Error Control and Flow Control

Overview

Layer	Flow Control Example	Error Control Example	Scope
Data Link Layer (L2)	Yes ✅ Stop-and-Wait, Sliding Window	Yes ✅ Parity Bit, Checksum / CRC, ARQ (3 types)	Node-to-Node (Hop-to-Hop)
Transport Layer (L4)	Yes ✅ TCP Receive Window	Yes ✅ TCP Checksum + Retransmission	End-to-End

Important Difference ⭐

Data Link Layer

• Error Control:
  • Error Detection: CRC
  • Retransmission: ARQ
• Flow Control:
  • Stop-and-Wait, Sliding Window

Transport Layer

• Error Control:
  • Error Detection: TCP Checksum
  • Retransmission: Timeout + ACKs
• Flow Control:
  • TCP Sliding Window

Note:

• Error Detection in L2 can be done using Parity Bit, Checksum, or CRC.
• However, for GATE and interviews, associate: L2 Error Detection → CRC L4 Error Detection → TCP Checksum
• CRC is emphasized because it is widely used in practice (Ethernet, Wi-Fi) and is much better at detecting burst errors.

Data Link Layer (L2)

• Controls data transfer between two directly connected devices.
• Detects/corrects errors on a single link.
• Ensures one device doesn’t overwhelm the next device.

A ---- B ---- C

L2 Flow/Error Control:
A <-> B
B <-> C

If frame A→B is corrupted, B asks A to retransmit.

Data Link Layer (L2) Flow Control

Suppose:

A -------- B
(Sender)   (Receiver)

If A sends frames too fast and B’s buffer becomes full, flow control mechanisms like:

• Stop-and-Wait
• Sliding Window

make A slow down or wait for ACKs.

Data Link Layer (L2) Error Control

A sends:

Frame + CRC

• B recalculates CRC.
• If CRC mismatches, frame is considered corrupted.
• B discards it and requests retransmission using ARQ techniques:
  • Stop-and-Wait ARQ
  • Go-Back-N ARQ
  • Selective Repeat ARQ

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Transport Layer (L4)

• Controls data transfer between source and destination applications.
• End-to-end reliability.
• Ensures the sender does not overwhelm the receiver.
• Retransmits lost segments if needed.

A ---- B ---- C ---- D

L4 Flow/Error Control:
A <-----------------> D

Even if B and C forward correctly, if D misses data, A retransmits.

Memory Shortcut

• L2: “My neighbor got the frame correctly?”
• L4: “Did the final destination get the entire message correctly?”

Detailed

Transport Layer (L4) Flow Control

In TCP:

Client <---------> Server

The receiver advertises:

Window Size = 5000 bytes

The sender can send at most 5000 bytes without waiting.

If the receiver is busy:

Window Size = 0

The sender stops sending temporarily.

This is called TCP Flow Control using the sliding window mechanism.

Transport Layer (L4) Error Control

TCP sends:

Segment + TCP Checksum

If:

• Segment is corrupted, or
• Segment is lost, or
• ACK is not received before timeout,

then TCP retransmits the segment.

A -----> B   (Segment #3 lost)

A <----- B   (No ACK for #3)

Timeout occurs

A -----> B   (Retransmit #3)