A Gentle Introduction to MTU and MSS

What

MTU
- Maximum Transmission Unit
- maximum payload length
- defined in datalink layer (layer 2)

en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500

MSS
- Maximum (TCP) Segment Size
- Used in TCP
- MTU - (TCP header + IP header) = MSS

Why: frame can not be infinitely large (hard to buffer, process and determine timeout, high loss penalty).
Larger MTU is associated with reduced overhead. Smaller MTU values can reduce network delay.
- Large MTU: packet carries more payload, while protocol overheads, such as headers or underlying per-packet delays, remain fixed. Overall lower overhead.
- Small MTU: improved network Congestion Control in layer 2, because it is less likely to retransmit.
Why 1500: historical reason / tradeoff / compatibility issue with middle-box.
- What if packet size > 1500: mostly dropped
- When will packet size > 1500: in VPN or overlay network such as VXLan

How TCP (layer 4) knows the size of MTU (layer 2)
- How
How to guarantee the packet size is no larger than MTU
- breaks packets into smaller pieces
  - For TCP (layer 4): MSS Announcement
  - For UDP (layer 4): It doesn’t care, packet size is handled by the protocol (such as QUIC, DNS). Leaky abstraction.
  - For IP (layer 2): Fragmentation (generally a bad thing)