Chapter 3 Notes - IS450

Chapter 3 Notes - IS450 - 1 IS 450/650 Transport Layer...

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Unformatted text preview: 1 IS 450/650 Transport Layer Announcements & Reminders: Starting Monday 10/25 no laptops, no exceptions. Too many people are doing less than well in the course. Quiz 2 median 5, mean 5.31 Homework due on paper Monday 10/25 at 4:30pm: R14 from p. 298 of the Kurose and Ross text. 11/15 Second Test 12/13 Grad paper due. Monday 12/20 6pm final exam Office Hours for the Rest of The Semester: M 1112, 7:158 W 1112, 24 Th 24 F 1112, 24 Cell phones & laptops off . Reminder: if printing the notes, consider printing 4-up (4 pages per side) or 2-up or 6-up or some such. Generated October 25, 2010. 2 Transport Layer: The User Datagram Protocol The point of the transport layer: connect processes to processes . How does this differ from the network layer? Why is UDP called the user datagram protocol? Weve seen datagrams before, e.g., the DNS, postcards, and the like. Well see them again, notably with IP. But UDP is the first transport layer protocol we discuss in detail. And the transport layer is where the application layer interfaces with the rest of the network stack. Typical in the Unix/Linux/Windows world: transport layer and below: part of the OS above the transport layer: part of the application 3 UDP The User Datagram Protocol sits on top of IP and multi- plexes that lower layer among multiple application layer services. UDP is a best-effort service that can lose datagrams, deliver datagrams out of order, deliver duplicates of data- grams, etc. Each UDP datagram must contain addressing informa- tion for the sending and receiving processes. This is in the form of port numbers . Similarly, the layer below, IP, must contain information about which transport layer service to use. This is IPs Protocol or NextHeader field, and for UDP it is 17. UDP also contains a checksum . 4 The Checksum The checksum is calculated by 1. taking the data 16b at a time, 2. adding them together, 3. adding any carry from the leftmost bit in at the right- most bit, and then 4. complementing the result. 5 Example Suppose we are sending the following three 16b words: 0110 0001 1100 0001 0010 0101 1101 1110 0011 1000 1001 1101 First, add them up. 0110 0001 1100 0001 +0010 0101 1101 1110 = 01000 0111 1001 1111 The extra bit at the left of the sum is the carry out of the high bit. Now add the sum to the third number: 1000 0111 1001 1111 +0011 1000 1001 1101 = 1100 0000 0011 1100 (2) Complement the result: 0011 1111 1100 0011 Send this as the checksum. 6 Second Example Consider these three words: 1011 0000 0101 1110 0110 0110 0101 1000 0001 1100 0010 1011 The first partial sum, the sum of the first two words: 1011 0000 0101 1110 +0110 0110 0101 1000 =1 0001 0110 1011 0110 The carry out of the high bits was 1, and this has to be added into the next sum: 1 0001 0110 1011 0110 +0001 1100 0010 1011 = 0011 0010 1110 0010 Finally, complement the sum: 1100 1101 0001 1101 7 Checksum, the Receiving End The receiver also calculates a checksum, but includes the...
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This note was uploaded on 08/28/2011 for the course IS 450 taught by Professor Martens,j during the Fall '08 term at UMBC.

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Chapter 3 Notes - IS450 - 1 IS 450/650 Transport Layer...

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