Download Introduction to MPLS

ATM and Multi-Protocol Label Switching
(MPLS)
By
Behzad Akbari
Spring 2011
These slides are based in parts on the slides of J. Kurose (UMASS) and Shivkumar (RPI)
1
Outline





ATM basics
IP over ATM
MPLS basics
MPLS VPN
MPLS traffic engineering
2
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
3
ATM architecture
AAL
AAL



ATM
ATM
ATM
ATM
physical
physical
physical
physical
end system
switch
switch
end system
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
4
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
IP
network
ATM
network
5
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
AAL
AAL
ATM
ATM
ATM
ATM
physical
physical
physical
physical
end system
switch
switch
end system
6
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)
User data
AAL PDU
ATM cell
7
ATM Layer
Service: transport cells across ATM network
 analogous to IP network layer
 very different services than IP network layer
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
8
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
9
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
10
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
11
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
12
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
13
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)
14
IP-Over-ATM
Classic 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
ATM
network
Ethernet
LANs
Ethernet
LANs
15
IP-Over-ATM
app
transport
IP
Eth
phy
IP
AAL
Eth
ATM
phy phy
ATM
phy
ATM
phy
app
transport
IP
AAL
ATM
phy
16
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
17
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!
ATM
network
Ethernet
LANs
18
Re-examining Basics: Routing vs Switching
19
IP Routing vs IP Switching
20
MPLS: Best of Both Worlds
PACKET
ROUTING
IP
HYBRID
MPLS
+IP
CIRCUIT
SWITCHING
ATM TDM
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!
21
History: Ipsilon’s IP Switching: Concept
Hybrid: IP routing (control plane) +
ATM switching (data plane)
22
Ipsilon’s IP Switching
ATM VCs setup when new IP “flows” seen, I.e.,
“data-driven” VC setup
23
Issues with Ipsilon’s IP switching
24
Tag Switching
Key difference: tags can be setup in the background
using IP routing protocols (I.e. control-driven VC setup)
25
Multi-Protocol Label Switching (MPLS)
26
Background

It was meant to improve routing performance on the
Internet



MPLS is similar to virtual circuits


Routing is difficult using CIDR (longest prefix matching)
Using the label-swapping paradigm to optimize network
performance
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
27
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
28
IP Regular Destination Based Forwarding
Address
Prefix
I/F
Address
Prefix
I/F
Address
Prefix
I/F
128.89
1
128.89
0
128.89
0
171.69
1
171.69
1
…
…
…
…
0
128.89
0
1
128.89.25.4 Data
0 128.89.25.4 Data
1
128.89.25.4 Data
128.89.25.4 Data
Packets Forwarded
Based on IP Address
171.69
29
MPLS Example: Routing Information
Out
In Address Out
I’face Label
Label Prefix
Out
In Address Out
I’face Label
Label Prefix
128.89
1
128.89
0
171.69
1
171.69
1
…
…
…
…
Out
In Address Out
I’face Label
Label Prefix
128.89
0
…
…
0
128.89
0
1
You Can Reach 128.89 Thru
Me
You Can Reach 128.89 and
171.69 Thru Me
Routing Updates
(OSPF, EIGRP, …)
1
You Can Reach 171.69 Thru
Me
171.69
30
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.
31
MPLS Example: Assigning Labels
Out
In Address Out
Label
I’face
Label Prefix
Out
In Address Out
Label
I’face
Label Prefix
-
128.89
1
4
4
128.89
0
9
-
171.69
1
5
5
171.69
1
7
…
…
…
…
…
…
…
…
Out
In Address Out
Label
I’face
Label Prefix
9
128.89
0
-
…
…
…
…
0
128.89
0
1
Use Label 9 for 128.89
Use Label 4 for 128.89 and
Use Label 5 for 171.69
Label Distribution
Protocol (LDP)
1
171.69
Use Label 7 for 171.69
(downstream allocation)
32
MPLS Example: Forwarding Packets
Out
In Address Out
Label
I’face
Label Prefix
Out
In Address Out
Label
I’face
Label Prefix
-
128.89
1
4
4
128.89
0
9
-
171.69
1
5
5
171.69
1
7
…
…
…
…
…
…
…
…
Out
In Address Out
Label
I’face
Label Prefix
9
128.89
0
-
…
…
…
…
0
128.89
0
1
128.89.25.4
9
128.89.25.4
Data
Data
1
128.89.25.4 Data
4
128.89.25.4
Data
Label Switch Forwards
Based on Label
33
MPLS Operation
1a. 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.
4. Edge LSR at egress
removes(POP) label
and delivers packet.
3. LSR switches packets using
label swapping(SWAP) .
34
Remarks

Rather than longest prefix-matching we use label
matching


Regular IP routing is still used



Labels can be very efficient, simply an index into the
routing table
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
35
Placement of “labels”
For Ethernet, the “protocol number used” is 0x8847 for MPLS
I.e., the “protocol number” of IP is not used.
Thus, IP never sees the message!
36
Label Header
0
1
2
3
0 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
Label = 20 bits
S = Bottom of Stack, 1 bit
EXP S
TTL
EXP = Class of Service, 3 bits
TTL = Time to Live, 8 bits
• Header= 4 bytes, Label = 20 bits.
• Can be used over Ethernet, 802.3, or PPP links
• Contains everything needed at forwarding time
37
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.
38
Traffic Aggregates: Forwarding Equivalence Classes
LSR
LER
LSR
LER
LSP
IP1
IP1
IP1
#L1
IP1
#L2
IP1
#L3
IP2
#L1
IP2
#L2
IP2
#L3
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”
• 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
39
Label Switched Path (LSP)
Intf Label
In
In
3
0.50
Intf Dest Intf
In
Out
3
47.1 1
Label
Out
0.50
Dest Intf
Out
47.1 1
Label
In
0.40
Dest Intf
Out
47.1 1
3
1
47.3 3
Intf
In
3
IP 47.1.1.1
1 47.1
3
1
Label
Out
0.40
2
2
47.2
2
IP 47.1.1.1
40
Label Merging

When multiple input streams corresponding to the
same FEC exit using the same MPLS label.
InLabel NextHop Label
10
Port 3
30
25
Port 3
30
Dest NextHop Label
D
Port 1
10
R2
Netw D
R4
Port 3
R1
Port 1
Port 5
Dest NextHop
D
Port 5
Label
25
R3
41
Non-Label Merging

Each source-destination pair has its own label at
each LSR router.
InLabel NextHop Label
10
Port 3
5
25
Port 3
8
Dest NextHop Label
D
Port 1
10
R2
Netw D
R4
Port 3
R1
Port 1
Port 5
Dest NextHop
D
Port 5
Label
25
R3
42
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
Netw D
R2
R4
R1
43
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.
44
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
45
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
46
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
47
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.
48
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
49
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
50
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
51
Explicitly Routed (ER-) LSP
Route=
{A,B,C}
#14
#216
#972
B
A
#14
#972
C
#462
ER-LSP follows route that source chooses. In other words, the
control message to establish the LSP (label request) is source
routed.
52
Explicitly Routed (ER-) LSP Contd
Intf Label
In
In
3
0.50
Intf
In
3
3
Dest
47.1.1
47.1
Intf
Out
2
1
Dest Intf Label
Out Out
47.1 1
0.40
Label
Out
1.33
0.50
Intf
In
3
Label
In
0.40
Dest Intf
Out
47.1 1
IP 47.1.1.1
1 47.1
3
1
3
1
47.3 3
2
2
47.2
2
IP 47.1.1.1
53
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
54
Virtual Private Networks


We can do VPN’s with MPLS.
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”
55
Virtual Private Networks
C
A
B
K
L
M
C
K
L
A
B
M
56
Tunneling

IP Tunnel

Virtual point-to-point link between an arbitrarily
connected pair of nodes
Network
1
R1
Internetwork
Network
2
R2
IP Tunnel
10.0.0.1
IP Dest = 2.x
IP Payload
IP Dest = 10.0.0.1
IP Dest = 2.x
IP Payload
IP Dest = 2.x
IP Payload
57
Tunneling

Advantages of tunneling

Transparent transmission of packets over heterogeneous
networks




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.



The data carried may not even be IP messages!
Multicast
Local Addresses
Disadvantages



Increases packet size
Processing time needed to encapsulate and decapsulate
packets
Management at tunnel-aware routers
58
Virtual Private Networks with MPLS



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.
59
“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
60
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.
61
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.
62
63
Emulate a VC (steps)
1.
2.
3.
4.
5.
6.
An ATM cell arrives to the input LSR with VCID
101
The head router attaches the demultiplexing label
and identifies the emulated circuit
The head router attaches the tunnel label (to reach
the tail router)
Routers in the middle forward as usual
The tail router removes the tunnel label, finds the
demultiplexing label, and identifies the VC
The tail router modifies the VCID to the next ATM
switch value (202) and sends it to the ATM switch.
64
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
65
“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
66
67