IETF Differentiated Services

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IETF Differentiated Services

• Concerns with Intserv
• Scalability signaling, maintaining per-flow
  router state difficult with large number of
  flows
• Flexible Service Models Intserv has only two
  classes. Also want qualitative service classes
• behaves like a wire
• relative service distinction Platinum, Gold,
  Silver
• Diffserv approach
• simple functions in network core, relatively
  complex functions at edge routers (or hosts)
• Dot define define service classes, provide
  functional components to build service classes

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Diffserv Architecture
Edge router - per-flow traffic management -
marks packets as in-profile and out-profile
Core router - per class traffic management -
buffering and scheduling based on marking at
edge - preference given to in-profile packets -
Assured Forwarding
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Edge-router Packet Marking

• profile pre-negotiated rate A, bucket size B
• packet marking at edge based on per-flow profile

User packets
Possible usage of marking

• class-based marking packets of different classes
  marked differently
• intra-class marking conforming portion of flow
  marked differently than non-conforming one

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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

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Classification and Conditioning

• may be desirable to limit traffic injection rate
  of some class
• user declares traffic profile (eg, rate, burst
  size)
• traffic metered, shaped if non-conforming

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Forwarding (PHB)

• PHB result in a different observable (measurable)
  forwarding performance behavior
• PHB does not specify what mechanisms to use to
  ensure required PHB performance behavior
• Examples
• Class A gets x of outgoing link bandwidth over
  time intervals of a specified length
• Class A packets leave first before packets from
  class B

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Forwarding (PHB)

• PHBs being developed
• Expedited Forwarding pkt departure rate of a
  class equals or exceeds specified rate
• logical link with a minimum guaranteed rate
• Assured Forwarding 4 classes of traffic
• each guaranteed minimum amount of bandwidth
• each with three drop preference partitions

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Diffserv and MPLS

• Both are WAN QoS mechanisms. While Diffserv is
  used for traffic aggregation and provisioning of
  differentiated services, MPLS is mainly used for
  traffic aggregation and load balancing.

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MPLS

• Originally introduced as a WAN mechanism for
  forwarding packets using label switching instead
  of the IP address-based routing and provide
  differentiated QoS.
• It has found its most use in Traffic Engineering
  (TE)
• TE requires that traffic follows specific,
  possibly nonoptimal, routes to enable diverse
  routing, traffic load balancing, and other means
  of optimizing network resources.
• MPLS forces traffic into these routes or Label
  Switched Paths (LSPs).

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Routers or LSRs

• In the MPLS network, routers are called label
  switching routers (LSR).
• Edge LSRs (also called LERs) provide the
  interface between the external IP network and the
  LSP.
• Core LSRs provide transit services through the
  MPLS cloud using the pre-established LSP.
• In a SP network, on the ingress the Edge LSR
  accepts IP packets and appends MPLS labels.
• On the egress, an edge LSR terminates the LSP by
  removing MPLS labels and resorting to the normal
  IP forwarding.

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FEC

• The forward equivalence class (FEC) is a
  representation of a group of packets that share
  the same requirements for their transport. All
  packets in such a group are provided the same
  treatment en route to the destination.
• Each LSR builds a table to specify how a packet
  must be forwarded. The table, label information
  base (LIB) comprises of FEC-to-label bindings.

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Labels and Label Bindings

• A label identifies the path a packet should
  traverse
• It is encapsulated in a layer-2 header of the
  packet -- special MPLS header (aka shim) includes
  a label, an experimental field (Exp), an
  indicator of additional labels(S), and Time to
  live (TTL).
• Receiving router uses the label content to
  determine the next hop.
• Label values are of local significance only
  pertaining to hops between LSRs.
• Labels are bound to an FEC asa result of some
  event or policy

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Label Assignment

• Based on forwarding criteria such as
• destination unicast routing
• traffic engineering
• multicast
• virtual private network
• QoS

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MPLS Signaling

• A signaling protocol performs a variety of
  functions such as
• setting up LSPs traversing specified sequences of
  LSRs derived from the constraint-based routing
  (CR) analysis
• create the path state in each LSR by performing
  label allocation, distribution, and binding
• reserve resources in each LSR including
  bandwidth, delay, and packet loss bounds
• eassign the network resources as necessary
• dynamically reroute during network congestion and
  failures
• monitor and maintain explicitly routed LSP state

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CR-LDP

• CR-LDP LDP using constraint-based routing
• LDP provides a common understanding between LSR
  peers of the meaning of labels used to forward
  traffic between them
• Message categories
• Discovery -- sent periodically by LSRs to
  announce their presence
• Session -- to establish, maintain, and terminate
  a session between two LDP peers
• Advertisement -- to create, change, and delete
  label mappings to FECs after a session has been
  established
• Notification -- to signal and provide advisory
  info.
• Forward path, hard state with no state refreshes

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RSVP-TE

• Signals between LSRs
• Creates a state for a collection of flows between
  the ingress and egress points of a traffic trunk
• An LSP aggregates multiple host-to-host flows and
  thus reduces the amount of RSVP states in the
  network
• Uses firm state where Path and Resv messages are
  periodically refreshed but their volume is
  significantly reduced

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QoS Routing

• As defined in RFC 2386, QoS is a set of service
  requirements to be met by the network while
  transporting a flow. A flow is a packet stream
  from source to a destination with an associated
  QoS.
• Measurable level of service delivered to network
  users which can be characterized by packet loss
  probability, available bandwidth, end-to-end
  delay, etc. Expressed as a Service Level
  Agreement(SLA) between network users and service
  providers.
• QoS-based routing is defined as 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. A dynamic routing
  scheme with QoS considerations.

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QoS Metrics

• Bandwidth, delay, jitter, cost, loss probability
• three types of metrics additive, multiplicative,
  concave
• Let m(n1,n2) be a metric for link(n1, n2). For
  any path P (n1, n2, .., ni, nj), metrci m is
• additive, if m(P) m(n1,n2) m(n2,n3) ..
  m(ni,nj) (examples are dealy, jitter, cost,
  hop-count)
• multiplicative, if m(P) m(n1,n2) m(n2,n3)
  m(ni,nj) (example is reliability, in which case
  0ltm(ni,nj)lt1)
• concave, if m(P) minm(n1,n2), m(n2,n3), ,
  m(ni,nj) (example is bandwidth meaning that the
  bandwidth of the path as a whole is determined by
  the link with the minimum available bandwidth)

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Objectives

• To meet QoS requirements of end users.
• To optimize network resource usage
• to gracefully degrade network performance under
  heavy load

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Design Issues(1)

• IP routing protocols such as OSPF, RIP, and BGP
  are called best-effort routing protocols. They
  use only the shortest path to the destination --
  single objective optimization algorithms which
  consider only one metric (like hop-count).
• Much more difficult to design and implement than
  Best-effort routing. Many tradeoffs have to be
  made. In most cases the goal is not to find the
  best solution but to find a viable solution with
  acceptable cost.

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Design Issues(2)

• Metrics and path computation
• how do we measure and collect network state
  information?
• how do we compute routes based on the information
  collected?
• Mapping of QoS requirements to well defined QoS
  Metrics
• Computation complexity associated with path
  computation (much of QoS routing based on
  multiple constraint optimization is NP-complete).
  Many heuristic algorithms exist.

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Design Issues (3)

• Path computation is followed by resource
  reservation which means that when the path is
  chosen the network state in terms of available
  resources is changed and such information needs
  to propagated throughout the network.
• Knowledge propagation and Maintenance
• how often the routing information is exchanged
  between the routers?
• The tradeoff here is between information accuracy
  and efficiency.
• For instance, what is available bandwidth? Is it
  what is left after reservation or the actual
  physically available?
• How do we maintain the info collected?(on demand
  path computation, aggregation, routing tables?)

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Design Issues (4)

• Scaling by hierarchical aggregation
• Imprecise state information model. Sources of
  inaccuracy
• network dynamics
• aggregation of routing information
• hidden information
• approximate calculation
• Administrative control -- flow priorities and
  preemption, resource control and fairness
• Integrate QoS-based routing and Best-effort
  routing

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Intra-domain Vs. Inter-domain

• Dynamic path computation to statically
  provisioned paths for a few service classes for
  intra-domain
• Some common features for intra-domain
• admission control, optimal resource usage,
  failure notices, support for best-effort flows,
  support for multicast routing with receiver
  heterogeneity and shared reservation styles
• Inter-domain routing scheme have to be scalable
  and therefore, simple.
• Cannot be based on highly dynamic network state
  info
• info exchange between domains should be
  relatively static

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Routing Strategies

• Source routing
• distributed routing
• hierarchical routing
• they are classified based on the way the state
  information is maintained and the search foe
  feasible path is carried out

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Source Routing

• Each node maintains the complete global state,
  including the network topology and the state
  information of every link
• Based on the global state, a feasible path is
  locally computed at the source node
• A control message is sent out along the selected
  path to inform the intermediate nodes of their
  precedent and successive nodes
• A link state protocol is used to update the
  global state at every node

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Source Routing (2)

• Strengths simplicity through centralization
  avoids many of the distributed computing
  problems guarantees loop-free routes
  conceptually simple, easy to implement, evaluate,
  debug and upgrade centralized heuristics are
  much easier to design for some NP-complete
  routing problems.
• Weaknesses communication overhead to maintain
  global state imprecision global state info high
  computation overhead at the source In short,
  source routing has scalability problem.

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Distributed Routing

• Path is computed by a distributed computation
• Control messages are exchanged among nodes and
  state information kept at each node is
  collectively used for path search
• Requires a distance-vector protocol or link-state
  protocol to maintain a global state in the form
  of distance vectors at every node. Based on the
  distance vectors, the routing is done on a
  hop-by-hop basis.

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Distributed Routing (2)

• Strengths path computation is distributed and
  result in shorter routing response time
  scalable searching multiple paths in parallel
  for a feasible path routing decision and
  optimization is done entirely based on local
  states
• Weaknesses dependence on global state flooding
  based algorithms which do not maintain global
  state have higher communication overheads
  difficult to design efficient heuristics in the
  absence of detailed topology or link-state info
  presence of loops due to inaccurate global state
  info at individual nodes (easily detected but
  alternate paths are difficult to find)

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Hierarchical Routing

• Nodes are clustered into groups which may be
  clustered into higher level groups recursively
  creating a multi-level hierarchy.
• Each physical node maintains an aggregated global
  state -- contains the detailed state info about
  the nodes in the same group and aggregated state
  info about other groups.
• Source routing is used to find a feasible path.
• A control message is sent along this path to
  establish the connection. A border node in a
  group represented by a logical node receives the
  message and uses source routing to extend the
  path through the group.

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Hierarchical Routing (2)

• Strengths Scales well retains many advantages
  of source routing as well as distributed routing.
  
• Weaknesses aggregated network state introduces
  additional imprecision gets more complicated
  when multiple QoS constraints are involved.

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QoS Routing Algorithms

• For Unicast, the problem is to find a network
  Path that meets the requirement of a connection
  between two end users
• For multicast, the problem is to find a multicast
  tree rooted at the sender and the tree covers all
  receivers with every internal path from the
  sender to a receiver satisfying the requirement
• QoS requirement as a set of constraints
• link constraint (concave metrics)
• path constraint (additive and multiplicative
  metrics)
• tree constraint

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Algorithms

• Feasible path is one that has sufficient residual
  resources to satisfy the QoS constraints of a
  connection
• In addition to a feasible path, we also want to
  optimize resource utilization -- measured by an
  abstract metric cost
• Cost could be in dollars or a function of the
  buffer or b/w utilization. Cost of a path is the
  total cost of all links on the path
• the optimization problem is to find the
  least-cost path among all feasible paths.

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Difficulties

• Diverse applications and different QoS
  requirements. Multiple constraints often make the
  routing problem intractable -- finding a path
  with two independent path constraints is
  NP-complete.
• Difficult to determine the optimal operating
  point for both QoS and Best effort traffic if
  their distributions are different. Best-effort
  traffic will suffer if overall traffic
  distribution is misjudged
• Maintaining up-to-date network state as it
  changes dynamically due to transient load
  fluctuation, connections in and out and links up
  and down.

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Graph-based Models

• A network modeled as a graph ltV, Egt. Nodes (V)
  represent switches, routers, and hosts. Edges (E)
  represent communication links. Symmetric or
  asymmetric links.
• Link state may be a triple consisting of residual
  b/w, delay, cost
• Node state can be combined into the state of the
  adjacent links
• The delay of a link consists of the link
  propagation delay and queueing delay at the node.
  The cost of alink is determined by the total
  resource consumption at the link and the node.

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State Information

• Local state each node is assumed to maintain its
  up-to-date local state including all delays,
  residual b/w on the outgoing links, and the
  availability of other resources
• Global state The combination of the local states
  of all nodes. Every node is able to maintain the
  global state by either a link-state protocol or a
  distance-vector protocol which exchanges the
  local states among the nodes periodically.
• Link state protocols broadcast the local state of
  every node to every other node. Distance vector
  protocols periodically exchange distance vectors
  among adjacent nodes. Figures 1 and 2

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Aggregate global state

• Figure 3

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Links and paths

• For some metrics, the state of a path is
  determined by the state of the bottleneck link
• link optimization routing -- find a path that has
  the largest bandwidth on the bottleneck link --
  widest path
• link-constrained routing -- find a path whose
  bottle neck bandwidth is above a required value
  (reduced to link optimization problem after
  pruning)
• for some other metrics, the state of the path is
  determined by the combined state over all links
  on the path
• path optimization -- least cost routing
• path constrained -- delay constrained

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NP-Complete problem classes

• PCPO -- delay-constrained least-cost routing
  find the least cost path with bonded delay
• MPC -- delay-delayjitter constrained routing find
  a path with both bounded delay and bounded delay
  jitter
• These two classes are NP-complete if the QoS
  metrics are independent and if they are allowed
  to be real numbers or unbounded integer numbers.
• Solvable in polynomial time if all but one metric
  take bounded integers Also if all metrics are
  dependent on a common metric (ex. worst-case
  delay and delay jitter are functions of b/w in
  WFQ)

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Chen-Nahrstedt

• Heuristic for multi-path constrained routing
  problem. Example delay-cost constrained
• map the cost (or delay) of every link from an
  unbounded real number to a bounded integer
  Solvable in polynomial time

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Source Routing Algorithms

• Maintain a global state at every node
• most algorithms transform the routing problem to
  a shortest path problem and then solve it by
  Dijkstras or Bellman-Ford algorithm.

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Salama et. al. Algorithm

• Distributed heuristic algorithm for
  delay-constrained least cost routing problem.
• A cost vector and a delay vector are maintained
  at every node by a distance vector protocol
• The cost(delay) vector contains for every
  destination the next node on the least-cost
  (least-delay) path.
• A control message is sent from the source toward
  the destination to construct a delay-constrained
  path. Loops may occur and detected if the control
  message visits a node twice. Routing process is
  rolled back until reaching a node from which the
  least-cost path was followed.

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Sun-Landgendorfer

• Improves worst-case performance of Salama et. al.
  by avoiding loops instead of detecting and
  removing loops.
• A control message is sent to construct the path
• The message travels along the least-delay path
  until reaching a node from which the delay of the
  least-cost path violates the delay constraint.

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PNNI and QOSPF

• Hierarchical link-state routing protocol
• Topology information is flooded through the
  network -- change (LSA)propagated based on a
  threshold model
• Traffic classes may be defined to indicate
  network resource requirements
• Widest-shortest path (which is a minimum hop
  count path with maximum bandwidth) may be
  pre-computed for every possible destination.