Differentiated Services

1
Differentiated Services

• Yogesh Bhumralkar
• WebTP Meeting - 03/06/2000

2
What is Diffserv?

• An approach to delivering QoS in a scalable,
  incrementally deployable way that
• keeps control of QoS local
• pushes work to the edges or network boundaries
  keeps forwarding path simple
• requires minimal standardization, encourages
  maximal innovation doesnt specify the means so
  much as it specifies the ends.
• Diffserv model leverages the fact that the
  Internet is composed of independently
  administered domains.

3
Aggregation

• Behavior Aggregates (BA) packets are grouped
  according to the forwarding behavior they are to
  receive within the cloud.
• Per-Hop Behavior (PHB) description of the
  forwarding treatment for a particular BA - this
  is the observable behavior.
• Basic idea Nodes in the center of a network only
  have to deal with the small number of traffic
  aggregates rather than keeping track of every
  separate traffic flow that passes through.

4
Aggregation

• Per-flow state is maintained at the edges
• Flows are classified into aggregates and are
  conditioned to meet the rules of that traffic
  aggregate.
• Traffic Conditioning occurs at the edges.
• Can occur either upon ingress or egress.
• Within the network the aggregate is treated as
  one
• no distinction made between different flows.
• treatment of behavior aggregate should not result
  in different performance for different traffic
  compositions within the behavior aggregate.

5
How are flows conditioned?

• Classifier identifies the flows that packets
  belong to.
• Classification based on the DS Field in IP header
  and perhaps other info such as source/dest. IP
  addresses etc.
• DS Field value represents a BA and maps to a
  particular PHB
• Meter measures the resource consumption of the
  flow flow rate and size of bursts.
• Marker modifies the DS Field.
• Shaper
• paces out the traffic
• usually done for incoming bursty traffic
• Dropper
• packets are dropped when a burst exceeds a
  maximum rate or maximum burst threshold

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Conditioning Process
7
Packet Marking DS Field

• DS Field is a replacement for the original IPv4
  TOS octet or IPv6 Traffic Class octet in the IP
  Header.
• Leftmost 6 bits represent the DSCP (Diffserv
  codepoint)
• Rightmost 2 bits currently unused (CU)
• Codepoints are an index into a table of packet
  forwarding treatments at each router.
• Table maps a DSCP to a particular PHB although
  this mapping may only be local to that domain.
• DSCP indicates a packets behavior aggregate
  within a cloud - from this point on the packet is
  treated only as part of this aggregate not as
  part of a particular flow.

8
Marking

• Marking can be done by a DS-node anywhere in the
  network but is usually done at network boundaries
• Re-marking
• the codepoints for a given PHB might be different
  in different domains.
• Hence, the need to re-specify the codepoint.
• There are recommended codepoint values for
  certain PHBs but these need not be strictly
  followed.

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Re-Marking
Domain A
Domain B
Ingress Node
Egress Node
110101
111001
Phb1 110101
Phb1 111001
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PHB Class Selector Compliant

• Motivation was to preserve backward compatibility
  with the IP Precedence Field (bits 0-2) in TOS
  octet.
• used for network control traffic, routing traffic
  etc.
• routers can implement filters based on this IP
  Precedence field.
• Specified by DSCP xxx000
• Preserve Default PHB requirement on code 000000
  - Best Effort traffic
• CSC codepoints describe minimum requirements on a
  set of PHBs that are compatible with forwarding
  treatments on selected by IP Precedence fields .

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CS PHB Requirements

• The set of PHBs mapped to by the 8 CSC codepoints
  must give at least 2 independently forwarded
  classes of traffic.
• PHBs for codepoints 11x000 must give packets
  preferential forwarding treatment compared to the
  default PHB (000000).
• Packets with different codepoints may be
  re-ordered but those with the same codepoints
  should remain in order
• Example Strict Priority scheduling with 2 queues

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Assured Forwarding (AF) PHB

• Four independently forwarded AF classes with
  three levels of drop precedence (two okay) within
  each class.
• Each class allocated certain amount of resources
  at the node (bandwidth and buffer space).
• Drop precedence determines the relative order of
  importance of a packet within a class. A
  congested node discards packets with a higher
  drop precedence.
• No re-ordering of AF packets of a microflow when
  they belong to the same AF class regardless of
  their drop precedence.
• Packets with the lowest drop precedence are
  assumed to be within a subscribed profile.

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AF Example Olympic Service

• 3 Classes Gold (AF1), Silver (AF2), Bronze (AF3)
• Packets are assigned to these classes so that
  gold class experiences lighter load than the
  others.
• Each class can have 3 drop precedences implement
  using a leaky bucket controller with parameters
  rate and size, which is the sum of two burst
  values committed burst size and excess burst
  size (ebs).
• If tokens available gt ebs --gt low drop
  precedence
• If 0 lt tokens lt ebs --gt medium drop
  precedence
• If tokens 0 --gt high drop precedence

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Expedited Forwarding (EF) PHB

• Forwarding treatment for an aggregate where the
  departure rate of the aggregates packets must
  equal or exceed the configured rate for that
  aggregate.
• EF traffic should receive this rate independent
  of the presence of any other traffic.
• Time scale Traffic should avg at least the
  configured rate when measured over a time
  interval equal to or longer than the time taken
  to send an MTU sized packet on the output link at
  the configured rate.
• Can implement this with a PQ with a token bucket
  rate limiter or a WRR scheduler with EFs share
  of the bandwidth being the configured rate.

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Resource Management Bandwidth Brokers

• Keep track of allocated bandwidth in a network
  domain.
• Process new bandwidth allocation requests.
• Configured with organizational policies -
  establish relationships with brokers in adjacent
  domains.
• Brokers configure routers within their domain to
  deliver particular services to a flow.
• Process is indicated in the following diagram

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Bandwidth Brokers
1. Request A wants to transfer data to B. 2.
Request requires negotiation between
Bba and BBb. 3, 4. Brokers configure
routers within their domains. 5, 6, 7.
Transfer of data.
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Acknowledgements

• Kathleen Nichols presentation slides on Diffserv
  at the UCB BMRC seminar (1998).