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NetBSD 6.1.5 - man page for mpls (netbsd section 4)

MPLS(4) 			   BSD Kernel Interfaces Manual 			  MPLS(4)

     mpls -- Multiprotocol Label Switching

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

     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 oth-
		      ers.  This way, multiple labels can be prepended to a single packet.

     TTL	      8 bits, representing Time to Live, decremented at every LSR.

     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-inter-
     faces, 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 user-
     land 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.

     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 for-
	warding 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.

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

	  # ifconfig mpls0 create up
	  # ifconfig mpls0 inet

     2.   Route IP packets into MPLS domain with a specific tag

	  # route add -ifp mpls0 -tag 25 -inet

     3.   Create a static MPLS forwarding rule - swap the incoming label 50 to 33 and forward the
	  frame to and verify the route

	  # route add -mpls 50 -tag 33 -inet
	  add host 50: gateway
	  # route -n get -mpls 50
	     route to: 50
	  destination: 50
		  Tag: 33
	   local addr:
	    interface: sk0
	  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 gener-
	  ated packets.

	  # route add -ifa -ifp mpls0 -tag 25 -inet
	  For the latter example, setting an IP address for the mpls0 interface is not necessary.

     5.   Route MPLS packets encapsulated with label 60 to and POP label

	  # route add -mpls 60 -tag 3 -inet

     6.   Route IP packets into MPLS domain and prepend more tags

	  # route add 10/8 -ifa -ifp mpls0 -tag 20,30,40 -inet
	  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

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

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

     Multiprotocol Label Switching Architecture, RFC 3031.

     MPLS Label Stack Encoding, RFC 3032.

     The mpls support appeared in NetBSD 6.0.

     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 Au-
     tonomous 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 for-
     warding 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.

BSD					  June 29, 2010 				      BSD

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