1. COP 5611 Operating Systems Spring 2010
Dan C. Marinescu
Office: HEC 439 B
Office hours: M-Wd 2:00-3:00 PM

2. Lecture 12 Reading Assignment: Chapters 8 from the online textbook
Homework 3 due on March 3
Midterm: Wednesday March 17, the first week after Spring Break
Last time:
More about layering and broadcast channels
Network layer
Today:
End-to-end-layer
Resource Management - Congestion
Next time
Faults, Failures and Fault-Tolerant Design
Measures of Reliability and Failure Tolerance
Tolerating active Faults 2 2 2 2 2

3. 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

4. Flat versus hierarchical addressing schemes
The scheme we discussed so far based on flat addresses have 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

5. Hierarchical addressing with two levels

6. Advantages and disadvantages of hierarchical addressing
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.

7. 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

8. 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

9. Dynamic IP address assignment –DHCP servers

10. From Wide Area Networks (WAN) to Local Area Networks (LAN) – the Ethernet

11. Ethernet frame

16. 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

17. 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?

18. Network layer errors
Messages originate at the network level and are delivered to the end-to-end layer.
Router error codes
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 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!!

19. 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
We need to identify individual segments by sequence numbers
Transport protocols  operate between two attachment points
TCP
UDP
Request-response protocol
RTP – Real-Time Transport protocol.

20. 3-way handshake in TCP

21. At 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
Provide the assurance that
if possible to get through messages will be delivered
If not possible to confirm delivery the application will know
Does not provide any assurance about duplicates
Round Trip Time
Timers could trigger retransmission
Static timers RTT
Adaptive timers
Exponentially weighted moving average using a decay factor

22. At most once delivery protocols
Requires both the sender and the receiver to maintain state.
The receiver must maintain a table of previously seen nonces , segments that have been received and passed to the application layer
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

23. 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.

24. 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

25. Lock-step-transmission (send segment and wait for acknowledgment)

26. Overlapped transmission

27. Flow control with a fixed window

28. Sliding window and self-pacing
Once a segment buffer is freed the receiver should send the permission to advance the window.
Regardless of the bottleneck the solution is:
Window size > = RTT x bottleneck data rate

29. 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

31. Cross-layer cooperation
Feedback
Source quench
Congested routers add a bit in the packet header indicating that congestion
Discard a packet
Control
Automatic rate adaptation  requires an estimation of the RTT

32. Congestion control in TCP
Slow start
Duplicate acknowledgments
Equilibrium
Additive increase
Multiplicative decrease