1. ECE4605: IP & TCP

2. Announcements 2 students per group
Same group for both project and paper presentations
If you have not sent the group information, please do so by tonight!

3. Reading List Up on our class website
Group 1 will present paper 1, etc.
First 4 groups will present the coming week
Tue, September 5th
Group 1: TCP Vegas
Group 2: TCP-SACK
Thu, September 7th
Group 3: IPv6
Group 4: pTCP

4. Presentations Plan for 25-30 minutes ~15 slides Introduction
setting up context
relating to lectures
Concepts/Algorithms/Main body of work
Results/Performance Evaluation
Critique/Suggestions for Improvement
Questions (at least 5 per presentation, distributed along talk)
Rehearse if possible

5. Internet Protocol (IP)
Addressing
Routing
Fragmentation and Reassembly
Quality of Service
Multiplexing and Demultiplexing

6. IP Routing Direct Indirect
If source and destination hosts are connected directly
Still need to perform IP address to physical address translation. Why?
Indirect
Table driven routing
Each entry: (NetId, RouterId)
Default router
Host-specific routes

7. IP Routing Algorithm RouteDatagram(Datagram, RoutingTable)
Extract destination IP address, D, from the datagram and compute the netID N
If N matches any directly connected network address deliver datagram to destination D over that network
Else if the table contains a host-specific route for D, send datagram to next-hop specified in table
Else if the table contains a route for network N send datagram to next-hop specified in table
Else if the table contains a default route send datagram to the default router specified in table
Else declare a routing error

8. Routing Protocols Interior Gateway Protocol (IGP)
Within an autonomous domain
RIP (distance vector protocol), OSPF (link state protocol)
Exterior Gateway Protocol (EGP)
Across autonomous domains
BGP (border gateway protocol)

9. IP Fragmentation
The physical network layers of different networks in the Internet might have different maximum transmission units
The IP layer performs fragmentation when the next network has a smaller MTU than the current network
MTU = MTU=500
IP fragmentation

10. IP Reassembly Fragmented packets need to be put together
Where does reassembly occur?
What are the trade-offs?

11. Multiplexing Web Email MP3 Web Email MP3 TCP UDP TCP UDP IP IP
IP datagrams
IP datagrams

12. IP Header Used for conveying information to peer IP layers Source
Destn
Application
Application
Transport
Router
Router
Transport IP IP IP IP
DataLink
DataLink
DataLink
DataLink
Physical
Physical
Physical
Physical

13. IP Header (contd.) 4 bit version 4 bit hdr length 16 bit total length
TOS
16 bit identification
3 bit
flags
13 bit fragment offset
8 bit TTL
8 bit protocol
16 bit header checksum
32 bit source IP address
32 bit destination IP address
Options (if any) (maximum 40 bytes)
data

14. Transmission Control Protocol (TCP)
End-to-end transport protocol
Responsible for reliability, congestion control, flow control, and sequenced delivery
Applications that use TCP: http (web), telnet, ftp (file transfer), smtp ( ), chat
Applications that don’t: multimedia (typically) – use UDP instead

15. Ports, End-points, & Connections
IP Layer
TCP
UDP
http ftp smtptelnet A1 A2 A3
Transport
Port
IP address
Protocol ID
Thus, an end-point is represented by (IP address,Port)
Ports can be re-used between transport protocols
A connection is (SRC IP address, SRC port, DST IP address, DST port)
Same end-point can be used in multiple connections

16. TCP Connection Establishment Connection Maintenance
Reliability
Congestion control
Flow control
Sequencing
Connection Termination

17. Active and Passive Open
How do applications initiate a connection?
One end (server) registers with the TCP layer instructing it to “accept” connections at a certain port
The other end (client) initiates a “connect” request which is “accept”-ed by the server

18. Connection Establishment & Termination
3-way handshake used for connection establishment
Randomly chosen sequence number is conveyed to the other end
Similar FIN, FIN+ACK exchange used for connection termination
Server does passive open
Accept connection request
Send acceptance
Start connection
Active open
Send connection
request
SYN
SYN+ACK
ACK
DATA

19. Fundamental Mechanism
ack
data
data
retx
Simple stop and go protocol
Timeout based reliability (loss recovery)
Multiple unacknowledged packets (W)
ack
data
Sliding Window Protocol: ….

20. Congestion Control Slow Start Start with W=1 For every ACK, W=W+1
Congestion Avoidance (linear increase)
For every ACK,
W = W+1/W
Congestion Control (multiplicative decrease)
ssthresh = W/2
W = 1
Alternative: Fall to W/2 and start
congestion avoidance directly

21. Why LIMD? (fairness) W=1 W=W/2 100 10 diff = 90 1 1 diff = 0
Problem? – inefficient
W=W/2
diff = 45
51 6 diff = 45
52 7 diff = 45 ..
diff = 45
diff = 23.5
diff = 23.5
diff = 11.2

22. Reliability (Loss Recovery)
ack
data
Sequence Numbers
TCP uses cumulative Acknowledgments (ACKs)
Next expected in-sequence packet sequence number
Pros and cons?
Piggybacking
Timeout calculation
Rttavg = k*Rttavg + (1-k)*Rttsample
RTO = Rttavg + 4*Rttdeviation 5 1 2 3 4 3 1 2 4

23. Flow Control Prevent sender from overwhelming the receiver
Receiver in every ACK advertises the available buffer space at its end
Window calculation
MIN(congestion control window, flow control window)

24. Sequencing Byte sequence numbers
TCP receiver buffers out of order segments and reassembles them later
Starting sequence number randomly chosen during connection establishment
Why? 3 1 2 4
1 given to app
2 given to app
Loss
4 buffered (not given to app)
3 & 4 given to app
4 discarded

25. TCP Segment Format 16 bit SRC Port 16 bit DST Port
32 bit sequence number
32 bit ACK number HL
resvd
flags
16 bit window size
Flags: URG, ACK,
PSH, RST, SYN,
FIN
16 bit TCP checksum
16 bit urgent pointer
Options (if any)
Data

26. TCP Flavors TCP-Tahoe TCP-Reno TCP-newReno TCP-Vegas, TCP-SACK
W=1 adaptation on congestion
TCP-Reno
W=W/2 adaptation on fast retransmit, W=1 on timeout
TCP-newReno
TCP-Reno + fast recovery
TCP-Vegas, TCP-SACK

27. TCP Tahoe Slow-start Congestion control upon time-out or DUP-ACKs
When the sender receives 3 duplicate ACKs for the same sequence number, sender infers a loss
Congestion window reduced to 1 and slow-start performed again
Simple
Congestion control too aggressive

28. TCP Reno Tahoe + Fast re-transmit
Packet loss detected both through timeouts, and through DUP-ACKs
Sender reduces window by half, the ssthresh is set to half of current window, and congestion avoidance is performed (window increases only by 1 every round-trip time)
Fast recovery ensures that pipe does not become empty
Window cut-down to 1 (and subsequent slow-start) performed only on time-out

29. TCP New-Reno TCP-Reno with more intelligence during fast recovery
In TCP-Reno, the first partial ACK will bring the sender out of the fast recovery phase
Results in timeouts when there are multiple losses
In TCP New-Reno, partial ACK is taken as an indication of another lost packet (which is immediately retransmitted).
Sender comes out of fast recovery only after all outstanding packets (at the time of first loss) are ACKed

30. User Datagram Protocol (UDP)
Simpler cousin of TCP
No reliability, sequencing, congestion control, flow control, or connection management!
Serves solely as a labeling mechanism for demultiplexing at the receiver end
Use predominantly by protocols that do no require the strict service guarantees offered by TCP (e.g. real-time multimedia protocols)
Additional intelligence built at the application layer if needed

31. UDP Header Src Port Dst Port Length: length of header + data (min = 8)
Checksum

32. Puzzle Man in a boat floating in a swimming pool 
He has a large solid iron ball
If he drops the ball into the water, what happens to the level of water in the swimming pool? (increases, decreases, stays the same?)