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17
votes
2answers
8k views

What is the “in-the-wire” size of a ethernet frame? 1518 or 1542?

According to the table here, it says that MTU = 1500 bytes and that the payload part is 1500 - 42 bytes or 1458 bytes (<- this is actually wrong!). Now on top of that you have to add IPv4 and UDP ...
14
votes
2answers
2k views

Why does lowering the MTU from 1500 to 1499 allow me to access most websites?

I had this issue where I was only able to connect to websites like google.com and ibm.com when the mtu was set at 1500, but if I tried to connect to anything else, it would just show a blank page. ...
14
votes
1answer
2k views

What am I misunderstanding about calculating MTU?

Okay, I just finished resolving a jumbo frames issue between a few Xserves, a Netgear GSM7224, and a Drobo B800i. It turned out that the Xserves (Mac OS X 10.6.8 Server) and the Drobo B800i accept the ...
10
votes
5answers
2k views

Are Jumbo Frames (MTU up to 9k) realistic/common over the open Internet?

I've got an application that would benefit from larger Ethernet frames. (In theory, we could reduce the number of outbound packets by > 50%, maybe even 66%.) I'm also in the process of specifying ...
8
votes
2answers
2k views

Jumbo frames between KVM guest and host?

I am trying to implement a 9000 byte MTU for storage communication between KVM guests and the host system. The host has a bridge (br1) with a 9000 byte MTU: host# ip link show br1 8: br1: ...
7
votes
1answer
125 views

Does setting MTU on logical interfaces affect physical interfaces

I've been using a combination of interface bond-, vlan- and bridge-interfaces to provide redundancy and different logical network layers to xen domU's. This setup is working well however i'm a bit ...
6
votes
6answers
3k views

OpenVPN over TCP, terrible latency but only on Win7 x64

I know TCP encapsulated over TCP is a terrible idea in general, but I can deal with the occasional decreased performance and MTU issues, and the snowball effect TCP within TCP provides where error ...
6
votes
1answer
3k views

How do I prevent TCP connection freezes over an OpenVPN network?

New details added at the end of this question; it's possible that I'm zeroing in on the cause. I have a UDP OpenVPN-based VPN set up in tap mode (I need tap because I need the VPN to pass multicast ...
5
votes
3answers
2k views

What's going on with this strange MTU?

I have a server mike at our office communicating with a remote server through a VPN. There have been various problems which seem to be linked to the MTU size. Mike is a RHEL 3 server and the customer ...
5
votes
0answers
82 views

POST of MTU + ~798 bytes gets lost

I have a very weird issue with certain packets not arriving on the destination host. It happens when we transmit a POST that is somewhat larger than the MTU. We can reproduce it with this script: ...
4
votes
2answers
230 views

Impact of MTU on IP packet fragmentation and reassembly

Let's say computer S (sender) sends some IP packets to computer R (receiver). Between S and R, there are 6 networks, divided by 5 routers. So the IP packets leave R, go through network 1, pass router ...
4
votes
1answer
618 views

Understanding Ethernet Headers, What MTU Do I Need?

I see many articles on line stating that the overhead of Ethernet is 18 bytes (example: http://www.learnios.com/viewtopic.php?f=8&t=18887) Looking at this wiki article ...
4
votes
3answers
2k views

What's the best MTU setting for a web server?

I manage several web servers and their MTU settings were set at 1500 by default. Some of the machines have an MTU setting of 576. I've read a bunch about MTU and I feel I understand it well enough but ...
4
votes
3answers
920 views

What IP MTU sizes are seen in real networks

I am writing an application which will be deployed in third party networks. It will receive IP packets and I want to know what the maximum size in reality they are likely to be is. I'm not asking what ...
4
votes
1answer
1k views

Relationship between MTU and NFS rsize/wsize options

I'm trying to understand networking settings related to NFS and various buffer sizes (and there are quite a few). I'm running wireshark and inspecting the TCP packets arriving at the NFS server. ...
4
votes
3answers
915 views

What is the Maximum MTU supported in DSL Standards

Do any of the ITU DSL standards limit the maximum MTU that can be supported? I know that RFC2516 (PPPoE) limits the MTU to 1492 bytes; however when PPPoE is not used this limit should not exist. The ...
3
votes
4answers
1k views

Is there such a thing as a “typical” or “de facto” default for MTU size?

When writing networking software, is there a default, typical, or otherwise de facto standard for MTU size, of which I should be aware? If so, what is it? Is an MTU size of 1500 (as suggested here: ...
3
votes
4answers
3k views

How is the MTU is 65535 in UDP but ethernet does not allow frame size more than 1500 bytes

I am using a fast ethernet of 100 Mbps, whose frame size is less than 1500 bytes (1472 bytes for payload as per my textbook). In that, I was able to send and receive a UDP packet of message size 65507 ...
3
votes
3answers
7k views

MTU problems? 'Request timed out'

I've been trying to figure out why some(very few) of the people who visit my site have random slowdowns and error pages('page cannot be loaded'). The random slowdowns and error pages never happen for ...
3
votes
3answers
9k views

How to increase MTU size on Linux 2.6?

How to increase MTU size on Linux 2.6? Is such a thing possible? If yes, which files (in /etc or elsewhere) must one edit?
3
votes
1answer
1k views

PIX 506E, MTU, VPN Packet Fragmentation and Shoretel IP telephone system

We have two sites, a large Southern site and a small Northern site, that have a VPN between them defined on two Cisco PIX Firewalls. Over this VPN Shoretel IP phone traffic travels as well as all ...
3
votes
1answer
3k views

Is there a way of setting an MTU lower for traffic destined to a specific IP address on Cisco ASA?

I have a number of VPN sites where the MTU is lower than standard (1500). I have had at least one site where fragmentation of packets has had an effect on the success of building an IPSEC tunnel. ...
2
votes
2answers
1k views

Getting interface MTU under Linux with PCAP

I am interested in storing packets I sniff using PCAP. As I preallocate the memory I need - i.e. a multiple of the MTU of the interface I am sniffing from, I would like to discover how much memory I ...
2
votes
1answer
92 views

Maximum MTU size

Thought one of the issues I'm experiencing with the following question is due to MTU rightfully so. ESXi 5 VM Putty session hangs, vSphere client timing out However, when I tried testing the ...
2
votes
1answer
188 views

Can fragments of a packet be refragmented again?

In IPv4, fragmentation is done by routers on way to the destination if DF(do not fragment) flag is not set in the IP packet. Once a packet is fragmented, its fragments may take different paths (due ...
2
votes
1answer
695 views

VMWare ESXi 4.1 VMXNET Ignoring MTU and Sending Jumbo Frames

On my CentOS 5.6 ESXi 4.1 VMs using VMXNET, it looks like they are attempting to send jumbo frames: However, all the interfaces are set to MTU 1500. What is causing this behavior? Update: I ...
2
votes
2answers
781 views

SFTP fails on listing directories with large number of files

I try to connect to an sftp server (vsftpd) on Centos server through port 22. there is no problem in connection and initial directory listing; there is also no problem if I click on a directory with ...
2
votes
1answer
138 views

IPv6: using a link mtu greater than the mtu of the default route

When configuring an IPv6 LAN it may be desirable to use router advertisements to announce a link MTU of either the standard 1500 bytes or something larger to enable usage of jumbo frames, perhaps ...
2
votes
1answer
283 views

OpenVPN: How to mitigate path MTU issues on a per-client basis?

We have dozens of embedded devices installed at customers, all calling home to our OpenVPN service. That works fine in general, but a few of our customers have severe path MTU issues. Our influence on ...
2
votes
1answer
235 views

Full MTU not utilized with jumbo frames enabled

I've been testing to see if I can get any benefit from enabling jumbo frames. I've set up two identical Dell R210 servers with Xeon quad core E3122 CPUs, 8G of RAM and Broadcom NetXtreme II BCM5716 ...
2
votes
1answer
874 views

other than manually changing it, how is MTU set?

I recently discovered a Windows 7 client that couldn't connect to one of our servers. It turned out the problem was both that the server was set for an MTU of 1514 and that the client was set to 1504. ...
2
votes
2answers
2k views

Why do I see an IP packet with size 2960, way above the MTU setting of 1500 on the interface and it gets through?

I am analyzing traffic between a client and a Linux webserver running on a HP blade server, the client sometimes gets stuck waiting for more data when the webserver has closed the connection. The ...
2
votes
1answer
99 views

seeing remote MTUs of 250 and 68. is this ever valid?

I'm seeing an ICMP storm from a mobile phone netblocks belonging to "TELEFONICA MOVILES". Periodically we will get upwards of 5 million in a few seconds, all something like this: 08:12:05.740781 IP ...
2
votes
1answer
498 views

Telecom provider not supporting MTU higher than 1515 for leased line

We have ordered a 100 Mbps leased line and did not specify our requirement to have a minimum of 1530 MTU supported. The telco now sent us a feedback that they do not support 1530, but only 1515. Did ...
2
votes
1answer
251 views

MTU value causing applications to hang

I am using CentOS 5 in my environment and it acts as a gateway for my LAN. My eth0 interface MTU is set to 1500 (default). UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Recently I am facing a ...
2
votes
2answers
670 views

MTU, DSL router and stalling TCP

I am discovering stalling TCP connection problem. The problem arises when I try to scp stuff from remote system from my home network. My home network is connected to internet via PPPoE (ADSL+) and ...
2
votes
4answers
2k views

How to check who blocks ICMP during MTU path negotiation?

I am getting MTU problems with connections to my web host provider. The issue is that I am behind ppoe with MTU size set to 1492 which preventing me from using 'scp'. Once changing MTU from 1492 to ...
1
vote
3answers
427 views

disadvantages MTU over 1500

What disadvantages have to set the MTU to the maximum possible value in ALL network adapters on a network (including virtual switches vmware esx)? Why do not the producers as a default? I do not ...
1
vote
4answers
2k views

Smaller network packets vs bigger packets - confused

Some are saying that bigger packets are better to send then smaller. But in this app: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/message/messagesegmentation.html The lower the packet ...
1
vote
2answers
186 views

Weird network interaction between Vista and Ubuntu

I have a network where my WAN (a CableModem) goes into a (dubiously configured) Ubuntu box which acts as the router for the whole company. We recently switched our ISP, and we're getting a very weird ...
1
vote
2answers
210 views

How can I set the MTU for just one remote server in Linux?

Hi I have a linux server that is behind a customer firewall. I can SSH into it but when I try to scp, the connection dies. I believe this is an MTU issue as described here: ...
1
vote
1answer
518 views

How do I change the MTU for only localhost connections on a Red Hat box?

I want to change the MTU value for only the connections that are going between localhost / 127.0.0.1 on a Red Hat box. How do I do that?
1
vote
2answers
1k views

Permanently set MTU on Ubuntu 14.04 machine within an Amazon VPC

I have some Ubuntu 14.04 machines running in an Amazon VPC. When they start, they have an MTU of 9001, but this appears to cause some problems when communicating with other servers which have an MTU ...
1
vote
1answer
2k views

Linux - set an interface to not fragment packets

I am using a Linux as a router. The machine has two NICs. This router will be used for experiments, so I have set one of it's interfaces to have a MTU that is lower than the standard (let's decide ...
1
vote
1answer
405 views

Setting MTU Per Destination Host

I'd like to change the MTU per host as I have a mixed infrastructure where not all devices support the same MTU. I'm on OS X 10.7 and like to set some MTU per destination host and a default one.
1
vote
1answer
1k views

Maximum Transmission Unit of VDSL

A simple question (hopefully) but with no clear answers when doing a Google Search. What is the MTU of VDSL (not VDSL2)? With ADSL it was 1492, but most ISPs added PPPoE/PPPoA headers reducing it to ...
1
vote
1answer
1k views

VLAN MTU on Linux with Intel Pro/1000 PT Server Adapter

For some reason MTU still seem to be an occasional issue with trunked connections. In this case I have a trunk connected to an Enterprise Linux 5 server. I use standard vlan seperation in Linux, and ...
1
vote
2answers
95 views

CentOS ssh connection stalls after server bandwidth upgrade

I have a few linux servers inside a customer that recently MASSIVELY increased his bandwidth - from about 0.8/5mbps to 25mpbs/100mbps... (yes 100mpbs!) this is very good, but ever since my ...
1
vote
2answers
1k views

XenServer 6.2 changing mtu

So I've looked around for a way to change the MTU of interfaces on XenServer 6.2, but can't find anything that works for me ... One method spoke about changing the ifcnfg-intX files in the ...
1
vote
1answer
768 views

Cisco IPSEC VPN Slow Speeds

We have a site to site IPSEC VPN, both endpoints are Cisco PIX 515e's. The links at both end are 100mb, however speeds over the VPN (logged using jperf) are at most 4mb. Obviously this represents a ...