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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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IPTEST(1)						      General Commands Manual							 IPTEST(1)

NAME
iptest - automatically generate packets to test IP functionality SYNOPSIS
iptest [ -1234567 ] [ -d <device> ] [ -g <gateway> ] [ -m <MTU> ] [ -p <pointtest> ] [ -s <source> ] <destination> DESCRIPTION
iptest ... OPTIONS
-1 Run IP test group #1. This group of tests generates packets with the IP header fields set to invalid values given other packet characteristics. The point tests are: 1 (ip_hl < ip_len), 2 (ip_hl > ip_len), 3 (ip_v < 4), 4 (ip_v > 4), 5 (ip_len < packetsize, long packets), 6 (ip_len > packet size, short packets), 7 (Zero length fragments), 8 (packet > 64k after reassembly), 9 (IP offset with MSB set), 10 (ttl variations). -2 Run IP test group #2. This group of tests generates packets with the IP options constructed with invalid values given other packet characteristics. The point tests are: 1 (option length > packet length), 2 (option length = 0). -3 Run IP test group #3. This group of tests generates packets with the ICMP header fields set to non-standard values. The point tests are: 1 (ICMP types 0-31 & 255), 2 (type 3 & code 0 - 31), 3 (type 4 & code 0, 127, 128, 255), 4 (type 5 & code 0, 127, 128, 255), 5 (types 8-10,13-18 with codes 0, 127, 128 and 255), 6 (type 12 & code 0, 127, 128, 129, 255) and 7 (type 3 & codes 9-10, 13-14 and 17-18 - shortened packets). -4 Run IP test group #4. This group of tests generates packets with the UDP header fields set to non-standard values. The point tests are: 1 (UDP length > packet size), 2 (UDP length < packetsize), 3 (sport = 0, 1, 32767, 32768, 65535), 4 (dport = 0, 1, 32767, 32768, 65535) and 5 (sizeof(struct ip) <= MTU <= sizeof(struct udphdr) + sizeof(struct ip)). -5 Run IP test group #5. This group of tests generates packets with the TCP header fields set to non-standard values. The point tests are: 1 (TCP flags variations, all combinations), 2 (seq = 0, 0x7fffffff, 0x8000000, 0xa0000000, 0xffffffff), 3 (ack = 0, 0x7fffffff, 0x8000000, 0xa0000000, 0xffffffff), 4 (SYN packet with window of 0, 32768, 65535), 5 (set urgent pointer to 1, 0x7fff, 0x8000, 0xffff), 6 (data offset), 7 (sport = 0, 1, 32767, 32768, 65535) and 8 (dport = 0, 1, 32767, 32768, 65535). -6 Run IP test group #6. This test generates a large number of fragments in an attempt to exhaust the network buffers used for holding packets for later reassembly. WARNING: this may crash or cause serious performance degradation to the target host. -7 Run IP test group #7. This test generates 1024 random IP packets with only the IP version, checksum, length and IP offset field correct. -d <interface> Set the interface name to be the name supplied. -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. -p <test> Run a... SEE ALSO
ipresend(1), ipsend(1), bpf(4), ipsend(5) DIAGNOSTICS
Only one of the numeric test options may be given when iptest is run. Needs to be run as root. BUGS
If you find any, please send email to me at darrenr@pobox.com IPTEST(1)
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