transport layer protocols

PART V: TRANSPORT LAYER
TRANSPORT-LAYER PROTOCOLS
Subject: Data Communications and Networking
Tutor: Bilal Munir Mughal
1
Ch-24

Content Outline
 INTRODUCTION
 Services
 Port Numbers
 USER DATAGRAM PROTOCOL
 User Datagram
 UDP Services
 UDP Applications
2

Content Outline
 TRANSMISSION CONTROL PROTOCOL
 TCP Services
 TCP Features
 Segment
 A TCP Connection
 State Transition Diagram
 Windows in TCP
 Flow Control
 Error Control
 TCP Congestion Control
 TCP Timers
 Options
3

Content Outline
 SCTP
 SCTP Services
 SCTP Features
 Packet Format
 An SCTP Association
 Flow Control
 Error Control
4

INTRODUCTION
 The transport layer in the TCP/IP suite is located
between the application layer and the network
layer.
 It provides services to the application layer and
receives services from the network layer.
 The transport layer acts as a liaison between a
client program and a server program, a process-
to-process connection.
 The transport layer is the heart of the TCP/IP
protocol suite; it is the end-to-end logical vehicle
for transferring data from one point to another in
5

SERVICES
 UDP(UserDatagramProtocol)
 UDP is an unreliable connectionless transport-layer
protocol used for its simplicity and efficiency in
applications where error control can be provided by
the application-layer process.
 TCP(TransmissionControlProtocol)
 TCP is a reliable connection-oriented protocol that
can be used in any application where reliability is
important.
 SCTP(StreamControlTransmissionProtocol)
 SCTP is a new transport-layer protocol that
combines the features of UDP and TCP.
7

PORT NUMBERS
 Port numbers provide end-to-end addresses at the
transport layer and allow multiplexing and demultiplexing
at this layer.
8

USER DATAGRAM PROTOCOL
 Example 24.1
 The following is the content of a UDP header in
hexadecimal format.
CB84000D001C001C
a. What is the source port number?
b. What is the destination port number?
c. What is the total length of the user datagram?
d. What is the length of the data?
e. Is the packet directed from a client to a server or
vice versa?
f. What is the client process?
10

USER DATAGRAM PROTOCOL
 Solution
a. The source port number is the first four hexadecimal
digits (CB84)16, which means that the source port
number is 52100.
b. The destination port number is the second four
hexadecimal digits (000D)16, which means that the
destination port number is 13.
c. The third four hexadecimal digits (001C)16 define the
length of the whole UDP packet as 28 bytes.
d. The length of the data is the length of the whole packet
minus the length of the header, or 28 − 8 = 20 bytes.
e. Since the destination port number is 13 (well-known
port), the packet is from the client to the server.
11

USER DATAGRAM PROTOCOL:
UDPSERVICES
 Process-to-Process Communication
 Connectionless Services
 No Flow Control
 No Error Control except for the checksum
 Checksum
 No Congestion Control
 Encapsulation and Decapsulation
 Queuing
 Multiplexing and Demultiplexing
12

UDPSERVICES
 Checksum
 UDP checksum calculation includes three sections:
a pseudoheader, the UDPheader, and the data
coming from the application layer.
 The pse udo he ade r is the part of the header of the
IP packet (discussed in Chapter 19) in which the
user datagram is to be encapsulated with some
fields filled with 0s.
 If the checksum does not include the
pseudoheader, a user datagram may arrive safe
and sound. However, if the IP header is corrupted, it
may be delivered to the wrong host.
13

UDPSERVICES
 The protocol field is added to ensure that the packet
belongs to UDP, and not to TCP.
 The value of the protocol field for UDP is 17. If this
value is changed during transmission, the
checksum calculation at the receiver will detect it
and UDP drops the packet. It is not delivered to the
wrong protocol.
14

UDPSERVICES
 OptionalInclusionof Checksum
 The sender of a UDP packet can choose not to
calculate the checksum. In this case, the
checksum field is filled with all 0s before being
sent.
 In the situation where the sender decides to
calculate the checksum, but it happens that the
result is all 0s, the checksum is changed to all
1s before the packet is sent.
15

UDPAPPLICATIONS
 UDP is suitable for a process that requires
simple request-response communication with
little concern for flow and error control. It is not
usually used for a process such as FTPthat
needs to send bulk data (see Chapter 26).
 UDP is suitable for a process with internal flow-
and error-control mechanisms. For example, the
Trivial File TransferProtocol (TFTP) process
includes flow and error control. It can easily use
UDP.
16

UDPAPPLICATIONS
 UDP is a suitable transport protocol for
multicasting. Multicasting capability is
embedded in the UDP software but not in the
TCP software.
 UDP is used for management processes such
as SNMP(see Chapter 27).
 UDP is used for some route updating protocols
such as Routing Information Protocol (RIP)
(see Chapter 20).
 UDP is normally used for interactive real-time
applications that cannot tolerate uneven delay
17

TRANSMISSION CONTROL PROTOCOL
 Transmission Control Protocol (TCP) is a
connection-oriented, reliable protocol.
 TCP explicitly defines connection establishment,
data transfer, and connection teardown phases
to provide a connection-oriented service.
 TCP uses a combination of GBN and SR
protocols to provide reliability.
 To achieve this goal, TCP uses checksum (for
error detection), retransmission of lost or
corrupted packets, cumulative and selective
acknowledgments, and timers.
18

TCPSERVICES
 StreamDelivery Service
 Full-Duplex Communication
 Multiplexing and Demultiplexing
 Connection-Oriented Service
 Reliable Service
19

TCP SERVICES
 StreamDelivery Service
 TCP, allows the sending process to deliver data
as a stream of bytes and allows the receiving
process to obtain data as a stream of bytes.
20

TCP SERVICES
 StreamDelivery Service…
 Sending and Receiving Buffers
21

TCP SERVICES
 StreamDelivery Service…
 Segments
22

TCPFEATURES
 Numbering System
 Byte Number
 TCP numbers all data bytes (octets) that are transmitted in a
connection. Numbering is independent in each direction.
 The numbering does not necessarily start from 0. Instead,
TCP chooses an arbitrary number between 0 and 232
− 1 for
the number of the first byte.
 Sequence Number
 TCP assigns a sequence number to each segment that is
being sent. The sequence number, in each direction, is
defined as follows:
1.The sequence number of the first segment is the ISN
(initial sequence number), which is a random number.
2.The sequence number of any other segment is the
sequence number of the previous segment plus the
23

TCP FEATURES
 Numbering System…
 Acknowledgment Number
 The value of the acknowledgment field in a segment
defines the number of the next byte a party expects to
receive.
 The acknowledgment number is cumulative. which
means that the party takes the number of the last byte
that it has received, safe and sound, adds 1 to it, and
announces this sum as the acknowledgment number.
24

TCPSEGMENT
25

TCP SEGMENT
26

TCPCONNECTION
 In TCP, connection-oriented transmission
requires three phases:
connection establishment,
data transfer,
and connection termination.
27

TCP CONNECTION
 Connection Establishment
28

TCP CONNECTION
 Data Transfer
29

TCP CONNECTION
 Connection Termination
30

TCP CONNECTION
 Connection Termination Half-Close
31

TCP CONNECTION
 Connection Reset
 TCP at one end may deny a connection request,
may abort an existing connection, or may
terminate an idle connection.
 All of these are done with the RST (reset) flag.
32

STATE TRANSITION DIAGRAM
33

34

35

WINDOWS IN TCP
36

FLOWCONTROL
37

FLOW CONTROL
38

ERRORCONTROL
 Error control in TCP is achieved through the use
of three simple tools:
 Checksum
 Acknowledgment
 Cumulative Acknowledgment (ACK)
 Selective Acknowledgment (SACK)
 Retransmission
 Retransmission after RTO(Retransmission time-out)
 Retransmission after Three Duplicate ACK Segments
39

ERROR CONTROL
 TCP implementations today do not discard out-
of-ordersegments.
 They store them temporarily and flag them as
out-of-order segments until the missing
segments arrive.
40

ERROR CONTROL
41

ERROR CONTROL
 Some TCPOperation Scenarios
42

ERROR CONTROL
 Some TCPOperation Scenarios
43

CONGESTION CONTROL
 TCP uses different policies to handle the
congestion in the network.
 Congestion Window
 The TCP sender uses the occurrence of two events
as signs of congestion in the network: time-out and
receiving three duplicate ACKs.
 The lack of regular, timely receipt of ACKs, which
results in a time-out, is the sign of a strong
congestion; the receiving of three duplicate ACKs is
the sign of a weak congestion in the network.
44

CONGESTION CONTROL
 Congestion Policies
 Slow Start: Exponential Increase algorithm
 Congestion Avoidance: Additive Increase algorithm
 Fast Recovery algorithm
45

CONGESTION CONTROL
 Congestion Policies…
 The fast-recovery algorithm is optional in TCP.
 It starts when three duplicate ACKs arrive, which is
interpreted as light congestion in the network.
 Like congestion avoidance, this algorithm is also an
additive increase, but it increases the size of the
congestion window when a duplicate ACK arrives
(after the three duplicate ACKs that trigger the use
of this algorithm)
46

CONGESTION CONTROL
 Three versions of TCP:
 Taho TCPuses only slow start and congestion
avoidance
 Reno TCPadded fast-recovery state
 New Reno TCPadded three duplicate ACKs arrive
47

CONGESTION CONTROL
48

TCPTIMERS
 To perform their operations smoothly, most TCP
implementations use at least fourtimers:
 Retransmission Timer
 Persistence Timer
 Keepalive Timer
 TIME-WAIT Timer
49

TCP TIMERS
 Retransmission Timer
 To retransmit lost segments, TCP employs one
retransmission timer (for the whole connection period) that
handles the retransmission time-out (RTO), the waiting
time for an acknowledgment of a segment. We can define
the following rules for the retransmission timer:
1. When TCP sends the segment in front of the sending queue, it
starts the timer.
2. When the timer expires, TCP resends the first segment in front
of the queue, and restarts the timer.
3. When a segment or segments are cumulatively acknowledged,
the segment or segments are purged from the queue.
4. If the queue is empty, TCP stops the timer; otherwise, TCP
restarts the timer.
50

TCP TIMERS
 Retransmission Timer…
 Round-Trip Time (RTT)
 Measured RTT.
 We need to find how long it takes to send a segment and receive an
acknowledgment for it. This is the measured RTT
 In TCP, there can be only one RTT measurement in progress at any time.
 We use the notation RTTM to stand for measured RTT.
 Smoothed RTT.
 The measured RTT, RTTM, is likely to change for each round trip.
 The fluctuation is so high in today’s Internet that a single measurement
alone cannot be used for retransmission time-out purposes.
 Most implementations use a smoothed RTT, called RTTS, which is a
weighted average of RTTM and the previous RTTS as shown below:
 The value of α is implementation-dependent, but it is normally set to 1/8.
In other words, the new RTTS is calculated as 7/8 of the old RTTS and 1/8
51

TCP TIMERS
 Round-Trip Time (RTT)…
 RTT Deviation.
 Most implementations do not just use RTTS; they also calculate the
 RTT deviation, called RTTD, based on the RTTS and RTTM, using the
following formulas.
 (The value of β is also implementation-dependent, but is usually set to
1/4.)
52

TCP TIMERS
 Retransmission Time-out (RTO)
 The value of RTO is based on the smoothed roundtrip time and its deviation.
 Most implementations use the following formula to calculate the RTO:
 In other words, take the running smoothed average value of RTTS and add four
times the running smoothed average value of RTTD (normally a small value).
 Karn’s Algorithm
 Do not consider the round-trip time of a retransmitted segment in the
calculation of RTTs.
 Do not update the value of RTTs until you send a segment and receive an
acknowledgment without the need for retransmission.
53

TCP TIMERS
 Retransmission Time-out (RTO)
 Exponential Backoff
 What is the value of RTO if a retransmission occurs?
 Most TCP implementations use an exponential backoff strategy.
 The value of RTO is doubled for each retransmission.
 So if the segment is retransmitted once, the value is two times the RTO.
 If it is retransmitted twice, the value is four times the RTO, and so on.
54

TCP TIMERS
 Persistence Timer
 To correct the deadlock, TCP uses a persistence timer for
each connection. When the sending TCP receives an
acknowledgment with a window size of zero, it starts a
persistence timer.
 When the persistence timer goes off, the sending TCP
sends a special segment called a probe. This segment
contains only 1 byte of new data.
 It has a sequence number, but its sequence number is
never acknowledged; it is even ignored in calculating the
sequence number for the rest of the data.
 The probe causes the receiving TCP to resend the
acknowledgment.
55

TCP TIMERS
 Persistence Timer…
 The value of the persistence timer is set to the value of the
retransmission time.
 However, if a response is not received from the receiver,
another probe segment is sent and the value of the
persistence timer is doubled and reset.
 The sender continues sending the probe segments and
doubling and resetting the value of the persistence timer
until the value reaches a threshold (usually 60 s).
 After that the sender sends one probe segment every 60
seconds until the window is reopened.
56

TCP TIMERS
 Keepalive Timer
 A keepalive timer is used in some implementations to
prevent a long idle connection between two TCPs e.g. a
aerver and a client.
 Each time the server hears from a client, it resets this
timer.
 The time-out is usually 2 hours. If the server does not hear
from the client after 2 hours, it sends a probe segment.
 If there is no response after 10 probes, each of which is 75
seconds apart, it assumes that the client is down and
terminates the connection.
57

TCP TIMERS
 TIME-WAIT Timer
 The TIME-WAIT (2MSL) timer is used during connection
termination. The maximumsegment lifetime (MSL) is the
amount of time any segment can exist in a network before
being discarded.
 The implementation needs to choose a value for MSL.
Common values are 30 seconds, 1 minute, or even 2
minutes.
 The 2MSL timer is used when TCP performs an active
close and sends the final ACK.
 The connection must stay open for 2 MSL amount of time
to allow TCP to resend the final ACK in case the ACK is
lost.
 This requires that the RTO timer at the other end times out
58

OPTIONS
 The TCP header can have up to 40 bytes of
optional information.
 Options convey additional information to the
destination or align other options.
 These options are included on the book website
for further reference.
59

STREAMCONTROL TRANSMISSION
PROTOCOL
 StreamControl Transmission Protocol (SCTP)
is a new transport-layer protocol designed to
combine some features of UDP and TCP in an
effort to create a better protocol for multimedia
communication.
60

STREAM CONTROL TRANSMISSION
PROTOCOL:
SERVICES
 Multiple Streams
 Multihoming
 Full-Duplex Communication
 Connection-Oriented Service
 Reliable Service
61

PROTOCOL:
SERVICES
 Multiple Streams
 SCTP allows multistreamservice in each
connection, which is called associationin SCTP
terminology.
 If one of the streams is blocked, the other streams
can still deliver their data.
62

PROTOCOL:
SERVICES
 Multihoming
 In TCP connection a multihomed host (connected to
more than one physical address with multiple IP
addresses), only one of these IP addresses per end
can be utilized during the connection.
 An SCTP association, supports multihoming
service. The sending and receiving host can define
multiple IP addresses in each end for an
association.
63

PROTOCOL:
FEATURES
 Transmission Sequence Number(TSN)
 TSNs are 32 bits long and randomly initialized between 0
and 232 − 1. Each data chunk must carry the
corresponding TSN in its header.
 StreamIdentifier(SI)
 Each stream in SCTP needs to be identified using a
stream identifier (SI). Each data chunk must carry the SI
in its header so that when it arrives at the destination, it
can be properly placed in its stream. The SI is a 16-bit
number starting from 0.
 StreamSequence Number(SSN)
 SCTP defines each data chunk in each stream with a
stream sequence number (SSN).
64

PROTOCOL:
FEATURES
 Acknowledgment Number
 SCTP acknowledgment numbers are chunk-oriented.
They refer to the TSN.
 Packets
65

PROTOCOL:
FEATURES
 Packets…
 In SCTP, we have data chunks, streams, and packets.
66

PROTOCOL:
PACKET FORMAT
 General Header
67

PROTOCOL:
PACKET FORMAT
 Chunks
 Types of Chunks
68

PROTOCOL:
AN SCTPASSOCIATION
 A connection in SCTP is called an association to
emphasize multihoming.
 Association establishment in SCTP requires a
four-way handshake.
69

PROTOCOL:
AN SCTP ASSOCIATION
 Data Transfer
 SCTP recognizes and maintains boundaries.
 Each message coming from the process is treated
as one unit and inserted into a DATA chunk unless it
is fragmented.
 In SCTP data chunks are related to each other.
70

PROTOCOL:
AN SCTP ASSOCIATION
 Data Transfer…
 Multihoming Data Transfer
 Multihoming allows both ends to define multiple IP
addresses for communication. However, only one of
these addresses can be defined as the primary address;
the rest are alternative addresses.
 Data transfer, by default, uses the primary address of the
destination. If the primary is not available, one of the
alternative addresses is used.
 The process, however, can always override the primary
address and explicitly request that a message be sent to
one of the alternative addresses.
 A process can also explicitly change the primary address
of the current association.
71

PROTOCOL:
AN SCTP ASSOCIATION
 MultistreamDelivery
 SCTP differentiates between data transferand data
delivery.
 SCTP uses TSN numbers to handle data transfer,
movement of data chunks between the source and
destination.
 The delivery of the data chunks is controlled by stream
identifier(SI) and stream sequence numbers (SSN).
 SCTP can support multiple streams, a message can
belong to one of these streams. Each stream is assigned
a unique stream identifier (SI).
 SCTP supports two types of data delivery in each
stream: ordered (default) and unordered. In ordered data
72

PROTOCOL:
AN SCTP ASSOCIATION
 Fragmentation
 Fragmentation in IP and SCTP belong to different levels:
the former at the network layer, the latter at the transport
layer.
 SCTP preserves the boundaries of the message from
process to process when creating a DATA chunk from a
message if the size of the message does not exceed the
MTU of the path.
 If the total size exceeds the MTU, the message needs to
be fragmented. (For more details about fragmentation,
see the Extra Materials section.)
73

PROTOCOL:
AN SCTP ASSOCIATION
 Association Termination
 Unlike TCP, SCTP does not allow a “halfclosed”
association.
 If one end closes the association, the other end must
stop sending new data.
 If any data are left over in the queue of the recipient of
the termination request, they are sent and the
association is closed.
74

PROTOCOL:
FLOWCONTROL
 Flo w co ntro lin SCTP is similar to that in TCP.
 In TCP, we need to deal with only one unit of
data, the byte. In SCTP, we need to handle two
units of data, the byte and the chunk.
 The values of rwnd and cwnd are expressed in
bytes; the values of TSN and acknowledgments
are expressed in chunks.
75

PROTOCOL:
FLOW CONTROL
 ReceiverSite
 The receiver has one buffer (queue) and three
variables.
 first variable holds the last TSN received, cumTSN
 second variable holds the available buffer size, winSize
 third variable holds the last cumulative acknowledgment,
lastACK
76

PROTOCOL:
FLOW CONTROL
 SenderSite
 The sender has one buffer (queue) and three
variables (Figure 24.48). We assume each chunk is
100 bytes long.
 first variable, curTSN, refers to the next chunk to be sent
 second variable, rwnd, holds the last value advertised by
the receiver (in bytes)
 third variable, inTransit, holds the number of bytes in
transit,
77

PROTOCOL:
ERRORCONTROL
 SCTP, like TCP, is a reliable transport-layer
protocol.
 It uses a SACK chunk to report the state of the
receiver buffer to the sender.
 Each implementation uses a different set of
entities and timers for the receiver and sender
sites.
 Here, a very simple design is used to convey
the concept to the reader.
78

PROTOCOL:
ERROR CONTROL
 ReceiverSite
79

PROTOCOL:
ERROR CONTROL
 SenderSite
 At the sender site, our design demands two buffers
(queues): a sending queue and a retransmission
queue.
 We also use three variables: rwnd, inTransit, and
curTSN, as described in the previous section.
80

PROTOCOL:
ERROR CONTROL
 SenderSite…
 To see how the state of the sender changes,
assume that the SACK in Figure 24.49 arrives at
the sender site in Figure 24.50. Figure 24.51 shows
the new state.
81

PROTOCOL:
ERROR CONTROL
 Sending Data Chunks
 An end can send a data packet whenever there are
data chunks in the sending queue with a TSN
greater than or equal to curTSN or if there are data
chunks in the retransmission queue.
 The retransmission queue has priority.
 However, the total size of the data chunk or chunks
included in the packet must not exceed the (rwnd −
inTransit) value and the total size of the frame must
not exceed the MTU size, as we discussed in
previous sections.
82

PROTOCOL:
ERROR CONTROL
 Sending Data Chunks…
 To control a lost or discarded chunk, SCTP, like
TCP, employs two strategies: using retransmission
timers and receiving three SACKs with the same
missing chunks.
 Retransmission.
 SCTP uses a retransmission timer, which handles the
retransmission time, the waiting time for an
acknowledgment of a segment. The procedures for
calculating RTO and RTT in SCTP are the same as we
described for TCP. SCTP uses a measured RTT (RTTM),
a smoothed RTT (RTTS), and an RTT deviation (RTTD)
to calculate the RTO. SCTP also uses Karn’s algorithm
to avoid acknowledgment ambiguity. Note that if a host is
83

PROTOCOL:
ERROR CONTROL
 Sending Data Chunks…
 FourMissing Reports.
 Whenever a sender receives four SACKs whose gap
ACK information indicates one or more specific data
chunks are missing, the sender needs to consider those
chunks as lost and immediately move them to the
retransmission queue. This behavior is analogous to
“fast retransmission” in TCP.
84

PROTOCOL:
ERROR CONTROL
 Generating SACKChunks
 Another issue in error control is the generation of SACK
chunks. We summarize the rules as listed below.
1.When an end sends a DATA chunk to the other end, it must
include a SACK chunk advertising the receipt of
unacknowledged DATA chunks.
2.When an end receives a packet containing data, but has no
data to send, it needs to acknowledge the receipt of the
packet within a specified time (usually 500 ms).
3.An end must send at least one SACK for every other packet it
receives. This rule overrides the second rule.
4.When a packet arrives with out-of-order data chunks, the
receiver needs to immediately send a SACK chunk reporting
the situation to the sender.
5.When an end receives a packet with duplicate DATA chunks
and no new DATA chunks, the duplicate data chunks must be
85

PROTOCOL:
ERROR CONTROL
 Congestion Control
 SCTP, like TCP, is a transport-layer protocol with
packets subject to congestion in the network. The
SCTP designers have used the same strategies for
congestion control as those used in TCP.
86

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