Transcript ATM-MPLS

15-441 Computer Networks
ATM and MPLS
Professor Hui Zhang
[email protected]
1
High Speed IP Router?
incoming links
Node
outgoing links
Memory
2

Dest IP address lookup for every packet

Switching and buffer management for variable size packet
Asynchronous Transfer Mode: ATM

ITU standard for high-speed (155Mbps to 622 Mbps and higher)
Broadband Integrated Service Digital Network architecture


3
Most of the work done in 90’s
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
Four Ideas

Flexible and efficient  packet switching

Support voice, video, data

 needs end-to-end Quality of Service

 connection-oriented network

 virtual circuit network

Scale to high performance switch  fixed size packet

Support low jitter voice  small size packet
4
Virtual Circuit Concept

Logical Connection

Connection is first established using signaling protocol

5

Route from the source to the destination is chosen

The same route is used for all packets of the connection
No routing decision for every cell
Virtual Circuit Concepts

No dedicated capacity


6
Packet switching to enable statistical multiplexing
Each packet contains enough information for
node (switch) to forward it towards the
destination
Virtual Circuit Switching: Label Swapping
E
C
VC4
A
B
VC5
D
F
IN
LINK
IN VC
OUT
LINK
OUT
VC
CA
7
AB
4
CA
2
AB
5
DA
3
AB
3
Table at Node A
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Signaling Protocol

Signaling protocol establishes/tears down virtual circuit




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Signaling message are routed
Signaling protocol fills the forwarding table
Parameters used for establishing Virtual Circuits

Source and destination Addresses

Traffic Characteristics

QoS Parameters

Others?
Parameters can be stored in forwarding table to help
forwarding decision
ATM architecture


Adaptation layer: only at edge of ATM network

data segmentation/reassembly

roughly analagous to Internet transport layer
ATM layer: “network” layer


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cell switching, routing
Physical layer
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 performance
Permanent VCs (PVCs)


Switched VCs (SVC):

10
long lasting connections
dynamically set up on per-call basis
Cell Size: 32 bytes or 64 bytes?

11
Cell size of 32 and 64 bytes:

64 bytes cells have better transmission efficiency

32 bytes cells have small delay

both sizes are integer power of 2

Europe wanted 32 bytes size, US and Japan
wanted 64 bytes size

Compromise: 48 bytes
ATM Cell Format
Header :5 bytes
at
GFC
UNI
4
at
NNI
GFC
VCI
CLP
UNI
12
Payload (Information) 48 bytes
VPI
8
VPI
12
:
:
:
:
Generic Flow Control
Virtual Circuit Identifier
Cell Loss Priority
User Network Interface
VCI
PT
16
3
VCI
PT
16
3
VPI
PT
HEC
NNI
:
:
:
:
C
L
P
HEC
1
C
L
P
8
bits
8
bits
HEC
1
Virtual Path Identifier
Payload Type
Header error Check
Network-Network Interface
Questions
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
How many Virtual Circuits can one ATM switch
support?

What is the purpose of Virtual Path?
Protocol vs. Service


Service – says what a layer does

Ethernet: unreliable subnet unicast/multicast/broadcast
datagram service

IP: unreliable end-to-end unicast datagram service

TCP: reliable end-to-end bi-directional byte stream service

Guaranteed bandwidth/latency unicast service
Protocol – says how is the service implemented

a set of rules and formats that govern the communication
between two peers
– Packet format, how to interpret packet fields
– State machine of protocol messages
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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

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analogy: TCP segment in many IP packets
AAL 5 Packet
User Data
0-65535
bytes
16

PAD : padding

User Data

8 Bytes header
PAD
0-47
bytes
UU CPI
1
1
Length
2
CRC
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
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
“IP over ATM”

ATM as switched link
layer, connecting IP
routers
Where is ATM Today?
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
DSL (Digital Subscriber Loop)

Multi-service switching

Interconnection of IP routers
MPLS
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
Multi-Protocol Label Switching

Bringing virtual circuit concept into IP

Driven by multiple forces

QoS

traffic engineering

High performance forwarding

VPN
MPLS Vocabulary

Label-switched path (LSP)

Simplex path through interior network
New York
San
Francisco
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MPLS Vocabulary

Label-switching router (LSR) performs

MPLS packet forwarding

LSP setup
New York
San
Francisco
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MPLS Vocabulary

Label Edge Router (LER)

Ingress and egress node of LSP
New York
Ingress
San
Francisco
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Egress
MPLS Header

IP packet is encapsulated in MPLS
header and sent down LSP
IP Packet
…
32-bit
MPLS Header

IP packet is restored at end of LSP by egress
router

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TTL is adjusted also
MPLS Header
Label
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
Label

Class of service

Stacking bit

Time to live

Decrement at each LSR, or

Pass through unchanged
CoS S
TTL
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.
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LABEL SWITCHED PATHS Driven by Routing
#216
#14
#311
#99
#311
#963
#311
#963
#14
#612
#5
#462
#99
#311
- A LSP is actually part of a tree from every source to that
destination (unidirectional).
- Control protocol (e.g. LDP) builds that tree using existing
IP forwarding tables to route the control messages.
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MPLS BUILT ON STANDARD IP
Dest
47.1
47.2
47.3
Dest
47.1
47.2
47.3
Out
1
2
3
Out
1
2
3
1 47.1
3
1
Dest
47.1
47.2
47.3
Out
1
2
3
2
3
2
1
47.2
47.3 3
2
• Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.
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IP FORWARDING USED BY HOPBY-HOP CONTROL
Dest
47.1
47.2
47.3
Dest
47.1
47.2
47.3
Out
1
2
3
1 47.1
1
Dest
47.1
47.2
47.3
Out
1
2
3
IP 47.1.1.1
2
IP 47.1.1.1
3
Out
1
2
3
2
IP 47.1.1.1
1
47.2
47.3 3
2
IP 47.1.1.1
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Label Switched Path (LSP)
Intf Label Dest Intf Label
In In
Out Out
3
50
47.1 1
40
Intf Dest Intf Label
In
Out Out
3
47.1 1
50
31
2
2
47.2
2
IP 47.1.1.1
3
1
47.3 3
Label Dest Intf
In
Out
40
47.1 1
IP 47.1.1.1
1 47.1
3
1
Intf
In
3
Route=
{A,B,C}
EXPLICITLY ROUTED OR ER-LSP
#14
#972
#216
B
#14
A
C
#972
#462
- ER-LSP follows route that source chooses. In other
words, the control message to establish the LSP (label
request) is source routed.
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EXPLICITLY ROUTED LSP ER-LSP
Intf Label Dest Intf Label
In In
Out Out
3
50
47.1 1
40
Intf
In
3
3
Dest
47.1.1
47.1
Intf
Out
2
1
Label
Out
33
50
Intf
In
3
Label Dest Intf
In
Out
40
47.1 1
IP 47.1.1.1
1 47.1
3
3
2
1
1
47.3 3
47.2
2
IP 47.1.1.1
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2
Protocol Comparison
Ethernet
IP
Forwarding
Control Protocols
Dest MAC address
Learning
Exact match
Spanning tree
Dest IP address
Routing protocol
Longest prefix match
TDM
ATM
MPLS
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Time slot, exact match
E2E signaling protocol
Time Slot Exchange
(TSE)
Routing protocol
Label, exact match
E2E signaling protocol
Label swapping
Routing protocol
Label, Dest IP Address
Flexible signaling
Routing Protocol