1. Transport Layer 3-1

2.  What is the Transport layer for?  Where does the code for the transport layer live?  What are the two protocols at this layer?  What is the basic idea behind these?  How does the transport layer share a network connection among programs?  What are well-known port numbers for? Transport Layer 3-2

3.  What information is needed to decide which socket a packet is associated with?  What is port scanning? Why is it a concern?  Why does DNS use UDP instead of TCP?  What is a checksum? Transport Layer 3-3

4. How can we take this apart? http://www.pms.ifi.lmu.de/mitarbeiter/ohlbach/multimedia/IT/IBMtutorial/3376c45.html http://en.wikibooks.org/wiki/Computer_Networks/UDP Transport Layer 3-4 0000 00 04 23 a7 a2 d1 00 13 02 5a ec 97 08 00 45 00..#......Z....E. 0010 00 3a d5 2a 00 00 80 11 4a 32 c0 a8 46 9b 0a 01.:.*.... J2..F... 0020 0a 12 09 35 00 35 00 26 07 66 4d 3f 01 00 00 01...5.5.&.fM?.... 0030 00 00 00 00 00 00 08 70 65 74 65 72 73 6f 6e 03.......p eterson. 0040 63 6f 6d 00 00 01 00 01 com.....

5.  What sort of things do we worry about to achieve reliability?  How do we address these issues?  What is going on with these state machines?  Explain what happens if a packet is lost between sender and receiver Transport Layer 3-5

6.  What if an acknowledgement is lost?  Why do we need to pipeline packets?  What is RTT? MSS?  What is the difference between Go Back N and Selective Repeat?  How does TCP handle out of order packets?  How do we adjust the timeout value when doing TCP? Transport Layer 3-6

7.  What are send and receive buffers?  How does the protocol deal with these?  What are the sequence # and ack # fields used for?  What does the ACK bit mean?  What is a cumulative acknowledgement?  How does TCP adapt to changing network conditions? Transport Layer 3-7

8.  How does TCP determine the link capacity?  How is the window size managed?  Why does the fast retransmit wait for 3 duplicate acks instead of 2?  How do we avoid overwhelming a slow program with data?  What is the “SYN” bit?  What is the 3-way handshake? Transport Layer 3-8

9. Scenarios: Attacker is in the “middle” with packets passing through Attacker is able to sniff the packets and inject new ones into the stream Attacker is blind and doesn’t understand anything about the session Transport Layer 3-9

10.  The initial sequence number is chosen randomly so that a hostile packet can’t appear to be part of the communication stream.  It’s easy to send a packet that impersonates a random IP address – you just can’t get a reply Transport Layer 3-10

11.  Suppose someone injects a random bit of data into a session – what will happen? Transport Layer 3-11

12. Transport Layer 3-12 0000 00 13 02 5a ec 97 00 04 23 a7 a2 d1 08 00 45 00...Z.... #.....E. 0010 05 14 6b 02 40 00 2e 06 7c d5 81 aa d7 1e c0 a8..k.@... |....... 0020 46 9b 00 50 11 17 64 1f 13 b1 61 3d 07 ac 50 10 F..P..d...a=..P. 0030 1b 94 8b b6 00 00 67 2e 20 53 59 4e 20 66 6c 6f......g. SYN flo 0040 6f 64 29 0d 0a 20 20 32 2e 20 50 72 65 64 69 63 od).. 2. Predic 0050 74 20 74 68 65 20 6e 65 78 74 20 53 45 51 20 6f t the ne xt SEQ o 0060 66 20 58 20 69 6e 20 73 6f 6d 65 20 77 61 79 0d f X in s ome way. 0070 0a 20 20 33 2e 20 53 65 6e 64 20 61 20 53 59 4e. 3. Se nd a SYN 0080 20 66 72 6f 6d 20 41 20 2d 3e 20 58 20 28 73 70 from A -> X (sp 0090 6f 6f 66 65 64 20 66 72 6f 6d 20 54 29 0d 0a 20 oofed fr om T).. 00a0 20 34 2e 20 77 61 69 74 2c 20 61 6e 64 20 73 65 4. wait, and se 00b0 6e 64 20 61 20 53 59 4e 7c 41 43 4b 20 66 72 6f nd a SYN |ACK fro 00c0 6d 20 41 20 2d 3e 20 58 20 28 73 70 6f 6f 66 65 m A -> X (spoofe See http://www.ssfnet.org/Exchange/tcp/tcpTutorialNotes.htmlhttp://www.ssfnet.org/Exchange/tcp/tcpTutorialNotes.html )..