How TCP really works: MTU vs MSS

Wireshark TCP Deep Dive continues: You need to understand this - whats the difference between Maximum Transmission Unit (MTU) vs Maximum Segment Size (MSS).
// MENU //
00:00 ▶️ Coming Up
00:25 ▶️ Intro
00:32 ▶️ Chris introduction
00:47 ▶️ Topic: Maximum Segment Size (MSS)
01:27 ▶️ Explaining Maximum Transmission Unit (MTU)
08:42 ▶️ Interface layout
10:25 ▶️ David Bombal "War Story"
12:00 ▶️ Wireshark demo
13:26 ▶️ Increasing the MTU on your device for larger connections
16:27 ▶️ Difference between MTU and MSS
19:36 ▶️ Wireshark demo (cont'd)
24:58 ▶️ Using Path MTU Discovery
27:02 ▶️ Ping and Wireshark demo
33:32 ▶️ Cool trick for Mac system
35:08 ▶️ TCP/MSS Clamping
38:21 ▶️ Chris Greer "War Story"
51:09 ▶️ What happens if you can't capture a server
55:08 ▶️ MSS Adjustment commands
56:55 ▶️ Tunnel Path MTU Discovery
57:40 ▶️ Figuring out 1432
01:02:52 ▶️ Conclusion
01:04:48 ▶️ "Cool features" in Wireshark
Previous video: • How TCP really works /...
// Wireshark PCAP files //
MTU PCAP: github.com/packetpioneer/yout...
War Story PCAP Client: github.com/packetpioneer/yout...
War Story PCAP Server: github.com/packetpioneer/yout...
Special “Thumbs Up” and “Subscribe” PCAP: github.com/packetpioneer/yout...
// VLAD SOCIAL //
Twitter: / packet_vlad
PMTUD Blog: www.packettrain.net/2016/09/21...
Thanks Vladimir Gerasimov!
// GOOD READING //
Network Implications of PMTUD: www.ipspace.net/kb/Internet/P...
Path MTU Discovery: www.ipspace.net/kb/Internet/P...
Resolve IPv4 Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPsec: www.cisco.com/c/en/us/support...
Configuring TCP MSS Adjustment: www.cisco.com/c/en/us/td/docs...
Ethernet MTU and TCP MSS Adjustment Concept for PPPoE Connections: www.cisco.com/c/en/us/support...
// MY STUFF //
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// Chris SOCIAL //
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KZbin: / chrisgreer
Twitter: / packetpioneer
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#wireshark #tcp #mtu

[@imnainggolanpm2245]

7:55 is very IMPORTANT reminder. MTU is per hop, while MSS, since it is on TCP, it's end-to-end. Networking people (me too) sometimes jump the gun that when they pinged next hop and able to get 1500 bytes MTU (do not fragments), immediately assume it is the same treatment along the hops throughout the network. I remember dealing with MPLS backbone team and they assumed basically that same thing. UNTIL we went P routers by P routers pinged each hop and identified there was an issue where somehow the MTU on that hop was dropped down to ~400 bytes. Once the "expert" backbone team rerouted the traffic away from that backbone path, then the application can resume.

[@mikkio5371]

It was really an eye opener. I have read a book on ccnp by Bradley another author I respect so much. He actually recommended tunnels to be set ip mtu to 1400 and mss 1360. But it was until today .I know why .also we have other encapsulation like mpls ,gre that has their sizes that need to be put into consideration. Thanks Bombia and Chris .

[@EWCRC]

Yes, more content with Chris and Wireshark. The real world examples are great and really helps show the thought process for going through the capture. It is also great how you go back and forth and ask questions that we the viewer may ask.

[@ryanflint386]

Great video! I had a client running a Cisco 1100 in their home office - everything was working perfectly except for Netflix and Disney+ which seemed very strange. After a fair bit of investigation the issue turned out to be the MSS. They were on a home internet plan using pppoe so had to use IP TCP ADJUST-MSS 1452 command to account for the 8 byte overhead of pppoe.

[@gorge5412]

RF: Thank you for providing your own war story, complete with your winning battle command. Good on ya', mate! .

[@moneymac1114]

I like that you explained the battle and the winning command so if somebody has this problem in the field they can also use your strategy to fix it. God bless you

[@JaCkFlApPiNg]

David and Chris, you won't believe how many networking professionals have no understanding on MTU or MSS, so thank you so much for this! Please more on TCP and ideally the next with some focus on TTL (You touched on it in this video "My TTL started at 64 and it's now 54, so I'm going through 10 Routers") Great video, subscribed and liked and will check out Chris' KZbin channel

[@rusnakhraj7401]

This was a great video. It reminded me and issue that I was dealing with around 2 years ago with Aruba APs. Our client has setup like this. One local WLAN aruba controller on site Spain and one redundant controller which was on remote site in Sweden. Clients reported issue with slow wifi. I found out that clients that are connected to APs which are connected to remote controller experience slowness. Other users connected to APs which used local wlan controller had no issue. So we started live troubleshooting and see that ping was ok but there was real slowness when opening web page or doing skype call (user was green but with packet loss). We started to manipulate ping request with more bytes and then packet loss appeared. Later I found out that APs has set "do not fragment" bit by default and we did calculation with wan team that packet is too big. So on our side we turn on preemption which will move APs to local WLC if it is online and WAN team did their config to divide packets to smaller size. This was really great issue and I enjoyed it :) Btw. redundat remote wlc served for several small sites. Usually medium and big sites had 2 local WLCs but because of fact that some customer sites were too small they had setup with one local and one remote wlc.

[@WillKohut]

These videos answer the question: what if Robin Williams was a network engineering vlogger? In all seriousness though, great content; appreciate the clear and real-world explanations!

[@MrBitviper]

it's always a treat when chris comes over for one of these deep dive videos thanks David for the great content. much appreciated

[@zoranzasovski]

Great episode David and Chris. The way Chris explains the topics is just amazing! Can't wait for the sequel.

[@techlearner4806]

27:02 ▶ Ping and Wireshark demo. This part made my day. Best ever explanation I found on this concept. Thanks to you both 👍👍

[@davidbombal]

// MENU // 00:00 ▶ Coming Up 00:25 ▶ Intro 00:32 ▶ Chris introduction 00:47 ▶ Topic: Maximum Segment Size (MSS) 01:27 ▶ Explaining Maximum Transmission Unit (MTU) 08:42 ▶ Interface layout 10:25 ▶ David Bombal "War Story" 12:00 ▶ Wireshark demo 13:26 ▶ Increasing the MTU on your device for larger connections 16:27 ▶ Difference between MTU and MSS 19:36 ▶ Wireshark demo (cont'd) 24:58 ▶ Using Path MTU Discovery 27:02 ▶ Ping and Wireshark demo 33:32 ▶ Cool trick for Mac system 35:08 ▶ TCP/MSS Clamping 38:21 ▶ Chris Greer "War Story" 51:09 ▶ What happens if you can't capture a server 55:08 ▶ MSS Adjustment commands 56:55 ▶ Tunnel Path MTU Discovery 57:40 ▶ Figuring out 1432 01:02:52 ▶ Conclusion 01:04:48 ▶ "Cool features" in Wireshark Previous video: kzbin.info/www/bejne/qJ6piWSMaZ5-brc // Wireshark PCAP files // MTU PCAP: github.com/packetpioneer/youtube/blob/main/PMTUD.pcapng War Story PCAP Client: github.com/packetpioneer/youtube/blob/main/slowfile-clientside.pcapng War Story PCAP Server: github.com/packetpioneer/youtube/blob/main/slowfile-serverside.pcapng Special “Thumbs Up” and “Subscribe” PCAP: github.com/packetpioneer/youtube/blob/main/tcp-streamexport.pcapng // GOOD READING // Network Implications of PMTUD: www.ipspace.net/kb/Internet/PMTUD/30-network-implications.html Path MTU Discovery: www.ipspace.net/kb/Internet/PMTUD/20-mtu-discovery.html Resolve IPv4 Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPsec: www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html Configuring TCP MSS Adjustment: www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3650/software/release/16-9/configuration_guide/ip/b_169_ip_3650_cg/configuring_tcp_mss_adjustment.pdf Ethernet MTU and TCP MSS Adjustment Concept for PPPoE Connections: www.cisco.com/c/en/us/support/docs/ip/transmission-control-protocol-tcp/200932-Ethernet-MTU-and-TCP-MSS-Adjustment-Conc.html // MY STUFF // www.amazon.com/shop/davidbombal // David SOCIAL // Discord: discord.com/invite/usKSyzb Twitter: twitter.com/davidbombal Instagram: instagram.com/davidbombal LinkedIn: www.linkedin.com/in/davidbombal Facebook: facebook.com/davidbombal.co TikTok: tiktok.com/@davidbombal KZbin: kzbin.info // Chris SOCIAL // Udemy course: davidbombal.wiki/chriswireshark LinkedIn: www.linkedin.com/in/cgreer/ KZbin: kzbin.info Twitter: twitter.com/packetpioneer // VLAD SOCIAL // Twitter: twitter.com/Packet_vlad PMTUD Blog: www.packettrain.net/2016/09/21/pmtud-or-not/ Thanks Vladimir Gerasimov! // SPONSORS // Interested in sponsoring my videos? Reach out to my team here: [e-mail] Please note that links listed may be affiliate links and provide me with a small percentage/kickback should you use them to purchase any of the items listed or recommended. Thank you for supporting me and this channel!

[@N1v3dh]

i just want to say thank you from the bottom of my heart lots of love david ... i am getting started in cyber security and i have everything i wanted here in your channe thanks for your effort ......

[@chekov6668]

Videos with Chris and you will never get to long! Great to watch and thanks for sharing your knowlege

[@brunomedeirosfraga]

Amazing job, guys ! We see a lot of information written on web but when we see a live demo it's much more interesting and captivating. I've recently found David and Chris's channels and can't stop watching. Keep doing wireshark analyses for network problems ! It's awesome !

[@adajatobi7866]

Thank you so much Chris and David. This video is a goldmine and I learnt a lot from it. Watching it over and over again, and it just keeps making more sense.

[@nilmega]

Guys, this stuff is awesome. More videos, please, and continue this great work you guys are doing.

[@TomWhi]

Boys, this video was incredible! Chris, you did an amazing job of practically explaining this and David’s input was very helpful.

[@dwaynesudduth1028]

Anohter fantastic collaboration and learning time. And the 'War Story" shows a real-world example for us to dive into and understand. Thank you David and Chris!

[@sirsquirrel0]

I love the way you both engaged us all. I have really struggled in and out to know mtu and mss in detail. Now, I’m all good and proud. Thanks for the video. Keep up the great work!

[@smanolito]

Brilliant video. thanks guys. An hour with you guys and I learnt more than when I spend a year with my manager.

[@esesimon3523]

Wow. I'm indeed wowed. This is an impressive and exceptional delivery. It's unbelievable to find this kind of quality content for free. I felt I was in a classroom. I would appreciate a deep dive on the entire TCP/IP and Ethernet header. Thanks for putting this out.

[@jonathantx]

Excellent content, Thanks again for sharing this valuable knowledge specially for newbies like me. I've been working on learning and mastering the fundamentals and this material makes it possible. Thanks Again.

[@davidbombal]

Great to hear that! You're welcome!

[@juanrebella2589]

Thanks, David. This hour lesson had so much information that I had to watch in slow-mode and analyse it for about 7 days! true gold. Thanks again and I am subscribing to Chris' channel as well. Have a great day.

[@alaaroumeeh2752]

Such Chris-David videos are the best thing happened in 2022. Keep it up⁦❤️⁩

[@touchdown74]

Excellent topic!! Reminding me why a Tunnel wasn't created however both routers were able to ping each other. Adjust for the overhead data

[@saramaeks9826]

I love these interviews, thank you so much for what you do David :) your channel has really helped me learn and grow as a web developer!

[@davidbombal]

Fantastic! Very happy to hear that Sara!

[@vyasG]

Thank you both for this detailed video. Appreciate the time and effort both put into for this interview! At many places I was lost - if the header bytes should be added or not. Should watch this video at-least twice to comprehend the depth of knowledge that this video contains. Pls continue your interviews with Chris. Absolutely love them and learning a lot from them. Looking forward for more Wireshark deep dive videos.

[@jacobhenriksen2324]

Nothing better than waking up, grabbing a cup of coffee and watch David and Chris talk about networking for an hour. Absolute fantastic content!

[@cyphodias1640]

1:06:37 Yes please let’s get into the weeds. Great content as always from you and Chris. Thanks so much. 👍

[@tenaciousmbob1826]

Very educational and good for troubleshooting we need more of Chris

[@cassioOsoares]

Thank you David and Chris!

[@yomisaurio]

Great video guys, very explanatory and easy to comprehend, I confirm this TCP issues are nigthmares, having dealing with a similar problem months ago where one of the branch offices experience a low vpn throughput and fragmentation, the issue was caused by incorrect mss clamping value, it took few days to figure out whats was causing it. Like Chris said, its easier to find out what's wrong when the path is broken rather that in this trickier issues. Thanks for sharing your vastly knowledge

[@albertsamuel7090]

Hi David & Chris, Thank you for sharing your experience and knowledge. Now I understand why MTU & MSS-adjust commands are used in the mGRE tunnels comes into play. Thanks alot guys and i can't wait to see you twos in your next collaboration. Cheers! 🇵🇬

[@yatharthsharma7442]

This was an amazing session! The clarity of thought and concept is too good and helpful !! I owe you guys !😇

[@JBNetwork]

Wow! Thanks crystal clear for me! i had some cloudy areas on this topic and now its clear. Amazingly explained. Thanks again to both of you.

[@Azy_Azhar]

Awesome guys, you guys made my day..First time watching a long video of yours, I hope this will help a lot. Thanks, Chris and David for teaching new kinds of stuff in TS.

[@mk-or8nm]

you two are like ... superman meets batman, no fighting though, only starting Justice League right away. Thanks for a great video.

[@alaahaider]

Wow, so much details and information. Amazing video. Thank you David

[@lanpaule3428]

Great video! Learned a lot! Thank you David and Chris!

[@peterpt4264]

As always , great videos with a lot of explanation . Thank you for going into detail of TCP communication .

[@sergioeduard4422]

Thank you for another great video, always a pleasure to watch 🖤

[@sunnyzheng7511]

David combines real war story to explain the symptoms we will experience if we have MTU and MSS issue. that is cool. Connects the theory with real life. More war stories please, that makes technical video more interesting.

[@ghanthor]

Great vid. Loving it! Keep taking us down, all the way to the bottom!

[@davidbombal]

Thanks!

[@z0nerider]

Loved the WarStories, loved the vid, keep em comming David & Chris

[@davidbombal]

Thank you! Will do!

[@twd3212]

Really fantastic content. Thank you so much! I had network technology and Cisco trainings in my education two times, but it was always boring theory an I thought, it's a hard way to go any deeper in this stuff. But your videos show me, that it's all about good coaches.

[@amitpatil1900]

Excellent content, Thank you both of you!! Love to learn more..Waiting for next.

[@sidreb7642]

Thanks guys for your generosity, you’re awesome!

[@jerekyox]

I so needed this! For those of us who have spent days to tshoot an MTU issue.

[@Xotty]

New video with Chris! Yessss! Thank you David for keep bringing him back.

[@warclan5429]

Thank you for the review. I ve seen this in the 2000 during ATM implementations and many tunnel and datalinks. it is a great video to review and or learn in a practical matter what you just setup as default as MTU 1500 in your routers. and as you described to know why datalinks are slow when pings show Ips are reachable

[@ehcankur]

We need time with Chris to get learn more about the packets, that transmit everything ..Great knowledge on handling the payload in defined mtu

[@carlb641]

This is an awesome topic to deep dive into. Thank you for sharing this, definitely will go and subscribe to Chris' channel.

[@jos3366]

Really love this. Thanks David!

[@nvelchev]

Best explanation heard ever. Thanks a lot.

[@toenos]

Love the video. Thanks for sharing!

[@robertroy1878]

More!? Of course we want more!

[@coldham77]

Great discussion and I learned about jumbo packets. Thanks David! I really enjoy your show.

[@davidbombal]

You're welcome! Glad you enjoyed it!

[@TAL74]

Very nice!! I love it . Thanks for all the great content 💎

[@shiroryuu2846]

Thank you for sharing knowledge, excellent!

[@Halomaster4ever]

Excellent topic - a subject that I often revisit, especially dealing with VPNs.

[@lxn7404]

you guys are amazing teachers, thanks for sharing all that for free

[@mitchellroberson]

Awesome video. Confirmed my beliefs and what u have seen. Would like to see a video on pmtud, with mtu, and miss discussion

[@MrFRankor]

I love this deep dive!!

[@ram-gc7gl]

awww you guys are such a cool duo! enjoyed watching videos with you both

[@efrainflores7881]

Thank you guys! amazing stuff!

[@hackingnut4164]

I love learning with you guys. Maybe there is something slow in my brain, but I need to watch these more than once.

[@davidbombal]

Glad you are enjoying the content. This is a deep dive - so you may need to watch it a few times as we cover a lot of information.

[@bda996]

David and Chris are just amazing i've enjoyed every sec

[@hamidabatemam2947]

amazing! both of you are fantastic teachers.

[@shaheerkhan2896]

Great Video David! Pretty awesome series for understanding TCP. Just one question though, are you planning on releasing more videos in the series? These videos are a great help and would love to know if we have more coming. Anyways, thanks for all the content. Cheers :)

[@random5107]

Good job guys, keep it up.

[@drbabbers]

Superb! Really enjoyed this thanks guys 😎

[@faran4536]

David and Chris .. dropping dope content like never before 🔥🔥🔥🔥💯💯💯

[@davidbombal]

Thank you Faran!

[@djdawso]

Wow, David! What a blast from the past! That Cisco MTU/MSS white paper you showed a clip of was my "go to" doc for many years for helping to explain fragmentation issues to customers and peers alike. I still have the link to it in my bookmarks, since you never know when MTU's are gonna raise their ugly heads! As a long-time CCIE (#1937 just over 26 years ago) your content is refreshingly good!

[@micah6465]

I know I’m late to the game but dang this was well explained. Enjoyed all of it! Thank you for making this video!

[@NickyDekker89]

Chris is a great teacher 👍

[@pafooo1043]

WOW ! Grest video, thank you both so much, I just want more 👍🏼

[@liamobrien1506]

Excellent video, been running into this a lot lately in a number of scenarios when replacing branch routers, first thing that breaks is Eap-TLS for wireless, those certificates do not like fragmentation, so have had to adjust MTU in a bunch of places.

[@dustcore]

This was a great video, thank you for sharing !!

[@tommyhuffman7499]

Your videos are so deep. Thank you!

[@mahmudulhasan9171]

YES WOULD LOVE MORE OF THIS VIDEOSS

[@derekmayabi4913]

Great interview

[@Obsessedwithcoding]

GOOD STUFF. LOVING ALL THE NETWORKING CONTENT

[@saimkhan89]

Super cool explanation, thanks!

[@miloslukic5171]

Awesome video, really educational, love it!

[@rohanmahakal8113]

Thanks for the 2nd part

[@bur3aawik141]

Amazing. Cheers guys

[@martintouko8359]

So enriching.Thanks

[@sobevj]

the bridge explanation is so vivid :D

[@brendonbarends1845]

Great video David do more war stories please!!!

[@jamespostlethwaite7799]

This is just ridiculously good content. Thank you

[@hunkdahunk]

God bless you both! It was educational! Thank you and best regards!

[@landrover827]

I’ve always wondered how this worked! Thanks!

[@davidbombal]

Hope you enjoy the video :)

[@spiraldynamics6008]

Thank you ! Good job !

[@xszleader7887]

great video david ❤️

[@davidbombal]

Thanks! Glad the video helped :)

[@chaithanyakatari7896]

Finally, after a longtime looking for a right video for interviews

[@davidbombal]

It's taken too long :) But, hope you enjoy the video!

[@chaithanyakatari7896]

@@davidbombal Absolutely it was a fantastic video again.

[@godmodemaster8750]

Love you guys ❤👏

[@chrisfx9097]

Sir, I really love your videos... please bring moreeeeeee!!!!!!!

[@Thriller627]

Cheers! Thanks to you two! Great teachers.

[@davidbombal]

Thank you very much!

[@KingBHD]

Thank you!! I really needed this video.

[@davidbombal]

You're welcome! Hope you enjoyed the video :)

[@emaayan]

this is great for watching while doing cardio on my elliptical , i need to stack up on those vids..

[@honourvirtue3820]

Excellent as always 😎

[@davidbombal]

Thanks!

[@Skynightburst]

This was a very interesting convo. Well I guess I'm into networking now XD.