MTU & MSS in networking

MTU (Maximum Transmission Unit) refers to the largest size of a data packet that can be transmitted over a network, including headers and payload. MSS (Maximum Segment Size) is the largest size of the payload that a TCP segment can carry, excluding TCP and IP headers. In essence, MSS is derived from MTU by subtracting the size of the TCP and IP headers.

MTU (Maximum Transmission Unit):

Definition: The maximum size of a data packet (including headers and payload) that can be transmitted over a specific network link.

Layer: Data Link Layer.

Headers: Includes IP and link-layer headers.

Example: A common Ethernet MTU is 1500 bytes, which includes 1460 bytes of payload and 20 bytes each for TCP and IP headers.

MSS (Maximum Segment Size):

Definition: The largest size of the data payload that a TCP segment can carry, excluding TCP and IP headers.

Layer: Transport Layer.

Headers: Excludes TCP and IP headers.
Example: With a 1500 byte MTU, the MSS would be 1460 bytes (1500 - 20 (TCP) - 20 (IP) = 1460).

Key Differences:

Scope:

MTU applies to the entire packet, including headers and payload. MSS only applies to the payload within a TCP segment.

Purpose:
MTU defines the maximum size of a frame/packet that can be transmitted. MSS defines the largest data payload that can be carried in a TCP segment.

Usage:
Both are important for efficient data communication and to prevent fragmentation.

Relationship:
MSS is calculated by subtracting the size of the TCP and IP headers from the MTU.

In simpler terms:
Think of MTU as the size of the box you can put your package in, including the box itself. MSS is the size of the package you can put inside the box without the box breaking.