1. ECE 4450:427/527 - Computer Networks Spring 2017
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 7.1: End-to-end Protocols
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

2. Discussions Up to now, we have learned various technologies to
Connect together a collection of computers/nodes
The collection ranges from simple Ethernet and wireless networks to much larger scale one, e.g., global scale
Host-to-host packet delivery: logical communication between hosts
Now, we turn the above host-to-host packet delivery into a process-to-process communication channel
Logical communication between application processes running on different hosts – Transport-layer protocols
Because it supports communication between application program running in the end nodes, it is also called end-to-end protocols
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

3. Transport Layer vs. Network Layer
Household analogy:
12 kids sending letters to 12 kids
processes = kids
app messages = letters in envelopes
hosts = houses
transport protocol = Ann and Bill who demux to in-house siblings
network-layer protocol = postal service
Network layer: logical communication between hosts
Transport layer: logical communication between processes
relies on, enhances, network layer services
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

4. Transport Layer
Ann and Bill do work within respective homes - Protocols live in the end system: move messages from application processes to the network edge
Assume Ann and Bill are on vacation, younger Susan and Tom substitute for them:
Pick up/deliver mail less frequent, or even lose letters
Susan and Tom do not provide the same service!!!
From network point of view: multiple transport protocols, each offering a different service to applications.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

5. Transport Layer
Common properties that a transport protocol can be expected to provide
Guarantees message delivery
Delivers messages in the same order they were sent
Delivers at most one copy of each message
Supports arbitrarily large messages
Supports synchronization between the sender and the receiver
Allows the receiver to apply flow control to the sender
Supports multiple application processes on each host
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

6. Internet Transport Protocols
Simple Demultiplexer (UDP – User Datagram Protocol)
Allow multiple application processes on each host to share the network
Reliable, in-order delivery (TCP – Transmission Control Protocol)
Connection set-up
Discarding of corrupted packets
Retransmission of lost packets
Flow control
Congestion control
Remote Procedure Call (RPC): Request/reply service
Real-time Transport Protocol (RTP): real-time applications
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

7. UDP Simplest possible protocol: Simple Demultiplexer (UDP)
Allow multiple application processes on each host to share the network
Aside it, no other functionality
The only interesting issue: the form of the address used to identify the target process
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

8. Addressing Processes: Ports
Application processes communicate by exchanging messages across the network
Web: a client browser process exchanges messages with a web server process.
We need something to identify receiving and sending processes: Port numbers
A web-server is identified by port 80
A mail server is identified by port 25
Port number is needed, since a host might run many network applications
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

9. Port Discovery Use well-publicized ports for different services
DNS uses port 53
uses port 25
HTTP uses port 80
Alternatively, use one port as a “port-mapper” service
Call 411 to learn the port of any other process
Allows for dynamic allocation of ports to different services
Allows for the assignment of ports to newly created services
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

10. UDP Processes are identified based on ports
16 bits for each field yields 64K different identifiers
<IP, port> combination allows de-multiplexing at receiving host
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

11. Multiplexing and Demultiplexing
Host may be running multiple processes at the same time
These processes
Generate multiple messages for the same host
Generate multiple messages for multiple hosts
Transport layer multiplexing
Multiplex messages from multiple processes
Break down messages to segments and pass to network layer
Transport layer de-multiplexing
Reassemble messages at the receiving host and pass to the communication processes
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

12. UDP Multiplexing/De-Multiplexing
Client
IP:B P2
client
IP: A P1 P3
server
IP: C
SP: 6428
DP: 9157
SP: 9157
DP: 6428
DP: 5775
SP: 5775
SP provides “return address”
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

13. TCP Connection oriented Explicit set-up and tear-down of TCP session
Stream-of-bytes service
Sends and receives a stream of bytes, not messages
Full-duplex
Reliable, in-order delivery
Checksums to detect corrupted data
Acknowledgments & retransmissions for reliable delivery
Sequence numbers to detect losses and data reordering
Demultiplexing mechanism
Flow control
Prevents overflow of the receiver’s buffer: Allow receiver to limit how much data sender can transmit at a given time – End to end issue
Congestion control
Prevent too much data from being injected into the network
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

14. TCP Multiplexing/De-Multiplexing
client
IP: A P4 P5 P6 P2 P1 P3
SP: 5775
DP: 80
S-IP: B
D-IP:C
SP: 9157
SP: 9157
DP: 80
DP: 80
Client
IP:B
server
IP: C
S-IP: A
S-IP: B
D-IP:C
D-IP:C
2 clients using the same destination port number (80)
to communicate with the same Web server applications.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

15. TCP : Threaded Web Server
2 clients using the same destination port number (80)
to communicate with the same Web server applications.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

16. TCP Duplicate Lower Layer Services?
How is TCP different?
Runs over a route rather than a single physical link  Needs to establish a connection and negotiate parameters (sliding window size)
Adapts to heterogeneous conditions
RTT varies with connected hosts, time of the day etc, unpredictable
Flow control on computers with different resources
Reorders and retransmits packets end-to-end
Even if packets are in order on one physical link this does not guarantee end-to-end ordering
Provides congestion control
No immediate feedback from the link, no knowledge of conditions along the way
Congestion control based on feedback
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

17. TCP Connection oriented Explicit set-up and tear-down of TCP session
Stream-of-bytes service
Sends and receives a stream of bytes, not messages
Full duplex
Reliable, in-order delivery
Checksums/CRC to detect corrupted data
Acknowledgments & retransmissions for reliable delivery
Sequence numbers to detect losses and data reordering
Demultiplexing mechanism
Flow control
Prevents overflow of the receiver’s buffer: Allow receiver to limit how much data sender can transmit at a given time – End to end issue
Congestion control
Prevent too much data from being injected into the network
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

18. TCP Segment
TCP is a byte-oriented protocol, which means that the sender writes bytes into a TCP connection and the receiver reads bytes out of the TCP connection.
Although “byte stream” describes the service TCP offers to application processes, TCP does not, itself, transmit individual bytes over the Internet.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

19. TCP Segment
Source: TCP buffers enough bytes from sending process to fill a reasonably sized packet and then sends this packet to destination: segment.
Destination: TCP empties the contents of the packet into a receive buffer, and the receiving process reads from this buffer at its leisure.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

20. TCP Segment IP packet No bigger than Maximum Transmission Unit (MTU)
IP Data
TCP Data (segment)
TCP Hdr
IP Hdr
IP packet
No bigger than Maximum Transmission Unit (MTU)
E.g., up to 1500 bytes on an Ethernet
TCP packet
IP packet with a TCP header and data inside
TCP header is typically 20 bytes long
TCP segment
No more than Maximum Segment Size (MSS) bytes
E.g., up to 1460 consecutive bytes
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

21. TCP Header TCP Header Format Dr. Nghi Tran (ECE-University of Akron)

22. TCP Header
The SrcPort and DstPort fields identify the source and destination ports, respectively.
The Acknowledgment, SequenceNum, and AdvertisedWindow fields are all involved in TCP’s sliding window algorithm.
Because TCP is a byte-oriented protocol, each byte of data has a sequence number; the SequenceNum field contains the sequence number for the first byte of data carried in that segment.
The Acknowledgment and AdvertisedWindow fields carry information about the flow of data going in the other direction.
HdrLen: Length of header in 32-bit words, e.g., 5
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

23. TCP Header
The 6-bit Flags field is used to relay control information between TCP peers.
The possible flags include SYN, FIN, RESET, PUSH, URG, and ACK.
The SYN and FIN flags are used when establishing and terminating a TCP connection, respectively.
The ACK flag is set any time the Acknowledgment field is valid, implying that the receiver should pay attention to it.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

24. TCP Header
The URG flag signifies that this segment contains urgent data. When this flag is set, the UrgPtr field indicates where the nonurgent data contained in this segment begins.
The urgent data is contained at the front of the segment body, up to and including a value of UrgPtr bytes into the segment.
The PUSH flag signifies that the sender invoked the push operation, which indicates to the receiving side of TCP that it should notify the receiving process of this fact.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

25. TCP Header
The RESET flag signifies that the receiver has become confused, it received a segment it did not expect to receive—and so wants to abort the connection.
Finally, the Checksum field is used in exactly the same way as for UDP—it is computed over the TCP header, the TCP data, and the pseudoheader, which is made up of the source address, destination address, and length fields from the IP header.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

26. Sliding Window Revisited
TCP’s variant of the sliding window algorithm, which serves several purposes:
(1) it guarantees the reliable delivery of data,
(2) it ensures that data is delivered in order, and
(3) it enforces flow control between the sender and the receiver.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

27. Sliding Window Revisited
Relationship between TCP send buffer (a) and receive buffer (b).
Sending Side
LastByteAcked ≤ LastByteSent
LastByteSent ≤ LastByteWritten
Receiving Side
LastByteRead < NextByteExpected
NextByteExpected ≤ LastByteRcvd + 1
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

28. TCP: Flow Control What is Overflow?
Receiver: Set a side a receive buffer for the connection
TCP connection receives bytes, places in the buffer
Application process will read data from buffer, but not necessarily at the instant data arrives
It might be busy with some other task and might not even attempt to read data
Therefore, the sender can very easily overflow the receive buffer by sending to much data.
Flow-control service is needed.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

29. Flow Control - Receiver
Receiver advertises window no larger than its buffer size
LastByteRcvd – LastByteRead  MaxRcvBuffer
AdvertisedWindow = MaxRcvBuffer – ((NextByteExpected – 1) – LastByteRead)? or
AdvertisedWindow = MaxRcvBuffer – ((LastByteRcvd) – LastByteRead)?
Example: LastByteRcvd = 5, LastByteRead = 1, NextByteExpected = 6, MaxRcvBuffer = 10
5 – 1  10
AdvertisedWindow = ((6 – 1) – 1) = 10 – 4 = 6
Too busy: go to 0
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

30. Flow Control - Sender Sender adheres to the AdvertisedWindow by
LastByteSent – LastByteAcked  AdvertisedWindow
EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked)
Example: sent 7, ACK 5, EffectiveWindow = 6 – (7 - 5) = 4
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

31. Flow Control
What if AdvertisedWindow is 0, i.e., when the receive buffer is full?
A well-recognized protocol design rule to make the receive side as simple as possible: Smart Sender/Dumb Receiver
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

32. Connection Establishment/Termination in TCP (extra slide)
Timeline for three-way handshake algorithm
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527