Protocol Identification

What is IPv4 Protocol Identification?

IPv4 Protocol Identification refers to the field in the IPv4 header that specifies the type of transport layer protocol contained in the packet payload, such as TCP, UDP, ICMP, etc.

Why is IPv4 Protocol Identification useful?

It allows the receiving host to determine how to process the packet payload by identifying which protocol handler to pass the data to, enabling proper communication between layers.

How does IPv4 Protocol Identification work?

The IPv4 header contains an 8-bit Protocol field that holds a number corresponding to the encapsulated protocol. Routers forward the packet unchanged, and the destination host uses this field to direct the payload to the correct transport protocol module.

Where is IPv4 Protocol Identification used?

It is used in all IPv4 packets to indicate the payload protocol, making it essential for processing data correctly on the receiving end, across all IP-based networks.

Which OSI layer does IPv4 Protocol Identification belong to?

It is part of the Network Layer (Layer 3) in the OSI model, within the IPv4 header, facilitating interaction between the Network and Transport Layers.

Is IPv4 Protocol Identification Windows specific?

No, IPv4 Protocol Identification is a standard feature of the IP protocol, implemented across all operating systems, including Windows.

Is IPv4 Protocol Identification Linux specific?

No, Linux and all other OSes support this as part of the IPv4 protocol stack.

Which Transport Protocol is used by IPv4 Protocol Identification?

IPv4 Protocol Identification itself indicates which transport protocol is used (e.g., TCP, UDP, ICMP), but it does not use a transport protocol.

Which Port is used by IPv4 Protocol Identification?

IPv4 Protocol Identification does not use ports; ports are part of the transport layer protocols identified by this field.

Is IPv4 Protocol Identification using client-server model?

IPv4 Protocol Identification itself is a header field and not tied to any communication model, including client-server.

What is the Protocol field in the IPv4 header?

The Protocol field in the IPv4 header is an 8-bit field that identifies the higher-layer protocol (e.g., TCP, UDP, ICMP) encapsulated in the IP packet’s payload.

How does the Protocol field assist in packet delivery?

The Protocol field helps the receiving host determine the correct handler for the payload, ensuring that the correct protocol module processes the packet’s data.

Can the Protocol field be customized?

No, the values in the Protocol field are standardized and predefined by the IETF. However, custom values can be used in experimental or proprietary protocols but they are not common.

What are some common Protocol field values?

Common Protocol field values include: - 1 for ICMP - 6 for TCP - 17 for UDP - 58 for ICMPv6

How does IPv4 Protocol Identification affect security?

The Protocol field can be used for filtering or blocking traffic based on the protocol type. Security devices like firewalls use this field to decide which protocols are allowed or denied.

What happens if the Protocol field is misinterpreted?

If the Protocol field is misinterpreted or incorrectly set, the receiving host may not properly handle the packet, leading to errors or dropped packets.

How does IPv4 Protocol Identification relate to IP routing?

The Protocol field is not used for routing decisions, which are based on the destination IP address. However, it is important for ensuring that the packet is processed correctly once it reaches the destination.

Is IPv4 Protocol Identification used in all types of network communication?

Yes, the Protocol field is used in all IPv4 packet communications, whether for web browsing, email, video streaming, or any other type of communication.

Can the Protocol field handle multiple transport protocols in a single packet?

No, each IPv4 packet contains a single Protocol field. If multiple transport protocols need to be used, they must be encapsulated in separate packets.

Does IPv4 Protocol Identification provide any error detection or correction?

No, IPv4 Protocol Identification only indicates the protocol type. Error detection is handled by other fields in the IPv4 header, like the checksum.

How does IPv4 Protocol Identification interact with IPv6?

IPv6 has a similar concept but uses a different header format. The “Next Header” field in IPv6 serves the same purpose as the Protocol field in IPv4, identifying the upper-layer protocol.

Can IPv4 Protocol Identification be used for traffic analysis?

Yes, the Protocol field can be used by network administrators for traffic analysis. By monitoring the Protocol field, admins can understand the distribution of different types of traffic (TCP, UDP, ICMP, etc.).

Is IPv4 Protocol Identification critical for troubleshooting?

Yes, the Protocol field is useful for troubleshooting network issues, such as identifying which transport protocol is causing problems (e.g., TCP connection issues or dropped UDP packets).

Does IPv4 Protocol Identification affect packet size?

No, the Protocol field only specifies the type of payload. The size of the packet is determined by the payload data and the IP header size, which are not influenced by the Protocol field.

Can the Protocol field be used to distinguish between IPv4 and IPv6?

No, the Protocol field is specific to IPv4. IPv6 uses the “Next Header” field to identify the next layer protocol, but the concept is similar.

How does IPv4 Protocol Identification relate to firewall configurations?

Firewalls often use the Protocol field to create rules based on transport protocols. For example, a firewall may allow or block all incoming TCP packets (Protocol = 6) while blocking UDP packets (Protocol = 17).

Can IPv4 Protocol Identification be used to route traffic?

No, the Protocol field does not influence routing decisions. Routing is based solely on the destination IP address, but the Protocol field is used for processing at the destination.

How does IPv4 Protocol Identification affect packet filtering?

Network devices like routers, firewalls, and intrusion detection systems (IDS) use the Protocol field for packet filtering, allowing or denying packets based on the transport protocol type.

Does IPv4 Protocol Identification provide information about encryption?

No, the Protocol field only specifies the transport protocol. If encryption is used (e.g., via IPsec), this is handled at the Network Layer and is not indicated by the Protocol field.

Topics in this section,

  • In this section, you are going to learn

  • Terminology

  • Version Info

Objective

Confirm that the IPv4 header’s Protocol field correctly identifies the encapsulated transport-layer protocol (ICMP, TCP, UDP).

Test Setup

  • Start Wireshark capture on the VM’s network interface.

  • Generate traffic for different protocols: - ICMP (ping) - UDP (DNS query) - TCP (Telnet or HTTP request)

Procedure

Run the following commands sequentially in a terminal. Wait for each command to finish before proceeding.

  • ICMP (Ping)

    test:~$ ping -c 1 8.8.8.8

    Note

    ICMP tests basic network connectivity. The Protocol field in IPv4 header is 1.

  • UDP (DNS Query)

    test:~$ dig @8.8.8.8 www.google.com

    Note

    DNS uses UDP by default. The Protocol field in IPv4 header is 17.

  • TCP (Telnet / HTTP Request)

    test:~$ telnet google.com 80
Trying 142.250.207.142...
Connected to google.com.
Escape character is '^]'.
hello
HTTP/1.0 400 Bad Request
...

    Note

    TCP connection establishes a session. The Protocol field in IPv4 header is 6. The SYN packet marks the start of a TCP handshake.

Analysis

In Wireshark, use display filters to examine each protocol individually:

  • ICMP Packet Analysis

    • Filter: icmp

    • Locate the Echo Request packet.

    • Expand the IPv4 header; the Protocol field = 1.

  • TCP Packet Analysis

    • Filter: tcp.port == 80

    • Locate a packet with the SYN flag set.

    • Expand the IPv4 header; the Protocol field = 6.

  • UDP Packet Analysis

    • Filter: udp.port == 53

    • Locate a DNS query packet.

    • Expand the IPv4 header; the Protocol field = 17.

Note

  • The Protocol field allows routers and end-hosts to identify which transport-layer protocol is encapsulated.

  • This test demonstrates IPv4’s ability to carry multiple protocols simultaneously.

  • Useful for troubleshooting mixed-protocol networks and ensuring proper packet handling by intermediate devices.

Wireshark Capture

Download wireshark capture

Protocol Identification - Testcases

Protocol Identification - Test Cases

#

Test Case

Description

Expected Result

1

Protocol = 1 (ICMP)

IPv4 packet with ICMP

Routed to ICMP handler

2

Protocol = 6 (TCP)

IPv4 packet with TCP

Routed to TCP stack

3

Protocol = 17 (UDP)

IPv4 packet with UDP

Routed to UDP stack

4

Protocol = 2 (IGMP)

IPv4 packet with IGMP

Routed to IGMP handler

5

Protocol = 89 (OSPF)

IPv4 packet with OSPF

Routed to OSPF process

6

Protocol = 47 (GRE)

IPv4 packet with GRE

Routed to GRE tunnel handler

7

Protocol = 50 (ESP)

IPv4 packet with IPsec ESP

Routed to IPsec stack

8

Protocol = 51 (AH)

IPv4 packet with IPsec AH

Routed to IPsec stack

9

Protocol = 132 (SCTP)

IPv4 packet with SCTP

Routed to SCTP stack

10

Protocol = 115 (L2TP)

IPv4 packet with L2TP

Routed to L2TP handler

11

Protocol = 0

Reserved value

Packet dropped or logged

12

Protocol = 255

Reserved for testing

Packet dropped or logged

13

Protocol = Unknown Value

Unsupported protocol

Packet dropped

14

Protocol = 6 with Invalid TCP Header

Malformed TCP

Packet dropped

15

Protocol = 17 with Invalid UDP Header

Malformed UDP

Packet dropped

16

Protocol = 1 with Echo Request

ICMP Echo Request

ICMP reply generated

17

Protocol = 1 with Destination Unreachable

ICMP error

Routed to ICMP handler

18

Protocol = 6 with SYN Packet

TCP handshake

Routed to TCP stack

19

Protocol = 6 with FIN Packet

TCP termination

Routed to TCP stack

20

Protocol = 17 with DNS Query

UDP DNS request

Routed to DNS service

21

Protocol = 17 with DHCP Discover

UDP broadcast

Routed to DHCP server

22

Protocol = 6 with HTTP Traffic

TCP port 80

Routed to web server

23

Protocol = 6 with HTTPS Traffic

TCP port 443

Routed to TLS handler

24

Protocol = 6 with FTP Traffic

TCP port 21

Routed to FTP server

25

Protocol = 6 with SSH Traffic

TCP port 22

Routed to SSH daemon

26

Protocol = 6 with SMTP Traffic

TCP port 25

Routed to mail server

27

Protocol = 6 with POP3 Traffic

TCP port 110

Routed to mail client

28

Protocol = 6 with IMAP Traffic

TCP port 143

Routed to mail client

29

Protocol = 6 with Telnet Traffic

TCP port 23

Routed to Telnet server

30

Protocol = 6 with RDP Traffic

TCP port 3389

Routed to RDP service

31

Protocol = 6 with SQL Traffic

TCP port 1433

Routed to database server

32

Protocol = 6 with Malformed TCP Flags

Invalid flags

Packet dropped or logged

33

Protocol = 6 with TCP Reset

TCP RST flag

Connection reset

34

Protocol = 6 with TCP Retransmission

Duplicate segment

Handled by TCP stack

35

Protocol = 17 with TFTP Traffic

UDP port 69

Routed to TFTP server

36

Protocol = 17 with SNMP Traffic

UDP port 161

Routed to SNMP agent

37

Protocol = 17 with NTP Traffic

UDP port 123

Routed to NTP service

38

Protocol = 17 with Syslog Traffic

UDP port 514

Routed to logging service

39

Protocol = 6 with Fragmented Packet

TCP in fragments

Reassembled before processing

40

Protocol = 17 with Fragmented Packet

UDP in fragments

Reassembled before processing

41

Protocol = 6 with NAT

TCP translated

Protocol field preserved

42

Protocol = 17 with NAT

UDP translated

Protocol field preserved

43

Protocol = 1 with Firewall

ICMP filtered

Packet dropped or allowed

44

Protocol = 6 with Firewall

TCP filtered

Packet dropped or allowed

45

Protocol = 17 with Firewall

UDP filtered

Packet dropped or allowed

46

Protocol = 6 with QoS

TCP prioritized

QoS applied

47

Protocol = 17 with QoS

UDP prioritized

QoS applied

48

Protocol = 6 with Logging Enabled

TCP packet logged

Protocol field recorded

49

Protocol = 17 with Logging Enabled

UDP packet logged

Protocol field recorded

50

Protocol = 6 with IDS/IPS

TCP packet inspected

Protocol used for rule matching

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