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Diffstat (limited to 'sem5/net/eksamnen/noter_spgs.md')
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1 files changed, 212 insertions, 4 deletions
diff --git a/sem5/net/eksamnen/noter_spgs.md b/sem5/net/eksamnen/noter_spgs.md index 7f063f2..14b2aad 100644 --- a/sem5/net/eksamnen/noter_spgs.md +++ b/sem5/net/eksamnen/noter_spgs.md @@ -83,6 +83,8 @@ Causal order Have a counter which is incremented before each event. Include this counter in each message sent. +*Lamport clock* + When a recipeint receives a message set its counter to either its own or the senders depending on which is largest. Scaler clocks are not strictly consistent, but can be used to define total order. @@ -626,7 +628,98 @@ Between AS numbers OSPF (Open Shortest Path First) is used. AS numbers communicate routes using BGP. -# Spørgsmål +# Topic 10 + +Adhoc have the advantage of creating spontaneous networks, while giving the same functionality of network infrastructure. + +This is hard because as this must be created from a one-to-one hop network, where multiple hops must be traveled for a packet to reach its destination. + +This introduces a *location service* where nodes must map a logical location with a real device located somewhere in the network. + +Because of the wireless nature, these network can have a very low reliability which requires a special transport layer. + +## Routing + +Why is routing so hard: + +- Can have asymmetric links +- Redundant links +- Changing and dynamic network topology + +A route formed may not work later, or for a reply. + +Forwarding is the idea that nodes should only know the next link in the route. + +Different methods: + +- Link state routing + - Bad fit for adhoc as it requires all nodes to know while topology. +- Distance vector + - Better fitting for adhoc as it has lower routing overhead. + - But should watch out with routing loops. + +### Types + +Proactive +: Keep a table of routes, so a forward or route is a simple lookup away. +: Requires that topology changes are propagated through network. + +Reactive +: Information is acquired when it is needed. + +## Solutions to broadcast problem + +- Wait a random amount of time when retransmitting. +- Only retransmit with a change. Known as *gossiping*. +- Do not forward if it has heard it from more than *k*. +- If it hears packet within *x* physical distance, do not rebroadcast. + +# Topic 11 Transport layer + +## UDP + +Connectionless and unreliable, as is does not have any flow and congestion control. + +Serves as the base for RDP, which adds timestamps and sequence numbers. + +## TCP + +Handles connection between endpoints, supporting multiplexing with the socket API. + +Connection oriented with communication going both ways, and supports reliable in order communication. + +Individual data streams are identified with two IP addresses, protocol number and two port numbers. + +### Header + +Important information in the header is: + +- Sequence number +: Used to set the initial seq number(SYN=1) and the counted sequence number is the session. +- ACK number +: The next expected sequence number, which basically says: "I have heard everything up until now". +- Window size +: The amount of bytes the receiver is willing to receive. + +### Handshake + +Utilizes a 3 way handshake, `SYN, SYN-ACK, ACK`. +This sets up the initial sequence number, and maximum segment size. + +`FIN` is signalled if a direction is done, so two `FIN`s to close the connection. +Can also close with the `RST` flag. + +### Flow/congestion control + +Also explained a bit in the question section. + +*Flow control* adapts sending rate to receiver speed and prevents buffer overflow at the receiver. +This is done by having the receiver notify sender about available buffer space. + +*Congestion control* tries to avoid network overload, and prevent packet loss underway. +This is done by dynamically adjusting sending rates, depending on network behavior (also called *elastic traffic*). + +# Questions If we answer a question nicely and quickly, we just get another question. We should not work hard to scretch time. @@ -904,21 +997,136 @@ SHIIIT token bucker er tom. Det tager 40 sec for token bucket at give alle pakkerne ud ved 5 per sec. Efter 40 sec har leaky bucket 5 tilbage, og det vil tage `5/20` sec. +Så det vil tage 40.25 sekundter. + 19. **Explain flooding and broadcast storm problem in ad hoc networks.** + +Flooding is a simple method of propagating information to a whole network. +If one wants to broadcast information to the whole network, it will send it to all its neighbours. + +These neighbours will forward this to their neighbours. +Here it is important for nodes to ignore flooding packets it has already seen, to avoid a infinite loop. + +This can also be avoided with a time to live. + +Flooding can create problems, some of them labeled *broadcast storm* problem. +These have different types. + +- Redundant rebroadcast +: Same message is sent to the same node multiple times. +- Contention +: Neighbours wanting to send the same packet may lead to contention on the channel. +- Collision +: High change of multiple nodes sending a broadcast packet to a single node, which leads to collision. + + 20. **Explain the difference between proactive and reactive routing approaches.** + +Proactive routing protocols continuously propagates routing information to all nodes, also if no traffic is sent. +Therefore packets can immediately be sent as the next hop is found with a simple lookup, which gives a low latency. + +However constant traffic is needed to keep routes up to date. + +Reactive routing protocols do this on demand. +If a route is needed it will send control traffic to find a route. +Less constant control traffic is therefore needed, however a higher latency can be expected. + +Reactive often use Routing request packets. + 21. **Explain the main principles of Dynamic Source Routing protocol.** +Is a reactive routing protocol as route discovery is done when source wants to send a packet. + +When a packet is to be sent, the source sends a RREQ (route request) which is flooded through the network. +Each node appends its own ID. + +When the RREQ reaches the destination a RREP is sent back with the route in the RREQ. +This then follows the reverse route, requiring that links are bidirectional. + +If uni direction is possible, the reply should also do route discovery. + +While source is sending packets is detect if a route does not exist anymore. +This is called *route maintanance*. + +*Route maintanance* is implemented with confirmation receipts for single hop, and *route error* for notifying source. + +The appending of routes has the effect that packet length grows with route length. + +DSR also employs route caching. +If nodes underway get a RREQ with a incomplete route in it, it will save it as a route to source. +This is also done when forwarding data packets. + +DSR has the advantage of only maintaining routes of who is talking together. + ## Transport layer 22. **Explain how TCP achieves reliable in-order delivery of TCP segments. What is the role of a receiver buffer and how is receiver buffer overflow prevented?** -23. **Explain the congestion control mechanismof TCP (slow-start phase and congestion avoidance).** + +It is normal for programs to read TCP as they process the information coming in. +It is therefore not guarantied that the data is read directly as it is received, as the program may be doing calculations. + +The receiver should therefore keep a buffer of received but not processed data. + +To keep this buffer from being overloaded by incoming packets TCP deploys *flow control*, where the receiver signals available buffer space. + +The sender keeps a sliding window, which keeps track of packets sent. +The window tail is moved forward as sent packets are ACK'ed by receiver and the head moves forward as the receiving window grows. + +The sender keeps track of this window making sure not to go to much forward (by sending to many packets). + +This is illustrated in notebook and below. + +``` +Seq: 1 '2 '3 '4 '5 '6 '7 '8 '9 '10'11'12 + |< Sender window >| + | ! | + Last Last Recv + ACK sent buffer + end +``` + +23. **Explain the congestion control mechanism of TCP (slow-start phase and congestion avoidance).** + +The size of the sender congestion window is used to control sending rate, and thus used as congestion control. + +TCP implements *slow-start* where a initial window size (cwnd) is set to 1. +Whenever a ACK is received increase cwnd with number of ACKs (or bytes). + +*Slow-start* phase ends when `cwnd > ssthreshold`. +And is started again at a timeout. + +At a timeout set `threshold = cwnd/2`, and cold start. +And at a 3 dup ack, half cwnd. + +If high bandwidth is desired, increase cwnd with 1 after each successful ack in threshold period, and double cwnd before threshold for each ack. + 24. **What are the possible problems of executing TCP over a wireless technology? How can these be mitigated?** +Wireless links have large delays, low throughput and high packet loss due to noise, interference and fading. + +This is a challenge for IP protocols not designed for these links. + +Problem with TCP is that it assumes congestion on transmission errors, which degrades performance. + +This can be solved in different ways, at different places in the protocol stack. + +- Link layer + - Do local retransmission of packets, which hides losses from sender. + - Cannot be used with IPSEC + - Retransmission create fluctuation in RTT (round trip time) which confuses TCP. +- Notify TCP of network condition/loss type. +- Change TCP ifself. + - Have to do this end-to-end. But can be put in the middle of existing tcp connection. *Split connection* + +*Split connection* has the advantage of not requiring changes outside wireless network, and allows for smaller headers as one controls the TCP protocol on the wireless link. +However one loses TCP symantics and does not work with encryption. + +*TCP SACK* is designed to be used with wireless networks. + ## Introduction to Fault Tolerance -25. **A server node shows has a down-time of 20 hours per year. Show how to calculate the resulting availability Pr(Server operational). ** +25. **A server node shows has a down-time of 20 hours per year. Show how to calculate the resulting availability Pr(Server operational).** Extend your derivation to the case of aredundant structure of 3 servers. Show how to calculate its availability assuming independent faults. - 26. **Discuss advantages and disadvantages of cluster structures (that hide the redundancy to accessing nodes) as opposed to an architecture where failover is done via the Clients (such as RSerPool)** |