Data Communication Networks

Transport Layer

M. R. Pakravan
Department of Electrical Engineering
Sharif University of Technology

Data Networks 1

Transport Layer
 Tasks:
 Provide reliable and cost effective end-to-end communication service to the
application layer
 Independent of used network (shielding)
 Boundary between network and applications
 TPDU (transport protocol data unit) denotes message sent from entity to entity
 Can be both connection oriented and connectionless service

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Transport Service
 Transport Service and Data Link Services:
 Similarities:
 Both provide point-to-point connection
 Both have to deal with error control, sequencing, flow control, retransmission, etc.
 Differences:
 Transport connection is indirect
 Network has memory: packets may be stored and and arrive with varying delays
and out of order
 Many connections
have to be managed
(instead of a fixed
number of links)

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Transport Service
 Primitives for a simple transport service:
 LISTEN: block until some process connects
 CONNECT
 SEND
 RECEIVE: block until data TPDU arrives
 DISCONNECT

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Elements of transport protocols
 Addressing: TSAP (transport  TCP addressing:
service access point);  (IP address, port number)
 How do I know the TSAP of the  Example ports of well knows services:
destination?  port 7: Echo
 Using well known addresses (works  port 23: Telnet
only for (stable) key services)
 port 25: SMTP (email)
 Use name server (or directory  port 80: HTTP (www)
server)
 port 110: POP (reading remote email)
 Connect to this server
 Send message and ask for TSAP
address of needed server
 Set up a connection with needed
server.
 New services have to register
with name server

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Transmission Control Protocol (TCP)

 TCP Provides a logical full duplex connection between two application layer
processes across an unreliable datagram network (IP network)
 TCP Provides flow control using Selective Repeat
 TCP Can support multiple connections at the same time
 Each direction of the connection can be terminated independently
 TCP Service is:
 Connection oriented
 Reliable Application Application
 In-sequence
byte stream byte stream
 Byte stream

segments

Transmitter Receiver
Send buffer Receive buffer

Data Networks ACKs 6

TCP Service Model
 TCP connection is :
 Full-duplex, point-to-point (No support for multicast or broadcast)
 Byte stream : message boundaries are not preserved (Application layer
should parse and detect messages)
 Both sender and receiver have to create sockets
 socket # = IP address + Port# (16-bit, = TSAP)
 Connection is identified by: (socket 1, socket 2)
 Port# < 1024 : well-known ports
 FTP: 21, Telnet:23, SMTP: 25, HTTP: 80
 TCP may buffer at both sides; consequently transmission may be
delayed
 TCP should handle lost, out of order and delayed segments with
non-aligned boundaries.

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TCP Service Model

 Every byte has 32-bits sequence number
 On 10 Mbps it takes about an hour to wrap around
 Separate 32 bit sequence numbers are used for acknowledgement and
for the window mechanism
 Data exchange in segments
 Segment contains 20 byte header, options, + data
 Variable payload size (the size is decided by the TCP software and is
usually 1500 bytes)
 Must fit into MTU: maximum transfer unit size of a network
 Sliding window protocol with timeout
 Receiver sends back ack# equal to next expected segment#
 Receiver uses piggybacking
 Sender retransmits if timeout occurs

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 TCP Congestion Control
 TCP controls the data flow between two endpoints of a connection
 Selective repeat sliding window is used for flow control
 Receiver specifies its reception capacity called “Advertised Window”
(WA) in the header of the messages that it sends back to transmitter.
This handles the end-to-end flow control
 Network congestion can also be controlled by slowing down the flow
of data that TCP generates into the network. This requires feedback
from the network behavior.
 The maximum amount of bytes that a TCP sender can transmit
without congesting the network is called “Congestion Window” (WC)
 TCP works by adapting these two windows to control the traffic
generation process.

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TCP Operation
Transmitter Receiver
Send Window
Receive Window
Slast+Wa-1 Rlast Rlast+WR-1

... ... ...

Octets Rnext Rnew Octets

transmitted Slast Srecent Slast+Ws-1 Received
and ACKed

Slast oldest unacknowledged octet Rlast highest-numbered octet not yet

Srecent highest-numbered transmitted octet read by the application
Slast+Wa-1 highest-numbered octet that Rnext next expected octet
can be transmitted Rnew highest numbered octet received
Slast+Ws-1 highest-numbered octet that correctly
can be accepted from the application Rlast+WR-1 highest-numbered octet that
can be accommodated in receive buffer

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TCP Operation
 Rnew can be greater than Rnext because the receiver accepts out of
sequence bytes
 Receiver buffer has a size of WR
 Rlast+WR-1 is the maximum numbered byte the receiver can
accept
 Advertised window: capacity of buffer available at the receiver:
WA=WR-(Rnew-Rlast)
 Transmitter should make sure that the number of outstanding
bytes is less than the advertised window size:
Srecent-Slast≤WA
 TCP separates acknowledgment from flow control.
 Advertised window is used to effectively control the traffic flow. It is
contained in the header of the frames going back to the sender.

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TCP Operation
 String of bytes from application layer are grouped together to form a
“segment”
 A 32 bit sequence number is assigned to each byte of the segment.
 Initial seq# in each direction is chosen randomly and negotiated.
 Timer management:
 Round trip delay of each segment (Transmission to ack) is measured.
 Appropriate time is chosen based on history and variation of this delay
 Typical moving average parameters: =7/8 and =1/4

t RTT (new)  t RTT (old )  (1   ) n

d RTT (new)  d RTT (new)  (1   ) |  n  t RTT |
tout  t RTT  4d RTT

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Congestion Control in TCP
 Sender can transmit up to the minimum of “Advertised Window” (WA) and
“Congestion Window” (WC). This is called Current Window
 TCP transmitter controls WC dynamically. Steps are:
 Slow Start: At the start, WC =1
 After each successful ACK from receiver, WC = WC *2
 Congestion Avoidance: If WC =congestion threshold (66,535 bytes), increase WC
linearly with each successful ACK.
 Congestion Reaction: When there
is congestion (Time out on ACK)
set congestion threshold to the be
WCU/2 and set WC =1
 Assumption is that ACK loss is due
to congestion.
 This is valid for wired networks, but
not for wireless networks where PHY
is not as reliable.

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TCP Frame
 Source Port, Destination Port:
<IP+TCP Port> uniquely identifies a process on each end.
 Seq_no : Position of the first byte of this segment in the sender’s
byte stream (Can be different in each direction)
 ACK_no : Sequence number of the next data byte that the sender
expects to receive if the ACK bit is set
 Window Size: The number of bytes the receiver is willing to accept
 Urgent Pointer: The value of this field added to that in the Seq No.
Field points to the last byte of URGENT DATA that needs
immediate delivery
 Data Offset:
Length of TCP header

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TCP Frame
 PSH: if set to 1, receiver should pass the data to application
immediately without buffering
 ACK: ACK # is valid if ACK=1
 URG: Urgent # is valid if URG=1
 RST: if set to 1, reset connection
 FIN: If set to one, the sender has no more byte to send (Can still
receive). Connection is terminated if the other end sends a FIN
 SYN: Indicate a connection request or connection acknowledge
during the startup procedure
 Connection Request:
[SYN,ACK]=[1,0]
 Connectione Accepted:
[SYN,ACK]=[1,1]

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TCP Checksum

 TCP checksum calculation notes:

 If segment length is not a multiple of 16, it is padded with zero to
make it a multiple of 16 bits.
 A pseudoheader is created by using the following format and is
included in the CRC calculation to make sure that the segment is
delivered to the right IP address

0 8 16 31

Source IP Address

Destination IP Address

00000000 Protocol = 6 TCP Segment Length

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TCP Connection Setup
 Host B is waiting for incoming
connection
 Host A: Sends a request,
SYN=1, Seq_no=x
 Host B: Replies by setting
ACK=1, ACK_no=1+x, SYN=1,
Seq_no=y
 Host A: Acknowledges by
setting the ACK=1,
ACK_no=1+y, Seq_no=1+x
 If during a connection
establishment phase, one of
the hosts decides to refuse a
connection, it will send a
RESET segment by setting the
RST bit.

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User Datagram Protocol (UDP)

 UDP in an unreliable, connectionless transport protocol
 UDP Services:
 Routes the received packet to the desired application on the host
(Destination Port)
 Checks the integrity of the datagram. This is optional! (UDP Checksum)
 If a host does not wish to calculate the checksum, it sets it to all 0’s.
 Applications that use UDP: Domain Name Services (DNS), Simple
Network Management Protocol (SNMP), Real Time Protocol (RTP)
 UDP checksum calculation: Similar to TCP (pad to 16, psuedo-
header for IP verification)

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Real Time Transport Protocol (RTP)

 A generic protocol for real time applications such as voice and video
 Uses UDP and acts as an interface between user application and transport
protocol (Mostly UDP)
 Header specifies the profile and encoding format of the payload (single
audio stream, mp3)
 Packets are numbered to allow detection of missing packets
 Time stamping is used to allow synchronization and jitter compensation
 No flow control, no error control, no acknowledgement and no repeat-
requests
 Services:
 Payload type identification
 Sequence numbering
 Time stamping
 Delivery monitoring

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Real Time Transport Protocol (RTP)

 P bit: the packet has been padded to a multiple of 4-bytes. The last padding
byte tells how many bytes were padded
 X bit: an extension header is present
 CC field: How many contributing sources of data are present
 M bit: Marker bit used by the application
 Time stamp of the first sample of the payload content is noted in the header:
 Appropriate time alignment of the samples in the receiver for smooth playback
and jitter reduction
 Synchronization between multiple streams such as video and audio in a
videoconference
 Synch Source ID: Tells which
stream this packet belongs to.

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Real Time Transport Control Protocol

 RTP is used in conjunction with the RTP Control Protocol (RTCP).

 RTP carries the media streams (e.g., audio and video) and RTCP is
used to monitor transmission statistics and quality of service (QoS)
and aids synchronization of multiple streams.
 When both protocols are used in conjunction, RTP is originated and
received on even port numbers and the associated RTCP
communication uses the next higher odd port number.
 RTCP feedback can let the source know about delay, jitter,
bandwidth, congestion, etc.
 This data can be used by the source to adjust encoding or transport
properties of the stream
 Different streams can use different clocks with different granularities
and different drift rates.

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