1. COT 5611 Operating Systems Design Principles Spring 2014
Dan C. Marinescu
Office: HEC 304
Office hours: M-Wd 3:30 – 5:30 PM

2. Lecture 13 Reading assignment: Last time:
Chapter 7 and 8 from the on-line text
The detailed description of the FCFS algorithm on class Web site.
Last time:
Formal description of the FCFS algorithm
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3. Today Control mechanisms and decisions in the Internet.
The network layer
The end-to-end layers
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4. Mechanisms to be implemented by protocols of a protocol stack
Error control
Flow control
Congestion control
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5. Decisions in the Internet
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6. The network layer The network layer transports packets
A network can be viewed as consisting of
Network core – carries out segments from one communicating entity to another
Network periphery – hosting the communicating entities at attachment points
The network core consists of routers which forward packets using routing tables
The entities at the periphery of the network use the transport service provided by the core to communicate with one another.
For a host at the periphery of the network:
the input for network layer on is a segment in a segment buffer at the transport layer.
The output is a link layer buffer.
Addressing – each communicating entity must have a unique address. The network layer transports packets
The network layer communicates with
the transport layer
the data link layer
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8. Encapsulation
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11. The forwarding table at the nodes
Contains pairs (destination, link)
A link is identified by a number.
The ids of all links terminating at a node are determined by that node.
A link connecting two nodes may have different ids, one at each end.
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12. Forwarding table at node A
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13. Forwarding table at node G
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14. How to construct efficient forwarding tables
Path vector exchange
Path vector a table providing a path to a destination in the network
Each node maintains in addition to the forwarding table a path vector
The algorithms consists of two steps
Path advertizing
Path selection
A node learns about optimal paths
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15. Initial state of path vector for node G
G starts with a path vector that contains just one item, an entry for itself
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16. Path vectors received by G in the first round
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17. First round path vector and forwarding table for G
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18. Path vectors received by G in the second round
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19. Second-round path vector and forwarding table for G
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20. Sketch of path vector algorithm implementation
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23. Requirements for routing algorithms
Routing algorithms should be dynamic and deal with:
Topological changes: links go down and new links are added
Congestion
Run asynchronously
Converge rapidly
Avoid oscillatory behavior
Ensure that no packet bounces around forever: add a hop count in the network header
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24. Flat versus hierarchical addressing schemes
The scheme we discussed so far is based on flat addresses and has disadvantages:
Each attachment point must have a unique address and the number of attachment points is equal to the number of systems connected to the network
The number of path vectors increases linearly with the size of the network.
Hierarchical addressing: e.g., two levels
Region
Address of the station/node within the region
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25. Hierarchical addressing with two levels
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26. Hierarchical addressing: advantages/disadvantages
Reduces the number of path vectors; a router for one region needs only to advertize a single path vector.
Assignation of network addresses is manageable:
a central authority assigns region addresses
station addresses are assigned by a regional authority.
The table lookup more complicated:
(1) extract the region
(2) forward to another region or forward within own region.
The path may no longer the shortest, the information provided by the path vector is less accurate.
The network addresses are location dependent. A mobile device must get a new address when it is connected in a different location.
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27. Physical and logical addresses
All devices have a physical address of the interface
Must obtain a logical address
ARP – address resolution protocol   is a protocol used by the Internet Protocol (IP), specifically IPv4, to map IP addresses to the hardware addresses used by a data link protocol.
ARP operates only across the local link that a host is connected to.
Types of ARP messages
ARP request
ARP response
RARP request
RARP response
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28. Internet Protocol (IPv4) over Ethernet ARP packet
bit offset
0 – 7
8 – 15
Hardware type (HTYPE) 16
Protocol type (PTYPE) 32
Hardware address length (HLEN)
Protocol address length (PLEN) 48
Operation (OPER) 64
Sender hardware address (SHA) (first 16 bits) 80
(next 16 bits) 96
(last 16 bits)
112
Sender protocol address (SPA) (first 16 bits)
128
144
Target hardware address (THA) (first 16 bits)
160
176
192
Target protocol address (TPA) (first 16 bits)
208
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29. Dynamic IP address assignment –DHCP servers
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30. From Wide Area Networks (WAN) to Local Area Networks (LAN) – the Ethernet
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31. Ethernet frame
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35. Internet addresses Internet provides a layered naming environment.
Internet address= Network number + host numbers
Most network numbers are global names.
Network 10 reserved for private networks
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36. Network address translation
Private networks
NAT (Network Address Translators)  map host’s private address to/from a temporarily assigned public address.
Problems
Communications may involve multiple parties with the addresses buried into application protocols
The router running NAT is a bottleneck
What if two private organizations merge?
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37. Network layer errors
Messages originate at the network layer delivered to the end-to-end layer.
Packets sent to the sender of the message with an error code:
Packets are discarded – the router has depleted its buffer space
The buffers of a router are depleted rapidly – stop sending
The region identifier is invalid
The station identifier is invalid
The end type identifier is not valid
The packet is larger than the MTU
The packet hop has been exceeded
The error packets include also a copy of the doomed packet
The network layer implements a best-effort policy. It would be too complicated to provide guarantees of service. There is no message regarding packets dropped. It would only increase congestion!!
ICMP – Internet Control Message Protocol  to ping a host.
Hop limit  “hope limit exceeded” error message.
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38. The network/transport interface
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39. End-to-end layer
Applications are too diverse to have a unique end-to-end protocol
End-to end multiplexing is more complicate:
You can have multiple instances of the same application running
Ports – logical end points for an end-to end protocol
SEND_MESSAGE (destination, service_port,reply_port,message)
DELIVER_MESSAGE(source, reply_port,message)
We need to identify individual segments by sequence numbers.
Connection-oriented versus connectionless communication,
Asymmetry of connection oriented end-to-end protocols: the server accepts the establishment of connections; the client initiates a connection.
Transport protocols  operate between two attachment points
TCP
UDP
Request-response protocol
RTP – Real-Time Transport protocol.
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41. IPV4 and IPv6 formats
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42. 3-way handshake in TCP
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43. Once delivery protocols
Persistent sender
It requires the sender to maintain state in the form of a table including the sequence numbers of segments sent but not acknowledged yet. Limited number of retrys.
Provide the assurance that
If possible to get through messages will be delivered
If not possible to confirm delivery the application will know; the sender
Does not provide any assurance about duplicates
Round Trip Time
Timers could trigger retransmission
Static timers RTT
Adaptive timers
Exponential backoff.
Exponentially weighted moving average using a decay factor
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44. At most once delivery protocols
Requires both the sender and the receiver to maintain state.
Nonces  segments that have been received and passed to the application layer
The receiver must maintain a table of previously seen nonces.
The list of nonces may grow; it may never be possible to delete the last nonce
Long messages need to be reassembled from out of order segments
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45. Reliable transport protocols
Data integrity assurance that the received message contains precisely the same information as the one sent.
Sender adds trailer containing a checksum;
Receiver computes the checksum and verifies that it is the same as the value in the trailer
Should send a NAK if they do not correspond? What if the address of the sender was damaged?
There is a check sum at the link layer but is this suffient?
Reliable transport protocols protocol with a persistent sender and with support for data integrity.
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46. Flow control
Ensure that the sender, the communication channel, and the receiver could work at the same pace.
The bottleneck could be any of them
Stop and wait protocols (lock step)
Overlapped transmission
Window based – send a range of frame. The protocol:
Sender – tell me how many frames I could send
Receiver – send 100 frames
Window based protocols
Fixed window size
Variable window size
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47. Lock-step-transmission (send segment and wait for acknowledgment)
A message with N segments requires N round-trip times.
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48. Overlapped transmission
The sender needs to maintain a list of segments sent.
Remove a segment from this list when it gets an acknowledgment for it.
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49. Flow control with a fixed window
The sender asks permission to send and the receiver responds with a window size; the sender sends all the data in the window size until it receives permission to send more.
The receiver consumes the data in its window and then sends permission to send more.
Problems: (i) missing permission message  leaves the sender blocked; (ii) duplicate permission message.
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50. Acknowledgments Purposes for an acknowledgment
OK to stop the timer associated with this segment
OK to release the buffer holding a copy of the segment
OK to send another segment
The segment has been accepted
The work requested in the acknowledged segment has been completed.
A packet may carry several types of acknowledgments
Piggybacking
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51. Sliding window and self-pacing
Sliding window a window that can have space added to it.
Once a segment buffer is freed the receiver should send the permission to advance the window.
Bottlenecks that affect the flow control
Sender’s rate of sending data
Receiver‘s rate of accepting data
The transmission capacity of the network
RTT (round-trip time)
Regardless of the bottleneck the solution is:
Window size > = RTT x bottleneck data rate
Self-pacing  the rate at which the window adjusts itself to the bottleneck data rate
Timer setting based on an estimation of the RTT
For packet retransmission  err by being too large to avoid unnecessary retransmissions
For the window size  err by being too small to avoid too large buffers for large window sizes
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52. Jitter control Jitter variation of the end to end delay.
Important for real-time applications such as audio and video streaming.
Acceptable failure rate e.g., we may miss one out of 100 frames
A standard TV consumes a “frame” every msec.
Strategy at the receiving site delay all arriving segments to make it look as all had the maximum allowable delay.
Data buffering:
Construct a histogram of the segment delivery delay (nr. of segments vs. delay).
Choose the acceptable failure rate  e%
Determine Dlong  the delay to ensure that (1-e) segment will be delivered in less than Dlong
Determine Dshort  the shortest delay observed
Call Dheadway  the average time between segments
The number of buffers is
N= (Dlong- Dshort)/ Dheadway
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53. Managing shared resources
Congestion -
a complex phenomena
leads to wasting of resources  congestion collapse
Managing resources in a network rather complex
More than one resource
Congestion collapse occurs more easily
Limited options to reduce capacity
The means to reduce congestion are difficult to implement
Feedback path is long
Source of the traffic must be
able to reduce the load
willing to cooperate
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55. Cross-layer cooperation
Feedback
Source quench
Congested routers add a bit in the packet header indicating congestion
Discard a packet
Control
Automatic rate adaptation  requires an estimation of the RTT
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56. Congestion control in TCP
Slow start
Duplicate acknowledgments
Equilibrium
Additive increase
Multiplicative decrease
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57. Bandwidth and latency
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58. Application requirements
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