MPLS(4) Kernel Interfaces Manual MPLS(4)

NAME

mpls — Multiprotocol Label Switching

SYNOPSIS

options MPLS
pseudo-device ifmpls
#include <sys/types.h>
#include <netmpls/mpls.h>

DESCRIPTION

MultiProtocol Label Switching represents a mechanism which directs and carries data in high-performance networks, its techniques being applicable to any network layer protocol.

In an MPLS domain the assignment of a particular packet a particular Forward Equivalence Class is done just once, as the packet enters the network. The FEC to which the packet is assigned is encoded as a short fixed length value known as a “label”. When a packet is forwarded to the next hop, the label is sent along with it; that is, the packets are “labeled” before they are forwarded.

A router capable of receiving and forwarding MPLS frames is called “Label Switch Router” or LSR. Label scope is generally router-wide meaning that a certain label has a specific meaning only for a certain LSR.

Currently, NetBSD supports MPLS over Ethernet interfaces and GRE tunnels. For these kind of interfaces, a label is contained by a fixed sized “shim” that precedes any network layer headers, just after data link layer headers.

MPLS shim header structure

In network bit order:

------------------------------------------- 
|               |        |       |        | 
| Label         | Exp.   | BoS   | TTL    | 
| 20 bits       | 3 bits | 1 bit | 8 bits | 
|               |        |       |        | 
-------------------------------------------
Label
20 bits representing FEC, consequently the only information used to forward the frame to next-hop
Experimental
3 bits that are sometimes used for specifying a type of service
Bottom of Stack
1 bit that is set for the last entry in the shim stack and 0 for all others. This way, multiple labels can be prepended to a single packet.
TTL
8 bits, representing Time to Live, decremented at every LSR.

USAGE

The MPLS behavior is controlled by the net.mpls sysctl(8) tree:
net.mpls.accept
If zero, MPLS frames are dropped on sight on ingress interfaces.
net.mpls.forwarding
If zero, MPLS frames are not forwarded to next-hop.
net.mpls.ttl
The default ttl for self generated MPLS frames.
net.mpls.inet_mapttl
If set, TTL field from IP header will be mapped into the MPLS shim on encapsulation, and the TTL field from MPLS shim will be copied into IP header on decapsulation.
net.mpls.inet6_mapttl
The IPv6 version of the above.
net.mpls.inet_map_prec
If set, precedence field from IP header will be mapped into MPLS shim EXP bits on encapsulation, and the MPLS EXP field will be copied into IP Precedence field on decapsulation.
net.mpls.inet6_map_prec
The IPv6 version of the above.
net.mpls.icmp_respond
Returns ICMP TTL exceeded in transit when an MPLS frame is dropped because of TTL = 0 on egress interface.
In order to encapsulate and decapsulate to and from MPLS, an mpls pseudo-interface must be created and packets that should be encapsulated must be routed to that interface.

“Pure” MPLS routes can be created using AF_MPLS sa_family sockaddrs for destination and tag fields. Other protocols can be encapsulated using routes pointing to mpls pseudo-interfaces, and AF_MPLS sockaddrs for tags. Decapsulation can be made using values of reserved labels set in the tag field (see below). For more information about doing this using userland utilities see the EXAMPLES section of this manual page.

The netstat(1) and route(8) utilities should be used to manage routes from userland.

ldpd(8) should be used in order to automatically import, manage and distribute labels among LSRs in the same MPLS domain.

RESERVED LABELS

MPLS labels 0 through 15 are reserved. Out of those, only four are currently defined:
0
IPv4 Explicit NULL label. This label value is only legal at the bottom of the label stack. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv4 header.
1
Router Alert Label. Currently not implemented in NetBSD.
2
IPv6 Explicit NULL label. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv6 header.
3
Implicit NULL label. This is a label that an LSR may assign and distribute, but which never actually appears in the encapsulation. When an LSR would otherwise replace the label at the top of the stack with a new label, but the new label is “Implicit NULL”, the LSR will pop the stack instead of doing the replacement.

EXAMPLES

  1. Create an MPLS interface and set an IP address:

    # ifconfig mpls0 create up 
# ifconfig mpls0 inet 192.168.0.1/32
  2. Route IP packets into MPLS domain with a specific tag

    # route add 10.0.0.0/8 -ifp mpls0 -tag 25 -inet 192.168.1.100
  3. Create a static MPLS forwarding rule - swap the incoming label 50 to 33 and forward the frame to 192.168.1.101 and verify the route

    # route add -mpls 50 -tag 33 -inet 192.168.1.101 
add host 50: gateway 192.168.1.101 
# route -n get -mpls 50 
   route to: 50 
destination: 50 
    gateway: 192.168.1.101 
        Tag: 33 
 local addr: 192.168.1.180 
  interface: sk0 
      flags: <UP,GATEWAY,HOST,DONE,STATIC> 
recvpipe  sendpipe  ssthresh  rtt,msec    rttvar  hopcount      mtu     expire 
      0         0         0         0         0         0         0         0 
sockaddrs: <DST,GATEWAY,IFP,IFA,TAG>
  4. Route IP packets into MPLS domain but use a different source address for local generated packets.

    # route add 10.0.0.0/8 -ifa 192.168.1.180 -ifp mpls0 -tag 25 -inet 192.168.1.100
    For the latter example, setting an IP address for the mpls0 interface is not necessary.
  5. Route MPLS packets encapsulated with label 60 to 192.168.1.100 and POP label

    # route add -mpls 60 -tag 3 -inet 192.168.1.100
  6. Route IP packets into MPLS domain and prepend more tags

    # route add 10/8 -ifa 192.168.1.200 -ifp mpls0 -tag 20,30,40 -inet 192.168.1.100
    For the above example, tag 20 will be inserted at Bottom of Stack, while tag 40 will be set into the outermost shim.
  7. Replace label 60 with label 30, prepend two more labels: 40 and 41 (in this order) and forward the result to 192.168.1.100

    # route add -mpls 60 -tag 30,40,41 -inet 192.168.1.100

SEE ALSO

netstat(1), route(4), ldpd(8), route(8), sysctl(8)

Multiprotocol Label Switching Architecture, RFC 3031.

MPLS Label Stack Encoding, RFC 3032.

HISTORY

The mpls support appeared in NetBSD 6.0.

SECURITY CONSIDERATIONS

User must be aware that encapsulating IP packets in MPLS implies a major security effect when using firewalls. Currently neither ipf(4) nor pf(4) implement the heuristics in order to look inside an MPLS frame. Moreover, it's technically impossible in most cases for an LSR to know information related to encapsulated packet. Therefore, MPLS Domains should be strictly controlled and, in most cases, limited to trusted connections inside the same Autonomous System.

Users must be aware that the MPLS forwarding domain is entirely separated from the inner (IP, IPv6 etc.) forwarding domain and once a packet is encapsulated in MPLS, the former forwarding is used. This could result in a different path for MPLS encapsulated packets than the original non-MPLS one.

IP or IPv6 forwarding is not necessary for MPLS forwarding. Your system may still forward IP or IPv6 packets encapsulated into MPLS if net.mpls.forwarding is set.

June 29, 2010 NetBSD 6.1