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Network Parameters MTU, MRU, and MSS in Linux

Various transmission parameters must be set for data transmission to, from, and within the internet.
These parameters are:

MSS - Maximum Segment Size:
The maximally allowed size of a segment. MSS is closely related to MTU.
MTU - Maximum Transmit Unit:
The maximally allowed size of a network packet in bytes. This is normally 1500, but must be set to 1492 or lower for DSL.
(The correct setting should be a maximum of 1492, but actually it needs to be 8 bytes less than any router you pass through on the way to the site.)
MRU - Maximum Receive Unit:
Must be set to 1492 or lower for DSL
Rwin - Receive Window:
Should be set to 32767 for DSL.
TTL - Time To Live:
Normally 32, should be set to 64 for DSL (Chip 12/03 p.248 and Chip 10/02 p.250)

Improper values, especially too high a value for MTU in a DSL-environment, may lead to slow data transfer from/to the internet, or may even make certain servers unreachable.

For more information refer to
New SuSE Network Configuration and Features (Version 8.0)
Das neue SuSE Netzwerk-Konfigurationsschema und Features (Version 8.0)
Informationen zur Netzwerk-Konfiguration erstellen (SuSE Linux 7.3 und 8.0)
Langsame Internetanbindungen effektiv nutzen (SuSE Linux 7.3)
T-DSL (u.a.): Manche Server sind nicht erreichbar (SuSE Linux 7.3)
Setting the MTU in SuSE Linux 8.0
Enabling High Performance Data Transfers
Linux Performance Tips - Setting the TCP Rx Window
Linux Tuning Guide
DSL HOWTO for Linux - Performance Tuning and Troubleshooting - TCP Receive Window
TCP Tuning parameters for different OS:es

Also look at the "man pages" of the route command.

SuSE Linux 9.0

Connection directly to DSL (without Router)

When the PC is directly connected to DSL (and by that means to the internet)
the lines

    MTU 1492
    MRU 1492 
should be added to the data sets /etc/ppp/options and /etc/ppp/peers/pppoe, possibly with smaller values than the maximum of 1492.

Ethernet Connection

When the PC is connected by an Ethernet card or Ethernet attachment with a LAN and/or via a router with the internet
in /etc/route.conf the line

    default ...  ...  ...  eth0              can be changed to
    default ...  ...  ...  eth0  mss 1400    
(or with another value for mss). Similar, when the mss value shall be set only for certain distant computers.
Also in etc/rc.config the line
    IFCONFIG_0="... broadcast ... netmask ..."            can be supplemented to, e.g.,
    IFCONFIG_0="... broadcast ... netmask ... mtu 1400"   

In SuSE Linux 9.0 I only have configured some values in /etc/sysconfig/network/ifcfg-eth0
to achieve good internet performance via DSL (the IP-address of the router is 192.168.1.1):

# Network Configuration File /etc/sysconfig/network/ifcfg-eth0 (LAN connection)
BOOTPROTO='static'
# MTU=''
# Changed by GM to:
# MTU - Maximum Transmission Unit: normally 1500, must be set to 1492 or lower for DSL, for safety set a lot lower:
MTU='1400'
# Added by GM:
# MRU - Maximum Receive Unit: must be set to 1492 or lower for DSL, for safety set a lot lower:
MRU='1400'
REMOTE_IPADDR=''
STARTMODE='onboot'
UNIQUE=''
BROADCAST='192.168.1.255'
IPADDR='192.168.1.21'
NETMASK='255.255.255.0'
NETWORK='192.168.1.0'    

When a PC is connected to the internet via a router, communication problems with the router are another possible reason for (very) slow internet access. Refer to Slow Internet Access from SUSE Linux 9.0 and 10.0 through router DrayTek Vigor 2500We.

SuSE Linux 10.0

Ethernet Connection

In SuSE Linux 10.0 I have configured an MTU of 1442 (to be safely below 1492) for the ethernet attachment within Yast.
This was sufficient for good internet performance via DSL.

When a PC is connected to the internet via a router, communication problems with the router are another possible reason for (very) slow internet access. Refer to Slow Internet Access from SUSE Linux 9.0 and 10.0 through router DrayTek Vigor 2500We.

 

Document www.gmusoft.de/information/linux/MTU-MRU-MSS-e.htm
Created by: Gerhard Mueller, Germany, Herrenberg, mail@gmusoft.de¹
Responsibility for this page and comments to: http://www.gmusoft.de/gmuwebe.htm
Creation/change date: 05.01.2006/26.01.2006
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