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Operating Systems Linux Red Hat Extremely low throughput between AIX 7.2 and RHEL Maipo Post 303044541 by Neo on Wednesday 26th of February 2020 12:13:40 AM
Old 02-26-2020
Quote:
Originally Posted by MadeInGermany
I wonder why the shown MTU is 1460 while the standard is 1500.
But if your LAN switch/router works better with 1460 then try to set it on the other box, too.

I remember a similar issue (severe packet loss), where all Linux systems had the standard MTU 1500. The LAN guy changed the MTU on the LAN switch (or router?), and that fixed it.
Google Cloud mandates this MTU:

Quote:
Gateway MTU vs. system MTU

You must configure your peer VPN gateway to use a MTU of no greater than 1460 bytes. A value of 1460 bytes is recommended because that matches the default MTU setting for Google Cloud VM instances.

The effective MTU for peer systems and Google Cloud VMs is typically lower than the MTU of your VPN gateway:

For TCP traffic, MSS clamping rewrites the SYN packet of the initial TCP handshake. This allows systems to dynamically adjust Maximum Segment Size (MSS) to accommodate encapsulation.

For UDP traffic, Path MTU Discovery (PMTUD) can negotiate smaller MTU sizes, under certain circumstances, provided that your firewall permits ICMP traffic.
REF: MTU considerations | Cloud VPN | Google Cloud
 

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IPSEND(1)						      General Commands Manual							 IPSEND(1)

NAME
ipsend - sends IP packets SYNOPSIS
ipsend [ -dITUv ] [ -i <interface> ] [ -f <offset> ] [ -g <gateway> ] [ -m <MTU> ] [ -o <option> ] [ -P <protocol> ] [ -s <source> ] [ -t <dest. port> ] [ -w <window> ] <destination> [TCP-flags] DESCRIPTION
ipsend can be compiled in two ways. The first is used to send one-off packets to a destination host, using command line options to specify various attributes present in the headers. The destination must be given as the last command line option, except for when TCP flags are specified as a combination of A, S, F, U, P and R, last. The other way it may be compiled, with DOSOCKET defined, is to allow an attempt at making a TCP connection using a with ipsend resending the SYN packet as per the command line options. OPTIONS
-d enable debugging mode. -f <offset> The -f allows the IP offset field in the IP header to be set to an arbitrary value, which can be specified in decimal or hexadeci- mal. -g <gateway> Specify the hostname of the gateway through which to route packets. This is required whenever the destination host isn't directly attached to the same network as the host from which you're sending. -i <interface> Set the interface name to be the name supplied. -m <MTU> Specify the MTU to be used when sending out packets. This option allows you to set a fake MTU, allowing the simulation of network interfaces with small MTU's without setting them so. -o <option> Specify options to be included at the end of the IP header. An EOL option is automatically appended and need not be given. If an option would also have data associated with it (source as an IP# for a lsrr option), then this will not be initialised. -s <source> Set the source address in the packet to that provided - maybe either a hostname or IP#. -t <dest.port> Set the destination port for TCP/UDP packets. -w <window> Set the window size for TCP packets. -I Set the protocol to ICMP. -P <protocol> Set the protocol to the value given. If the parameter is a name, the name is looked up in the /etc/protocols file. -T Set the protocol to TCP. -U Set the protocol to UDP. -v enable verbose mode. SEE ALSO
ipresend(1), iptest(1), bpf(4), protocols(5), ipsend(5) DIAGNOSTICS
Needs to be run as root. BUGS
If you find any, please send email to me at darrenr@pobox.com IPSEND(1)
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