Applications and layered architectures

1
Applications and layered architectures

• Various services various applications require
  various services
• Flexibility powerful networks need to be
  flexible enough to support the current services
  and future services
• Architectures an overall network architecture
• is necessary to achieve the flexibility

2
Layered architectures

• Layers grouping the common functions
• Benefits of layers
• Simplicity easy to design once layers and their
  interaction are defined clearly
• Flexibility easy to modify and develop networks
  by separate layers modifications
• Incremental changes add new layers, add new
  functions to a layer

3
Big picture of layered architectures

• Web browsing and e-mail examples
• OSI reference model (Seven layers)
• TCP/IP architecture
• Detailed end-to-end examples to complete big
  picture of layered architectures
• Socket API and other utilities

4
Terminology

• Client/Server model the most typical interaction
  between two parties within networks
• Client the process making requests
• Server (Daemon) the process waiting and
  receiving requests, processing the requests and
  returning results
• Protocols a set of rules governing how two
  communicating parties are to interact.
• Service a protocol will provide a service
• Layers protocols each layer carries out a
  specific set of functions using its own protocol,
  and builds on the services of the layer below it
  (provides a service to its upper layer).

5
ExampleHTTP and Web Browsing

• HTTP HyperText Transfer Protocol
• Rules by which the client and server interact so
  as to retrieve a document and how the request and
  response are phased
• Client sets up a two-way connection before
  request
• Client generally carries out DNS to find IP
  address of server

6
Retrieve a document from the Web

• Event
• User selects a document
• HTTP Client locates the server host and sets up a
  two way connection
• HTTP client sends message requesting document
• HTTP server listing on TCP port 80 interprets
  message

• Message content
• Get /infocom/index.htm HTTP/1.0

7
Retrieve a document from the Web (cont.)

• Event
• HTTP server sends a result code and a
  description of information that the client will
  receive
• HTTP server reads the requested file and sends
  the file through TCP port
• HTTP server disconnects the connection
• Text is displayed by the client browser, which
  interprets HTML

• Message content
• HTTP/1.1 200 OK
• Server Apache/1.2/5 FrontPage 3.0.4
• Content-Length 414
• Content-Type text/html
• lthtmlgt
• ltheadgt
• lttitlegtIEEE Infocom99The future is Now

8
HTTP client/server interaction

Request
HTTP server
HTTP client
Response
Figure 2.1
9
HTTP server
HTTP client
Ephemeral
Port 80
Port
GET 80,
TCP
TCP
, 80 STATUS

• TCP provides a pipe between HTTP client and HTTP
  server
• Transfer of message between HTTP client/server is
  virtual, indirect
• HTTP is said to use the service provided by TCP
  (lower layer)

Figure 2.2
10
ExampleDNS query

• DNSDomain Name Service
• Domain name?IP address such as
• cse.unl.edu?192.93.33.1
• It is a distributed database on multiple
  machines, each of which can act as a DNS server
  that other systems can query
• May recursively query to resolve an IP address

11
DNS query and response

• Message content
• Header OPCODESQUERY
• Question
• QNAMEcse.unl.edu., QCLASSIN, QTYPEA
• Header OPCODESQUERY,RESPONSE AA
• Question QNAMEcse.unl.edu., QCLASSIN,QTYPEA
• Answer cse.unl.edu. 86400 IN A 192.93.33.1

• Event
• 1.Application requests name to address
  translation
• 2. Resolver composes query message
• 3. Resolver sends datagram encapsulating the
  query
• 4. DNS server looks up address and prepares
  response
• 5. DNS sends UDP datagram encapsulating the
  response message

12
DNS server
DNS client
Port 53
Ephemeral
Port
Query 53,
UDP
UDP
, 53 STATUS

• UDP provides a pipe between DNS client and DNS
  server
• Transfer of message between DNS client/server is
  virtual, indirect
• DNS is said to use the service provided by UDP
  (lower layer)
• UDP is connectionless, so no connection between
  client/server

Figure 2.2
13
ExampleSMTP and Email

• SMTP Simple Mail Transfer Protocol

• Email application (preparing email message)

By SMTP
Local email server
By SMTP
By POP
Destination email server
Email (retrieving) application
14
Layered structures

• HTTP, DNS, EMAIL
• TCP/UDP
• IP

15
Unified view of layers, protocols, and services

• Various layers a layer groups a set of relevant
  functions
• Peer processes (entities) the two corresponding
  communicating processes (entities) in a layer
• Protocol rules governing the behavior of two
  peer entities in that layer
• Services the functionality provided by a layer
• Protocol Data Unit (PDU) the data exchanged
  between peer entities
• Header protocol control/address information in
  a PDU
• Service Data Unit (SDU) actual user information
  in a PDU
• PDU Header SDU, PDUn1 SDUn
• Service Access Point (SAP) a place (unique
  identifier, software port) of layer n where layer
  n1 can access the services offered by layer n

16
Peer-to-peer communication
n-PDUs

n entity
n entity
PDU Header SDU
Figure 2.3
17
Layer services
n1 entity
n1 entity
n1-PDU
n1-PDU
n-SDU
n-SDU
n-SAP
n-SAP
n-SDU H
n entity
n entity
H n-SDU
n-PDU

• n1-PDU is passed to layer n through n-SAP
• n-SDU (n1-PDU) is encapsulated, layer n does not
  touch it (usually)
• Layer n1 just relies on the success of transfer
  by layer n, but does not
• concern the implementation of layer n

Figure 2.2
18
Services connection-oriented connectionless
services

• Steps of a service 1. Accepting n1-PDU from
  layer n1, 2. Transferring to its peer, 3. The
  peer delivers to the user at layer n1
• Connection-oriented service 1. Set up a
  connection between two n-SAPs, 2. Transferring
  n-PDUs using layer n protocol, 3. Tear down the
  connection and release resources
• Connectionless service no set up, each PDU is
  transferred directly from SAP to SAP control
  information from layer n1 to layer n must
  contain all the address information required to
  transfer the PDUs
• Example http.

19
Confirmed unconfirmed service

• Confirmed service the sender must be informed of
  the outcome.
• Unconfirmed service the sender need not to be
  informed of the outcome.
• Example connection setup is a confirmed service.
  The connectionless service may be confirmed or
  unconfirmed depending whether sender requires
  acknowledgment.
• QUESTION? Does it make sense for a network to
  provide a confirmed, connectionless transfer
  service?

20
Segmentation and blocking

• Different networks may have different limitation
  on the size of a block of information.
• MTU maximum Transfer Unit
• Segmentation if the size of block information is
  too large, need to break into several segments
  and transfer them separately.
• Blocking if SDUs are too small as to result in
  inefficiency, then combine several SDUs into a
  single SDU.

21
(a)
Segmentation
Reassembly
n-SDU
n-SDU
n-PDU
n-PDU
n-PDU
n-PDU
n-PDU
n-PDU
(b)
Blocking
Unblocking
n-SDU
n-SDU
n-SDU
n-SDU
n-SDU
n-SDU
n-PDU
n-PDU
Segmentation/reassembly and blocking/unblocking
Figure 2.5
22
Application A
Application B
Application Layer
Application Layer
Presentation Layer
Presentation Layer
Session Layer
Session Layer
Transport Layer
Transport Layer
Communication Network
Network Layer
Network Layer
Network Layer
Network Layer
Data Link Layer
Data Link Layer
Data Link Layer
Data Link Layer
Physical Layer
Physical Layer
Physical Layer
Physical Layer
Electrical and/or Optical Signals
The OSI reference model (proposed by ISO)
Figure 2.6
23
OSI reference modelseven layers

• All applications are built on the top of the
  seven layers, specifically, on the top of the
  application layer.
• The top 4 layers are end-to-end and involves the
  interaction of peer entities across the network,
  however, the bottom 3 layers are point-to-point
  and involve the interaction of peer entities
  across a single hop.

24
OSI reference model layer 7 and 6

• Application layer provides services that are
  frequently required by applications. e.g., WWW
  applications (browser and web server) are built
  on HTTP layer.
• Presentation layer provides application layer
  with independence from difference in the
  representation of data.
• For example, application A uses
  machine-dependent data format DFA and application
  B uses machine-dependent format DFB, then at end
  A, the representation layer will convert the data
  in DFA to machine-independent data, then when
  data arrive at end B, the representation layer
  will convert the machine-independent data into
  format DFB. DFA ? machine-independent ?DFB.

25
OSI reference model layer 5

• Session layer provides dialog control and
  enhances the reliable transfer service provided
  by transport layer. For example, half-duplex
  dialog where two parties take turn transferring
  information introduction of synchronization
  points used to mark the progress of an
  interaction and for error recovery.

26
OSI reference model layer 4

• Transport layer end-to-end transfer of message
  from the source machine to the destination
  machine.
• Only being executed at end computer systems
• Certain end-to-end services
• Connection-oriented
• Error-free transfer of byte stream
• Error detection and recovery
• Sequence and flow control.
• Unconfirmed connection-less
• Transfer of individual messages
• Provides appropriate address information

27
OSI reference model layer 4 (cont.)

• Transport layer
• Segmentation/reassembly and blocking/unblocking
• Possibly setting up and releasing connections
• Possibly multiplexing multiple transport layer
  connections into one network connection
• Possibly split one transport layer connection
  into several network connections
• Accessing transport layer by socket interface

28
Connection-oriented VS. Connectionless
29
Connection-oriented VS. Connectionless (cont.)
30
OSI reference model layer 3

• Network layer provides the transfer of data in
  the form of packets across the communication
  networks.
• Routing (which makes this layer most complex)
• Congestion control
• Internet sub-layer routing between the different
  networks, hiding the details of each specific
  network such as address differences, size and
  format differences
• Being implemented at each intermediate node

31
PS packet switch C computer
C
PS
C
PS
PS
PS
C
C
C
A packet-switching network using a uniform
routing procedure
Figure 2.7
32
The networks may be quite different. A
gateway/router may connect several networks.
G gateway/router
net 3
G
net 1
G
G
G
net 5
net 2
net 4
G
An internetwork
Figure 2.8
33
Switches/routers/gateways

• Circuit switch, used in telephone networks
• Packet switch/router/gateway
• Generally consider them as the same meaning
• Packet switch deals with a uniform routing
  procedure, within one homogenous network, one
  pair of data link and physical layer
• Router/gateway deals with routing in multiple
  heterogeneous networks, more than one pair of
  data link and physical layers
• Gateway sometime contains firewall function

34
Switches/routers/gateways (cont.)

• Mainly from the point of software, i.e.,
  containing functions of lower three layers
• Of course, there are some requirements for
  hardware such as speed, disk, memory, multiple
  interfaces.
• Bridge used to connect multiple similar LANs.

35
OSI reference model layer 2

• Data link layer provides the transfer of frames
  across a transmission link that directly connects
  two nodes
• Point-to-point transfer
• Framing to indicate the boundaries of frames
• Inserting control and physical address
  information
• Inserting check bits for recovering from error
• Flow control (in point-to-point basis)
• Links point-to-point, shared media,wireless

36
point-to-point link
transceivers
? ? ? ? ? ?
Shared media link
Figure 1.17
37
Wireless link
BSS
BSS
MSC
SS7
STP
HLR
wireline terminal
VLR
EIR
PSTN
AC
MSC mobile switching center PSTN public
switched telephone network STP signal transfer
point VLR visitor location register
AC authentication center BSS base station
subsystem EIR equipment identity register HLR
home location register
Figure 4.52
38
OSI reference model layer 1

• Physical layer deals with the transfer of bits
  over a communication channel.
• Setting up and release of physical connection
• Mechanical factors
• Systems parameters
• Again, each intermediate node in the networks has
  the network layer, data link layer and physical
  layer

39
Transfer of what across what in each layer
messages
from
end (source) to end (destination).
Transport layer transfer of
Network layertransfer of
packets
across
multiple networks.
a transmission link.
frames
across
Data link layer transfer of
bits
over
a communication channel.
Physical layer transfer of

low three layers.
The intermediate node (or router) has
The end node has
high four layers low three layers.
40
Headers and trailers added to data
Application B
Application A
data
Application Layer
Application Layer
ah
data
Presentation Layer
Presentation Layer
ph
data
Session Layer
Session Layer
sh
data
Transport Layer
Transport Layer
th
data
Network Layer
Network Layer
data
nh
Data Link Layer
Data Link Layer
dh
dt
data
Physical Layer
Physical Layer
bits
Figure 2.9