Network Layer. The Internet Network layer. IP datagram format

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2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 1

Network Layer

Goals:

❒ understand principles behind network layer services:

❍routing (path selection) ❍dealing with scale ❍how a router works ❍advanced topics: IPv6,

multicast ❒ instantiation and implementation in the Internet Overview: ❒ last time

❍network layer services ❍IP addressing ❍routing principle: path

selection

❒ today

❍IP

❍hierarchical routing ❍Internet routing protocols

reliable transfer

• intra-domain • inter-domain

❍what’s inside a router? ❍IPv6

❍multicast routing

The Internet Network layer

routing table

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 Network layer

IP datagram format

ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum time to live

32 bit source IP address IP protocol version number header length (32-bit words) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes)

upper layer protocol to deliver payload to

head. len type ofservice

“type” of data _{flgs fragment} offset upper

layer

32 bit destination IP address

Options (if any) E.g. timestamp, record route taken, specify list of routers to visit.

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Fragmentering och hopsättning

❒MTU varierar i olika nät, två val

❍Alla paket tillräckligt små ❍Fragmentering och hopsättning ❒Alla fragment har samma Ident ❒Tappas ett fragment slängs alla

❒Varje fragment fullständigt IP-datagram ❒Hopsättning hos mottagaren

❒Designat för ytterligare fragmentering

Ident Flags Offset

IP Fragmentation and Reassembly

ID =xfragflag=0 offset=0 length =4000 ID =x fragflag=1 offset=0 length =1500 ID =x fragflag=1 offset=1480 length =1500 ID =xfragflag=0 offset=2960 length =1040

One large datagram becomes several smaller datagrams

ICMP: Internet Control Message Protocol

❒ used by hosts, routers, gateways 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 description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion

control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

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Skalbarhet

❒27+ 214+ 221olika nätverksadresser i IP ❒Det blir stora tabeller om varje router ska

hitta alla nätverk

❒Hur får man routing att skala?

Forwarding algortim

❒Om direkt kopplad till destinationens nätverk - skicka direkt till destination ❒annars om destinationen finns i forwarding

tabellen - skicka till NextHop routern ❒annars -skicka till default routern

R2 R1 H4 H5 H3 H2 H1 Network 2 (Ethernet) Network 1 (Ethernet) H6 Network 3 (FDDI) Network 4 (point-to-point) H7 R3 H8 NetworkNum NextHop 1 2 3 4 R3 R1 interface 1 interface 0 Forwarding tabell R2

DHCP

A new node needs an IP-address: ❒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 ❒An example on scaling of network

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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

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

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 a a C A B d A.a A.c C.b B.a c b c

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Intra-AS and Inter-AS routing

Host h2 a b b a a C A B d _c A.a A.c C.b B.a c b Host h1 Intra-AS routing within AS A Inter-AS routing between A and B Intra-AS routing within AS B

2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 14 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

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

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2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 16 RIP ( Routing Information Protocol)

❒Distance vector algorithm

❒Included in BSD-UNIX Distribution in 1982 ❒Distance metric: # of hops (max = 15 hops)

❍Can you guess why?

❒Distance vectors: exchanged every 30 sec via

Response Message (also called advertisement)

❒Each advertisement: route to up to 25 destination

nets

❒RIP v1 RFC 1058 ❒RIP v2 RFC 1723

2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 17 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)

RIP Table processing

❒RIP routing tables managed by application-level

process called route-d (daemon)

❒advertisements sent in UDP packets, periodically

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2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 19 RIP Table example (continued)

Router: giroflee.eurocom.fr

❒ Three attached class C networks (LANs) ❒ Router only knows routes to attached LANs ❒ Default router used to “go up”

❒ Route multicast address: 224.0.0.0 ❒ Loopback interface (for debugging)

Destination Gateway Flags Ref Use Interface -- -- --- --- --- ---127.0.0.1 127.0.0.1 UH 0 26492 lo0 192.168.2. 192.168.2.5 U 2 13 fa0 193.55.114. 193.55.114.6 U 3 58503 le0 192.168.3. 192.168.3.5 U 2 25 qaa0 224.0.0.0 193.55.114.6 U 3 0 le0 default 193.55.114.129 UG 0 143454

2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 20 OSPF (Open Shortest Path First)

❒“open”: publicly available, v2 RFC 2178 ❒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 entireAS (via

flooding)

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 multicastsupport:

❍Multicast OSPF (MOSPF) uses same topology data base as

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 ASs.

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

Internet AS Hierarchy

Intra-AS border (exterior gateway) routers

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Internet inter-AS routing: BGP

❒BGP (Border Gateway Protocol):the de facto

standard

❒Path Vectorprotocol:

❍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 sendits path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z

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

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) ❍KEEPALIVEkeeps connection alive in absence of

UPDATES; also ACKs OPEN request

❍NOTIFICATION:reports errors in previous msg;

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2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 28 Why different Intra- and Inter-AS routing ?

Policy:

❒Intra-AS: single admin, so no policy decisions needed

❒Inter-AS: admin wants control over how its traffic

routed, who routes through its net. Scale:

❒hierarchical routing saves table size, reduced update

traffic Performance:

❒Intra-AS: can focus on performance

❒Inter-AS: policy may dominate over performance

Hur får man routing att skala

mer?

❒Adressanvändning ❍Adresser får inte ta slut

❍Klass C med 2 hosts (2/255 = 0.78%) ❍Klass B med 256 hosts (256/65535 = 0.39%) ❒Skalabilitet för routing

❍Routingtabeller skalar ej ❍IGP skalar ej

Subnetting

❒Subnät ny nivå till adress/routing hierarkin ❒Subnätmasken definierar variabel del av

hostdelen => subnät nummer ❒Subnät syns bara i aktuellt nätverk

Network number Host number Class B address

Subnet mask (255.255.255.0)

Subnetted address 111111111111111111111111 00000000

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Subnät exempel

Forwarding table at router R1 Subnet Number Subnet Mask Next Hop 128.96.34.0 255.255.255.128 interface 0 128.96.34.128 255.255.255.128 interface 1 128.96.33.0 255.255.255.0 R2 Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0 128.96.34.15 128.96.34.1 H1 R1

128.96.34.130 Subnet mask: 255.255.255.128_{Subnet number: 128.96.34.128}

128.96.34.129 128.96.34.139 R2 H2 128.96.33.1 128.96.33.14 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0 H3

Forwarding algoritm

D = destination IP address

for each entry (SubnetNum, SubnetMask, NextHop) D1 = SubnetMask & D

if D1 = SubnetNum

if NextHop is an interface deliver datagram directly to D else

deliver datagram to NextHop

❒Använd default router om inget matchar ❒Inte nödvändigt med sammanhängande 1’or ❒Möjligt att ha flera subnät på ett fysiskt nätverk ❒Subnät syns ej ifrån resten av Internet

Supernetting

❒Om alla som vill ha klass B adresser får det blir det dåligt utnyttjande

❒Om alla istället får klass C adresser ökar routingtabellernas storlek

❒Lösning: Supernetting - CIDR ❍Classless InterDomain Routing

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Classless InterDomain Routing

❒Dela ut block av kontinuerliga

nätverksnummer till närliggande nätverk ❒Representera block med

❍(first_network_address, count), 130.239.40/21 ❒Begränsa storlekar till potens av 2

❒Alla routrar måste förstå CIDR ❒Möjligt att slå ihop adresser rekursivet ❒Längsta möjliga match i router

Hierarchical addressing: more specific

routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anything with 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 7 Internet Organization 1

ISPs-R-Us “Send me anything_{with addresses} beginning 199.31.0.0/16 or 200.23.18.0/23” 200.23.20.0/23 Organization 2 .. . .. .

Subnetting och CIDR:

skalabiliteten

❒Subnetting

❍Hjälper till att lösa skalabilitetsproblemet • Nya fysiska nätverk kan läggas till utan att använda

nya Klass B eller C nätverksadresser • Samlar ihop information

❒CIDR

❍Kortare routingtabeller • Slår ihop routinginformation ❍Spar adresser

• Dela ut block av kontinuerliga nätverksnummer istället för adresser för större nätverksklass

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Skalbara nätverk

❒Interdomän routing för att hitta rätt AS

❍Färre AS än nätverk

❒Intradomän routing för att hitta rätt nätverk i AS

❍Färre nätverk i en domän än i hela Internet

❒CIDR

❍Slår ihop routing information

❒Subnetting

❍Subnät syns ej utåt

❒ARP (nästa lager)

❍Hitta rätt host i ett nätverk (subnät)

❒Skalabilitet viktigare än optimal väg

Router Architecture Overview

Two key router functions:

❒run routing algorithms/protocol (RIP, OSPF, BGP) ❒switching datagrams from incoming to outgoing link

Input Port Functions

Decentralized switching:

❒ given datagram dest., lookup output port using routing 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., Ethernet see chapter 5

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Input Port Queuing

❒ Fabric slower that 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!

Three types of switching fabrics

Via memory First generation routers: ❒packet copied by system’s (single) CPU ❒speed limited by memory bandwidth (2 bus crossings per datagram) Modern routers: ❒input port processor performs lookup, copy into memory Via bus ❒ datagram from input port memory to output port memory via a shared bus ❒ bus contention: switching speed limited by bus bandwidth Via an interconection network

❒ overcome bus bandwidth limitations

❒ Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.

Output Ports

❒Bufferingrequired when datagrams arrive from

fabric faster than the transmission rate

❒Scheduling disciplinechooses among queued

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Output port queueing

❒ buffering when arrival rate via switch exceeeds

ouput line speed

❒ queueing (delay) and loss due to output port buffer overflow!

IPv6

❒Initial motivation:32-bit address space completely allocated by 2008 or 2018 or … ❒Additional motivation:

❍header format helps speed processing/forwarding ❍header changes to facilitate QoS

❍new “anycast” address: route to “best” of several

replicated servers

❒IPv6 datagram format: ❍fixed-length 40 byte header ❍no fragmentation allowed

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?

❒Two proposed approaches:

❍Dual Stack: some routers with dual stack (v6,

v4) can “translate” between formats

❍Tunneling: IPv6 carried as payload in IPv4

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Dual Stack Approach

Tunneling

IPv6 inside IPv4 where needed

Multicast routing

❒Unicast ❍Enkelt, men

bandbreddskrävande

❒Multicast

❍Mindre antal paket,

men kräver support från nätverkslagret

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Multicast routing (forts)

❒Problem

❍Hur identifieras mottagarna av multicast-meddelanden? ❍Hur adresseras ett datagram till dem?

❒Lösning

❍”address indirection”, en identifierare för hel multicast

grupp

❍Class D multicast-adresser

Multicast routing - frågor

❒Hur startar och slutar en grupp? ❒Hur väljs gruppadress?

❒Hur ansluter man till befintlig grupp? ❒Kan vem som helst ansluta sig? ❒Känner man till vilka övriga som är

anslutna?

❒Hur samarbetar routrarna? ❒Involverar IGMP

IGMP

❒Internet Group Management Protocol ❒Mellan host och första router ❒Kan ej identifiera andra i gruppen ❒Tre typer av meddelanden

❍Membership query, membership report, leave

group

❒Körs över IP ❒Receiver-driven

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Multicast routing - generellt

❒Mål: Hitta ett träd med alla routrar ❒Ibland måste andra routrar involveras ❒Två varianter:

❍Group-shared tree, endast ett träd

❍Source-based trees, ett träd för varje sändare

Multicast routing – i Internet

❒DVMRP

❍Distance Vector Multicast Routing Protocol ❍Source-based

❒MOSPF – Multicast OSPF

❍Source-based

❒CBT – Core Based Trees

❍Group-shared

❒PIM – Protocol Independent Multicast

❍Dense mode – liknar DVMRP ❍Sparse mode – group-shared

❒Mellan AS – DVMRP

❍Men inte helt lyckat

Network Layer: Summary

Goals:

❒ understand principles behind network layer services:

❍routing (path selection) ❍dealing with scale ❍how a router works ❍advanced topics: IPv6,

multicast

❒ instantiation and implementation in the Internet

Overview:

❒ network layer services ❒ IP addressing ❒ routing principle: path

selection ❒ IP

❒ hierarchical routing ❒ Internet routing protocols

reliable transfer

❍intra-domain ❍inter-domain

❒ what’s inside a router? ❒ IPv6