MTU and MSS bug?

Question

Why do I get this error when I try to configure it like this 
 
 This is a standard configuration for PPPOE networks 
 
 &laquo; SpeedGuide.net TCP Analyzer Results &raquo; Tested on: 2019.08.06 07:14 IP address: 83.45.xxx.xxx Client OS/browser: Windows 10 (Firefox 68.0) TCP options string: 020405ac0103030801010402 MSS: 1452 MTU: 1492 TCP Window: 262656 (not multiple of MSS) RWIN Scaling: 8 bits (2^8=256) Unscaled RWIN : 1026 Recommended RWINs: 63888, 127776, 255552, 511104, 1022208 BDP limit (200ms): 10506kbps (1313KBytes/s) BDP limit (500ms): 4202kbps (525KBytes/s) MTU Discovery: ON TTL: 112 Timestamps: OFF SACKs: ON IP ToS: 00100000 (32) Precedence: 001 (priority) Delay: 0 (normal delay) Throughput: 0 (normal throughput) Reliability: 0 (normal reliability) Cost: 0 (normal cost) Check bit: 0 (correct) DSCP (DiffServ): CS1 001000 (8) - class 1 (RFC 2474). Similar forwarding behavior to the ToS Precedence field.

donald2612 · Accepted Answer

Keyur said: mss = tcp-mss which is actually = MTU - 40 (20 Byte for TCP + 20 Byte IP) 
 
 This is absolutely correct, but the PPPOE-header of 8 bytes gets subtracted from both MTU and MSS, thus still leaving a difference of 40. 48 makes no sense to me at all.

LuCar Toni · Answer

You should open a Bug with the Sophos Support to get this reported.

Aditya Patel · Answer

Helo Everyone, 
 Allow me to clarify the confusion in this thread. 
 While dealing with MTU and MSS value of any normal interface type, such as Static and DHCP then the MTU default would be 1500 and MSS value would be 1500-40=1460 value. 
 This is different in PPPOE case, we would need an additional 8 byte PPPoE i..e PPP-Max-Payload field and needs an additional 8 bytes and truncates the Ethernet MTU to 1492 and the value for MSS is 1452. 
 
 Difference between Ethernet MTU and IP MTU. 
 Ethernet MTU. 
 The main difference is interface MTU defined max packet size supported by an interface, while IP MTU is used to set MTU size of IP PACKET. Each interface has a default maximum packet size or MTU size. This number generally defaults to the largest size possible for that interface type. 
 IP MTU A.K.A MRU. 
 To set the maximum transmission unit (MTU) size of IP packets sent on an interface. The minimum is 128 bytes; the maximum depends on the interface medium. 
 Changing the MTU value (with the MTU interface configuration command) can affect the IP MTU value. If the current IP MTU value is the same as the MTU value, and you change the MTU value, the IP MTU value will be modified automatically to match the new MTU. However, the reverse is not true; changing the IP MTU value has no effect on the value for the MTU command. 
 "The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a larger size than 1492. Since Ethernet has a maximum payload size of 1500 octets, the PPPoE header is 6 octets and the PPP Protocol ID is 2 octets, the PPP MTU MUST NOT be greater than 1492." -> Taken from RFC 2516 
 
 Calculation 
 
 1. Interface MTU - default MTU size for an interface: 
 For PPPOE connection Ethernet MTU = IP MTU + 8. 8 bytes for overhead 
 For Normal Connection Ethernet MTU = IP MTU. 
 Mss value = IP MTU - 40. 
 So the difference between ethernet MTU and MSS value is 48, not 40. 
 In the screenshot mentioned above, the Ethernet MTU was set as 1492, so we would need an additional 48 bytes for overhead.