Physical & Data Link Layers

Layers that TCP/IP rely on

February 4,2001Professor Tom Mavroidis

TCP/IP daemons

TCP/IP services are controlled by daemons The most important of these is the Internet

super server, inedt. It is used to dispatch other server daemons to

handle communications. /etc/rc.local file runs inetd at boot time inetd listens for socket connections and

decides on what service to provide

Physical Layer

Is the actual physical medium. May be copper wire or fiber optic cable or

the carrier wave in a wireless connection

Repeaters

Transmit messages between networks boosting the signal in the process thereby increasing the distance between nodes.

Maximum # of repeaters in a segment = 4 Maximum nodes in a segment for Thick

coax is 100 and for thin 30 Twisted pair supports 1024 nodes

Bridges

More intelligent than repeaters Operate at the Data Link layer. Deal with frames rather than signals Whole frame is read and retransmitted examines the address to determine if on the

same segment thereby reducing network traffic by not needlessly transmitting to another segment.

Switches

Modern replacement for bridges Perform better than bridges Operates at Data Link layer Accepts many different types of input Routes traffic between and broadcasts

between hosts in that group only Use where LAN performance is critical

Routers

Operates at the network layer Frames are passed up from the Data Link

layer Can route between different types of

networks I.E. Ethernet to token ring. When a router also acts as a bridge it is

known as a brouter

inetd

Referred to as the network super daemon Listens to several ports and dispatches other

server daemons to handle incoming service requests for other protocols

example of inetd.conf

Inetd - continued

If changes are made to this file you can have it reread by sending the daemon a HUP signal

# killall -HUP inetd inetd doesn’t work on all services such as

HTTP, SQL, and LDAP. Advantage is only one service has to be running

for services that are used infrequently

Data Link Layer

Addresses, controls and synchronizes data flow

Handles Data Addressing associated with each host

Controls data flow over the physical network Data integrity, ensuring data arrives at the

destination intact and in the right order

Data Addressing

Addresses must be unique across the Physical Layer otherwise traffic will not be directed to the correct host

Duplication of addresses can cause network slowdown and may even cause a network segment to be inoperative.

Flow Control

Ensures source host does not send faster than the destination host can receive otherwise the destination host will be overloaded.

Sets the maximum number of frames the source can send at once before waiting for the destination to let it continue

Flow Control - continued

Often implemented in the upper protocol layers but can also be implemented at the Data Link layer.

The mechanism for handling overload is called Source Quench.

Source Quench lies within the ICMP protocol which lives in the Network layer.

Data Integrity

Provided by dividing data into frames and adding a Cyclic Redundancy Check (CRC)

Electronic signature (mathematically computed) for the frame.

Computed at both ends, if differs retransmission is requested by the receiving end

Timers and Time Outs

Timer is started when frame is sent If no ack received in time frame is resent Each frame is numbered so acknowledging

frame can be identified Usually handled by higher layers although

can be implemented in the Data Link layer

Frames

A packet of information Many formats, sizes and encoding Three ways to delineate frames 1)Mark the end of the frame with a special

delimiter 2)Put the length in the frame header 3)Use fixed length packets

Frame Sizes

Maximum frame size determines maximum data payload

Known as Maximum Transmission Unit (MTU)

If data is larger than MTU it must be spread across more than one frame

Manchester encoding

Popular way to encode a string of data Surround each packet byte with start, stop,

and parity bits Based on transitions rather than simple

voltage Another method is differential Manchester

encoding - lack of transition denotes 1

CSMA/CD - Physical layer

CSMA/CD Carrier Sense Multiple Access with Collision Detect

host listens for a quiet period before sending frame (CS)

CD - host detects when a collision occurred waits a random amount of time and resends further retries use an exponential back-off

Variation of CSMA/CD

1-persistent - always sends a frame in response to a clear line (Ethernet uses this)

non-persistent - each station waits a random interval before checking the line

p-persistent - uses a variable probability scheme

Strength of CSMA/CD

Good at handling variable loads Minimizes collisions at low to medium

loads Easy to plug in or remove hosts Requires little admin Simple concept - easy to implement

Token Passing

Logical token is passed between hosts in a ring

Host can only send frames while it holds on to the token.

No Collisions Much more complex than CSMA/CD

Ethernet

Most common LAN in the TCP/IP environment

Connected in bus or star topology Uses CSMA/CD Three types of cable can be used 10Base5, 10Base2, 10BaseT

Token Ring

Designed by IBM standardized by IEEE (referred to as IEEE 802.5)

Uses a logical and physical ring topology Network is physically constructed into a star

shape connected by MSAUs Multistation Access Units.

Speed are 4 and 16 Mbps Uses differential Manchester encoding

Serial Protocols

Used when a connection is between two points only

Based on direct links, leased lines, or modem- based access through POTS

Relatively slow speeds Improvements have led to more sophisticated

protocols across serial lines called nomadic workstations

Slip

Early protocol to send IP over a serial link Never formalized into a standard Start and end of an IP datagram is detected by

reading bytes from a serial connection No longer predominant Limitations arise from its simplicity Fixed mapped IP - no dynamic address

allocation

PPP

Point to Point Protocol Modern implementation of SLIP Standardized Based on the HDLC standards Dynamic IP - many things can be

negotiated about the current session

Features of PPP

Error Checking Synchronous/asynchronous Provides weak (PAP) and strong (CHAP)

authentication

Three components to PPP

Encapsulation mechanism - includes and additional field that identifies the protocol using the link

Link Control Protocol - Link control protocol us used to establish, configure and test the data-link connection

Network Control Protocol - used to negotiate the network layer protocol

Things to review

IP Addressing Netmasks Reserved Addresses Broadcast Addresses Multicasting Network Interfaces