# HTTP connection problem with mtu>1460

## dE_logics

I borrowed a modem and router from a friend until I get a new one. It negotiates connection over PPP.

I determined the MTU to be 1492 (1500-8, 8 bytes being PPP header). I can ping packages with ICMP payload 1464 bytes with don't frag bit set.

However if I set the MTU to be anything over 1460, establishing HTTP connection fails.

The only thing that can go wrong with this is TCP MSS size which must be such that it utilizes the full 1492 bytes. Once such a packet is received or sent, the modem drops the packet resulting in a timeout establishing the HTTP connection (TCP connection is established).

What's your opinion?

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## chithanh

It is possible that your ISP encapsulates your traffic further, resulting in even lower mtu. ICMP path mtu discovery will automatically determine the highest possible mtu (unless some firewall along the path blocks ICMP).

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## dE_logics

 *chithanh wrote:*   

> It is possible that your ISP encapsulates your traffic further, resulting in even lower mtu. ICMP path mtu discovery will automatically determine the highest possible mtu (unless some firewall along the path blocks ICMP).

 

But TCP traffic only?

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