INTERNER QOS
Vadim Koshkarev
22.04.2016
Introduction
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Integrated Services
Resource Reservation Protocol
Differentiated services
Multiprotocol label switching
Constraint based routing
• Today’s internet – best effort, no guarantee of
delivery or latency
• There is a demand for guaranteed high quality
service
• Bandwidth increase wont solve the problem
• Integrated services is about resource
reservation, and RSVP is a signaling protocol
which sets up paths and reserves resources
• Differentiated services is about setting flags in
network packets to create several quality
classes.
Integrated services and RSVP
In addition to best effort service, two classes are
proposed:
• Guaranteed service for application requiring
fixed delay bound
• Controlled-load service for applications
requiring reliable and enhanced best effort
service
• Four components of integrated services:
• Signaling protocol(e.g. RSVP)
• Admission control routine – decides whether
the request for resources can be granted.
• Classifier – router performs multifield
classification and puts packets on
appropriate queue based on classification
result.
• Packet scheduler – schedules packets
accordingly to meet their QoS requirements.
• Problems of integrated services are:
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Bad scalability(storage, processing
overhead)
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All routers must support the technology
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Incremental deployment is possible for
controlled-load services by tunneling RSVP
messages.
Differentiated Services
• Integrated services are difficult to implement
and to deploy.
• IPv4 header contains a Type of Service byte.
Applications can set three bits in TOS byte
indicating need for low-delay, highthroughput or low-loss-rate.
• Differentiated services define the layout of
the TOS byte and a base set of packet
forwarding treatments (per hop behaviors).
ISP may completely ignore this.
• Marking different bits in DS field can create
several DS classes.
• Service level agreement with internet service
provider is required to receive DS. SLA
specifies amount of traffic and service
classes supported.
• Static SLA
• Dynamic SLA
• Customers can mark DS fields of individual
packets, or leave it to leaf routers to mark
packets based on MF classification.
At the ingress of the ISP networks, packets are
classified, policed, and possibly shaped, all
based on SLA. When a packet enters one
domain from another domain, its DS field may
be remarked based on SLA between two
domains.
Example of services that can be provided:
• Premium service for applications requiring
low-delay and low-jitter service
• Assured service for applications requiring
better reliability than best-effort service
• Olympic service, which provides three tiers
of services, gold, silver and bronze.
• Differences between differentiated services
and integrated services:
• More scalable
• Easier to deploy
• Incremental deployment is possible
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Assured service
‘In’ and ‘Out’ packets
Random Early detection
RIO
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Premium service
Provides low-delay, low-jitter.
Excess traffic will be dropped.
ISP guarantees contracted bandwidth.
Both dynamic and static SLA
Customers have to shape their traffic
Premium traffic is limited so that It doesn’t
starve normal traffic
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Service allocation in customer domains
Host can decide what services to use
Bandwidth broker is preferred
BB sets classification, marking and shaping
rules based on RSVP or LDAP
• BB requests resources from ISP if dynamic
SLA is used
• Resource allocation in ISP domains
• Static SLA – routers are manually configured
• Dynamic SLA – boundary routers or ISP’s BB
make admission control decisions.
• ISP core routers are not affected, to provide
scalability.
Examples of End-to-End Service Delivery
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Phase 1 Signaling
Sender requests resources
BB can reject path message
BB can aggregate multiple requests
Each domain behaves like a single node,
represented by BB.
• Soft state
• Phase 2 Data transmission
• Routers should support:
– BA classification
– two queues with strict priority.
– Leaf routers in customer domain need MF
classification, marking, shaping.
– ISP ingress routers need policing and remarking.
An architectural comparison of ST-II
and RSVP
• QoS and multipoint-to-multipoint
• Datagram networks are only best effort
• Circuit switched networks and ISDN provide
service guarantees, but is inefficient when
bursting data, cannot adapt to router failure,
no multipoint communication.
• Integrated Service Packet Network
– QoS + Multihome communication
• Required:
• a flow specification defining the source traffic stream
and receiver service requirements
• a routing protocol supporting QoS and multicast
paths
• a reservation protocol to create and maintain
resource reservations
• an admission control algorithm to maintain network
load at a proper level
• a packet service algorithm to schedule packet
transmissions in an order that maintains service
guarantees for individual data streams.
• ST-II stream protocol supports multicast in addition
to resource reservation.
• Protocol Overview
• Point to point is inefficient, multicast is
needed for both ST-II and RSVP.
• ST-II builds a multicast distribution tree based
upon unicast routing tables, and performs
the replication and forwarding of data
packets.
• RSVP is decoupled from multicast routing,
assuming it is provided by underlying
network.
• ST-II protocol
• Model is data stream rooted at the source and
extending to all receivers via a multicast
distribution tree.
• Source ST agent generates a Connect message
with flow specification and initial set of
participants.
• Each intermediate ST agent determines set of
next hop subnets, installs multicast forwarding
state and reserves resources.
• Upon receiving Connect message, receivers must
respond with Accept or Refuse messages.
Receivers may reduce resource request by
updating flow specification.
• ST source must wait for response from each
participant before transmitting.
• When someone asks for a lower QoS, ST source
will either reduce QoS for everyone, or it will
send a Disconnect message to this participant.
• Group members can be added or removed after
initial setup. Addition of users triggers sending a
Connect message, which is performed out-ofband using IP.
• Robustness and reliability is achieved using
retransmissions and acknowledgements, a Hello
protocol is used to query for reachability
changes between ST agents, and automatic
stream recovery is initiated when needed.
• RSVP protocol
• Similar to ST-II in a way that it is a simplex distribution
tree rooted at the source and extending to all
receivers, however mechanisms to establish resource
reservations are different from ST-II.
• RSVP sends a Path message, as was described earlier.
• Before establishing a reservation each receiver must
first join the associated multicast group to begin
receiving Path messages. Multicast is outside of the
RSVP scope.
• Each receiver must determine its own QoS
requirements and initiate a Reservation message,
which propagates towards group sender. Reservation
message ends as soon as it splices into an existing
distribution tree with sufficient QoS requirements.
This reservation style enables RSVP to support
heterogeneous requirements.
• RSVP reservation model consists of resource
allocation (determines what amount of resources
needed) and a packet filter (determines which packets
can use the resources).
• Several different reservation styles can be achieved
when changing packet filter, while resources
allocation stays the same.
• Wildcard – source specific reservation is not required,
any packets destined for multicast group can use
resources.
• Fixed Filter – cannot be changed during its lifetime,
without re-invoking setup and admission control. This
allows the reservation to be shared among multiple
requests for the same resource.
• Dynamic Filter – allows receiver to modify its packet
filter over time
• Static analysis
• Audio conference. Only one source at a time
needed to reserve resources for a few
simultaneous audio channels. RSVP can use
Wildcard for this. ST-II requires an
Independent Stream reservation for each
audio source.
• As we can see ST-II has a scalability issue,
while RSVP does not.
• ST-II resource requirements are unbounded,
while RSVP resource requirements are
bounded, meaning no additional resources
has to be stored in “core” RSVP nodes.
Support of heterogeneous groups
• Supporting channel selection
• Channel selection is, for example, when a
user cannot accommodate all audio sources
at the same time, but would like to
dynamically select a subset of them.
• Assured channel selection
• Non-assured selection
• Traditional way is - Independent Stream
reservation
• RSVP - Dynamic Filtering reservation
• Chosen Source reservation style – nonassured
Chosen Source and Independent Streams can be provided by
ST-II and RSVP, while Dynamic Filter is only by RSVP.
• Dynamic Analysis
– Network dynamics
• ST-II incorporates a failure detection
mechanism using Hello, Status and Notify
messages, all of which add considerable
complexity to the protocol.
• RSVP relies on soft state refreshes to
automatically adapt without additional
protocol complexity.
• Main difference is that ST-II requires that the
network be responsible for correctness, while
RSVP leaves responsibility to the end users,
which is much less complex protocol wise.
• RSVP merges Path and Reservation refreshes
to avoid overhead
• ST-II Hello messages are independent of
number of users
• Wildcard needs only one reservation on each
link
• Fixed Filter needs one reservation for each
source forwarding along a link
• Dynamic Filter requires a separate
reservation per receiver, thus, RSVP protocol
overhead scales with the number of
reservations.
– Group membership dynamics
• Dynamic addition of receivers in ST-II
requires of generation of Connect and Accept
messages between source and receiver. This
can result in processing bottleneck at the
source.
• RSVP requires a single Path message from the
source and a single reservation request sent
by each receiver.
• RSVP heterogeneous groups may ask for a
more demanding reservation.
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Summary
RSVP is better, uses less resources.
RSVP topics that require research:
Channel selection, tradeoff between assured and nonassured channel selection, overhead for dynamic
filter.
Possibility of further aggregating refresh messages.
Fault detection and refresh trigger mechanism could
be incorporated into the protocol, its effect on
complexity and latency.
Possibility to dynamically adapt timers to measure
network performance to reduce protocol overhead.
Additional reservation styles for ISPN application.
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MPLS
Fixed-length label
Forwarding scheme, traffic engineering
Header consists of a 20 bit label, 3 bit Class of
Service field, 1 bit stack indicator(last label in
the stack), and 8 bit time to life field.
• Label-switched routers examine only headers
• Label switched paths(LSPs)
• Label distribution protocol (LDP) or RSVP
could be potentially used in this goal.
• LSPs are unidirectional
• LSP setup can be
– control-driven by routing updates
– data-driven by request of a flow
– traffic trunk LSP can be hop-by-hop or the sender LSR
can specify an explicit route(ER) for the LSP.
• MPLS is faster than longest match in IP routing.
• Incoming label is replaced by an outgoing label
and the packet is switched to the next LSR.
• QoS is based on COS fields.
• MPLS provides
– faster packet classification and
– Forwarding
– efficient tunneling mechanism
• A service architecture based on MPLS
• MPLS can be used together with differentiated
services
• LSPs are configured between each ingressegress pair.
• Different LSPs for each traffic class can be
merged into a sink tree.
• Different traffic classes can use the same sink
tree, COS bits are used to differentiate traffic.
• In ISP networks admission control can be
handled by ingress and egress routers, therefore
BBs are not always needed.
• Traffic engineering and constraint-based
routing
• When traffic load is high, we may need QoS.
When traffic is low, QoS doesn’t really do much.
Traffic engineering tries to reduce traffic load.
• Network congestion caused by
– lack of network resources
– uneven distribution of traffic
• Constraint-based routing makes traffic
engineering automatic, it evolves from QoS
routing.
• Goals are:
– select routes that meet certain QoS requirements
– to increase utilization of the network
• Routers need topology and bandwidth availability information in
order to compute QoS routes.
– Can be done by extending the link state advertisements of protocols
such as OSPF and IS-IS
– Tradeoff between accurate information and frequent flooding
• Common route metrics are:
– monetary cost
– hop count
– bandwidth
– reliability
– delay
– jitter
Computing optimal routes subject to two or more of these constrains is
NP-complete, but there are ways to make it in polynomial time.
• Bandwidth and hop count are considered more useful
constraints than delay and jitter
• Real time applications are bandwidth sensitive, hop
count helps to reduce network resource demand.
• Routes can be computed on demand or precomputed
for each traffic class.
• Routing tables are computed more frequently than in
normal dynamic routing
• Computation load can become very high, can be
reduced by
– a larger timer value
– choosing only hop count and bandwidth as constrains
– pruning unsuitable links before computation.
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Pros and Cons
+meeting needs of QoS requirements better
+better utilization of a network
-increased communication and computation
overhead
• -longer paths may consume more resources
• -potential routing instability
• Size of the routing table can get quite high, this
can be reduced with
– coarse routing granularity(based solely on
destination address)
– hop quantization(merging hop-count values)
– computing routes for QoS requests on demand.
• Widest-shortest path – path with minimum
hop count, then with largest bandwidth (as
current dynamic routing)
• Shortest-widest path – path with largest
bandwidth, then with minimum hop count
• Shortest distance path - a tradeoff between
the two above, shortest path when network
load is heavy, and widest path when network
load is medium. This gives better
performance.
• Computations are frequent and overhead is
high, this can cause instability in the network.
Timer should be carefully chosen.
• It is possible that constraint based routing
will replace dynamic routing at least in
intradomain networks, an emerging routing
protocol is QOSPF.
• Constraint based routing doesn’t replace, but
help other mechanisms of QoS.
• RSVP can benefit from it by getting a better
route
• MPLS and constraint based routing works
well together