Chapter 4 - Network Layer / Data Plane Flashcards
Let’s review some of the terminology used in this textbook. Recall that the name of a transport-layer packet is segment and that the name of a link-layer packet is frame. What is the name of a network-layer packet? Recall that both routers and link-layer switches are called packet switches. What is the fundamental difference between a router and link-layer switch?
A network-layer packet is a datagram. A router forwards a packet based on the packet’s IP (layer 3) address. A link-layer switch forwards a packet based on the packet’s MAC (layer 2) address.
We noted that network layer functionality can be broadly divided into data plane functionality and control plane functionality. What are the main functions of the data plane? Of the control plane?
The main function of the data plane is packet forwarding, which is to forward datagrams from their input links to their output links. For example, the data plane’s input ports perform physical layer function of terminating an incoming physical link at a router, perform link-layer function to interoperate with the link layer at the other side of the incoming link, and perform lookup function at the input ports.
The main function of the control plane is routing, which is to determine the paths a packet takes from its source to its destination. A control plane is responsible for executing routing protocols, responding to attached links that go up or down, communicating with remote controllers, and performing management functions.
We made a distinction between the forwarding function and the routing function performed in the network layer. What are the key differences between routing and forwarding?
The key differences between routing and forwarding is that forwarding is a router’s local action of transferring packets from its input interfaces to its output interfaces, and forwarding takes place at very short timescales (typically a few nanoseconds), and thus is typically implemented in hardware. Routing refers to the network-wide process that determines the end-to-end paths that packets take from sources to destinations. Routing takes place on much longer timescales (typically seconds), and is often implemented in software
What is the role of the forwarding table within a router?
The role of the forwarding table within a router is to hold entries to determine the outgoing link interface to which an arriving packet will be forwarded via switching fabric.
We said that a network layer’s service model “defines the characteristics of end-to-end transport of packets between sending and receiving hosts.” What is the service model of the Internet’s network layer? What guarantees are made by the Internet’s service model regarding the host-to-host delivery of datagrams?
The service model of the Internet’s network layer is best-effort service. With this service model, there is no guarantee that packets will be received in the order in which they were sent, no guarantee of their eventual delivery, no guarantee on the end-to-end delay, and no minimal bandwidth guarantee
In Section 4.2, we saw that a router typically consists of input ports, output ports, a switching fabric and a routing processor. Which of these are implemented in hardware and which are implemented in software? Why? Returning to the notion of the network layer’s data plane and control plane, which are implemented in hardware and which are implemented in software? Why?
Input port, switching fabric, and output ports are implemented in hardware, because their datagram-processing functionality is far too fast for software implementation. A routing processor inside a traditional router uses software for executing routing protocols, maintaining routing tables and attached link state information, and computing the forwarding table of a router. In addition, a routing processor in a SDN router also relies on software for communication with a remote controller in order to receive forwarding table entries and install them in the router’s input ports.
Data plane is usually implemented in hardware due to the requirement of fast processing, e.g., at nanosecond time scale. Control plane is usually implemented in software and operates at the millisecond or second timescale, for example, for
What does each input port of a high speed router store to facilitate fast forwarding decisions?
With the shadow copy, the forwarding lookup is made locally, at each input port, without invoking the centralized routing processor. Such a decentralized approach avoids creating a lookup processing bottleneck at a single point within the router
What is meant by destination-based forwarding? How does this differ from generalized forwarding (assuming you’ve read Section 4.4, which of the two approaches are adopted by Software-Defined Networking)?
Destination-based forwarding means that a datagram arriving at a router will be forwarded to an output interface based on only the final destination of the datagram. Generalized-forwarding means that besides its final destination, other factors associated with a datagram is also considered when a router determines the output interface for the datagram. Software defined networking adopts generalized forwarding, for example, forwarding decision can be based on a datagram’s TCP/UDP source or destination port numbers, besides its destination IP address.
Suppose that an arriving packet matches two or more entries in a router’s forwarding table. With traditional destination-based forwarding, what rule does a router apply to determine which of these rules should be applied to determine the output port to which the arriving packet should be switched?
A router uses longest prefix matching to determine which link interface a packet will be forwarded to if the packet’s destination address matches two or more entries in the forwarding table. That is, the packet will be forwarded to the link interface that has the longest prefix match with the packet’s destination.
Switching in a router forwards data from an input port to an output port. What is the advantage of switching via an interconnection network over switching via memory and switching via bus?
Switching via memory; switching via a bus; switching via an interconnection network. An interconnection network can forward packets in parallel as long as all the packets are being forwarded to different output ports.
What is the role of a packet scheduler at the output port of a router?
If the rate at which packets arrive to the fabric exceeds switching fabric rate, then packets will need to queue at the input ports. If this rate mismatch persists, the queues will get larger and larger and eventually overflow the input port buffers, causing packet loss. Packet loss can be eliminated if the switching fabric speed is at least n times as fast as the input line speed, where n is the number of input ports.
What is a drop-tail policy? What are AQM algorithms? Which is the most widely studied and implemented AQM algorithm? How does it work?
When there is not enough memory to buffer an incoming packet, a decision must be made to either drop the arriving packet (a policy known as drop-tail) or remove one or more already-queued packets to make room for the newly arrived packet.
active queue management (AQM)
Random Early Detection (RED): pre-emptively dropping packets before the buffer becomes completely full. It uses predictive models to decide which packets to drop
Assuming input and output line speeds are the same, packet loss can still occur if the rate at which packets arrive to a single output port exceeds the line speed. If this rate mismatch persists, the queues will get larger and larger and eventually overflow the output port buffers, causing packet loss. Note that increasing switch fabric speed cannot prevent this problem from occurring
What is HOL blocking? Does it occur in input ports or output ports?
HOL blocking: Sometimes a packet that is first in line at an input port queue must wait because there is no available buffer space at the output port to which it wants to be forwarded. When this occurs, all the packets behind the first packet are blocked, even if their output queues have room to accommodate them. HOL blocking occurs at the input port.
In Section 4.2, we studied FIFO, Priority, Round Robin (RR), and Weighted Fair Queueing (WFQ) packet scheduling disciplines. Which of these queueing disciplines ensure that all packets depart in the order in which they arrived?
Only FIFO can ensure that all packets depart in the order in which they arrived.
Give an example showing why a network operator might want one class of packets to be given priority over another class of packets.
real-time applications such as voice or video calls may get a higher priority than for example an ftp application.
For example, a packet carrying network management information should receive priority over regular user traffic. Another example, a real-time voice-over-IP packet might need to receive priority over non-real-time traffic such as e-email.
Describe an IPV4 header (VHSDIFOTUC)
Describe an IPV6 header (VTFPNH)
IPv6 has a fixed length header, which does not include most of the options an IPv4 header can include. Even though the IPv6 header contains two 128 bit addresses (source and destination IP address) the whole header has a fixed length of 40 bytes only. Several of the fields are similar in spirit. Traffic class, payload length, next header and hop limit in IPv6 are respectively similar to type of service, datagram length, upper-layer protocol and time to live in IPv4.
