Now that you know what IP is, let’s look at the actual packet structure in more detail. The
following is an IP packet that was broken down by EtherPeek, a network analyzer. The entire
header has six layers, and each layer consists of 32 bits. Look at each section of the header
and get an explanation for each:
IP Header - Internet Protocol Datagram
Version: 4
Header Length: 5
Precedence: 0
Type of Service: 0
Unused:
Total Length: 60
Identifier: 0
Fragmentation Flags: 0
Fragment Offset: 0
Time To Live: 2
IP Type: 0x58 EIGRP
Header Checksum: 0x10dc
Source IP Address: 205.124.250.7
Dest. IP Address: 224.0.0.10
No Internet Datagram Options
At this point, we will define the key fields that appear in this listing. As you can see, the
packet IP header starts out with the Version field. Right now, the standard is IPv4. The version
parameter uses four of the 32 bits available.
The next field is the IP Header Length, or IHL. This field also uses another four bits, and it
specifies the datagram header length in 32-bit words.
The Type of Service (TOS field) follows the IHL. This field uses eight bits and indicates
datagram priority and how other OSI layers are to handle the datagram once they receive it.
Following the TOS field is the Total Length parameter. This field indicates how long the
packet is, including header and payload or data. The length is in units of bytes. The field itself
uses 16 bits, which brings the total for these fields to 32 bits or four bytes.
The second field begins with the Identifier or Identification field. The Identifier is
a 16-bit field that contains an integer value that identifies the packet. It is like a sequencing number
that is used when reassembling datagram fragments.
The Fragmentation Flags field follows, using only three bits. This field is used to control
fragmentation of a datagram. If the datagram can be fragmented, the first bit has a value of 0;
otherwise, a value of 1 is assigned to the first bit if the datagram is not to be fragmented. The
second bit is used to indicate the last fragment of a fragmented datagram. The third bit is an
undefined bit and is set to 0.
Fragment Offset follows the Flags field. This value uses 13 bits and specifies the fragment’s
position in the original datagram. The position is measured from the beginning of the datagram
and marked off in 64-bit increments. This again brings you to 32 bits, so you must move down
to the next layer in the IP packet.
The third field begins with the Time-to-Live (TTL) field, which is a counter whose units are
measured in hops. A starting value is given, and it counts decrements by 1 as it passes through each
hop or router. Once the value of this field is 0, the packet is discarded. This field uses eight bits.
The protocol field (IP Type) follows the TTL parameter. This field tells layer 3 which upper
layer protocol is supposed to receive the packet. It uses a decimal value to specify the protocol.
This field uses eight bits.
The Header Checksum field finishes the third layer. The checksum is used to help verify the
integrity of the IP header. This field uses 16 bits.
The next two fields are the Source IP Address and Dest. IP Address respectively. Both
of these fields are 32 bits long.
An Options field occupies the final field of the header. The field needs to be 32 bits long, so
any additional empty bits are padded.
Figure 36.13 gives a good visual representation of the IP packet structure.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Internet Protocol (IP)
It is important to distinguish between the Internet Protocol suite and the actual Internet Protocol
that is used in the Network layer of the OSI model.
The IP suite consists of several discrete protocols that are implemented at different levels of
the OSI model.
The Internet Protocol (IP) is a Network layer protocol of the IP suite. It is used to allow
routing among internetworks and heterogeneous systems. IP is a connectionless protocol,
even though it can provide error reporting, and it performs the segmentation and reassembly
of PDUs.
that is used in the Network layer of the OSI model.
The IP suite consists of several discrete protocols that are implemented at different levels of
the OSI model.
The Internet Protocol (IP) is a Network layer protocol of the IP suite. It is used to allow
routing among internetworks and heterogeneous systems. IP is a connectionless protocol,
even though it can provide error reporting, and it performs the segmentation and reassembly
of PDUs.
Layers 3 and 4: IP Routed Protocols
The Network layer is used by the Transport layer to provide the best end-to-end services and
path for PDU delivery. This means that the Network layer also uses protocols to accomplish this task. This section discusses protocols that are used within layer 3 of the OSI model. Some of
these protocols use other protocols within them for finer granularity of certain functions.
There is a significant difference between routing protocols and routed protocols. Routing
protocols are used to exchange route information and to create a network topology, thus
enabling routing decisions to be made. The routed protocols, on the other hand, contain information
regarding the end systems, how communication is established, and other information
relevant to the transfer of data. The routing protocols will be covered in Chapter 38, “TCP/IP
Routing Protocol Troubleshooting.”
path for PDU delivery. This means that the Network layer also uses protocols to accomplish this task. This section discusses protocols that are used within layer 3 of the OSI model. Some of
these protocols use other protocols within them for finer granularity of certain functions.
There is a significant difference between routing protocols and routed protocols. Routing
protocols are used to exchange route information and to create a network topology, thus
enabling routing decisions to be made. The routed protocols, on the other hand, contain information
regarding the end systems, how communication is established, and other information
relevant to the transfer of data. The routing protocols will be covered in Chapter 38, “TCP/IP
Routing Protocol Troubleshooting.”
Frame Structure
Look at Figure 36.12 to get a picture of the ISDN frame. As you can see, this frame is similar
to the HDLC frame that you studied earlier (Figure 36.11). ISDN uses LAP (Link Access Procedure)
on the D channel for layer 2 functions. Unlike the HDLC frame, the ISDN frame is
bounded by Flag fields.
After the Flag field, again going from right to left, we see the Address field. The Address
field contains several bits of key information:
SAPI This field is the service access point identifier. It defines which services are provided
to layer 3.
C/R This field designates the frame as a command or a response.
EA This is the last bit of the first byte of the Address field. This bit defines the Address field
as one or two bytes. If it is set to one byte, this is the last field within the Address field. If it is
set to two bytes, then one more field follows, ending with another EA bit.
TEI This is the terminal end point identifier, the layer 2 address used to identify individual
devices connecting to an ISDN network.
to the HDLC frame that you studied earlier (Figure 36.11). ISDN uses LAP (Link Access Procedure)
on the D channel for layer 2 functions. Unlike the HDLC frame, the ISDN frame is
bounded by Flag fields.
After the Flag field, again going from right to left, we see the Address field. The Address
field contains several bits of key information:
SAPI This field is the service access point identifier. It defines which services are provided
to layer 3.
C/R This field designates the frame as a command or a response.
EA This is the last bit of the first byte of the Address field. This bit defines the Address field
as one or two bytes. If it is set to one byte, this is the last field within the Address field. If it is
set to two bytes, then one more field follows, ending with another EA bit.
TEI This is the terminal end point identifier, the layer 2 address used to identify individual
devices connecting to an ISDN network.
Integrated Services Digital Network (ISDN)
Integrated Services Digital Network (ISDN) is a service that allows telephone networks to carry
data, voice, and other digital traffic. There are two types of ISDN interfaces: Basic Rate Interface
(BRI) and Primary Rate Interface (PRI). BRI uses two B channels and one D channel. Each
of the two B channels operates at 64Kbps bidirectionally; the D channel operates at 16Kbps.
The B channels are used for transmitting and receiving data. The D channel is used for protocol
communications and signaling.
In contrast, PRI uses 23 B channels and 1 D channel. All 23 B channels are added to a rotary
group, as well. The D channel runs at the same line speed as the B channels—64Kbps. Because of the
D channel’s additional line speed, PRI has the equivalent line speed of a T-1 circuit (1.544Mbps). In
Europe, PRI offers 30 B channels and 1 D channel, making it the equivalent of an E-1 circuit.
Just as there are two types of ISDN interfaces, there are two terminal equipment types. Type 1
(TE1) is equipment that was built specifically for use on ISDN. Type 2 (TE2) is equipment that
was made before the ISDN specifications, and it requires a terminal adapter to actually interface
with ISDN. Terminal equipment, which is comparable to DTE as described in the “Frame Relay”
section earlier in this chapter, includes computers or routers.
In order for terminal equipment to work, it must be able to connect to a network termination.
There are three types of ISDN network terminations, known as NT devices. Type 1 (NT1) devices
are treated as customer premises equipment. Type 2 (NT2) devices are more intelligent devices
than NT1 and can perform concentration and switching functions. The last type is a combination
of Types 1 and 2. It is known as a Type 1/2 or NT1/2.
More information about troubleshooting ISDN is covered in Chapter 40, “Troubleshooting
ISDN.”
data, voice, and other digital traffic. There are two types of ISDN interfaces: Basic Rate Interface
(BRI) and Primary Rate Interface (PRI). BRI uses two B channels and one D channel. Each
of the two B channels operates at 64Kbps bidirectionally; the D channel operates at 16Kbps.
The B channels are used for transmitting and receiving data. The D channel is used for protocol
communications and signaling.
In contrast, PRI uses 23 B channels and 1 D channel. All 23 B channels are added to a rotary
group, as well. The D channel runs at the same line speed as the B channels—64Kbps. Because of the
D channel’s additional line speed, PRI has the equivalent line speed of a T-1 circuit (1.544Mbps). In
Europe, PRI offers 30 B channels and 1 D channel, making it the equivalent of an E-1 circuit.
Just as there are two types of ISDN interfaces, there are two terminal equipment types. Type 1
(TE1) is equipment that was built specifically for use on ISDN. Type 2 (TE2) is equipment that
was made before the ISDN specifications, and it requires a terminal adapter to actually interface
with ISDN. Terminal equipment, which is comparable to DTE as described in the “Frame Relay”
section earlier in this chapter, includes computers or routers.
In order for terminal equipment to work, it must be able to connect to a network termination.
There are three types of ISDN network terminations, known as NT devices. Type 1 (NT1) devices
are treated as customer premises equipment. Type 2 (NT2) devices are more intelligent devices
than NT1 and can perform concentration and switching functions. The last type is a combination
of Types 1 and 2. It is known as a Type 1/2 or NT1/2.
More information about troubleshooting ISDN is covered in Chapter 40, “Troubleshooting
ISDN.”
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