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MTU / MSS Calculator

Compute the effective MSS from the MTU: subtract the IP header, the TCP header and the overhead of every encapsulation you tick. You will see the byte-by-byte breakdown (MTU − each component).

IP version
TCP options

The base TCP header is 20 B. With timestamps (RFC 7323), common on Linux, it adds 12 B.

Encapsulations

Each MPLS label adds 4 B.

ESP tunnel varies with the cipher (outer IP + ESP + IV + padding + ICV). Typical ~73 B; adjust it to your suite.

redzilla.cl — mtu 
 
  
 
 
Effective MSS
 
—
 
maximum TCP payload
 
 
 
Effective MTU
 
—
 
after encapsulation
 
 
 
Total overhead
 
—
 
IP + TCP + encap.
 
 
  
 
 MSS —  
 
 Eff. MTU —  
 
 clamp —  
 
  
 
Byte-by-byte breakdown
 
 
  
ComponentBytesRunning total
   
 
 
 
 How it is calculated · MTU, MSS and overhead 
 
 1. The MTU is the largest IP
            packet that fits in a frame. The MSS is the largest block of
            TCP data per segment.

 
 2. MSS = MTU − IP header − TCP header − encapsulation overhead.

 
 3. Headers: IPv4 = 20 B,
IPv6 = 40 B, TCP = 20 B (+12 B with timestamps).

 
 4. Encapsulation (B): VLAN 4, QinQ 8,
            PPPoE 8, MPLS 4/label, GRE 24,
            IPsec ESP tunnel ~73, VXLAN 50, WireGuard 60.

 
 5. Example: 1500 − 20 − 20 = 1460;
            with PPPoE 1500 − 8 − 20 − 20 = 1452.

 
 
 
 
 
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