virtual circuit (vc) switching
DESCRIPTION
Virtual-Circuit Switching : ATM (Asynchronous Transmission Mode) and MPLS (Multiprotocol Label Switching) 2007. 10. Virtual Circuit (VC) Switching. Hybrid of packets and circuits Circuits: establish and teardown along end-to-end path - PowerPoint PPT PresentationTRANSCRIPT
Virtual-Circuit Switching: ATM (Asynchronous Transmission Mode)
and MPLS (Multiprotocol Label Switching)
2007. 10
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
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
Timing of Virtual-Circuit Packet Switching
Packet 1
Packet 2
Packet 3
Host 1 Host 2Node
1Node
2
propagation delay between Host 1 and Node 1VC
establishment
VCtermination
Datatransfer
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
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
2
1: 72: 7
link 7 1: 142: 8
link 14link 8
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 ID1
2
1: 7, 202: 7, 53 link 7
20: 14, 7853: 8, 42
link 14link 8
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”
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
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
ATM reference model
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
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)
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
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.
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
trafficVC introduces call setup latency,
processing overhead for short lived connections
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
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
ATM Service
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 ?
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
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)
AAL PDU
ATM cell
User data
IP-Over-ATM
AALATMphyphy
Eth
IP
ATMphy
ATMphy
apptransport
IPAALATMphy
apptransport
IPEthphy
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 <IP> Internet technology + ATM philosophy
but ATM ideas continue to powerfully influence design of next-generation Internet
ex: MPLS, admission ctl., resource reservation, …...
Multiprotocol label switching Multiprotocol label switching (MPLS)(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!PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Label SubstitutionLabel 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.
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.
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
MPLS capable routers a.k.a. label-switched router forwards packets to outgoing interface based
only on label value (don’t 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
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 1
12 9 D 0
in out outlabel label dest interface
10 A 0
12 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
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!
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 applicationsSmall routing tables and fast look-upsVirtual Private NetworksTraffic engineeringPath protection and fast reroute
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
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???
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
1-D MEMSMicro-mirror
Array
Digital MirrorControl
Electronics1011
Wavelength Dispersive Element
Input Fiber
Output Fiber 1
Output Fiber 2
Input & Output fiber array
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
DWDMFibers
in
DWDMDemux
DWDMDemux
DWDMFibers
out
DWDMMux
DWDMMux
All-optical
OXC
MPS MPS = 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)
<Input port, > to <output port, > mapping