How is the TCP MSS of an AR interface changed


The TCP MSS specifies the maximum length of TCP packets that can be received by a device. When setting up a TCP connection, the local and peer devices negotiate an MSS value to determine the maximum data length of TCP packets. If the length of TCP packets sent from the peer device exceeds the MSS value, the packets are fragmented. To prevent TCP packets from being fragmented, you must configure a proper MSS based on the maximum transmission unit (MTU). The MTU determines whether IP packets will be fragmented. If the size of an IP packet exceeds the MTU, the IP packet will be fragmented. To ensure that a complete packet is transmitted properly, the MSS value plus all the header lengths (TCP header and IP header) cannot exceed the MTU. For example, if the default MTU value of an Ethernet interface is 1500 bytes, to ensure that packets are not fragmented, the maximum MSS value can be 1460 bytes. 1460 = 1500 �?20 (TCP header) �?20 (IP header) You are advised to set the MSS value to 1200 bytes.
An improper TCP MSS value may lead to slow network access. To set the TCP MSS, run the following commands:
[Huawei] interface gigabitethernet1/0/0
[Huawei-GigabitEthernet1/0/0] tcp adjust-mss 1200

Other related questions:
Does a change in the MSTP status of an AR router interface affect the MSTP status of other interfaces in the same MST region
If the MSTP status of an interface changes, the device will recalculate the status of all other interfaces in the MST region.

How is the MTU of an AR interface changed
The network layer limits the size of data frames. When a network layer device receives an IP packet, it determines the outbound interface and obtains the MTU configured on the interface. The IP layer then compares the MTU with the packet length. If the packet length is longer than the MTU, the device fragments the IP packet. Each fragment has the smaller or equal size as the MTU. If forcible unfragmentation is configured, some packets may be discarded when transmitted at the IP layer. Therefore, large packets must be forcibly fragmented to prevent packet loss on the network. The mtu command is used to set the size of a fragment. The MTU value can be configured for Layer 3 interfaces only. The commands are as follows: [Huawei] interface gigabitethernet1/0/0 [Huawei-GigabitEthernet1/0/0] mtu 1200 [Huawei-GigabitEthernet1/0/0] restart

Why do I need to consider the interface MTU when setting the MSS of TCP packets
The maximum segment size (MSS) is negotiated during TCP connection setup. The MSS determines the maximum length of a TCP packet. Some upper-layer applications such as HTTP reset the Don't fragment (DF) field of IP packets to prevent TCP packets from being fragmented. If the DF field is reset and the interface MTU is smaller than the MSS, the router discards TCP packets because TCP packets cannot be fragmented. A TCP packet has the TCP header and IP header; therefore, the MSS value plus all the header lengths cannot exceed the MTU. The MTUs supported by Ethernet and PPPoE are 1500 bytes and 1492 bytes respectively. You are advised to set the MSS to 1200 bytes. If the interface MTU is changed or encapsulation packets of some special applications cannot be fragmented in PPPoE, L3VPN, and IPSec scenarios, note the MSS setting.

How to determine the interface address used to establish a TCP connection
By default, the LSR ID is used to establish a TCP connection. If the mpls ldp transport-address command is configured in the interface view, the address of the specified interface is used to establish a TCP connection.

How to check the LDP TCP connections and determine whether TCP connections are normal
The port number for LDP-dedicated TCP connections is 646. Run the display tcp status command. All connections with port number of 646 are LDP TCP connections. In most cases, the server has a Listening port and one or more Established connections. The local port number is 646, and the remote port number is a randomly allocated five-digit numeral. The command output is displayed as follows: display tcp status TCPCB Tid/Soid Local Add:port Foreign Add:port VPNID State 0d3bda5c 32 /1 14849 Listening 0b6d91cc 23 /2 0 Established

If you have more questions, you can seek help from following ways:
To iKnow To Live Chat
Scroll to top