Differentiated Services

1
Differentiated Services

• IntServ is too complex
• More focus on services than deployment
• Functionality similar to ATM, but at the IP layer
• Per flow QoS guarantee per flow signaling,
  resource reservation, packet scheduling at IP
  level.
• Not scalable
• Focus on end to end
• Cannot be deployed incrementally.

2

• Differentiated Services
• Less focus on services more focus on deployment.
• Resource allocation for aggregated traffic
• Distinguish a small number of classes
• Forward class recorded in the packet header, no
  per flow classification and scheduling is needed.
  
• Traffic policing on the edge and class-based
  forwarding in the core
• Edge router is usually lower speed and can put
  more functionality
• Core router is usually higher speed make it
  simple.

3

• Differentiated Services
• Define forwarding behaviors not services
• opposite to IntServ
• Forwarding behaviors are much easier to enforce
  than services (local .vs. global)
• QoS guarantee by provisioning rather than
  reservation
• QoS by IP admission control
• QoS for aggregate traffic, not individual flows.
• Emphasis on service level agreements rather than
  dynamic signaling
• Service level agreement can be monitor by the
  edge router
• Signaling needs other things authentication
• Focus on a single domain, not end to end
• DS domain.

4

• Service Forwarding treatment
• Related, but not one-to-one mapping.
• Per-Hop Behaviors (PHBs)
• Externally observable forwarding treatments at a
  single node are described by PHBs.
• Each PHB is represented by a 6-bits value called
  Differentiated Services CodePoint (CSCP).
• All packets with the same codepoint are referred
  as a behavior aggregate.
• PHB may describe the forwarding treatments in
  either relative or absolute terms.
• Minimal bandwidth
• Proportional allocation of link bandwidth

5

• Per-Hop Behaviors (PHBs)
• A set of PHBs may for a PHB group. A PHB group
  may describe resource allocation in relative
  terms among than self.
• Multiple PHB groups may coexist in the same DS
  domain.
• PHBs are typically implemented by means of buffer
  management and packet scheduling
• WFQ may be used, it does not have to be.

6

• Service
• A service describes the overall treatment of a
  customers traffic within a DS domain.
• Customers see services not PHBs.
• To support a service, many components must work
  together
• Mapping of service to PHBs, traffic conditioning,
  network provisioning, PHB-based forwarding.
• Services in the DiffServ architecture is defined
  in the form of Service Level Agreement (SLA).

7

• Service
• SLA includes traffic conditioning agreement
  (TCA)
• Traffic profile
• Performance metrics
• Actions for nonconformant packets
• Additional marking and shaping services.
• Service can be defined in either quantitative or
  qualitative terms.
• Service may have scopes.

8

• Differentiated Services Field
• IP TOS field
• XXX D T R 0 0
• DS Field DSCP(6 bits) CU (2 bits)
• 32 standardized codepoints xxxxx 0
• 16 experimental codepoints xxxx11
• 16 experimental, but may be standarized xxxx 0 1

9

• Differentiated Services Field
• Historical codepoint definition
• Provides limited backward compatibility
• Default PHB codepoint (000000)
• Backward compatibility with the current best
  effort forwarding treatment
• Class selector codepoints (xxx 000)
• Backward compatibility with some known current
  use of IP precedence field.
• At least two forwarding classes
• Larger numerical value should have better or
  equal forwarding treatment
• (111000) and (110000) must be better than
  (000000)

10

• Differentiated Services Field
• Current codepoint allocation
• Two new standardized PHBs assure forwarding (AF)
  and expedited forwarding (EF).
• Table 3.3. Summarize the current codepoint
  allocation.