ch4-1 chapter 4: network layer r 4. 1 introduction r 4.2 virtual circuit and datagram networks r 4.3...
TRANSCRIPT
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Ch4-1 -1
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
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Network Layer 4-2
Network layer transport segment
from sending to receiving host
on sending side encapsulates segments into datagrams
on rcving side, delivers segments to transport layer
network layer protocols in every host, router
router examines header fields in all IP datagrams passing through it
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
networkdata linkphysical network
data linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
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Ch4-1 -3
Network layer connection and connection-less service
Datagram network provides network-layer connectionless service
VC network provides network-layer connection service
Analogous to the transport-layer services, but: Service: host-to-host (vs process-to-process) Implementation: in the core (vs edge)
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Ch4-1 -4
Key Network-Layer Functions
routing: determine route taken by packets from source to dest. Routing algorithms
connection setup: in some (VC) networks only
forwarding: move packets from router’s input to appropriate router output
analogy:
routing: process of planning trip from source to dest
connection setup: process of having police setting up road for parades
forwarding: process of getting through single interchange
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Ch4-1 -5
1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
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Virtual circuits: signaling protocols used to setup, maintain teardown VC not used in original IP, but supported by Generalized Multi-Protocol Label
Switching (G-MPLS) label switched paths (LSPs) = VCs
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1. Initiate call 2. incoming call
3. Accept call4. Call connected5. Data flow begins 6. Receive data
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Ch4-1 -7
Datagram networks - The Internet Model
no call setup at network layer routers: no state about end-to-end connections
no network-level concept of “connection” packets forwarded using destination host
address packets between same source-dest pair may take
different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1. Send data 2. Receive data
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Ch4-1 -8
Forwarding table
Destination Address Range Link Interface
11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111
otherwise 3
4 billion possible entries
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Network Layer 4-9
Longest prefix matching
Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3
DA: 11001000 00010111 00011000 10101010
Examples
DA: 11001000 00010111 00010110 10100001 Which interface?
Which interface?
0
1
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Ch4-1 -10
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
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Ch4-1 -11
Router Architecture Overview
Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing
link
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Input Port Functions
Decentralized switching: given datagram dest., lookup output
port using forwarding table in input port memory
goal: complete input port processing at ‘line speed’
queuing: if datagrams arrive faster than forwarding rate into switch fabric
Physical layer:bit-level reception
Data link layer:e.g., Ethernetsee chapter 5
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Input Port Queuing
Fabric slower than input ports combined -> queueing may occur at input queues
Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward
queueing delay and loss due to input buffer overflow!
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Ch4-1 -14
Three types of switching fabrics
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Ch4-1 -15
Switching Via Memory
First generation routers: traditional computers with switching under direct control of CPU
packet copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram)
InputPort
OutputPort
Memory
System Bus
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Ch4-1 -16
Switching Via a Bus
datagram from input port memory
to output port memory via a shared bus
bus contention: switching speed limited by bus bandwidth
1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone)
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Ch4-1 -17
Switching Via An Interconnection Network overcome bus bandwidth limitations Crossbar is an expensive, non-blocking, high-
speed interconnection network 3_Satge Close or other interconnection nets
initially developed to connect processors in multiprocessor (parallel computer) systems Some may be “blocking” but with less
complexity. Cisco 12000: switches Gbps through the
interconnection network
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Ch4-1 -18
Output Ports
Buffering required when datagrams arrive from fabric faster than the transmission rate
Scheduling discipline chooses among queued datagrams for transmission
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Ch4-1 -19
Output port queueing (need internal speedup)
buffering when arrival rate via switch exceeds output line speed
queueing (delay) and loss due to output port buffer overflow
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Ch4-1 -20
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
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Ch4-1 -21
The Internet Network layer
forwardingtable
Host, router network layer functions:
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
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Ch4-1 -22
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
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Ch4-1 -23
IP datagram format
ver length
32 bits
data (variable length,typically a TCP
or UDP segment)
16-bit identifier
Internet checksum
time tolive
32 bit source IP address
IP protocol versionnumber
header length (bytes)
max numberremaining hops
(decremented at each router)
forfragmentation/reassembly
total datagramlength (bytes)
upper layer protocolto deliver payload to
head.len
type ofservice
“type” of data flgsfragment
offsetupper layer
32 bit destination IP address
Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.
how much overhead with TCP?
20 bytes of TCP 20 bytes of IP = 40 bytes +
app layer overhead
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Ch4-1 -24
IP Fragmentation & Reassembly network links have MTU
(max.transfer size) - largest possible link-level frame. different link types,
different MTUs large IP datagram divided
(“fragmented”) within net one datagram becomes
several datagrams “reassembled” only at
final destination IP header bits used to
identify, order related fragments
fragmentation: in: one large datagramout: 3 smaller datagrams
reassembly
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Ch4-1 -25
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
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Ch4-1 -26
IP Addressing: introduction IP address: 32-bit
identifier for host, router interface
interface: connection between host/router and physical link router’s typically
have multiple interfaces
host may have multiple interfaces
IP addresses associated with each interface
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
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
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Ch4-1 -27
Subnets IP address:
subnet part (high order bits)
host part (low order bits)
What’s a subnet ? device interfaces
with same subnet part of IP address
can physically reach each other without intervening router
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
network consisting of 3 subnets
LAN
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Ch4-1 -28
Subnets 223.1.1.0/24223.1.2.0/24
223.1.3.0/24
Recipe To determine the
subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.
Subnet mask: /24
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Ch4-1 -29
SubnetsHow many? 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
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Ch4-1 -30
IP addressing: CIDRUsed to have classes:
A (0xxxxxxx/8), B (10xxxxxx, yyyyyyyy/16), C (110xxxxx, yyyyyyyy, zzzzzzzz/24) etc.
Now: CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet
portion of address
11001000 00010111 00010000 00000000
subnetpart
hostpart
200.23.16.0/23
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Ch4-1 -31
IP addresses: how to get one?
Q: How does host get IP address? hard-coded by system admin in a file
Wintel: control-panel->network->configuration->tcp/ip->properties
UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol:
dynamically get address from as server “plug-and-play”
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Ch4-1 -32
IP addresses: how to get one?
Q: How does network get subnet IP addr?A: From its ISP’s address space
ICANN: (Internet Corporation for Assigned Names and Numbers) allocates addresses to ISPs and manages DNS
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….
Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
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Ch4-1 -33
Hierarchical addressing: route aggregation
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”
200.23.20.0/23Organization 2
...
...
Hierarchical addressing allows efficient advertisement of routing information:
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Hierarchical addressing: more specific routes After Organization 1 switched to ISPs-R-Us:
longest substring matching between the destination address and the routing table entries becomes necessary to ensure a packet to 200.23.18.0/23 won’t end up in Fly-By-Night-ISP.
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Ch4-1 -35
NAT: Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4
138.76.29.7
local network(e.g., business network)
10.0.0/24
rest ofInternet
Datagrams with source or destination in this networkhave 10.0.0/24 address for
source, destination (as usual)
All datagrams leaving localnetwork have same single source
NAT IP address: 138.76.29.7,different source port numbers
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Ch4-1 -36
NAT: Network Address Translation
Motivation: local network uses just one IP address as far as outside world is concerned: no need to be allocated range of addresses from
ISP: - just one IP address is used for all devices can change addresses of devices in local network
without notifying outside world can change ISP without changing addresses of
devices in local network devices inside local net not explicitly
addressable, visible by outside world (a security plus).
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Ch4-1 -37
NAT: Network Address TranslationImplementation: NAT router must:
outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using
(NAT IP address, new port #) as destination addr.
remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
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Ch4-1 -38
NAT: Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
S: 10.0.0.1, 3345D: 128.119.40.186, 80
1
10.0.0.4
138.76.29.7
1: host 10.0.0.1 sends datagram to 128.119.40, 80
NAT translation tableWAN side addr LAN side addr
138.76.29.7, 5001 10.0.0.1, 3345…… ……
S: 128.119.40.186, 80 D: 10.0.0.1, 3345
4
S: 138.76.29.7, 5001D: 128.119.40.186, 80
2
2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table
S: 128.119.40.186, 80 D: 138.76.29.7, 5001
3
3: Reply arrives dest. address: 138.76.29.7, 5001
4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345
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Ch4-1 -39
NAT: Network Address Translation
16-bit port-number field: 60,000 simultaneous connections with a
single LAN-side address! NAT is controversial:
routers should only process up to layer 3 violates end-to-end argument
• NAT possibility must be taken into account by app designers, eg, P2P applications
address shortage should instead be solved by IPv6
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Network Layer 4-40
NAT traversal problem client wants to connect to
server with address 10.0.0.1 server address 10.0.0.1
local to LAN (client can’t use it as destination addr)
only one externally visible NATted address: 138.76.29.7
solution 1: statically configure NAT to forward incoming connection requests at given port to server e.g., (123.76.29.7, port
2500) always forwarded to 10.0.0.1 port 25000
10.0.0.1
10.0.0.4
NAT router
138.76.29.7
Client?
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Network Layer 4-41
NAT traversal problem solution 2: Universal Plug
and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to: learn public IP address
(138.76.29.7) add/remove port
mappings (with lease times)
i.e., automate static NAT port map configuration
10.0.0.1
10.0.0.4
NAT router
138.76.29.7
IGD
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Network Layer 4-42
NAT traversal problem solution 3: relaying (used in Skype)
NATed client establishes connection to relay External client connects to relay relay bridges packets between two
connections
138.76.29.7
Client
10.0.0.1
NAT router
1. connection torelay initiated
by NATted host
2. connection torelay initiated
by client
3. relaying established
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Ch4-1 -43
DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address from network server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while
connected an “on”Support for mobile users who want to join network (more
shortly)
DHCP overview: host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg host requests IP address: “DHCP request”
msg DHCP server sends address: “DHCP ack” msg
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Ch4-1 -44
DHCP client-server scenario
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
BE
DHCP server
arriving DHCP client needsaddress in thisnetwork
223.1.2.5
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Ch4-1 -45
DHCP client-server scenarioDHCP server: 223.1.2.5 arriving
client
time
DHCP discover
src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654
DHCP offer
src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs
DHCP request
src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs
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Ch4-1 -46
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
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Ch4-1 -47
ICMP: Internet Control Message Protocol
used by hosts & routers to communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping)
network-layer “above” IP: ICMP msgs carried in IP datagrams
ICMP message: type, code plus first 8 bytes of IP datagram causing error
Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header
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Ch4-1 -48
Traceroute and ICMP
Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number
When nth datagram arrives to nth router: Router discards datagram And sends to source an ICMP message (type 11, code 0) Message includes name of router& IP address
When ICMP message arrives, source calculates RTT Traceroute does this 3 timesStopping criterion UDP segment eventually arrives at destination host Destination returns ICMP “host unreachable” packet
(e.g., type 3, code 3) When source gets this ICMP, stops.
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Ch4-1 -49
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
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Ch4-1 -50
IPv6 Initial motivation: 32-bit address space
soon to be completely allocated. Additional motivation:
header format helps speed processing/forwarding
header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed
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Ch4-1 -51
IPv6 Header (Cont)Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow.” (concept of“flow” not well defined).Next header: identify upper layer protocol for data
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Ch4-1 -52
Other Changes from IPv4
Checksum: removed entirely to reduce processing time at each hop
Options: allowed, but outside of header, indicated by “Next Header” field
ICMPv6: new version of ICMP additional message types, e.g. “Packet Too
Big” multicast group management functions
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Network Layer 4-53
TunnelingA B E F
IPv6 IPv6 IPv6 IPv6
tunnelLogical view:
Physical view:A B E F
IPv6 IPv6 IPv6 IPv6IPv4 IPv4
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Network Layer 4-54
TunnelingA B E F
IPv6 IPv6 IPv6 IPv6
tunnelLogical view:
Physical view:A B E F
IPv6 IPv6 IPv6 IPv6
C D
IPv4 IPv4
Flow: XSrc: ADest: F
data
Flow: XSrc: ADest: F
data
Flow: XSrc: ADest: F
data
Src:BDest: E
Flow: XSrc: ADest: F
data
Src:BDest: E
A-to-B:IPv6
E-to-F:IPv6
B-to-C:IPv6 inside
IPv4
B-to-C:IPv6 inside
IPv4