the network layer: ip, subnets, nat and routing based on slides from the computer networking: a top...
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The Network Layer:IP, subnets, NAT and Routing
Based on slides from the Computer Networking: A Top Down Approach Featuring the Internet by Kurose and Ross
Network layer
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
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
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Key Network-Layer Functions
forwarding: move packets from router’s input to appropriate router output
routing: determine route taken by packets from source to dest.
Routing algorithms
analogy:
routing: process of planning trip from source to dest
forwarding: process of getting through single interchange
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1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
4
Datagram networks 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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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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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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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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IP Fragmentation and Reassembly
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 datagram becomesseveral smaller datagrams
Example 4000 byte
datagram MTU = 1500
bytes
1480 bytes in data field
offset =1480/8
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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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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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Subnets223.1.1.0/24
223.1.2.0/24
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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Subnets
How 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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IP addressing: CIDR
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
200.23.16.0/2314
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”
(more in next chapter)
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IP addresses: how to get one?
Q: How does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
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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IP addressing: the last word...Q: How does an ISP get block of addresses?A: ICANN: Internet Corporation for Assigned
Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes
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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., home 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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NAT: Network Address Translation
Motivation: local network uses just one IP address as far as outside word 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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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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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
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S: 138.76.29.7, 5001D: 128.119.40.186, 80
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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
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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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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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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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Intra-AS Routing Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
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RIP ( Routing Information Protocol)
Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops)
DC
BA
u v
w
x
yz
destination hops u 1 v 2 w 2 x 3 y 3 z 2
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RIP: Example
Destination Network Next Router Num. of hops to dest.
w A 2y B 2
z B 7x -- 1…. …. ....
w x y
z
A
C
D B
Routing table in D
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RIP: Example
Destination Network Next Router Num. of hops to dest.
w A 2y B 2
z B A 7 5x -- 1…. …. ....
Routing table in D
w x y
z
A
C
D B
Dest Next hops w - - x - - z C 4 …. … ...
Advertisementfrom A to D
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RIP advertisements Distance vectors: exchanged among
neighbors every 30 sec via Response Message (also called advertisement)
Each advertisement: list of up to 25 destination nets within AS
Loops Why loops?
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RIP: Link Failure and Recovery
If no advertisement heard after 180 sec --> neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if
tables changed) link failure info quickly propagates to entire net poison reverse used to prevent ping-pong loops
(infinite distance = 16 hops)
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RIP Table processing RIP routing tables managed by application-level
process called route-d (daemon) advertisements sent in UDP packets, periodically
repeated
physical
link
network forwarding (IP) table
Transprt (UDP)
routed
physical
link
network (IP)
Transprt (UDP)
routed
forwardingtable
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OSPF (Open Shortest Path First) “open”: publicly available Uses Link State algorithm
LS packet dissemination Topology map at each node Route computation using Dijkstra’s algorithm
OSPF advertisement carries one entry per neighbor router
Advertisements disseminated to entire AS (via flooding) Carried in OSPF messages directly over IP (rather
than TCP or UDP
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OSPF “advanced” features (not in RIP) Security: all OSPF messages authenticated
(to prevent malicious intrusion) Multiple same-cost paths allowed (only one
path in RIP) For each link, multiple cost metrics for
different TOS (e.g., satellite link cost set “low” for best effort; high for real time)
Integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology
data base as OSPF Hierarchical OSPF in large domains.32
Hierarchical OSPF
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Hierarchical OSPF
Two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas. Area border routers: “summarize” distances to
nets in own area, advertise to other Area Border routers.
Backbone routers: run OSPF routing limited to backbone.
Boundary routers: connect to other AS’s.
Hierarchical Routing
scale: with 200 million destinations:
can’t store all dest’s in routing tables!
routing table exchange would swamp links!
administrative autonomy
internet = network of networks
each network admin may want to control routing in its own network
Our routing study thus far - idealization all routers identical network “flat”… not true in practice
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3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
Intra-ASRouting algorithm
Inter-ASRouting algorithm
Forwardingtable
3c
Interconnected ASes
Forwarding table is configured by both intra- and inter-AS routing algorithm Intra-AS sets entries for
internal dests Inter-AS & Intra-As sets
entries for external dests
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3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
3c
Inter-AS tasks Suppose router in
AS1 receives datagram for which dest is outside of AS1 Router should forward
packet towards on of the gateway routers, but which one?
AS1 needs:1. to learn which dests
are reachable through AS2 and which through AS3
2. to propagate this reachability info to all routers in AS1
Job of inter-AS routing!
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Why different Intra- and Inter-AS routing ?
Policy: Inter-AS: admin wants control over how its traffic
routed, who routes through its net. Intra-AS: single admin, so no policy decisions
needed
Scale: hierarchical routing saves table size, reduced
update trafficPerformance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance