Virtual-Circuit Switching: ATM (Asynchronous Transmission Mode) and MPLS (Multiprotocol Label Switching) 2007. 10 - PowerPoint PPT Presentation

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Virtual-Circuit Switching: ATM (Asynchronous Transmission Mode) and MPLS (Multiprotocol Label Switching) 2007. 10

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Title: Virtual-Circuit Switching: ATM (Asynchronous Transmission Mode) and MPLS (Multiprotocol Label Switching) 2007. 10


1
Virtual-Circuit Switching ATM (Asynchronous
Transmission Mode) and MPLS (Multiprotocol
Label Switching)
2007. 10

2
Virtual Circuit (VC) Switching
  • Hybrid of packets and circuits
  • Circuits establish and teardown along end-to-end
    path
  • Packets divide the data into packets with
    identifiers
  • Packets carry a virtual-circuit identifier
  • Associates each packet with the virtual circuit
  • Determines the next link along the path
  • Intermediate nodes maintain state VC
  • Forwarding table entry
  • Allocated resources

3
Timing of Virtual-Circuit Packet Switching
Host 1
Host 2
Node 1
Node 2
propagation delay between Host 1 and Node 1
VC establishment
Data transfer
VC termination
4
Establishing the Circuit
  • Signaling
  • Creating the entries in the forwarding tables
  • Reserving resources for the virtual circuit, if
    needed
  • Two main approaches to signaling
  • Network administrator configures each node
  • Source sends set-up message along the path
  • Set-up latency
  • Time for the set-up message to traverse the path
  • and return back to the source
  • Routing
  • End-to-end path is selected during circuit set-up

5
Virtual Circuit Identifier (VC ID)
  • Virtual Circuit Identifier (VC ID)
  • Source set-up establish path for the VC
  • Switch mapping VC ID to an outgoing link
  • Packet fixed length label in the header

1 7 2 7
1 14 2 8
link 7
1
link 14
2
link 8
6
Swapping the Label at Each Hop
  • Problem using VC ID along the whole path
  • Each virtual circuit consumes a unique ID
  • Starts to use up all of the ID space in the
    network
  • Label swapping
  • Map the VC ID to a new value at each hop
  • Table has old ID, and next link and new ID

1 7, 20 2 7, 53
20 14, 78 53 8, 42
link 7
1
link 14
2
link 8
7
Virtual Circuits Similar to IP Datagrams
  • Data divided in to packets
  • Sender divides the data into packets
  • Packet has address (e.g., IP address or VC ID)
  • Store-and-forward transmission
  • Multiple packets may arrive at once
  • Need buffer space for temporary storage
  • Multiplexing on a link
  • No reservations statistical multiplexing
  • Packets are interleaved without a fixed pattern
  • Reservations resources for group of packets
  • Guarantees to get a certain number of slots

8
Virtual Circuits Differ from IP Datagrams
  • Forwarding look-up
  • Virtual circuits fixed-length connection id
  • IP datagrams destination IP address
  • Initiating data transmission
  • Virtual circuits must signal along the path
  • IP datagrams just start sending packets
  • Router state
  • Virtual circuits routers know about connections
  • IP datagrams no state, easier failure recovery
  • Quality of service
  • Virtual circuits resources and scheduling per VC
  • IP datagrams difficult to provide QoS

9
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

10
ATM reference model
11
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

12
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)

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 (PMD) 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 Layer Virtual Circuits
  • analogous to IP network layer
  • very different services than IP network layer
  • 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.

15
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
  • VC introduces call setup latency, processing
    overhead for short lived connections

16
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
17
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

18
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
19
ATM Service
  • 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
20
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

21
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 (phone)
  • AAL2 for VBR (Variable Bit Rate) services, e.g.,
    MPEG video
  • AAL5 for data (eg, IP datagrams)

User data
AAL PDU
ATM cell
22
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
23
IP-Over-ATM
24
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

25
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
26
How far along are we?
  • Standardization bodies - ATM Forum, ITU-T
  • We may never see end-to-end ATM (1997)
  • Backbone - 1995 vBNS (ATM)
    - 1998 Abilene (SONET) - 2000 IP over DWDM
  • ATM - too complex - too expansive ltIPgt
  • Internet technology ATM philosophy
  • but ATM ideas continue to powerfully influence
    design of next-generation Internet
  • ex MPLS, admission ctl., resource reservation,
    ...

27
Multiprotocol label switching (MPLS)
  • initial goal speed up IP forwarding by using
    fixed length label (instead of IP address) to do
    forwarding
  • borrowing ideas from Virtual Circuit (VC)
  • but IP datagram still keeps IP address!

28
Label Substitution
  • Have a friend go to B ahead of you using one of
    the previous two techniques. At every road they
    reserve a lane just for you. At every
    intersection they post a big sign that says for a
    given lane which way to turn and what new lane to
    take.

29
(No Transcript)
30
A LABEL BY OTHER NAME There are many examples
of label substitution protocols already in
existence ATM - label is called VPI/VCI and
travels with cell. Frame Relay - label is
called a DLCI and travels with frame.
TDM - label is called a timeslot its implied,
like a lane. X25 - a label is an
LCN Proprietary PORS, TAG etc.. One day
perhaps Frequency substitution where label
is a light frequency?
31
  • MPLS Terminology
  • ? Label Short fixed length, locally significant
  • ? Label Switching Router (LSR) Routers - use
    labels
  • ? Forwarding Equivalence Class (FEC)
  • Same Path treatment gt Same Label
  • ? MPLS Domain Contiguous set of MPLS nodes in
    one
  • administrative domain
  • ? MPLS edge node Egress or ingress node
  • ? Label distribution protocol gt Routing protocols

32
Label Encapsulation
  • MPLS Encapsulation is specified over various
    media types. Top labels may use existing format,
    lower label(s) use a new shim label format.

33
MPLS Link Layers ? MPLS -- run over multiple
link layers ? Following link layers currently
exist ATM label -- in VCI/VPI field of
ATM header Frame Relay label -- in DLCI
field in FR header PPP/LAN uses shim
header inserted
between L2 and L3 headers ? Translation between
link layers types must be supported ? MPLS is
between L2 and L3 It intended to be
multi-protocol below and above
34
MPLS capable routers
  • a.k.a. label-switched router
  • forwards packets to outgoing interface based only
    on label value (dont inspect IP address)
  • MPLS forwarding table distinct from IP forwarding
    tables
  • signaling protocol needed to set up forwarding
  • Hop-by-hop or source routing to establish
    labels
  • forwarding possible along paths that IP alone
    would not allow (e.g., source-specific routing)
    !!
  • use MPLS for traffic engineering
  • RSVP-TE
  • must co-exist with IP-only routers

35
MPLS forwarding tables
in out out label
label dest interface
10 A 0
12 D 0
8 A 1
R6
0
0
D
1
1
R3
R4
R5
0
0
A
R2
R1
36
MPLS FOR TRAFFIC ENGINEERING A USEFUL WAY OF
LOOKING AT MPLS Conventional IP routing is
driven by topology (i.e., hops), and does not
take into account of bandwidth availability and
traffic characteristics MPLS can support
traffic engineering via L2 trunking
Classify packets (only once!) Map packet
classes onto trunks Map trunks onto physical
routes
37
Best of Both Worlds
  • MPLS IP form a middle ground that combines the
    best of IP and the best of virtual circuit
    switching technologies
  • ATM and Frame Relay cannot easily come to the
    middle so IP has!

38
  • Summary of Motivations for MPLS (1)
  • Simplified forwarding based on exact match of
  • fixed length label
  • - initial drive for MPLS was based on
  • existence of cheap, fast ATM switches
  • Separation of routing and forwarding in IP
  • networks
  • - facilitates evolution of routing
    techniques
  • by fixing the forwarding method
  • - new routing functionality can be
    deployed
  • without changing the forwarding
  • techniques of every router in the
  • Internet

39
  • Summary of Motivations for MPLS (2)
  • Facilitates the integration of ATM and IP
  • - allows carriers to leverage their large
    investment
  • of ATM equipment
  • - eliminates the adjacency problem of
    VC-mesh
  • over ATM
  • Enables the use of explicit routing/source
    routing
  • in IP networks
  • - can be easily used for such things as
    traffic
  • management, QoS routing
  • Promotes the partitioning of functionality
    within
  • the network
  • - move granular processing of packets to
    edge
  • restrict core to packet forwarding
  • - assists in maintaining scalability of IP
    protocols
  • in large networks

40
  • Summary of Motivations for MPLS (3)
  • Improved routing scalability through stacking of
    labels
  • - removes the need for full routing tables
    from
  • interior routers in transit domain only
    routes
  • to border routers are required
  • Applicability to both cell and packet
    link-layers
  • - can be deployed on both cell (eg. ATM)
    and
  • packet (eg. FR, Ethernet) media
  • - common management and techniques
    simplifies
  • engineering
  • Many drivers exist for MPLS above and beyond
  • high speed forwarding


41
Multi-Protocol Label Switching
  • Key ideas of MPLS
  • Label-switched path spans group of routers
  • Explicit path set-up, including backup paths
  • Flexible mapping of data traffic to paths
  • Motivating applications
  • Small routing tables and fast look-ups
  • Virtual Private Networks
  • Traffic engineering
  • Path protection and fast reroute

42
Status of MPLS
  • Deployed in practice
  • Small control and data plane overhead in core
  • Virtual Private Networks
  • Traffic engineering and fast reroute
  • Challenges
  • Protocol complexity
  • Configuration complexity
  • Difficulty of collecting measurement data
  • Continuing evolution
  • Standards
  • Operational practices and tools

43
Optical Networks
  • 1 st Generation optical fibers substitute copper
    as physical layer
  • Submarine Systems
  • SONET (synchronous optical) in TDM
  • FDDI for LAN, Gbit Ethernet etc.
  • 2 nd Generation optical switching and
    multiplexing/ WDM
  • broadcast-and-select networks
  • WDM rings
  • wavelength routing networks
  • 3 th Generation optical packet switching???

44
Optical Switch
  • 1-input 2-outoput illustration with four
    wavelengths
  • 1-D MEMS (micro-electromechanical system) with
    dispersive optics
  • Dispersive element separates the ?s from inputs
  • MEMS independently switches each ?
  • Dispersive element recombines the switched ?s
    into outputs

45
All-Optical Switching
  • Optical Cross-Connects (OXC)
  • Wavelength Routing Switches (WRS)
  • route a channel from any I/P port to any O/P port
  • Natively switch ?s while they are still
    multiplexed
  • Eliminate redundant optical-electronic-optical
    conversions

46
MP?S
  • MP?S Multi-Protocol Lambda Switching
  • MPLS OXC
  • Combining MPLS traffic eng control with OXC
  • All packets with one label are sent on one
    wavelength
  • Next Hop Forwarding Label Entry (NHFLE)
  • ltInput port, ? gt to ltoutput port, ? gt mapping

47
(No Transcript)
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