Download Diffserv

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
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“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)
 Don’t define define service classes, provide
functional components to build service classes
Differentiated Services
 Intended to address the following difficulties
with Intserv and RSVP;
 Scalability: maintaining states by routers in high
speed networks is difficult sue to the very large
number of flows
 Flexible Service Models: Intserv has only two
classes, want to provide more qualitative service
classes; want to provide ‘relative’ service
distinction (Platinum, Gold, Silver, …)
 Simpler signaling: (than RSVP) many applications
and users may only w ant to specify a more
qualitative notion of service
Diffserv Architecture
Edge router:
r
- 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
b
marking
scheduling
..
.
Edge Functions
 At DS-capable host or first DS-capable router
 Classification: edge node marks packets according
to classification rules to be specified (manually by
admin, or by some TBD protocol)
 Traffic Conditioning: edge node may delay and
then forward or may discard
Edge-router Packet Marking
 profile: pre-negotiated rate A, bucket size B
 packet marking at edge based on per-flow profile
Rate A
B
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
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
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
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:
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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
Forwarding (PHB)
PHBs being developed:
 Expedited Forwarding: pkt departure rate of a
class equals or exceeds specified rate
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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
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.
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)
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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).
Routers or LSRs
 In the MPLS network, routers are called label
switching routers (LSR).
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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.
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.
Labels and Label Bindings
 A label identifies the path a packet should
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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
Label Assignment
 Based on forwarding criteria such as
 destination unicast routing
 traffic engineering
 multicast
 virtual private network
 QoS
MPLS Signaling
 A signaling protocol performs a variety of
functions such as:
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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
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
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
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.
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:
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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 0<=m(ni,nj)<=1)
concave, if m(P) = min{m(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)
Objectives
 To meet QoS requirements of end users.
 To optimize network resource usage
 to gracefully degrade network performance under
heavy load
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.
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.
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
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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?)
Design Issues (4)
 Scaling by hierarchical aggregation
 Imprecise state information model. Sources of
inaccuracy:
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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
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:
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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.
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Cannot be based on highly dynamic network state info
info exchange between domains should be relatively
static
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
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
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.
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-byhop basis.
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)
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.
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.