Transcript Internet Quality of Service – the big picture

Internet Quality of Service – the
big picture
• What is quality of service?
• What does it take for the Internet to support
QoS?
• Existing Internet QoS architectures:
– Integrated services, differentiated services, and
MPLS overview.
What is QoS
• User point of view:
– Assurance of end-to-end service.
• E.g. Guaranteed delay (VoIP), Guaranteed bandwidth (VPN)
• Relaxed definition:
– Service differentiation: different packets is treated
differently.
– end-to-end service guarantees may be achieved by
provisioning.
• e.g. only a small portion of high priority packets.
• Existing IP networks only support best effort service.
Adding service differentiation is non-trivial.
• Applications that need QoS
– VoIP: bounded delay
– VPN: bounded bandwidth
– Video conferencing: bounded delay and bounded loss
rate
• Common QoS parameters:
– delay/delay varation (jitter)
– Bandwidth
– error rate
• Per flow QoS guarantees and aggregate QoS
guarantees
• Statistical QoS guarantees .vs. deterministic QoS
guarantees
• What is needed to support QoS:
– Between the network and its clients: Traffic contract.
Traffic specification/desired QoS/supported QoS
– At network edge:
• Signaling and admission control: if everything is allowed,
QoS cannot be guaranteed. Signaling is usually needed to setup
the network so that QoS can be guaranteed.
• Packet classification/marking: What kind of service do you
want to apply to the packet?
• Traffic shaping: traffic with a certain shape is easier to
guarantee service
• Packet classification/marking and traffic shaping is also called
traffic conditioning.
• Traffic policing: packets out of the service agreement should
be stopped at the edge.
• What is needed to support QoS:
– At routers:
• classification and scheduling: FCFS won't work, need more
advanced packet scheduling scheme (Fair Queuing)
• Routing algorithm need to improve: find a path that satisfies
QoS constraints (QoS/policy/constraint based routing).
• Buffer management.
• Traffic monitoring: find problems as early as possible
• Traffic reshaping (at merge and fork points)
• QoS in the Internet: Do we really need it?
– Alternative: buy excessive bandwidth
– Everything is simple in the Internet without QoS,
everything seems to be much harder in the Internet with
QoS support.
– What is the main problem?
• Complexity and scalability of QoS mechanisms
• Which is cheaper: higher network speed or network with QoS
support.
– Where is the balance?
• A guess: Some form of QoS support will be there, per flow
QoS guarantee may or may not ever be deployed.
• Intergrated Services (IntServ):
– trying to match the user demand by providing per flow
QoS guarantees.
– Two service models:
• controlled-load service: performance is as good as unloaded
network
• guaranteed service: firm bound of throughput and delay
– Signaling protocol: RSVP
• IntServ is a reservation based approach
– Main problem:
• router complexity (scalability)
• Differentiated Services (Diffserv)
– Matching to the user demand has been proven to be too much.
– Define per-hop behavior instead of end-to-end service model
• no signaling is needed.
– Support a small number of forwarding classes at each router.
• Service models to be accomplished through provisioning
– Edge routers map packets into forwarding classes based on service
level agreement (SLA).
– Forwarding class is encoded in the packet header.
– Six bits in the TOS file in the IP packet is used in DiffServ:
• Examples of forwarding classes:
– 101 110 - expedited forwarding
– 010 010 - assured forwarding
– Problems with DiffServ:
• end-to-end service guaranteed is hard to maintain.
• Multi-Protocol Label Switching (MPLS):
– originally designed for IP over ATM
– A short (fixed length) label is encoded for the packet
header for packet forwarding
– Allow Label switched path (LSP) to be setup (explicit
routing).
– allow datagram and virtual circuit to be coexisted in an
IP network.
– MPLS can be combined with IntServ and DiffServ to
support QoS.