1. Applications Layer Functionality & Protocols
2019/4/20

2. Human Generates Data
2019/4/20

3. Convert communication to digital format
2019/4/20

4. Application layer initiate data transfer
2019/4/20

5. OSI vs TCP/IP
Application
Protocols
2019/4/20

6. Well-known TCP/IP Application Layer Protocols
2019/4/20

7. Who is in charge of TCP/IP protocols?
IETF (Internet Engineering Task Force)
Protocol forming process
Proposal
Internet Draft
RFC (Requests for Comments)
Example, RFC 2616 (Hypertext Transfer Protocol, HTTP/1.1)
2019/4/20

8. Application layer software
Two forms of software programs or processes that provide access to the network:
Applications
Services
2019/4/20

9. Application Layer Software
Application Programs
Network-aware
can communicate directly with the lower layers of the protocol stack.
clients, like Outlook Express
Web browser, like Internet Explorer
Non network-aware
Excel, word, etc
2019/4/20

10. Application Layer Software
Application Services
Provide assistance to non network-aware Application layer programs to use network resources
like file transfer or network print spooling.
These services are the programs that interface with the network and prepare the data for transfer.
2019/4/20

11. Summary
Application layer 就是由很多 Application programs 與 Application services 等程式 (or processes) 所組成
而所謂 Application layer protocols 則實作於上述 Application programs 與 Application services 等程式中
2019/4/20

12. Applications 與 Services 關係
Applications provide the user interface
Services follow protocols to prepare data for the network
2019/4/20

13. Processes running in Windows OS
2019/4/20

14. 補充 svchost.exe 是從動態連結程式庫 (DLL) 執行之服務的一般性主處理程序名稱
多個 svchost.exe 可以同時執行，且每一個 svchost.exe 工作階段都可以包含一組服務。
2019/4/20

15. More on svchost.exe …
Some time ago, Microsoft started moving all of the functionality from internal Windows services into .dll files instead of .exe files.
From a programming perspective this makes more sense for reusability…
but the problem is that you can't launch a .dll file directly from Windows, it has to be loaded up from a running executable (.exe).
Thus the svchost.exe process was born.
2019/4/20

16. Application layer protocol functions
Application layer protocols are used by both the source and destination devices during a communication session
In order for the communications to be successful, the application layer protocols implemented on the source and destination host must match
2019/4/20

17. Application layer protocols
Many different types of applications communicate across data networks.
Therefore, Application layer services must implement multiple protocols to provide the desired range of communication experiences.
Each protocol has a specific purpose and contains the characteristics required to meet that purpose.
2019/4/20

18. Accessing remote information
Modes of interaction
Client-server model
Peer-to-peer model
2019/4/20

19. The Client/Server model
In the client/server model, the device requesting the information is called a client and the device responding to the request is called a server.
Client and server processes are considered to be in the Application layer.
2019/4/20

20. Server
Server runs a service, or process, sometimes called a server daemon.
Like most services, daemons typically run in the background and are not under an end user's direct control.
Daemons are described as “listening” for a request from a client, because they are programmed to respond whenever the server receives a request for the service provided by the daemon.
2019/4/20

21. 2019/4/20

22. Peer-to-peer model Peer-to-peer networks Peer-to-peer applications
2019/4/20

23. Peer-to-peer networks
The roles of client and server
are set on a per request basis.
2019/4/20

24. Peer-to-peer applications
A device to act as both a client and a server
within the same communication
2019/4/20

25. Application Layer Protocol Examples
DNS services & protocol
WWW services & HTTP
services & SMTP/POP protocols
FTP
DHCP
File sharing services & SMB protocol
Telnet
2019/4/20

26. Application Layer Protocols/Services & Port number
Domain Name System (DNS) - TCP/UDP Port 53
Hypertext Transfer Protocol (HTTP) - TCP Port 80
Mail
Simple Mail Transfer Protocol (SMTP) - TCP Port 25
Post Office Protocol (POP) - UDP Port 110
File Transfer Protocol (FTP)
TCP Ports 20 and 21
Dynamic Host Configuration Protocol - UDP Port 67
SMB – 445
Telnet - TCP Port 23
2019/4/20

27. DNS Protocol Procedure
2019/4/20

28. DNS Protocol Procedure
2019/4/20

29. Hierarchy of DNS Servers
2019/4/20

30. nslookup demo
Name Server Lookup
2019/4/20

31. DNS Queries Recursive Queries Iterative (non-recursive) Queries
Inverse Queries (obsolete)
2019/4/20

32. Resolver program or library
Located on each host and provides a means of translating a users request for, say, into one or more queries to DNS servers using UDP (or TCP) protocols
2019/4/20

33. Recursive Queries 1. Resolver  DNS server (www.test.com?)
2. DNS server found no answer
3. DNS  sends query to a root-server (
4. The root-server replies with a “referral” to .com DNS
5. DNS  sends query to .com DNS
6. .com DNS replies with a referral to test.com DNS
7. DNS  sends query to test.com DNS
8. test.com DNS  DNS
9. DNS send response to original client resolver
2019/4/20

34. Iterative (non-recursive) Queries
1. Resolver  DNS server (
2. DNS server found no answer
3. DNS  sends referral info (root-server)  Resolver
4. Resolver  root-server
5. Root server  resolver (referral info of .com server)
6. Resolver  .com server
7. …. (well, you get the picture …)
The resolver on Windows and most *nix systems is a stub resolver,
a minimal resolver which cannot follow referrals.
If you reconfigure your local PC or Workstation to point to a DNS server
that only supports Iterative queries - it will not work.
2019/4/20

35. HTTP
2019/4/20

36. HTTP message types The three common message types are GET POST, PUT
a client request for data
A web browser sends the GET message to request pages from a web server
POST, PUT
send messages that upload data to the web server
2019/4/20

37. GET vs POST
The HTML specifications technically define the difference between "GET" and "POST" so that
GET means that form data is to be encoded (by a browser) into a URL
POST means that the form data is to appear within a message body.
2019/4/20

38. Email related processes
Mail User Agent (MUA)
client
like outlook express, etc
Two processes of server
Mail Transfer Agent (MTA)
Mail Delivery Agent (MDA)
2019/4/20

39. Email related processes
2019/4/20

40. FTP
To successfully transfer files, FTP requires two connections between the client and the server:
one for commands and replies,
the other for the actual file transfer.
2019/4/20

41. FTP
The client establishes the first connection to the server on TCP port 21.
This connection is used for control traffic, consisting of client commands and server replies.
2019/4/20

42. FTP
The client establishes the second connection to the server over TCP port 20.
This connection is for the actual file transfer and is created every time there is a file transferred.
2019/4/20

43. FTP
2019/4/20

44. DHCP
The Dynamic Host Configuration Protocol (DHCP) service enables devices on a network to obtain IP addresses and other information from a DHCP server.
Like network mask, gateway, DNS, etc
2019/4/20

45. DHCP
2019/4/20

46. DHCP
2019/4/20

47. SMB
The Server Message Block (SMB) is a client/server file sharing protocol.
IBM developed Server Message Block (SMB) in the late 1980s
to describe the structure of shared network resources, such as directories, files, printers, and serial ports.
It is a request-response protocol.
2019/4/20

48. SMB
Unlike the file sharing supported by FTP, clients establish a long term connection to servers.
Once the connection is established, the user of the client can access the resources on the server as if the resource is local to the client host.
2019/4/20

49. File sharing using SMB
2019/4/20

50. SMB & Microsoft
SMB file-sharing and print services have become the mainstay (支柱) of Microsoft networking.
In previous versions, the SMB services used a non-TCP/IP protocol to implement name resolution
Beginning with Windows 2000, all subsequent Microsoft products use DNS naming.
This allows TCP/IP protocols to directly support SMB resource sharing.
2019/4/20

51. SMB & Other OSs
The LINUX and UNIX operating systems also provide a method of sharing resources with Microsoft networks using a version of SMB called SAMBA.
The Apple Macintosh operating systems also support resource sharing using the SMB protocol.
2019/4/20

52. Telnet services & protocol
Long before desktop computers with sophisticated graphical interfaces existed, people used text-based systems which were often just display terminals physically attached to a central computer.
Once networks were available, people needed a way to remotely access the computer systems in the same manner that they did with the directly attached terminals
Telnet was developed to meet that need.
2019/4/20

53. Telnet services & protocol
2019/4/20

54. Homework Verify what kind of DNS query your resolver & DNS use
Use WireShark
2019/4/20