1. Introduction of Transport Protocols
Computer Networking
Introduction of Transport Protocols

2. Transport Protocols Network provides best-effort delivery
Transport at end-systems implement many functions 7 7 6 6 5 5
Transport
Transport IP IP IP
Datalink 2 2
Datalink
Physical 1 1
Physical
router

3. Minimum Transport Protocol Function
Multiplexing/de-multiplexing for multiple applications
How does the protocol stack know what application should receive a packet?
Why not using process ID?

4. Multiplexing using Ports
Transport layer uses the port in the transport header to identify the application socket.
(Destination IP, destination port) identifies the destination
Port numbers are well-known port numbers
In UDP packets delivered at a socket can come from multiple sources (connectionless).
Why do we need a source port number?
Needed to return a packet!
Flip source and destination (IP, port) pairs

5. User Datagram Protocol (UDP)
Source Port
Dest. Port
Length
D. Checksum
Still connectionless datagram
Multiplexing/demultiplexing
port numbers = connection/application endpoint
End-to-end reliability through optional end-to-end checksum.
protects against data corruption errors between source and destination (links, switches/routers, bus)
does not protect against packet loss, duplication or reordering
checksum is very simple so it can be implemented efficiently in software

6. Using UDP Examples: Use the port addressing provided by UDP
remote procedure calls
multimedia
distributed computing communication libraries
Use the port addressing provided by UDP
implement their own reliability, flow control, ordering, congestion control

7. More Transport Protocol Functions
Multiplexing/de-multiplexing for multiple applications
Error control
Hide unreliability of the network layer from applications
Many types of errors: corruption, loss, duplication, reordering.
End-to-end flow control
Avoid flooding the receiver
Congestion control
Avoid flooding the network
Connection management
Logical end-to-end connection
Connection state to optimize performance

8. Problems to Be Solved by Error Control At Transport Layer
Best effort network layer
Packets can get corrupted
Packets can get lost
Packets can get re-ordered

9. Mechanisms used For Error Control
Packets can get corrupted
CRC or Checksum to detect, retransmission to recover
Error correction code to recover
Packets can get lost
Sequence number to detect
Acknowledgement + Timeout to detect, retransmission to recover
Packets can get re-ordered
Sequence number to detect, receiver buffer to re-order

10. Sliding Window Protocol: Sender
Each packet has a sequence number
Assume infinite sequence numbers for simplicity
Sender maintains a window of sequence numbers
SWS (sender window size) – maximum number of packets that can be sent without receiving an ACK
LAR (last ACK received)
LFS (last frame sent)
Acknowledged packets
Packets not acknowledged yet
LAR
LFS
seq. numbers

11. Example Assume SWS = 3 Sender Receiver 1 frame 1 frame 2 frame 3 ACK 14 5 1 2 3 1 2 3 4 1
frame 4
Note: usually ACK contains the sequence number of the first packet in
sequence expected by receiver

12. Sliding Window Protocol: Receiver
Receiver maintains a window of sequence numbers
RWS (receiver window size) – maximum number of out-of-sequence packets that can received
LFR (last frame received) – last frame received in sequence
LAF (last acceptable frame)
LAF – LFR <= RWS

13. Sliding Window Protocol: Receiver
Let seqNum be the sequence number of arriving packet
If (seqNum <= LFR) or (seqNum >= LAF)
Discard packet
Else
Accept packet
ACK largest sequence number seqNumToAck, such that all packets with sequence numbers <= seqNumToAck were received
Packets in sequence
Packets out-of-sequence
LFR
LAF
seq. numbers

14. Sender/Receiver State
Max ACK received
Next seqnum
Next expected
Max acceptable … … … …
Sender window
Receiver window
Sent & Acked
Sent Not Acked
Received & Acked
Acceptable Packet
OK to Send
Not Usable
Not Usable

15. Lecture 16: Transport Protocols
Sequence Numbers
How large do sequence numbers need to be?
Must be able to detect wrap-around
Depends on sender/receiver window size
E.g.
Max seq = 7, send win=recv win=7
If pkts 0..6 are sent succesfully and all acks lost
Receiver expects 7,0..5, sender retransmits old 0..6!!!
Max sequence must be  send window + recv window
Xxx picture
Lecture 16: Transport Protocols

16. Cumulative ACK + Go-Back-N Retransmission
On reception of new ACK (i.e. ACK for something that was not acked earlier)
Increase sequence of max ACK received
Send next packet
On reception of new in-order data packet (next expected)
Hand packet to application
Send cumulative ACK – acknowledges reception of all packets up to sequence number
Increase sequence of max acceptable packet
Lecture 16: Transport Protocols

17. Lecture 16: Transport Protocols
Loss Recovery
On reception of out-of-order packet
Send nothing (wait for source to timeout)
Cumulative ACK (helps source identify loss)
Timeout (Go-Back-N recovery)
Set timer upon transmission of packet
Retransmit all unacknowledged packets
Performance during loss recovery
No longer have an entire window in transit
Can have much more clever loss recovery
Lecture 16: Transport Protocols

18. Lecture 16: Transport Protocols
Go-Back-N in Action
Lecture 16: Transport Protocols

19. Selective Ack + Selective Repeat
Receiver individually acknowledges all correctly received pkts
Buffers packets, as needed, for eventual in-order delivery to upper layer
Sender only resends packets for which ACK not received
Sender timer for each unACKed packet
Sender window
N consecutive seq #’s
Again limits seq #s of sent, unACKed packets
Lecture 16: Transport Protocols

20. Selective Repeat: Sender, Receiver Windows
Lecture 16: Transport Protocols

21. Summary of ARQ Protocols
Mechanisms:
Sequence number
Timeout
Acknowledgement
Sender window: fill the pipe
Receiver window: handle out-of-order delivery
Lecture 16: Transport Protocols

22. Many Nuances What type of acknowledgements?
Selective acknowledgement
Cumulative acknowledgement
Negative acknowledgement
How big should be the timeout value, SWS, RWS, sequence number field?
Reliability mechanism used to implement other functions: flow control, congestion control
Function overloading introduces ambiguity and complexity

23. Transmission Control Protocol (TCP)
The most utilized transport protocol
Provides a bidirectional reliable byte stream service
Made specific design decisions that have tradeoffs
Ports for multiplexing and-demultiplexing
Error control
Each byte (not a packet) has a sequence number
32 bits sequence number space
Cumulative ack + Go-Back-N
Flow control
Receiver gives send a window size to limit range of bytes that sender can send
16 bits window size
Congestion control (next lecture)
What is the key difference between this and flow control?
Why is it more difficult to implement congestion control than flow control?
Connection management (next lecture)

24. Sequence Number Space Each byte in byte stream is numbered.
32 bit value
Wraps around
Initial values selected at start up time
TCP breaks up the byte stream in packets (“segments”)
Packet size is limited to the Maximum Segment Size
Each segment has a sequence number.
Indicates where it fits in the byte stream
13450
14950
16050
17550
segment 8
segment 9
segment 10

25. Bidirectional Communication
Send bytes 1000:2000
Ack bytes 1000:2000
Send bytes 40000:42000
Ack bytes 40000:42000
Each Side of Connection can Send and Receive
What this Means
Maintain different sequence numbers for each direction
Single segment can contain new data for one direction, plus acknowledgement for other
But some contain only data & others only acknowledgement

26. Window Flow Control: Send Side
Packet Sent
Packet Received
Source Port
Dest. Port
Source Port
Dest. Port
Sequence Number
Sequence Number
Acknowledgment
Acknowledgment
HL/Flags
Window
HL/Flags
Window
D. Checksum
Urgent Pointer
D. Checksum
Urgent Pointer
Options..
Options..
App write
acknowledged
sent
to be sent
outside window

27. TCP Flow Control Sliding window protocol
For window size n, can send up to n bytes without receiving an acknowledgement
When the data are acknowledged then the window slides forward
Each packet advertises a window size
Indicates number of bytes the sender is prepared to receive
Original TCP always sent entire window
Congestion control now limits this

28. Window Flow Control: Send Side
Sent but not acked
Not yet sent
Sent and acked
Must retain for possible retransmission
Next to be sent

29. Window Flow Control: Receive Side
Receive buffer
Acked but not
delivered to user
Not yet
acked
window

30. Maximum Window Size Mechanism for receiver to exert flow control
Prevent sender from overwhelming receiver’s buffering & processing capacity
Max. transmission rate = window size / round trip time
Mechanism to Guarantee Receive Buffer Does Not Overflow
Receive buffer
Acked but not
delivered to user
Not yet
acked
window