circuit switching • virtual circuit switching • datagram switching ... · 2012. 3. 25. · •...

© Grotto Networking 2004Page - 1

Fundamental Switching Types

• Circuit Switching• Virtual Circuit Switching• Datagram Switching• Implications for Signaling, Routing, Path

Computation, and Restoration– MPLS and GMPLS control planes


Differences in Switching Types

• Is connection set up required?• Is statistical multiplexing possible?• What are the QoS measures? How is

bandwidth allocated?• How much work is needed to provide QoS

guarantees?• How can reliability/protection/restoration be

provided and what are the trade offs?


Forwarding at each switch

• Datagram (e.g., IP)– Based on complete destination address within the packet.

Any valid destination must be forwarded correctly.• Virtual Circuits (e.g., MPLS, ATM, Frame Relay)

– Based only on a label with the packet header. Onlypackets whose “virtual circuit” has been set up ahead oftime must be forwarded correctly.

• Circuits (not packets)– Based implicitly on either time slot or wavelength. No

forwarding information needed in data. Only thosecircuits whose path has been set up ahead of time must beforwarded correctly.


Example Network

– Datagram, Virtual Circuits, or Circuits– Switches 1-5, Hosts A-J


Datagram Forwarding ExampleSwitch #1

Dest Port A 1 B 2 C 3 D 3 E 4 F 4 G 4 H 4 I 3 J 3

Switch #2 Dest Port A 2 B 2 C 1 D 3 E 2 F 2 G 4 H 4 I 4 J 4



Switch #5 Dest Port A 1 B 1 C 1 D 1 E 2 F 2 G 2 H 2 I 3 J 4 Graph of our

example networkwith switch portsand hosts shown

II I I

I

I


Virtual Circuit forwarding Example• Connections

– Host A to Host J, Host B to Host C, Host E to Host I,Host D to Host H, and Host A to Host G


Virtual Circuit Forwarding

– Packets are forwarded based on a label in the header– Labels are not destination addresses, usually much

shorter– Labels need to be unique on a link but not in a network,

i.e., we can reuse labels on each link.– Switch forwarding tables consist of a map between

(input port, packet label) to (output port, new packetlabel)

– Table entry for each virtual circuit rather than for eachdestination (the datagram case)

– Technologies: MPLS, Frame Relay, ATM, X.25


VC Forwarding Table ExampleSwitch #2

In Port In Label Out Port Out Label2 5 4 1 2 1 1 1 3 6 4 3

Switch #3 In Port In Label Out Port Out Label 1 1 3 3 2 1 3 1

Switch #5 In Port In Label Out Port Out Label 1 1 4 2 1 3 2 1 2 1 3 1

Switch #1 In Port In Label Out Port Out Label1 2 3 5 2 1 3 1 1 1 4 1

Switch #4 In Port In Label Out Port Out Label1 3 2 5 1 1 3 1 3 1 4 1

6

33

1

1

1


“Real” Circuit Forwarding

• No more packets• Bit streams are distinguished by port and

– Time slots in the TDM case– Wavelength in the WDM case– Frequency in the FDM case

• Switching independent of bit stream contents• TDM example (same connections as VC case)

– Host A to Host J, Host B to Host C, Host E to Host I,Host D to Host H, and Host A to Host G


“Real” Circuit Tables Example

Switch #2 In Port In Slot Out Port Out Slot 2 5 4 1 2 1 1 1 3 6 4 3

Switch #3 In Port In Slot Out Port Out Slot 1 1 3 3 2 1 3 1





Time Division Multiplexing

Regenerator (3R) #1

Regenerator (3R) #2

TDM de-multiplexor

TDM Multiplexor

= Optical Fiber

= Regenerator section overhead

= Multiplex section (line) overhead= User traffic (path layer)= Unused time slots

Path

MS

RS RS RS

Path

MS

RS

TDM Path

Multiplex Section

Regenerator Section


Real Circuits and Virtual Circuits

• Virtual Circuits– Packet based, label (not destination address) in packet

header– Doesn’t always consume bandwidth, i.e., traffic can be

bursty

• Real Circuits– No packets raw bit stream, implicit label with either

time slot or wavelength– Is always consuming a fixed bandwidth, easy to keep

track of bandwidth but not necessarily the mostefficient utilization of link capacity.


QoS with Real Circuits

• Bandwidth– Hard bandwidth guarantees are given by default

(even if you don’t want them).• Delay

– Very little delay variation. Most delayattributable to propagation. Switching delays inmost circuit switches is minimal.

• Bit Error Rate– Is the primary “signal quality measure”


QoS with Virtual Circuits

• Bandwidth– Is by default shared with other users. Effort required to

make guarantees. Very good statistical multiplexinggain can be obtained.

• Delay– In addition to propagation and switch processing delay

we now have queueing induced delays– Queueing delays: can be quite large, can be quite

variable– By default no guarantees made

• Dropped/Errored Packets– Packets can be errored (bits errors), or dropped due to

buffer overflows.


Protection/Restoration

• Failure detection– Most circuit technologies have very fast built-in failure

detection.– For packet technologies this hasn’t been the case but

new work, e.g., BFD at IETF is underway.• Alternative Routes

– Alternate routes for circuit consume bandwidth or mustbe set up on the fly costing time.

– Alternate routes for virtual circuits do not consumebandwidth until they are used, hence can be set upahead of time.

– Alternate routes can not be preconfigured for datagramnetworks and all switches (routers) must recalculaterouting tables based on link failure info.


Forwarding Tables

• All switching types use them• Datagram Forwarding Tables

– Need to account for all destinations no matter who’scommunicating at any given time.

• Circuit and Virtual Circuit Forwarding Tables– Entry for each circuit or VC that traverses a particular

switch.– Note that if there are N host and they all want to talk

to each other at exactly the same time then the networkwill need to support N(N-1) circuits or VCs.


Scaling Forwarding Tables

– Modern networks like the Internet and Telephonenetworks consists of 100 of Millions or more hosts howcan we keep our routing tables under control?

• Datagram Tables– We route based on networks and groups of networks.

Addresses are given out accordingly. This allows theaggregation of destination addresses.

• Circuit Tables– We multiplex circuits onto larger and larger trunks in a

hierarchy. Switches generally only work at a couplelevels of the hierarchy. Example a switch working withSONET OC48 links (2.5Gbps) will switch with50Mbps granularity but not 64kbps granularity!


Setting up the Routing Tables

• Finding Paths from Source to Destination– How do we choose our “route”?– Algorithms– Protocols

• Datagram Routing– Must make sure that the tables are consistent so

we don’t get datagram loops.• “Real” Circuit Routing

– Need to have enough bandwidth available onthe links to support the circuits.


Differences Between Optical NetworkRouting and IP Routing

• IP routing– Per hop forwarding of datagrams based on destination

IP address– Every router must have exactly the same network

topology information (links, nodes, and link wts.)– Every router must run exactly the same path

computation algorithm– Failure to insure these last two requirements can result

in routing loops and “black holes”


Differences Between Optical NetworkRouting and IP Routing...

• Optical routing– Circuits are source routed; no loops possible– No standardization of path computation required– Okay for information to be slightly out of date, e.g.,

available capacity information; worst case “crank-back”of connection

– Unless restoration action is taken based on link stateupdates, routing is not service impacting in transportdomain


What is GMPLS?

• GMPLS = Generalized MPLS– Refers to adaptation of MPLS control plane for

the control of other technologies– Includes signaling and routing mechanisms

developed for MPLS traffic engineering– GMPLS protocols developed under IETF– Previously called “MPλS”


What is MPLS?

• MPLS = “Multi-Protocol Label Switching”

• A virtual circuit form of packet switchingsuch as frame relay or ATM but with amore IP centric control plane and built in IP“adaptation”.


MPLS and IP

• A combine IP router / MPLS switch assigns IPpackets to MPLS flows (virtual circuits)– This process is known as “classification” and can be

very simple or very complex depending upon thecontext.

– This box is known as a Label Edge Router (LER)

• The IP packet header is not touched or looked atwhile in the MPLS network– The LSR (label switched routers) only switch based on

MPLS labels.


An example…

UnlabeledPacket arrives

IP

Egress routerremoves label

IP

IP 20

Label switching & packet forwarding

Ingressrouter addslabel to packet

IP 10

Autonomoussystem boundary


Label Switched Path (LSP)

A Label Switched Path is like a pipe or tunnel to IP packets.However its just another term for a virtual circuit. While travelingon a label switched path, forwarding is based on the label only,not on destination IP address in packet.

Label switched path


10.1.1.2

10.1.1.6

10.1.1.3

10.1.1.710.1.1.4

10.1.1.5

10.1.1.1

12.0.0.1

Controlling LSP Set-Up: ExplicitRouting

POP

Explicit route10.1.1.7 strict10.1.1.6 strict10.1.1.5 strict10.1.1.2 strict10.1.1.1 strict

Strict hopLSP takes direct route to 10.1.1.7

10.1.1.2

10.1.1.6

10.1.1.7

10.1.1.5

Similar procedure can be used for optical connection set-up.


“Generalized” MPLS

• Virtual Circuits ! Real Circuits– From real labels in MPLS to “virtual labels” in GMPLS

• “Labels” in GMPLS– TDM where time slots are the implicit labels (e.g.,

SONET)– FDM where frequencies (or λs) are the implicit labels

(e.g., WDM)– Space-division multiplexing where port numbers are

the implicit labels (e.g., OXCs)• Generalized labels used in MPLS messaging =

Generalized MPLS

circuit switching • virtual circuit switching • datagram switching ... · 2012. 3. 25. · •...

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