MPLS

1
MPLS

• H/W update
• Brief description of the lab
• What it is?
• Why do we need it?
• Mechanisms and Protocols

2
Multi-Protocol Label Switching(MPLS)

• A mechanism for building tunnels
• Label Switched Path (LSP)
• Forwarding based on labels
• 20 bits for label
• Packet contains MPLS header
• Label
• 3 EXP (experimental) bits
• 8 bits for TTL
• 1 bit for bottom of stack
• EXAMPLE
• LSPs are unidirectional

3
Label Operations

• PUSH
• Add a label on the packet
• At the entry point of the tunnel
• Packet may already have a label
• Label stack
• POP
• Remove a label from the packet
• At the exit point of the tunnel
• SWAP
• Exchange the label on the packet with another one
  
• During forwarding inside the tunnel

4
LSP

• Has an ingress
• Entrance
• Egress
• Exit
• It goes over some transit nodes
• What traffic does it carry
• Forwarding Equivalence class (FEC)
• Anything that matches some rules that use the IP
  header information
• We not have the EDGE where information is put
  into the LSP
• Edge classifies packet into a FEC and there is no
  need to do it again inside the network

5
MPLS forwarding

• Routers than can do MPLS forwarding are called
  Label Switch Routers (LSRs)
• Both a router and an LSR
• Before we had the RIB and FIB
• Now we (also) have the LIB and the LFIB
• Lookup incoming label in LFIB
• Determine where the packet goes and
• What label it should carry
• SWAP labels and send the packet

6
Label Stack

• Can have tunnel hierachies
• Push another label on a labeled path
• I have a label stack
• Forwarding decisions are always based on the
  label at the top of the stack
• Swap operations apply to the label at the top of
  the stack

7
Signaling

• In order to create the LSP somebody has to setup
  the labels
• They are put in the LFIB Label FIB
• MPLS signaling protocols
• Label Distribution Protocol (LDP)
• RSVP-TE
• Even BGP (for VPNs)
• They distribute labels and establish tunnels

8
Reserved Labels

• 0 IPv4 explicit NULL
• 1 Router Altert
• 2 IPv6 explicit NULL
• 3 - Implicit NULL
• What are these NULLs?
• Determine what the Penultimate hop (PHP) will do
• May pop the label completely
• Or may keep an empty label in the packet

9
PHP

• Penultimate Hop Popping
• The egress scales better
• But does not know that it was an MPLS packet
• Potentially useful information (e.g. EXP bits)
  are lost

10
Label Distribution modes

• Ordered vs. Independent
• Will I wait for the downstream nodes to give me
  their label?
• Unsolicited vs. on-demand
• Will I wait for a neighbor to ask me for a label
  or I will send my labels to them anyway?
• Label Space
• Per platform
• Per interface
• Labels have link-local scope
• They are unique for the same link only

11
What is the big deal

• Initially people though MPLS forwarding will be
  faster than IP forwarding
• Not an issue anymore
• Now people like MPLS because it can be used to
  setup tunnels
• Tunnels for Traffic Engineering
• Tunnels for Recovery from failures
• Tunnels for VPNs

12
How can I use the tunnels

• Use them if I am their ingress
• Can have a static route pointing to a tunnel
  next-hop
• Share traffic
• Between IGP
• Between IGP/LSPs
• Between LSPs
• Can share with unequal weights
• Advertise them in IGP and use them as links
• forwarding adjacencies
• I already can do things that I could not do with
  IGP
• Can control quite well where my traffic goes
• EXAMPLE

13
Intro to TE

• An ISP has a number of core routers that attach
  to PoPs
• ISP usually knows the traffic matrix
• Traffic demands between PoPs
• ISP would like to control exactly how this
  traffic moves around in his network
• Can setup a full mesh of LSPs and control exactly
  what is going on
• Of course scalability is a issue

14
RSVP-TE

• Based on Resource Reservation Protocol
• Extended to carry labels (TE)
• It is a signaling protocol
• Sets up network state
• Does not have anything to do with
  routing/forwarding
• Basic Idea
• I am telling RSVP the path I want and it will
  setup an LSP over this path
• Specify the whole path
• Loose of strict source route
• RSVP calls it Explicit Route object (ERO)

15
PATH and RESV messages

• Ingress sends a PATH message downstream
• Follows the route we specified until it hits the
  egress
• ERO or simply IP forwarding towards the
  destination
• Egress sends a RESV message upstream
• Until it hits the ingress
• Intermediate routers install state
• Labels are allocated
• Downstream ordered mode

16
Soft state

• PATH and RESV messages create state in RSVP
  routers
• PATH and RESV messages are send periodically to
  refresh this state
• If state is not refreshed in time it expires and
  it is cleared
• Simpler
• Do not need to worry about reliable messages etc
• Do not need to check with hellos if my neighbor
  is alive
• But I have the overhead of state refreshes

17
A big example

• How messages are sent
• How labels are allocated