MPLS Architecture

1
MPLS Architecture

• Gautham Pamu
• CS590F - Design of MultiService Networks

2
Goals of MPLS

• Scalability of network layer routing.
• Using labels as a means to aggregate forwarding
  information,while working in the presence of
  routing hierarchies.
• Greater flexibility in delivering routing
  services.
• Using labels to identify particular traffic which
  are to receive special services, e.G. QoS.
• Increased performance.
• Using the label-swapping paradigm to optimize
  network performance.

3
Goals of MPLS

• Simplify integration of routers with cell
  switching based technologies.
• Making cell switches behave as routers.
• By making information about physical topology
  available to network layer routing procedures.

4
Motivation Behind MPLS

• MPLS improves internet scalability by eliminating
  the need for each router and switch in a packet's
  path to perform traditionally redundant address
  lookups and route calculation.
• Improves scalability through better traffic
  engineering.
• MPLS also permits explicit backbone routing,
  which specifies in advance the hops that a packet
  will take across the network.
• This should allow more deterministic, or
  predictable, performance that can be used to
  guarantee QoS.

5
Introduction to MPLS

• These paths function at layer 3 or can even be
  mapped directly to layer 2 transport such as ATM
  or frame relay.
• Explicit routing will give IP traffic a semblance
  of end-to-end connections over the backbone.
• The MPLS definition of IP QoS parameters is
  limited.
• Out of 32 bits total, an MPLS label reserves just
  three bits for specifying QoS.

6
Introduction to MPLS

• Label-switching routers (LSRs) will examine these
  bits and forward packets over paths that provide
  the appropriate QoS levels. But the exact values
  and functions of these so-called 'experimental
  bits remain to be defined.
• The MPLS label could specify whether traffic
  requires constant bit rate (CBR) or variable bit
  rate (VBR) service, and the ATM network will
  ensure that guarantees are met.

7
MPLS Architecture
MPLS Egress Node
MPLS Ingress Node
8
Labels

• A label is short, fixed length physically
  continuous identifier which is used to identify a
  FEC ( forwarding equivalence class), usually of
  local significance.
• Ru can transmits a packet labeled L to Rd, if
  they can agree to a binding between label L and
  FEC F for packets moving from Ru to Rd.
• Ru (upstream LSR) ? Rd (downstream LSR with
  respect to a given binding).
• L becomes Rus outgoing label representing FEC
  F, and L becomes rds incoming label
  representing FEC F.
• Rd must make sure that the binding from label to
  FEC is one-to-one.

9
Labels

• Rd must not agree with Ru1 to bind L to FEC F1,
  while agreeing with some other LSR Ru2 to bind L
  to a different FEC F2, unless rd can always tell,
  when it receives a packet with incoming label L,
  whether the label was put on the packet by Ru1 or
  Ru2.

L for FEC F1
Ru1
Rd
L for FEC F2
Ru2
10
Labeled Packet

• A packet into which a label has been encoded.
• The label resides in an encapsulation header
  which exists specifically for this purpose.
• Or the label may reside in a existing data link
  or network layer header.
• The particular encoding technique which is used
  must be agreed to by both the entities which
  encodes the label and the entity which decodes
  the label.

11
Label Assignment and Distribution

• The decision to bind a particular label L to a
  particular FEC F is made by the LSR which is
  downstream with respect to that binding.
• The downstream LSR informs the upstream LSR of
  the binding.
• The labels are downstream assigned and label
  binding are distributed in the downstream to
  upstream direction.

12
Label Distribution Protocols

• It is set of procedures by which one LSR informs
  another LSRs of the bindings (label/FEC) it has
  made.
• Two LSRs which use a distribution protocol to
  exchange label/FEC binding information are known
  as label distributing peers with respect to the
  binding information they exchange.
• There exists many different distribution
  protocols ( MPLS- BGP, MPLS-RSVP,
  MPLS-RVSP-TUNNELS, MPLS-CR-LDP).

13
Unsolicited Downstream Vs. Downstream-on-demand

• Downstream-on-demand label distribution.
• An LSR explicitly request (a label binding for
  that FEC ),from its next hop for a particular
  FEC.
• Unsolicited downstream label distribution.
• LSR distribute bindings to LSRs that have not
  explicitly requested them.
• Both these label distribution techniques can be
  used in the same network at the same time.
• Which protocol is provided by the MPLS
  implementation depends on the characteristics of
  the interfaces which are supported by a
  particular implementation.

14
Label Retention Mode

• An LSR Ru receives a label binding for a
  particular FEC from an LSR Rd, even though Rd is
  not Rus next hop. Ru then has the choice of
  whether to keep track or discard it.
• Liberal label retention mode.
• It maintains the bindings.
• Allows for quicker adaptation to routing changes.
• Conservative label retention mode.
• It discards such bindings.
• Requires an LSR to maintain many few labels.

15
The Label Stack

• Label stack carries a number of labels organized
  as a last-in, first out stack.
• The processing is always based on the top label.
• An unlabeled packet can be thought as a packet
  whose label stack is empty.

L1
L2
Packet
L3
16
NHLFE

• NHLFE (Next Hop Label Forwarding Entry) is used
  when forwarding a labeled packet.
• It contains the following information.
• The packets next hop.
• The operation to perform on the packets label
  stack.
• Replace the label at the top of the label stack
  with a specified new label.
• Pop the label stack.
• Replace the label at the top of the label stack
  with a specified new label, and then push one or
  more specified new labels onto the label stack.

17
Incoming Label Map (ILM)

• Maps each incoming label to a set of NHLFEs.
• Used when forwarding packets that arrive as
  labeled packets.
• Exactly one element of set must be chosen before
  the packet is forwarded.
• It is used to load balance over multiple
  equal-cost paths.

Set of NHFLE
Label
18
FEC-to-NHFLE Map (FTN)

• FTN maps each FEC to a set of NHFLEs.
• It is used when forwarding packets that arrive
  unlabeled, but are labeled before being
  forwarded.

Set of NHFLE
FEC
19
Label Swapping

• In order to forward a labeled packet, a LSR
  examines the label at the top of the label stack.
  It uses the ILM to map this label to an NHLFE.
• Using the information in the NHFLE, it determines
  where to forward the packet, and performs an
  operation on the packets label stack. It then
  encodes the new label stack into the packet, and
  forwards the result.

20
Label Swapping

• In order to forward an unlabeled packet, a LSR
  analyzes the network layer header, to determine
  the packets FEC. It then uses FTN to map this
  label to an NHFLE.
• Using the information in the NHFLE, it determines
  where to forward the packet, and performs an
  operation on the packets label stack. It then
  encodes the new label stack into the packet, and
  forwards the result.

21
Uniqueness of Labels

• A given LSR Rd may bind label L to FEC F1, and
  distribute that binding to label distribution
  peer Ru1. Rd may also bind a label to FEC F2, and
  distribute that binding to label distribution
  peer Ru2. If RD can tell when it receives a
  packet whose top label is L, whether the label
  was put there by RU1 or RU2, then the
  architecture does not require that F1F2.
• Rd may be using different label space for the
  labels it distributed to Ru1 than to Ru2.

22
LSP, LSP Ingress, LSP Egress

• A label switched path (LSP) of level m for a
  particular packet P is a sequence of routers.
• ltR1, ., Rngt
• With following properties
• R1, the LSP ingress, is an LSR which pushes a
  label onto Ps label stack, resulting in a label
  stack of depth m.
• For all I, 1lt I lt n, P has a label stack of depth
  m when received by LSR Ri.
• At no time during Ps transit from R1 to rn-1
  does it label stack ever have a depth of less
  than m.

23
LSP, LSP Ingress, LSP Egress

• A label switched path (LSP) of level m for a
  particular packet P is a sequence of routers.
• Which begins with an LSR ( an LSP ingress) that
  pushes on a level m label.
• All of whose intermediate LSRs make their
  forwarding decision by label switching on a level
  m label.
• Which ends ( at an LSP egress) when a
  forwarding decision is made by label switching on
  a level m-k label, where k gt 0 or when a
  forwarding decision is made by ordinary,
  non-MPLS forwarding procedures.

24
Penultimate Hop Popping

• If ltR1, , Rngt is level m LSP for packet P, P may
  be transmitted from rn-1 to Rn with a label
  stack of depth m-1. That is, the label stack may
  be popped at the penultimate LSR of the LSP,
  rather than at the LSP egress.
• Once rn-1 has decided to send the packet to Rn,
  the label no longer has any function, and need no
  longer be carried.
• Allows the egress to do a single lookup, and also
  requires only a single lookup by the penultimate
  node.
• In this case, LSP egress need not even be an LSR.

25
Penultimate Hop Popping

• There may be situations when penultimate hop
  popping is not desirable.
• Therefore the penultimate node pops the label
  stack only if this is specifically requested by
  the egress node.
• Or
• If the next node in the LSP does not support MPLS.

26
LSP Next Hop

• The LSP next hop for a particular labeled packet
  in a particular LSR is the LSR which is the next
  hop, as selected by the NHFLE entry used for
  forwarding that packet.
• The LSP next hop for a particular FEC is the next
  hop as selected by the NHFLE entry indexed by the
  label which corresponds to that FEC.
• The LSP next hop may differ from the next hop
  which would be chosen by the network layer
  routing algorithm.

27
Invalid Incoming Labels

• What should an LSR do if it receives a labeled
  packet with a particular incoming label, but has
  no binding for that label ?
• It must be discarded.
• It is not safe to strip off the label and the
  packet is forwarded as an unlabeled packet.
• It could cause a loop.

28
Independent LSP Control

• Independent LSP control each LSR, upon noting
  that it recognizes a particular FEC, make an
  independent decision to bind a label to that FEC
  and to distribute that binding to its label
  distribution peers.
• Similar to conventional IP datagram routing,
  where each node makes an independent decision as
  to how to treat each packet, and relies on the
  routing algorithm to converge rapidly so as to
  ensure that each datagram is correctly delivered.

29
Ordered LSP Control

• Ordered LSP Control an LSR only binds a label
  to a particular FEC if it is the egress LSR for
  that FEC, or if it has already received a label
  binding for that FEC from its next hop for that
  FEC.
• To ensure that traffic in a particular FEC follow
  a path with some specified set of properties,
  ordered control must be used.
• It has to be initiated either by the egress or
  the ingress LSR.

30
LSP Control

• To have ordered control all LSRs in an LSP
  should use ordered control otherwise the overall
  effect on the network behavior is largely that of
  independent control, since one cannot be sure
  that an LSP is not used until it is fully setup.
• Both methods interoperate.
• A given LSR needs support of only one or other.

31
Aggregation

• In MPLS Domain, all the traffic in a set of FECs
  might follow the same route.
• Example a set of distinct address prefixes
  might all have the same egress node. In such
  case, the union of those FECs it itself a FEC.
• The procedure of binding a single label to a
  union of FECs which is itself a FEC, and applying
  that label to all traffic in the union is known
  as Aggregation.
• It reduces the no of labels and reduces the
  amount of label distribution control traffic
  needed.

32
Whose Granularity Is Used ?

• In ordered control each LSR should adopt, for a
  given set of FECs, the granularity used by its
  next hop for those FECs.
• In independent control it is possible that
  there will be two adjacent LSRs, Ru and Rd, which
  aggregate some of FECs differently.

33
Granularity of Aggregation

• If Ru has finer granularity than Rd, this does
  not cause a problem.
• Ru distribute more labels for that set of FECs
  than Rd does. This means that when Ru needs to
  forward labeled packets in those FECs to Rd, it
  may withdraw the set of n labels into m labels,
  where n gt m. Or it may with draw the set of n
  labels it has distributed and then distribute a
  set of m labels.

34
Granularity

• If Ru has coarser granularity than Rd, it has two
  choices.
• It may adopt Rds finer level of granularity,
  This would require it to withdraw m labels it has
  distributed and distribute n labels.
• It may simply map its m labels into a subset of
  Rds n labels, if it can determine that this will
  not produce the same routing.

35
Route Selection

• Route selection refers to the method used for
  selecting the LSP for a particular FEC.
• Hop by Hop Routed LSP each node independently
  choose the next hop for that FEC.
• Explicitly routed LSP each LSR does not
  independently choose the next hop, rather, a
  single LSR, generally the LSP ingress or the LSP
  egress, specifies several of the LSRs in the LSP.

36
Advantages of Explicit Routing

• It is useful for policy routing and traffic
  engineering.
• The explicit route has to be specified at the
  time that labels are assigned, but the explicit
  route does not have to be specified with each IP
  packet.
• It makes MPLS explicit routing much more
  efficient than the alternative of IP source
  routing.

37
Lack of Outgoing Label

• If a labeled packet reaches an LSR at which the
  ILM does not map the packets incoming label into
  an NHFLE, even though the incoming label is
  valid.
• Discard the packet to be safe
• It is unsafe to strip off the label stack and
  attempt to forward the packet further via
  conventional forwarding, based on its network
  layer header.

38
TTL

• Provides some level of protection against
  forwarding loops that exist due to
  misconfigurations or due to failure or slow
  convergence of the routing algorithm.
• It also supports traceroute commands and
  multicast scoping.
• The MPLS label values are carried in an
  MPLS-specific shim header.
• If the MPLS labels are carried in an L2 header,
  such as ATM header or a frame relay

39
Loop Control

• On a non-TTL LSP Segment, TTL cannot be used to
  protect against forwarding loops.
• It depends on the hardware used to provide the
  LSR functions along the non-TTL LSP segment.
• Example ATM hardware is used to provide MPLS
  switching function, with the label being carried
  in the VPI/VCI field. Since ATM switching
  hardware cannot decrement TTL, there is no
  protection again loops.
• If it provides fair access to the buffer pools
  for incoming labels, this looping may not cause
  deleterious effect on other traffic otherwise
  even transient loops may cause severe degradation
  of the LSRs total performance.

40
Label Encoding

• Architecture supports different encoding
  techniques, the choice of encoding technique
  depends on the particular kind of device being
  used to forward labeled packets.
• MPLS-specific hardware/software to forward
  packets.
• To encode the label stack, we need to define a
  new protocol to be used as a shim between the
  data link layer and network layer headers.
• It is protocol independent, used to encapsulate
  any network layer.

41
ATM switches as LSRs

• MPLS forwarding procedures are similar to those
  legacy label swapping switches such as ATM
  switches.
• ATM switches use the input port and the incoming
  VPI/VCI value as the index into a switching
  table, from which they obtain the output port and
  an outgoing VPI/VCI value.
• If one or more labels can be encoded directly
  into the fields which are accessed by these
  switches, then the switches can be used as LSRs.

42
Encoding labels in ATM Cell header

• SVC Encoding
• Use the VPI/VCI to encode the label which is at
  the top of the label stack.
• Can be used in any network.
• Label distribution protocol becomes ATM
  signaling.
• ATM LSRs cannot perform push and pop
  operation on the label stack.

43
Encoding labels in ATM Cell header

• SVP Encoding
• Use VPI field to encode the label which is at the
  top of the label stack and VCI field to encode
  the second label on the stack.
• Cannot always be used, when the network includes
  an ATM Virtual path through a non-MPLS ATM
  network.
• Since VPI field is not necessarily available for
  use by MPLS.

44
Encoding labels in ATM Cell header

• SVP Multipoint Encoding
• Use the VPI field to encode the label which is at
  the top of the label stack, use part of the VCI
  field to encode the second label on the stack, if
  one is present, and use the remainder of the VCI
  to identify the LSP egress.
• It enables us to do label merging, without
  running into cell interleaving problems, on ATM
  switches which can provide multipoint-to-point
  VPs, but which do not have the VC merge
  capability.

45
Switching Table

• VPI/VCI label is looked up in switching table
• Output port is chosen
• VPI/VCI is relabeled
• VPI/VCI has local significance only

VPI.in
VCI.in
Port.in
VPI.out
VCI.out
Port.out
46
ATM Switching example
Port 1
Port 1
(2,1)
(3,1)
(2,1)
Port 1
Port 1
(1,1)
(2,1)
(1,1)
(2,2)
(1,2)
(1,2)
(2,2)
Port 2
Port 2
(1,3)
(4,1)
Port 1
Port 1
(2,1)
(4,2)
(1,1)
Port 2
Port 2
(4,1)
(2,1)
(4,2)
(4,2)
(1,3)
Port 2
Port 2
47
Interoperability Among Encoding Techniques

• If ltR1, R2, R3gt is a segment of a LSP, it is
  possible that R1 will use one encoding of the
  label stack when transmitting packet P to R2 but
  R2 will use a different encoding when
  transmitting a packet P to R3.
• Architecture supports LSPs with different label
  stack encoding used on different hops.
• ATM switches have no capability of translating
  from one encoding technique to another.

48
Interoperability Among Encoding Techniques
LSR with SHIM interface
LSR
L1
P
P
L2
LSR
The LSR may swap off an ATM encoded label stack
on the Incoming interface and replace With MPLS
shim header
The LSR may swap off an MPLS shim encoded label
stack on the Incoming interface and replace With
ATM encoded label
LSR
LSR with ATM Interface
49
Label Merging

• With label merging, the no of outgoing labels per
  FEC need only be one.

L1 for FEC F
LSR
L1
L2 for FEC F
LSR
LSR
L2
L4
L4
L4
LSR
L3
With label merging the no of outgoing Labels per
FEC need only be one.
L3 for FEC F
50
Merge over ATM

• Methods to eliminate the cell interleaving
  problem in ATM.
• VP Merge, using the SVP Multipoint Encoding
• Multiple virtual paths are merged into a virtual
  path, but packets from different sources are
  distinguished by using different VCIs with the
  VP.
• VC Merge
• Switches are required to buffer cells from one
  packet until the entire packet is received.

51
Applications of MPLS

• MPLS and Hop by Hop Routed Traffic
• MPLS and Explicitly Routed LSP
• MPLS and Multicast
• MPLS and Multi-Path Routing

52
Diff-Serv and MPLS

• Two major IETF standardization efforts are making
  IP QoS a reality. Sometimes perceived as rivals.
• Both are in fact complementary developments that
  approach the QoS challenge from two different
  network perspectives.
• DiffServ and MPLS are in fact independent
  developments that can function with or without
  each other's help. Neither specification requires
  the other, but MPLS networks should be able to
  derive QoS status from DiffServ traffic.
• There is hope that they can be used together as
  access (DiffServ) and backbone (MPLS)
  counterparts.

53
Diff-Serv

• DiffServ is a layer 3 solution that addresses QoS
  requirements in a connectionless environment.
• Its main purpose is to standardize a set of QoS
  building blocks with which providers can fashion
  QoS-enhanced IP services.
• DiffServ QoS is meant to be implemented at the
  network edge by access devices and then supported
  across the backbone by DiffServ-capable routers.
• Since it operates purely at layer 3, DiffServ can
  be deployed on any layer 2 infrastructure.
  DiffServ and non-DiffServ routers and services
  can be mixed in the same environment.

54
Conclusion

• MPLS is a strategy for streamlining the backbone
  transport of IP packets across a layer 3/layer 2
  network. Although it does involve QoS issues,
  that is not its main purpose.
• MPLS is focused mainly on improving internet
  scalability through better traffic engineering.
• MPLS will help to build backbone networks that
  better support QoS traffic, but it entails
  significant changes in existing network
  architecture.

55
Conclusion

• MPLS is essentially a hybrid of the network
  (layer 3) and transport (layer 2) structure, and
  may represent an entirely new way of building IP
  backbone networks.
• In the near term, DiffServ may have more
  relevance. It tackles IP QoS head-on, and it
  provides mechanisms for achieving both access QoS
  and backbone QoS across the network.
• The specification is drafted, early
  implementations of the technology have proven
  stable for over half a year, and standards-based
  products will soon be available.

56
Conclusion

• MPLS, on the other hand, is not expected to reach
  RFC status until some time later this year.
  Backed by established and emerging players, such
  as Cisco systems, inc. And juniper networks,
  inc., MPLS should become a major element of
  internet backbone growth next year.