tcp/ip network layer

Network Layer

Session No.4

Network Layer

Network Layer

Chapter 4: Network Layer

Chapter goals: r understand principles behind network layer

services:m network layer service modelsm forwarding versus routingm how a router worksm routing (path selection)m dealing with scalem advanced topics: IPv6

Network Layer


r 4. 1 Introductionr 4.2 Virtual circuit and

datagram networksr 4.3 What’s inside a

routerr 4.4 IP: Internet

Protocolm Datagram formatm IPv4 addressingm ICMPm IPv6

r 4.5 Routing algorithmsm Link statem Distance Vectorm Hierarchical routing

r 4.6 Routing in the Internetm RIPm OSPFm BGP

Network Layer

Network layerr transport segment from

sending to receiving host r on sending side

encapsulates segments into datagrams

r on rcving side, delivers segments to transport layer

r network layer protocols in every host, router

r router examines header fields in all IP datagrams passing through it

applicationtransportnetworkdata linkphysical


networkdata linkphysical network

data linkphysical

networkdata linkphysical







networkdata linkphysicalnetwork

data linkphysical

Network Layer

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

analogy:

r routing: process of planning trip from source to dest

r forwarding: process of getting through single interchange

Network Layer

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer

Connection setup

r 3rd important function in some network architectures:m ATM, frame relay, X.25

r before datagrams flow, two end hosts and intervening routers establish virtual connectionm routers get involved

r network vs transport layer connection service:m network: between two hosts (may also involve

intervening routers in case of VCs)m transport: between two processes

Network Layer








Network Layer

Network layer connection and connection-less service

r datagram network provides network-layer connectionless service

r VC network provides network-layer connection service

r analogous to the transport-layer services, but:m service: host-to-hostm no choice: network provides one or the otherm implementation: in network core

Network Layer

Virtual circuits

r call setup, teardown for each call before data can flowr each packet carries VC identifier (not destination host

address)r every router on source-dest path maintains “state” for

each passing connectionr link, router resources (bandwidth, buffers) may be

allocated to VC (dedicated resources = predictable service)

“source-to-dest path behaves much like telephone circuit”m performance-wisem network actions along source-to-dest path

Network Layer

Virtual circuits: signaling protocols

r used to setup, maintain teardown VCr used in ATM, frame-relay, X.25r not used in today’s Internet



1. Initiate call 2. incoming call3. Accept call4. Call connected

5. Data flow begins 6. Receive data

Network Layer

Datagram networksr no call setup at network layerr routers: no state about end-to-end connections

m no network-level concept of “connection”r packets forwarded using destination host address

m packets between same source-dest pair may take different paths



1. Send data 2. Receive data

Network Layer

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

Network Layer

Longest prefix matching

Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?

Network Layer

Datagram or VC network: why?

Internet (datagram)r data exchange among

computersm “elastic” service, no strict

timing req. r “smart” end systems

(computers)m can adapt, perform

control, error recoverym simple inside network,

complexity at “edge”r many link types

m different characteristicsm uniform service difficult

ATM (VC)r evolved from telephonyr human conversation:

m strict timing, reliability requirements

m need for guaranteed service

r “dumb” end systemsm telephonesm complexity inside

network

Network Layer








r 4.7 Broadcast and multicast routing

Network Layer

Router Architecture Overview

Two key router functions: r run routing algorithms/protocol (RIP, OSPF, BGP)r forwarding datagrams from incoming to outgoing link

Network Layer








Network Layer

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 conventionsICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer








Network Layer

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifierheader

checksumtime to

live32 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 fragment offset

upper layer

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

IP Fragmentation & Reassemblyr network links have MTU (max.

transfer size) - largest possible link-level frame.m different link types,

different MTUs r large IP datagram divided

(“fragmented”) within netm one datagram becomes

several datagramsm “reassembled” only at final

destinationm IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Network Layer








Network Layer

IP Addressing: introductionr IP address: 32-bit

identifier for host, router interface

r interface: connection between host/router and physical linkm router’s typically have

multiple interfacesm host typically has one

interfacem 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

Network Layer

Subnetsr IP address:

m subnet part (high order bits)

m host part (low order bits)

r What’s a subnet ?m device interfaces with

same subnet part of IP address

m 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

subnet

Network Layer

Subnets 223.1.1.0/24 223.1.2.0/24

223.1.3.0/24

Reciper 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

Network Layer

SubnetsHow 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

Network Layer

IP addresses: how to get one?

Q: How does a host get IP address?

r hard-coded by system admin in a filem Windows: control-panel->network->configuration-

>tcp/ip->propertiesm UNIX: /etc/rc.config

r DHCP: Dynamic Host Configuration Protocol: dynamically get address from as serverm “plug-and-play”

Network Layer

DHCP: Dynamic Host Configuration Protocol

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

m host broadcasts “DHCP discover” msgm DHCP server responds with “DHCP offer” msgm host requests IP address: “DHCP request” msgm DHCP server sends address: “DHCP ack” msg

Network Layer

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

Network Layer

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 offersrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP requestsrc: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACKsrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

Network Layer

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

Network Layer


r Motivation: local network uses just one IP address as far as outside world is concerned:m range of addresses not needed from ISP: just one IP

address for all devicesm can change addresses of devices in local network

without notifying outside worldm can change ISP without changing addresses of

devices in local networkm devices inside local net not explicitly addressable,

visible by outside world (a security plus).

Network Layer

NAT: Network Address TranslationImplementation: NAT router must:

m 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.m remember (in NAT translation table) every (source

IP address, port #) to (NAT IP address, new port #) translation pair

m 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

Network Layer


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

110.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr138.76.29.7, 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, 802

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

Network Layer


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

LAN-side address!r NAT is controversial:

m routers should only process up to layer 3m violates end-to-end argument

• NAT possibility must be taken into account by app designers, eg, P2P applications

m address shortage should instead be solved by IPv6

Network Layer

NAT traversal problemr client wants to connect to

server with address 10.0.0.1m server address 10.0.0.1 local

to LAN (client can’t use it as destination addr)

m only one externally visible NATted address: 138.76.29.7

r solution 1: statically configure NAT to forward incoming connection requests at given port to serverm e.g., (123.76.29.7, port 2500)

always forwarded to 10.0.0.1 port 25000

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

Client ?

Network Layer

NAT traversal problemr solution 2: Universal Plug and

Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to:❖ learn public IP address

(138.76.29.7)❖ add/remove port mappings

(with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

IGD

Network Layer

NAT traversal problemr solution 3: relaying (used in Skype)

m NATed client establishes connection to relaym External client connects to relaym relay bridges packets between to connections

138.76.29.7Client

10.0.0.1

NAT router

1. connection torelay initiatedby NATted host

2. connection torelay initiatedby client

3. relaying established

Network Layer








Network Layer

ICMP: Internet Control Message Protocol

r used by hosts & routers to communicate network-level informationm error reporting:

unreachable host, network, port, protocol

m echo request/reply (used by ping)

r network-layer “above” IP:m ICMP msgs carried in IP

datagramsr 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

Network Layer

Traceroute and ICMP

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








Network Layer

IPv6r Initial motivation: 32-bit address space soon

to be completely allocated. r Additional motivation:

m header format helps speed processing/forwardingm header changes to facilitate QoS IPv6 datagram format: m fixed-length 40 byte headerm no fragmentation allowed

Network Layer

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

Network Layer

Other Changes from IPv4

r Checksum: removed entirely to reduce processing time at each hop

r Options: allowed, but outside of header, indicated by “Next Header” field

r ICMPv6: new version of ICMPm additional message types, e.g. “Packet Too Big”m multicast group management functions

Network Layer

Transition From IPv4 To IPv6

r Not all routers can be upgraded simultaneousm no “flag days”m How will the network operate with mixed IPv4 and

IPv6 routers? r Tunneling: IPv6 carried as payload in IPv4

datagram among IPv4 routers

Network Layer

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6IPv4 IPv4

Network Layer

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


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6B-to-C:

IPv6 insideIPv4

B-to-C:IPv6 inside

IPv4

Network Layer








Network Layer

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing, forwarding

Network Layer

Routing Algorithm classification

Global or decentralized information?

Global:r all routers have complete

topology, link cost infor “link state” algorithmsDecentralized: r router knows physically-

connected neighbors, link costs to neighbors

r iterative process of computation, exchange of info with neighbors

r “distance vector” algorithms

Static or dynamic?Static: r routes change slowly

over timeDynamic: r routes change more

quicklym periodic updatem in response to link

cost changes

Network Layer








Network Layer

A Link-State Routing Algorithm

Dijkstra’s algorithmr net topology, link costs

known to all nodesm accomplished via “link

state broadcast” m all nodes have same info

r computes least cost paths from one node (‘source”) to all other nodesm gives forwarding table

for that noder iterative: after k

iterations, know least cost path to k dest.’s

Notation:r c(x,y): link cost from node

x to y; = ∞ if not direct neighbors

r D(v): current value of cost of path from source to dest. v

r p(v): predecessor node along path from source to v

r N': set of nodes whose least cost path definitively known

Network Layer








Network Layer

Distance vector algorithm (4)

Basic idea: r From time-to-time, each node sends its own

distance vector estimate to neighborsr Asynchronousr When a node x receives new DV estimate from

neighbor, it updates its own DV using B-F equation:Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N

r Under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)

Network Layer

Distance Vector: link cost changes

Link cost changes:r node detects local link cost change r updates routing info, recalculates

distance vectorr if DV changes, notify neighbors

“goodnews travelsfast”

x z14

50

y1

At time t0, y detects the link-cost change, updates its DV, and informs its neighbors.

At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV.

At time t2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z.

Network Layer

Distance Vector: link cost changes

Link cost changes:r good news travels fast r bad news travels slow -

“count to infinity” problem!r 44 iterations before

algorithm stabilizes: see text

Poisoned reverse: r If Z routes through Y to

get to X :m Z tells Y its (Z’s) distance

to X is infinite (so Y won’t route to X via Z)

r will this completely solve count to infinity problem?

x z14

50

y60

Network Layer

Comparison of LS and DV algorithms

Message complexityr LS: with n nodes, E links, O

(nE) msgs sent r DV: exchange between

neighbors onlym convergence time varies

Speed of Convergencer LS: O(n2) algorithm requires

O(nE) msgsm may have oscillations

r DV: convergence time variesm may be routing loopsm count-to-infinity problem

Robustness: what happens if router malfunctions?

LS: m node can advertise

incorrect link costm each node computes only

its own tableDV:

m DV node can advertise incorrect path cost

m each node’s table used by others

• error propagate thru network

Network Layer








Network Layer

Hierarchical Routing

scale: with 200 million destinations:

r can’t store all dest’s in routing tables!

r routing table exchange would swamp links!

administrative autonomyr internet = network of

networksr each network admin may want

to control routing in its own network

Our routing study thus far - idealization r all routers identicalr network “flat”… not true in practice

Network Layer

Hierarchical Routing

r aggregate routers into regions, “autonomous systems” (AS)

r routers in same AS run same routing protocolm “intra-AS” routing

protocolm routers in different AS

can run different intra-AS routing protocol

Gateway routerr Direct link to router in

another AS

Network Layer

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

r forwarding table configured by both intra- and inter-AS routing algorithmm intra-AS sets entries

for internal destsm inter-AS & intra-As

sets entries for external dests

Network Layer

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Inter-AS tasksr suppose router in AS1

receives datagram destined outside of AS1:m router should

forward packet to gateway router, but which one?

AS1 must:1. learn which dests are

reachable through AS2, which through AS3

2. propagate this reachability info to all routers in AS1

Job of inter-AS routing!

Network Layer








Network Layer

Intra-AS Routing

r also known as Interior Gateway Protocols (IGP)r most common Intra-AS routing protocols:

m RIP: Routing Information Protocol

m OSPF: Open Shortest Path First

m IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

Network Layer








Network Layer

RIP ( Routing Information Protocol)

r distance vector algorithmr included in BSD-UNIX Distribution in 1982r distance metric: # of hops (max = 15 hops)

DC

BA

u vw

x

yz

destination hops u 1 v 2 w 2 x 3 y 3 z 2

From router A to subnets:

Network Layer

RIP advertisements

r distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)

r each advertisement: list of up to 25 destination subnets within AS

Network Layer

RIP: Example

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/Forwarding table in D

Network Layer

RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B A 7 5

x -- 1…. …. ....

Routing/Forwarding table in D

w x y

z

A

C

D B

Dest Next hops w - 1 x - 1 z C 4 …. … ...

Advertisementfrom A to D

Network Layer

RIP: Link Failure and Recovery If no advertisement heard after 180 sec -->

neighbor/link declared deadm routes via neighbor invalidatedm new advertisements sent to neighborsm neighbors in turn send out new advertisements (if

tables changed)m link failure info quickly (?) propagates to entire netm poison reverse used to prevent ping-pong loops

(infinite distance = 16 hops)

Network Layer

RIP Table processing

r RIP routing tables managed by application-level process called route-d (daemon)

r advertisements sent in UDP packets, periodically repeated

physicallink

network forwarding (IP) table

Transprt (UDP)

routed

physicallink

network (IP)

Transprt (UDP)

routed

forwarding

table

Network Layer








Network Layer

OSPF (Open Shortest Path First)

r “open”: publicly availabler uses Link State algorithm

m LS packet disseminationm topology map at each nodem route computation using Dijkstra’s algorithm

r OSPF advertisement carries one entry per neighbor router

r advertisements disseminated to entire AS (via flooding)m carried in OSPF messages directly over IP (rather than TCP

or UDP

Network Layer

OSPF “advanced” features (not in RIP)

r security: all OSPF messages authenticated (to prevent malicious intrusion)

r multiple same-cost paths allowed (only one path in RIP)

r For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time)

r integrated uni- and multicast support: m Multicast OSPF (MOSPF) uses same topology data

base as OSPFr hierarchical OSPF in large domains.

Network Layer

Hierarchical OSPF

Network Layer

Hierarchical OSPF

r two-level hierarchy: local area, backbone.m Link-state advertisements only in area m each nodes has detailed area topology; only know

direction (shortest path) to nets in other areas.r area border routers: “summarize” distances to nets

in own area, advertise to other Area Border routers.r backbone routers: run OSPF routing limited to

backbone.r boundary routers: connect to other AS’s.

Network Layer








Network Layer

Internet inter-AS routing: BGP

r BGP (Border Gateway Protocol): the de facto standard

r BGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.2. Propagate reachability information to all AS-

internal routers.3. Determine “good” routes to subnets based on

reachability information and policy.r allows subnet to advertise its existence to

rest of Internet: “I am here”

Network Layer

BGP basicsr pairs of routers (BGP peers) exchange routing info

over semi-permanent TCP connections: BGP sessionsm BGP sessions need not correspond to physical

links.r when AS2 advertises a prefix to AS1:

m AS2 promises it will forward datagrams towards that prefix.

m AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3ceBGP session

iBGP session

Network Layer

Path attributes & BGP routes

r advertised prefix includes BGP attributes. m prefix + attributes = “route”

r two important attributes:m AS-PATH: contains ASs through which prefix

advertisement has passed: e.g, AS 67, AS 17 m NEXT-HOP: indicates specific internal-AS router

to next-hop AS. (may be multiple links from current AS to next-hop-AS)

r when gateway router receives route advertisement, uses import policy to accept/decline.

Network Layer

BGP route selection

r router may learn about more than 1 route to some prefix. Router must select route.

r elimination rules:1. local preference value attribute: policy decision2. shortest AS-PATH 3. closest NEXT-HOP router: hot potato routing4. additional criteria

Network Layer

BGP messages

r BGP messages exchanged using TCP.r BGP messages:

m OPEN: opens TCP connection to peer and authenticates sender

m UPDATE: advertises new path (or withdraws old)m KEEPALIVE keeps connection alive in absence of

UPDATES; also ACKs OPEN requestm NOTIFICATION: reports errors in previous msg;

also used to close connection

Network Layer

Chapter 4: summary







tcp/ip network layer

Software

network layerservices

network architectures

network layernetwork

network layerdatagram

network layersession

network corenetwork

servicer datagram network

network layerchapter