CSCI-131 Networking: the End-to-End Layer. Rodrigo Fonseca March 12 th, 2013

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1 CSCI-131 Networking: the End-to-End Layer Rodrigo Fonseca March 12 th, 2013

2 Today Transport layer Provides useful abstractions for applications Uses (the fairly minimal) services provided by the network layer Corresponding text: Saltzer and Kaashoek, 7.5 and 7.6

3 Plan Transport layer introduction Factoring functionality End-to-end argument Multiplexing Reliability At least once At most once In-order Integrity Efficient use of medium Don t send faster than receiver can handle Don t send faster than the network can handle

4 Layers Application what the users sees, e.g., HTTP Presentation crypto, conversion between representations Session can tie together multiple streams (e.g., audio & video) Transport demultiplexes, provides reliability, flow and congestion control Network sends packets, using routing Data Link sends frames, handles media access Physical sends individual bits

5 Why not provide (*) on the network layer? Cost These functionalities are not free: don t burden those who don t need them Conflicting Timeliness and in-order delivery, for example Insufficient Example: reliability

6 End-to-end argument Functions placed at lower levels of a system may be redundant or of little value They may need to be performed at a higher layer anyway But they may be justified for performance reasons Or just because they provide most of what is needed Example: retransmissions Lesson: weigh the costs and benefits at each layer Also: the end also varies from case to case

7 Transport Protocols Why then not let the applications handle (*)? Applications have different requirements But many have common requirements "#$ -##$ ). #"#$ #%$ &'$ ($! )*# + )*#, )*#!

8 Transport protocol examples UDP Best-effort, datagram-based service Provides multiplexing Basic integrity (checksums) TCP Reliable, connection-based stream service In-order, exactly once delivery Congestion control Also provides multiplexing and basic integrity DCCP Unreliable, connection-based datagram service Congestion control Multiplexing and basic integrity Many others

9 Plan Transport layer introduction Factoring functionality End-to-end argument Multiplexing Reliability At least once At most once In-order Integrity Efficient use of medium Don t send faster than receiver can handle Don t send faster than the network can handle

10 Network Layer Service Best effort, packets may: Not arrive Arrive out of order Arrive corrupted Take too long

11 Multiplexing Several applications on a single node want to use the same network endpoint Possible solution: have application identifiers Well known names? Program name? There may be multiple instances of the same application Another solution: ports Opaque ids to the transport layer Chosen by the applications/os Well-known ports make it easy to find services

12 Reliability: At least once How to guarantee that a packet reaches the destination at least once? Acknowledgments and timeouts Semantics: If it is possible to get through, the packet will Otherwise the application will know Problems? Duplicate packets. Why do they arise? Differentiating failures: forward or reverse path?

13 Challenge: How to Choose Timeouts? What are the effects of wrong timeouts? What are the sources of delay? Fixed timers are bad Adaptive timers EWMA: running estimate of the average EstRTT = (1 α) EstRTT + α SampleRTT Use 2x EstRTT TCP does something more Also estimate Standard Error, and uses EstRTT + 4x EstErr

14 At most once semantics Acks and timeouts work, but create duplicates How do we eliminate these duplicates? Keep state at the receiver List of received packet ids How long should this list be? For how long should we keep it? What if the sender uses incremental ids? Receiver can forget about packets that are far enough in the past (more about this later) Can this always work?

15 Reliability Question: did we really achieve exactly once semantics? If the data arrives, it will arrive exactly once If not, the sender will learn of this by a lack of acknowledgment But it can t be fundamentally perfect, people choose limits that work well in practice

16 Integrity* Question: did you get the same data that was sent? Must have a way to check: Make not all messages valid Discard the messages that are invalid How to do that? Checksums are a simple way Are they perfect? Can you have false positives? False negatives? * Not the cryptographic no0on of integrity, although you can layer it on top

17 In-order delivery Why can packets arrive out of order? How to ensure that data is delivered in order to the application? Receiver should keep a buffer, and only deliver a packet once all the others before it. How can we limit the size? What if we miss the first packet? Solution: have a window of valid packets Receiver doesn t ack until all preceding packets have been received. Sender only sends packet i+w after i s acknowledgment

18 Efficiency We want to utilize the network resources as much as possible but not more!

19 Sliding Window See drawing What is the data rate achieved? data rate = window size / round trip time What is the best you can do? fill the pipe : window size = round trip x bottleneck rate A sliding window is self-paced Acknowledgment allows sender to send new packet Determines buffer size at sender and receiver Controls the rate

20 Efficiency 3 goals: Utilize the network Don t overwhelm the receiver: flow control Don t overwhelm the network: congestion control

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