Transcript transport layer

Chapter 3
Transport Layer
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Computer
Networking: A Top
Down Approach
6th edition
Jim Kurose, Keith Ross
Addison-Wesley
March 2012
Thanks and enjoy! JFK/KWR
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
Transport Layer 3-1
3.1 Introduction and Transport-Layer Services
: Transport services and protocols



provide logical communication
between app processes running on
different hosts (meaning as if the
hosts running the processes were
directly connected)
transport protocols run in end
systems
 send side: breaks app messages
into segments, passes to
network layer
 rcv side: reassembles segments
into messages, passes to app
layer
more than one transport protocol
available to apps
 Internet: TCP and UDP
application
transport
network
data link
physical
application
transport
network
data link
physical
Transport Layer 3-2
3.1 Introduction and Transport-Layer Services
: Transport vs. network layer


network layer: logical
communication
between hosts
transport layer:
logical
communication
between processes
 relies on, enhances,
network layer
services
household analogy:
12 kids in Ann’s house sending
letters to 12 kids in Bill’s
house:
 hosts = houses
 processes = kids
 app messages = letters in
envelopes
 transport protocol = Ann
and Bill who demux to inhouse siblings
 network-layer protocol =
postal service
Transport Layer 3-3
3.1 Introduction and Transport-Layer Services
: Internet transport-layer protocols

reliable, in-order
delivery (TCP)
 congestion control
 flow control
 connection setup

unreliable, unordered
delivery: UDP
 no-frills extension of
“best-effort” IP

services not available:
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
 delay guarantees
 bandwidth guarantees
Transport Layer 3-4
3.2 Multiplexing/Demultiplexing
multiplexing at sender:
handle data from multiple
sockets, add transport header
(later used for demultiplexing)
demultiplexing at receiver:
use header info to deliver
received segments to correct
socket
application
application
P1
P2
application
P3
transport
P4
transport
network
transport
network
link
network
physical
link
link
physical
socket
process
physical
Transport Layer 3-5
3.2 Multiplexing/Demultiplexing
: How demultiplexing works

host receives IP datagrams
 each datagram has source IP
address, destination IP
address
 each datagram carries one
transport-layer segment
 each segment has source,
destination port number

host uses IP addresses &
port numbers to direct
segment to appropriate
socket
32 bits
source port #
dest port #
other header fields
application
data
(payload)
TCP/UDP segment format
Transport Layer 3-6
3.2 Multiplexing/Demultiplexing
: Connectionless demultiplexing

recall: created socket has
host-local port #:

DatagramSocket mySocket1
= new DatagramSocket(12534);

when host receives UDP
segment:
 checks destination port #
in segment
 directs UDP segment to
socket with that port #
recall: when creating
datagram to send into
UDP socket, must specify
 destination IP address
 destination port #
IP datagrams with same
dest. port #, but different
source IP addresses
and/or source port
numbers will be directed
to same socket at dest
Transport Layer 3-7
3.2 Multiplexing/Demultiplexing
: Connectionless demux: example
DatagramSocket
mySocket2 = new
DatagramSocket
(9157);
DatagramSocket
serverSocket = new
DatagramSocket
(6428);
application
application
DatagramSocket
mySocket1 = new
DatagramSocket
(5775);
application
P1
P3
P4
transport
transport
transport
network
network
link
network
link
physical
link
physical
physical
source port: 6428
dest port: 9157
source port: 9157
dest port: 6428
source port: ?
dest port: ?
source port: ?
dest port: ?
Transport Layer 3-8
3.2 Multiplexing/Demultiplexing
: Connection-oriented demux

TCP socket identified
by 4-tuple:





source IP address
source port number
dest IP address
dest port number
demux: receiver uses
all four values to direct
segment to appropriate
socket

server host may support
many simultaneous TCP
sockets:
 each socket identified by
its own 4-tuple

web servers have
different sockets for
each connecting client
 non-persistent HTTP will
have different socket for
each request
Transport Layer 3-9
3.2 Multiplexing/Demultiplexing
: Connection-oriented demux: example
application
application
P4
P5
application
P6
P3
P3
P2
transport
network
network
link
network
link
physical
link
physical
host: IP
address A
transport
transport
server: IP
address B
source IP,port: B,80
dest IP,port: A,9157
source IP,port: A,9157
dest IP, port: B,80
three segments, all destined to IP address: B,
dest port: 80 are demultiplexed to different sockets
physical
source IP,port: C,5775
dest IP,port: B,80
host: IP
address C
source IP,port: C,9157
dest IP,port: B,80
Transport Layer 3-10
3.2 Multiplexing/Demultiplexing
: Connection-oriented demux: example
threaded server
application
application
P3
application
P4
P3
P2
transport
network
network
link
network
link
physical
link
physical
host: IP
address A
transport
transport
server: IP
address B
source IP,port: B,80
dest IP,port: A,9157
source IP,port: A,9157
dest IP, port: B,80
physical
source IP,port: C,5775
dest IP,port: B,80
host: IP
address C
source IP,port: C,9157
dest IP,port: B,80
Transport Layer 3-11
3.3 Connectionless Transport
: User Datagram Protocol [RFC 768]



“no frills,” “bare bones”
Internet transport
protocol
“best effort” service,
UDP segments may be:
 lost
 delivered out-of-order
to app
connectionless:
 no handshaking
between UDP sender,
receiver
 each UDP segment
handled independently
of others

UDP use:
 streaming multimedia
apps (loss tolerant, rate
sensitive)
 DNS
 SNMP

reliable transfer over
UDP:
 add reliability at
application layer
 application-specific error
recovery!
Transport Layer 3-12
3.3 Connectionless Transport
: UDP segment header
32 bits
source port #
dest port #
length
checksum
application
data
(payload)
length, in bytes of
UDP segment,
including header
why is there a UDP?



UDP segment format

no connection
establishment (which can
add delay)
simple: no connection
state at sender, receiver
small header size
no congestion control:
UDP can blast away as
fast as desired
Transport Layer 3-13
3.3 Connectionless Transport
: UDP checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment
sender:



treat segment contents,
including header fields,
as sequence of 16-bit
integers
checksum: take the 1’s
complement for the
addition of segment
contents
sender puts checksum
value into UDP
checksum field
receiver:


compute checksum of
received segment
check if computed
checksum equals checksum
field value:
 NO - error detected
 YES - no error detected.
But maybe errors
nonetheless? More later
….
Transport Layer 3-14
3.3 Connectionless Transport
: Internet checksum (example)
example: add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0
1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0
checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
(1’s complement)
Note: when adding numbers, a carryout from the most
significant bit needs to be added to the result
Transport Layer 3-15
3.5 Connection-Oriented Transport
: TCP Overview RFCs: 793,1122,1323, 2018, 2581

connection-oriented:
 handshaking (exchange of
control msgs) inits sender,
receiver state before data
exchange

point-to-point:
reliable, in-order byte
stream:
 no “message boundaries”

flow controlled:
 sender will not overwhelm
receiver
full duplex data:
 bi-directional data flow in same
connection
 MSS: maximum segment size



pipelined:
 TCP congestion and flow
control set window size
 one sender, one receiver
 multicasting (one sender to
many receivers ) is not
possible with TCP
Transport Layer 3-16
3.5 Connection-Oriented Transport
: how a TCP connection is established




The client application first informs the client transport layer that it wants to
establish a connection to process in the server
(ex) a Python client program: clientSocket.connect((serverName, serverPort))
The client first sends a special TCP segment; the server responds with a second
special TCP segment; and finally the client responds again with a third special
segment  three-way handshake (three segments between two hosts)
The client process passes a stream of data through the socket. Once the data
passes through the socket, the data is in the hands of TCP running in the client
TCP directs this data to the connection’s send buffer. From time to time, TCP
will grab chunks of data from the send buffer and pass the data to the network
layer. The maximum amount of data that can be grabbed and placed in a
segment is limited by the maximum segment size (MSS)
Transport Layer 3-17
3.5 Connection-Oriented Transport
: TCP segment structure
32 bits
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
source port #
dest port #
sequence number
acknowledgement number
head not
UAP R S F
len used
checksum
counting
by bytes
of data
(not segments!)
receive window
Urg data pointer
options (variable length)
# bytes
rcvr willing
to accept
application
data
(variable length)
Transport Layer 3-18
3.5 Connection-Oriented Transport
: TCP segment structure





The MSS limits the maximum size of a segment’s data fields. When TCP sends a
large file it typically breaks the file into chunks of size MSS. (note: data chunks
smaller than MSS are possible)
The 32-bit sequence number field and the 32-bit acknowledgement number field
are for implementing a reliable data transfer service
16-bit receive window field is for flow control (i.e. indicates the number of bytes
that a receiver is willing to accept)
The 4 -bit header length filed. The TCP header can be of variable length due to
the TCP options filed
The optional and variable-length options field is to negotiate the MSS or as a
window scaling factor. A time-stamping option is also defined.
Transport Layer 3-19
3.5 Connection-Oriented Transport
: TCP segment structure

The flag field contains bits:
 The ACK bit is used to indicate that the value carried in the
acknowledgement filed is valid
 The RST, SYN, and FIN bits are used for connection setup and tear down
 The PSH bit indicates that the receiver should pass the data to the upper
layer immediately
 The URG bit is used to indicate that there is data in this segment that the
sending-side upper-layer entity has marked as “urgent”
Transport Layer 3-20