05 network
TRANSCRIPT
Network Layer 4-1
Network layer
Network Layer 4-2
Network layerr transport segment from
sending to receiving host r on sending side puts
segments into datagramsr on rcving side, delivers
segments to transport layerr network layer protocols in
every host, router
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 4-3
Network layer functionsr Connection setup
m datagramm connection-oriented, host-
to-host connectionr Delivery semantics:
m Unicast, broadcast, multicast, anycast
m In-order, any-orderr Security
m secrecy, integrity, authenticity
r Demux to upper layerm next protocolm Can be either transport or
network (tunneling)
r Quality-of-servicem provide predictable
performance r Fragmentation
m break-up packets based on data-link layer properties
r Routingm path selection and packet
forwarding r Addressing
m flat vs. hierarchicalm global vs. localm variable vs. fixed length
Network Layer 4-4
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
Network Layer 4-5
IP datagram format
ver length
32 bits
data (variable length,typically a TCP
or UDP segment)
16-bit identifierInternetchecksum
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 flgs fragmentoffset
upperlayer
32 bit destination IP address
Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.
how much overhead with TCP?
r 20 bytes of TCPr 20 bytes of IPr = 40 bytes + app
layer overhead
Network Layer 4-6
Recall network layer functions
rHow does IPv4 support..m Connection setupm Delivery semanticsm Securitym Demux to upper layer mQuality-of-servicem Fragmentationm Addressingm Routing
Network Layer 4-7
IP connection setup
rHourglass designrNo support for network layer connectionsm Unreliable datagram servicemOut-of-order delivery possiblem Connection semantics only at higher layerm Compare to ATM and phone network…
Network Layer 4-8
Connectionless network layersr Postal service abstraction (Internet)
m Model• no call setup or teardown at network layer• no service guarantees
m Network support• no state within network on end-to-end connections• packets forwarded based on destination host ID• packets between same source-dest pair may take different
paths
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
1. Send data 2. Receive data
Network Layer 4-9
Connection-oriented network layersr Circuit abstraction
m Examples: ATM, frame relay, X.25, phone networkm Model
• call setup and signaling for each call before data can flow• guaranteed performance during call• call teardown and signaling to remove call
m Network support• every router on source-dest path maintains “state” for each passing
circuit• link, router resources (bandwidth, buffers) allocated to VC to
guarantee circuit-like performance
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
1. Initiate call2. incoming call3. Accept call4. Call connected
5. Data flow begins 6. Receive data
Network Layer 4-10
IP delivery semantics
rNo reliability guaranteesm Loss
rNo ordering guaranteesmOut-of-order delivery possible
rUnicast mostlym IP broadcast (255.255.255.255) not forwardedm IP multicast supported, but not widely used
• 224.0.0.0 to 239.255.255.255
Network Layer 4-11
IP securityrWeak support for integritym IP checksum
• IP has a header checksum, leaves data integrity to TCP/UDP• http://www.rfc-editor.org/rfc/rfc1141.txt
mNo support for secrecy, authenticityr IPsecm Retrofit IP network layer with encryption and
authenticationm http://www.rfc-editor.org/rfc/rfc2411.txt
Network Layer 4-12
IP demux to upper layerr http://www.rfc-editor.org/rfc/rfc1700.txtm Protocol type field
• 1 = ICMP• 4 = IP in IP• 6 = TCP• 8 = EGP• 9 = IGP • 17 = UDP
Network Layer 4-13
IP quality of servicer IP originally had “type-of-service” (TOS) field to
eventually support qualitymNot used, ignored by most routers
rNeed to provide applications with performance guaranteesmMid 90s: Add circuits to the Internet!
• Integrated services (intserv) and RSVP signalling• Per-flow end-to-end QoS support• Per-flow signaling and network resource allocation
Network Layer 4-14
Network service model
Example services for individual datagrams:
r guaranteed deliveryr guaranteed delivery
with less than 40 msec delay
Example services for a flow of datagrams:
r in-order datagram delivery
r guaranteed minimum bandwidth to flow
r restrictions on changes in inter-packet spacing (jitter)
Network Layer 4-15
IP quality of servicer Protocols developed and standardized
m RSVP signalling protocolm Intserv service models
r Failed miserably…Why?m Complexity
• Scheduling• Routing (pinning routes)• Per-flow signalling overhead
m Lack of scalability• Per-flow state
m Economics• Providers with no incentive to deploy• SLA, end-to-end billing issues
m QoS a weak-link property• Requires every device on an end-to-end basis to support flow
Network Layer 4-16
IP quality of servicerNow it’s diffserv…mUse the “type-of-service” bits as a priority markingm http://www.rfc-editor.org/rfc/rfc2474.txtm http://www.rfc-editor.org/rfc/rfc2475.txtm http://www.rfc-editor.org/rfc/rfc2597.txtm http://www.rfc-editor.org/rfc/rfc2598.txt
Network Layer 4-17
IP Addressingr IP address:m 32-bit identifier for
host/router interface
m routers typically have multiple interfaces
m Addresses hierarchical (like post office)
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
Network Layer 4-18
IP Addressingr IP address:
m network part (high order bits)
m host part (low order bits) r What’s a network ?
m all interfaces that can physically reach each other without intervening router
m each interface shares the same network part of IP address
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
Network Layer 4-19
How did networks get IP addresses?
r Total IP address size: 4 billionr Initially one large class (8-bit network, 24-bit host)
m ISP given an 8-bit network number to managem Each router keeps track of each network (28=256 routes)m Each network has 16 million hostsm Problem: one size does not fit all
r Classful addressingm Accommodate smaller networks (LANs)m Class A: 128 networks, 16M hostsm Class B: 16K networks, 64K hostsm Class C: 2M networks, 256 hostsm Total routes potentially > 2,113,664 networks and network routes !
Network Layer 4-20
IP address classes
Network ID Host ID8 16
Class A32
0
Class B 10
Class C 110
Multicast AddressesClass D 1110
Reserved for experimentsClass E 1111
24
Network ID
Network ID
Host ID
Host ID
1.0.0.0 to 127.255.255.255
128.0.0.0 to 191.255.255.255
192.0.0.0 to 223.255.255.255
224.0.0.0 to 239.255.255.255
Network Layer 4-21
Special IP AddressesrPrivate addresses
– http://www.rfc-editor.org/rfc/rfc1918.txt– Class A: 10.0.0.0 - 10.255.255.255 (10.0.0.0/8 prefix)– Class B: 172.16.0.0 - 172.31.255.255 (172.16.0.0/12
prefix)– Class C: 192.168.0.0 - 192.168.255.255 (192.168.0.0/16
prefix)
r127.0.0.1: local host (a.k.a. the loopbackaddress)
Network Layer 4-22
IP Addressing problems
r Inefficient use of address spacem Class A (rarely given out, sparse usage)m Class B = 64k hosts (sparse usage)
• Very few LANs have close to 64K hosts
rAddress space depletionm Classes A and B take huge chunks of space but
not used muchmNot many class C addresses left to give out
r Explosion of routesm Increasing use of class C explodes # of routes
Network Layer 4-23
variablenetwork
part
IP addressing: CIDR
rOriginal classful addressingmUse class structure (A, B, C) to determine
network ID for route lookup
rCIDR: Classless InterDomain RoutingmArbitrarily aggregate and split up adjacent
network addresses
11001000 00010111 00010000 00000000
hostpart
200.23.16.0/23
Network Layer 4-24
CIDR
rAssign any range of addresses to networkmAllows one to split large network blocks into multiple
smaller ones (increase usage of Class A & B)mAllows one to combine small network blocks into a
single large one (reduce routes from Class C usage)
Network Layer 4-25
Getting IP addressesQ: How does network get IP addresses?A: organization gets allocated portion of its provider
ISP’s address spacem ISPs get it from ICANN: Internet Corporation for
Assigned Names and Numbers• Allocates addresses, manages DNS, resolves disputes
m Customers get sub-blocks from ISPs
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
Network Layer 4-26
IPv6r IPv4 running out of addressesrNeed to replace it with a new network protocolrWhat changes should be made in….
• IP addressing• IP delivery semantics• IP quality of service• IP security• IP routing• IP fragmentation• IP error detection
Network Layer 4-27
IPv6r Initial motivation: 32-bit address space soon
to be completely allocated rAdditional motivation:m Remove ancillary functionality
• Speed processing/forwardingm Add missing, but essential functionality
• header changes to facilitate QoS • new “anycast” address: route to “best” of several
replicated servers
Network Layer 4-28
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 next protocol for data
Network Layer 4-29
IPv6 Changes
rScale – addresses are 128bitmHeader size?
rSimplificationm Removes infrequently used parts of headerm Removes checksumm 40 byte fixed header vs. 20+ byte variable
headerm Eliminates fragmentation
Network Layer 4-30
Transition From IPv4 To IPv6rNot all routers can be upgraded simultaneousm no “flag days”mHow will the network operate with mixed IPv4 and
IPv6 routers? m Tunneling: IPv6 carried as payload in an IPv4
datagram among IPv4 routers
Network Layer 4-31
TunnelingA B E F
IPv6 IPv6 IPv6 IPv6
tunnelLogical view:
Physical view:A B E F
IPv6 IPv6 IPv6 IPv6IPv4 IPv4
Network Layer 4-32
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:IPv6B-to-C:
IPv6 insideIPv4
B-to-C:IPv6 inside
IPv4
Network Layer 4-33
Routing
Network Layer 4-34
Two Key Network-Layer Functions
r forwarding: move packets from router’s input to appropriate router output
r routing: determine route taken by packets from source to dest.
m routing algorithms
Network Layer 4-35
Internet routing with IP addressesrHop-by-hop forwarding based on destination IP
carried by packetm Each packet has destination IP addressm Each router has forwarding table of..
• destination IP à next hop IP addressm IP route table calculated in network routers
rMost prevalent way to route on the InternetmDistributed routing algorithm for calculating
forwarding tables
Network Layer 4-36
Routing protocols
Graph abstraction for routing algorithms:
r Routing algorithms find minimum cost paths through graph
Goal: determine “good” path (sequence of routers) thru network from source to dest.
A
ED
CB
F2
21
3
1
1
2
53
5
Network Layer 4-37
Routing Algorithm classification
Global or decentralized information?Global:
m all routers have complete topology, link cost infom “link state” algorithms
Decentralized:m router knows physically-connected neighbors, link costs
to neighborsm iterative process of computation, exchange of info with
neighborsm “distance vector” algorithms
Network Layer 4-38
Hierarchical Routing
scale: with 200 million destinations:
r can’t store all dest’s in routing tables!
r routing table exchange would swamp links!
r Flat routing does not scale
administrative autonomyr internet = network of
networksr each network admin may
want to control routing in its own network
Network Layer 4-39
Routing Hierarchies
r Key observationmNeed less information with increasing distance to
destinationmHierarchical routing
• saves table size• reduces update traffic• allows routing to scale
Network Layer 4-40
AreasrDivide network into areasmWithin area, each node has routes to every other node mOutside area
• Each node has routes for other top-level areas only (not nodes within those areas)
• Inter-area packets are routed to nearest appropriate border router
Network Layer 4-41
Internet Routing Hierarchy
r Internet areas called “autonomous systems”(AS)m administrative
autonomyr routers in same AS run
same routing protocolm “intra-AS” routing
protocol (IGP)
Border routersm Special routers in AS
that directly link to another AS
• also run inter-ASrouting protocol or border gateway protocol (BGP) with other gateway routers in other AS’s
Network Layer 4-42
Internet Routing Hierarchy
a
b
b
aaC
A
Bd
A.aA.c
C.bB.a
cb
c
Network Layer 4-43
Inter-AS routing
r Done using BGP (Border Gateway Protocol)m Uses distance-vector style algorithms
r BGP messages exchanged using TCP.m Advantages:
• Simplifies BGP• No need for periodic refresh - routes are valid until
withdrawn, or the connection is lost• Incremental updates
m Disadvantages• BGP TCP spoofing attack• Congestion control on a routing protocol?• Poor interaction during high load (Code Red)• No authentication of route advertisements
– Pakistan Youtube incident
Network Layer 4-44
ICMP: Internet Control Message Protocol
r Essentially a network-layer protocol for passing control messages
r used by hosts & routers to communicate network-level informationm error reporting: unreachable
host, network, port, protocolm echo request/reply (used by
ping)r network-layer “above” IP:
m ICMP msgs carried in IP datagrams
r ICMP message: type, code plus first 8 bytes of IP datagram causing error
r http://www.rfc-editor.org/rfc/rfc792.txt
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
Network Layer 4-45
ICMP and traceroute
r What do “real” Internet delay & loss look like? r Traceroute program: provides delay
measurement from source to router along end-end Internet path towards destination. For all i:m sends three packets that will reach router i on path
towards destinationm router i will return packets to senderm sender times interval between transmission and reply.
3 probes
3 probes
3 probes
Network Layer 4-46
ICMP and traceroute
r Source sends series of UDP segments to destm First has TTL =1m Second has TTL=2, etc.m Unlikely port number
r When nth datagram arrives to nth router:m Router discards datagramm And sends to source an
ICMP message (type 11, code 0)
m Message includes name of router& IP address
r When ICMP message arrives, source calculates RTT
r Traceroute does this 3 times
Stopping criterionr UDP segment eventually
arrives at destination hostr Destination returns ICMP
“host unreachable” packet (type 3, code 3)
r When source gets this ICMP, stops.
Network Layer 4-47
Examples
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms17 * * *18 * * *19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms
traceroute: gaia.cs.umass.edu to www.eurecom.frThree delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu
* means no response (probe lost, router not replying)
trans-oceaniclink
Network Layer 4-48
Try it
rSome routers labeled with airport code of city they are located inm traceroute www.yahoo.com
• Packets go to SEA, back to PDX, SJCm traceroute www.oregonlive.com
• Packets go to SMF, SFO, SJC, NYC, EWR.m traceroute www.uoregon.edu
• Packets go to Pittock block to Eugenem traceroute www.lclark.edu
• Packets go to SEA and back to PDX
Network Layer 4-49
Internet overview complete
rTechnical background for the rest of the course