Quality of Service CS215 Winter, 2001 Ning. Wang

Need for QoS Multimedia communication –Audio and video conferencing –IP telephony –Audio/video streaming –Distance learning QoS requirements –Fixed delay bound –Bandwidth –Packet loss

Challenges TCP/UDP/IP suite –Best-effort –No delay and bandwidth guarantees Primitive QoS support in Internet –Most routers only implement FCFS for packet transmission scheduling

Outline Architecture Components –Signaling –Scheduling –Routing –Traffic Engineering

Integrated Service: Classes Guaranteed services –Assured level of bandwidth –A firm end-to-end delay bound –No queuing loss for confirming packets Controlled-load service –No firm guarantees –Best-effort service in a lightly loaded network

Integrated Service per-flow basis according to requests from the end applications. It relies on resource reservation, routers need to maintain state info, maintaining records of allocated resources and responding to new Call setup requests on that basis

Differentiated Service Philosophy: keep the forwarding path simple, push complexity to the edges of network Provide simple way to categorize and prioritize network traffic (flow) aggregate Service classes are not defined, instead it provides functional components with which service classes can be built (Service Level Agreement) Edge functions vs. Core functions –Classification, marking, policy, shaping –Forwarding Bandwidth Brokers (BB) –Parcel out their region’s Marked traffic allocation –Manage the messages that are sent across boundaries to adjacent regions’ BBs.

At DS-capable host or first DS-capable router Classification: edge node marks packets according to classification rules to be specified Traffic Conditioning: edge node may delay and then forward or may discard Edge Routers

Core Functions Forwarding –according to “Per-Hop-Behavior” or PHB specified for the particular packet class; such PHB is strictly based on class marking (no other header fields can be used to influence PHB) PHB –Expedited Forwarding (EF): single codepoint –Assured Forwarding (AF): twelve codepoints No state info to be maintained by routers!

Classification and Conditioning Packet is marked in the Type of Service (TOS) in IPv4, and Traffic Class in IPv6 6 bits used for Differentiated Service Code Point (DSCP) and determine PHB that the packet will receive, 2 bits are currently unused Packets are metered and shaped if non-confirming

IntServ vs. DiffServ Scalability –Per-flow vs. per aggregate Service Model Signaling

Responsible for requesting allocation and release of network resources along the data distribution path to ensure QoS requirements are met –RSVP –ST-II Comparison

RSVP Path message (flowspec, packet filter) Reservation message (flowspec, reservation style, a packet filter) Filter and different reservation styles Merge path and reservation messages as they traverse the network (reduce overhead)

ST-II An Internet Protocol at the same layer as IP –Incorporates the concept of streams across an Internet to pre-allocate resource. A stream is initiated when a source ST agent generates a connect message containing flow spec Flow spec in the Connect packet is updated if actual source allocation is less than the amount requested Upon receiving a Connect indication, receiver returns either an Accept or Refuse message to the stream source. Disconnect messages are used to tear down the stream when it is done

RSVP vs. ST-II simplex distribution tree rooted at the source and extending to all receivers. Receiver initiated –Heterogeneous receiver Soft-state –Dynamic –Timer, Refresh –Overhead Reservation styles Sender initiated Hard-state –Tear down connections Lack of reservation styles

Scheduling Performance bounds –Deterministic and statistic bounds for bandwidth, delay, delay jitter and loss Scheduling methods –Generalized Processor Sharing (GPS) and Weighted Fair Queuing (WFQ) –Earliest Deadline First (EDF) –Core Stateless Fair Queuing (CSFQ)

GPS and WFQ GPS is an idealized fluid discipline –provision of minimum service guarantees –Fair sharing of resources WFQ –Compute the packet finishing time –Rank packets according to finishing times –The resulting sequence number is the packet’s turn to be transmitted.

Earliest Deadline First Assign each packet a deadline The scheduler serves packets in order of their deadlines –Arrive closer to deadline-low delay Along with per-hop traffic shaping permits the provision of end-to-end delay guarantee During call setup, each source negotiate a service contract with the scheduler to ensure a delay bound if the source obeys a peak rate descriptor

CSFQ Per flow accounting and rate labeling at edge routers Packets carry rate labels Stateless FQ at core routers: no per flow state kept; packet drop probability computed from packet label Iterative computation is used to determine the fair share Packet drop probability = max(0,1– a(t)/r(i,t))

Edge and Core Routers

Routing QoS Routing –Unicast: find a network path that meets the requirement of a connection between two end users –Multicast: find a multicast tree, rooted at a sender, which covers all receivers with every internal path from the sender to receiver satisfying the requirement Metrics –Link propagation delay, available bandwidth, hop count Routing strategies –Source routing (centralized routing) –Distributed routing –Hierarchical routing

QOSPF Link state routing Routing based on more than one constraint is an intractable problem. However, due to the nature of constraints, bandwidth, hop count and delay are related Pre-computed vs. on-demand routing Both Bellman-Ford (BF) or DijKstra algorithms can be used to compute path of maximum available bandwidth for minimum hop counts. Delay can be handled by D(p) = α(h(p))/b + Σd l α(h) = σ + h*c D(p) – end2end delayd l - propagation delay h(p)- # of hopsσ - burst size, c – max pack size

Traffic Engineering Arrange how traffic flows so that congestion caused by uneven network utilization can be avoided MPLS –Forwarding protocol between link layer and network layer –Its technique can be used with any network protocol –Header contains 20-bit label 3-bit Class of Service (COS) field 8-bit Time to Live (TTL) field (protection from forwarding loops) –Can be used for traffic engineering

MPLS Packets are classified and routed at the ingress of Label Switching Router (LSR) When an LSR receives a labeled packet, it will use the label as index to look up the forwarding table. –Old label is replaced by new label –Packet is processed, no header analysis is needed by subsequent routers Label Distribution Protocol (LDP) Label Switching Path (LSP) for each Forwarding Equivalent Class (FEC) Tunneling effect –Can control the complete path of a packet without explicitly specify the intermediate routers

MPLS and DiffServ MPLS can be used together with differentiated services to provide QoS At the ingress of the ISP network, MPLS header is inserted At the core, routers process the packet based on its label and COS field rather than it DS field At the egress, unless inter-domain LSPs are configured, the MPLS header is removed BBs may not be needed in MPLS-based ISP networks, because of tunneling

Summary Architecture –IntServ –DiffServ Like pieces of a puzzle, all different components (signaling, scheduling, routing and traffic engineering) must fit together to construct a QoS enabled network