TLDR;
This video explains the difference between Maximum Transmission Unit (MTU) and Maximum Segment Size (MSS) in networking. MTU refers to the maximum size of a packet that can be transmitted over a network interface, while MSS is a TCP option that specifies the maximum amount of data a host is willing to receive in a single TCP segment.
- MTU is a network-level setting that defines the maximum packet size for Ethernet frames or IP packets.
- MSS is a TCP-level option negotiated during the TCP handshake to limit the amount of data a host receives in each segment.
- MTU is configured on network interfaces, while MSS is negotiated between TCP endpoints.
Introduction to MTU [0:00]
The video starts by introducing the concept of MTU, explaining that it defines the maximum size of data that can be transmitted over a network interface. It specifies the largest amount of encapsulated data within an Ethernet frame. MTU is set per interface on network devices like switches and routers, and it's not negotiated between interfaces.
Ethernet MTU and IP MTU [0:17]
The discussion covers Ethernet MTU, which is typically what's being referred to when discussing MTU in general. The video also touches on IP MTU, which can be adjusted based on the protocols in use, such as GRE tunnels, to accommodate additional header overhead without exceeding the Ethernet MTU. An example of MTU mismatch is given, where one router has an MTU of 9000 bytes while the other has a different setting.
MTU vs MSS: TCP-Level [2:49]
The video transitions to explaining MSS, which is a TCP-level setting established during the three-way handshake. MSS informs the sender of the maximum amount of data the receiver wants to receive in each TCP segment, not including TCP, IP, or Ethernet headers. A common MSS value is 1460 bytes, which, when combined with 20-byte TCP and IP headers, results in a total packet size of 1500 bytes.
MSS Adjustment and Packet Size [3:49]
The explanation details how MSS can be adjusted by network devices, such as routers, to accommodate additional header overhead from tunnels or other protocols. For example, a router might reduce the MSS from 1460 to 1432 to allow room for extra headers, ensuring the total packet size remains within acceptable limits.
Wireshark Demonstration of MSS [4:53]
The video uses Wireshark to demonstrate the MSS option in a TCP handshake. By examining the SYN packet, the MSS value is found in the options field, indicating the maximum segment size the client wants to receive. The SYN-ACK packet from the server may also include an MSS value, potentially adjusted by network devices along the path. If the MSS option is absent in the SYN packet, the default value of 536 bytes is used.
MTU and MSS Recap [6:14]
The video concludes by summarizing the key differences between MTU and MSS. MTU is a network-level setting that defines the maximum packet size for Ethernet frames or IP packets, while MSS is a TCP-level option negotiated during the TCP handshake to limit the amount of data a host receives in each segment.