1 ATM and Multi-Protocol Label Switching (MPLS) By Behzad Akbari Fall 2008 These slides are based in...

1

ATM and Multi-Protocol Label Switching(MPLS)

By

Behzad Akbari Fall 2008

These slides are based in parts on the slides of J. Kurose (UMASS) and Shivkumar (RPI)

2

Outline

ATM basics IP over ATM MPLS basics MPLS VPN MPLS traffic engineering

3

Asynchronous Transfer Mode: ATM 1990’s/00 standard for high-speed (155Mbps to

622 Mbps and higher) Broadband Integrated Service Digital Network architecture

Goal: integrated, end-end transport of carry voice, video, data meeting timing/QoS requirements of voice,

video (versus Internet best-effort model) “next generation” telephony: technical roots in

telephone world packet-switching (fixed length packets, called

“cells”) using virtual circuits

4

ATM architecture

adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer

ATM layer: “network” layer cell switching, routing

physical layer

physical

ATM

AAL

physical

ATM

AAL

physical

ATM

physical

ATM

end system end systemswitch switch

5

ATM: network or link layer?Vision: end-to-end transport:

“ATM from desktop to desktop” ATM is a network

technology

Reality: used to connect IP backbone routers “IP over ATM” ATM as switched link

layer, connecting IP routers

ATMnetwork

IPnetwork

6

ATM Adaptation Layer (AAL)

ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below

AAL present only in end systems, not in switches AAL layer segment (header/trailer fields, data)

fragmented across multiple ATM cells analogy: TCP segment in many IP packets

physical

ATM

AAL

physical

ATM

AAL

physical

ATM

physical

ATM

end system end systemswitch switch

7

ATM Adaptation Layer (AAL) [more]Different versions of AAL layers, depending on ATM

service class: AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video AAL5: for data (eg, IP datagrams)

AAL PDU

ATM cell

User data

8

ATM LayerService: transport cells across ATM network analogous to IP network layer very different services than IP network layer

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

9

ATM Layer: Virtual Circuits VC transport: cells carried on VC from source to dest

call setup, teardown for each call before data can flow each packet carries VC identifier (not destination ID) every switch on source-dest path maintain “state” for each

passing connection link,switch resources (bandwidth, buffers) may be allocated to

VC: to get circuit-like perf. Permanent VCs (PVCs)

long lasting connections typically: “permanent” route between to IP routers

Switched VCs (SVC): dynamically set up on per-call basis

10

ATM VCs

Advantages of ATM VC approach: QoS performance guarantee for connection

mapped to VC (bandwidth, delay, delay jitter) Drawbacks of ATM VC approach:

Inefficient support of datagram traffic one PVC between each source/dest pair) does

not scale (N*2 connections needed) SVC introduces call setup latency, processing

overhead for short lived connections

11

ATM Layer: ATM cell 5-byte ATM cell header 48-byte payload

Why?: small payload -> short cell-creation delay for digitized voice

halfway between 32 and 64 (compromise!)

Cell header

Cell format

12

ATM cell header

VCI: virtual channel ID will change from link to link thru net

PT: Payload type (e.g. RM cell versus data cell) CLP: Cell Loss Priority bit

CLP = 1 implies low priority cell, can be discarded if congestion

HEC: Header Error Checksum cyclic redundancy check

13

ATM Physical Layer (more)

Two pieces (sublayers) of physical layer: Transmission Convergence Sublayer (TCS): adapts

ATM layer above to PMD sublayer below Physical Medium Dependent: depends on physical

medium being used

TCS Functions: Header checksum generation: 8 bits CRC Cell delineation With “unstructured” PMD sublayer, transmission

of idle cells when no data cells to send

14

ATM Physical Layer

Physical Medium Dependent (PMD) sublayer SONET/SDH: transmission frame structure (like a

container carrying bits); bit synchronization; bandwidth partitions (TDM); several speeds: OC3 = 155.52 Mbps; OC12 = 622.08

Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps TI/T3: transmission frame structure (old telephone

hierarchy): 1.5 Mbps/ 45 Mbps unstructured: just cells (busy/idle)

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IP-Over-ATMClassic IP only 3 “networks” (e.g., LAN segments) MAC (802.3) and IP addresses

IP over ATM replace “network”

(e.g., LAN segment) with ATM network

ATM addresses, IP addresses

ATMnetwork

EthernetLANs

EthernetLANs

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IP-Over-ATM

AALATMphyphy

Eth

IP

ATMphy

ATMphy

apptransport

IPAALATMphy

apptransport

IPEthphy

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Datagram Journey in IP-over-ATM Network

at Source Host: IP layer maps between IP, ATM dest address (using ARP) passes datagram to AAL5 AAL5 encapsulates data, segments cells, passes to ATM layer

ATM network: moves cell along VC to destination at Destination Host:

AAL5 reassembles cells into original datagram if CRC OK, datagram is passed to IP

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IP-Over-ATM

Issues: IP datagrams into

ATM AAL5 PDUs from IP addresses to

ATM addresses just like IP

addresses to 802.3 MAC addresses!

ATMnetwork

EthernetLANs

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Re-examining Basics: Routing vs Switching

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IP Routing vs IP Switching

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MPLS: Best of Both WorldsPACKET

ROUTINGCIRCUITSWITCHING

MPLS+IP

IP ATM

HYBRID

Caveat: one cares about combining the best of both worlds only for large ISP networks that need both features!Note: the “hybrid” also happens to be a solution that bypasses IP-over-ATM mapping woes!

TDM

22

History: Ipsilon’s IP Switching: Concept

Hybrid: IP routing (control plane) + ATM switching (data plane)

23

Ipsilon’s IP SwitchingATM VCs setup when new IP “flows” seen, I.e., “data-driven” VC setup

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Issues with Ipsilon’s IP switching

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Tag Switching

Key difference: tags can be setup in the background using IP routing protocols (I.e. control-driven VC setup)

26

Multi-Protocol Label Switching (MPLS)

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Background

It was meant to improve routing performance on the Internet Routing is difficult using CIDR (longest prefix matching) Using the label-swapping paradigm to optimize network

performance MPLS is similar to virtual circuits

Only a fixed-sized label is used (like a VCID) with local scope

It is very datagram oriented though It uses IP addressing and IP routing protocols

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Goals of MPLS

To enable IP capability on devices that cannot handle IP traffic Making cell switches behave as routers

Increased performance Using the label-swapping paradigm to optimize network

performance Forward packets along “explicit routes” (pre-calculated routes not

used in “regular” routing) 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 To support certain virtual private network services

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IP Regular Destination Based Forwarding

0

171.69Packets Forwarded Based on IP Address

Data

Address

Prefix128.89128.89

171.69

1

1

I/F

…

Address

Prefix128.89128.89

171.69

0

1

…

01

I/F

128.890

1

128.89.25.4 Data

Address

Prefix128.89128.89 0

… …

I/F

Data Data128.89.25.4128.89.25.4128.89.25.4128.89.25.4

128.89.25.4128.89.25.4

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MPLS Example: Routing Information

128.89

1

01

0

Routing Updates (OSPF, EIGRP, …)

You Can Reach 128.89 and 171.69 Thru Me

You Can Reach 171.69 Thru Me




In Label

Address

Prefix128.89

171.69

1

1

OutI’face

OutLabel

In Label

Address

Prefix128.89

171.69

0

1

OutI’face

OutLabel

In Label

Address

Prefix128.89 0

OutI’face

OutLabel

… … … … … …

171.69

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Labels for Destination-Based Forwarding A label is allocated for each prefix in its table

The label is chosen locally Think of them as indices into the routing table

Router advertises this to its neighbors “label distribution protocol” (LDP)

Packets addressed to the prefix should, for efficiency, be tagged with the label.

The label of an incoming packet is “swapped” before being forwarded to the next router.

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MPLS Example: Assigning Labels

128.89

1

01

0

Label Distribution Protocol (LDP)

(downstream allocation)

Use Label 4 for 128.89 and Use Label 5 for 171.69

Use Label 4 for 128.89 and Use Label 5 for 171.69

Use Label 7 for 171.69Use Label 7 for 171.69

Use Label 9 for 128.89Use Label 9 for 128.89

In Label

Address

Prefix128.89

171.69

1

1

OutI’face

OutLabel

In Label

Address

Prefix128.89

171.69

0

1

OutI’face

OutLabel

In Label

Address

Prefix128.89 0

OutI’face

OutLabel

-9

… … … … … …… …… … … …

9

7

4

5

4

5

-

-

171.69

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In Label

Address

Prefix128.89128.89

171.69

1

1

OutI’face

OutLabel

… …… …

4

5

-

-

MPLS Example: Forwarding Packets

Label Switch Forwards Based on Label

In Label

Address

Prefix128.89

171.69

0

1

OutI’face

OutLabel

… …… …

9

7

44

5

In Label

Address

Prefix128.89 0

OutI’face

OutLabel

-9

… …… …

Data 128.89.25.4 Data

128.89.25.4 Data

128.89.25.4 Data

128.89

1

01

0

128.89.25.4128.89.25.4 44

99

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MPLS Operation1a. Existing routing protocols (e.g. OSPF, IS-IS) establish reachability to destination networks.

1b. Label Distribution Protocol (LDP) establishes label to destination network mappings.

2. Ingress Edge LSR receives packet, performs Layer 3 value-added services, and labels(PUSH) packets.

3. LSR switches packets using label swapping(SWAP) .

4. Edge LSR at egress removes(POP) label and delivers packet.

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Remarks

Rather than longest prefix-matching we use label matching Labels can be very efficient, simply an index into the

routing table

Regular IP routing is still used E.g., we could use OSPF to determine the routes Then we use labels for efficiency in per-hop routing

Note that a “Setup” phase (like in VC’s) is not used

36

Placement of “labels”

For Ethernet, the “protocol number used” is 0x8847 for MPLSI.e., the “protocol number” of IP is not used.

Thus, IP never sees the message!

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Label Header

• Header= 4 bytes, Label = 20 bits.• Can be used over Ethernet, 802.3, or PPP

links• Contains everything needed at forwarding

time

Label = 20 bits EXP = Class of Service, 3 bitsS = Bottom of Stack, 1 bit TTL = Time to Live, 8 bits

0 1 2 30 1 2 30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 10 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

Label EXP S TTL

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Some Definitions

Forwarding Equivalence Class (FEC): a group of IP packets which are forwarded in the same manner (e.g., over the same path, with the same forwarding treatment)

Labeled Switched Router (LSR): A router capable of supporting MPLS labels.

Labeled Switched Path: a sequence of LSR’s so that data can traverse the entire path using labels.

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Traffic Aggregates: Forwarding Equivalence Classes

• FEC = “A subset of packets that are all treated the same way by a router”

• The concept of FECs provides for a great deal of flexibility and scalability

• 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

Packets are destined for different address prefixes, but can bemapped to common pathPackets are destined for different address prefixes, but can bemapped to common path

IP1

IP2

IP1

IP2

LSRLSRLER LER

LSP

IP1 #L1

IP2 #L1

IP1 #L2

IP2 #L2

IP1 #L3

IP2 #L3

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Label Switched Path (LSP)

IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

47.1

47.247.3

1

2

31

2

1

23

3IntfIn

Dest IntfOut

LabelOut

3 47.1 1 0.50

IP 47.1.1.1

IP 47.1.1.1

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Label Merging

When multiple input streams corresponding to the same FEC exit using the same MPLS label.

Dest NextHop LabelD Port 1 10


InLabel NextHop Label10 Port 3 3025 Port 3 30

R1

R2

R3

R4

Netw D

Port 1

Port 3

Port 5

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Non-Label Merging



InLabel NextHop Label10 Port 3 525 Port 3 8

R1

R2

R3

R4

Netw D

Port 1

Port 3

Port 5

Each source-destination pair has its own label at each LSR router.

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Pushing-Requesting Labels

R2 can “push” a label to R1, indicating which label to use to reach D

R1 can “request” a label from R2 to be used to reach D.

If using non-merging, usually R1 requests a label from R2

R1

R2 R4

Netw D

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ATM

Most importantly, we can use ATM switches for IP

We can turn “ATM Cell switches” into “label switching routers” usually only by changing the software and not the hardware of the switch.

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IP over ATM (Before MPLS) We had every router with a VC over an ATM network to every other router

Known as an “overlay” network Whole ATM network looked like a single “subnet” to the IP Routers

ATM switches are not aware that the payload is an IP packet

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IP disassembly into ATM cells IP becomes an “application” to the ATM layer. IP packets have to be broken into small 48-byte pieces, and placed

into ATM Cells Cells are sent over the ATM circuit (e.g. from R1 to R6), the

switches only see ATM Cells, not IP packet At R6, the cells are regrouped and the IP packet restored

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ATM switches as LSRs (using MPLS) ATM switches are now “peers” of MPLS routers

No longer viewed as a single subnet, each link is now a subnet

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Advantages of MPLS vs overlay Each MPLS router has fewer “adjacencies” (i.e. neighbors)

This reduces the OSPF traffic to the router significantly In OSPF you receive the topology of the entire network via each

of your neighbors.

Each router now has a view of the entire topology Not possible in overlay networks (ATM network “black box”) Routers have better control of paths in case of link failures

In overlay networks, the ATM switches would do the rerouting

ATM switches may still support native ATM if desired.

49

How to route IP packets?

Can we send IP messages to our neighbors? We can use a special VCID (say 0) to send IP

messages to our neighbor. Each node has a VCID 0 with each of its neighbors (a

“single hop” VCID Thus, to send an IP message to a neighbor

Disassemble the IP packet into ATM Cells Send them on VCID 0 of the link of the desired neighbor The neighbor reassembles the IP packet

Since we can send an IP message to any neighbor This implies ATM LSR’s can execute ANY Internet

protocol based on IP (e.g., OSPF, RIP, etc) and forward IP datagrams

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End-to-end VC’s Disassembly/reassembly at each hop is wasteful

It is better to establish an e-2-e VC for each source/destination pair, e.g., from R1 to R6

From OSPF (or other mechanism), each router knows which other router is ATM or regular router R1 “requests” a label from LSR1 for destination R6 LSR1 requests a label from LSR3 for destination R6 LSR3 requests a label from R6

51

GMPLS

“Generalized” MPLS

Used in optical networks Turn an optical switch (e.g. SONET) into a LSR

Give the IP routers a better view/control of the optical network

52

Explicit Routing Similar to “source routing” but done by a router “Fish” network due to its shape R1 -> R7 : R1 R3 R6 R7 R2 -> R7 : R2 R3 R4 R5 R7

Perhaps we want to balance the load somehow Cannot be done with regular IP

IP routing does not look at the source of the message

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#216

#14

#462

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

#972

#14 #972

A

B

C

Route={A,B,C}

Explicitly Routed (ER-) LSP

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IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

47.1

47.247.3

1

2

31

2

1

23

3

IntfIn

Dest IntfOut

LabelOut

3 47.1.1 2 1.333 47.1 1 0.50

IP 47.1.1.1

IP 47.1.1.1

Explicitly Routed (ER-) LSP Contd

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Explicit Route Advantages

Traffic Engineering You can control how much traffic travels through some

point in the network This is done by controlling the paths taken by traffic

Fast-rerouting You can bypass broken links quickly with explicit routing.

No need to wait for a routing protocol (OSPF) to react. How?

Keep track of two paths, regular path and backup path If the regular path fails use the backup

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Virtual Private Networks

We can do VPN’s with MPLS. Let us review VPN’s’ with regular IP first. Goal

Controlled connectivity Virtual Private Network

A group of connected networks

Connections may be over multiple networks not belonging to the group (e.g. over the Internet)

E.g., joining the networks of several branches of a company into a “private internetwork”

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C

A B

M

K L

C

A BM

K L

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Tunneling

IP Tunnel Virtual point-to-point link between an arbitrarily

connected pair of nodes

Network 1

Network 1

Network 2

Network 2InternetworkInternetwork

R1 R2

IP Tunnel

IP Dest = 2.xIP Payload

IP Dest = 10.0.0.1

10.0.0.1



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Tunneling Advantages of tunneling

Transparent transmission of packets over heterogeneous networks The data carried may not even be IP messages!

Only need to change relevant routers (end points) Coupled with encryption, gives you a secure private

internetwork. End-points of tunnels my have features not available in other

Internet routers. Multicast Local Addresses

Useful for mobile routing. Disadvantages

Increases packet size Processing time needed to encapsulate and decapsulate

packets Management at tunnel-aware routers

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We can do similarly with MPLS We can connect different sites with an MPLS

tunnel We can send regular IP traffic through the

tunnel, or any other type of traffic.

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“Layer 2” tunnel

Use MPLS to provide a tunnel between two LANs (Ethernet, etc) ATM points

Any data can be “wrapped” with a label It need not be IP datagrams LSR does not look “beyond” the label

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Demultiplexing Label

What to do with the packet once it reaches the other side of the tunnel?

A “demultiplexing” label needs to be added to inform the end-point router what to do with the packet.

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E.g., Emulate a VC

ATM cells with a specific VCID come in at the entrance of the tunnel

ATM cells at the end of the tunnel should have the appropriate VCID for the next switch after the router.

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Emulate a VC (steps)

1. An ATM cell arrives to the input LSR with VCID 101

2. The head router attaches the demultiplexing label and identifies the emulated circuit

3. The head router attaches the tunnel label (to reach the tail router)

4. Routers in the middle forward as usual5. The tail router removes the tunnel label, finds the

demultiplexing label, and identifies the VC6. The tail router modifies the VCID to the next ATM

switch value (202) and sends it to the ATM switch.

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Label Stacks

The previous example has a stack of two labels

You can have larger stacks of labels in the header.

In the example It enables to have a tunnel And many types of traffic within the tunnel

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“Layer 3” VPN’s

The packet being carried is an IP packet Hence the name “layer 3” VPNs

Service provider (see picture next ..) Has many customers Each customer has many sites

These sites are linked with tunnels to appear to be one large Internetwork

Each customer can only reach its own sites The customer is isolated from the rest of the Internet and from

other customers

1 ATM and Multi-Protocol Label Switching (MPLS) By Behzad Akbari Fall 2008 These slides are based in...

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