Content Overview This module covers the
operation of various transport-layer networking technologies
used on routers and hosts, including: - Transport
Control Protocol
- User Datagram Protocol
- NetBIOS
- Network Address Translation
- Extended Access Lists
This module also
discusses tools and methodologies that can be used to aid in
troubleshooting transport-layer networking issues.
Content 6.1 Characteristics of Transport
Layer Technologies 6.1.1 Common transport layer
technologies The transport layer provides end-to-end
traffic accountability. Layer 4 technologies ensure reliable
data delivery using acknowledgments, sequence numbers, and flow
control mechanisms. The transport layer is the first layer that
provides end-user functions. Problems at the transport layer
can present symptoms ranging from sub-optimal network operation
to complete network communications failure. There are at least
35 recognized transport layer protocols. Some of the more
common of these are: - User Datagram Protocol (UDP)
- Transport Control Protocol (TCP)
- Sequenced
Packet Exchange (SPX)
- AppleTalk Transaction Protocol
(ATP)
- NetBIOS
This section will discuss
the characteristics of these protocols and related transport
layer technologies. A specific network protocol can communicate
with another network protocol at the layer above or below it.
Within the TCP/IP protocol suite, Layer 4 operations are
primarily handled by UDP and TCP. UDP and TCP rely on IP at the
network layer and use port numbers to identify what higher
layer application traffic is contained in the packet. ICMP is a
protocol from the TCP/IP suite that operates at the network
layer and it too relies on IP. Unlike UDP and TCP, ICMP does
not carry user data. ICMP is primarily used by network devices
for self-management and self-tuning functions and by network
engineers for troubleshooting network problems. UDP, TCP, and
ICMP are all used heavily on the Internet, supporting a wide
variety of traffic types and applications. The Network Basic
Input/Output System (NetBIOS) was developed for IBM in 1983 by
Sytek Corporation and officially defines a session level
interface and a data transport protocol. NetBIOS was extended
by IBM in 1985 to create the NetBIOS Extended User Interface
(NetBEUI) protocol. NetBEUI supports NetBIOS operations at the
network layer. NetBEUI and NetBIOS are commonly used in
Microsoft and IBM LANs. NetBEUI operates at the network layer
and interfaces directly with ISO’s Logical Link Control 2
(LLC2) at the data-link layer. NetBIOS interfaces with NetBEUI
and with IBM’s Server Message Block (SMB) protocol at the
application layer. Together, NetBIOS and NetBEUI can be
considered to be operating from the network layer through to
the presentation layer. Because both NetBIOS and NetBEUI are
non-hierarchical broadcast-based protocols, they depend on
other hierarchical protocols, such as IP or IPX, to operate in
a routed network. Novell’s proprietary protocol suite uses
Sequenced Packet Exchange (SPX) at the transport layer to
implement reliable data delivery. In the early days of local
area networking, Novell’s suite of protocols was commonly
implemented. Until version 5 of Novell’s network operating
system, IPX/SPX was the default protocol suite installed for
Novell networks. Because IPX is not compatible with IP,
networks running the Novell protocols are unable to communicate
with the Internet without being translated. Due to the growing
need for corporate, academic, and government networks to be
connected to the Internet, almost all Novell network
installations now use the TCP/IP protocol suite. This has led
to the steady decline of the number of new network
installations using the IPX/SPX protocol suite. AppleTalk
Transaction Protocol (ATP) is used at the transport layer of
legacy AppleTalk networks and relies on AppleTalk’s Datagram
Delivery Protocol (DDP) at the network layer. Because ATP is
incompatible with IP, new Mac networks usually use the TCP/IP
protocol suite in preference to using the AppleTalk protocol
suite. Although legacy networks using IPX/SPX and AppleTalk
still exist, troubleshooting these protocol suites is not in
the scope of this course and will not be discussed further in
this curriculum.
Content 6.1
Characteristics of Transport Layer Technologies
6.1.2 User Datagram Protocol UDP is connectionless
and is considered unreliable because it does not guarantee
packet delivery. UDP operates on ‘best effort delivery’ basis
and does not use packet sequencing, acknowledgment or
retransmission mechanisms for flow control and error
detection/correction. If flow control and error
detection/correction features are required for a UDP-based data
flow, these features must be implemented in higher layer
protocols or applications. - Because UDP does not retransmit
lost packets and does not consume bandwidth with
acknowledgments, it is relatively light-weight, fast, and is
suitable for both one-to-one and one-to-many communications.
Over congestion-free and error-free networks, UDP is ideal for
transferring small amounts of data, and for supporting
streaming applications such as voice communications and video
multicasts. However, using UDP over congested or error-prone
networks often results in high degrees of data loss with
higher-layer protocols, applications, or even users having to
initiate data retransmission. Many higher-layer protocols and
applications make use of UDP, including: - Trivial File
Transfer Protocol (TFTP)
- Domain Name Service (DNS)
- NetBIOS Name Resolution (NetBIOS-NS)
- Windows Internet Name Service (WINS)
- Bootstrap
Protocol (BootP)
- Dynamic Host Configuration Protocol
(DHCP)
- Network Time Protocol (NTP)
- Remote
Authentication Dial-In User Service (RADIUS)
- Terminal
Access Control Access Control Server (TACACS)
Web Links Protocol Numbers
http://www.iana.org/assignments/protocol-numbers
Content
6.1 Characteristics of Transport Layer
Technologies 6.1.3 Transport Control
Protocol Unlike UDP, TCP is connection-oriented. Because
TCP implements packet sequencing, acknowledgment, and
retransmission mechanisms at the transport layer, it is
considered to be an inherently reliable protocol. These
additional features at Layer 4 give TCP a larger operational
overhead, which do not carry a data payload and consume
bandwidth. Because of these reliability features, TCP is better
suited to one-to-one communications and is rarely used for
streaming and one-to-many communications. - TCP implements two
main mechanisms for maximizing reliability and efficiency. They
are the three-way handshake and windowing. Understanding these
technologies is important to troubleshooting network
performance issues and failures. TCP three-way handshake
The TCP three-way handshake occurs during TCP
connection establishment and consists of three stages:
- Session request (TCP SYN) – the initiating host sends a TCP
synchronization packet, which has the SYN bit set on, and
contains the initiating host’s own sequence number (seq = x)
for the connection. This packet also contains information about
the initiating hosts TCP receive-window size.
- Session
request acknowledgment (TCP SYN-ACK) – the target host responds
to the packet from the initiating host by sending its own
‘synchronization acknowledgment’ packet. This packet has both
the ACK and SYN bits set on, and contains the acknowledgment
number generated by incrementing the sequence number from the
initiating host by one (ack = x+1) plus the new sequence number
from the target host (seq = y). The purpose of this packet is
to inform the initiating host that the target host has received
and understood the information from the initiating host and
inform the initiating host of the TCP receive-window size of
the target.
- Session acknowledgment (TCP ACK) – the