CCNA 3 Labs and Study Guide - BINARYBB.INFO – @jagalbraith

192 Switching Basics and Intermediate Routing CCNA 3 Labs and Study Guide
Field C Type of Service (TOS) or Differentiated Service
This 8-bit field can be broken down into two parts: Precedence and TOS. The first 3 bits specify the
Precedence value (e.g. routine-0, priority-1, immediate-2, etc.); 4th bit specifies minimal delay; 5th bit
specifies maximize throughput; 6th bit specifies maximize reliability; 7th bit specifies minimize monetary
costs; 8th bit is currently unused. Although this field is not commonly used and is usually set to all zeros,
the Precedence bits are occasionally used for QoS applications.
Field D Total Length
This field is a 16-bit number specifying the total length of the packet (header + data) in bytes; can be up to
65,535 bytes. By subtracting the header length for this value, you can determine the size of the payload.
Field E Identifier
This field is used to identify the fragments of one datagram from those of another. The originating protocol
module of an Internet datagram sets the Identifier field to a value that must be unique for that sourcedestination
pair and protocol for the time the datagram will be active in the Internet system. The originating
protocol module of a complete datagram sets the MF bit to 0 and the Fragment Offset field to 0.
Field F Flags
These 3 bits indicate whether the packet can be fragmented and whether it has more fragments coming.
The 3 bits are as follows:
■ 1st bit: Reserved (unused)
■ 2nd bit: Fragment? 1=no, 0=yes
■ 3rd bit: More Fragments Coming? 0=no, 1=yes
Field G Fragment Offset
This is a byte count from the beginning of the original packet so that the destination knows where to place
this particular fragment when reconstructing the packet. If a router’s interface is set to a maximum transmission
unit that is smaller than the sent packet, then it will be fragmented by the router. However, if the
Do Not Fragment bit is set in the Flag field, then the packet will be dropped and an ICMP message will be
sent to the source.
Field H Time to Live
This 8-bit field helps prevent routing loops. This field is set with a certain number when the packet is first
encapsulated at the source. Each router along the path from the source to the destination decrements this
field. If this field reaches 0 before the packet reaches the destination, then the packet is dropped and an
ICMP messages is sent to the source. The trace utility uses this field to trace a route to a specified destination.
Field I Protocols
Also called Service Access Point (SAP), the Protocol field identifies the upper-layer protocol that the data
packet is destined for. A few of the values of this field are shown in the following table: