Network Layer continued

1
Network Layer (continued)

• Traffic control
• Handling nonuniform traffic conditions

2
Congestion Control

• When arriving traffic (packets sent, G, ?)
  approaches or exceeds service rate (µ) delays
  become unbounded from queuing theory
• Congestion when the result of more offered
  traffic results in less delivered traffic
• This implies that too much traffic causes
  degeneration of service. In networks this may be
  due to
• 1) discarded packets due to buffer overflow or
  time-to-live field expiration
• 2) more control packets trying to handle
  congestion adding to congestion
• 3) time-outs and resends
• 4) failure to complete connections (losing part
  of the work completed)
• 5) congestion feeds on itself- as routers get
  overloaded they may malfunction

3
Congestion Control (cont.)

• Well confine this discussion to the network
  layer, but clearly other layers have an affect
  (for example the time-out period and window size
  of transport or DL layer)
• Routing
• Spreading routes over uncongested areas
• Oscillations and cycles add to congestion
• Discard policy
• Wine and milk
• CLP bit (ATM), DE bit (frame relay)
• Flow control (adjacent machines)

4
Congestion control (cont.)

• Virtual circuits- advantages and disadvantages
• Pre-allocation of resources such as buffers
• Less routing information must be sent
• Can deny the entire circuit
• But it is less flexible
• Throttling
• Choke packets/ dynamic windows/FECN bit (frame
  relay)
• reverse packet bit (BECN Backward Explicit
  Congestion Notification -frame relay)
• Get more resources

5
Prevention methods

• Strategies may be static (VC) or dynamic
• Congestion control strategies dynamically invoked
  based on perhaps the following metrics
• Queuing delays
• of buffers available
• of line capacities available
• 1 or more ICMP packets that indicate packet
  discarded because of time-to-live field
  expiration
• of time-outs and resends

6
Congestion Control sample algorithm

• Leaky bucket algorithm (ATM)
• Input is asynchronous, varying size, rate packet
  (multiple faucets into bucket)
• Output is cells of fixed size and rate (leak at
  bottom of bucket)
• Bursts of traffic are discarded (overflow of
  bucket)
• CLP bit for preferred cells

7
Traffic shaping

• Client and network agree to send at a specific
  rate/ transmit at agreed upon rate
• ATMs leaky bucket
• May discard packets if client does not hold to
  agreement
• ATMs token bucket
• Cannot send without token

8
Reserving bandwidth

• Internet Engineering Task Force (IETF)
• Resource reSerVation Protocol for the Internet
• RFC 2205, 2246, etc.
• Multicast routing using spanning trees
• Bandwidth is reserved from destination back to
  sender
• Controls congestion
• Allows destination to switch sources and keep
  reserved part of bandwidth

9
Reserving buffers, processor capacity, bandwidth
with labels

• Virtual circuits
• For each VC, buffers other resources are
  allocated
• Label flows (IPv6)
• Class of service may be allocated a buffer pool
• Priority-based
• Resources in general can be pre-allocated based
  on service classes (differentiated) services
• Consider airlines, postal services

10
Jitter control

• Applications such as voice and video
• Predictable delays
• Times must be included on packets
• Late packets sped up
• Early packets delayed
• Perhaps 99 of packets should be delivered within
  less than .5 msec variation of 25msec
• (2 variation)

11
Handling jitter

• Routers try to keep the flow uniform
• Speed up slow packets
• Delay fast ones
• Streaming audio or video buffer for about 10
  seconds before starting to send
• Requires buffers at receiver send (player)
• Requires processor

12
Quality of Service (QoS)

• Primary parameters
• reliability
• delay (for synchronous traffic)
• jitter (for isynchronous traffic)
• bandwidth (thruput)

13
Applications reliance on QoS

• Applications that need high reliability
• E-mail
• File transfer
• Web access
• Remote login
• Those that can handle low reliability
• Audio, video on demand
• Telephony, videoconferencing

14
Applications reliance on QoS

• Those that need strict parameters for delay
• Telephony
• Videoconferencing
• Probably web access
• Those that can tolerate a small amount of delay
• Email, file transfer

15
Applications reliance on QoS

• Jitter of successive packets must be kept within
  strict parameters for
• Telephony and videoconferencing
• Audio and video on demand
• Jitter is not an issue for
• Email, file transfer, web access

16
Applications reliance on bandwidth

• Those that need high thruput
• Video on demand
• Web access
• Low bandwidth needs
• Telephony, email

17
ATM categories of flow (see 802.16)

• Constant bit rate (delay jitter)
• Real-time variable bit rate (delay)
• Non-real-time variable bit rate
• Available bit rate

18
QoS for VoIP

• Average and worst case
• Delay
• Jitter
• Packet loss