Graeme Flashcards
1
Q
What is Internetworking?
A
- Hides routing from the Transport layer
- The internet as a ‘network of networks
2
Q
What are the function of the internet layer?
A
- Internetworking
- Transmitting packets
- Receiving packets
3
Q
What happens when the internet layer transmits packets?
A
- Packetisation into datagrams
- Processing and Routing IP datagrams
- Fragmentation
4
Q
What happens when the internet layer receives packets?
A
- Error checking
- Fragment Reassembly
5
Q
What are the properties of the internet protocol?
A
- Packet-switched: connectionless
- Unreliable
- Routed
6
Q
What does unreliable in for the internet protocol?
A
- No guarantee any given datagram will arrive
- Packets sent and routed on a best effort basis
- IP packets may get dropped or delayed (due to congestion/errors)
- Quality of Service methods can help
7
Q
What does routed mean for the internet protocol?
A
- Usually destination-routed
- Routers have routing tables and determine routing
8
Q
What are the Internet Layer Protocols?
A
Internet Protocol:
- IPv4
- IPv6
Diagnostics, Control:
- ICMP
- ICMPv6
IPSEC - encryption/security
IGMP - helps establish IPv4 multicast groups
9
Q
What are the characteristics of IPv4: RFC 791?
A
- 32-bit addresses, written as dotted quads
- Originated as part of ARPANET in the 1970s
- Variable size header, minimum 20-bytes
10
Q
What are the characteristics of IPv6: RFC 2460, 8200?
A
- 128-bit addresses, written as ‘colon-hex’
- Originated in the late 199-s
- Fixed size, simpler header: 40 bytes
11
Q
How can IPv6 addresses be simplifed?
A
We can omit leaders 0s in a block replacing them with a single set of “::”
12
Q
How is fragmentation handled by IPv4?
A
- Packets can be fragmented at any routing hop
- Generally re-assembled at the receiving host, but could be done at any intermediate router
13
Q
How is fragmentation handled by IPv6?
A
- Packets may only be fragmented at the sending host
- Must use Path MTU Discovery before sending
14
Q
What is Classless Inter Domain Routing?
A
CIDR (introduced in 1993) allows variable length prefixes, reducing IPv4 address consumption
15
Q
What do subnets do?
A
They limit the propagation of Ethernet broadcast traffic and segment hosts by logically dividing a larger IP allocation network
16
Q
What do routers do in the internet protocol layer?
A
- You need them any time there is a change in address space
- It needs an interface for each network segment
- Each interface needs an address that is reachable by hosts in that segment
17
Q
What is Routing?
A
Routing describes how packets move between subnets
- Occurs at the internet layer and where there is a change in IP address space and each host has a routing table
18
Q
What are Netmasks?
A
- Specifies how many bits identify the network prefix
19
Q
What is the Internet Control Message Protocol (ICMP)?
A
ICMP packets are encapsulated in standard IP packets
20
Q
How is ICMP used for IPv4?
A
Used for information and error messages
21
Q
How is ICMP is used for IPv6?
A
Also used for Router advertisement and neighbour discovery
22
Q
What is Multicast?
A
- One-to-many communication
- Packets are only sent to hosts that are interested in them
- Afterthought for IPv4, required for IPv6
23
Q
What are uses for multicast?
A
- One-to-many multimedia
- Local service discovery
- Inherently part of IPv6 for router advertisements, neighbour discovery and duplicate address detection
24
Q
What is Address Resolution Protocol (ARP)?
A
- Maps an IPv4 address on the local subnet to a MAC address
- The host looking for a MAC address broadcasts an ARP “who has” request
- The target sends a (unicast) reply to the requestor
25
What is Dynamic Host Configuration Protocol (DHCP)?
Automates the process for configuring addresses on a network for IPv4
26
How does Dynamic Host Configuration Protocol work?
- When a host connects to a network, it broadcasts a DHCP DISCOVER message
- The DHCP server reserves an address and replies with a DHCP OFFER
- The client then needs to DHCP REQUEST the address
- Finally, the server sends DHCP ACK, which contains lease duration and config
27
What is Neighbour Discovery Protocol?
- Maps an IPv6 address on the local subnet to a MAC address
- Uses ICMP and multicast, rather than encapsulated frames and broadcast
- Router solicitations sent to a solicited node address
- Router advertisements sent to the all nodes address
28
What does NDP replace?
ARP and ICMP router discovery
29
What are the 5 packet types for ICMPv6?
- Router Solicitation
- Router Advertisement
- Neighbour Solicitation
- Neighbour Advertisement
- Redirect
30
What is Router Solicitation?
Host request for router information
31
What is Router Advertisement?
Router information. It carries the IPv6 network prefix (/64) to use and its source address implies the default router address
32
What is Neighbour Solicitation?
Equivalent to ARP "who has"
33
What is Neighbour Advertisement?
Equivalent to ARP reply
34
What is Redirect?
Router informing host of a better first hop
35
What is StateLess Address AutoConfiguration (SLAAC)?
SLAAC allows a host to autoconfigure basic network settings without a DHCPv6 server, specified by RA
36
What does RFC7217 do?
- Uses a pseudo-random function that takes the prefix, Network card identifier, Network identifier, DAD counter and secret key as arguments
37
What is DHCPv6 (RFC8415)?
Similar to DHCP but
- Requires Router Advertisement
- Uses DHCP Unique Identifier instead of MAC address
38
What are the benefits of IPv6?
- Removes the need for NAT
- More plug-and-play than IPv4
- Streamlined header
- Fragmentation only occurs at sender
39
What are the barriers to IPv6 Deployment?
- Time and Money
- Hardware support
- Regular users don't want to learn something new
- Chicken and egg situation with ISPs and content providers
- "Peering wars" between Tier-1 network providers
40
What is basic routing?
- P1's packet is sent to the nearest router
- The packet is then passed on through the network
- Eventually it arrives at P2
(P1 and P2 are two different computers)
41
What is IP Routing?
- Occurs where there is a change in IP address space
- A router has an IP address in each address space it routes between
- There can be many routers between hosts on the internet
- Parts of the IP header are re-written at each hop
42
What is a routing table
- Tells the host how to route traffic
- All hosts on a network have one
- May be built from information from DHCP or IPv6 RA
43
What do routing tables include?
- Destination prefixes and the interface or next hop to use
- The local subnet that the host is connected to
- a catchall default route
44
How does a routing table work?
- A set of rules
- There can be multiple routes for one destination
- The most specific matching route is picked first (Longest prefix)
- The metric determines the priority of routes with the same specificity
45
What are Autonomous Systems?
A large network or group of networks that has a unified routing policy. The internet is made of interconnected Autonomous Systems
46
What are the three general categories of Autonomous Systems?
- Multihomed
- Transit
- Single-homed/stub
47
What is a routing protocol?
Allows router to build and exchange routing information automatically
48
What protocols are used within an Autonomous System
Interior gateway protocols
49
What protocols are used between Autonomous System
Exterior gateway protocols
50
What are the two types of Interior Gateway protocols?
- Distance Vector
- Link state
51
What is Distance Vector?
- Talk only to directly neighbouring routers
- Exchange "best" route information for any known prefixes with direct neighbours
52
What is Link State?
- Talk to all routers to establish full knowledge of the routers/topology in a site
- Routers flood information messages describing their connected neighbours around entire site network
53
What is Routing Information Protocol (RIP)?
- Router sends its whole routing table periodically (every 30s) to directly connected routers
- Destination network (prefix) and distance (cost) in hops
- Receiving routers update their view of the best route to a given network
54
What are the limitations of RIP?
- Updates only every 30s
- Updates are not acknowledged
- Metrics are simple hop count values
- Routers don't have knowledge of network topology
- Authentication is MD5, which is broken
55
How does Link state routing work?
- Messages are sent periodically, or any time a change in connectivity is detected
- Both ends of a link must agree for it to be valid
-All routers learn the full network topology
56
How are Neighbours discovered?
With broadcast packets sent on all interfaces
57
How is Link State better than Distance Vector?
- Link State converges faster
- Link state is better at avoiding loops
58
What us Border Gateway Protocol? (BGP)
- Works between Autonomous Systems
- Distance-Vector-like, but includes information about the AS path associated with a given route, cost of paths and other rich attributes
58
How does BGP work?
- The IP of a neighbour is specified in config
- Creates a BGP peering session (over TCP, port 179)
- Then you advertise routes you know to your neighbours
59
What are the downsides of BGP?
- Relies on trust
- Slow
- Routers have limited BGP routing table size
60
What is Traceroute?
- Determines the route between two hosts
- Sends packets with gradually increasing TTL
61
What does the transport layer do?
- Sends segments via the Network layer
- Provides host-unique port numbers
- Provides logical communication between applications
62
How many bit port numbers do TCP and UDP use?
16 bit
63
What are the features of TCP?
- Connection oriented
- Includes acknowledgements and retransmissions
- Provides flow control/congestion control for segments it sends
- TCP will adjust sending rate over time
- Retransmission
- Provides performance and reliability
64
How does TCP establish connection?
There is a three way handshake: SYN, SYN-ACK and ACK. SYN opens connection
Each side uses a sequence number
65
How is TCP reliable?
- ACKS are sent back by receiver
- Sender must detect lost packets
66
What are the properties of UDP?
- Connectionless
- Retransmission/adaptation is up to the application (layer)
- No flow control, often a fixed bit rate
- Low overhead
67
How big is the TCP header?
32 bits
68
What is the difference between the UDP and TCP header?
The UDP header is much simpler where checksum is optional
69
What is UDP loss?
- Lossy/congested links can drop packets
- Lower bandwidth links may drop packets as their buffers fill up
70
What is the TCP/UDP service model?
- Sender and receiver each create a socket to act as a communication endpoint
- Socket has IP address and port number
- Need five bits of information to identify communication
- The sockets and protocol used, uniquely identify the applications subsequent data transmissions
71
What is TCP Flow control?
Prevents a faster sender overwhelming a slow receiver
72
How does TCP flow control work?
- TCP uses a sliding window protocol to control sending rate
- Sender should not send data unless receiver indicates it has buffer space to accept it
- The sliding window is effectively the buffer space, the receiver says it has available at any given time
73
What is TCP congestion control?
Reduces send rate to cope with network congestion
74
How does TCP congest control work?
- Maintained by the sender
- The TCP congestion window indicates the number of bytes a sender may put into the network at any time
- The congest window size starts slow, if successful, the this doubles the window every round trip
75
What would be better for sending a video on YouTube?
TCP
