¿Cuál es el valor de MSS?

What is the MSS value?

The MSS parameter is a part of the options filed in the TCP initial handshake that specifies the largest amount of data that a TCP speaker can receive in a single TCP segment. Each direction of TCP traffic uses its own MSS value, as this is a receiver-specified value.

Following flow charts demonstrates how MSS is determined based on configuration and destination IP. Example of SYN Packets with MSS: When connection is established, each end announces the MSS it expects to receive. 0

The MSS announcement (often mistakenly called a negotiation) is sent during the three-way handshake by both sides, saying: “I can accept TCP segments up to size x”. The size (x) may be larger or smaller than the default. The MSS can be used completely independently in each direction of data flow.

To find the optimum MSS/MTU value of the network, do the ‘ping test’ to find the MTU and then reduce 40 from that value and set that as tcp-mss in the firewall policy. If the packet needs to be fragmented but the DF bit set, it means that the MTU is less than 1500 bytes.

The MTU (Maximum Transmission Unit) states how big a single packet can be. Since the backbone of the internet is now mostly made up of ethernet links, the de facto maximum size of a packet is now unofficially set to 1500 bytes to avoid packets being fragmented down links.

Generally, if your MTU is too large for the connection, your computer will experience packet loss or dropping internet connection. You can start from 1472 until you can reach the exact packet size which will not result to the prompt “Packet needs to be fragmented but DF set.” You may decrease the value by 10.

The maximum size of frames is called the Maximum Transmission Unit (MTU). Historically, Ethernet has a maximum frame size of 1500 bytes. An Ethernet packet larger than 1500 bytes is called a jumbo frame. An Ethernet frame uses a fixed-size header.

In the TCP Maximum Segment Size (MSS) field, type your MSS value. The range is 512 to 1500 and the default value is 1024. If you enter a value out of the range, you will receive an out-of-range error.

Contrary to popular belief, the MSS value is not negotiated between hosts. The sending host is required to limit the size of data in a single TCP segment to a value less than or equal to the MSS reported by the receiving host .”

Use the ip tcp adjust-mss command in interface configuration mode to specify the MSS value on the intermediate router of the SYN packets to avoid truncation. When a host (usually a PC) initiates a TCP session with a server, it negotiates the IP segment size by using the MSS option field in the TCP SYN packet.

Generally speaking, when you are talking to devices on your own LAN the MTU will be around 1500 bytes and the internet runs almost universally on 1500 as well. For example, 802.11 (better known as WiFi) has a MTU of 2304 bytes, or if your network is using FDDI then you have a MTU around 4352 bytes.

Following flow charts demonstrates how MSS is determined based on configuration and destination IP. When connection is established, each end announces the MSS it expects to receive. An MSS option can only appear in SYN packet from each end.

As a result the network devices have the option to rewrite the value of TCP MSS packets that are processed through them. For example, in a Cisco Router the command “ip tcp mss-adjust 1436” in the interface level will rewrite the value of the TCP MSS of any SYN packet that will go via this interface.

The ip tcp adjust-mss command helps prevent TCP sessions from being dropped by adjusting the MSS value of the TCP SYN packets. The ip tcp adjust-mss command is effective only for TCP connections passing through the router.

To adjust the maximum segment size (MSS) value of TCP synchronize/start (SYN) packets going through a router, use the ip tcp adjust-mss command in interface configuration mode. To return the MSS value to the default setting, use the no form of this command. Syntax Description. Maximum segment size, in bytes.