Wireshark · Wireshark-users: Re: [Wireshark-users] How can a packet size be greater than the NIC's MTU?

[Aaron Wasserott <aaron.wasserott@xxxxxxxxxxx>]

The IP packet size can be much larger than the MTU of an Ethernet frame, which is what the MTU refers to.

 

From: wireshark-users-bounces@xxxxxxxxxxxxx [mailto:wireshark-users-bounces@xxxxxxxxxxxxx] On Behalf Of Mohamed Lrhazi
Sent: Tuesday, December 03, 2013 7:30 PM
To: wireshark-users@xxxxxxxxxxxxx
Subject: [Wireshark-users] How can a packet size be greater than the NIC's MTU?

 

I guess the subject line is all I need to say :)

 

am debugging an issue which seems to be rooted at some MTU problem... and I notice that a host, according to the pcaps I take, using tcpdump, on redhat linux 6.x, the packet size is shown to be over 2500 bytes, when the MTU of the network interface is only 1500.... or is a "packet" as displayed by wireshark or tcpdump, unrelated to the L2 frames? could there have been more frames for that one "packet"? How can I have "tcpdump -r" or wireshark, show me the exact frames, so I can see their actual sizes?

 

Example, notice the packet with a "tcp size" of 2896, "IP size" is 2948.

 

➜  tmp  tcpdump  -qnr ubuntu-1.mtu1500.pcap

reading from file ubuntu-1.mtu1500.pcap, link-type EN10MB (Ethernet)

18:09:07.874894 IP 192.168.77.204.55992 > 192.168.77.201.80: tcp 0

18:09:07.874990 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 0

18:09:07.878527 IP 192.168.77.204.55992 > 192.168.77.201.80: tcp 0

18:09:07.878819 IP 192.168.77.204.55992 > 192.168.77.201.80: tcp 85

18:09:07.878842 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 0

18:09:07.879982 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 17

18:09:07.880299 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 2896

18:09:07.880506 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 395

18:09:07.882022 IP 192.168.77.204.55992 > 192.168.77.201.80: tcp 0

18:09:07.882048 IP 192.168.77.204.55992 > 192.168.77.201.80: tcp 0

18:09:07.883506 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 1448

18:09:07.883523 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 1448

18:09:08.087483 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 1448

18:09:08.495509 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 1448

18:09:09.311515 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 1448

18:09:10.947503 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 1448

18:09:14.223563 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 1448

18:09:16.463307 IP 192.168.77.204.55992 > 192.168.77.201.80: tcp 0

18:09:16.463353 IP 192.168.77.201.80 > 192.168.77.204.55992: tcp 0

➜  tmp  

 

 

 

Thanks a lot,

Mohamed.