CE150 Chapter 4: The Network Layer
What kind of service does Network Layer provide to the Transport Layer?
- Connection and Connection-less service - Datagram Network (Connection-less) - Virtual Circuit (Connection based)
What are the components of a router?
1) Input Port 2) Switching Fabric 3) Output port 4) Routing processor
How does "Switching via Bus" work?
1) Input port pre-pends switch-internal label to the packet indicating the local output port to which the packet is being transferred 2) Input port transmits packet onto a bus 3) All output ports get the packet, but only the output port that matches the label will get to keep the packet 4) Label is removed What about multiple packets?? Bus can only take one packet at a time, so all others must wait for their turn **Switching speed depends on bus speed** **Sufficient for routers operating in small local area and enterprise networks**
How does "Switching via Memory" work?
1) Input port sends a signal to the routing process by "interrupting" it 2) Packet is copied from the input port to the routing processor 3) The routing processor extracts the destination address from the header 4) The routing processor looks up the output port in the forwarding table 5) The routing processor copies the packet onto the output port buffer **If memory bandwidth is B packets per second, then the overall forwarding throughput must be less than B/2**
What type of device is a router? And how does it decide where to forward the packet?
1) Network device 2) Uses the header fields of an incoming datagram to make forwarding decision
What are examples of control plane functions?
1) Responding to attached links 2) Executing router protocols 3) Perform management functions
What are the different ways the switching fabric operates?
1) Via Memory 2) Via Bus 3) Via Interconnection Network
What are some services the Network Service Model provides?
1. Guaranteed Delivery 2. Guaranteed delivery with bounded delay 3. In-order packet delivery 4. Guaranteed minimal bandwidth 5. Security (Encrypt all datagrams at source, decrypt at destination)
How to implement Virtual Circuits?
1. Path from src to dest 2. VC numbers, one number for each link (used to forward packets) 3. Entries in forwarding table along each router in path 4. Packet belongs to VC
What are the two packet-switching devices?
1. Routers 2. Link-Layer Switches
What is a packet-switching device?
A device that transfers the packet from input link interface to output link interface
What is a subnet mask?
An address that IP addressing assigns to the subnet
Who reassembles the fragments?
End System
What is generalized forwarding?
Forwarding the packet based on its attributes?
What is an interface?
The boundary between a link and host, or the boundary between a router and any of its links
What is non-preemptive priority queuing?
The transmission of a packet is not interrupted once it has begun. So if a low prio packet is transmitted and a higher prio packet appears, then the low prio packet's transmission is not stopped
What is the throughput for each class?
(R*Wi)/sum(Wj) - R is the transmission rate -Wi is how many packets are queued for class i - Wj is how many packets are queued across all classes
How is buffering calculated at the output or input port of a router?
(RTT * C)/SQRT(N) RTT: Round Trip Time (e.g. 250 msec) C: Link Capacity (e.g. 10 Gbps) N: number of flows
What is Weighted Fair Queuing (WQF)?
- A generalized form of round robin - Each class may receive a differential amount of service in any interval of time - Each class i is assigned a weight Wi - During any interval of time which there are class i packets to send, class i will be guaranteed a fraction of service (Wi/sum(Wj)) -denominator is taken over all classes that also have packets queued for transmission
Dynamic Host Configuration Protocol (DHCP)
-Client Server Protocol Allows a host to automatically receive an IP address -Could also be configured so: host gets same IP address each time it connects to the network -temp address is assigned -DHCP also allows host to learn subnet mask, address of first-hop router, and address of local DNS server
How does "Switching via Interconnection Network" work?
-Crossbar Switch: Interconnection network consisting of 2N buses that connect N input ports to N output ports -There are horizontal and vertical buses - Each vertical bus intersects each horizontal bus a crosspoint which can be opened or closed by switch fabric controller -Non-blocking: A packet being forwarded to an output port will not be blocked from reaching output port as long as no other packet is being forwarded to that output port
How are blocks of addresses obtained by a device?
-Network admin contacts ISP -ISP could divide its address block into eight equal-sized contiguous address blocks and give one of these addresses block
Where does the queuing occur?
1) At the input ports 2) At the output ports
How do Virtual Circuits work?
1) Call setup/breakdown for each call before data flows 2) Each packet carries VC identifier (ID) 3) Each router from source to dest has a "state" 4) Links and other router resources may be allocated to VC
What happens when a newly arriving host connects to the network?
1) DHCP Server Discovery 2) DHCP Server Offer(s) 3) DHCP request 4) DHCP ACK
What are the two types of forwarding?
1) Destination-Based 2) Generalized Forwarding
What are the types of packet scheduling?
1) FIFO (First-In-First-Out) 2) Priority Queuing 3) Round Robin and Weighted Fair Queuing (WFQ)
How does the destination host use the identification, flag, and offset fields to make sure it has all the fragments of a datagram?
1) Identification number: Incremented for each packet that is sent 2) Flag: Last fragment has flag set to 0, while all others have flag set to 1. This is how the dst host knows the last fragment has been received 3) The offset is used to figure out where the fragment fits within the original datagram
What is FIFO (First In First Out)?
A type of packet scheduling Order of packets transmitted onto link is same order in which packets arrived
What is Priority Queuing?
A type of packet scheduling Packets arriving at output link are classified into priority classes upon arrival at the queue -packets carrying network management information get priority over user traffic -real time voice-over IP traffic packets might receive priority over non-real traffic like SMTP - priority queuing discipline will transmit packet from highest priority class that has a nonempty queue -if choice is among packets in same prio class, then the scheduling is done in FIFO manner
Where does packet loss and drops happen?
At the router! When no memory is available to store arriving packets
Plug-and-play + zeroconf
Basically means that network admin doesn't have to perform finding IP addresses manually, DHCP does it for it
How is an interface's IP address chosen?
By the subnet to which the interface is connected
Who is responsible for creating the routing algorithm?
Controller
What is the Network Service Model?
Defines the characteristics && end-to-end delivery of packets between sending and receiving hosts
What does the Datagram Network forwarding table look like?
Destination addresses are grouped into "range" and classified under a link interface -Why: There are over 4 billion IP addresses. listing all individually is not feasible
What format are IP addresses?
Dotted-decimal notation. Each digit is a representation of 8 bits of the address. Example: 193.32.216.9
Who requires an IP address?
Each host and router interface must have one Why: Because each router and host is capable of sending and receiving IP datagrams ~IP address is technically associated with an interface, rather than the host or router~
What is a temp IP address?
Every time the host connects to the network, it is assigned a different temporary IP address?
What is destination based forwarding?
Forwarding the packet according to where its final destination is
What are two main functions a network layer performs?
Forwarding: Move packets from router's input to appropriate router output Routing: Determine route taken by packets from source to destination
What is the solution the problem with MTU?
Fragment! -Fragment payload into two or more smaller IP datagrams, encapsulate each one into separate link-layer frame, and send frames over outgoing link
How does ISP get its block of addresses?
ICANN (Internet Corporation for Assigned Names)
What is a subnet?
In IP terms, a network that connects one or more host interfaces to one router
What is an input port in a router?
Input port's role is to eliminate the physical link that is coming to the router once the packet is received.
What is Classless Interdomain Routing (CIDR)?
Internet's address assignment strategy
What is the switching fabric's role in the router?
It is a network of the network router. It connects the router input port and router output port.
Is Virtual Circuit packet-switching or circuit-switching?
It's packet switching, but emulates circuit switching.
What is output port processing?
It's taking packets that have been stored in the output port's memory and transmitting them over the output link.
How is the link interface chosen (in datagram network)?
Longest prefix matching -Longest address that matches the dest address
How do routers maintain state info in VC?
Maintain info such as VC number and incoming interface
What is the Maximum Transmission Unit?
Maximum amount of data that a link-layer frame can carry.
How does a Datagram Network work?
No call setup at network layer No state maintained by routers packets forwarded using dest address
What happens when the queue is full at either end of the router?
Option 1: Drop-Tail (Drop the arriving packet) Option 2: Drop one or more already queued packets to make room for the newly arrived packet **Could be wise to drop a packet before the buffer gets full as a congestion signal to the sender**
What is Round Robin?
Packets are sorted into priority classes still, but instead of there being a strict service, round robin alternates service among the classes. -Work conserving: Never lets the link remain idle whenever there are packets of any class queued for transmission
What is classful addressing?
Pre-CIDR Network portions of IP addresses were constrained to be 8, 16, or 24 bits in length
What is one way to make forwarding faster?
Prefixes. Router uses prefix to match entries in table. If there is more than one matching prefix, then the longest-prefix match is used to determine outbound link interface
How does a router use a forwarding packet?
Router takes one or more header fields from an incoming packet and indexes the forwarding table. After indexing the forwarding table, the router will get the outgoing link interface to know where to send the packet
Who does the network layer support?
Supports Transport Layer. It carries the segment from the sending to receiving host
A demo of delay on the input port
Suppose Rswitch is the rate at which packets can be moved from input port to output port. If Rswitch is N times faster than Rline (transmission rate), then there is negligible queuing at the input ports
What is head-of-the-line (HOL) blocking?
Suppose there is 1 dark packet in one queue, and one dark and one light packet in another queue. The dark packets have the same output port. Only one can go first. Suppose the first dark packet. The second one has to wait. But so does the light packet behind it, even if it's not trying to go to the same output port!
What is a forwarding table?
Table used by router to find outgoing link interface at that router to which that packet is to be forwarded
What is best-effort service and what is an example of one?
The best-effort service is a service that does not guarantee in-order packet delivery or delivery itself. The Internet is a best-effort service.
What is the problem with Maximum Transmission Unit?
The challenge is that each link has a different protocol, and each protocol has different MTU
What if a destination host receives more than one datagram from the source host?
The destination host uses the identification, flag, and fragmentation offset fields in the IP datagram header!
A demo of delay on the output port
The output port can only move one packet at a time onto the outgoing link. What if N times a packet come into the output port at once? They must wait in a queue
What is the the output port's role in the router?
The output port's role is to use physical layer functions to create an outbound link in order to send the packet out
What is the routing processor's role in the router?
The routing processor component directs the packet within the router. If it's a traditional router: The routing processor maintains the routing tables and attached link state info. It computes the forwarding table If it's an SDN router: The routing processor needs to go to the remote controller, and install entries into the input port
What does the x mean in a.b.c.d/x?
The x most significant bits, it constitutes the network portion of the IP address Also called the prefix When a router outside an org forwards a datagram whose dest address is inside the org, only the leading x bits need to be considered
What factors are a part of queuing delay?
Traffic load, relative speed of the switching fabric, and the line speed
True or False? Each interface on every host and router must have an IP address that is globally unique
True
What are signaling protocols for in VC?
Used to setup, maintain teardown in VC -Used in ATM