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CS3102: Networking > Data plane > Flashcards

Data plane Flashcards

1
Q

What is the network layer?

A

Transports segments from sender to receiver.

segments are encapsulated by the sender into datagrams, and then passed up to transport layer by receiver

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2
Q

What devices use network layer protocols?

A

Use by every host and router, as it allows for datagrams to be addressed

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3
Q

What are the functions of the network layer?

A

Forwarding

Routing

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4
Q

What is forwarding?

A

Move packets from router’s input to appropriate output.

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5
Q

What is routing?

A

Determine route to be taken by packets from source to destination

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6
Q

What is the data plane?

A

Local, per-router function. Determines how datagrams on a router should be forwarded

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7
Q

What is the control plane?

A

Network-wide logic, Determine how datagram is routed among routers along end-to-end path

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8
Q

What are the two approaches to control-plane implementation?

A

Traditional routing algorithms: Implemented in routers

Software-defined networking (SDN): Implemented in remote server

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9
Q

How do traditional routing algorithms work?

A

Individual algo. components in each router interact with the control plane.

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10
Q

What is the architecture of a router?

A

Division into control plane and data plane.

Control plane contains routing processor

Data plane contains input/output ports and a high-seed switching fabric which speaks with the routing processor.

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11
Q

What is the structure of an input port?

A

Line termination, into link layer protocol (receive), into input buffer which is for queueing lookups and forwarding.

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12
Q

What happens at the input buffer in the input port?

A

Input port is given instruction by routing processor.

lookup output port using forwarding table in memory.

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13
Q

What types of forwarding is there for a router?

A

Destination-based forwarding: Based on IP address

Generalised forwarding: Forward based on any set of header field values.

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14
Q

How do forwarding tables decide what output port to use?

A

Longest prefix matching, find the longest address prefix matching the destination address (basically, most specific)

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15
Q

What type of memory is used in routers?

A

TCAMS
Ternary content addressable memories

Allows for the retrieval of an address in one clock cycle.

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16
Q

What is a switching fabric?

A

Transfers packet from input buffer to output buffer.

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17
Q

What is switching rate?

A

Rate is the rate at which packets can be transferred through the switching fabric.

Often measured as multiple of input/output line rate.

N inputs should aim for N * line rate.

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18
Q

What are the three types of switching fabrics?

A

Memory
Bus
Crossbar

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19
Q

How does switching via memory work?

A

Packet copied to system’s memory, speed limited by memory bandwidth as it 2 busses have to be crossed per datagram.

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20
Q

How does switching via bus work?

A

Datagram from input port memory to output port memory via a shared bus.

BUs contention: Switching speed is limited by bus bandwidth.

As bus is shared, data flows from input to output without leaving it. (think of Harry Potter staircase)

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21
Q

How is Switching via crossbar done?

A

Can split datagrams into n length cells and switch them through the network.

Overcomes bus bandwidth limitations.

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22
Q

How does input port queuing occur and what can be done about it?

A

Occurs when the switching fabric is slower than the input ports combined, causes delay and loss on buffer overflow.

Can also cause HOL blocking.

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23
Q

What is Head-of-the-line (HOL) blocking?

A

Queued datagram at front of the queue prevents others in queue from moving forward.

Caused by output port contention, as only one packet can use an output port at a time.

24
Q

How are output ports structured?

A

Datagram buffer -> Link layer protocol (send) -> Line termination.

25
Q

Why do we need a buffer in the output port?

A

Buffering required when datagrams reach the output port faster than the transmission rate.

26
Q

How are packets chosen for transmission and do we choose what ones to transmit?

A

Scheduling discipline chooses the best way to schedule packets for transmission

Might want different discpilines to prioritise packets due to the application needs

27
Q

What happens when the output buffer overflows?

A

Packets are dropped, however these have to be decided by the router. Can do random drop, or drop last one in.

28
Q

What are the different scheduling mechanisms for output ports?

A

FIFO
Priority
Round Robin
Weighted Fair Queuing

29
Q

How does FIFO deal with output port buffers?

A

Send in order of arrival to queue.

Discard policies:
Drop arriving packet
Drop based on priority basis from queue.
Random drop.

30
Q

How are Priority queues used to deal with output buffers?

A

Send highest priority queued packet. Classify packets into different priorities.

31
Q

How does Round Robin scheduling deal with output buffers?

A

Multiple classes of packets. Scan over queue, sending one complete packet from each class if it’s available.

32
Q

How is Weighted Fair Queueing used for output buffers?

A

Generalised version of round robin, each class gets weighted amount of service in each cycle. IE: 1 here, 2 here, 1 here.

33
Q

What is the format of an IP datagram

A

Version | Head. len | ToS | Length | 16-bit ID | flgs | fragment offset | ttl | upper layer | head checksum | 32b source IP | 32b dest IP | options | Data.

34
Q

What is the length field of an IP datagram?

A

Total datagram length in bytes

35
Q

What is the 16-bit Identifier used for in an IP datagram?

A

Used for fragmentation and reassembly of the content at the receiver side.

36
Q

What is the upper layer field used for in an IP packet?

A

Tells the host what protocol to deliver the payload to.

37
Q

What options are available as per the IP field?

A

Timestamp, record route, specify must-visit routers.

38
Q

How long is the header of a IP packet?

A

20 bytes.

39
Q

Why is data fragmented by IP?

A

Different link types have different MTU’s (Max transfer size), so large IP datagram is divided within the network. Only reassembled at the final destination.

40
Q

What’s important about the offset field?

A

Divide the number of bytes by 8 to get the number of offset.

41
Q

What is an IP address?

A

32-bit identifier for host, router interface.

42
Q

What is an interface?

A

Connection between host/router and physical link.

IP address for each interface.

43
Q

What is a subnet?

A

Group of interfaces with the same subnet part of IP address, ie /16. Can physically reach each other without intervening router.

44
Q

What is CIDR?

A

Classless InterDomain Routing

Subnet portion of address is of arbitrary length, a.b.c.d/x where /x is the number of bits that say what the subnet is.

45
Q

How does a host get an IP address?

A

Can be hardcoded or leased via DHCP.

46
Q

What is DHCP and how does it work?

A

Dynamic Host Configuration Protocol.

Allows host to dynamically obtains its IP address from network server - allowing for reuse of addresses.

47
Q

What are the different DHCP messages?

A

DHCP discover - is there a DHCP server?
DHCP offer - Here’s an address
DHCP request - Can I have that IP?
DHCP ack - Ok, it’s yours.

48
Q

What does DHCP provide other than IP addresses on a subnet?

A

Address of first-hop router for client.

Name and IP address of DNS server.

Subnet mask.

49
Q

How do ISP’s get IP’s to distribute?

A

ICANN allocated addresses, manages DNS and assigns domain names, resolves dipsutes.

50
Q

What is NAT and how does it work?

A

Network Address Translation.

All datagrams leaving network have same source IP, with different source ports.

NAT keeps mapping of port number/host

51
Q

Why do we need NAT?

A

Don’t need a range of IP’s from ISP, just one.

Can change local addresses without notifying outside world.

Can’t explicitly address internal hosts externally.

SCALABLE

52
Q

Why is NAT controversial?

A

Breaks 5-layer model.

Address shortage should be solved by IPv6 and not a ‘hacky’ short-term solutioon.

Violates end-to-end argument, must be taken into account by app designers.

53
Q

What is the motivation for IPv6?

A

Running out of IPv4 addresses.

Header format helps speed processing/forwarding and QoS.

54
Q

What is the stucture of IPv6 datagrams?

A

Fixed-length 40 byte header, no fragmentation allowed.

ver | pri | flow label | payload len | next hdr | hop limit | 128 bits saddr | 128 bits dest addr | data.

55
Q

What is the flow label in the IPv6 header?

A

Identify datagrams in the same flow, which isn’t well defined.

56
Q

What is the next header field user for in the IPv6 header?

A

Identifies the upper layer protocol to be used for the data.

Basically, the upper layer field from IPv4

57
Q

What is tunnelling?

A

Allows for IPv4 routers to deal with IPv6.

IPv4 packets carries IPv6 as payload.

CS3102: Networking (6 decks)

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