78 lines
2.3 KiB
Markdown
78 lines
2.3 KiB
Markdown
# lec7
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## Transport Layer
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Transport protocols usually lie in an end node's operating system.
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Send side:
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* Here we break the messages into _segments_, then we pass that segment into the network layer.
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Receive side:
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* Reassembles the segments into messages then passes it off to the application layer.
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If the network layer is communication then the transport layer is essentially running processes talking to each other on different nodes.
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Keep in mind as well that we don't concern ourselves with the responsibilities of other layers.
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## Transport Layer Protocols
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* TCP
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Reliable, in-order delivery. Also provides congestion and flow control, but requires connections to be opened.
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* UDP
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_No frills_ extension
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Two things that both of the above do not offer however
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* Delay guarantees
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* Bandwidth guarantees
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## Multiplexing and Demultiplexing
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### Multi
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Handle data from multiple sockets, add transport header
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For connectionless oriented multiplexing the segments must contain both the source and destination port numbers.
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However, conecction oriented multiplexing we need source/destination IP and port numbers.
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### Demultiplexing
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Host receives IP datagrams:
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* Each DG has soure IP and destination IP
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* Each DG carries one transport-layer segment
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* Each segment source and destination port number
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There is also connectionless de-multiplexing
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* Host must check destination port # in the given segment
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* It then passes that segment to the socket with that port number
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If we have nultiple segments with the same destination socket then they just get sent to that segment in some fashion.
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__TCP__ sockets are identified by a tuple of 4 items:
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* source IP
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* source PORT
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* destination IP
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* destination PORT
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The receiver uses this tuple to direct segments to the proper socket.
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For something like a webserver this also means that creating these sockets will mean there is some overhead as well which usually means we maintain sockets for some time to avoid that overhead.
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The benefit to this however is that with open connections we are able to serve content more quickly.
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## UDP Segment
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* Header
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64 bits of length
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Lower 16 bits of the header is dedicaated to the check sum is gets verified on the receiver's end.
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Lower 16 bits is essentially an addc checksum with a not at the end.
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