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MPLS BASICS:

Multi Protocol Label Switching (MPLS) represents a simple technology, which is
today indispensable in network cores. MPLS protocol adds a 32-bit header
(Figure 1) between the second and the third network layer headers. 
Within these 32 bits, 20 bits are reserved for the label, 3 bits for
the Traffic Class (TC) field, 1 bit for the Bottom of Stack (S) field
and the remaining 8 bits are reserved for the Time to Live (TTL) field.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
|                Label                  | Exp |S|       TTL     | Stack
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

                    Label:  Label Value, 20 bits
                    Exp:    Experimental Use, 3 bits
                    S:      Bottom of Stack, 1 bit
                    TTL:    Time to Live, 8 bits

                              Figure 1

A traffic class carried by means of a MPLS packet is defined based on the TC field.
Definition of an S field enables assigning of several labels to the packets. In the
label grouping, only the lowest label, closest to the IP packet, has S bit set to 1.
All other labels in the label stack have S set to 0. TTL field is used just like a
TTL field of the IP packet.

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BUILDING: 

MPLS can be built as a kernel module, or it can be built in a kernel.

To build MPLS first you need to run:

$ make menuconfig

and enable MPLS compiling from:

Network setup -> Networking options -> MPLS (Experimental)

If you're running Debian based system, nice HOW-TO on compiling and installing custom kernel
can be found here:

https://help.ubuntu.com/community/Kernel/Compile

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RUNNING: 

To be able to run MPLS, you will need modified iproute2 programme which can be downloaded from:

https://github.com/i-maravic/iproute2

Creation of MPLS tunnel ingress points is done with:

# ip link add tunnel_name type mpls push 300 192.168.2.2

or with

# ip route add 1.2.3.4 mpls push 300 192.168.2.2

To get more comprehensive explanation use:

$ ip link add tunnel_name type mpls help

or

$ ip route help

In order to be able to receive MPLS packets, interface must be MPLS enabled. This is done with:

# ip link set eth0 mpls on

To be able to receve MPLS packets recursively mpls0 interface must be brought up!

To build MPLS tunnel cross connect and egress points use:

# ip -M route add 200 mpls swap 300 dev eth0 192.168.3.3

MPLS SNMP stats are located in:

/proc/net/mpls_stats

MPLS sysctl parameters are located in:

/proc/sys/net/mpls

MPLS fully supports net namespaces.

IP-MPLS multipath routes are suported

Reserved labels, except the RouterAlert label, are handled properly.

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TODO:
1. Firewall support
2. ICMPv6 message sending, when TTL is exceeded or packet can't be forwarded because of MTU,
   from the inside of MPLS tunnel
3. MPLS multipath
3. Bridging support