Network Layer Lecture # 2 MAHS. 4: Network Layer 4b-2 Hierarchical Routing scale: with 200 million...

Network Layer

Lecture # 2

MAHS

4: Network Layer 4b-2

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 (cost metrics, etc.) in its own network

Our routing study thus far - idealization all routers identical network “flat”

… … notnot true in practice true in practice

Why?Why?


Hierarchical Routing

Organization: aggregate routers into regions, called “autonomous systems” (AS)

routers in same AS run same routing protocol “intra-AS” routing (i.e.,

within an AS) protocol routers in different AS

can run different intra-AS routing protocol

special routers in (on the edge of) an AS

run intra-AS routing protocol with all other routers in AS

also responsible for routing to destinations outside AS run inter-AS routing

(i.e., between AS) protocol with other gateway routers

gateway routers


Intra-AS and Inter-AS routing

Gateway routers:•perform inter-AS routing amongst themselves•perform intra-AS routing with other routers in their AS

inter-AS, intra-AS routing in

gateway A.c

network layer

data link layerphysical layer

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c


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 (section 4.5)


IP datagram format

ver length

32 bits

data (variable length,

typically a TCP segment, a UDP segment,

or an ICMP message)

16-bit identifier

Header checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (4-byte multiples)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

(RFC 1700, 3232)

head.len

type ofservice

DS codepoint, ECN 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


IP Fragmentation & Reassembly network links have MTU

(Max. Transfer Unit) size - largest possible link-level frame. different link types,

different MTUs large IP datagram is divided

(“fragmented”) within network one datagram becomes

several datagrams “reassembled” only at

the final destination IP header bits are used

to identify and order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly


IP Fragmentation and Reassembly

ID=x

offset=0

More bit=0

bytes*=3980

ID=x

offset=0

More bit=1

bytes*=1480

ID=x

offset=1480

More bit=1

bytes*=1480

ID=x

offset=2960

More bit=0

bytes*=1020

One large datagram becomesseveral smaller datagrams

Note: Offset is actuallyspecified as number of 8-byte (64-bit) units.

Example 4000 byte

datagram MTU = 1500

bytes

* This is the number of data bytes in the IP datagram. The IP length field would show this + 20. Why?


DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when it joins a 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


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


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


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



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).



Implementation: 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



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



16-bit port-number field: 60,000 simultaneous connections with a

single LAN-side address! Reserved address space (rfc 1918) 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

ftp://ftp.rfc-editor.org/in-notes/rfc1918.txt

ftp://ftp.rfc-editor.org/in-notes/rfc1918.txt

http://www.icann.org/registrars/accredited-list.html


Intra-AS Routing

Also known as Interior Gateway Protocols (IGP) Most common IGPs:

RIP: Routing Information Protocol (legacy, RIPv2 still in use)

OSPF: Open Shortest Path First (common)

EIGRP: Enhanced Interior Gateway Routing Protocol (proprietary – Cisco Systems)


RIP ( Routing Information Protocol)

Distance vector algorithm Included in BSD-UNIX Distribution in 1982

RFC 1058 (version 1), RFC 2453 (version 2)

Distance metric: # of hops (max = 15 hops) Can you guess why?

Distance vectors: exchanged every 30 seconds via Response Message (also called advertisement)

Each advertisement: routing info for maximum of 25 destination nets within the AS

Uses UDP transport, port 520


Problems/limitations with RIP

Good for small systems, but doesn’t scale well

Count-to-infinity problem… poisoned reverse only

Comparatively slow convergence

1979 – RIP version 1 1988 – IETF initiates work on

replacement 1990 – OSPF became new standard 1990’s – RIP version 2


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

However…. OSPF advertisement carries only one entry per

neighbor router Advertisements disseminated to entire AS (via

flooding) Sent as payload in IP datagram


EIGRP (Enhanced Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (mid 80’s) uses Distance Vector, like RIP several cost metrics (delay, bandwidth,

reliability, load etc) uses TCP (!) to exchange routing updates Loop-free routing via a distributed update

routing algorithm (called DUAL) based on diffused computation


Inter-AS routing

Network Layer Lecture # 2 MAHS. 4: Network Layer 4b-2 Hierarchical Routing scale: with 200 million...

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