reading, MA: Addison-Wesley. 1999.
  • Detailed networking text.

Homework

• homework for this class, due next class