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Questions tagged [mtu]

"Maximum Transmission Unit" (MTU) is the maximum number of bytes a frame or packet may have on a network (in several layers of the OSI model). Wrong MTU can lead to packet loss.

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Does TCP's MSS hide a mismatched intra-LAN MTU?

I'm trying to understand how mismatched MTUs behave in a network. Suppose two hosts are able to talk to each other through an Ethernet-based network. The network and host A are configured for jumbo frames up to but not including a switch on the LAN…
Stanley Yu
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Can't mount read-write iSCSI with MTU=9000

I'm attempting to connect my desktop (running debian 7) to a QNAP NAS (running QTS 4.1.1) using iSCSI with an MTU=9000. The network infrastructure consists of a cat5e crossover cable between the desktop and the nas. If I set the MTU on local machine…
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CentOS 7 MTU setting not works

I am using CentOS 7 to test some network performance. The interface setting is normal like that : saberin@localhost ~]$ ifconfig enp0s25: flags=4163 mtu 1500 inet 10.40.196.150 netmask 255.255.255.0 …
Saberin Lin
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Debian don't forward through L2TPv3 more than mtu l2tp virtual interface

R3--R1--Internet--R90 R3: ip addr 10.123.0.3 R1: sudo ip l2tp add tunnel tunnel_id 699 peer_tunnel_id 699 encap udp local 5.254.174.8 remote 5.254.174.90 udp_sport 699 udp_dport 699 sudo ip l2tp add session tunnel_id 699 session_id 699…
Nico Zyrtec
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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 WinSCP/Putty connections go stalled if I leave them for a…
user26676
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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 HUGE gulf in the speeds we feel we should be…
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How do I force a specific MTU for only certain TCP ports?

Background I have a set of embedded hardware deployed in the field. These remote machines connect back to my servers at AWS running Ubuntu and I use the iptables mangle chain to lower the MTU to 500 so these devices are happy. For reference, this…
Dave S.
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What is the right way to temporarily disable/enable PMTU Discovery on a Linux box?

I've a few pcaps that contain packets larger than standard ethernet MTU size of 1500B. I searched about this and found refs pointing towards Jumbo Frames. I now understand that since I'm on a GBE network that supports sending Jumbo frames (upto 9k),…
7h3rAm
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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 the MTU has been set to 200, for the sake of…
summerbulb
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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.
bot47
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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: http://www.snailbook.com/faq/mtu-mismatch.auto.html. How can I set the MTU…
MattSmith
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What is the difference between IP_MTU_DISCOVER values (IP_PMTUDISC_xxx)?

The IP_MTU_DISCOVER description from man 7 ip is too short. As far as I understand IP_PMTUDISC_DONT / IP_PMTUDISC_DO values enable/disable packet fragmentation on a route. What are IP_PMTUDISC_WANT and IP_PMTUDISC_PROBE? What do they need for? How…
Rom098
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Configure LeftHand iSCSI SAN for jumbo frames after clients are configured

This is similar to enabling jumbo frames on an iSCSI network after the fact but in our case we configured the switches and then went through all the hosts, enabling jumbo frames (we may have screwed ourselves here). The SAN is a Lefthand (actually…
murisonc
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(RFC-791) Application Layer PDU size, guaranteed to pass without fragmentation

I am writing a client-server python application, a part of which is using UDP Multicast to communicate. And my current concern is about fragmentation. RFC 791 (page 24) says: Every internet module must be able to forward a datagram of 68…
athspk
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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?
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