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ATM and MultiProtocol Label Switching MPLS

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meeting timing/QoS requirements of voice, video (versus Internet best-effort model) ... at Destination Host: AAL5 reassembles cells into original datagram ... – PowerPoint PPT presentation

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Title: ATM and MultiProtocol Label Switching MPLS


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
  • 1990s/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

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

IP network
ATM network
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

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)

User data
AAL PDU
ATM cell
8
ATM Layer
  • Service transport cells across ATM network
  • analogous to IP network layer
  • very different services than IP network layer

Guarantees ?
Network Architecture Internet ATM ATM ATM ATM
Service Model best effort CBR VBR ABR UBR
Congestion feedback no (inferred via
loss) no congestion no congestion yes no
Bandwidth none constant rate guaranteed rate gua
ranteed minimum none
Loss no yes yes no no
Order no yes yes yes yes
Timing no yes yes no no
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 (N2 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 -gt 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)

15
IP-Over-ATM
  • IP over ATM
  • replace network (e.g., LAN segment) with ATM
    network
  • ATM addresses, IP addresses
  • Classic IP only
  • 3 networks (e.g., LAN segments)
  • MAC (802.3) and IP addresses

ATM network
Ethernet LANs
Ethernet LANs
16
IP-Over-ATM
17
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

18
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
19
Re-examining Basics Routing vs Switching
20
IP Routing vs IP Switching
21
MPLS Best of Both Worlds
CIRCUITSWITCHING
PACKETROUTING
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!
22
History Ipsilons IP Switching Concept
Hybrid IP routing (control plane) ATM
switching (data plane)
23
Ipsilons IP Switching
ATM VCs setup when new IP flows seen, I.e.,
data-driven VC setup
24
Issues with Ipsilons IP switching
25
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)

27
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

28
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

29
IP Regular Destination Based Forwarding
Address Prefix
Address Prefix
Address Prefix
I/F
I/F
I/F
128.89
128.89
128.89
1
0
0
171.69
1
171.69
1





128.89
0
0
1
128.89.25.4
Data
0
Data
128.89.25.4
1
128.89.25.4
Data
Data
128.89.25.4
Packets Forwarded Based on IP Address
171.69
30
MPLS Example Routing Information
In Label
Address Prefix
Out Iface
OutLabel
In Label
Address Prefix
Out Iface
OutLabel
In Label
Address Prefix
Out Iface
OutLabel
128.89
1
128.89
0
128.89
0
171.69
1
171.69
1







128.89
0
0
1
You Can Reach 128.89 Thru Me
You Can Reach 128.89 and 171.69 Thru Me
1
Routing Updates (OSPF, EIGRP, )
171.69
You Can Reach 171.69 Thru Me
31
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.

32
MPLS Example Assigning Labels
In Label
Address Prefix
Out Iface
In Label
Address Prefix
Out Iface
In Label
Address Prefix
Out Iface
OutLabel
OutLabel
OutLabel
128.89
1
128.89
0
128.89
0
-
9
9
4
4
-
171.69
1
171.69
1
7
5
5
-












128.89
0
0
1
Use Label 9 for 128.89
Use Label 4 for 128.89 and Use Label 5 for 171.69
1
Label Distribution Protocol (LDP) (downstream
allocation)
171.69
Use Label 7 for 171.69
33
MPLS Example Forwarding Packets
In Label
Address Prefix
Out Iface
In Label
Address Prefix
Out Iface
In Label
Address Prefix
Out Iface
OutLabel
OutLabel
OutLabel
4
128.89
1
4
-
128.89
0
9
128.89
0
-
9
5
171.69
1
5
-
171.69
1
7












128.89
0
0
1
128.89.25.4
Data
128.89.25.4
Data
9
1
Data
128.89.25.4
Data
4
128.89.25.4
Label Switch Forwards Based on Label
34
MPLS Operation
35
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 VCs) is not
    used

36
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!
37
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
EXP
S
TTL
Label 20 bits EXP Class of Service, 3
bits S Bottom of Stack, 1 bit 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

38
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 LSRs so
    that data can traverse the entire path using
    labels.

39
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

40
Label Switched Path (LSP)
41
Label Merging
  • When multiple input streams corresponding to the
    same FEC exit using the same MPLS label.
  • InLabel NextHop Label
  • Port 3 30
  • 25 Port 3 30

Netw D
Dest NextHop Label D Port 1 10
R2
R4
R1
Port 3
Port 1
Port 5
R3
Dest NextHop Label D Port 5 25
42
Non-Label Merging
  • Each source-destination pair has its own label at
    each LSR router.
  • InLabel NextHop Label
  • Port 3 5
  • 25 Port 3 8

Netw D
Dest NextHop Label D Port 1 10
R2
R4
R1
Port 3
Port 1
Port 5
R3
Dest NextHop Label D Port 5 25
43
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
44
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.

45
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

46
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

47
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

48
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 LSRs can execute ANY Internet
    protocol based on IP (e.g., OSPF, RIP, etc) and
    forward IP datagrams

50
End-to-end VCs
  • 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 -gt R7 R1 R3 R6 R7
  • R2 -gt 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

53
Explicitly Routed (ER-) LSP
54
Explicitly Routed (ER-) LSP Contd
55
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

56
Virtual Private Networks
  • We can do VPNs with MPLS.
  • Let us review VPNs 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

57
Virtual Private Networks
58
Tunneling
  • IP Tunnel
  • Virtual point-to-point link between an
    arbitrarily connected pair of nodes

IP Tunnel
10.0.0.1
59
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

60
Virtual Private Networks
  • 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.

61
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

62
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.

63
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.

64
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65
Emulate a VC (steps)
  • 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.

66
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

67
Layer 3 VPNs
  • 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

68
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