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Lecture 16: Network LayerOverview, Internet Protocol
COMP 332, Spring 2018Victoria Manfredi
Acknowledgements: materials adapted from Computer Networking: A Top Down Approach 7th edition: ©1996-2016, J.F Kurose and K.W. Ross, All Rights Reserved as well as from slides by Abraham Matta at Boston University, and some material
from Computer Networks by Tannenbaum and Wetherall.
1. Announcements– homework 6 posted
• discuss: UDP ping server, chat server + reliability– midterm graded
2. Network layer– overview– what’s inside a router– Internet protocol (IP)
3. Addressing– IPv4 addressing– usage in routing– how to get an IP address– IPv6 addressing– Dynamic Host Configuration Protocol (DHCP)– Network Address Translation (NAT)
2
Goal– move pkt from one host to another
How done on Internet?– routers
• examine header fields in every IP pkt• determines outgoing link
Internet e2e argument– some functionality only properly
implemented in end systems– smart hosts vs. dumb routers
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical network
data linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
Network layer is in every host and router on Internet
Sender– encapsulates segments into packets, puts src, dest IP in IP pkt hdr
Receiver– decapsulates packets into segments, delivers to transport layer
5
TCP hdr
IP data
IP pkt
TCP data IP hdr
TCP segment
Max len of TCP data in bytes– MSS: Max Segment Size– MSS = MTU – IP hdr – TCP hdr
• TCP header >= 20bytes
Max len of IP packet in bytes– MTU: Max Transmission Unit– 1500 bytes if Ethernet used as
link layer protocol
1. Control plane– comprises traffic only between routers, to
compute routes between src and dst– network-wide: routers run routing algorithms
2. Data plane– comprises traffic between end hosts,
forwarded by routers– forwarding table set based on routes
computed in control plane– local: each router stores, forwards packets
Plan trip from src to dst
Trip analogy
Get through one interchange
Routing algorithm
Local forwarding tableDest IP Output link
129.133.*.*43.*.*.*43.56.*.*189.37.35.*
2331
123
Dest IP addr in header of arriving packet
Routing (slower time scale)• routers view Internet as graph• run shortest path algorithms
Forwarding (faster time scale)• routers use paths to choose best output
link for packet destination IP address• if one link fails, chooses another
2 control-plane approaches1. traditional routing algorithms
implemented in routers2. software-defined networking
(SDN) implemented in (remote) servers [email protected] 7
RoutingAlgorithm
Dataplane
Controlplane
4.1 • OVERVIEW OF NETWORK LAYER 309
tables. In this example, a routing algorithm runs in each and every router and both forwarding and routing functions are contained within a router. As we’ll see in Sec-tions 5.3 and 5.4, the routing algorithm function in one router communicates with the routing algorithm function in other routers to compute the values for its forward-ing table. How is this communication performed? By exchanging routing messages containing routing information according to a routing protocol! We’ll cover routing algorithms and protocols in Sections 5.2 through 5.4.
The distinct and different purposes of the forwarding and routing functions can be further illustrated by considering the hypothetical (and unrealistic, but technically feasible) case of a network in which all forwarding tables are configured directly by human network operators physically present at the routers. In this case, no routing protocols would be required! Of course, the human operators would need to interact with each other to ensure that the forwarding tables were configured in such a way that packets reached their intended destinations. It’s also likely that human configu-ration would be more error-prone and much slower to respond to changes in the net-work topology than a routing protocol. We’re thus fortunate that all networks have both a forwarding and a routing function!
Values in arrivingpacket’s header
1
23
Local forwardingtable
header
0100011001111001
1101
3221
output
Control plane
Data plane
Routing algorithm
Figure 4.2 ♦ Routing algorithms determine values in forward tables
M04_KURO4140_07_SE_C04.indd 309 11/02/16 3:14 PM
Individual routing algorithm components in each and every router interact in the control plane
1
2
0111
Values in arriving packet header
3
Dataplane
Controlplane
Remote Controller
CA
CA CA CA CA
1
2
0111
3
Values in arriving packet header
A distinct (typically remote) controller interacts with local control agents (CAs)
Example services– individual packets
• guaranteed delivery• guaranteed delivery with less than 40 ms delay
– flow of packets• in-order packet delivery• guaranteed minimum bandwidth to flow• restrictions on changes in inter-packet spacing
Q: What service model does network layer provide to transport layer for moving packets from sender to receiver?
NetworkArchitecture
Internet
ATM
ATM
ATM
ATM
ServiceModel
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constantrate
guaranteedrate
guaranteed minimum
none
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestionfeedback
no (inferredvia loss)
nocongestion
nocongestion
yes
no
Guarantees ?
ATM: Asynchronous Transfer Modee.g., used in public switched telephone network
Run routing protocols (control) and store and forward pkts (data)
high-speed switching
fabric: connects inputs to outputs
Routing processor:run routing algorithms, maintain routing state
Router input ports Router output ports
Data plane - simpleForwarding (hardware)
Operates in nanosecond timescale
Control plane - complexRouting management (software)
Operates in millisecond timescale
Port is an interface not a socket
pkt
pkt
phys
ical
link
netw
ork
13
High-level view of generic router architecture
linetermination
link layer
protocol(receive)
lookup,forwarding
queueing
Network layer– validate/update checksum, decrement TTL– switching: use header field values, lookup output port – queue: if packets arrive faster than forwarding rate
into switch fabric
Physical layer:bit-level reception,
terminate phys. conn.
Data link layer:e.g., Ethernet processing,
error-checking, de-capsulation,
switchfabric
Transfer packet – from input buffer to appropriate output buffer
Switching rate– rate at which packets can be transferred from inputs to outputs– N inputs: switching rate = N x line rate desirable
3 types of switching fabrics
Memory
memory
Bus Crossbar
15Forward multiple pkts in parallel
Speed limited by bus contention
Speed limited by memory bandwidth
Switching fabric slower than input ports combined – queueing may occur at input queues – queueing delay and loss due to input buffer overflow!
Head-of-the-Line (HOL) blocking– queued pkt at front of queue prevents others from moving forward
Output port contention: only one red datagram can be transferred.Lower red packet is blocked
switchfabric
One packet time later: green packet experiences HOL
blocking
switchfabric
16
Buffering – when packets arrive from fabric faster than transmission rate– packet loss: due to congestion, lack of buffers
Scheduling– chooses next among queued packets to transmit on link– net neutrality: who gets best performance
linetermination
link layer
protocol(send)
switchfabric
datagrambuffer
queueing
This slide in HUGELY important!
FIFO (first in first out) – send in order of arrival to queue
Priority – multiple classes, with different priorities (e.g., based on hdr info)
• send highest priority queued packet
Round robin scheduling– multiple classes, cyclically scan class queues
• send one packet from each class (if available)
Weighted fair queueing – generalized round robin
• each class gets weighted amount of service in each cycle
queue(waiting area)
packetarrivals
packetdepartureslink
(server)
18
In practice: hardware queues use FIFO,need software to do priority
THE network layer protocol of the Internet– protocol your device must implement to run on Internet– RFC published ~1980
Provides– best effort service
• to get pkts from one end host to another across many interconnected networks using dst IP address in IP hdr
– addressing• format and usage of addresses
– fragmentation• e.g., if pkt size exceeds Ethernet MTU of 1500 bytes
– some error detection
Q: what does IP not provide?– QoS, reliability, ordering, persistent state for e2e flows, connections20
Network layer functions on hosts and routers
forwardingtable
Routing protocols• path selection• RIP, OSPF, BGP
IP protocol• addressing conventions• datagram format• packet handling conventions
ICMP protocol• error reporting• router “signaling”
transport layer: TCP, UDP
link layer
physical layer
Networklayer
ver length
32 bits
data (variable length,
typically a TCP segment or UDP datagram)
16-bit identifierheader
checksumtime to
live
32 bit source IP address
head.len
type ofservice
flgs fragmentoffset
upperlayer
32 bit destination IP address
options (if any)
IP protocol versionnumber
header length(# of 32bit words)
upper layer protocolto deliver payload to
ICMP=1, UDP=17, TCP=6
total packetlength (bytes)
Not widely used
forfragmentation/
reassemblymax numberremaining hops
(decremented at each router)
e.g. timestamp,record route
taken, specify routers to visit.
When used usually indicate
suspicious activity
How much overhead?
• 20 bytes of TCP• 20 bytes of IP• = 40 bytes + app
layer overhead 1-22
Q: Why is version 1st?So you know how to parse pkt
Bits transmitted left to right, top to bottom
Network links have MTU – largest possible link-level frame– different link types have
different MTUs
Fragment when pkt > MTU– 1 pkt becomes several pkts
• IP header bits used to identify, order related fragments
– reassembled only at final dest– re-fragmentation possible– don’t recover from lost fragments– (IPv6 does not support)
fragmentation:in: one large pktout: 3 smaller pkts
reassembly
…
…
DoS attack: send fragmented pktsbut leave one out
ID=x
offset=0
fragflag=0
length=4000
ID=x
offset=0
fragflag=1
length=1500
ID=x
offset=185
fragflag=1
length=1500
ID=x
offset=370
fragflag=0
length=1040
One large pktbecomes several
smaller pkts
4000 byte packet• 3980 bytes payload• IP hdr >=20 bytes
MTU = 1500 bytes1480 bytes in
data field
offset =1480/8 = 185
25
Identify as last segment
Globally unique 32-bit identifier– associated with host or router interface – Interface: connection between host/router and physical link
– host: usually 1 or 2 interfaces– router: usually many interfaces
Hierarchical– subnet part
• variable len# of high order bits• assigned by ICANN
– host part• variable len # of low order bits• network + host
27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
Pros– scalable: routers don’t need to look at host part– all pkts on same network forwarded in same direction
• only when pkt reaches network does host matter
Cons – every IP addr belongs to specific network … but what if host
moves networks and wants to keep same addr?• mobile IP• contrast with fixed Ethernet link layer addr
28
Q: how are interfaces actually connected?A: see Ch 5, 6.
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A: wired Ethernet interfaces connected by Ethernet switches
A: wireless WiFi interfaces connected by WiFi base station
For now: don’t worry about how one interface is connected to another
(with no intervening router)
IP address– subnet part: high order bits– host part: low order bits
What’s a subnet?– set of interfaces that all
have same subnet part of IP addr
– devices can reach other without intervening routers
Subnet mask– divides IP addr into subnet
addr + host addr– included in routing protocol
info given to [email protected]
Network comprising 3 subnets
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
subnet
223.1.1.2
223.1.3.27223.1.2.2
223.1.2.1
Recipe to find subnets– detach each interface
from its host or router– create islands of
isolated networks
Each isolated network – is called a subnet
subnet mask: /24
223.1.1.0/24223.1.2.0/24
223.1.3.0/24
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
subnet
223.1.1.2
223.1.3.27223.1.2.2
223.1.2.1
223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Allows more fine-grained division of blocks of addresses
Subnet masks – define variable partition of host part of addresses – e.g.,
• /23 subnet mask: 11111111 11111111 11111110 00000000• logical “and” of subnet mask with addr
– if a and b are both 1 then 1 otherwise 0
Address format– a.b.c.d/x, where x is # bits in subnet portion of address
33
11001000 00010111 00010000 00000000
subnetpart
hostpart
200.23.16.0/23
Suppose – we must have 223.1.1 as network prefix– we need block of 90 addresses
What should subnet mask be?– how many bits for 90 addresses?
0 26 25 24 23 22 21 20
64 32 16 8 4 2 1
27 26 25 24 23 22 21 20
128 64 32 16 8 4 2 1
1 26 25 24 23 22 21 20
64 32 16 8 4 2 1
223.1.1.0/24 gives 256 addresses [0-255]
223.1.1.0/25 gives 128 addresses [0-127]
223.1.1.128/25 gives a different set of 128
addresses [128-255]
Private subnet (used in NAT), do not appear on Internet– 172.16-31.*.*– 10.*.*.*– 192.168.*.*
Loopback address:– 127.*.*.*
Addresses you can’t assign to devices– *.*.*.255: broadcast addr– *.*.*.0: used for subnet name
Broadcast address– 255.255.255.255: broadcast to all hosts on network indicated
• if no mask: local network• if mask: broadcast on that network
Address when device booting up– 0.0.0.0 35
