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INTRO TO MPLS. CSE 8344 Southern Methodist University Fall 2003. Introduction. We will be discussing material from MPLS, Technology and Applications Read Chapters 1-5 and we will resume this topic after the MidTerm

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intro to mpls

INTRO TO MPLS

CSE 8344 Southern Methodist University

Fall 2003

introduction
Introduction
  • We will be discussing material from MPLS, Technology and Applications
  • Read Chapters 1-5 and we will resume this topic after the MidTerm
  • These notes will be based on the MPLS book plus some presentations from Cisco, Nortel, Juniper as well as Dr. Nair’s notes from last time.
historical perspective
Historical Perspective
  • X.25, Frame and ATM have proven themselves as a viable Layer2 transport technology
  • Virtual Circuits are nice for a number of reasons:
    • The route through the network is predictable
    • IP Networks can be built on Top
    • Just Like Ethernet but spread over the wide area.
    • The trend has been for the carrier to supply the layer 2 links and the enterprise to build IP on top. (“Keep the carrier out of my routing.”)
    • Often the enterprise will get a separate T1 or VC to the internet.
  • But IP routers are getting much bigger
    • This create lots of problems and questions.
building big ip networks
Building big IP Networks
  • IP is beginning to dominate all other traffic
    • The need is for a cheap scalable IP network is the primary focus
  • 3 Different ways this could be built …..

1) Use physical layer interfaces to connect IP routers

Requires lots of physical layer interface cards in the router$$$$$$$$

Uses lots of bandwidth in backbone, because pipes may be sparsely filled.

2) Use a layer 2 mesh

Use ATM or Frame Switches in the core to create a LOGICAL MESH at Layer 2 with fewer physical layer connections between routers

OR..

building big ip networks cont
Building big IP Networks (cont)
  • Build a mesh of routers

This would use routers at both the edge of the network and the core

Use fewer physical layer connections

Lots of routers

Potentially lots of router hops through the network

Routers can be expensive and complicated

Flooded route updates can consume time and bandwidth

DataGram routing only

SO….

Most operators chose to go with Option 2

how can we optimize an ip network over layer 2 vcs
How can we optimize an IP network over Layer 2 VCs?
  • ATM has many features that are NOT necessary to support IP
    • SVC’s not needed
    • Most of IP has been best effort (so far anyway)
    • QoS and admission control is overkill
    • CBR not needed
  • Early attempts to integrate ATM and IP were interesting:
    • LANE, ATMARP, MPOA, etc.
    • All were attempts to preserve both ATM and IP

Doubly complicated because even more signaling and addressing schemes were required.

issues
Issues
  • Price and performance
  • Scalability
  • Flexibility of routing functionality
  • Tight coupling between routing and forwarding algorithms
issues ipoatm
Issues (IPOATM)
  • Exponential adjacencies
    • Means that more and more VCs must be built and managed to support the network.
    • If not, more hops
  • N^4 messages for topology changes
    • Changing the network can be a headache
  • Solution
    • Smart Label switching
extending router functionality
Extending Router Functionality

D

A

F

C

B

E

Plain old IP routing doesn’t allow us to route some traffic BD and some BE.

history
History
  • Cell switching router (CSR)
      • Toshiba
      • Mapped the IP signaling to control the ATM Network
      • Never really got off the ground
  • IP switching
    • Ipsilon
    • Strip the ATM hardware and use it as a router
    • Data driven connections through the network
  • Tag switching
    • Cisco
    • External control of Virtual Circuits (LSPs)
    • Not ATM specific
history cont
History (cont)
  • Multi-protocol Label Switching (MPLS) created in 1997
    • Consisted of Tag Switching plus other input from Ipsilon, IBM (ARIS) and others.
    • Worked in the Internet Engineering Task Force (IETF)
    • IETF creates the Request For Comments (RFCs) that are quasi-standards
ip switching1
IP Switching
  • Introduced by Ipsilon
  • Significant innovations
    • General switch management protocol (GSMP)
    • Label binding protocol, Ipsilon flow management protocol (IFMP)
  • GSMP allows an ATM switch to be controlled by an “IP switch controller”
ip switching premise
IP Switching Premise
  • IP over ATM models are complex and inefficient - involve running two control planes
    • ATM signaling and routing
    • IP routing and address resolution on top
  • In contrast IP Switching uses
    • IP component plus label binding protocol
    • Completely removes ATM control plane
  • Goal: To integrate ATM switches and IP routing in a simple and efficient way
removing atm control plane
Removing ATM Control Plane

IP

ATM MARS NHRP

ARP

PNNI

Q.2931

ATM hardware

  • (a) IP over Standard ATM
  • (b) IP Switching

IP IFMP

ATM hardware

(a)

(b)

ip switching architecture
IP Switching Architecture
  • Switch controller
    • Control processor of the system
    • Uses GSMP to communicate with ATM switch
    • Runs IP routing and forwarding code
  • Default VC
    • To get control traffic before IP Switching is performed
    • Uses well known VCI/VPI value
    • Used for data that doesn’t have a label yet
ip switch architecture
IP Switch Architecture

Switch controller

Flow Classification and control

To

downstream

switch

To

upstream

switch

Routing

and

forwarding

IFMP

GSMP

GSMP

Default

VC

Default

VC

Data

VC

Data

VC

Switch

switching basics
Switching Basics
  • Relies on IP protocols
    • To establish routing information
    • To determine next hop
  • Flow classification and control module selects flows from incoming traffic
  • IP flow refers to a sequence of datagrams
    • from one source to one destination, identified by the ordered pair <source address, destination address>
    • can also refer to a flow at finer granularity, e.g., different applications between same pair of machines, identified by < source address, source port, destination address, destination port>
flow redirection
Flow Redirection
  • Redirection: Process of binding labels to flows and establishing label switched paths
  • Example:
    • data is flowing from A via B to C on default VC
    • B sends a redirect to A specifying flow y and the label (VPI/VCI) on which it expects to receive
    • If C issues a redirect to B for flow y, B forwards y on the VPI/VCI specified by C
    • Since same flow y enters B on one VC and leaves on another, B uses GSMP to inform its switching element to set up the appropriate switching path
flow redirection1
Flow Redirection

Redirect:Flow y VPI/VCI 3/57

Switch

Controller

B

Switch B issues a REDIRECT message to switch A

A

A

C

Redirect:Flow y VPI/VCI 3/57

Redirect:Flow y VPI/VCI 2/22

Default VC

Default VC

Switch

Element

Switch

Controller

B

C

3/57

Default VC

Default VC

Switch

Element

3/57

2/22

Switch B and C redirect the same flow, allowing it to be switched at B

ipsilon flow management protocol ifmp
Ipsilon Flow Management Protocol (IFMP)
  • Designed to communicate flow to label binding information
  • IFMP is a soft state protocol
  • IFMP’s Adjacency Protocol:
    • Used to communicate and discover information about neighbors
    • Adjacency message sent as limited broadcast
  • IFMP’s Redirection Protocol
    • Used to send appropriate messages for flow-label bindings
adjacency protocol
Adjacency Protocol
  • To exchange initial set of information
  • ADJACENCY message encapsulated into IP datagram and sent to limited broadcast address
  • Also used to agree on the sequence numbers
ifmp s redirection protocol
IFMP’s Redirection Protocol
  • Different message types defined:
    • REDIRECT: used to bind label to a flow
    • RECLAIM: enables label to be unbound for subsequent re-use
    • RECLAIM ACK: Acknowledgement for RECLAIM message
    • ERROR: Used to deal with various error conditions
  • Common header format
ifmp redirect protocol message format
IFMP Redirect Protocol Message Format

IFMP REDIRECT message body

encapsulation of redirected flows
Encapsulation of Redirected Flows

Encapsulation of IP packet on the default VC

Encapsulation of IP packet on the redirected VCs

general switch management protocol gsmp
General Switch Management Protocol (GSMP)
  • GSMP is a master/slave protocol
    • ATM switch is the slave
    • Master could be any general purpose computer
  • The protocol allows the master to
    • Establish and release VC connections across the switch
    • Perform port management (Up, Down, Reset, Loopback)
    • Request Data (configuration information, statistics)
    • Allows slave to inform master of events such as link failure
gsmp cont d
GSMP (cont’d)
  • GSMP packets are LLC/SNAP encapsulated and sent over ATM link using AAL5
  • GSMP Adjacency Protocol
    • Used to gain information about the system at the other end of the link and
    • To monitor link status
  • GSMP Connection Management Protocol
    • Used to ensure consistency between the GSMP master and slave
    • Specifies the QoS using a priority field
design goals
Design Goals
  • Adding functionality
    • Explicit routing
  • Improve scalability
    • Hierarchy of routing knowledge
  • Link layer independent
    • Not just ATM
  • Implemented in a variety of devices such as routers and ATM switches
destination based routing
Destination Based Routing
  • A TSR uses information from unicast routing protocols to construct its mapping between FECs and next hops
  • This mapping is used by the Tag Switching Control component for constructing the TFIB which is used for actual packet forwarding
construction of tfib
Construction of TFIB
  • A local binding between the FEC and a tag
    • Takes a tag from the pool of free tags and uses it as an index in the TFIB to set the incoming tag entry
  • A mapping between the FEC and the next hop for that FEC (provided by the routing protocol(s) running on the TSR)
  • A remote binding between the FEC and a tag that is received from the next hop
example
Example

A

B

if0

if1

if2

if1

E

if2

if0

if0

if1

if2

if0

if2

if1

if0

TSR

C

D

192.6/16

initial tfib entries
Initial TFIB Entries

For FEC 192.6/16

behavior with routing change
Behavior With Routing Change

A

B

if0

if1

if2

if1

E

if2

if0

if0

if1

if2

if0

if2

if1

if0

TSR

C

D

Link Down

hierarchical routing
Hierarchical Routing
  • Scalability
  • Faster convergence
  • Fault isolation
hierarchy of routing knowledge
Hierarchy of Routing Knowledge
  • All TSRs within a routing domain participate in a common intra-domain routing protocol and construct TFIB corresponding to destinations within the domain
  • All border TSRs or TERs within a domain and directly connected TERs from other domains also exchange Tag binding information via inter-domain routing protocol
hierarchy cont d
Hierarchy (Cont’d)
  • To support forwarding,Tag switching allows a packet to carry several tags organized as a tag stack
  • At the ingress, a tag is pushed onto the tag stack, and at the egress a tag is popped off the stack
hierarchical routing model
Hierarchical Routing Model

Routing

domain

C

Routing

domain

B

Routing domain A

V

T

X

Y

W

Z

TSR

label stack
Label Stack

Top of

Stack

10

Top of

Stack

2

2

Stack after processing in

TSR T

Stack after processing in

TSR W

TSR Z distributes label 2 to TSR W and TSR W gives

label 5 to TSR T for the purpose of inter-domain routing

multicast in tag switching
Multicast in Tag Switching
  • Selects the distribution tree based only on
    • tag carried in a packet
    • interface on which the packet arrives
  • TSR maintains its TFIB on a per interface basis
  • TSRs connected to a common sub-network agree among themselves on a common tag associated with a particular multicast tree
multicast cont d
Multicast (Cont’d)
  • Partition the set of tags for use with multicast into disjoint subsets
    • Avoid overlap with the help of HELLO packets
  • TSR connected to a common sub-network and those which are a part of the same distribution tree elect one TSR that will create the tag bindings and distribute them
    • any TSR can join the group using the JOIN command
multicast model
Multicast Model

A

B

if0

D

if0

if1

if2

if0

if0

TSR

E

F

rsvp with tag switching
RSVP With Tag Switching
  • RSVP supported with the help of a RSVP object - the tag Object
  • The tag object binding for an RSVP flow carried in the RSVP “RESV” message
  • The RESV message carries the tag object containing the tag given by a TSR and also information about the local resources to be used
  • The reservation state is refreshed once the flow is set up using the RESV message
explicit routes
Explicit Routes
  • Tag switching supports explicit routes with the help of Explicit Route Object
  • The object is carried in the RSVP “PATH” message
  • The tag information is carried in the Tag Object by the RSVP “RESV”
tag switching over atm
Tag Switching Over ATM
  • VCI field used as tag field
    • For stacks, use VPI
  • Cell interleave problem
    • VC merge
    • Different tags on the same path
mpls terminology
MPLS Terminology

Label Distribution Protocol LDP, CR-LDP or RSVP

Label Switch Router (LSR)

Label Switch Path

Forwarding

Equivalence

Class

Label Switch Hop

Label Edge Router

(non-standard but useful term)

forwarding equivalence classes
Forwarding Equivalence Classes

LSR

LSR

LER

LER

LSP

IP1

IP1

IP1

IP2

IP1

IP1

IP2

IP2

#L3

#L2

#L2

#L3

#L1

#L1

IP2

IP2

Packets are destined for different address prefixes, but can be

mapped to common path

  • FEC = “A subset of packets that are all treated the same way by a router”
  • In conventional routing, a packet is assigned to a FEC at each hop (i.e. L3 look-up), in MPLS it is only done once at the network ingress
  • Mapping a packet to an FEC is known as “classification”.
    • This is nominally done via examination of the IP header, but could be done by other means. (e.g. direct stream adaptation at the LER).
label switched path vanilla
LABEL SWITCHED PATH (vanilla)

#14

#311

#216

#99

#311

#963

#311

#963

#14

#612

#462

#311

#99

#5

- A Vanilla LSP is actually part of a tree from every source to that destination (unidirectional).

- Vanilla LDP builds that tree using existing IP forwarding tables to route the control messages.

ip forwarding used by hop by hop control
IP FORWARDING USED BY HOP-BY-HOP CONTROL

IP 47.1.1.1

47.1

1

IP 47.1.1.1

2

IP 47.1.1.1

1

3

2

IP 47.1.1.1

1

47.2

3

47.3

2

  • Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.
label switched path lsp
Label Switched Path (LSP)

IP 47.1.1.1

IP 47.1.1.1

1

47.1

3

3

2

1

1

2

47.3

3

47.2

2

explicitly routed or er lsp
EXPLICITLY ROUTED OR ER-LSP

Route={A,B,C}

#972

#14

#216

#14

#972

#462

B

C

A

- ER-LSP follows route that source chooses. In other words, the control message to establish the LSP (label request) is source routed.

label encapsulation
Label Encapsulation

IP | PAYLOAD

“Shim Label” …….

VPI

VCI

DLCI

Label

L2

ATM

FR

Ethernet

PPP

MPLS Encapsulation is specified over various media types. Top labels may use existing format, lower label(s) use a new “shim” label format.

MPLS may use the QoS/CoS mechanisms of the media type.