Scribe Notes For Lecture 32

Kuldip Patel

Circuit Switching vs Packet Switching

In a circuit switched network there is a physical end to end connection created between the communicating parties.
This connection is maintained for the duration of the call and is of a fixed data rate. The PSTN (telephone network) is the classic example.
Circuit-switched networks require dedicated point-to-point connections during calls.
In Circuit swithcing state per connection is maintained so it is limited to scaling.

      In packet switching there is no dedicated end to end connection between communicating parties.
      Individual packets are treated separately. The data rate can vary.
     Voice connections - circuit switched is ok (circuit has a high percentage of utilisation because somebody is generally always talking!)
     With data connections the line is frequently idle, e.g. surfing the Web! We don’t need a circuit-switching approach because it is inefficient.
      Packet switching is used to optimise the use of the bandwidth available in a network and to minimise the latency.
      Packets are routed to their destination through the most expedient route.
     Not all packets travelling between the same two hosts will necessarily follow the same route.
     The destination computer reassembles the packets into their appropriate sequence.
     It is connection-less, like IP       
     Packet Switching is Destination based routing hence it is slower.


It stands for Multi-Protocol Label Switching.
It is the latest technique that provides virtual path capability to packet(label) switches.


To avoid some drawbacks of both circuit switching and packet switching and to increase the utilization of bandwidth MPLS came into picture.
MPLS is basically to manage the traffic within the ISP .
It combines the benifits of both Circuit switching and packet switching .
It uses Circuit switching within ISP. and IP based packet switching within ISPs.

The idea behind MPLS is to attach a discrete set of labels to IP packets to perform a specific function, without forcing routers and switches to dive into IP addresses or other information in each packet to obtain instructions relating to that particular function.

MPLS packet Forwarding

MPLS does Label switching in which label is assigned to each IP flow.
It direct a flow of IP packets along a predetermined path across a network.
This path is called a label-switched path (LSP).
LSPs are simplex; that is, the traffic flows in one direction from the head-end (ingress) router to a tail-end (egress) router.
Duplex traffic requires two LSPs: one LSP to carry traffic in each direction.
An LSP is created by the concatenation of one or more label-switched hops, allowing a packet to be forwarded from one router to another across the MPLS domain.

When an ingress router receives an IP packet, it adds an MPLS header to the packet and forwards it to the next router in the LSP.
The labeled packet is forwarded along the LSP by each router until it reaches the tail end of the LSP, at which point the MPLS header is removed and the packet is forwarded based on Layer 3 information such as the IP destination address.
Here the label not the Destnation IP address determine the next route.


MPLS Header

MPLS Header is 32-bit.
Label (20-bit)->First 20-bits are to specify  Label per flow.
CoS(Class of Service (3bit)-> It is proposed to use is to  indicate perhop behavior of labeld packet traversing Label Switch Routers.
Stack bit-> Indiacte the presence of Label Stack.
TTL(8-bit)-> It is Time To Leave filed , decremented at each LSR hop and used to throuw away the looping packets.

Label (20 bit)
S TTL(8-bit)

MPLS Packet.
MPLS fits between Layer 2 and Layer 3.

L2 Header
MPLS Header
L3 Header
L3 Data

L2 Header
MPLS Header
MPLS Header
L3 Header
L3 Data

MPLS Path Recovery

Send a Mail If Image is missing
Figure 1.

Suppose There is a link failure ,as shown in figure, on the path from ingress router to egress router then there are 2 ways of replacing it.

Local Recovery(green in figure):-
Choose alternate path between 2 LSR which were connected by teh failed link. It enables a router upstream from the failure to route around the failure quickly to the router downstream of the failure. The upstream router then signals the outage to the ingress router, thereby maintaining connectivity before a new LSP is established.
End to End recovery(blue in figure):- Use an Alternate path from Ingress router to Egress Router.In Later option there is some propagation dealy to know the link failure.

Detecting such a failure is by insertion of some kind of periodic keep alive in the data path of the LSP (Note that keep alives such
as OSPF hello, RSVP refresh etc. Monitoring the presence of these keep alives at downstream nodes.
All this needs extra processing.


What is Lembda MPLS

Multiprotocol Label Switching (MPLS) promises to improve the performance, reliability and service quality in packet-switched networks by bringing many of the advantages of ATM networks to an arbitrary switched link layer while avoiding many of their disadvantages. MPLS achieves this by taking advantage of existing routing protocols to set up virtual label-switched paths across a set of label switching routers to identified destinations, thus requiring a packet's layer 3 header to be interpreted only at the ingress and egress of an MPLS switching domain. This not only reduces the packet-processing overhead but allows for multiple paths to be established for a single destination. It allows the  support for path protection, segregation of traffic by class of service and traffic engineering.
Multiprotocol Lambda Switching extends this paradigm into the optical domain by using an extended MPLS (or MPLS-like) control plane to control optical cross-connects. In effect instead of "route once switch many" we then have "electrical once, optical many" or "switch once, then transmit".
This lambda switching technology is an offshoot of an important traffic-engineering technique called Multi-protocol Label Switching. They share the same acronym and are almost the same thing, only with wavelengths substituting for numerical labels in the lambda version.