Content Overview The application layer
is the top layer in the TCP/IP reference model. When the ISO
developed the OSI Reference Model, the application layer
functions were divided into three separate, more detailed
layers. Although the OSI version is more detailed, it is more
common to refer to the application layer of TCP/IP since it is
more encompassing. The application layer is the interface that
separates application software from the transport layer, and
deals with high-level protocols rather than segments, bytes,
packets, or bits. It provides network services for users and
their programs and is the layer in which user-access network
processes reside. These processes include all of those that
users interact with directly, as well as other processes of
which the users are not aware. This layer includes all
application layer protocols that use the host-to-host transport
protocols to deliver data. Other functions that process user
data such as data encryption, decryption, compression and
decompression, can also reside at the application layer. Most
of the application layer protocols provide user services.
Application layer protocols are typically used for network
management, file transfer, distributed file services, terminal
emulation, and electronic mail. However, new user services are
often added (for example, VPNs, VoIP, and so on). The most
widely known and implemented TCP/IP application layer protocols
are listed below: - Telnet enables users to
establish terminal session connections with remote hosts.
- HyperText Transfer Protocol (HTTP) supports
the exchanging of text, graphic images, sound, video, and other
multimedia files on the World Wide Web.
- File
Transfer Protocol (FTP) performs interactive file transfers
between hosts.
- Trivial File Transfer Protocol
(TFTP) performs basic interactive file transfers typically
between hosts and networking devices (for example, routers,
switches, and so on).
- Simple Mail Transfer
Protocol (SMTP) supports basic message delivery services.
- Post Office Protocol (POP) is used to connect
to mail servers and download e-mail.
- Simple
Network Management Protocol (SNMP) is used to collect
management information from network devices.
- Domain Name Service (DNS) maps IP addresses to the
names assigned to network devices. Commonly called name
service.
- Network File System (NFS) enables
computers to mount drives on remote hosts and operate them as
if they were local drives. Originally developed by Sun
Microsystems, it combines with two other application layer
protocols, external data representation (XDR), and
remote-procedure call (RPC), to allow transparent access to
remote network resources.
Other application layer
protocols are listed below. - Finger — User
Information Protocol
- IMAP4 — Internet Message
Access Protocol
- IPDC — IP Device Control
- ISAKMP — Internet Message Access Protocol
- LDAP — Lightweight Directory Access Protocol
- NTP — Network Time Protocol
- POP3 —
Post Office Protocol version 3
- RLOGIN — Remote
Login
- RTSP — Real-time Streaming Protocol
- SCTP — Stream Control Transmission Protocol
- S-HTTP — Secure Hypertext Transfer Protocol
- SLP — Service Location Protocol
- TFTP
— Trivial File Transfer Protocol
- WCCP —
Web Cache Coordination Protocol
- X-Window
These and other network applications use the
services of TCP/IP and other lower-layer Internet protocols to
provide users with basic network services.
Content
7.1 Troubleshooting the Application Layer
7.1.1 Overview The primary responsibility of the
upper layers of the OSI model is to provide services such as
e-mail, file transfer, and data transport. Application layer
problems result when data is not delivered to the destination
or network performance degrades to a level where productivity
is affected. The same general troubleshooting process used to
isolate problems at the lower layers can be used to isolate
problems at the application layer. The ideas stay the same, but
the technological focus has shifted to involve things such as
refused or timed out connections, access lists, and DNS issues.
Problem isolation is vital to successfully troubleshoot any
problem. Merely isolating the problem will not bring the types
of changes necessary to return network functions to the
documented baseline. To meet the troubleshooting objective of
resolving the problem, use the tools and resources that are
provided to correctly configure the properties of a properly
functioning network. Application layer problems prevent
services from being provided to application programs. A problem
at the application layer can result in unreachable or unusable
resources when the physical, data link, network, and transport
layers are functional. It is possible to have full network
connectivity but the application simply cannot provide data.
Another type of problem at the application layer occurs when
the physical, data link, network, and transport layers are
functional, but the data transfer and requests for network
services from a single network service or application do not
meet the normal expectations of a user. A problem at the
application layer may cause users to complain that the network
or the particular application that they are working with is
sluggish or slower than usual when transferring data or
requesting network services. Lab Activity Lab Exercise:
Isolating Problems at the Transport and Application Layers
After completing this lab, the student will be able to analyze
user-feedback and end-system data to decide which OSI layer to
begin isolating problems.
Content 7.1
Troubleshooting the Application Layer 7.1.2
Eliminating Layers 1-3 When an application program cannot
successfully connect to the destination host, establish at
which layer the problem resides. Is it a lower layer problem or
a higher layer problem? For example, assume the problem is the
inability to connect to a remote FTP server. To determine
whether this is an application layer problem and not a lower
layer problem, the first step is to verify Layer 3
connectivity. If successful, Layer 3 and lower can be
eliminated as the source of the problem. To troubleshoot use
the following steps: - Ping the default gateway. If
successful, Layer 1 and Layer 2 services are functioning
properly.
- Verify end-to-end (host-to-host)
connectivity. Use an extended ping if attempting the
ping from a Cisco router.
If these pings are
successful, then Layer 1 through Layer 3 can be eliminated.
Since they are functioning properly, the issue must exist at a
higher layer. Lab Activity Lab Exercise: Correcting
Problems at the Transport and Application Layers In this lab,
the student will use various Cisco commands to correct network
problems.
Content 7.1 Troubleshooting the
Application Layer 7.1.3 Eliminating Layer
4 Layer 4 is the home of UDP and TCP protocols and is not
as easy to eliminate. For example, assume there are FTP
connection problems. To troubleshoot Layer 4, use the following
steps: - Use the show access-list command. Are
there any access-lists that could be stopping traffic? Notice
which access lists have matches.
- Clear the
access-list counters with the clear access-list
counters command and try to establish an FTP connection
again.
- Verify the access-list counters. Have any
increased? Should they increase?
Improperly
configured access lists are common problem areas. Be sure the
implications of each access list statement are understood. This
may sound strange but it sometimes helps to think like the
packet. However, if the access lists are functioning as
expected, then the problem must lie in a higher layer.