What is MSS and MTU in networking?

As mentioned earlier, the MSS is like the MTU, but used with TCP at layer 4. Put simply, the MSS is the maximum size that the payload can be, after subtracting space for the IP, TCP, and other headers. So, if the MTU is 1500 bytes, and the IP and TCP headers are 20 bytes each, the MSS is 1460 bytes.

What is the difference between MTU and MRU?

On the other hand maximum receive unit (MRU) is the largest packet size that an interface can receive, so it’s an ingress interface parameter. In most of the cases MRU equals MTU but it’s not a requirement. You can configure different values for both MTU and MRU to achieve some benefits.

What is MSS in networking?

The maximum segment size (MSS) is a parameter of the options field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment.

What MTU means?

maximum transmission unit
A maximum transmission unit (MTU) is the largest packet or frame size, specified in octets (eight-bit bytes) that can be sent in a packet- or frame-based network such as the internet. The internet’s transmission control protocol (TCP) uses the MTU to determine the maximum size of each packet in any transmission.

What is MSS clamping?

TCP MSS clamping is a feature that sets the maximum segment size used by a TCP session. The way that it achieves this is during the TCP 3 way handshake, a server can set the MSS in the outgoing TCP SYN packets signalling the maximum segment size of the data packets that it can receive.

Why is PPPoE MTU 1492?

With PPPoE connections, the PPP and PPPoE header increases the frame size by 8 bytes, so we must lower the MTU to 1492. With the ethernet header added to this, we get a frame size of 1518 bytes. The network between the DSL multiplexer and the ISP aggregation router is ATM.

What is MTU clamping?

Essentially what MTU clamping is doing is manipulating the MSS field in the IP packets traversing the network. If you set it too large for an upstream network peer it won’t be able to process it and is forced to drop the packet (assuming “DF=1” (Don’t Fragment this packet= TRUE).

What is MSS clamping OpenWrt?

I found this video really helpful in explaining why wan firewall zones use MSS Clamping by default in OpenWrt. In short, it prevents round-trips to a remote server when possible by marking TCP packets with the MTU size of the local client when they go out of the router.