Every network link has a Maximum Transmission Unit (MTU) — the largest packet it can carry. Ethernet’s standard MTU is 1500 bytes. If an IP packet is larger than the next hop’s MTU, it must be fragmented.
MTU (Maximum Transmission Unit)
The maximum size of a Layer 3 (IP) packet that can be transmitted over a specific link.
| Link type | MTU |
|---|---|
| Ethernet | 1500 bytes |
| WiFi (802.11) | 2304 bytes |
| PPPoE | 1492 bytes |
| Loopback | 65536 bytes |
| Jumbo frames (Ethernet) | 9000 bytes |
IP Fragmentation
When a packet exceeds the MTU of the next link, the router fragments it:
- The original packet is split into smaller fragments
- Each fragment has its own IP header (with the same ID field)
- The More Fragments (MF) flag is set on all fragments except the last
- The Fragment Offset field tells the receiver where each fragment belongs
Problems with fragmentation: If any fragment is lost, the entire original packet must be retransmitted. It also increases CPU overhead on routers.
MSS (Maximum Segment Size)
MSS is the maximum payload size that TCP will send, calculated to avoid fragmentation.
MSS = MTU - IP header (20 bytes) - TCP header (20 bytes)
= 1500 - 20 - 20 = 1460 bytesTCP negotiates MSS during the handshake. If the path MTU changes, Path MTU Discovery (PMTUD) detects this using ICMP “Fragmentation Needed” messages and reduces the MSS.
Why This Matters in Interviews
Fragmentation is inefficient. Modern networks avoid it by:
- Using TCP’s MSS to send segments that fit the MTU
- Path MTU Discovery to find the minimum MTU along the path
- Using UDP applications that limit datagram sizes to ~1400 bytes
Q: What is MTU and why does it matter?
A: Maximum Transmission Unit — the largest packet a link can carry. If a packet exceeds the MTU, it’s fragmented (split) into smaller pieces. Ethernet’s MTU is 1500 bytes. Path MTU is the minimum MTU along the entire route.
Q: What is the difference between MTU and MSS?
A: MTU is the Layer 3 maximum packet size (IP header + payload). MSS is the Layer 4 maximum payload size (TCP data only). MSS = MTU - 20 (IP header) - 20 (TCP header) = 1460 bytes for standard Ethernet.
Q: How does IP fragmentation work?
A: When a packet is larger than the outgoing link’s MTU, the router splits it into fragments. Each fragment has its own IP header with the same ID, a fragment offset, and an MF flag (set on all but the last). The receiver reassembles them.
Q: Why should we avoid IP fragmentation?
A: (1) If one fragment is lost, the entire original packet is dropped. (2) Fragmenting/defragmenting uses CPU on routers and receivers. (3) Firewalls often drop fragments. TCP avoids this by using MSS and Path MTU Discovery.
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