5. Advanced Routing Mechanisms II.
PA159: Net-Centric Computing I.
Eva Hladká
Faculty of Informatics Masaryk University
Autumn 2010
Eva Hladká (Fl MU)
5. Advanced Routing Mechanisms II
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Lecture Overview I
l   Traffic Engineering in IP Networks o Introduction
o Discovering Network Utilization o Discovering Network Topology • Links' Weights Computation
2^ Multiprotocol Label Switching o MPLS
o Generalized MPLS o Grid-enabled GMPLS
3^ QoS-Based Routing
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Traffic Engineering in IP Networks Introduction
Lecture Overview I
l   Traffic Engineering in IP Networks o Introduction
o Discovering Network Utilization o Discovering Network Topology
• Links' Weights Computation
2^ Multiprotocol Label Switching a MPLS
• Generalized MPLS
<» Grid-enabled GMPLS
3^ QoS-Based Routing
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Traffic Engineering in IP Networks Introduction
Traffic Engineering in IP Networks
Introduction I.
o (interior) routing protocols used in IP networks are based on Shortest Path First (SPF) routing
in an unused network, the SPF is ideal:
datagrams are delivered expeditiously with the least use of network resources o Problem statement: once traffic increases, a link/router on the shortest path may become saturated
while longer paths remain unused/underused o Equal-Cost MultiPath (ECMP) is usable, but NOT problem-solving solution
Host U     Router A        Router B Router D Router E      Host X
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Traffic Engineering in IP Networks
Introduction II.
Traffic Engineering is all about discovering what other paths and links are available in the network, what the current traffic usage is within the network, and directing traffic to routes other than the shortest so that optimal use of the resources in the network is made.
• achieved by a combination of:
• extensions to existing IGP protocols
• traffic monitoring tools
• traffic routing techniques
• occurs outside the actual network
• does not address issues such as traffic surge lasting a few seconds/minutes
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Traffic Engineering in IP Networks Introduction
Traffic Engineering in IP Networks
Introduction III. Performed steps:
[i> traffic measurements are collected to estimate the traffic matrix
topology and configuration is obtained from the network []> a link weight determination process determines link weights
o the computed link weights for each link are injected into the network
o i.e., each router receives a metrics for its outgoing links o once injected, using a normal OSPF/IS-IS flooding process the metrics are disseminated through link-state advertisements
Question: How often should the TE system update the link weights? o up to the network provider/administrator o usually once a day or once a week
o to avoid short-term traffic fluctuations o since traffic matrix determination is a fairly complex and time-consuming process
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Traffic Engineering in IP Networks Introduction
Traffic Engineering in IP Networks
Introduction IV.
Eva Hladka  (FI MU)
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Traffic Engineering in IP Networks Introduction
TE - Complexity illustration I.
Host U    Router A        Router B Router D Router E      Host X
Which costs should be assigned to a, b, c, and d ? o not trivial even in such a simple network
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Traffic Engineering in IP Networks Introduction
TE - Complexity illustration II.
Host U    Router A       Router B Router D        Router E      Host X
o a = b = c = d = 1 == all traffic tends toward the link FG
• a = c = d =1, b = 7
o U — X routed through B, C, D (total cost 6) o W — Z routed through J (total cost 5) o V — Y routed through J (total cost 5)
o not ideal - some congestion is moved to router J
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Traffic Engineering in IP Networks Introduction
TE - Complexity illustration III.
o a = c = 2, b =7, d = 10 achieves the desired result
o U — X routed through B, C, D (cost 6), V — Y routed through F, G (cost
9), and W — Z routed through J (cost 5) o But: imagine W — X traffic - takes the path WHFABCDEX (cost 9) instead of the shorter path WHFGEX (cost 12) o we can increase a to 6 o but what about U —> Y traffic?
(will prefer UABCDEGY over shorter UAFGY ©)
MM) 5   Advanced Routiner Mechanisms II
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Traffic Engineering in IP Networks      Discovering Network Utilization
TE - Discovering Network Utilization I.
o a network-wide view of resource utilization is needed a challenging problem
several methods to collect and consolidate network usage information exist:
P> Simple Network Management Protocol (SNMP)
o an application polls each router and converts the returned information
into a view of usage across the network o does not determine, which flows need to be redistributed to ease any congestion (just an absolute measure of the traffic load is obtained) ^> NetFlow
• Cisco's tool collecting the information at key points within the network o includes aggragation points (NetFlow collectors) consolidating the information from a subset of the network
sFlow, ntop, etc.
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Traffic Engineering in IP Networks      Discovering Network Utilization
TE - Discovering Network Utilization II.
o network traffic is nonstationary and (usually) time-dependent o data rate is different depending on the time of the day o == usually, a peak of the traffic data rate (or, say 90% of the peak)
over the 24-hour window is considered as a traffic volume needed for
traffic engineering considerations
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Traffic Engineering in IP Networks      Discovering Network Topology
TE - Discovering Network Topology
o the application making TE decisions must have a clear view of the topology and capabilities of the links within the network
o small, static networks == manual configuration is sufficient o large and dynamic networks == an automatic system has to be used o naturally, extending the IGP routing protocols to distribute additional
information about the links will do the job o both OSPF and IS-IS have been extended to provide (for each link): o traffic engineering metric, maximum bandwidth, maximum reservable bandwidth, unreserved bandwidth, etc.
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
o single-commodity -just a single node pair in the network has positive demand volume
o commodity « demand for a link's capacity o let's assume the following network:
V12
Let's denote:
o c ... a capacity of each link (here the same for all the links) oh .. .the demand volume for node pair 1 : 2
o x12, x132 .. .the amount of the demand volume to be routed over the path 1 - 2 (resp. 1 - 3 - 2)
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
Problem Constraints
Then the following constraints have to be satisfied:
o the demand volume h has to be carried over these two paths:
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
Problem Constraints
v12
Then the following constraints have to be satisfied:
o the demand volume h has to be carried over these two paths:
o        X12 + X132 = h
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
Problem Constraints
v12
Then the following constraints have to be satisfied:
o the demand volume h has to be carried over these two paths:
o  == X12 + X132 = h
o a path may not carry any negative demand:
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
Then the following constraints have to be satisfied:
o the demand volume h has to be carried over these two paths:
o  == X12 + X132 = h
o a path may not carry any negative demand:
° == X12 > 0, X132 > 0
Problem Constraints
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Traffic Engineering in IP Networks
Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
Then the following constraints have to be satisfied:
o the demand volume h has to be carried over these two paths:
o        X12 + X132 = h
o a path may not carry any negative demand:
°       X12 > 0, X132 > 0
o any flow on the path cannot exceed the capacity on any of the links the path uses:
Problem Constraints
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
Problem Constraints
Then the following constraints have to be satisfied:
o the demand volume h has to be carried over these two paths:
o  == X12 + Xi32 = h
o a path may not carry any negative demand:
° == X12 > 0, X132 > 0
o any flow on the path cannot exceed the capacity on any of the links the path uses:
° == x12 < c, x132 < c (same capacity on each link)
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
Let's assume the goal of minimizing the cost of routing flows:
o let's introduce a cost per unit of flow on each path: £12 and £132, both > 0
o « a price payed for data transferred over the path o => TotaLcost = £12X12 + £132X132
o = the objective function (in general denoted by F)
The Goal - Minimize the cost of routing I.
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Traic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
The Goal - Minimize the cost of routing II.
The complete problem could be written as follows:
The above system solves a goal of minimizing the cost (price) of routing for the above topology when a traffic demand h is given.
o it finds proper values of x12 and x132 satisfying the given conditions
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5. Advanced Routing Mechanisms II.
Autumn 2010
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Single-Commodity Network Flow
The Goal - Load Balancing
Another goals could be also considered:
o load balancing - minimization of maximum link utilization
o average delay - minimization of the average packet delay Example: Minimization of maximum link utilization:
o utilization of the link 1 - 2: *f
o utilization of the links 1 - 3 or 3 - 2: ^
• maximum utilization overall links: max {^, ^}
minimize^   F = max{, ^} subject to     x\2 + *132 = h
X\2 < C, X132 < C X\2 > 0, X132 > 0.
The above system solves a goal of balancing the load over paths 1 — 2 and 1 — 3 — 2 when a traffic demand h is given.
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Multicommodity Network Flow
o multicommodity - all the three demand pairs can have positive demand volumes
o  hi2, hi3, /?23
o for each demand pair, the volume of demand can be accommodated using two paths:
Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Multicommodity Network Flow
Problem Constraints
Then the following constraints have to be satisfied:
o the demand volume for each node pair may be carried over two paths:
Eva Hladkaá  (FI MU)
5. Advanced Routing Mechanisms II.
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Multicommodity Network Flow
Problem Constraints
Then the following constraints have to be satisfied:
o the demand volume for each node pair may be carried over two paths:
° X12 + X132 = h12 °        X13 + X123 = h13
Eva Hladkaá  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 2010     20 / SS
Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Multicommodity Network Flow
Problem Constraints
Then the following constraints have to be satisfied:
o the demand volume for each node pair may be carried over two paths:
° X12 + X132 = hi2 ° X13 + X123 = h13 °        X23 + X213 = ^23
• links' capacity limits must also be satisfied:
Eva Hladkaá  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 2010     20 / SS
Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Multicommodity Network Flow
Problem Constraints
Then the following constraints have to be satisfied:
o the demand volume for each node pair may be carried over two paths:
° X12 + X132 = hi2 ° X13 + X123 = hi3 °        X23 + X213 = h23
• links' capacity limits must also be satisfied:
O =>> X12 + X123 + X213 < C12 ° =>> X13 + X132 + X213 < C13 O   =>> X23 + X132 + X123 < C23
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5. Advanced Routing Mechanisms II.
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Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Multicommodity Network Flow
Problem Constraints
Then the following constraints have to be satisfied:
o the demand volume for each node pair may be carried over two paths:
° Xi2 + X132 = hi2 ° Xi3 + X123 = hi3 °        X23 + X213 = h23
• links' capacity limits must also be satisfied:
° =>► Xi2 + X123 + X213 < Ci2 ° ^ Xi3 + X132 + X213 < C13 °   ^ X23 + X132 + X123 < C23
o Total cost:
o        TútaLcOSt = ^12 X12 + £i32 X132 + £13 X13 + £i23 X123 + ^23X23 + 6i3 X213 Eva Hladká (FI MU) 5. Advanced Routing Mechanisms II. Autumn 2010       20 / 55
Traffic Engineering in IP Networks     Links' Weights Computation
Network Flow Modeling - Multicommodity Network Flow
The Goal - Minimize the cost of routing
Then, the goal of minimizing the cost of routing can be formulated as follows:
minimize^   F = %12x12 + £132*132 + £13*13 + £123*123 + £23*23 + £213*213 subject to     x 12 + * 132 = h12
*13+*123 =h[3 *23 +*213=^23 *12+*123+*213 <Cu *13+*132+*213 <Cl3 *23 + *132 +*123 < c23
*12 > 0, Xl32 > 0, X13 > 0, Xi23 > 0, X23 > 0, X2\3 > 0.
(Another goals (load balancing, average delay, etc.) can be formulated as well.)
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Traffic Engineering in IP Networks     Links' Weights Computation
TE - Shortest Path Routing and Network Flow
o in an IP network based on OSPF or IS-IS, the shortest paths are computed based on links' weights
o this computation does NOT consider traffic volume or (usually)
capacity of the network o the previous examples did NOT consider the links' weights
o How is the shortest path routing related to network flow modeling? o link weights drive the flows let s denote w to be an array of link weights of all links in the network o w = (wi, w2, w3, . . . )
o a dependency of a flow xu on the link weights will be denoted as xn(w)
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Traffic Engineering in IP Networks      Links' Weights Computation
TE - Shortest Path Routing and Network Flow
MCSPRF optimization problem I.
The goal: to determine link weights for given traffic volume demand and capacity limits where a certain objective is optimized.
The Multicommodity shortest path-based routing flow (MCSPRF) optimization problem having the objective to minimize the maximum link utilization (load balancing) can be formulated as follows:
minimize^?)   F = r
Pk
subject to J2 xkp(w) = ftk, k=l,2,...,K
P=l K Pk
E £ skpt *kp(w)=yi,   I = 1,2,L
k=\ p=\
ye<cer, t = l,2,...,L w\, n>2,wp 6 W
Xkp(,w)>0, p = l,2,...,Pk,   k = l,2.....K
>V>0, 1 = 1,2,... ,L r > 0.
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5. Advanced Routing Mechanisms II.
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Traffic Engineering in IP Networks      Links' Weights Computation
TE - Shortest Path Routing and Network Flow
MCSPRF optimization problem II. Where
Notation	Explanation
K	Number ot demand pairs with positive demand volume
L	Number of links
ft*	Demand volume of demand index k — 1,2.....K
Q	Capacity of link I = 1, 2,L
Pk	Number of candidate paths for demand k, k — 1,2,K
&kpl	Link-path indicator, set to 1 if path p for demand pair k uses the link I; 0,
	otherwise
^kp	Unit cost of flow on path /) for demand k
h	Unit cost of flow on link I
wi	Link weight for link t = 1,2,..., L
	Flow amount on path /; for demand k for given link weight system w
Xkp	Flow amount on path p for demand k
yi	Link flow variable for link t
r	maximum link utilization variable
*	Use as a superscript with a variable to indicate optimal solution, e.g.,
The weights are determined by solving a dual problem. 9 details: PA163: Constraint programming (dr. Rudova)
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Traffic Engineering in IP Networks      Links' Weights Computation
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5. Advanced Routing Mechanisms II.
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Traffic Engineering in IP Networks      Links' Weights Computation
TE - Shortest Path Routing and Network Flow
MCSPRF - Minimum cost AND load balancing objective
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Multiprotocol Label Switching MPLS
Lecture Overview I
Traffic Engineering in IP Networks
• Introduction
• Discovering Network Utilization
• Discovering Network Topology
• Links' Weights Computation
2 Multiprotocol Label Switching o MPLS
o Generalized MPLS o Grid-enabled GMPLS
3^ QoS-Based Routing
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
Introduction I.
Multiprotocol Label Switching (MPLS)
o a new forwarding mechanism originally presented as a way of
improving the forwarding speed of core IP routers o in MPLS network, packets are forwarded based on labels
o a label is added in front of a packet (i.e., as another header so that routers know how to act based on this label)
□ assigned when packet enters the MPLS-capable network • internal MPLS routers don't inspect packet's IP address
o short and fixed-length label lookup is much faster than longest-prefix match performed on every router o labels usually correspond to IP destination networks
but can also correspond to other parameters, such as QoS or source address
o requires new protocols to distribute label information
or extensions to existing protocols
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
Introduction II.
Multiprotocol Label Switching (MPLS) - cont'd.
o MPLS flows are connection-oriented and packets are routed along pre-configured Label Switched Paths (LSPs) o the MPLS connection (LSP) is unidirectional
o =$> two-way communication requires a pair of LSPs to be established o the paths for forward and reverse directions may differ
MPLS allows new forwarding paradigms not available with conventional IP routing
o e.g., the ability of network operators to dictate the path that traffic takes through their network, Virtual Private Network support, etc.
o for example, low-priority data may be sent on a longer path to keep the shortest path clear for higher-priority traffic
MPLS has emerged into a crucial standard technology for large-scale IP networks
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
Basic functionality
o an analysis of packets entering the network
o and their classification to FEC classes (Forward Equivalence Class)
o the classification may be based on more information than just on the
destination address o for example, type of service, VPN, etc.
• labels' creation for all the FEC classes
o determination/creation of Label Switched Paths (LSPs)
• labels' distribution
setting the forwarding information tables in the routers
the tables are known as Label Information Base (LIB) or Label Forwarding Information Base (LFIB) o the tables map [incomingjnterface, incomingjabel} to {outgoing-interface, outgoingJabel}
o each MPLS core router maintains a valid mapping from the label of an incoming packet ("incoming label") to a label to be attached to the packet before being sent out ("output label") packets' forwarding (based on the label) MPLS header (called shim header) creation
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
MPLS Example
Figure: Label swapping and label switched paths.
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5. Advanced Routing Mechanisms II.
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
MPLS Network Components I.
Edge Label-Switched Routers (Edge-LSRs) = border routers o Ingress-LSR
□ analyses information in IP packet header
o based on analysed information, the packet is assigned to particular FEC o depending on the assigned FEC, a proper label is inserted into MPLS header
o Egress-LSR
removes MPLS header and forwards original IP packet to an egress link • decrements packet's TTL field
Core Label-Switched Routers (Core-LSRs)
• ensures packets' forwarding based on the assigned label o the IP header is neither modified nor analysed by the Core-LSRs o just MPLS labels are analysed and modified, if necessary
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Multiprotocol Label Switching MPLS
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
MPLS Shim Header
o Label - carries the actual value of the Label o Traffic Class field - previously named as Experimental o Stack - set to one for the last entry in the label stack, and zero for all other label stack entries
o receiving router examines the top label only o TTL - used to encode a time to live value
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
MPLS Labels
o usually, just a single MPLS label is assigned to a packet o scenarios, that may produce more than one label: o MPLS VPNs - 2 labels
o the top label points to the egress router and the second label identifies the VPN
□ MPLS Traffic Engineering - 2 labels
□ the top label points to the endpoint of the traffic engineering tunnel and the second label points to the destination
o MPLS TE combined with MPLS VPNs - 3 or more labels o etc.
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
MPLS Label Distribution
o before an LSP can be used, the LFIBs must be populated at each LSR along the path
o == a label distribution protocol has to be used o several protocols could be used:
• BGP (Border Gateway Protocol) - its extension allowing labels' distribution
o RSVP-TE (RSVP-Traffic Engineering) - a modified version of the RSVP protocol
o LDP (Label Distribution Protocol) - a specialized protocol for MPLS networks
TDP (Tag Distribution Protocol) - Cisco's specialized protocol for
MPLS networks
o LDP/CR (Label Distribution Protocol/Constrained Routing) - LDP's
extension for QoS support □ etc.
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Multiprotocol Label Switching
MPLS
Multiprotocol Label Switching (MPLS)
MPLS Label Distribution - Basic approaches I.
Downstream-on-demand, ordered control approach
o MPLS devices do not signal a FEC-to-label binding until requested to
do so by an upstream device o an LSR does not advertise a label for a FEC unless it is the egress
LSR for the FEC or until it has received a label for the FEC from its
downstream peer o the same label has to be used only between adjacent LSRs!
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
MPLS Label Distribution - Basic approaches II.
Downstream-unsolicited, independent control approach
o MPLS devices do not wait for a request from an upstream device before signaling FEC-to-label bindings
o as soon as the LSR learns a route, it sends a binding for that route to all peer LSRs, both upstream and downstream the LSR sending the label acts independently of its downstream peer it does not wait for a label from the downstream LSR before it sends a label to its peers
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
MPLS Label Distribution - LDP protocol
Label Distribution Protocol (LDP)
o a protocol defined by the IETF (RFC 5036) for the purpose of distributing labels in
an MPLS environment o relies on the underlying routing information provided by an IGP in order to forward
label packets
o makes use of the TCP or UDP transport protocols
o can operate in both Downstream-on-demand and Downstream-unsolicited modes o main protocol activities:
• discovery of LDP-capable LSRs that are "adjacent"
• LDP's Discovery message
o establishment of a control conversation between adjacent LSRs, and negotiation of capabilities and options
• LDP's Adjacency message advertisement of labels withdrawal of labels
both performed by LDP's Label Advertisement message q error notifications
• LDP's Notification message
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
Traffic Engineering in MPLS I.
MPLS is able to supply much of the function of the traffic engineered overlay model in an integrated manner:
o MPLS has the ability to establish an LSP that follows a path other than the one
offered as "preferred" by the routing protocol and forwarding algorithm o resources within the network can be dynamically reserved as LSPs are established and can be dynamically updated as the needs of the LSPs change o traffic flows can be guaranteed a level and quality of service • traffic can be groomed onto "parallel" LSPs
multiple LSPs can be established between a pair of source and destination end points
o traffic can be distributed over the LSPs by a defined algorithm o recovery procedures can be defined describing how traffic can be transferred to alternate LSPs in the event of a failure
indicating how and when backup and standby LSPs should be set up and routed
o load-sharing and traffic grooming decisions need to be made just once (at the entry point into the LSP) rather than at each node within the network etc.
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Multiprotocol Label Switching MPLS
Multiprotocol Label Switching (MPLS)
Traffic Engineering in MPLS II.
Figure: Explicit path control in an MPLS network.
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Multiprotocol Label Switching     Generalized MPLS
Generalized MPLS (GMPLS)
Generalized MPLS (GMPLS) o MPLS has been designed to switch packets using a labeling mechanism
o however, there is the need for an MPLS control-type functionality for controls that is beyond just switching packets
o e.g., wavelength switching, time division multiplexing, fiber (port) switching, etc.
o traditionally referred to as circuit switching or circuit routing (a dedicated path and physical resources must be allocated for a service from one end to another)
o GMPLS thus intended for the following switching capabilities:
o Packet-Switch Capable - (i.e., GMPLS encompasses MPLS) o Time-Division Multiplexing Capable - for timeslot-based circuit switching
Lambda-Switch Capable - for wavelength switching at optical cross-connects
o Fiber-Switch Capable - for fiber-level switching at optical cross-connects
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Multiprotocol Label Switching     Grid-enabled GMPLS
Grid-enabled GMPLS (G2MPLS)
Grid-enabled GMPLS (G2MPLS)
o a network control plane solution that enhances the GMPLS and provides a single-step resource reservation, co-allocation, and maintenance of both network and Grid resources □ designed by 1ST Phosphorus project
o seamlessly serves Grid jobs by co-allocating and provisioning network and Grid resources in a single-step
o not widely used (yet)
Eva Hladka  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 2010        43 / 55
QoS-Based Routing
Lecture Overview I
Traffic Engineering in IP Networks
• Introduction
• Discovering Network Utilization
• Discovering Network Topology
• Links' Weights Computation
2^ Multiprotocol Label Switching a MPLS
• Generalized MPLS
<» Grid-enabled GMPLS
3^ QoS-Based Routing
Eva Hladka  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 2010
44 / 55
QoS-Based Routing
QoS-Based Routing - Introduction I.
QoS-Based Routing is defined as:
o a routing mechanism under which paths for flows are determined based on some knowledge of resource availability in the network as well as the QoS requirement of the flows, or a dynamic routing protocol that has expanded its path-selection criteria to include QoS parameters such as available bandwidth, link and end-to-end path utilization, node resources consumption, delay and latency, and induced jitter
Autumn 2010       45 / 55
QoS-Based Routing
QoS-Based Routing - Introduction II.
Objectives of QoS-based Routing: o to meet the QoS requirements of end users
o QoS-based routing is supposed to dynamically find a path from source to destination which can satisfy user's requirements on bandwidth, end-to-end delay, etc.
o to optimize the network resource usage
o QoS-based routing is expected to direct network traffic in an efficient way that can maximize the total network throughput
to gracefully degrade network performance when things like
congestion happen
when network is in heavy load, QoS-based routing is expected to give better performance (e.g., better throughput) than the best-effort routing, which can degrade the performance dramatically
Eva Hladká  (Fl MU)
5. Advanced Routing Mechanisms II.
Autumn 2010       46 / 55
QoS-Based Routing
QoS-Based Routing - Issues I.
Metric and path computation
o How to measure and collect network state information?
o How to compute routes based on the information collected?
o a suitable metric has to be chosen (e.g., available bandwidth, delay, jitter, etc.)
o path computation also closely related to resource reservation
once a feasible path is chosen, the corresponding resources (bandwidth, buffer space in routers etc.) must be reserved for the traffic flow thus are not available to other flows
Knowledge propagation and maintenance
o How often is the routing information exchanged between the routers?
q more information has to be exchanged than in the case of best-effort routing o QoS information (available BW) has to be exchanged along with common routing information like connection topology changes
o the metrics used by QoS-based routing could be changing very quickly
if the routing information is exchanged every time the values of metrics change, it will cause a great burden for the network links and routers == a common way is to set a threshold to distinguish significant changes from minor changes (routing information accuracy becomes lower, however)
Eva Hladka  (FIMU) 5. Advanced Routing Mechanisms II. Autumn 2010        47 / 55
QoS-Based Routing
QoS-Based Routing - Issues II.
Scaling by hierarchical aggregation
o QoS-based routing is expected to be scalable
o in order to keep the complexity of path computation and the amount of information need to be exchanged and maintained under control, a hierarchical aggregation is used
o however, such aggregation brings inaccuracy in regard of routing information
Administrative Control
o different flows in the network should have different priorities
o in the framework having multiple service classes (e.g., DiffServe), the resources should be allocated fairly among all the classes to avoid starvation of lower priority classes
Integration of QoS-based routing and Best-effort routing
o for compatibility, QoS-based routing must be able to support best-effort routing i.e., both routing schemes must be able to coexist
Eva Hladka  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 2010       48 / 55
QoS-Based Routing
QoS-Based Routing - Routing Algorithms
Basic types
o QoS-based routing algorithms classified according to the way how the state information is maintained and how the search of feasible paths is carried out
o source-based routing algorithms
o hop-by-hop routing algorithms (also called distributed routing algorithms)
hierarchical routing algorithms
Eva Hladká  (Fl MU)
5. Advanced Routing Mechanisms II.
Autumn 2010       49 / 55
QoS-Based Routing
QoS-Based Routing - Routing Algorithms
Basic types - Source-based routing
Source-based routing algorithms
o every router has global state information about the network, and the path is locally selected based on the state information
o once the path is determined, the source router notifies the other
router along that path how to forward the traffic flow o features:
• simpler in the sense that it's decided solely by the source o drawbacks:
o requires that each router has complete state information of the network (hard to maintain)
the computation overhead at the source routers is very high o == scalability problems (not suitable for large networks)
Eva Hladká  (Fl MU)
5. Advanced Routing Mechanisms II.
Autumn 2010        50 / 55
QoS-Based Routing
QoS-Based Routing - Routing Algorithms
Basic types - Hop-by-hop routing
Hop-by-hop routing algorithms
o each router just knows the next hop towards the destination
o features:
• used by most current "best-effort" routing protocols == it's more natural to design and more compatible with existing routing protocols
o the routing computation burden is distributed among all the routers along the path
o drawbacks:
o it has the routing loop problem (when the routing state information in
different routers is not consistent) o besides, it also has the scalability problem
Eva Hladka  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 20l0     5l / 55
QoS-Based Routing
QoS-Based Routing - Routing Algorithms
Basic types - Hierarchical routing I.
Hierarchical routing algorithms
o the routing structure consists of multiple levels o the bottom level contains the actual routers
o these routers are organized into some logical groups, which in turn form the next level
the groups can be further organized into some higher level groups
the routing information is integrated at the border nodes of each groups
o every node contains the detailed information about its group and integrated information about other groups
features:
• scalability == it's suitable for large networks o drawbacks:
aggregation decreases the accuracy of the routing state information
Eva Hladka  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 2010     52 / 55
QoS-Based Routing
Figure: Hierarchical routing structure.
Eva Hladka  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 2010        53 / 55
QoS-Based Routing
QoS-Based Routing - Routing Protocols
PNNI (Private Network-Network Interface)
Private Network-Network Interface (PNNI)
o a hierarchical, dynamic routing protocol for ATM networks based on link-state algorithm
topology information (including information about nodes, links, addresses) is flooded through the network o network resources are defined by metrics and attributes (delay, available bandwidth, jitter, etc.)
o grouped by supported traffic class threshold algorithms are used to determine if the change in a metric or attribute is significant enough to require propagation of updated information o hierarchical ==
PNNI has the concepts of levels and logical nodes supports aggregation of topology and reachability information o drawbacks:
• doesn't support multicast and policy routing, and control of alternate routing inherits the common problem with link state QoS-based routing
o an issue with efficient broadcast of state information (especially for dynamic metrics)
Eva Hladka  (FIMU) 5. Advanced Routing Mechanisms II. Autumn 2010        54 / 55
QoS-Based Routing
QoS-Based Routing - Routing Protocols
QOSPF (QoS routing extensions to OSPF)
QoS routing extensions to OSPF (QOSPF)
QoS extension to OSPF
o also based on link-state algorithm, and is hierarchical protocol
o supposed to be working in an environment in which both QoS-based routing and best-effort routing are needed
o for simplicity, link bandwidth and propagation delay are the only metrics extension added to Link State Advertisements (LSAs)
o in order to decrease protocol overhead, LSAs are triggered only when there is a significant change in the value of the metrics since the last advertisement
o a concept of QoS paths pre-computation is used:
• for every possible destination, the algorithm pre-computes a "widest-shortest path" (a minimum hop count path with maximum bandwidth)
Eva Hladka  (FI MU)
5. Advanced Routing Mechanisms II.
Autumn 2010       55 / 55