Future Internet Chapter 3: (Virtual) circuit switching or: a tale - - PowerPoint PPT Presentation

Computer Networks Group Universität Paderborn

Future Internet Chapter 3: (Virtual) circuit switching

• r: a tale from the past

Holger Karl

SS 19, v 1.3 FI - Ch 3: (Virtual) circuit switching 2

Overview

• (Virtual) circuit vs. packet switching
• The old: ATM
• The current: MPLS

SS 19, v 1.3 FI - Ch 3: (Virtual) circuit switching 3

The thorns of IP

• What made IP forwarding hard?
• Packets of different lengths
• Complex destination address -> outgoing port lookup in forwarding

plane

• Very hard to impossible to provide any kind of guarantees
• No central view on network
• Historically, we did not always build networks like that
• … nor do we do that today
• A look at the past: Asynchronous Transfer Mode (ATM)
• Started in the 1960s, had its big day in the 1990s
• .. and at the present: Multi-protocol label switching (MPLS)

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ATM – Basic concepts

• Virtual circuits (VC)
• Switches know how to handle data arriving in a VC
• VCs have to be setup before data can travel along it
• VCs have explicit names – Virtual Circuit Identifiers (VCI)
• Packets contain this VCI in header, not a destination address
• Can be much smaller than a full destination address
• Forwarding deterministic based on VCIs, all data on one VC

follows same route

• Results in separated control and data handling
• Operating at different time scales; eases HW/SW separation
• VCs can be setup per call/connection or permanently

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ATM – Basic concepts

• Fixed-size packets – or cells
• Makes buffer management, switching fabric, line scheduling

MUCH easier

• But causes segmentation/reassembly overhead
• Small cells
• Necessitates small addressing for acceptable overhead
• Historical decisions; was meant for voice traffic (follow-up to PSTN

STM network)

• Size: industry conflict between
• Small cells ! low delay ! no echo cancellation needed vs.
• Big country ! needs echo cancellation anyway ! wants small
• verhead
• Ended up as compromise: 48 bytes payload, 5 bytes header

SS 19, v 1.3 FI - Ch 3: (Virtual) circuit switching 6

ATM – Basic concepts

• Statistical multiplexing
• Deal with outgoing link data rate < sum of incoming data rates
• Multiplexing gain = sum of incoming / outgoing rate
• Integrated services
• Provide support for different service types (voice, data, …)

explicitly

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Uniqueness of VCIs

• Simple approach: all VCs sharing at least a single link

must have different VCIs

• Scaling problems
• Better: VCI swapping
• Switches can rewrite VCI identifiers in incoming cells, based on

incoming port: (Incoming port, Incoming VCI) ! (Outgoing port, Outgoing VCI)

• These rules are collected in a switch’s translation table
• Entails per-VC state in each switch!
• Switching becomes a simple lookup operation in

translation table!

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Setting up VCs

• Option 1: Let each switch locally do routing on the call

setup cells

• Handle translation tables locally as well
• Option 2: “Global” view ?

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ATM state of affairs

• Currently, no longer a relevant system for research or new

deployment

• … but still sits in some backdoor closets somewhere
• Largely replaced by its successor, MPLS
• See IETF RFC 3031 plus follow-up RFCs

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From ATM to MPLS

• ATM turned out to be inflexible, evolved towards MPLS
• Main similarities
• MPLS is a label-switched protocol, just like ATM uses the virtual

circuit identifiers (a terminology difference only)

• MPLS has a similar translation table for its labels
• Forwarding still stays a simple table lookup
• Main differences
• MPLS can support varying payload size
• MPLS can carry (“tunnel”) arbitrary network layer protocols; MPLS

prepends a so-called shim header

• Shim: Label, QoS, stack bit, time to live
• VC setup works differently

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MPLS control vs. data

• Like ATM, MPLS maintains a clear and strict separation of

data and control plane

• Data plane: Only forwarding, based only on labels, very simple
• Much simpler than IP: no longest prefix matching, no routing

algorithms at all in the switch

• Control plane:
• Protocols to determine reachability information (aka routing) or to

import it from other networks (e.g., talk to IP BGP routers to understand which network prefixes are attached where)

• Protocols to distribute labels (Label distribution protocols, LDP)
• For discovered network prefixes, distribute labels to relevant routers
• Proactively or on-demand
• Various options, e.g., RSVP, MP-BGP, …
• Traffic engineering functionality

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MPLS translation table – power over ATM

• Translation tables under MPLS much more powerful
• ATM: only action was to replace a VCI by another and forward
• MPLS: Actions by a label-switched router (LSR)
• Replace label by another and forward
• Insert another shim header! (Push a label onto a label stack)
• Remove a shim header and look at the resulting packet
• Label stack allows nesting of virtual circuits inside each other
• Great tool for traffic engineering
• Once last shim header is removed, pass contained packet to

whatever network protocol, e.g., IP

An MPLS label

https://en.wikipedia.org/wiki/Multiprotocol_Label_Switching SS 19, v 1.3 FI - Ch 3: (Virtual) circuit switching 13

Switching table in an MPLS LSR

• What’s hence needed in an MPLS LSR switching table?
• What are differences to an Ethernet switch?
• Ethernet switching table columns:
• Destination MAC address
• Outgoing port
• MPLS switching table columns - Next Hop Label

Forwarding Entry (NHLFE)

• Incoming label
• Incoming port
• Outgoing label
• Outgoing port
• Optional: Operations on label stack (push, pop)

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Why?

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Where do MPLS labels come from?

• Typical setup: an MPLS network interconnects various other networks
• E.g., IP networks interconnected by MPLS
• At some point, non-MPLS packet arrives at the first MPLS router
• So called label edge router (LER)
• LER can analyse packet and decide its forwarding equivalence class

(FEC)

• FEC: Set of packets to be forwarded in the same fashion (same path)
• Decision based on destination IP, source IP, QoS, time of day, load situation
• f network, non-local information, …
• Information that is not necessarily available to an IP router inside a network!
• For each FEC, there is a label-switched path (LSP) with according

label

• LER inserts shim header with this label and starts switching the packet
• Typically, multiple FECs on same LSP

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Example: Traffic engineering with MPLS

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MPLS’ hidden jewel (?): Path Computation Element

• Determining routes when setting up a label: so far, distributed

process similar to a packet router assumed

• Actually, MPLS reuses IP routing (not forwarding!) protocols like

OSPF to distribute topology information (or better, OSPF-TE with traffic engineering extensions)

• Typically: Label Edge Router decides on path
• Alternative proposal: Decide on paths in a centralized way!
• Introduce a central instance where routing decisions are taken,

entries for translation tables are then redistributed

• So-called Path Computation Element (PCE)
• Presentation here based on RFC 4655
• Note: This makes a lot more sense in a “virtual circuit” model

rather than a packet forwarding model

• Unlike packets, decisions on path setup only happens rarely – can be

done centrally, simplifies network management a lot

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Why centralized path computations?

• More complex algorithms could be used
• E.g., shared backup paths for paths that do not interact with each
• ther
• Bundles of requests can be considered and optimized

jointly

• E.g.: Setup a VLAN between multiple sites; only relevant if all of

paths obtain certain quality

• Limited visibility in distributed solutions
• E.g., multi-domain, multi-carrier, multi-layer
• Dealing with legacy equipment without control functionality
• In particular, optical nodes
• Management simplified in general; one place to put control

functions

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Path Computation Element

• To quote RFC 4655:
• A Path Computation Element (PCE) is an entity that is capable of

computing a network path or route based on a network graph, and

• f applying computational constraints during the computation.
• The PCE entity is an application that can be located within a

network node or component, on an out-of-network server, etc.

• For example, a PCE would be able to compute the path of a TE

LSP by operating on the TED and considering bandwidth and

• ther constraints applicable to the TE LSP service request.

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Path Computation Element

• Terminology:
• TED: Traffic Engineering Database, contains topology and

resource information of given network domain. Derived from routing protocols like IGP (at slow time scales)

• TE LSP: Traffic Engineering MPLS Label Switched Path
• Domain: Collection of network elements with common sphere of

management, e.g., an AS

• NMS: Network Management System

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PCE Architecture

• PCE receives/answers requests

via the Path Computation Element communication protocol (PCEP)

• PCE talks to Path Computation

Clients (PCC) and NMS

• Main operation sequence:
• PCC requests path towards given destination
• Typically, the LER is the PCC
• Request can contain optional parameters (bandwidth, excluded
• bjects, … )
• PCE computes it, returns path
• Path can contain strict nodes or loose nodes (loose: additional nodes

maybe inserted into path before it, see RSVP-TE, RFC 3209)

• PCC uses signalling protocol to request corresponding path

From [3]

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Single or multiple PCEs

From [3]

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Example use case: Multi-domain Path Computation

From [3]

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Example use case: Multi-domain Path Computation

From [3]

SS 19, v 1.3 FI - Ch 3: (Virtual) circuit switching 25

From MPLS to GMPLS

• MPLS hugely successful
• Got extended in multiple directions – Generalized MPLS
• Additional packet switching technologies on top (not just IP, as in

MPLS)

• Additional layer 2 technologies
• Interesting example: Doing label switching directly on optical fibre

– GMP¸S

• Using the basic circuit setup infrastructure of MPLS, but doing the

actual switching not by looking at labels, but rather by optical switches directly switching on wavelengths (¸)

• Generalized label can here represent an entire fibre, a wavelength in

a fibre, or timeslots in such a wavelength (building a “virtual channel”)

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Summary

• Virtual circuit switching is still a powerful technique
• Determining fate of data at the edge turns out to be a big

lever indeed

• The idea to have a global view of the network is much

easier in circuit-switched networks than in packet-switched networks

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References

• 1. Material by Cisco and Juniper, various, on their websites
• 2. A. Farrel, J.-P. Vasseur, J. Ash, A Path Computation

Element (PCE)-Based Architecture, Aug. 2006, RFC 4655.

• 3. F. Paolucci, F. Cugini, A. Giorgetti, N. Sambo, and P.

Castoldi, “A Survey on the Path Computation Element (PCE) Architecture,” IEEE Communications Surveys & Tutorials, pp. 1–23, 2013.