Chapter 4: Network Layer

Function of the Application Layer

-Enables user to access the network -Provides user interfaces and support for services (e.g. the Web, email) -SMTP: Simple Mail Transfer Protocol

Function of the Session Layer

-Establishes end connections between two hosts

Function of the Presentation Layer

-Formats data so it can be viewed by user -Encrypt and decrypt -JPG: compressed image -GIF(graphics interchange format): combined images/frames into single image/animation -HTTP(Hyper Text Transfer Protocol Secure): secures messages in code

Function of the Network Layer

-Reads IP address from packet -Routers layer 3 switches

Function of the Data Link Layer

-Reads MAC address from data packet -Switches

Function of the Transport Layer

-Responsible for the transport protocol and error handling -End-to-end communication over a network provides logical communication -Uses TCP and UDP protocol -TCP: connection oriented, reliable transfer protocol, segment sequencing, flow control -UDP: lightweight protocol, unreliable, connectionless, no sequencing

Function of the Physical Layer

-Sends data onto physical wire -Hubs, NICS, Cable

Name the 7 layers of the OSI model from top to bottom.

Application(7) Presentation(6) Session(5) Transport(4) Network(3) Data Link(2) Physical(1)

Describe how packet loss can occur at output ports. Can this loss be prevented by increasing the switch fabric speed?

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.

Difference between Destination-based forwarding and Generalized forwarding

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

Give an example showing why a network operator might want one class of packets to be given priority over another class of packets.

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.

How does generalized forwarding differ from destination-based forwarding?

Forwarding has two main operations: match and action. With destination-based forwarding, the match operation of a router looks up only the destination IP address of the to-be-forwarded datagram, and the action operation of the router involves sending the packet into the switching fabric to a specified output port. With generalized forwarding, the match can be made over multiple header fields associated with different protocols at different layers in the protocol stack, and the action can include forwarding the packet to one or more output ports, load-balancing packets across multiple outgoing interfaces, rewriting header values (as in NAT), purposefully blocking/dropping a packet (as in a firewall), sending a packet to a special server for further processing and action, and more.

Data Plane

Function: packet forwarding, forwarding datagrams from their input links to their output link

Control Plane

Function: routing, which is to determine the paths a packet takes from its source to its destination -executing router protocols -responding to up and down attached links -communicating with remote controllers -performing management functions

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.

Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in common?

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.

Describe how packet loss can occur at input ports. Describe how packet loss at input ports can be eliminated (without using infinite buffers).

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 ntimes as fast as the input line speed, where nis the number of input ports.

Suppose Host A sends Host B a TCP segment encapsulated in an IP datagram. When Host B receives the datagram, how does the network layer in Host B know it should pass the segment (that is, the payload of the datagram) to TCP rather than to UDP or to some other upper-layer protocol?

The 8-bit protocol field in the IP datagram contains information about which transport layer protocol the destination host should pass the segment to.

Role of Switching Fabric

connects the router's input ports to its output ports

Role of Forwarding Table within Router

hold entries to determine the outgoing link interface to which an arriving packet will be forwarded via switching fabric (A router forwards a packet by examining the value of one or more fields in the arriving packet's header, and then using these header values to index into its forwarding table. The value stored in the forwarding table entry for those values indicates the outgoing link interface at that router to which that packet is to be forwarded)

Switching via a bus

input port transfers a packet directly to the output port over a shared bus, without intervention by the routing processor; all output ports receive the packet but only the correct one will keep it; only one packet can cross the bus at a time

Switching via memory

oldest form of switching; two packets cannot be forwarded at the same time because only one memory read/write can be done at a time over the shared system bus; lookup of the destination address and the storing of the packet into the appropriate memory location are performed by processing on the input line cards

Switching via an interconnection network

overcome bandwidth restrictions; crossbar switch is an interconnection network consisting of 2N buses that connect N input ports to N output ports. Each vertical bus intersects each horizontal bus at an intersection, which can be opened or closed at any time by the switch fabric controller; non-blocking: a packet being forwarded will not be blocked as long as no other packet is being forwarded to the same port; capable of forwarding multiple packets in parallel

What is meant by the "match plus action" operation of a router or switch? In the case of destination-based forwarding packet switch, what is matched and what is the action taken? In the case of an SDN, name three fields that can be matched, and three actions that can be taken.

"Match plus action" means that a router or a switch tries to find a match between some of the header values of a packet with some entry in a flow table, and then based on that match, the router decides to which interface(s) the packet will be forwarded and even some more operations on the packet. In the case of destination-based forwarding packet switch, a router only tries to find a match between a flow table entry with the destination IP address of an arriving packet, and the action is to decide to which interface(s) the packet will be forwarded. In the case of an SDN, there are many fields can be matched, for example, IP source address, TCP source port, and source MAC address; there are also many actions can be taken, for example, forwarding, dropping, and modifying a field value.

What is a private network address? Should a datagram with a private network address ever be present in the larger public Internet? Explain.

A private network address of a device in a network refers to a network address that is only meaningful to those devices within that network. A datagram with a private network address should never be present in the larger public Internet, because the private network address is potentially used by many network devices within their own private networks.

What is the difference between a router and link-layer switch?

A router forwards a packet based on its IP address; a link-layer switch forwards a packet based on its MAC 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

Which, if any, can send multiple packets across the fabric in parallel? (name the type of switching method)

An interconnection network is able to send multiple packets

What is the name of a network-layer packet?

Datagram

What is the difference between a forwarding table that we encountered in destination-based forwarding?

Each entry in the forwarding table of a destination-based forwarding contains only an IP header field value and the outgoing link interface to which a packet (that matches the IP header field value) is to be forwarded. Each entry of the flow table in OpenFlow includes a set of header field values to which an incoming packet will be matched, a set of counters that are updated as packets are matched to flow table entries, and a set of actions to be taken when a packet matches a flow table entry.

Disciplines: 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

FIFO can ensure that all packets depart in the order in which they arrived.

What is "inside" of your router?

Input ports: performs several functions physical layer: terminating incoming physical link to router data link layer: performs lookup and forwarding function, so that data forwarded into switch fabric of router emerges at correct output port Outport ports: stores datagram that have been forwarded to it through the switching fabric, and then transmits the datagrams on the outgoing link. The output port thus performs the reverse data link and physical layer functionality as the input port Routing Processor: executes routing protocols, maintains routing tables, performs network management functions within router

Router and link-layer switches are referred to as _______.

Packet switches

Describe the three switching methods

Switching via a bus Switching via memory Switching via an interconnection network

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.

Name three header fields in an IP datagram that can be "matched" in OpenFlow 1.0 generalized forwarding. What are three IP datagram header fields that cannot be "matched" in OpenFlow?

Three example header fields in an IP datagram that can be matched in OpenFlow 1.0 generalized forwarding are IP source address, TCP source port, and source MAC address. Three fields that cannot be matched are: TTL field, datagram length field, header checksum (which depends on TTL field).

What field in the IP header can be used to ensure that a packet is forwarded through no more than N routers?

Time-to-live

Do routers have IP addresses? If so, how many?

Yes. They have one address for each interface.

Difference between Routing and Forwarding

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