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Reliable transfer
Open questions
Consider a `b` bits per second link between two hosts that has a propagation delay of `t` seconds. Derive a formula that computes the time elapsed between the transmission of the first bit of a `d` bytes frame from a sending host and the reception of the last bit of this frame on the receiving host.
Transmission links have sometimes different upstream and downstream bandwidths. A typical example are access networks that use ADSL (Asymmetric Digital Subscriber Lines). Consider two hosts connected via an ADSL link having an upstream bandwidth of 1 Mbps and a downstream bandwidth of 50 Mbps. The propagation delay between the two hosts is 10 milliseconds. What is the maximum throughput, expressed in frames/second, that the alternating bit protocol can obtain on this link if each data frame has a length of 125 bytes and acknowledgments are 25 bytes long. Same question if the protocol is modified to support 1500 bytes long data frames.
How would you set the duration of the retransmission timer in the alternating bit protocol ?
A version of the Alternating Bit Protocol supporting variable length frames uses a header that contains the following fields :
a `number` (0 or 1)
a `length` field that indicates the length of the data
a Cyclic Redundancy Check (`CRC`)
To speedup the transmission of the frames, a student proposes to compute the CRC over the data part of the frame but not over the header. What do you think of this proposed solution ?
Derive a mathematical expression that provides the `goodput`, i.e. the amount of payload bytes that have been transmitted during a period of time, achieved by the Alternating Bit Protocol assuming that :
Each frame contains `D` bytes of data and `c` bytes of control information
Each acknowledgment contains `c` bytes of control information
The bandwidth of the two directions of the link is set to `B` bits per second
The delay between the two hosts is `s` seconds in both directions
there are no transmission errors
Consider a go-back-n sender and a go-back receiver that are directly connected with a 10 Mbps link that has a propagation delay of 100 milliseconds. Assume that the retransmission timer is set to three seconds. If the window has a length of 4 frames, draw a time-sequence diagram showing the transmission consisting of 10 data frames (each frame contains 10000 bits):
when there are no losses
when the third and seventh frames are lost
when every second acknowledgment is discarded due to transmission errors
Same question when using selective repeat instead of go-back-n. Note that the answer is not necessarily the same.
Practice
Reliable protocols depend on error detection algorithms to detect transmission errors. The following questions will reinforce your understanding of these algorithms.
Reliable protocols rely on different types of checksums to verify whether frames have been affected by transmission errors. The most frequently used checksums are :
the Internet checksum used by UDP, TCP and other Internet protocols which is defined in :rfc:`1071` and implemented in various libraries.
the 16 bits or the 32 bits Cyclical Redundancy Checks (CRC) that are often used on disks, in zip archives and in datalink layer protocols. See http://rosettacode.org/wiki/CRC-32 for CRC-32 implementations in various languages.
the Fletcher checksum [Fletcher1982]_, see https://en.wikipedia.org/wiki/Fletcher%27s_checksum for implementation details.
By using your knowledge of the Internet checksum, can you find a transmission error that will not be detected by these checksums?
The Cyclic Redundancy Checks (CRCs) are efficient error detection codes that are able to detect :

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Source string location
../../exercises/reliability.rst:31
String age
5 years ago
Source string age
5 years ago
Translation file
locale/pot/exercises/reliability.pot, string 14