This page has notes on various network protocols

VLAN

A virtual lan (VLAN) is a broadcast domain that is partitioned and isolated in a computer network at the data link layer. A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer.

The VLAN tag a 32-bit field is added after the Source MAC address and before the EtherType/Size field.

  • 16b: Tag protocol identifier set to 0x8100 (TPID)

The following 16b comprise the tag control information (TCI):

  • 3b: Priority code point (PCP) indicates class of service
  • 1b: drop eligible indicator (DEI) to allow dropping during congestion
  • 12b: VLAN Identifer (VID). values of 0x000 and 0xfff are reserved

References

IEEE 802.1Q

VXLAN (Virtual Extensible LAN)

VXLAN is a L2 in L3 tunneling protocol that has a VLAN-like encapsulation to encapsulate Ethernet frames in UDP packets (RFC 7348).

DHCP

DHCP lets a network element discover its IP address, among other parameters. A DHCP exchange consists of four steps (“DORA”) (RFC 2131):

  1. discovery by client
  2. offer address by server
  3. request address by client
  4. acknowledge by server

DHCP client in Linux

dhclient is the Ubuntu DHCP client daemon. Search for dhcp in /var/log

Address Resolution Protocol (ARP)

ARP lets a network element determine the Ethernet address that corresponds to an IP address (RFC 826).

An ARP exchange consists of two messages:

  1. query: sent to the data link broadcast address indicating a network layer address for which the transmitter needs the corresponding data link address
  2. response: response that contains the desired data link layer address.

ARP is carried in an Ethernet frame with protocol type 0x0806 and contains no IP header.

TODO

802.1AD