4.7 NETWORK LAYER PROTOCOLS
• IP (Internet Protocol)
• ICMP (Internet Control Message Protocol)
• ARP (Address Resolution Protocol)
• RARP (Reverse Address Resolution Protocol)

4.7.1 The IP PROTOCOL
• IP represents the heart of the Internet Protocol suite
• Provides the main service of the layer: data transmission in connectionless mode – datagram

IP Header Format IP Header Format
(Including user data, this makes an IP packet)
IP Header fields:

• Version - Indicates the version of IP currently used (now 4);
• IP Header Length - Indicates the datagram header length in 32-bit words (value = 5);
• Type-of-service - Specifies how a particular upper-layer protocol would like the current datagram to be handled. Datagrams can be assigned various levels of importance through this field;
• Total length - Specifies the length of the entire IP packet, including data and header, in bytes.
• Identification - Contains an integer that identifies the current datagram (sequence number). This field is used for re-assembling the datagram fragments;
• Flags - A 3-bit field for fragmentation control:
 000 - last fragment;
 001 - not last fragment;
 010 - the fragmentation not allowed.
• Fragment offset - measured in double-words (offset from main datagram);
• Time-to-live - Maintains a counter that gradually decrements down to zero, at which point the datagram is discarded. This keeps packets from looping endlessly;
• Protocol - Indicates which upper-layer protocol receives incoming packets after IP processing is complete ( 6 - TCP, 17 - UDP);.
• Header checksum - Ensures IP header integrity to make sure all fields are correct;
• Source address - Specifies the sending node;
• Destination address - Specifies the receiving node;
• Options - Allows IP to support various options, such as security;
• Padding - Completes the length of Options field to 32 bits;
• User Data - Contains upper-layer information. The maximum length is 64 KB (header included).
4.7.2 ADDRESS RESOLUTION PROTOCOLS (ARP, RARP)
• Addresses specified in IEEE 802. (Ethernet, Token Ring) have format that differs from IP address (physical vs. network addresses)
• An Ethernet address is 48 bits and an IP address - 32 bits
• Addresses from higher layers of the model have to be transformed to the real hardware addresses and vice versa
• Process is called address resolution :
 IP  IEEE - direct address resolution;
 IEEE  IP - reverse address resolution.

• Address resolution protocol (ARP)/reverse ARP (RARP)
• Generally used to translate IP addresses to Ethernet MAC addresses
• two-part protocol:
 Request from source asking for hardware address; (ARP)
 Reply from destination carrying hardware address. (RARP) Principle of ARP

4.7.3 INTERNET CONTROL MESSAGE PROTOCOL (ICMP)
• Reporting routing failures back to the source
• Provides helpful messages such as the following:
 Echo and reply messages to test node reachability across an internetwork;
 Redirect messages to stimulate more efficient routing;
 Time exceeded messages to inform sources that a datagram has exceeded its allocated time to exist within the internetwork;
 Router advertisement and router solicitation messages to determine the addresses of routers on directly attached subnetworks.

4.8 IP Addressing
• Address is 32 bits in length
• Divided into either two or three parts
First part designates the network address
Second part (if present) designates the subnet address
Final part designates the host address
• IP addresses are written in dotted decimal format, e.g. 172.16.254.1
• The above example address is equivalent to 10101100. 00010000. 11111110. 00000001 in binary form
• Different network address classes: A, B, C, D, E: a, b, c and d represent the four bytes in the dotted-decimal address notation

Class First byte in binary Range of first byte Network ID (bits) Host
ID (bits) No. of
Networks No. of Hosts
A 0XXXXXXX 0 - 127 a (8) b.c.d (24) 128 = (27) 16,777,214 = (224-2)
B 10XXXXXX 128 – 191 a.b (16) c.d (16) 16,384=(214) 65,534 = (216-2)
C 110XXXXX 192 - 223 a.b.c(24) d (8) 2,097,152=(221) 254=(28-2)

• No. of networks calculated from: 2(8-1), 2(16-2), 2(24-3) subtract the number of first bits from the total bits for network ID
• Description of address ranges
 Class A: first byte starts as 0xxxxxxx
 The range (in binary) is
 0000000 to 01111111 (0 to 127)

 Class B: first byte starts as 10xxxxxx
 The range (in binary) is
 10000000 to 10111111 (128 to 191)

 Class C: first byte starts as 110xxxxx
 The range (in binary) is
 11000000 to 11011111 (192 to 223)

4.9 THE FUTURE OF IP - IPv6
• Motivation for change
 Address space
 Type of services
After many proposals for IP developing, a new version of IP protocol is created - IP version 6 - IPv6
New features
• New address size o IPv6 addresses are 128 bits. The address includes network prefix and host suffix
• New header format o A support of old-version headers and a new feature is introduced: header extension
• Support for video and audio o A flow label and quality of services are introduced as protocol fields

4.10 TRANSPORT LAYER PROTOCOLS
• Internet transport layer is implemented by
 Transmission Control Protocol (TCP) - connection-oriented data transport
 User Datagram Protocol (UDP) - operation is connectionless

4.10.1 Transmission Control Protocol (TCP)
• Responsible for
 Breaking up the message into datagrams
 Reassembling them at the other end
 Resending anything that gets lost – retransmissions
 Putting things back in the right order - reassembly
 Provides full-duplex, acknowledged, and flow-controlled service to upper-layer protocols
 Can also support numerous simultaneous upper-layer conversations

TCP header format
Source Port / 16 b / Destination Port / 16 b/
Sequence Number / 32 b /
Acknowledgement Number / 32 b /
Header length /4b/ Reserved /6b/ Code Bits /6b/ Window / 16 b /
Check Sum / 16 b / Urgent Pointer /16b/
Options ( + Padding) / 32 b /
Data (variable)

Source port and destination port - Identify the points at which upper-layer source and destination processes receive TCP services
Sequence number - Usually specifies the number assigned to the first byte of data in the current message. Under certain circumstances, it can also be used to identify an initial sequence number to be used in the upcoming transmission
Acknowledgment number - Contains the sequence number of the next byte of data (or packet) the sender of the packet expects to receive
Header length - Indicates the length of the TCP header in of 32-bits words in the TCP header
Reserved - Reserved for future use
Code Bits - Carries a variety of control information
e.g. to determine the purpose and contents of the segment the bits tell how to interpret other fields in the header according to: (left to right – meaning if bit set to 1) (e.g. 111111)
URG – Urgent pointer field is valid
ACK – Acknowledgement field is valid
PSH – This segment requires a push
RST – Reset the connection
SYN – Synchronise sequence numbers
FIN – Sender has reached end of its byte stream
Window - Specifies the size of the sender's receive window (that is, buffer space available for incoming data)
Checksum - Indicates whether the header was damaged in transit
Urgent pointer - Points to the first urgent data byte in the packet
Options - Specifies various TCP options
Data - Contains upper-layer information (user data)

• Maximum Segment Size
• Segments (packets) differ in size, therefore communicating ends must specify the maximum segment size they will transfer
• This is done in the OPTIONS field; TCP software specifies the Maximum Transfer Unit (MTU)
• Important for computers connected high-speed LANs in order to make good use of the bandwidth
• If the two endpoints are on the same physical network, TCP computes a maximum segment size such that the resulting IP datagrams will match the network MTU
• If the two endpoints are on different physical networks, they can attempt to compute a minimum MTU, or choose an MTU of 536 (default – 576 minus standard IP and TCP headers)
4.10.2 User Datagram Protocol (UDP)
• Much simpler protocol than TCP
• Useful in situations where the reliability mechanisms of TCP are not necessary
• Provide protocol ports used to distinguish among multiple programs executing within a single machine
• Provides unreliable, connectionless delivery
• Does not use acknowledgements to make sure a message arrives
• Does not order incoming messages
• Does not provide feedback to control the rate at which information flows between machines
• UDP messages (packets) can be lost, duplicated, or arrive out of order
• UDP header has only four fields
 Source port: same as TCP
 Destination port: same as TCP
 Length: UDP header and data
 UDP checksum: optional