1.The TCP/IP Architecture

• The PDUs exchanged by the peer TCP protocols are called TCP segments or segments
• The PDUs exchanged by UDP protocols are called UDP datagrams or datagrams.
• The PDUs exchanged by IP protocols are called IP packets or packets.

IP multiplexes TCP segments and UDP datagrams and performs fragmentation. IP packets are sent to the network interface for delivery across the physical network.

At the receiver: (1) packets passed up by the network interface are demultiplexed to the appropriate protocol(IP, ARP, RARP).

(2) The receiving IP entity determines whether a packet should be sent to TCP or UDP.

(3) The TCP(UDP) sends each segment(datagram) to the appropriate application based on the port number. such as Ethernet, Token Ring, ATM, PPP.

The logic addresses need to be converted into specific physical addresses to carry out the transfer of bits from one device to the other. This conversion is done by an address resolution protocol.

Each host in the Internet is identified by a globally unique IP address.

An IP address is divided into two parts:

• a network ID
• a host ID

The Internet layer provides for the transfer of information across multiple networks through the routers.

• At each router the network interface layer is used to encapsulate the IP packet into a packet or frame of the underlying network or link.
• The IP packet is recovered at an exit router of the given network. This router must determine the next hop in the route to the destination and then encapsulate the IP packet into the frame of the type of the next network or link.

To enhance the scalability of the routing algorithms and to control the size of a routing tables, additional levels of hierarchy are introduced in the IP addresses. Within a domain the host address is further subdivided into a subnetwork part and an associated host part.

Addresses of multiple domains can be aggregated to create supernets.

2.The Internet Protocol

IP corresponds to the network layer in the OSI reference model and provides a connectionless best-effort delivery service to the transport layer.

• best-effort: IP will try its best to forward packets to the destination, but does not guarantee that a packet will be delivered to the destination.

2.1 IP Packet

The header has a fixed-length component of 20 bytes plus a variable-length component consisting of options that can be up to 40 bytes. IP packets are transmitted according to network byte order in the following groups: bits 0–7, bits 8–15, bits 16–23, and finally bits 24–31 for each row

• Version: The version field indicates the version number used by the IP packet so that revisions can be distinguished from each other.
• Internet Header Length: The Internet Header Length(IHL) specifies the length of the header in 32-bit words. If no options are present, IHL will have a value of 5
• Type of Service: The type of service (TOS) field traditionally specifies the priority of the packet based on delay, throughput, reliability, and cost requirements.
  • Three bits are assigned for priority levels (called “precedence”)
  • Four bits for the specific requirement (i.e., delay, throughput, reliability, and cost).
  • delay bit to one to get the high priority.
• Total Length: The total length specifies the number of bytes of the IP packet including header and data. with 16 bits assigned, the maximum packet length is 65535 bytes. However, the Ethernet limits(Physical Networks) has limitation of 1500 bytes.
• Identification, Flags, and Fragment Offset: These fields are used for fragmentation and reassembly.
• Time to live: The time-to-live (TTL) field is defined to indicate the amount of time in seconds the packet is allowed to remain in the network. However, most routers interpret this field to indicate the number of hops the packet is allowed to traverse in the network.
  • this field prevents packets from wandering aimlessly in the Internet
• Protocol: The protocol field specifies the upper-layer protocol that is to receive the IP data at the destination host. Examples of the protocols include TCP (protocol = 6), UDP (protocol = 17), and ICMP (protocol = 1).
• Header checksum: The header checksum field verifies the integrity of the header of the IP packet.
• Source IP address and destination IP address: These fields contain the addresses of the source and destination hosts.
• Options: The options field, which is of variable length, allows the packet to request special features such as security level, route to be taken by the packet, and timestamp at each router.
• Padding: This field is used to make the header a multiple of 32-bit words.

The IP packet is passed to the Router, the following process take place:

(1) The header checksum is computed and the fields in the headerr (version and total length) are checked to see if they contain valid values.

(2) the router identifies the next hop for the IP packet by consulting its routing table.

(3) Update the TTL and header checksum fields. The IP packet is then forwarded along the next hop.

2.2 IP Addressing

A node (such as a router or a multihomed host) may have multiple network interfaces with each interface connected to a different network.

• An IP address has a fixed length of 32 bits.
• The network ID identifies the network the host is connected to. , all hosts connected to the same network have the same network ID.
• The host ID identifies the network connection to the host rather than the actual host.

An implication of this powerful aggregation concept is that a router can forward packets based on the network ID only, thereby shortening the size of the routing table significantly.

• Class A addresses have seven bits for network IDs and 24 bits, for host IDs, allowing up to 126 networks and about 16 million hosts per network.
• Class B addresses have 14 bits for network IDs and 16 bits for host IDs, allowing about 16,000 networks and about 64,000 hosts for each network.
• Class C addresses have 21 bits for network IDs and 8 bits for host IDs, allowing about 2 million networks and 254 hosts per network.
• Class D addresses are used for multicast services that allow a host to send information to a group of hosts simultaneously.
• Class E addresses are reserved for experiments.

2.3 Subnet Addressing

Problem: With the original addressing scheme, it would be a gigantic task for the local network administrator to manage all 64,000 hosts.

Solution: Subnetting. Add another hierarchical level called ‘subnet’.

• Subnet mask: the router needs to store an additional quantity to find the subnet number.

Calculation Process:

(1) The IP Address is:

10010110 01100100 00001100 10110000

(2) The Subnet mask is:

11111111 11111111 11111111 10000000

(3) The subnet number can be determined by performing a binary AND between the subnet mask and the IP address.

10010110 01100100 00001100 10000000

The Router Process Example

(1) a Packet having a destination IP address of 150.100.15.11 arrives from the outside network R1 has to know the next-hop router to send the packet to.

(2) Change the IP address to binary string, and apply the subnet mask. Finally get the subnet binary string.

(3) Router R1 looks up this subnet number in its routing table, and findsthe corresponding entry to specify the next-hop router address for R2.

(4) When R2 recieves the packet, R2 performs the same process and finds out that the destination host is connected to one of its network interfaces.

2.4 IP Routing

The IP layer in the end-system hosts and in the routers work together to route packets from IP network sources to destinations. The IP layer in each host and router maintains a routing table that it uses to determine how to handle each IP packet.

• If its routing table indicates that the destination host is directly connected to the originating host by a link or by a LAN, then the IP packet is sent directly to the destination host using the appropriate network interface.

• Otherwise, the routing table typically specifies that the packet is to be sent to a default router that is directly connected to the originating host.

Now consider the action of a router. When a router receives an IP packet from one of the network interfaces:

• the router examines its routing table to see whether the packet is destined to itself, and if so, delivers the packet to the appropriate higher-layer protocol.
• If the destination IP address is not the router’s own address, then the router determines the next-hop router and the associated network interface, and then forwards the packet.

Each row in the routing table must provide the following information:

(1) destination IP address

(2) IP address of next-hop router

(3) several flag fields

(4) outgoing network interface

(5) other information such as subnet mask, physical address, and statistics information.

• the H flag indicates whether the route in the given row is to a host (H = 1) or to a network (H = 0).
• The G flag indicates whether the route in the given row is to a router (gateway; G = 1) or to a directly connected destination (G = 0).

The routing table is searched in the following order:

(1) The destination column is searched to see whether the table contains an entry for the complete destination IP address. If so, then the IP packet is forwarded according to the next-hop entry and the G flag.

(2) If the table does not contain the complete destination IP address, then the routing table is searched for the destination network ID. If an entry is found, the IP packet is forwarded according to the next-hop entry and the G flag.

(3) If the table does not contain the destination network ID, the table is searched for a default router entry, and if one is available, the packet is forwarded there.

(4) If none of the above searches are successful, then the packet is declared undeliverable and an ICMP “host unreachable error” packet is sent back to the originating host.

2.5 Classless Interdomain Routing(CIDR)

CIDRE: An arbitrary prefix length to indicate the network number. Using a CIDR notation, a prefix 205.100.0.0 of length 22 is written as 205.100.0.0/22.

With CIDR, packets are routed according to the prefix of the IP address without distinguishing different address classes.

The entries in a CIDR routing table contain a 32-bit IP address and a 32-bit mask.

CIDR enables a technique called supernetting to allow a single routing entry to cover a block of classful addresses.

Example:

(1) the original contiguous four class c entries:

Class C address 205.100.0.0 = 11001101 01100100 00000000 00000000

Class C address 205.100.1.0 = 11001101 01100100 00000001 00000000

Class C address 205.100.2.0 = 11001101 01100100 00000010 00000000

Class C address 205.100.3.0 = 11001101 01100100 00000011 00000000

(2) the route aggregation:

Mask 255.255.252.0 = 11111111 11111111 11111100 00000000

Supernet address 205.100.0.0 = 11001101 01100100 00000000 00000000