Internet Protocol

Computer Networks Network Layer In The Internet by Abdul Ghaffar

Network Layer In The Internet Overview This section is TCP specific, It’s how the Internet works. Defined by RFC 791. Most Popular Layer 3.

Network Layer In The Internet The Internet Protocol (IP) is a network layer protocol. Hosts and gateways process packets called Internet datagrams (IP datagrams). IP provides connectionless, best-effort delivery service to the layers above it. The Transmission Control Protocol (TCP) is a transport layer protocol. Provides reliable stream service between processes on two machines. It uses acknowledgments and retransmissions methods to overcome the unreliability of IP. The Universal Datagram Protocol (UDP) is a Transport Layer Protocol. It provides connectionless datagram service between processes. The IP Protocol

Network Layer In The Internet INTERNET PROTOCOL (IP) The goal of IP is to interconnect networks of diverse technologies and create a single, virtual network to which all hosts connect. Hosts communicate with other hosts by handing datagrams to the IP layer; The sender doesn't worry about the details of how the networks are actually interconnected. IP provides unreliable, connectionless delivery service. IP defines a universal packet called an Internet Datagram. The IPV4 Protocol All Internet hosts and gateways process IP datagrams.

Network Layer In The Internet 1. Version number (4-bits): The current protocol version is 4. Including a version number allows a future version of IP be used along side the current version, facilitating migration to new protocols. 2. Header length (4-bits): Length of the datagram header (excluding data) in 32-bit words. The minimum length is 5 words = 20 bytes, but can be up to 15 words = 60 bytes, if options are used. In practice, the length field is used to locate the start of the data portion of the datagram. The IPV4 Protocol

Network Layer In The Internet 3. Type-of-service (8-bits): (last 2 unused) A hint to the routing algorithms as to what type of service we desire. Precedence (3-bits): A priority indication, where 0 is the lowest and means normal service, while 7 is highest and is intended for network control messages (e.g., routing, congestion control). Delay (1-bit): An Application can request low delay service (e.g., for interactive use). Throughput (1-bit): Application requests high throughput. Reliability (1-bit): Application requests high reliability. The IPV4 Protocol

Network Layer In The Internet Note: These last three TOS bits will generally be mutually exclusive. Does setting the low-delay bit guarantee getting such service? No. The type-of-service field is meant as a request or hint to the routing algorithms, but does not guarantee that your request can be honored (e.g., there may not be a low-delay path available). In practice, routers ignore the TOS field in IPV4. The IPV4 Protocol

Network Layer In The Internet 4. Total length (16-bits): Total length of the IP datagram (in bytes), including data and header. The size of the data portion of the datagram is the total length minus the size of the header. The IPV4 Protocol

Network Layer In The Internet 5 - 8. Identification (16-bits), Flags (3-bits), Fragment offset (13-bits): These three fields are used for fragmentation and reassembly. Gateways along a path are free to fragment datagrams as needed; hosts are required to reassemble fragments before passing complete datagrams to the higher layer protocols. Each fragment contains a complete copy of the original datagram header plus some portion of the data. A receiving host must match arriving fragments with the proper original datagram. These fragments may be out of order and interleaved with other fragments. All fragments of a datagram will have the same source and destination IP address The IPV4 Protocol

Network Layer In The Internet 5 - 8. Identification (16-bits), Flags (3-bits), Fragment offset (13-bits) (Continued): The identification field uniquely identifies fragments of the same original datagram. Whenever a host sends a datagram, it sets the identification field of the outgoing datagram and increments its local identification counter. The offset field shows order of the fragments. When a gateway fragments a datagram, it sets the offset field of each fragment to reflect at what data offset with respect to the original datagram the current fragment belongs. Fragmentation occurs in 8-byte chunks, so the offset holds the “chunk number”. Gateways can further fragment fragments! A 400-byte fragment having an offset of 300 chunks could be split into two 200-byte fragments having offsets of 300 and 325 chunks, respectively. The IPV4 Protocol

Network Layer In The Internet We need to know when we’ve received all of the fragments. To help with this, the flags field may contain: A Don't Fragment indication (set by host, honored by gateways). (A 1-bit flag.) The More Fragments field indicates that another fragment follows this one. This fragment is not the last fragment of the original datagram. An unfragmented datagram has an offset of 0, and a More Fragment bit of 0. The last fragment of a fragmented datagram contains More Fragment = Clear and the Offset non-zero. Note: The total length field of the IP header refers to the current datagram, not the original. Thus, the More Fragment bit is needed in order for the recipient host to determine when it has all fragments of a datagram. The IPV4 Protocol

Network Layer In The Internet 5 - 8. Identification (16-bits), Flags (3-bits), Fragment offset (13-bits) (Continued): Example: Original Frame: IHL = 5, Length = 656, Fragment Offset = 0, More = 0 Fragment 1: IHL = 5, Length = 256, Fragment Offset = 0, More = 1 Fragment 2: IHL = 5, Length = 256, Fragment Offset = 32, More = 1 Fragment 3: IHL = 5, Length = 144, Fragment Offset = 64, More = 0 The IPV4 Protocol

Network Layer In The Internet 9. Time-to-live (8-bits): A counter that is decremented by each gateway. If this hop count reach 0, discard the datagram. Originally, the time-to-live field was intended to reflect real time. In practice, it is now a hop count. The time-to-live field squashes looping packets. It also guarantees that packets don't stay in the network for longer than 255 seconds, a property needed by higher layer protocols that reuse sequence numbers. 10. Protocol (8-bits): What type of data the IP datagram carries (e.g., TCP, UDP, etc.). Needed by the receiving IP to know the higher level service that will next handle the data. The IPV4 Protocol

Network Layer In The Internet 11. Header Checksum (16-bits): A checksum of the IP header (excluding data). The IP checksum is computed as follows: Treat the data as a stream of 16-bit words Compute the 1's complement sum of the 16-bit words. Take the 1's complement of the computed sum. It has the property that the order in which the 16-bit words are summed is irrelevant. We can place the checksum in a fixed location in the header, set it to zero, compute the checksum, and store its value in the checksum field. On receipt of a datagram, the computed checksum calculated over the received packet should be zero The header must be recalculated at every router since the time_to_live field is decremented. The IPV4 Protocol

Network Layer In The Internet 12. Source address (32-bits): Original sender's address. 13. Destination address (32-bits): Datagram's ultimate destination. The IP embedded datagram contains the source of the original sender (not the forwarding gateway) and the destination address of the ultimate destination. The IPV4 Protocol

Network Layer In The Internet 14. IP Options IP datagrams allow the inclusion of optional, varying length fields that need not appear in every datagram. We may sometimes want to send special information, but we don't want to dedicate a field in the packet header for this purpose. Options start with a 1-byte option code,followed by1-byte option length field, followed by zero or more bytes of option data. The option code byte contains three parts: copy flag (1 bit): If 1, replicate option in each fragment of a fragmented datagram. That is, this option should appear in every fragment as well. If 0, option need only appear in first fragment. option class (2 bits): Purpose of option: 0 = network control 1 = reserved 2 = debugging and measurement 3 = reserved option number (5 bits): A code indicating the option's type. See Figure 5.46 for these. The IPV4 Protocol

Network Layer In The Internet Class A addresses start with a `0' in the most significant bit, followed by a 7-bit network address and a 24-bit local part. Class B addresses start with a `10' in the two most significant bits, followed by a 14-bit network number and a 16-bit local part. Class C addresses start with a `110' in the three most significant bits, followed by a 21-bit network number and an 8-bit local part. Class D addresses start with a `1110' in the four most significant bits, followed by a 28-bit group number. Used for multicast. Class E addresses start with a ‘11110’ and are reserved for future use. IPV4 Addresses Address Classes The Internet designers were unsure whether the world would evolve into a few networks with many hosts (e.g., large networks), or many networks each supporting only a few hosts (e.g., small networks). Thus, Internet addresses handle both large and small networks. Internet address are four bytes in size, where:

Network Layer In The Internet IPV4 Addresses

Network Layer In The Internet Address Classes The use of fixed-sized IP addresses makes the routing operation efficient. In the ISO world, addresses are of varying format and length and extracting the address from the packet may not be straightforward. Registration of addresses is through the NIC (Network Information Center.) See Figure 5.48 for the use of special addresses. IPV4 Addresses

Network Layer In The Internet Goals: We want to be able to reduce the number of networks seen by the outside world; We want to simplify the management of those many networks within the organization; We want to be able to slice the network/node “pie” in various ways. A large organization or campus might have 30 or more LANs (one for each department). An organization will probably have only a single connection to the rest of the Internet. In order for every local host to be able to communicate with other Internet machines, routing entries for each of the 30 networks must exist in the core gateways. In order for other sites to be able to respond to our queries, they must be able to route packets back to us. Wouldn't it be nice if we only needed to advertise a single network number for all 30 networks? The Answer: Subnet addressing is a technique that allows a set of multiple, interconnected networks to be covered by a single IP network number. IP addresses have a well-defined structure that allows a gateway to extract the network portion of an address by simply looking at its class and an optional netmask. Subnets This usage of “Subnets” is different from that we used before to define the routers and lines in a network.

Network Layer In The Internet With subnetting, the local part of an IP address is further subdivided into a network and a host part: Consider two addresses 128.204.2.29 and 128.204.3.109. Are they on the same network? NO. They refer to hosts on the same network address (128.204), but they can actually be on different ethernets connected by a bridge. To do this, we divide the local part (the two bytes to the right of 128.204) into a 1-byte network part and a 1-byte host part. When sending data to 128.204.3.109 local gateways first route datagrams to the (sub)network 128.204.3 rather than (IP network) 128.204. 128.204.2 and 128.204.3 are distinct (sub)networks. To the outside world, there is only a single network 128.204. Each of the individual networks is called a subnet. Subnets

Network Layer In The Internet With subnetting, the local part of an IP address is further subdivided into a network and a host part: Consider two addresses 128.204.2.29 and 128.204.3.109. Are they on the same network? YES. They refer to hosts on the same network address (128.204), but they can actually be on different Ethernets connected by a bridge. To do this, we divide the local part (the two bytes to the right of 128.204) into a 7-bit network part and a 9-bit host part. Our example above is a Class B address; the technique applies also to Classes A and C. Subnets

Network Layer In The Internet Subnets

Internet Protocol

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Internet Protocol