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![Page 1: תזכורת r שיעור השלמה יום שישי הקרוב r שעות 9-12 r מקום : אורנשטיין 111.](https://reader035.fdocuments.in/reader035/viewer/2022062300/56649e055503460f94af1b6f/html5/thumbnails/1.jpg)
תזכורת
שיעור השלמה יום שישי הקרוב 9-12שעות 111מקום: אורנשטיין
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Last Week
Basic Routing Schemes Link State:
• broadcast link information• Local computation on global topology
Distance vector: • Exchange distance information with neighbors• Local updates based on neighbors information
Hierarchical Routing Broadcast and multicast
Using a tree topology
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This week
Hierarchical routing IP addresses
Definition of network Network Address Translation (NAT)
Routing algorithms implementations
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Lecture 6: Network Layer #4
The Internet Network layer
routingtable
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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Lecture 6: Network Layer #5
Hierarchical Routing
scale: with 50 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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Lecture 6: Network Layer #6
Hierarchical Routing
aggregate routers into regions, “autonomous systems” (AS)
routers in same AS run same routing protocol “intra-AS” routing
protocol routers in different AS
can run different intra-AS routing protocol
special routers in AS run intra-AS routing
protocol with all other routers in AS
also responsible for routing to destinations outside AS run inter-AS routing
protocol with other gateway routers
gateway routers
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Lecture 6: Network Layer #7
Intra-AS and Inter-AS routing
Gateways:•perform inter-AS routing amongst themselves•perform intra-AS routers with other routers in their AS
inter-AS, intra-AS routing in
gateway A.c
network layer
link layer
physical layer
a
b
b
aaC
A
Bd
A.a
A.c
C.bB.a
cb
c
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Lecture 6: Network Layer #8
Intra-AS and Inter-AS routing
Host h2
a
b
b
aaC
A
Bd c
A.a
A.c
C.bB.a
cb
Hosth1
Intra-AS routingwithin AS A
Inter-AS routingbetween A and B
Intra-AS routingwithin AS B
We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly
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Lecture 6: Network Layer #9
Routing: Example
AS A(OSPF)
AS B(OSPF intra routing)
AS D
AS Ci
b
b->i: I can reach hosts
in D; my path: BCD
a1
a2
d
d->a2: I
can re
ach
hosts in
D; m
y path: D
a1->i: I can reach hosts
in D; my path: AD
E
F
Export to E: i->e: I can
reach hosts in D; path: IBCD
AS I
a2->a1: I can reach
hosts in D; path: D
choose BCD
using i2
b->i2: I can reach hosts
in D; my path: BCD
i2
i2->i: I can reach
hosts in D; path: BCD
No Exportto F
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Lecture 6: Network Layer #10
Routing: Example
AS A(OSPF)
AS B(OSPF intra routing)
AS D
AS C
i
b
How to specify?
a1
a2
d
a1->i: I can
reachhosts in D; my
path: AD
E
F
AS I
d1
d2
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Lecture 6: Network Layer #11
IP Addressing Scheme
We need an address to uniquely identify each destination
Routing scalability needs flexibility in aggregation of destination addresses we should be able to aggregate a set of
destinations as a single routing unit
Preview: the unit of routing in the Internet is a network---the destinations in the routing protocols are networks
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Lecture 6: Network Layer #12
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 interface, not host, or 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
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
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Lecture 6: Network Layer #13
IP Addressing IP address:
network part • high order bits
host part • low order bits
What’s a network ? (from IP address perspective) device interfaces with
same network 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 IP networks(for IP addresses starting with 223, first 24 bits are network address)
LAN
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Lecture 6: Network Layer #14
IP AddressingHow to find the
networks? Detach each
interface from router, host
create “islands of isolated networks
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
Interconnected system consisting
of six networks
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Lecture 6: Network Layer #15
IP Addresses
0network host
10 network host
110 network host
1110 multicast address
A
B
C
D
class1.0.0.0 to127.255.255.255
128.0.0.0 to191.255.255.255
192.0.0.0 to223.255.255.255
224.0.0.0 to239.255.255.255
32 bits
given notion of “network”, let’s re-examine IP addresses:
“class-full” addressing:
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Lecture 6: Network Layer #16
IP addressing: CIDR classful addressing:
inefficient use of address space, address space exhaustion
e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network
CIDR: Classless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network
portion of address
11001000 00010111 00010000 00000000
networkpart
hostpart
200.23.16.0/23
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Lecture 6: Network Layer #17
CIDR Address Aggregation
AS A(OSPF)
AS D
i
a1
a2
d
i->a1: I can reach130.132/16; my path: I
AS I
d1
130.132.1/24
130.132.2/24
130.132.3/24
intradomain routing uses /24
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Lecture 6: Network Layer #18
CIDR Address Aggregation
x00/24: B
x01/24: C
x10/24: E
x/22: A
x11/24: F
A
B
C
E
F
G
x11/24: F
x11/24: GF
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Lecture 6: Network Layer #19
IP addresses: how to get one?
Hosts (host portion): hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol:
dynamically get address: “plug-and-play” 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 The common practice in LAN and home access (why?)
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Lecture 6: Network Layer #20
IP addresses: how to get one?
Network (network portion): get allocated portion of 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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Lecture 6: Network Layer #21
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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Lecture 6: Network Layer #22
Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
“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 199.31.0.0/16or 200.23.18.0/23”
200.23.20.0/23Organization 2
...
...
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Lecture 6: Network Layer #23
Network Address Translation: Motivation
192.168.1.2
192.168.1.3
192.168.1.4
192.168.1.1
138.76.29.7
local network(e.g., home network)
192.168.1.0/24
rest ofInternet
Datagrams with source or destination in this network
have 192.168.1/24 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
A local network uses just one public IP address as far as outside world is concerned Each device on the local network is assigned a private IP address
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Lecture 6: Network Layer #24
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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Lecture 6: Network Layer #25
NAT: Network Address Translation
192.168.1.2
S: 192.168.1.2, 3345D: 128.119.40.186, 80 1
192.168.1.1
138.76.29.7
1: host 192.168.1.2 sends datagram to 128.119.40.186, 80
NAT translation tableWAN side addr LAN side addr
138.76.29.7, 5001 192.168.1.2, 3345…… ……
S: 128.119.40.186, 80 D: 192.168.1.2, 3345
4
S: 138.76.29.7, 5001D: 128.119.40.186, 80
2
2: NAT routerchanges datagramsource addr from192.168.1.2, 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 192.168.1.2, 3345
192.168.1.3
192.168.1.4
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Lecture 6: Network Layer #26
Network Address Translation: Advantages No need to be allocated range of
addresses from ISP: - just one public IP address is used for all devices 16-bit port-number field allows 60,000
simultaneous connections with a single LAN-side address !
can change ISP without changing addresses of devices in local network
can change addresses of devices in local network without notifying outside world
Devices inside local net not explicitly addressable, visible by outside world (a security plus)
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Lecture 6: Network Layer #27
NAT: Network Address Translation
If both hosts are behind different NAT, they will have difficulty establishing connection
NAT is controversial: routers should process up to only layer 3 violates end-to-end argument
• NAT possibility must be taken into account by app designers, e.g., P2P applications
address shortage should instead be solved by having more addresses --- IPv6 !
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Lecture 6: Network Layer #28
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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Lecture 6: Network Layer #29
Getting a datagram from source to dest.
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
IP datagram:
miscfields
sourceIP addr
destIP addr data
datagram remains unchanged, as it travels source to destination
addr fields of interest here mainly dest. IP addr
Dest. Net. next router Nhops
223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2
routing table in A
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Lecture 6: Network Layer #30
Getting a datagram from source to dest.
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
Starting at A, given IP datagram addressed to B:
look up net. address of B find B is on same net. as A link layer will send datagram
directly to B inside link-layer frame B and A are directly
connected
Dest. Net. next router Nhops
223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2
miscfields223.1.1.1223.1.1.3data
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Lecture 6: Network Layer #31
Getting a datagram from source to dest.
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
Dest. Net. next router Nhops
223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2
Starting at A, dest. E: look up network address of
E E on different network
A, E not directly attached
routing table: next hop router to E is 223.1.1.4
link layer sends datagram to router 223.1.1.4 inside link-layer frame
datagram arrives at 223.1.1.4
continued…..
miscfields223.1.1.1223.1.2.2 data
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Lecture 6: Network Layer #32
Getting a datagram from source to dest.
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
Arriving at 223.1.4, destined for 223.1.2.2
look up network address of E
E on same network as router’s interface 223.1.2.9 router, E directly
attached link layer sends datagram
to 223.1.2.2 inside link-layer frame via interface 223.1.2.9
datagram arrives at 223.1.2.2!!! (hooray!)
miscfields223.1.1.1223.1.2.2 data network router Nhops interface
223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9
223.1.3 - 1 223.1.3.27
Dest. next
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Lecture 6: Network Layer #33
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.
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Network Layer 4-34
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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Network Layer 4-35
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=1060
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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Lecture 6: Network Layer #36
Routing in the Internet
The Global Internet consists of Autonomous Systems (AS) interconnected with each other: Stub AS: small corporation Multihomed AS: large corporation (no transit) Transit AS: provider
Two-level routing: Intra-AS: administrator is responsible for choice Inter-AS: unique standard
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Lecture 6: Network Layer #37
Internet AS HierarchyInter-AS border (exterior gateway) routers
Intra-AS interior (gateway) routers
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Lecture 6: Network Layer #38
Intra-AS Routing
Also known as Interior Gateway Protocols (IGP) Most common IGPs:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol (Cisco propr.)
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Lecture 6: Network Layer #39
RIP ( Routing Information Protocol)
Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops)
why?
Distance vectors: exchanged every 30 sec via Response Message (also called advertisement)
Each advertisement: route to up to 25 destination nets
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Lecture 6: Network Layer #40
RIP (Routing Information Protocol)
Destination Network Next Router Num. of hops to dest. w A 2
y B 2 z B 7
x -- 1…. …. ....
w x y
z
A
C
D B
Routing table in D
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Lecture 6: Network Layer #41
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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Lecture 6: Network Layer #42
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)
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Lecture 6: Network Layer #43
OSPF “advanced” features (not in RIP)
Security: all OSPF messages authenticated (to prevent malicious intrusion); TCP connections used
Multiple same-cost paths allowed only one path in RIP
For each link, multiple cost metrics for different ToS (eg, 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.
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Lecture 6: Network Layer #44
Hierarchical OSPF
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Lecture 6: Network Layer #45
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 ASs.
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Lecture 6: Network Layer #46
IGRP (Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (mid 80s) Distance Vector, like RIP several cost metrics (delay, bandwidth,
reliability, load etc) uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg.
(DUAL) based on diffused computation
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Lecture 6: Network Layer #47
Inter-AS routing
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Lecture 6: Network Layer #48
Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard
Path Vector protocol: similar to Distance Vector protocol each Border Gateway broadcast to
neighbors (peers) entire path (I.e, sequence of ASs) to destination
E.g., Gateway X may send its path to dest. Z:
Path (X,Z) = X,Y1,Y2,Y3,…,Z
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Lecture 6: Network Layer #49
Internet inter-AS routing: BGP
Suppose: gateway X send its path to peer gateway W W may or may not select path offered by X
cost, policy (don’t route via competitors AS), loop prevention reasons.
If W selects path advertised by X, then:Path (W,Z) = W, Path (X,Z)
Note: X can control incoming traffic by controlling its route advertisements to peers: e.g., don’t want to route traffic to Z -> don’t advertise any routes to
Z
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Lecture 6: Network Layer #50
Internet inter-AS routing: BGP
BGP messages exchanged using TCP. BGP messages:
OPEN: opens TCP connection to peer and authenticates sender
UPDATE: advertises new path (or withdraws old)
KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request
NOTIFICATION: reports errors in previous msg; also used to close connection
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Lecture 6: Network Layer #51
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
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Extra
Lecture 6: Network Layer #52
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Network Layer 4-53
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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Network Layer 4-54
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 times
Stopping criterion UDP segment eventually
arrives at destination host
Destination returns ICMP “dest port unreachable” packet (type 3, code 3)
When source gets this ICMP, stops.
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Example: tracert www.yahoo.com
Tracing route to www-real.wa1.b.yahoo.com [69.147.76.15]over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms 132.67.250.1 2 <1 ms 1 ms <1 ms dmz-cc-gw.math.tau.ac.il [132.67.252.2] 3 <1 ms <1 ms <1 ms tel-aviv.tau.ac.il [132.66.4.1] 4 1 ms <1 ms <1 ms gp1-tau-ge.ilan.net.il [128.139.191.70] 5 1 ms * 1 ms gp0-gp1-te.ilan.net.il [128.139.188.2] 6 87 ms 86 ms 87 ms iucc.rt1.fra.de.geant2.net [62.40.125.121] 7 87 ms 87 ms 87 ms TenGigabitEthernet7-3.ar1.FRA4.gblx.net
[207.138.144.45] 8 177 ms 177 ms 177 ms 204.245.39.226 9 180 ms 177 ms 265 ms ae1-p151.msr2.re1.yahoo.com [216.115.108.23] 10 177 ms 177 ms 177 ms te-9-4.bas-a2.re1.yahoo.com [66.196.112.203] 11 177 ms 177 ms 177 ms f1.www.vip.re1.yahoo.com [69.147.76.15]
Trace complete.
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Network Layer 4-56
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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Network Layer 4-57
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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Network Layer 4-58
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 4-59
Transition From IPv4 To IPv6
Not all routers can be upgraded simultaneous no “flag days” How will the network operate with mixed IPv4
and IPv6 routers? Tunneling: IPv6 carried as payload in IPv4
datagram among IPv4 routers
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Network Layer 4-60
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
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IPv6 status report Operating systems –
wide support – early 2000 Windows (2000, XP, Vista), BSD, Linux, Apple
Networking infrastructure Cisco
Deployment Slow
Penetration Host - minor (less than 1%) Used in 2008 in China Olympic games
Motivation: CIDR & NAT
Lecture 7: Network Layer II #61