Routing TCP/IP, Vol. 1: Chapter 1

What is the purpose of the TTL field in the IP header? How does the TTL process work?

The Time to Live (TTL) field prevents "lost" packets from being passed endlessly through the IP network. The field contains an 8-bit integer that is set by the originator of the packet. Each router through which the packet passes will decrement the integer by one. If a router decrements the TTL to zero, it will discard the packet and send an ICMP "time exceeded" error message to the packet's source address.

Instead of ARP, Novell NetWare uses a network address that includes a device's MAC address as the host portion. Why can't IP do this?

A MAC identifier is a fixed-length binary integer. If IP used MAC identifiers as the host part of the IP address, subnetting would not be possible because there would be no flexibility in using some of the host bits as network bits.

What is ARP?

ARP, or Address Resolution Protocol, is a function that maps the IP addresses of interfaces on a data link to their corresponding MAC identifiers.

What is an address mask, and how does it work?

An IP address mask identifies the network part of an IP address. Each one in the 32-bit mask marks the corresponding bit in the IP address as a network bit. A zero in the mask marks the corresponding bit in the IP address as a host bit. A Boolean AND is performed in all 32 bits of the address and the mask; in the result, all network bits of the mask will be repeated, and all host bits will be changed to zero.

What mechanisms does TCP use to provide connection-oriented service?

Correct sequencing is accomplished with sequence numbers. Reliability is accomplished by using checksums, acknowledgments, timers, and retransmissions. Flow control is accomplished by windowing.

What is Proxy ARP?

Proxy ARP is a function of an IP router. If the router hears an ARP request, and; The destination network or subnet is in the router's routing table, and; The table indicates that the destination is reachable via a different router interface than the one on which the ARP request was received; The router will respond to the ARP request with its own MAC address.

Why can't a subnet of all zeros or all ones be used in a classful routing environment?

A classful routing protocol has no way to differentiate between the all-zeros subnet and the major IP address, and between the all-ones subnet and the all-hosts, all-subnets broadcast address of the major IP address.

What is a redirect?

A redirect is an IP router function. If a device has sent a packet to the router and the router must forward the packet to a next-hop router on the same data link, the router will send a redirect to the originating device. The redirect will inform the device that it can reach the next-hop router directly

What is a subnet? Why are subnets used in IP environments?

A subnet is a subgrouping of a class A, B, or C IP address. Without subnetting, the network part of a major class A, B, or C IP address can only identify a single data link. Subnetting uses some of the host bits of a major IP address as network bits, allowing the single major address to be "subdivided" into multiple network addresses.

What is fragmentation? What fields of the IP header are used for fragmentation?

Routers perform fragmentation when a packet is longer than the maximum packet length (Maximum Transmission Unit, or MTU) supported by a data link onto which the packet must be transmitted. The data within the packet will be broken into fragments, and each fragment will be encapsulated in its own packet. The receiver uses the Identifier and Fragment Offset fields and the MF bit of the Flags field to reassemble the fragments.

What is the essential difference between TCP and UDP?

TCP, or Transmission Control Protocol, provides a connection-oriented service over the connectionless internet layer. UDP, or User Datagram Service, provides a connectionless service.

What is the first octet rule?

The first octet rule determines the class of an IP address as follows: Class A: The first bit of the first octet is always 0. Class B: The first two bits of the first octet are always 10. Class C: The first three bits of the first octet are always 110. Class D: The first four bits of the first octet are always 1110. Class E: The first four bits of the first octet are always 1111.

What are the five layers of the TCP/IP protocol suite? What is the purpose of each layer?

The five layers of the TCP/IP protocol suite are the following: Physical layer Data-link layer Internet (or IP) layer Host-to-host layer Application layer The physical layer contains the protocols of the physical medium. The data link layer contains the protocols that control the physical layer: how the medium is accessed and shared, how devices on the medium are identified, and how data is framed before being transmitted on the medium. The internet layer contains the protocols that define the logical grouping of data links into a network and the communication across that network. The host-to-host layer contains the protocols that define and control the logical, end-to-end paths across the network. The application layer corresponds to the OSI session, presentation, and application layers.

What is the most common IP version presently in use?

The most common IP version now in use is version 4.

What purpose does UDP serve by providing a connectionless service on top of what is already a connectionless service?

The only purpose of the UDP header is to add fields for the source and destination port numbers.