데이터 링크 계층 (Data-Link Layer)

Data Link Layer

1. Nodes and Links

• Communication at the data link is note to note

2. Services

• node is responsible for delivering a datagram to the next node
• each node may be using different protocol with different frame format
  • each node need to encapsulate and decapsulate

3. Framing

• Definitely, The first service provided by data-link layer is framing (encapsulate, decapsulate)

4. Flow Control

• Definitely, we cannot have unlimited buffer size at the receiving side. We have two choices
• The first choice is to let the receiving data-link layer drop the frame if its buffer is full
• The second choice is to let the receiving data-link layer send a feedback to sending data-link layer to ask it to stop or slow down

5. Error Control

• A frame is susceptible to error
• After detected, it needs to be either corrected at the receiver node or discarded and retransmitted by the sending node

6. Congestion Control

• Most date-link-layer protocols do not directly use a congestion control to alleviate congestion
• Ingeneral, congestion control is considered an Issue in the network layer or the transport layer. because of its end to end nature

Three Types of addresses

• Unicast Address : one-to-one commumcatlon
• Mulmicast Address : one-to-many communlcatlon
• Broadcast Address : one-to-all communlcatlon

ARP (Address Resolution Protocol)

• The IP address of the next node is not helpful in movlng a frame through a link; we need the link-layer address of the next node
• Anytime host or router needs to find the link-layer address of another host or router in its network, sends an ARP packet.
• The request packet includes the link-layer and lP addresses of the sender and the IP address of the receiver (broadcast)
• The response packet contains the recipient's IP and link-layer addresses (unicast)