UDP
The 8-byte header and its four fields, the lack of connection, acknowledgement and ordering, the checksum, what the application must do itself and the uses (DNS, DHCP, voice, QUIC).
- 24 cards
- 5 topics
- Free
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Each topic is weighted the way the exam weighs it.
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UDP protocol: what is it?
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A minimal transport protocol: it adds ports and a checksum to the data, with no connection, no acknowledgement and no ordering.
It does little more than steer data to the right port: that is what makes it fast and light, and what leaves everything else to the application.
Here is a captured UDP header. How big is the payload?
```text
d4 31 00 35 00 1e 8f 2a`- 22 bytes
- 30 bytes
- 8 bytes
- 38 bytes
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22 bytes
Bytes 4 and 5 form the length: 0x001e = 30, header included. The payload is 30 - 8 = 22 bytes. Answering 30 forgets the header is counted, 38 adds it a second time.
The UDP header is … bytes and holds four 16-bit fields: source port, destination port, … and checksum.
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The UDP header is 8 bytes and holds four 16-bit fields: source port, destination port, length and checksum.
Four 2-byte fields and nothing else: no sequence number, no acknowledgement, no window. It is the smallest transport header.
Datagram: what is it?
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A self-contained message, delivered or lost as a whole, with no link to the one before or the one after.
Unlike TCP's byte stream, boundaries are preserved: a 100-byte send comes out as one 100-byte read, never split or merged with another.