Networking datalink layer

The protocol used to transmit frames over the wire.
It sends network frames to MAC Addresses (media access control).

Tools

arp

nd

tcpdump

Components

routers

Protocols

ethernet

most commonly used protocol for the datalink layer

Terminology

routing schemes	broadcast, unicast, multicast, ...
duplex	If traffic can be sent/received at the same time
MTU	the largest allowed frame-size (datalink) on a network.

Configuration

vlan	using one NIC/cord to share multiple connections to separate networks (ex: WAN/LAN on firewall)

Troubleshooting

Error Types

Datalink Errors
frame errors	Frames received with an invalid checksum.
drops	Frames that were discarded by the netwk iface. unexpected VLAN tags, ipv6 packets when iface is not configured for ipv6.
overruns	Too much data received too quickly (before kernel can handle them). The buffer is full, so packets are dropped.
collisions	On half-duplex netwk, when 2x sides of a connection are talking at the same time.

Finding Errors

netstat -i

TX-     ## packets sent
RX-     ## packets received


*-OK   ## correctly recceived frames
*-ERR  ## incorrect packets received
*-DRP  ## packets dropped at this interface
*-OVR  ## packets this interface was unable to receive