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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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IPRESEND(1)						      General Commands Manual						       IPRESEND(1)

NAME
ipresend - resend IP packets out to network SYNOPSIS
ipresend [ -EHPRSTX ] [ -d <device> ] [ -g <gateway> ] [ -m <MTU> ] [ -r <filename> ] DESCRIPTION
ipresend was designed to allow packets to be resent, once captured, back out onto the network for use in testing. ipresend supports a num- ber of different file formats as input, including saved snoop/tcpdump binary data. OPTIONS
-d <interface> Set the interface name to be the name supplied. This is useful with the -P, -S, -T and -E options, where it is not otherwise possi- ble to associate a packet with an interface. Normal "text packets" can override this setting. -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. -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. -r <filename> Specify the filename from which to take input. Default is stdin. -E The input file is to be text output from etherfind. The text formats which are currently supported are those which result from the following etherfind option combinations: etherfind -n etherfind -n -t -H The input file is to be hex digits, representing the binary makeup of the packet. No length correction is made, if an incorrect length is put in the IP header. -P The input file specified by -i is a binary file produced using libpcap (i.e., tcpdump version 3). Packets are read from this file as being input (for rule purposes). -R When sending packets out, send them out "raw" (the way they came in). The only real significance here is that it will expect the link layer (i.e. ethernet) headers to be prepended to the IP packet being output. -S The input file is to be in "snoop" format (see RFC 1761). Packets are read from this file and used as input from any interface. This is perhaps the most useful input type, currently. -T The input file is to be text output from tcpdump. The text formats which are currently supported are those which result from the following tcpdump option combinations: tcpdump -n tcpdump -nq tcpdump -nqt tcpdump -nqtt tcpdump -nqte -X The input file is composed of text descriptions of IP packets. SEE ALSO
ipftest(1), ipsend(1), iptest(1), bpf(4), ipsend(5), tcpdump(8) DIAGNOSTICS
Needs to be run as root. BUGS
Not all of the input formats are sufficiently capable of introducing a wide enough variety of packets for them to be all useful in testing. If you find any, please send email to me at darrenr@pobox.com IPRESEND(1)
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