This document discusses Internet Protocol version 4 (IPv4) and the Internet Control Message Protocol (ICMP). It provides details on IPv4 including that it is an unreliable, connectionless protocol operating at layer 3. It describes IPv4 header fields and fragmentation. It also explains that ICMP is used for error reporting and network queries since IPv4 lacks these functions. Specific ICMP message types are outlined including echo request/reply, destination unreachable, and source quench.

1. Yarmouk University – Faculty of Information
Technology and CS Computer Science
Department
Network Layer Protocols

2. Internet Protocol (IPv4)
• It is a layer 3 protocol
– Host-to-host network layer
delivery protocol for the
internet.
– It is unreliable and
connectionless protocol.
• No error control.
• No flow control.
• It has error detection (discard)
– If reliability is of concern,
the IP has to be tied to a
connection-oriented
protocol (i.e. TCP)
- In reality, each packet (datagram) is
handled independently:
- Each packet can follow different
rout to destination.
-Thus, Packets may arrive out
of order, dropped and/or lost.
 IP relies on above layers to take care
of the above issue.

3. Internet Protocol (IPv4): Header Format
Layer 3 PDU

4. Internet Protocol (IPv4): Header Format
Field Length Description
Version 4 bit Identifies the version of IP used to generate the datagram
HLEN 4 bit Specifies the length of the IP header, including the length of any
options and padding. The normal value of this field when no options
are used is (20 bytes) value must be multiplied by 4 to give the length in bytes
Service
(TOS)
8 bit Type of Service (TOS): A field designed to carry information to
provide quality of service features, such as prioritized delivery, for IP
datagram.
Total Length (TL) 16 bit Specifies the total length of the IP datagram, in bytes. Since this field
is 16 bits, the maximum length of an IP datagram is 65,535 bytes
(2^16 – 1) of which 20-60 byte is the header.
Identification 16 bit This field is used by the receiver to reassemble messages without
mixing fragments from different messages.
Flags 3 bit Control flags to manage fragmentation
Fragmentation
offset
13 bit This field specifies the offset, or position, in the overall message
where the data in this fragment goes.
Time to live (TTL) 8 bits Specifies how long the datagram is allowed to “live” on the network,
in terms of router hops. Each router decrements the value of the TTL
by one prior to transmitting it. If the TTL =0, the datagram is
discarded.

5. Internet Protocol (IPv4): Header Format
Field Length Description
Protocol 8 bit Identifies the higher layers protocols (transport or encapsulated
network layer protocols) carried in the datagram
Header checksum 16 A checksum computed over the header to provide basic protection
against corruption in transmission
Source address 32 bit The 32-bit IP address of the originator of the datagram
(intermediate device will not change it)
Destination address 32 bit The 32-bit IP address of the intended recipient of the datagram.
(intermediate device will not change it)
options var One or more of several types of options may be included after the
standard headers in certain IP datagram
• The header length (HLEN) value must be multiplied by 4 to give the length in bytes
• Total Length (TL) defines the total datagram length including the data part in bytes.
• to find the length of the data coming from the upper layer (upper layer PDU)
Length of data = total length (Tl) – [header length (HLEN)*4]

6. Internet Protocol (IPv4): Header Format
8 bit 8 bit
Transport
layer
protocols

7. Internet Protocol (IPv4): Header Format
• The datagram has to be less then the defined MTU (L2). Otherwise a fragmentation is a must
• To make the IP independent of the physical network, the maximum length of the IP
datagram has to be equal to the MTU (65,535 bytes). Otherwise a fragmentation has to occur
• at the host if the IP datagram > the host MTU.
• at the router if the IP datagram > the router MTU.
•Can we guarantee that the fragmented datagram travel using the same route?
•NO

8. Internet Protocol (IPv4): Header Format
Data has to be always equal to MTU, if less then the MTU, then packet is padded
with 0’s until it is equal to the MTU.

9. Network Protocol: ICMP
• The IP protocol has no error-reporting or error-correcting
mechanism (unreliable and connectionless
delivery).
• The IP protocol also lacks a mechanism for host
and management queries ( a host need to know if a
router or another host is alive).
• The Internet Control Message Protocol (ICMP)
has been designed to compensate for the above
two deficiencies.
– It is a companion to the IP protocol.

10. Network Protocol: ICMP: Encapsulation
• ICMP is Layer 3 protocol.
– It is crafted in the network layer itself.
• ICMP Packets are encapsulated within the IP
datagram.
• The IP datagram is passed directly to Layer 2.

11. Network Protocol: ICMP: Message Format
• The Protocol field within the IP header is 1 to indicate it is an ICMP Message.
• ICMP always reports error messages to the original source.

12. Network Protocol: ICMP: Error Reporting
• Error Messages are always sent back to the source, WHY?
– The only information available in the datagram about the route is the
source and destination IP.
Important points about ICMP error messages:
❏ No ICMP error message will be generated in response to a datagram
carrying an ICMP error message.
❏ No ICMP error message will be generated for a fragmented
datagram that is not the first fragment.
❏ No ICMP error message will be generated for a datagram having a
multicast address.
❏ No ICMP error message will be generated for a datagram having a
special address such as 127.0.0.0 or 0.0.0.0.

13. Network Protocol: ICMP: Error Reporting
• Type 3: when a router cannot route a datagram, or a host cannot deliver a datagram, the
datagram is discarded and the sender is notified through ICMP message type 3.
•Type 4: when a router or a host discard a datagram due to congestion in the path from
source to destination. Thus the sender is notified on the error and quenched to slow down
the transmission.
• Type 11: to notify the sender when a router discard a datagram with TTL =0 and/or
when all fragments did not reach the destination within a certain time window.
• Type 12: to notify the sender when a router or any host discarded a datagram because
there is an error/ missing info from any field of the datagram.
• Type 5: to notify the sender when he sends a datagram to the wrong router. The
receiving router will forward the datagram to the correct router and notify the host to
update his routing table.

14. 14
Network Protocol: ICMP: Types
Type Code Meaning
0 0 echo reply
3 0 network unreachable
3 1 host is unreachable
3 3 port is unreachable
4 0 source quench
5 0 redirect
8 0 echo request
9/10 0 router discovery/advertisement
11 0 time exceed
12 0 parameter problem
13/14 0 time stamp request
17/18 0 network request/reply

15. Network Protocol: ICMP: Echo Request/Reply
• PING sends icmp type 8 echo request to a
node and expects an icmp type 0 echo reply
Send
Reply
8
0
0
0

16. Network Protocol: ICMP: Destination unreachable
• Router is unable to deliver datagram, it can
return the ICMP type 3 with failure code
Destination unreachable Reply
3
0/1/3
Sent

17. Network Protocol: ICMP: Source Quench
• Router detected hosts were overloaded would send
this message to hosts to reduce the rate at which
subsequence message are sent
Source quench
4 0
Sent