introduction to computer networking guy leduc chapter 4 ... · networking guy leduc chapter 4 ......

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© From Computer Networking, by Kurose&Ross Network Layer: Data Plane 4-1

Introduction to Computer Networking

Guy Leduc

Chapter 4 Network Layer: The Data Plane

Computer Networking: A Top Down Approach, 7th edition. Jim Kurose, Keith RossAddison-Wesley, April 2016.


Chapter 4: Network Layer Data Plane

Chapter goals: ❒  understand principles behind network layer

services:❍  network layer service models❍  addressing❍  forwarding versus routing❍  how a router works

❒  instantiation, implementation in the Internet

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❒  4.1 Overview of Network layer❍  Data plane❍  Control plane

❒  4.2 IP: Internet Protocol❍  IPv4 addressing and forwarding❍  Network Address Translation (NAT)❍  Datagram format❍  Fragmentation❍  IPv6

❒  4.3 What’s inside a router


Network layer❒  transport segment from

sending to receiving host ❒  on sending side

encapsulates segments into datagrams

❒  on receiving side, delivers segments to transport layer

❒  network layer protocols in every host, router

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

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

networkdata linkphysical network

data linkphysical

networkdata linkphysical







networkdata linkphysicalnetwork

data linkphysical

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Two Key Network-Layer Functions

❒  forwarding: move packets from router’s input to appropriate router output

❒  routing: determine route taken by packets from source to destination❍  routing algorithms

analogy:

❒  routing: process of planning trip from source to destination

❒  forwarding: process of getting through single interchange

Network layer: data plane, control plane

Data plane

❒  local, per-router function❒  determines how datagram

arriving on router input port is forwarded to router output port

❒  forwarding function

Control plane

!  network-wide logic! determines how datagram is routed

among routers along end-end path from source host to destination host

!  two control-plane approaches:•  traditional routing algorithms:

implemented in routers•  software-defined networking

(SDN): implemented in (remote) servers

•  not studied in this course

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0111

values in arriving packet header

4-6Network Layer: Data Plane© From Computer Networking, by Kurose&Ross

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Per-router control plane

RoutingAlgorithm

Individual routing algorithm components in each and every router interact in the control plane

dataplane

controlplane

4.1 • OVERVIEW OF NETWORK LAYER 309

tables. In this example, a routing algorithm runs in each and every router and both forwarding and routing functions are contained within a router. As we’ll see in Sec-tions 5.3 and 5.4, the routing algorithm function in one router communicates with the routing algorithm function in other routers to compute the values for its forward-ing table. How is this communication performed? By exchanging routing messages containing routing information according to a routing protocol! We’ll cover routing algorithms and protocols in Sections 5.2 through 5.4.

The distinct and different purposes of the forwarding and routing functions can be further illustrated by considering the hypothetical (and unrealistic, but technically feasible) case of a network in which all forwarding tables are configured directly by human network operators physically present at the routers. In this case, no routing protocols would be required! Of course, the human operators would need to interact with each other to ensure that the forwarding tables were configured in such a way that packets reached their intended destinations. It’s also likely that human configu-ration would be more error-prone and much slower to respond to changes in the net-work topology than a routing protocol. We’re thus fortunate that all networks have both a forwarding and a routing function!

Values in arrivingpacket’s header

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23

Local forwardingtable

header

0100011001111001

1101

3221

output

Control plane

Data plane

Routing algorithm

Figure 4.2 ♦ Routing algorithms determine values in forward tables

M04_KURO4140_07_SE_C04.indd 309 11/02/16 3:14 PM

4-7Network Layer: Data Plane

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© From Computer Networking, by Kurose&Ross

dataplane

controlplane

Logically centralized control planeA distinct (typically remote) controller interacts with local control agents (CAs)

Remote Controller

CA

CA CA CA CA


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0111

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© From Computer Networking, by Kurose&Ross

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Network service modelQ: What service model for “channel” transporting datagrams from sender to receiver?

Example services for individual datagrams:

❒  guaranteed delivery❒  guaranteed delivery

with less than 40 msec delay

Example services for a flow of datagrams:

❒  in-order datagram delivery

❒  guaranteed minimum throughput to flow

❒  restrictions on changes in inter-packet spacing


Network layer service models:

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

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❒  4. 1 Overview of Network layer❍  Data plane❍  Control plane




The Internet Network layer

forwardingtable

Host, router network layer functions (data and control):

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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Chapter 4: Network Layer





IPv4 addressing: introduction❒  IPv4 address: 32-bit

identifier for host, router interface

❒  interface: connection between host/router and physical link❍  routers typically have

multiple interfaces❍  host typically has one or

two interfaces (e.g., wired Ethernet, wireless 802.11)

❒  IP addresses associated with each interface

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

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

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IPv4 addressing: introduction

Q: how are interfaces actually connected?A: we’ll learn about thatin chapter 6 (and in next course)

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: wired Ethernet interfaces connected by Ethernet switches

A: wireless WiFi interfaces connected by WiFi base station

For now: don’t need to worry about how one interface is connected to another (with no intervening router)


Subnets❒  IP address:

❍  subnet part: high order bits

❍  host part: low order bits❒  What’s a subnet?

❍  device interfaces with same subnet part of IP address, and

❍  can physically reach each other without intervening router

network consisting of 3 subnets

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2223.1.3.1

subnet

223.1.1.2

223.1.3.27223.1.2.2

223.1.2.1

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Subnets

Recipe❒  To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks

❒  Each isolated network is called a subnet

In this example, the subnet part is 24 bit-long.Notation: /24

223.1.1.0/24223.1.2.0/24

223.1.3.0/24

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2223.1.3.1

subnet

223.1.1.2

223.1.3.27223.1.2.2

223.1.2.1


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

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Deprecated Classful addresses

Network Layer: Data Plane 4-19

In the past the network portions were constrained to be 8, 16, or 24 bits in length,known as class A, B, and C. It was not very flexible: class C was too small for many organizations, leading to class B rapid depletion. However class B was too large, thus leading to a poor utilization of the assigned address space


IP addressing: CIDRCIDR: Classless InterDomain Routing

❍  subnet portion of address of arbitrary length❍  address format: a.b.c.d/x, where x is # bits in subnet

portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Example with x = 23:

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Special IP addresses


e.g. « Localhost » maps to 127.0.0.1


IP addresses: how to get one?

Q: How does host get IP address?

❒  hard-coded by system admin in a file❍  Wintel: control-panel->network->configuration->tcp/ip->properties❍ UNIX: /etc/rc.config

❒  DHCP: Dynamic Host Configuration Protocol: dynamically get address from a server❍  “plug-and-play”

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DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when it joins network❍  Can renew its lease on address in use❍  Allows reuse of addresses

(only hold address while connected/“on”)❍  Support for mobile users who want to join network

DHCP overview:❍  host broadcasts “DHCP discover” msg [optional]❍  DHCP server responds with “DHCP offer” msg [optional]❍  host requests IP address: “DHCP request” msg❍  DHCP server sends address: “DHCP ack” msg


DHCP client-server scenario

223.1.1.0/24

223.1.2.0/24

223.1.3.0/24

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2223.1.3.1

223.1.1.2

223.1.3.27223.1.2.2

223.1.2.1

DHCPserver

arriving DHCPclient needs address in thisnetwork

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DHCP server: 223.1.2.5 arriving client

DHCP offer

DHCP ACK

DHCP client-server scenarioDHCP discover

Broadcast: is there a DHCP server out there?

Broadcast: I’m a DHCP server! Here’s an IP address you can use

DHCP request

Broadcast: OK. I’ll take that IP address!

Broadcast: OK. You’ve got that IP address!


Maybe several DHCP servers in subnet

Has no IP address

Doesn’t know IP address of DHCP server

DHCP uses UDPWhy?


DHCP client-server scenario (2)

DHCP server: 223.1.2.5 arriving client

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, 68yiaddr: 223.1.2.4transaction ID: 654lifetime: 3600 secs

DHCP requestsrc: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddr: 223.1.2.4transaction ID: 655lifetime: 3600 secs

DHCP ACKsrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddr: 223.1.2.4transaction ID: 655lifetime: 3600 secs

0.0.0.0 = myself

255.255.255.255 = broadcast on this subnet

yiaddr = yielded address

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DHCP: more than IP address

DHCP can return more than just allocated IP address on subnet:❍  address of first-hop router for client❍  name and IP address of default DNS server❍  network mask (indicating network versus host

portion of address)❍ …

DHCP: Wireshark output

Message type: Boot Reply (2) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 192.168.1.101 (192.168.1.101) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 192.168.1.1 (192.168.1.1) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP ACK Option: (t=54,l=4) Server Identifier = 192.168.1.1 Option: (t=1,l=4) Subnet Mask = 255.255.255.0 Option: (t=3,l=4) Router = 192.168.1.1 Option: (6) Domain Name Server Length: 12; Value: 445747E2445749F244574092; IP Address: 68.87.71.226; IP Address: 68.87.73.242; IP Address: 68.87.64.146 Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."

reply

Message type: Boot Request (1) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 0.0.0.0 (0.0.0.0) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 0.0.0.0 (0.0.0.0) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP Request Option: (61) Client identifier Length: 7; Value: 010016D323688A; Hardware type: Ethernet Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=50,l=4) Requested IP Address = 192.168.1.101 Option: (t=12,l=5) Host Name = "nomad" Option: (55) Parameter Request List Length: 11; Value: 010F03062C2E2F1F21F92B 1 = Subnet Mask; 15 = Domain Name 3 = Router; 6 = Domain Name Server 44 = NetBIOS over TCP/IP Name Server ……

request

4-28Network Layer: Data Plane© From Computer Networking, by Kurose&Ross

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!  connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP

router with DHCP server built into router

!  DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.1 Ethernet

!  Ethernet frame broadcast on LAN, received at router running DHCP server

!  Ethernet demuxed to IP demuxed, UDP demuxed to DHCP

168.1.1.1

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP: example


!  DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server

!  encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client

DHCP: example

router with DHCP server built into router

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

!  client now knows its IP address, name and IP address of DNS server, IP address of its first-hop router

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IP addresses: how to get one?Q: How does network get subnet part of IP

address?A: Gets allocated portion of its provider ISP’s

address space, so-called Provider Assigned (PA) addresses

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


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

Organization 0

Organization 7Internet

Organization 1


200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routing information:

Fly-By-Night-ISP200.23.16.0/20

ISPs-R-Us199.31.0.0/16

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Provider Independent (PI) addresses – Multihoming

Alternative: Organization 8 gets a Provider Independent (PI) range of addresses

Organization 8 may have multiple providers (so-called multihoming)

“Send me anythingwith addresses beginning 200.23.16.0/20or 100.56.10.0/23”

100.56.10.0/23

Fly-By-Night-ISP200.23.16.0/20

InternetOrganization 8

ISPs-R-Us199.31.0.0/16





IP addressing: the last word...

Q: How does an ISP get block of addresses? Or how does an organization get a PI address block?

A: ICANN: Internet Corporation for Assigned Names and Numbers, http://www.icann.org/

❍  allocates addresses❍ manages DNS❍  assigns domain names, resolves disputes

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IP forwarding table❒  Assume forwarding is only based on the destination addr❒  There are 232 (i.e., 4 billion) IPv4 addresses❒  Clearly all addresses in the same subnet can be

aggregated into a single forwarding entry in tables❍  Saves space in table❍  Makes address look-up easier too


Destination Address Range (Subnet) 223.1.1.0 /24

223.1.2.0 /24

223.1.3.0 /24

…

Outgoing link interface0

12

…

More aggregation is possible in forwarding table



200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Organization 0


Organization 1


...

...Fly-By-Night-ISP200.23.16.0/20

Here a unique forwarding entry is enough to reach all 8 organizations,namely 200.23.16.0/20

In Fly-By-Night-ISP, a forwarding entry per organization is still needed

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Hierarchical addressing: more specific routes

Assume -  Fly-By-Night-ISP acquires ISPs-R-Us-  Organization 1 now connects through its subsidiary ISPs-R-Us-  Organization 1 keeps its address range (easier management)


200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Organization 0


Organization 1



...

...Fly-By-Night-ISP200.23.16.0/20

ISPs-R-Us199.31.0.0/16

Issue?

IP forwarding table – Overlap❒  In larger Internet, routers will

have overlapping entries in their table

❒  Overlap could be avoided by splitting 200.23.16.0 /20 into 7 blocks of size /23❍  but leads to larger tables


Destination Address Range200.23.16.0 /20

200.23.18.0 /23

199.31.0.0 /16

…

Outgoing link interface0

12

…


Internet


{ overlap

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Longest prefix matching

Destination Address Range 11001000 00010111 00010*** ********

11001000 00010111 00011000 ********

11001000 00010111 00011*** ******** ******** ******** ******** ********

DA: 11001000 00010111 00011000 10101010

examples:DA: 11001000 00010111 00010110 10100001 which interface?

which interface?

when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.

longest prefix matching

Link interface0

12

3default =

{ overlap


Longest prefix match - Another ex.❒  Suppose a network has the address range 139.165.0.0/16❒  It is first split into 139.165.0.0/17 and 139.165.128.0/17❒  Then part of 139.165.128.0/17 (namely 139.165.128.0/20) is

reallocated elsewhere

139.165.128/17

139.165.0/17

139.165.128/20

R1

R2

Forwarding table in R1:139.165.0/17: forward to port 1139.165.128/17: forward to port 2139.165.128/20: forward to port 1

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Destination address 139.165.128.0 matches entries 2 and 3.Longest prefix match gives entry 3!

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Longest prefix matching❒  prefix matching: makes sense if entries with same prefix

are in same area, leads to smaller forwarding tables❍  one entry per subnet, or one per aggregation of subnets sharing

some common prefix❍  granularity: address block size is a power of 2

❒  longest prefix matching: ❍  allows even more aggregation, at the price of a more complex

matching (look-up) procedure❒  performance issue:

❍  matching often performed in hardware using Ternary Content Addressable Memories (TCAMs)

❍  Content addressable: present destination address to TCAM: retrieve action in one clock cycle, regardless of table size

•  Cisco Catalyst: stores up to ~1M forwarding entries in TCAM❍  efficient software implementations use tries







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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 local network have same single

source NAT IP address: 138.76.29.7, different source port numbers

Private IP addresses (RFC 1918): 10.0.0.0/8 = 10/8 172.16.0.0/12 = 172.16/12 192.168.0.0/16 = 192.168/16

Fact: Not enough IPv4 addresses to assign a block to every SOHO (Small Office Home Office)

Solution: NAT


NAT: Network Address Translation

❒  Motivation: local network uses just one public (i.e., routable) IP address as far as outside world is concerned:❍  range of addresses not needed from ISP: just one IP

address for all devices❍  can change private addresses of devices in local

network without notifying outside world❍  can change ISP without changing private addresses of

devices in local network❍  devices inside local net not explicitly addressable, visible

by outside world (a security plus)

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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 address

❍  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

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

NAT: network address translation

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NAT: Network Address Translation❒  possible to restrict incoming traffic even more

❍  e.g. only from contacted outside host, by adding fields in WAN side of table

❒  16-bit port-number field: ❍  60,000 simultaneous connections with a single LAN-side address!

❒  NAT is controversial:❍  routers should only process up to layer 3❍  address shortage should instead be solved by IPv6❍  violates end-to-end argument

•  NAT deployment must be taken into account by app designers, e.g., P2P applications

❒  NAT traversal: what if client wants to connect to server behind NAT?


NAT traversal problem❒  client wants to connect to

server with address 10.0.0.1❍  server address 10.0.0.1 local to

LAN (client can’t use it as destination address)

❍  only one externally visible NATed address: 138.76.29.7

❒  solution 1: statically configure NAT to forward incoming connection requests at given port to server❍  e.g., (138.76.29.7, 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?

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NAT traversal problem❒  solution 2: Universal Plug and

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

(138.76.29.7)o  add/remove port mappings

(with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

NAT router

IGD

138.76.29.7


NAT traversal problem❒  solution 3: relaying (used in Skype)

❍ NATed server establishes connection to relay❍  external client connects to relay❍  relay bridges packets between two connections

138.76.29.7client

1. connection torelay initiatedby NATed host

2. connection torelay initiatedby client

3. relaying established

NAT router

10.0.0.1

26







IPv4 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?

❒  20 bytes of TCP❒  20 bytes of IP❒  = 40 bytes + app

layer overhead

27







IP fragmentation, reassembly❒  network links have MTU (max

transfer unit) - 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 (why?)•  non transparent

❍  note: fragments could also be fragmented again

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

IP header bits needed to identify and reorder related fragments

28


IP fragmentation, reassembly (2)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=1040

One large datagram becomesseveral smaller datagrams

Example❒  4000 byte datagram❒  MTU = 1500 bytes

1480 bytes in data field

offset =1480/8

offset = 0 means it is the first fragmentfragflag = 0 means it is the last fragmentfragflag of last fragment is a copy of fragflag before fragmentation


❒  To avoid fragmentation, the source must know the minimal MTU of the path

❒  Path MTU discovery (trial and error)❍  Send an IP packet with the "Don't fragment flag" set❍  Routers may be forced to discard the packet❍  If the source receives an ICMP error message (see later), it tries

again with a size equal to the MTU indicated in the ICMP packet❒  Drawback

❍  Relies on routers properly returning ICMP error message❍  Also, congestion could discard ICMP messages❍  Also, the route may change afterwards❍  So fragmentation can happen anyway

Avoiding fragmentation

29







IPv6: motivation❒  Initial motivation: 32-bit address space soon to

be completely allocated ❒  Additional motivation:

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

❒ No change in higher and lower protocol layers

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

30


IPv6 datagram formatPriority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow” (but concept of “flow” not well defined)Next header: allows to daisy chain several extension headers and finally identify upper layer protocol for data

data

destination address(128 bits)

source address(128 bits)

payload len next hdr hop limitflow labelpriver

32 bits


Other changes from IPv4

❒  (Header) 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

31


IPv6 addresses❒  128 bits written as x:x:x:x:x:x:x:x

❍  where x is a 16-bit hexadecimal field❍  e.g., 2001:0000:130F:0000:0000:09C0:876A:130B

❒  Leading zeros in a field are optional:❍  2001:0:130F:0:0:9C0:876A:130B

❒  Successive fields of 0 are represented as ::but only once in an address:❍  2001:0:130F::9C0:876A:130B is allowed❍  2001::130F::9C0:876A:130B is not allowed❍  FF01:0:0:0:0:0:0:1 can be written as FF01::1❍  0:0:0:0:0:0:0:1 can be written as ::1 (loopback address)❍  0:0:0:0:0:0:0:0 can be written as :: (this host)


IPv6 addresses

❒  Hierarchical addresses to allow for aggregation❍  Prefix of 64 bits identifies a site (initial part, typically 48 bits, e.g.

given by ISP)❍  Suffix of 64 bits identifies an interface in this site

•  May be assigned in several ways: e.g. DHCPv6, or based on interface “layer 2” address (see chapter 6), or pseudo-random

•  Pseudo-random by default in many operating systems

❒  Link-local addresses❍  Not routable, have a scope limited to a link❍  Suffix derived from 48-bit interface “layer 2” address❍  Prefix is FE80:0:0:0

32


Transition from IPv4 to IPv6

❒ Not all routers can be upgraded simultaneously❍  no “flag days”❍  how will the network operate with mixed IPv4 and IPv6

routers? ❒ One solution is Tunneling: IPv6 carried as payload

in IPv4 datagram among IPv4 routers

IPv4 source, dest addr IPv4 header fields

IPv4 datagramIPv6 datagram

IPv4 payload

UDP/TCP payloadIPv6 source dest addr

IPv6 header fields


Tunneling

physical view:IPv4 IPv4

A B

IPv6 IPv6

E

IPv6 IPv6

FC D

logical view:IPv4 tunnel

connecting IPv6 routers E

IPv6 IPv6

FA B

IPv6 IPv6

33


flow: Xsrc: Adest: F

data

A-to-B:IPv6

Flow: XSrc: ADest: F

data

src:Bdest: E

B-to-C:IPv6 inside

IPv4

E-to-F:IPv6

flow: Xsrc: Adest: F

data

D-to-E:IPv6 inside

IPv4

Flow: XSrc: ADest: F

data

src:Bdest: E

physical view:A B

IPv6 IPv6

E

IPv6 IPv6

FC D

logical view:IPv4 tunnel

connecting IPv6 routers E

IPv6 IPv6

FA B

IPv6 IPv6

Tunneling

IPv4 IPv4

IPv6: adoption

❒  Long (long!) time for deployment, use

•  20 years and counting!

❒  Oct 2018:❍  Google: 20% of clients

access services via IPv6

❍  Belgium’s adoption rate: ± 57% (according to APNIC)


34


Chapter 4: Network Layer: Data Plane

❒  4. 1 Overview of the Network Layer❍  Data plane❍  Control plane




Router architecture overviewHigh-level view of a generic router architecture:

high-speed switching

fabric

routing processor

router input ports router output ports

forwarding data plane (hardware),

operates in nanosecond

timeframe

routing, managementcontrol plane (software)

forwarding tables computed,pushed to input ports

35


link layer

protocol(receive)

lookup,forwarding

queueing

Input port functions

decentralized switching: ❒  using IP header fields values, lookup output

port using forwarding table in input port memory (“match plus action”)

❒  goal: complete input port processing at ‘line speed’(also decrement TTL, update packet count, …)

❒  queuing: if datagrams arrive faster than forwarding rate into switch fabric

physical layer:bit-level reception

data link layer:e.g., Ethernetsee chapter 6

switchfabric

physical layer(bit decoding)


physical layer(bit decoding)

link layer

protocol(receive)

lookup,forwarding

queueing

Input port functions

decentralized switching: ❒  using IP header fields values, lookup output

port using forwarding table in input port memory (“match plus action”)

❒  destination-based forwarding: forward based only on destination IP address (traditional)

❒  generalized forwarding: forward based on any set of header field values

physical layer:bit-level reception

data link layer:e.g., Ethernetsee chapter 6

switchfabric

36


Switching fabrics❒  transfer packet from input buffer to appropriate

output buffer❒  switching rate: rate at which packets can be

transferred from inputs to outputs❍  often measured as multiple of input/output line rate❍  N inputs: switching rate N times line rate desirable

❒  three types of switching fabrics

memory

memory

bus crossbar


Switching Via MemoryFirst generation routers:❒  traditional computers with switching under direct

control of CPU❒  packet copied to system’s memory❒  speed limited by memory bandwidth, and 2 bus

crossings per datagram

inputport(e.g.,

Ethernet)memory

outputport(e.g.,

Ethernet)

system bus

37


Switching Via a Bus

❒  datagram from input port memory to output port memory via a shared bus

❒  bus contention: switching speed limited by bus bandwidth

bus


Switching Via An Interconnection Network

❒  overcome bus bandwidth limitations❍  use multiple buses in //

❒  Banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor architectures

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

crossbar

38


Input port queuing❒  fabric slower than input ports combined -> queueing may occur at input

queues ❍  queueing delay and loss due to input buffer overflow!

❒  Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

output port contention:only one red datagram can be

transferred.lower red packet is blocked

switchfabric

one packet time later: green packet experiences HOL

blocking

switchfabric


Output Ports

❒  Buffering required when datagrams arrive from fabric faster than the transmission rate❍  datagrams can be lost due to congestion, lack of buffers

❒  Fragmentation (if needed)❒  Scheduling discipline chooses among queued datagrams for

transmission❍  E.g. FIFO queuing, priority scheduling, …

physical layer(bit encoding)

link layer

protocol(send)

switchfabric

datagrambuffer

queueing

See chapter 6 for details

39


Output port queueing

❒  buffering when arrival rate via switch exceeds output line speed

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

at t, packets morefrom input to output

one packet time later

switchfabric

switchfabric


Chapter 4: Network Layer Data Plane❒  Data plane vs Control

Plane❒  IP: Internet Protocol

❍  IPv4 addressing•  Subnets, CIDR•  DHCP

❍  IP forwarding•  Longest prefix match

❍  NAT❍  Fragmentation❍  IPv6

❒  Router architecture

Question: how are forwarding tables computed?

Answer: by the control plane (next chapter)

introduction to computer networking guy leduc chapter 4 ... · networking guy leduc chapter 4 ......

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