TCP/IP Networking Fundamentals: Layers, Routing, Fragmentation, and Subnetting
TCP/IP Networking Fundamentals
In the TCP/IP model, the Network Interface layer corresponds to which layer(s) of the OSI model? - Physical & MAC layers
Give 3 examples of link layer protocols? - IEEE 802.3/Ethernet, Frame Relay, ATM, & S1t
The Network Layer and IP
In the TCP/IP layer, the Network layer is responsible for enabling the routing of data across (logical or physical)? - It is across a logical network path, as it uses packet format & IP address format.
What is the min/max length of an IP header? - 20/65535 octets
Which field in the IP header is responsible for defining the Quality of Service? - It is TOS (Type of Service).
How to prioritize the forwarding of an IP packet? - The priority is set through the Precedence field of the TOS. The TOS is a part of the DiffServ Quality of Service model & it is also used by the OSPF protocol.
What does per-hop behaviour (PHB) refer to? - It refers to the queuing & the treatment behaviour of a router.
How can a router notify its neighbours that it supports congestion? - By setting 21 in the ECN field (Explicit Congestion Notification).
Provide the definition of MTU & state for which purposes it is used? - It defines the maximum size of a frame in the data link layer. In case a packet exceeds the tolerated size, it is then fragmented.
Fragmentation and Reassembly
In which TCP/IP layer is an IP packet fragmented & based on which criteria is it reassembled in the correct order? - A packet is fragmented in the network layer. An identifier is added to each fragment.
Is it always possible that a router fragments a packet or not (we assume that the packet size is higher than the data link capability of a router)? - The fragmentation rule is not absolute. A router can deny fragmentation by setting the flag Don't Fragment (DF) = 1 (DF=1).
How does a receiver detect the end of a fragmented flow (a flow is a set of packets from the same source)? - The receiver detects the end of a fragmented flow when it encounters a fragment with MF=0.
Does a checksum of an IP packet get recalculated at each router or not? - Yes, because the TTL field is decremented at each hop & this field is part of the IP packet header.
Network Addressing and Masks
How does a router or a host determine the network of an IP address? - A router determines the network of a given IPv4 address by using the first octet & the network mask.
What are the 2 main restrictions that apply to network masks in a classful environment? - An IPv4 host address cannot have all its host bits set to 1 (reserved for broadcast), & the host bits cannot be set to 0 (reserved for routers).
ARP Protocol
How does a host discover the data-link identifier of another neighbor? - By using the ARP protocol.
How is an ARP request sent? - The request is encapsulated into a MAC frame. The source MAC address of the frame is the MAC address of the source, & the destination MAC address is the broadcast address (all-hosts address).
Which Cisco command debugs the activity of the ARP protocol? - debug arp
Which Cisco command deletes all the entries of an ARP table? - clear arp-cache
What are gratuitous ARP requests? - They are ARP requests issued by a host using its own IPv4 address as the target. They are used to detect address duplication.
Redundancy and Control Messages
For which purpose is the HSRP (Hot Standby Router Protocol) used? - It is used for fault tolerance. One router is configured to be active & another one is a standby. When the active router fails, the standby takes over.
What is a “Source Quench” message, and when is it used? - A destination host may send a Source Quench message if datagrams arrive too fast to be processed.
How do hosts discover local routers? - By listening to Router Advertisement & Router Selection messages (ICMP message types 9 & 10).
TCP Reliability and Flow Control
By which means does the TCP protocol provide reliability? - TCP uses a system of acknowledgments, checksums, retransmissions & timers to provide reliability.
How does the TCP protocol regulate the flow of IP packets? - The TCP protocol uses a mechanism called windowing to control the flow between pairs & avoid congestion.
What are the essential differences between TCP & UDP? - Correct sequencing, reliability & flow control through the windowing mechanism.
Addressing Design Questions
Instead of ARP, Novell NetWare uses a network address that includes a device's MAC address in the host portion. Why can't IP do this? - If IP used MAC identifiers as the host part of the IP address, subnetting would not be possible.
Subnetting Exercises
Select a subnet mask for 10.0.0.0 so there will be at least 16000 subnets with at least 700 host addresses available on each subnet. - A subnet mask of 255.255.252.0 provides 16382 subnets of the Class A, & 1022 host addresses on each subnet.
Select a subnet mask for 172.27.0.0 so there are at least 500 subnets with at least 100 host addresses available on each subnet. - 255.255.255.128 provides 510 subnets with 126 host addresses on each subnet.
Use a 28-bit mask to derive the available subnets of 192.168.147.0, then derive the available host addresses of each subnet. - 192.168.147.16, 192.168.147.32, 192.168.147.48, …, 192.168.147.224