Enable or disable ECN (Explicit Congestion Notification) on Windows

The TCP protocol can detect network congestion with different mechanisms:

• packets loss
• timeouts
• duplicate acknowledgments 

In order to avoid this behavior on a saturated link, TCP ECN  can be enable (on by default on Windows 2012 server). TCP ECN are generated by the network in order to signal to the receiver that the network component is close to drop packets. The receiver can notify the sender to slow down the traffic rate. This mechanisms can react faster than the traditional TCP timeout or DUP ack.

You can use the following command to enable it on Windows:
C:\Windows\system32>netsh int tcp set global ecncapability=enabled
Ok.
Verify:
C:\Windows\system32>netsh int tcp show global
Querying active state...

TCP Global Parameters
----------------------------------------------
Receive-Side Scaling State          : enabled
Chimney Offload State               : automatic
NetDMA State                        : enabled
Direct Cache Acess (DCA)            : disabled
Receive Window Auto-Tuning Level    : normal
Add-On Congestion Control Provider  : none
ECN Capability                      : enabled
RFC 1323 Timestamps                 : disabled
Disable:
C:\Windows\system32>netsh int tcp set global ecncapability=disabled
Ok.

Track an IP on your Network

Who has never received the following question from a colleague:
- Hey, the network guy, could you find where is this laptop for me ? I have only his IP address.

To resolve this, you start to follow the IP address by looking in the arp and mac-address table of your switchs, routers and firewall. I have finished this boring job by using the following open source tool:
http://netdbtracking.sourceforge.net/

Developpers have alreay preconfigured a VM:
http://sourceforge.net/projects/netdbtracking/files/vmware/

After 30 minutes of the VM installation and some hours (depending of the size of your network) of configuration to enter your different equipment, you can provide a webtools reachable from everybody to track an IP.

You will find below some examples of host configuration.

• HP Procurve (Layer 3)

hostname,devtype=procurvehpv2, arp

• HP Procurve (Layer 2):

On this example, I skipped uplinks interfaces (45 and 47). I have also limited to 2 mac address by interface.
hostname,devtype=procurvehpv2,skip_port=45,skip_port=47,use_trunks,max_macs=2

Windows: find the MTU for a specific IP

The following command, give us the MTU for a specific destination:
U:\>netsh interface ipv4 show destinationcache address='IP_addresss'

• Example on a classic LAN:

U:\>netsh interface ipv4 show destinationcache address=192.168.95.216
Destination              : 192.168.95.216
Next Hop Address         : 192.168.211.254
Source                   : 192.168.211.1
Interface                : Local Area Connection
Path MTU                 : 1500
Upper-layer MTU          : 1480
RTT mean                 : 3000
RTT deviation            : 0
Path transmit speed (Bps): 0
Path receive speed (Bps) : 0
Link transmit Speed (bps): 1000000000
Link receive Speed (bps) : 1000000000

• Example with an IPsec Tunnel:

U:\>netsh interface ipv4 show destinationcache address=192.168.95.216
Destination              : 192.168.95.216
Next Hop Address         : 192.168.95.216
Source                   : 192.168.243.197
Interface                : Local Area Connection
Path MTU                 : 1400
Upper-layer MTU          : 1380
RTT mean                 : 40
RTT deviation            : 20
Path transmit speed (Bps): 0
Path receive speed (Bps) : 0
Link transmit Speed (bps): 2000000000
Link receive Speed (bps) : 2000000000

OSPF Database Reminder in the same area

This is a reminder in order to interpret an ospf database on a Cisco router. I have created the following lab to test the 'show ip ospf datase':

In red, the router ID:

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

The list of all routers in the area:

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         129         0x80000002 0x0057AC 1
2.2.2.2         2.2.2.2         25          0x80000003 0x00536B 2
3.3.3.3         3.3.3.3         24          0x80000002 0x0016B6 2

All the network segments in the area and the DR for each segment. Here, 10.1.2.2 is the IP address of the DR on the segment and 2.2.2.2 is the router id:

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.2.2        2.2.2.2         103         0x80000001 0x0021F5
10.1.4.2        2.2.2.2         25          0x80000001 0x006F9D

Each subnet, even if there is no OSPF neighbor:

R1#show ip ospf database internal

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Stub Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Mask
10.1.2.255      1.1.1.1         355         0x0        0x006D69 /24
10.1.3.255      2.2.2.2         98          0x0        0x00448D /24
10.1.4.255      2.2.2.2         315         0x0        0x003997 /24
10.1.5.255      3.3.3.3         230         0x0        0x0010BB /24