1. TCP/IP

2. Recap Medium Access Control Scheduling
ALOHA, slotted-ALOHA, CSMA, CSMA/CD
Scheduling
FIFO, Priority, GPS, WFQ, WRR, WRR+spread, DRR, CBQ

3. TCP/IP Protocol Suite Physical layer Data-link layer – ARP, RARP, SLIP
Network layer – IP, ICMP, IGMP, BootP
Transport layer _ TCP, UDP, RTP
Application layer – http, smtp, ftp

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

5. Addressing
Need unique identifier for every host in the Internet (analogous to postal address)
IP addresses are 32 bits long
Hierarchical addressing scheme
Conceptually …
IPaddress =(NetworkAddress,HostAddress)

6. Address Classes Class A Class B Class C 0 netId hostId
7 bits 24 bits
netId hostId
14 bits 16 bits
netId hostId
21 bits 8 bits

7. IP Address Classes (contd.)
Two more classes
1110 : multicast addressing
1111 : reserved
Significance of address classes?
Why this conceptual form?

8. Addresses and Hosts
Since netId is encoded into IP address, each host will have a unique IP address for each of its network connections
Hence, IP addresses refer to network connections and not hosts
Why will hosts have multiple network connections?

9. Special Addresses hostId of 0 : network address
hostId of all 1’s: directed broadcast
All 1’s : limited broadcast
netId of 0 : this network
Loopback :
Dotted decimal notation: IP addresses are written as four
decimal integers separated by decimal points, where each
integer gives the value of one octet of the IP address.

10. Exceptions to Addressing
Subnetting
Splitting hostId into subnetId and hostId
Achieved using subnet masks
Useful for?
Supernetting (Classless Inter-domain Routing or CIDR)
Combining multiple lower class address ranges into one range
Achieved using 32 bit masks and max prefix routing

11. Examples Subnetting Supernetting 192.168.1.0/24 – class C network
/26 and /26 – 2 subnetworks with upto 62 stations each!
Supernetting
/24 and /24 – 2 class C networks
/23 – 1 super network with upto 512 stations!!

12. Weaknesses Mobility Switching address classes
Notion of host vs. IP address

13. 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

14. 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

15. 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)

16. 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

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

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

19. 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

20. 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

21. Transmission Control Protocol (TCP)

22. 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

23. 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

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

25. 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

26. 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

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

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

29. 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

30. 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

31. 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)

32. Sequencing Byte sequence numbers
TCP receiver buffers out of order segments and reassembles them later 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
Starting sequence number
randomly chosen
during connection
establishment
Why?

33. 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

34. 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 + intelligent fast recovery
TCP-Vegas, TCP-SACK

35. 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

36. 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

37. 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

38. TCP SACK
TCP (Tahoe, Reno, and New-Reno) uses cumulative acknowledgements
When there are multiple losses, TCP Reno and New-Reno can retransmit only one lost packet per round-trip time
What about TCP-Tahoe?
SACK enables receiver to give more information to sender about received packets allowing sender to recover from multiple-packet losses faster

39. TCP SACK (Example) Assume packets 5-25 are transmitted
Let packets 5, 12, and 18 be lost
Receiver sends back a CACK=5, and SACK=(6-11,13-17,19-25)
Sender knows that packets 5, 12, and 18 are lost and retransmits them immediately

40. Other TCP flavors TCP Vegas TCP FACK
Uses round-trip time as an early-congestion-feedback mechanism
Reduces losses
TCP FACK
Intelligently uses TCP SACK information to optimize the fast recovery mechanism further

41. 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

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

43. Puzzle Singly linked list Need to find if the list has a loop
Constraint1: only read operations on the nodes of the linked-list
Constraint2: constant order memory usage

44. Recap TCP Connection management Reliability Flow control
Congestion control
TCP flavors
UDP