ECE 4450:427/527 - Computer Networks
Spring 2016
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
Computer Networks
1
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-toend protocols
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
2
Transport Layer vs. Network Layer
• 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)
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 inhouse siblings
• network-layer protocol =
postal service
ECE 4450:427/527
Computer Networks
3
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
Computer Networks
4
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
Computer Networks
5
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
Computer Networks
6
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
Computer Networks
7
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
Computer Networks
8
Port Discovery
• Use well-publicized ports for different services
– DNS uses port 53
– Email 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
Computer Networks
9
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
Computer Networks
10
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
Computer Networks
11
Example
P1
P4
P5
P2
P6
P1P3
SP: 5775
DP: 80
S-IP: B
D-IP:C
SP: 9157
client
IP: A
DP: 80
S-IP: A
D-IP:C
SP: 9157
server
IP: C
DP: 80
S-IP: B
D-IP:C
Client
IP:B
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
Computer Networks
12
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
Computer Networks
13
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
Computer Networks
14
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
Computer Networks
15
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
Computer Networks
16
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
Computer Networks
17
TCP Segment
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
Computer Networks
18
TCP Header
TCP Header Format
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
19
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
Computer Networks
20
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
Computer Networks
21
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
Computer Networks
22
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
Computer Networks
23
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
Computer Networks
24
Sliding Window Revisited
Relationship between TCP send buffer (a) and receive buffer (b).
Sending Side
– LastByteAcked ≤ LastByteSent
– LastByteSent ≤ LastByteWritten
Dr. Nghi Tran (ECE-University of Akron)
Receiving Side
– LastByteRead < NextByteExpected
– NextByteExpected ≤ LastByteRcvd + 1
ECE 4450:427/527
Computer Networks
25
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
Computer Networks
26
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 = 10 - ((6 – 1) – 1) = 10 – 4 = 6
– Too busy: go to 0
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
27
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
Computer Networks
28
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
Computer Networks
29
Connection Establishment/Termination in
TCP (extra slide)
Timeline for three-way handshake algorithm
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
30