Transport Layer

1
Transport Layer

• End-to-end protocol
• Ensures that data units are delivered
• error-free
• in sequence
• with no loses or duplications
• Enhances the QOS provided by the Network Layer
  Comments on Network RESET NRESET caused by
• internal congestion
• hardware problems
• software bugs
• Establishes a new connection
• Resynchronizes and continues

2
Transport Layer (cont.)
A
B
Application (or Session) Layer
5 4 3 1-2
Application (or Session) Layer
TSAP
Interface
Services provided to the session layer
TPDU
Transport Entity
Transport Entity
Transport protocol
Network layer services used by transport layer
NSAP
Network Layer
Network Layer
3
Transport Layer (cont.)

• QOS parameters are specified by the Transport
  users when a connection is requested.
• Transport Service Primitives
• Provided for both
• Connection-oriented service
• Connectionless service

4
Transport Layer Quality of Service Parameters

• Connection establishment delay
• Connection establishment failure probability
• Throughput
• Transit delay
• Residual error ratio
• Transfer failure probability
• Connection release delay
• Connection release failure probability
• Protection
• Priority
• Resilience

5
Transport Layer (cont.)
IMP
Physical Communication channel
(a) Environment of the data link layer
IMP
Host
Subnet
(b) Environment of the transport layer
6
Types of serviceoffered by the network layer
Network type
Description
A Flawless, error-free service with no
N-RESETS B Perfect packet delivery, but
with N-RESETS C Unreliable service with lost
and duplicated packets and possibly N-RESETS
7
TSAP, NSAP, and Connections
8
TSAP, NSAP, andConnections (cont.)
3. Process server creates time-of-day server
tells it where to listen
1. Process server listens on well-known TSAP
4. Time-of-day server
5. Process server tells user where to listen and
closes this connection.
6. User connects to the time-of-day server
2. User connects to Well-known TSAP
How a user process in Host A establishes a
connection with a time-of-day server
9
Seven States in Transport Entity

• Each connection maintained by the transport
  entity is always in one of seven states, as
  follows
• 1. Idle - Connection not established yet.
• 2. Passive Establishment Pending - CONNECT has
  been executed and CALL REQUEST sent.
• 3. Active Establishment Pending - A CALL REQUEST
  has arrived LISTEN has not been done.
• 4. Established - The connection has been
  established.
• 5. Passive Disconnect Pending - The user is
  waiting for permission to transmit a packet.
• 6. Active Disconnect Pending - A RECEIVE has been
  done.
• 7. Idle - A DISCONNECT has been done locally.

10
Connection Management Scheme
Connect primitive executed
Connection request TPDU received
Idle
Active Establishment Pending
Passive Establishment Pending
Established
Connection request TPDU received
Connect primitive executed
Active Disconnect Pending
Passive Disconnect Pending
Disconnect primitive executed
Disconnection request TPDU received
Idle
Disconnection request TPDU received
Disconnect primitive executed
Transition labeled in italics are caused by
packet arrivals The solid lines show the clients
state sequence. The dashed lines show the
servers state sequence.
11
Networking in UNIX(Berkeley Sockets)

• Berkeley Primitives implemented as a set of
  system CALLs, and allow application programs to
  access communication protocols via SOCKET
  concept.
• Note Socket OSI TSAP

12
The Principal Transport Service Calls in Berkeley
UNIX
Socket Create a TSAP of a given type Bind
Associate an ASCII name to a previously
created socket Listen Create a queue to
store incoming connection
requests Accept Remove a connection request
from the queue or wait for one Connect
Initiate a connection with a remote
socket Shutdown Shutdown Send Send a
message through a given socket Recv Receive
a message on a given socket Select Check a
set of sockets to see if any can be
read or written
13
Implementation of an API

• Sockets (Sockets Interface by Berkeley)
• System V UNIX (Sockets Interface by ATT)
• WINSOCK (Windows Sockets Interface by Microsoft)

14
Implementation of an API (cont.)
Application1
Application2
Applicationn
Application Programs
.......
DLL containing socket interface procedures
Socket API
TCP/IP functions
DLL containing TCP/IP software
Operating System Functions
I/O functions
The organization of the socket API and TCP/IP
code in a Dynamic Linked Library under Windows
95. One copy of a DLL is loaded into memory when
needed all applications share the copy
15
Implementation of an API (cont.)
Application1
Application2
Applicationn
Application Programs
.......
DLL containing socket interface procedures
Socket API
TCP/IP functions I/O functions
Operating Systems
The organization of the socket API and TCP/IP
code under Windows NT. Although code for TCP/IP
is part of the operating system, procedures for
the socket API are part of a DLL
16
Internet Transport Protocols

• TCP (connection-oriented) Designated to provide
  a reliable end-to-end byte stream over an
  unreliable internetwork.
• UDP (connectionless) - Just IP with a short
  header added.
• TCP - Designed to dynamically adapt to properties
  of the internetwork and to be robust in the face
  of many kind of failures.

17
Internet Transport Protocols (cont.)

• Each machine supporting TCP has a TCP transport
  entity (e.g., user process or part of the kernel
  that manages TCP streams and interfaces to the IP
  layer).
• A TCP entity accepts user data streams from local
  processes, breaks them up into pieces not
  exceeding 64K bytes and sends each piece as a
  separate IP datagram.
• When IP datagrams containing TCP data arrive at a
  machine, they are given to the TCP entity, which
  reconstructs the original byte streams

18
The TCP Service Model

• TCP service is obtained by having both the sender
  and receiver create end pts, called sockets.
• Each socket has a socket number (address),
  consisting of the IP address of the host and a
  16-bit number local to that host, called a port
  (TCP name for a TSAP)
• To obtain TCP service, a connection must be
  explicitly established between a socket on the
  sending machine and the receiving machine.

19
The TCP Service Model (cont.)

• All TCP connections are full duplex and
  point-to-point
• TCP does not support multicasting or broadcasting
• Push Flag - tells TCP not to delay the
  transmission
• Urgent Data - (e.g., Interactive user hits the
  DEL or CTRL-C key) The sending application
  puts some CTL information in the data stream and
  gives it to TCP, along with the urgent flag.

20
The TCP Protocol (overview)

• The sending and receiving TCP entities exchange
  data in the form of segments
• Basic protocol - sliding window when the sender
  transmits a segment, it also starts a timer, When
  the segment arrives at the destination, the
  receiving TCP entity sends back a segment (with
  data if any exists, otherwise without data)
  bearing an ACK.

21
The TCP TPDU Structure
Source Port
Destination Port
Sequence Number
Piggyback Acknowledgement
TCP Header
F I N
S Y N
R S T
E O M
A C K
U R G
TCP Header Length
Window
Urgent Pointer
Checksum
Options (0 or more 32 bit words)
Data
22
TCP Congestion Control

• Congestion Severe delay caused by an overload
  of datagrams at one or more router.
• Internet TCP algorithms assume that timeouts are
  caused by congestion.
• To avoid congestion, the TCP standard now
  recommends using two techniques
• Slow start
• Multiplicative decrease
• Note TCP must remember the size of the
  receivers window. A second limit, congestion
  window, must be maintained.
• Allowed_window
• min (receiver_advertisement,congestion_window)

23
Transmission rate adjustment
Transmission network
Internal congestion
Small-capacity receiver
Large-capacity receiver
(b)
(a)

• A fast network feeding a low-capacity receiver.
• A slow network feeding a high-capacity receiver.

24
Slow Start
44
40
36
32
28
Congestion window (kilobytes)
24
20
16
12
8
4
0
0
2
4
6
8
10
12
14
16
18
20
22
24
Transmission number
An example of the Internet congestion algorithm
25
Estimation of Congestion Window Size

• TCP assumes that most datagram loss comes from
  congestion and uses the following strategy
• MULTIPLICATIVE DECREASE CONGESTION AVOIDANCE
• Upon loss of segment, reduce the congestion
  window by half (down to a minimum of at least one
  segment). For those segments that remain in the
  allowed window, backoff the retransmission timer
  exponentially.
• (comment) If congestion is likely, TCP reduces
  the volume of traffic exponentially and the rate
  of retransmission exponentially.

26
TCP Recovery When Congestion Ends

• SLOW-START (ADDITIVE) RECOVERY
• Whenever starting traffic on a new connection or
  increasing traffic after a period of congestion,
  start the congestion window at the size of a
  single segment and increase the congestion window
  by one segment each time an Ack arrives.
• (comment) Slow start avoids swamping the
  internet with additional traffic immediately
  after congestion clears or when new connections
  suddenly start.

27
Congestion Avoidance Phase

• To avoid increasing the window size too quickly,
  TCP adds one additional restriction
• Once the congestion window reaches one half of
  its original size before congestion, TCP enters a
  congestion avoidance phase and slow down the rate
  of increment.
• During congestion avoidance, it increases
  congestion window by 1 only if all segments and
  the window have been acknowledged.

28
The TCP/IP Protocol Suite

• Hierarchy Versus Layering
• TCP/IP--the task of communications is broken up
  into modules or entities that may communicate
  with peer entities in another system. One entity
  within a system provides services to other
  entities and, in turn uses the services of other
  entities. Good software design practice dictates
  that these entities be arranged hierarchically.

29
TCP/IP Architecture

• Based on the view of a communication that
  involves three agents
• Process
• Hosts
• Networks
• Note Processes (fundamental entities that
  communicate), execute on hosts, which often
  support multiple simultaneous processes.
  Communication between processes take place across
  the networks to which the hosts are attached.

30
TCP/IP Architecture (cont.)

• Protocols are Organized into 4 Layers
• Network access layer
• Internet layer IP (MIL-STD-1977)
• Host-host layer TCP (MIL-STD-1978)
• Process/application layer FTP
  (MIL-STD-1980) SMTP (MIL-STD-1981) TELNET
  (MIL-STD-1982)

31
TCP/IP Architecture (cont.)

• Network access layer
• Contains those protocols that provide access to a
  communication network. Protocols at this layer
  are between a communication node and an attached
  host. A function of all these protocols is to
  route data between host attached to the same
  network. Other services may include flow
  control, error control and various QoS features.

32
TCP/IP Architecture (cont.)

• Internet layer
• Consists of procedures required to allow data to
  traverse multiple networks between hosts. Thus,
  it provides a routing function, and usually
  implemented within hosts and gateways.

33
TCP/IP Architecture (cont.)

• Host-to-host layer
• Contains protocol entities with the ability to
  deliver data between two processes on different
  host computers. A protocol entity at this level
  may or may not provide a logical connection
  between higher-level entities. Other possible
  services include error and flow control and the
  ability to deal with control signals not
  associated with a logical data connection.

34
TCP/IP Architecture (cont.)

• Process/Application Layer
• Contains protocols for resource sharing (e.g.,
  computer-to-computer) and remote access (e.g.,
  terminal-to-computer).

35
Application-level Internet Services

• E-mail
• File Transfer
• Remote Login

36
TCP/IP Internet Domain Names

• The mechanism that implements a machine name
  hierarchy for TCP/IP internets is called the
  Domain Name System. This system uses a
  hierarchical naming system known as domain names.
• Hierarchical machines are assigned according to
  the structure of the organizations obtained
  authority for parts of the namespace, not
  necessarily according to the structure of the
  physical network interconnections.

37
Mapping Domain Names to Addresses

• The Domain mechanism for mapping names to
  addresses consists of independent, cooperative
  system called name server. A name server is a
  server program that supplies name-to-address
  translation to IP addresses.
• Often, name server software executes on a
  dedicated processor, and the machine itself is
  called the name server.

38
Domain Address Resolution

• When a domain server receives a query, it checks
  to see if the name lies in the sub-domain for
  which it is an authority. If so, it translates
  the name to an address according to its database,
  and appends an answer to the query before sending
  it back to the client.
• If the name server cannot resolve the name
  completely, it contacts a domain server that can
  resolve the name returns the answer to the
  client.

39
The Top-Level Internet Domains and Their Meanings
Domain Name Meaning
COM EDU GOV MIL NET ORG ARPA INT country code
Commercial organizations Educational
Institutions Government Institutions Military
groups Major network support centers Organizations
other than those above Temporary ARPANET domain
(obsolete) International organizations Each
country (geographic scheme)
Although labels are shown in upper case, domain
name system comparisons are insensitive to case,
EDU is equivalent to edu
40
Domain Name Servers in a Tree
Root Server
server for .com
server for .edu
server for .gov
server for .us
.......
server for dec.com
server for msu.edu
server for nsf.gov
server for va.us
The conceptual arrangement of domain name servers
in a tree that corresponds to the naming
hierarchy. In theory, each server knows the
addresses of all lower-level servers for all
sub-domains within the domain it handles
41
Hierarchical organizationof the DNS
unnamed root
....
....
Top Level Domains
arpa
com
edu
gov
mil
net
ae
us
zw
United Arab Emirates
Zimbabwe
2nd Level Domains
msu
va
in-addr
cps
reston
140
cps.msu.edu
cnri
252
cnri.reston.va.us
13
generic domains
country domains
33
33.13.252.140.in-addr.arpa
42
Caching The key to Efficiency

• The cost of looking up nonlocal names can be
  extremely high if resolvers send each query to
  the root server. So, Internet name server can use
  name caching to optimize the costs.
• Each server maintains a cache of recently used
  names as well as record of where the mapping
  information for that name was obtained. Note
  cache entries are timed stamped, and deleted
  after a specified time period.
• When a client asks the server to resolve a name,
  the server first check to see if it has authority
  to resolve it by the standard procedure. If not,
  the server checks the cache to see if the name
  has been resolved recently.

43
Internet Electronic Mail, with a relay system at
both ends.
Sending Host
user at a terminal
one organi- zation
user agent
queue of mail to be sent
local MTA
local MTA
local MTA
relay MTA
queue of mail
across the Internet
44
Internet Electronic Mail, with a relay system at
both ends (cont.)
across the Internet
relay MTA
queue of mail
one organi- zation
local MTA
local MTA
local MTA
user agent
user mailboxes
user at a terminal
Receiving Host
45
Simple Mail Transfer Protocol (SMPT)
NOTE Five SMPT commands are used to send the
mail HELO, MAIL, RCPT, DATA and QUIT
S 220 Beta.GOV Simple Mail Transfer Service
Ready C HELO Alpha.EDU S 250 Beta.GOV C MAIL
FROMltSmith_at_Alpha.EDUgt S 250 OK C RCPT
TOltJones_at_Beta.GOVgt S 250 OK C RCPT TO
ltGreen_at_Beta.GOVgt S 550 No such user here C RCPT
TOltBrown_at_Beta.GOVgt S 250 OK C DATA S 354
Start mail input end with ltCRgtltLFgt.ltCRgtltLFgt C
... sends body of mail message ... C ...
continues for as many lines as message
contains C ltCRgtltLFgt.ltCRgtltLFgt S 250 OK C
QUIT S 221 Beta.GOV Service closing transmission
channel
46
Layering of TCP/IP-based protocols
47
Network-level Internet Services

• Connectionless packet delivery service
• Reliable stream transport service
• Network technology independence
• Universal Interconnection
• End-to-end ACKs
• Application Protocols Standards

48
Approximate correspondences between the various
networks