Lecture 6 Flashcards
Network Layer
What are the 2 primary responsibilities of the router?
- To run routing algorithms and protocols
- To forward datagrams from incoming links to the correct outgoing links
What happens if the datagrams arrive faster than they can be forwarded?
Queuing occurs.
What are the input port functions?
- Line termination
- Data link processing
- Lookup and forwarding
What are the output port functions?
- Buffer management
- Data a link processing
- Line termination
What are the 3 main components of the Internet Network layer?
- IP
- Routing protocols
- Internet Control Message Protocol (ICMP)
What is contained in a datagram?
- IP Version number (4 bits)
- Number to determine the size of the header of the datagram (4 bits)
- Type of service to allow different types of IO datagrams (8 bits)
- Datagram length indicator (16 bits)
- 16 bit identifier
- Flags (3 bits)
- 13 bit Fragmentation offset
- Time to live (8 bits)
- Upper layer protocol (8 bits)
- Header checksum (16 bits)
- 32 bit Source IP address
- 32 bit Destination IP address
- Options, if any (32 bits)
- Data (32 bits)
What is the Time-to-live in a datagram?
A field to ensure that the datagram does not circulate forever. Each router it visits will decrement this value until it reaches zero, when it is dropped.
What is the Upper-layer protocol field in the datagram?
It indicates the specific transport layer protocol to which the data portion of this datagram should be passed to.
What is datagram fragmentation?
When a datagram cannot be sent through a link because it is too big, so it gets split up into multiple datagrams to fit through the link. The datagram is reassembled afterwards.
How does the receiver know which fragments to reassemble into a datagram?
Related datagram segments have header bits that identify them as part of the same datagram.
What is the maximum transmission unit fo the ethernet?
1500 bytes.
How many bits long is IPv4?
32 bits.
What it an interface (PS: it is not an user interface)?
A connection between a host or router and the physucal link. Interfaces have an associated IP address.
Routers can only have one interface?
TRUE or FALSE
FALSE.
Routers can have multiple interfaces running at the same time.
Hosts only have one interface?
TRUE.
This is generally the case with hosts.
Hosts only have one interface?
TRUE.
This is generally the case with hosts.
What do higher-order bit and lower-order bit IP addresses determine in a network?
Higher order bit IP addresses signify the sub networks in a network.
Lower order bit IP addresses signify the interfaces in a network.
Interfaces can only communicate directly if they are on the Same network.
TRUE or FALSE
TRUE.
Interfaces on the same network share the same subnet portion of there IP address to indicate the belong to the same network.
How do when know how many bits of the IP address of an interface identify the subnet it is a part of?
Using slash notation. A slash comes after the IP address and shows the bit length of the subnet portion of said IP:
223.1.1.0/24
Has a 24 bit long subnet section.
What are two disadvantages of xlassful addressing?
- It is not very flexible - only has three options
2. Inefficient use of address space - a class might not use all the space in the address
What is the format of Classful InterDonain Routing (CIDR) address?
a.b.c.d/x
Where a, b, c and d are bits in the address and x is the number of bits in the network part of the address.
Network space is not fixed by byte boundaries, and is thus not dynamic.
What is address aggregation?
IP address are assigned to ISP (Internet service providers) in contiguous blocks. The ISPs can use these IPs for there service.
There are many addresses with the first part corresponding to:
199.31.x.x
Where the xs are other parts of the IP.
What is the advantage if address aggregation?
It allows for efficient routing between subnets, due to these division of of subnets.
What are IP addresses managed by?
The Internet Coporation for Assigned Names and Numbers (ICANN).
How does ICANN manage IPs?
It allocates addresses to regubal address supporting organisations, which then in turn allocates blocks of IP to large ISPs. Large ISPs allocate to smaller ISPs or organisations, which then allocate to the end users.
What does ICANN do?
- Manages IP addresses.
- Can also manage DNS, domain names and disputes
What does the Dynamic Host Configuration Protocpqol (DHCP) do?
It assigns addresses throughout the hosts/routers in an organisation. Often configured by network administrators. Addresses assigned can be permanent or temporary.
What 4 things does a host need to communicate to the network?
- A unique IP address
- The subnet mask (Subnet signifier at the end of address)
- The broadcast address
- The default gateway
What is DHCP server discover?
When a new client doesn’t have a IP address and doesn’t not know where to sent the data. So I broadcasts to every other host and hopes that the DHCP server responds.
What is DHCP server discover?
When a new client doesn’t have a IP address and doesn’t not know where to sent the data. So I broadcasts to every other host and hopes that the DHCP server responds.
What is DHCP server offer(s)?
When a DHCP server sees a DHCP discover message, so it broadcasts an offer message (As the discover host still does not have an IP). The offer message contains an IP for the host, and the IP address of the DHCP server itself.
Multiple offers can be sent to the host from different DHCP servers.
What is an DHCP request?
When the host receives the offer(s) from the DHCP server(s) it will select one offer and respond to the DHCP server who sent it with a DHCP request message. This message contains the configuration parameters.
What is a DHCP ACK?
When the DHCP server receives the DHCP request it responds with a acknowledgement message. This acknowledgement message echoes back the configuration parameters.
What are private addresses?
They are the local network addresses owned by the local network hosts.
What are public addresses?
Outside facing IP addresses owned by gateway routers for use over the Internet.
All internal hosts use this same external IP address.
TRUE or FALSE
TRUE.
Each Local Area Network (LAN) uses one IP address.
TRUE or FALSE
TRUE.
- This address can change, but the internal address in the LAN network doesn’t need to change
Give 3 facts about LAN networks.
- Devices inside the LAN network are not visible to the internet
- All traffic comes through the main router in the LAN
- The router has a firewall from protection from things outside the LAN
What is universal plug and play?
Allows applications to specify which ports they wish to use, via the NAT translation tables. When the router accepts a request, an ebtry in the NAT table is created for that application.
What is stored in the NAT translation tables?
The port or IP translations that the router is maintaining. The NAT table is stored on the router.
What is the Internet Control Message Protocol (ICMP) used for?
IMCP is how routers communicate with each other:they exchange network layer information like routing and error reporting. The IMCP messages are consider part of IP but are carried on IP datagrams.
What fields does an IMCP message have?
- Type field
- Code field
Also sends the first 8 bytes of the datagram that caused the IMCP message in the first place.
What does Traceroute do?
It allows us to trace the route of a packet from host to host, encompassing all routers it passed through in its path. It does this by using IMCP messages by sending UDP packets with unlikely port numbers to the end host and receives the IMCP message back.
How exactly does Traceroute do its job?
It sends UDP segments to the host with an increasing TTL number and starts timers for each segment. The nth segment has a TTL of n, which will expire at the nth router which in turn sends a IMCP warning back to the source. The Traceroute uses this message to calculate the RTT. Eventually, the segment will reach the destination once it has a high enough TTL. Since the host is unlikely to be accepting random segments it will return a message back saying ‘host port unreachable’. Once the Traceroute receives this, it stops.
How many bits in a IPv6 address?
128 bits.
What is contained in the IPv6 datagram format?
- Version number
- A traffic class (Like type of service)
- A flow label
- The payload length (16 bit value)
- Next header (Indentifies the protocol the payload is to be delivered to)
- The hope limit (same as the TTL)
- Source address
- Destination address
- The payload
