[j-nsp] TCP-MSS adjust does not work on MPC10E

[Louis Ho]

Hi Olivier,


Yes I using Enhanced MX SCB 3


#run show forwarding-options hyper-mode
Current mode: hyper mode
Configured mode: hyper mode

user-louis at MX960.OL.lax.rnbn.net.RE0# show forwarding-options hyper-mode

{master}[edit]

Exactly it is running in hyper mode by default


I'll try to disable hyper mode If working fine, it means that hyper mode does not support TCP-MSS adjust. ya It's not documented.


I will share it if I got any results later.



Cheers



On Apr 17, 2021, at 12:52 AM, Olivier Benghozi <olivier.benghozi at wifirst.fr<mailto:olivier.benghozi at wifirst.fr>> wrote:

MPC10. SCBE3? SCBE3 => hyper-mode by default.
As there's no hypermode firmware in MPC-16x10, only the MPC10 would really be in hyper mode, and therefore the firmware loaded in the MPC10 would be the faster hypermode one, while the firmware in the 16x10 will still be the standard one with all the features.

I wonder if tcp-mss is expected to work in Hyper-Mode, in fact. It's not documented... but this would explain a difference of behaviour between both cards.

You should try:

show forwarding-options hyper-mode


Of course if you're in standard mode and/or not using SCBE3, these thinkings are to be thrown away :)


Le 16 avr. 2021 à 05:53, Louis Ho via juniper-nsp <juniper-nsp at puck.nether.net<mailto:juniper-nsp at puck.nether.net>> a écrit :


De: Louis Ho <louis at rootglobal.com<mailto:louis at rootglobal.com>>
Objet: TCP-MSS adjust does not work on MPC10E
Date: 16 avril 2021 à 05:53:13 UTC+2
À: "juniper-nsp at puck.nether.net<mailto:juniper-nsp at puck.nether.net>" <juniper-nsp at puck.nether.net<mailto:juniper-nsp at puck.nether.net>>




Does MPC10E support changing TCP-MSS on transit packets?



Today I need to set TCP-MSS to 1400,


My topological map


Server A > EX4300 A > MX960 A xe-4/0/0> > EX4300 B > server B

Serve A > EX4300 A > MX960 A et-5/0/0 > QFX-5110  > EX4300 C > server C


There are two line cards on my MX960 router, MPC 3D 16x 10GE and MPC10E 3D MRATE-10xQSFPP

I have configured tcp-mss on the two MPC interfaces, but the mss will only be adjusted if the traffic passes through xe-4/0/0,
but the traffic through et-5/0/0 does not work, the mss still keep at 1460.




show interfaces xe-4/0/0 | match mss
tcp-mss 1400;

tcpdump by serverB
https: Flags [S], seq 3239034622, win 29200, options [mss 1400,sackOK,TS


show interfaces et-5/0/0 | match mss
tcp-mss 1400;

tcpdump by serverC
Flags [S], seq 3355130726, win 29200, options [mss 1460,sackOK,TS val



Cheers


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