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Packet fragmentation through a router interface

how does fragmentations works when sending a large file (video or data) over 1500 bytes? does a router fragment the packets? Is it safe to increase the default MTU over 1500 bytes?

Franco

  • Application Networking
1 REPLY
Cisco Employee

Re: Packet fragmentation through a router interface

Franco,

we would need to review the complete ip/tcp book to explain this in detail.

Basically, your application takes a video of size X when sending it, the tcp stack create multiple packets of a size equal to min(MSS,MTU).

The MTU of a client/server is usually 1500 since those are attached to ethernet network.

So the clients and servers are the ones that slice the data into 1500 bytes packets.

Changing the mtu on a router won't change anything.

The router will not reassemble the packets because it has a bigger mtu.

All it does is fragments when the packet that comes has a size bigger than mtu.

But again, since clients and servers are also using a mtu of 1500, there is normally no need to fragment.

Honestly, you should not play with those parameters unless you understand clearly the mechanism involved in TCP/IP.

Gilles.

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