Format.md

#Format

Link Layer

Ethernet Frame Format

• Source Ethernet (MAC) Address
• Destination Ethernet Address
• Frame Type: used to identify the payload
• CRC: used for error control

Ethernet Encapsulation (RFC 894)

• Dest. Addr., Src. Addr.: MAC addresses are 48 bit

• Type: Identifies the content of the data field (must ≥ Ox0600)

• CRC: Cyclic Redundancy Check

IEEE 802.2/802.3 Encapsulation

• Destination address, Source address: MAC addresses are 48 bit (displayed as 12 hexadecimal characters)
• Length: frame length in number of bytes (<Ox0600, 1,500 bytes → Ox05dc)
• DSAP, SSAP: always set to 0xaa
• Ctrl: set t o 3
• Org code: set to 0
• Type field: identifies the content of the data field
• CRC: cyclic redundancy check
• Total frame size: 64 bytes to 1518 bytes Overhead: 38 bytes including 12 bytes Inter Frame Gap (IFG)

DISCUSS How to distinguish IEEE 802.2/802.3 and RFC 894?

By the length and type field field（No.13 byte, No.14 byte）. 802.2/802.3 < 0x600, RFC 894 >= 600. 

IEEE 802.11 Frame Format

• More fields than other data-link protocols

• High overhead:

  • 30 byte header
  • Four Address fields: BSSID, Source Address (SA), Destination Address (DA), Receiving Station Address (RA), Transmitting station Address (TA) depend on Frame Control setting

• Different frame types for different tasks: – Some fields are not presented in all types of frames

IPv4 Package Format

• Version: current version is 4, new version is 6.

• IHL (Internet Header Length, 4 bits): Number of 32-bit words in the IP header

• Type of Service (TOS): contains 4 TOS bits, where each bit indicates a desired service.

  • Only one bit can be set! Not supported in all applications.

• Total Length: Number of bytes in the IP datagram (header+payload)

• Identification: unique identification of a datagram from a host. Incremented whenever a datagram is transmitted.

• Flags and Fragrment Offset: Associated with fragmentation

• Time To Live (TTL): specifies longest path before datagram is dropped.

  • Used to prevent infinite looping of packets

    Discuss: Who can drop the packets? Host? Router?

  • TTL field is set at sending host and is decremented by 1 at each router

  • If a router gets a datagram whose TTL is either 0 or 1, the router will drop the packet.

  • If a destination host gets a datagram whose TTL is larger than 0, the host will deliver the datagram to the higher layer.

• Protocol: Specifies the higher-layer protocol. Used for demultiplexing to higher layers.

  Higher Layer Protocol	value
  TCP	6
  ICMP	1
  UDP	17
  IGMP (???)	2

• Header Checksum: verifies correctness of header.

• Source and Destination Addresses: identify the interfaces on the sending and receiving hosts

• Options:

  • Security: indicates security and handling restrictions, ...
  • Record Route: each router that processes the packet adds its IP address to the header.
  • Timestamp: each router that processes the packet adds its IP address and time to the header.
  • (loose) Source Routing: specifies a list of routers that must be traversed.
  • (strict) Source Routing: specifies a list of the only routers that can be traversed.

• Padding: ensures that header ends on a 4-byte boundary

UDP Header Format

• Port Numbers identify sending and receiving applications (processes). The maximum value for a port number is 216-1= 65,535
• Message Length is between 8 bytes (i.e., data field can be empty) and 65,535 bytes (length of UDP header and data in bytes)
• Checksum is for UDP header and UDP data

UDP checksum

• Optional:

1. set all 0’s if not calculated
2. A calculated checksum can never be all 0’s.

• Computed using the UDP header, UDP data and a pseudo-header as below.
• All fields of pseudo-header are available in UDP layer

TCP Packet Format

PPP Encapsulation

• PPP frame format
  • Flag: mark the beginning and ending of a frame
  • Protocol: used to multiplex different protocol data
  • No addressing, only two end hosts.

ARP Packet Format

• 28 bytes long.

• An ARP request or ARP reply is encapsulated in an Ethernet frame.

  – The protocol type in Ethernet frame is set to 0x0806 for ARP messages.

• Hardware Type - Specifies a hardware interface type for which the sender requires a response, i.e. Ethernet (1) in our case

  DISCUSS: What's the usage of Hardware Type?

• Protocol type - Specifies the type of high-level protocol address the sender has supplied, ie. IP (Ox0800) in our case

  DISCUSS: What's the usage of Protocol Type?

• Hlen - Hardware address length.

• Plen - Protocol address length.

• Operation field specifies ARP request (1), ARP reply (2), RARP request (3), or RARP reply (4).

ICMP

Format

Types

ICMP Unreachable Error

ICMP Port Unreachable

Echo Request and Reply