How TCP Works - MTU vs MSS

It is possible view MTU value on Wireshark?

Thanks! I was confused that the same word "MTU" was used on different layers. Now I got it :)

router can change mss？😂

Hi, I'm reporting a problem I've had in the past. An application is running at 1400 MTU. There was a problem with application access, I pulled the interface to 1400 MTU on the firewall. App access has been improved. Why did we have access problems when the application was 1400 and interface 1500? Will it not cause different access problems when you reduce the interface to 1400?

Thank you for explanation. :)

Hi Chris! Appreciate your work! It would be great to add in video explanation about how work adjusts MSS in network devices. I can provide you with simple gns3 topology with c7200 routers and an example of how routers change the mss value

hey, I got a question.
The syn,ack from the server to the host -- is that the MSS negotiated between the two devices ? or is that the MSS on server's side (MSS of server) ??. Please answer.

Hi Chris,
Thanks for sharing the knowledge, I wanted to one thing.
When I tried to change the MTU size to 9014bytes from 1500 bytes, am unable to see that change. I tried to change in configure option of particular ethernet interface.

Please help me with this issue

Could you share the trace file with us Chris ?

Very good explain 👌👌

Thanks Chris, your videos really helped me clear my interview for Akamai, really appreciate your contribution.

Hi Chris,

First of all thank you for these TCP amazing explanation. I never knew this much TCP does and this series of video made me to understand it deeper.

Could you please explain why MSS default value is 536 bytes?

Nice that was extremely helpfull.

Another great video thank you.

Amazing video Chris very informative! I never knew knew about MSS . Keep them coming please.

Some serious stuff is explained in the easiest manner!! Thank you.

Wow Chris, thank you so much, this has helped me so much. Looking forward to learning more from you. Now getting on the TCP with you and David.

Well explained. Thanks Guru🙏

you have course in Udemy ?

Thank you . Great explanation

gracias por el video..!,saludos.

Assume we have a host and 3 routers connection serially with different mtu. How does the tcp sender and receiver calculates mss value

Good video about Ethernet MTU and L3 MTU and TCP MSS! Thanks!

Thanks for this clear and concise break down between TCP MSS and Network MTU

Is there a concept like L2 MTU and L3 MTU? What I heard is L2 MTU is 1500Bytes + Ethernet header where as L3 MTU is 1500Bytes(Payload+TCP H+IP H). I always thought ethernet MTU is how much of data that can go inside an ethernet header(excluding the ethernet header). Can you correct me?

Thanks, Chris. Your videos help me a lot!!

Hi Chris. Can u explain the difference between Window Size Value and and MSS. Not able to find any tutorial for the same.

Amazing Chris

My understanding is that the MSS value is 'announced' and not 'negotiated' during the TCP handshake. I have seen material over the internet that will tell you that the 'lower' of the two (client and server) MSS values will be used. I don't believe its correct. Chris, would you clarify? BTW, your videos are just amazing

I have a doubt Chris. As per your explanation, MSS is set during the 3 way handshake. So the data being segmented would not cross the MSS value. When does the IP fragmentation come into play then?

Was told to watch this guys videos while reading my books for uni. Now I am watching this guy simply out of enjoyment lmao... 😄👍

Thank you!! Great!

Chris, Thank you for all the educational videos that you post. I have learnt so much by watching them.

Thanks chris for the vidio , we hope the plalylist include new vidio for fragmentation process and its details, and what the offset number refers to ? , how pcap will be different at the client before fragmentation and at the server side after fragmentation happend in between , and is that fragmentation must lead to slowness ? How can i decide ?

At about 1' it is only true for IPv4. When using IPv6, there can be no fragmentation along the way. The MTU must be set no just with the other interface locally, but along the whole path. (For the 0.01% that already use IPv6 out there ...).
Excellent videos by the way, I am watching the whole series backwards !

Thank you for this great content!

Great video ! I wish you could add some throughput and bandwidth troubleshooting videos as well.

Hi Chris,

I am getting into this and it is quite a lot of details if I try to put all things together. Nevertheless, can you please address one question?

Suppose interface MTU (L2) is set to 9000. Now, if MSS is having a value of say 1460, does it make any sense/advantage of having the interface MTU very large (i.e. 9000)? Segments are naturally going to be smaller because of limited MSS value right? I am now puzzled on how the concepts fit in. Does the 9000 L2 MTU have any significance or advantage in this case? Maybe yes - because, so many TCP segments would fit in to a single L3 and L2 PDU?

I hope my question would make sense to you.

Hi Chris,

So does this imply that MSS must match on both ends? And, the negotiation "should" be such that they should settle up to the lowest MSS value between the two?

I have a question:
What i learned recently was, that the network layer (i am talking about the IP layer 3, which i am not sure if that is the correct english term), would not "look into" the encapsulated headers (layer 4 and 5).
You however mentioned in your video that some network device along the way could have changed the MSS.
I am a little confused and would really appreciate if you answered my question.
Thanks in advance and thank you so much for the great videos on this topic. really appropriate speed and great visualisation!

Thank you Chris for this video. It really helps me to understand clearly about MTU and MSS.

Hi,
Assuming only one interface and a P2P connection b/w two devices.
If MTU size is configured to 9000, then changing the MSS to more than 1460 would make sense ??

The most confusing part about TCP and MSS is about Options in the IP layer and TCP. From my understanding, you only 'count' fixed header size of both TCP and ID (20 each) and then trim off any data from the TCP segment, by the byte amount of Options of IP and TCP (that are used). Right?
For example, if I have MSS of 1460 and IP without options and have one TCP option (4 bytes, or smaller but with added padding, for 32bit rule) then I can't really send 1460 but 1456 of actual data?

So tcp window and mss is same!?

Good