View
1
Download
0
Category
Preview:
Citation preview
Lecture 8
Traffic Engineering - QoS
ATM
Label switching
Dimitrios Klonidis – 14/04/2019
2 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Outline
▪ Traffic engineering principles‒ Classification, Scheduling and Polishing
‒ QoS
▪ ATM overview
▪ Label switching ‒ MPLS intro
3 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Traffic engineering principles
QoS, Scheduling, Policing, Classification
4 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Traffic Engineering
▪ Traffic Engineering: A set of processes that aim to put traffic where the network capacity is, or in other words provide a path for carrying data traffic
▪ Target: ‒ Optimize performance …
‒ …for certain traffic demand …
‒ …without affecting existing flows.
5 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Traffic Engineering
▪ Relationship among Capacity, Utilization and QoS (resource reservation).
‒ One can claim that for an underutilized network QoS is not needed.
‒ And if demand exceeds network’s capacity QoS cannot resolve the problem.
➔ Think why?
▪ Currently ISPs do not want to operate beyond 40-50% utilization levels. What if they like to push it to 80-90%?
6 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Quality of Service (QoS)
▪ Quality of Service: ‒ Definition: Provision (by the network) of traffic service guarantees,
such as bandwidth, buffers, delay bounds, jitter, acceptable packet loss
‒ Goal: Ensure fairness among users/traffic streams (fair resource allocation). Use policing, scheduling, priority queueing mechanisms
▪ Applications may have different QoS requirements: real-time applications vs. non-real-time e.g,, IP telephony vs. FTP)
▪ Classify incoming packets, assign them times scheduled for transmission ▪ Discard packets from ill-behaved sources
7 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Applying QoS
▪ Flow and Congestion control are types of QoS‒ Their target is to increase throughput, avoid congestion
• … but not enough themselves to provide true QoS (per service)
‒ Can be done by forming flows (groups) of traffic and treat them separately
• … but applying QoS to each flow can be slow and expensive for routers
▪MPLS has been presented as a solution for traffic engineering in the internet (protocol independent)
▪ SLAs: Service Level Agreements define specific (network) performance metrics (throughput, latency), including service availability and outage notification. Offered by ISPs to their customers [like a customer contract]
8 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
IP
Header
TCP
Header
MPLS
LABELTCP Payload
L2
Header
MPLS – The key idea
▪ It is a waste of resources to have to examine every IP packet header (e.g., routing table look-up)
▪ Solution:
➔ create a short, fixed-length identifier: “label” (20 bits). Use the label to identify packets as a unified flow and forward/switch them rapidly.
▪ Characteristics:‒ Uses Layer 2 (e.g., Ethernet, ATM) encapsulation.‒ MPLS applicable to any network layer protocol. It hides the details
(and header) of network layer protocol.‒ Enables integration of IP networks with connection-oriented
networks.
9 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
QoS, CoS and Policies
▪ Quality of Service (QoS) classifies network traffic and then ensures that some of it receives special handling.
‒ May track each individual dataflow (sender:receiver) separately.
‒ May include attempts to provide better error rates, lower network transit time (latency), and decreased latency variation (jitter).
▪ Differentiated Class of Service (CoS) is a simpler alternative to QoS.
‒ Doesn't try to distinguish among individual dataflows; instead, uses simpler methods to classify packets into one of a few categories.
‒ All packets within a particular category are then handled in the same way, with the same quality parameters.
▪ Policy-Based Networking provides end-to-end control.‒ The rules for access and for management of network resources are
stored as policies and are managed by a policy server.
10 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•Example: (1Mbps IP phone + data) to share 1.5 Mbps link
−bursts of FTP can congest router, cause audio loss
−want to give priority to audio over data
−Traffic mix (data, IP phone)
Packet marking & classification needed for router to distinguish between different classes; and new router policy to treat packets accordingly
Principle 1
Classification, Scheduling and Polishing
11 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•What if applications misbehave (audio sends higher than declared rate)
−policing: force source adherence to bandwidth allocations
•Marking/classification and policing at network edge:
−similar to ATM UNI (User Network Interface)− Note: If only classification is used providing guaranteed bandwidth to IP phone then if IP phone
exceeds 1Mbps as initially declared, then this bandwidth will be “stolen” from data.
Provide protection (isolation) for one class from others
Principle 2
Classification, Scheduling and Polishing
12 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Allocating fixed (non-sharable) bandwidth to flow: →
but … inefficient use of bandwidth if flows
doesn’t use its allocation
While providing isolation, it is desirable to use resources as efficiently as possible
Principle 3
Classification, Scheduling and Polishing
13 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•Basic fact of life: can not support traffic demands beyond link capacity
•Example: two (competing) IP phone sessions with aggregate
bandwidth request exceeding available capacity
Call Admission: flow declares its needs, network may block call (e.g., busy signal) if it cannot meet needs
Principle 4
Classification, Scheduling and Polishing
14 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Scheduling
▪ Scheduling: ‒ way packets are buffered and scheduled for transmission
(choose next packet to send)
‒ Motivation: • support different classes of service that have heterogeneous
requirements (e.g., bw, delay, loss)• Support for flows with different priorities
‒ Results in better buffer management (memory allocation among competing flows)
‒ Requires flow identification (classification)
15 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Polishing
▪ Policing: ‒ regulate or limit traffic so that it does not exceed declared
parameters.
‒ Can include drastic measures such as packet discard
16 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Resource allocation
▪ Allocation of resources‒ Identify (monitor) the physical and computation resources of a
network and assign the optimum amount to traffic demands according to their service requirements
‒ Requirements:• allow fair share (multiplexing) of resources• should not lead to starvation situations• should not affect existing users, flows or sessions • provide performance bounds (for guaranteed services)• ease of implementation
‒ Not a straightforward process• how much bandwidth is allocated per flow • when packets should be transmitted (choosing eligibility times)• which packets are discarded from the router
‒ Relies on a set of optimization rules and targets • … typically a heuristic approach
17 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Policing Criteria
▪ Three common-used criteria: ‒ (Long term) Average Rate:
• how many pkts can be sent per unit time (in the long run)
• what is the interval length: • E.g. 100 packets per sec or 6000 packets per min have same average!
‒ Peak Rate: • E.g. avg. rate of 6000 pkts per min., but Peak rate of 500 pkts per sec
‒ Maximum burst Size: max. number of packets sent consecutively (with no intervening idle)
▪ ! Recall that Internet traffic is bursty (especially at the network edge)
18 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•FIFO (first in first out) scheduling: send in order of arrival to queue
−real-world example? (bank branch, post office)
−discard policy: if packet arrives to full queue: who to
discard?
tail drop: drop arriving packet
priority: drop/remove on priority basis
random: drop/remove randomly
FiFo scheduling
19 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
(Strict) Priority scheduling: transmit highest priority queued packet
•Multiple classes, with different priorities
−class may depend on marking or other header info, e.g. IP source/dest,
port numbers, etc..
−real world example? (airline counter: business and economy class
passenger lines)
Priority scheduling
20 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Round Robin scheduling:
•Multiple classes
•Cyclically scan class queues, serving one from each class (if
available)
•Fair mechanism (→ applied in Waited Fair Queuing)
Round Robin scheduling
21 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
QoS implementation models
22 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
QoS implementation models
▪ Best Effort‒ The IP approach → All packets are treated equally
‒ No BW, delay or jitter predictions
‒ Some guaranties through the use of TCP
▪ Integrated Services (IntServ) + RSVP‒ Reservations are made per simplex flow (demand)
‒ Applications request reservations for network resources• granted or denied based on resource availability
‒ Senders specify the resource requirements via a PATH message that is routed to the receiver
‒ Receivers reserve the resources with a RESV message that follows the reverse path
23 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
QoS implementation models
▪ Differentiated Services (DiffServ)‒ The DiffServ Model offers a service that is better than Best-
Effort and more scalable than IntServ
‒ Traffic is first classified • Five forwarding classes are considered
• Process takes part at the edge of a DiffServ network
‒ Forwarding classes are encoded in IP packets • Use of the Differentiated Services Codepoint (DSCP) field of each
packet’s IP header
‒ DiffServ routers apply pre-provisioned Per-Hop Behaviors (PHBs) to packets according to the encoded forwarding class
24 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
ATM
A quick technology overview
25 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
ATM in a nutshell
▪ ATM : Asynchronous Transfer Mode‒ Layer 2, Virtual Circuit Switching technology‒ End-to-end network technology‒ Use fixed small packets (cells) of 48B payload and 5B header
▪ Key features:‒ Truly supports integrated services and QoS (it was designed that
way)• Able to meet bandwidth demands and stringent delay constraints• Able to provide guaranteed bandwidth and resource allocation.• Support for multiple classes of service• Support for circuit emulation (voice channels)
‒ Ultra high-performance (switching and sophisticated traffic control)
‒ Integration of multiple traffic streams (protocols for x–over–ATM ‒ Efficient packet switching and muxing
26 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Plane Management
Layer Management
Physical Layer
ATM Layer
ATM Adaptation Layer (AAL)
Higher-Layer
Protocols
Control plane
Higher-Layer
Protocols
User plane
The ATM protocol model
27 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
The ATM protocol model
End System End SystemSwitch
Voice
Data
Video
AA
AA
LL
P
H
Y
P
H
Y
A
T
M
P
H
Y
A
T
M
P
H
Y
A
T
M
AA
AA
LL
Voice
Data
Video
cells cells
Voice
Data
Video
End System End SystemSwitch
Voice
Data
Video
AA
AA
LL
P
H
Y
P
H
Y
A
T
M
P
H
Y
A
T
M
P
H
Y
A
T
M
AA
AA
LL
Voice
Data
Video
cells cells
Voice
Data
Video
•Adaptation Layer (AAL): Adapts upper layers (e.g., IP) into ATM. Performs data segmentation/reassembly into 48-byte payload. Present only in end-systems
•ATM Layer: Performs cell switching. Adds/ removes 5 byte header to payload
•Physical Layer: Converts to appropriate electrical or optical format
28 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
VC
switch
VP
switch
VC
switch
ATM
hostATM
host
VPC
VPI= 0
VCI=25
VPI= 4
VCI=12
VPI= 8
VCI= 2
VPI= 0
VCI=16
VCC
Virtual Paths (VPs) - Virtual Channels (VCs)
•Virtual Channel : a virtual circuit between source and destination (unidirectional)
•Virtual Channel Link (VCL) : specific instance of a VC between two VC connecting points
•Virtual Channel Identifier (VCI): virtual channel identifier, a 16-bit number that uniquely identifies a VCL
•Virtual Channel Connection (VCC) : concatenation of VCLs (unidirectional flow)
29 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•Bundle (thousands of) VCs in (point-to-point) VPs so that all VCs within a VP can benefit from the same management functions
UNIUNI
NNINNI
Virtual Paths (VPs) - Virtual Channels (VCs)
30 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Networking (layered) view
31 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
ATM header format
▪ GFC : Generic Flow Control: generally unused
‒ (0 = uncontrolled)
▪ VPI/VCI : used on a per link basis (NNI supports more VPs)
‒ actual number of VCI bits is negotiated between user and network
‒ some VCIs are pre-assigned (e.g., for signalling) -
‒ all cells belonging to same VC have same VCI (max number of VCs within path is 65,636)
▪ PTI : Payload Type Identifier: Indicates type of cell
‒ 1st bit if 0 user cell, if 1 management cell, ‒ 2nd bit indicates whether congestion has been
experienced, 3rd bit ),, Service Data Unit, if set (SDU=1)
‒ 3rd bit denotes the last cell in a multi-cell frame
▪ CLP : Cell Loss Priority‒ used for congestion control (0 = higher priority, 1 =
lower priority - discard first )
▪ HEC : Header Error Control (Check)‒ detects errors in the header only and is able to
correct all single-bit errors (FEC)
32 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
IP – ATM comparison
33 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•6 types of messages for establishing and releasing a connection
setupsetup
setup
connect ack
connect
call proceeding
call proceeding
connectconnect
connect ackconnect ack
source switch-1 switch-2 destination
setu
p
setupsetup
setup
connect ack
connect
call proceeding
call proceeding
connectconnect
connect ackconnect ack
setupsetup
setup
connect ack
connect
call proceeding
call proceeding
connectconnect
connect ackconnect ack
setupsetup
connect ack
connect
call proceeding
call proceeding
connectconnect
connect ackconnect ack
source switch-1 switch-2 destination
setu
p
releaserelease
release complete
release complete
release
release complete
rele
ase release
releaserelease complete
release complete
release
release complete
releaserelease
release complete
release complete
release
release complete
rele
ase
Connection set-up
34 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
The VP is managed by the network according to each resources and route calculation
Request
Acknowledgement
ATM Connection establishment
35 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•SVC : Switched VC → user-established VC via signaling (dynamically)
ATM
host-B
ATM
host-A
connect to B
Yes/No
UNI signaling NNI signalingconnect to B
Yes/No
set up
connect/reject
Switched VCs
36 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•PVC : Permanent VC → established by the network management (updates VPI/VCI tables)
- provisioned for indefinite use
NMS
ATM
host-B
ATM
host-A
VPI/VCI
VPI/VCI VPI/VCI
VPI/VCI
Permanent VCs
37 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•SPVC : allows PVC- capable end-systems to be supported across an SVC-capable ATM network
PVC PVCSVC
SPVC
UNI UNINNI
ATM network
Soft private VCs
38 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•According to the applications we distinguish into various service (traffic) classes (ATM Forum):
- Constant Bit Rate (CBR)
- Variable Bit Rate (VBR)
- Available Bit Rate (ABR)
- Unspecified Bit Rate (UBR)
•Real-time service classes: CBR, rt-VBR
•Non-real-time service classes: nrt-VBR, UBR and ABR
•Guaranteed services: CBR, rt-VBR, nrt-VBR
•Best Effort: UBR, ABR
ATM service categories
39 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
ATM Adaptation Layer (AAL)
40 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
ATM service categories
Concepts of CBR/VBR/ABR/UBR
41 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
VBR vs. ABR
▪ Both nrt - VBR service and ABR …‒ support variable rate data transmissions and does not preserve
any timing relationships between source and destination.
▪ However…‒ ABR service does not provide any guaranteed bandwidth to the
user. ‒ Rather, the network provides a “best effort” service, in which
feedback (flow control mechanisms) is used to increase the bandwidth available to the user (i.e. the Allowed Cell Rate (ACR)), if the network is not congested and to reduce the bandwidth when there is congestion.
▪ Through such flow control mechanisms, the network can control the amount of traffic that it allows into the network, and minimize cell loss within the network due to congestion.
42 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
ATM meets QoS requirements for a diverse range of traffic data
QoS in ATM
▪ QoS monitoring of‒ Cell loss rate‒ Delay‒ Jitter
▪ Traffic based on traffic contract that specifies: ‒ Peak cell rate‒ Average cell rate‒ Burst size
▪ Admission control‒ ATM network determines the set of connections that can be admitted
without QoS violation
▪ Flow control‒ Feedback from ATM network about the congestion levels can be used to
control the traffic from a user
43 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
ATM advantages
▪ ATM is scaleable, ‒ i.e., bandwidth can be adapted to meet user requirements
▪ Guaranteed transmission quality‒ Able to match quality demanded by the user
▪ Seamless integration of voice, video, data. ‒ Great for real time services (small cell size = small packetization delay)
▪ Ultra-fast switching‒ Due to synchronous operation
▪ Well standardized,
▪ Enhanced management ‒ (results in cost savings for operators)
▪ Direct compatibility with SONET/SDH backplane‒ (in favor to operators control and management)
44 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Current status
▪ ATM found it main use in metro and less core networks‒ Acted as service differentiation technology for 3-ple play ‒ Direct interface with SONET/SDH in core
▪ It was not fully adopted by the entire community
▪ ATM to the desktop (25 Mbps) never took-off.‒ (Fast) Ethernet prevailed as a much cheaper solution
▪ ATM gradually replaced in metro by MPLS‒ MPLS is a good alternative with lower cost and compatible to access
technology
▪ ATM was somehow successful but replaced by something better.
▪ The ATM characteristics may be repeated in future under a different shape and format!
‒ E.g. Optical Burst Switching OR Optical Packet Switching
45 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Label switching
Combining the benefits from IP and ATM
46 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Moving towards label switching
▪ Software-based routers became network bottlenecks as networks kept growing
‒ need for calculating connections, large number of header addresses, large routing tables
▪ Label switching concept was introduced as an alternative to IP able to facilitate and speed-up switching
▪ Main concept: ‒ use a special tag or label assigned to a packet that identifies and
switches the packet quickly
▪ Goal: ‒ separate route calculation from actual forwarding (done by hardware)
▪ Examples (in order of appearance): ‒ Tag Switching (CISCO), ‒ IP Switching (Ipsilon Networks)‒ MPLS (label swapping),‒ Flow-based switching,
47 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•In order to reduce the amount of information processed (using
software) in a router:
➔ Combine a router with a switch
Router = slow and expensive (for large scale high capacity links!)
Switch = fast and cheap
•Objective: speed data through the network as to reduce latency and
processing (routing)
•Once a flow is detected set up a VC
•Cut-through operation: detour around usual routing paths by taking
fast switching paths (e.g., through ATM switches)
•Use normal hop-by-hop IP forwarding for short-lived packets
Layer 3 intelligence in switching
48 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
IP Switch
Controller
ATM
Switch Speed up flows of information destine to the same output.
Process only small amounts of data
Claim:
• - added software is 10% the size of removed software!
• - latency drop (processing) by e factor up to 100!
Layer 3 intelligence in switching
49 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Issues with ATM and IP
▪ ATM‒ ATM has a scalability problem as a backbone technology.‒ ATM is connection-oriented:
• Between any pair of switches / routers, a separate PVC (Permanent Virtual Connection) must be set up.
• If optimal routing is required a full mesh of PVCs needs to be established • Very high complexity (proportional to n2). • In large networks this becomes unfeasible.
‒ ATM does not have powerful routing protocols:• ATM does not provide such powerful routing protocols as IP (OSPF, BGP).
▪ IP‒ QoS is difficult to meet with traditional routers:
• Traditional routers have become the bottleneck in the backbone.‒ Routing is costly:
• Routing is costly ($) since it needs a lot of performance and memory. (@ 2014 ca. 300‘000 BGP routes in Internet backbone; requires 60-120Mb memory to hold these routes)
50 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
Towards a labeled approach
▪ Combine the advantages of layer 3 routing + layer 2 packet forwarding
Forwarding:
Label Swapping
Control:
IP Router Software
Control:
IP Router Software
Forwarding:
Longest-match Lookup
Control:
ATM Forum Software
Forwarding:
Label Swapping
IP Router MPLS ATM Switch
Layer 3
Layer 2
▪ Routing is done with IP routing protocols. ‒ No change to the existing Internet backbone routing infrastructure is required
▪ QoS (Quality of Service) is achieved through layer 2 switching.
▪ The routing tables size can be reduced through layer 2 switching.
51 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
IP
Header
TCP
Header
MPLS
HeaderTCP Payload
L2
Header
Multi-Protocol Label Switching
▪MultiProtocol Label Switching (RFC3031): ‒ Proposed by IETF standard for carrying multiple network layer
protocols‒ In practice it deals only with IP
▪ Idea: ‒ Use of a short (20 bits) identifier, LABEL, inserted ahead of IP
header at specialized nodes and handled by all the other label switching nodes in the network
‒ The labeled packets are identified as a unified flow and switched rapidly
‒ The IP layer is tunneled with the much faster MPLS‒ MPLS gets information from IP in order to determine the path to
establish follow• No need to go with shortest path
52 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
MPLS flow aggregation
▪ Packets with common characteristics form a flow:‒ same source-destination pair,‒ type of service,‒ protocol id, … etc.
▪ Flows are assigned a label
➔So MPLS performs “flow aggregation”
▪ MPLS is possible to offer QoS guarantees for certain traffic
▪ MPLS is used for traffic engineering: ‒ establish performance characteristics for different classes of traffic‒ set up paths that are followed by specific traffic classes (not necessary
shortest paths)
▪ MPLS header typically carries a TTL field, which is reduced at every LSR hop
▪ Easier implementation of source routing ‒ While in IP networks source routing can be only used to force a packet over a
specific route, in MPLS can be easily done by assigning a label
53 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
LER
LSR
LER
LSR LSP
MPLS – How it works
▪ Label Edge Router (LER) assigns label to the packet.
‒ LERs handle the entrance/exit of packets into MPLS network
‒ Ingress LER performs the label assignment and packet forwarding classification
‒ Egress LER performs label removal and label distribution (upstream)
▪ Label Switch Router (LSR) switch labelled packets over the label switched paths (LSPs)
▪ LSRs switch packets based only on their labels. MPLS forwarding table distinct from IP forwarding tables
54 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
MPLS – How it works ctd.
▪ Since labels are of local significance, LSRs may have to perform label swapping (exchange of labels)
▪ The egress LER has to update the TTL value in IP header, in order to reflect the number of hops that the packet traversed
▪ LSP route discovery can be performed by same protocols as in IP (BGP, OSPF)
▪ Signaling protocol needed to set up forwarding ‒ forwarding is possible along paths that IP alone would not allow
(e.g., source-specific routing) !!‒ use MPLS for traffic engineering
55 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
•Packets maintain IP addresses
•3-bit Experimental field: not standard use, could be used as class-of-service
(COS) field
•1-bit S: stack: indicates that the bottom of a stack of labels has been reached
•TTL: IP TTL is copied at the egress and then updated at the egress. MPLS TTL
is decremented at each LSR
PPP or Ethernet
headerIP header remainder of link-layer frameMPLS Header
label Exp S TTL
20 3 1 5
Header position and format
56 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro
R1R2
D
R3R4R5
0
1
00
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS – Forwarding example
Recommended