Goals:

1
Network Layer

• Goals
• understand principles behind network layer
  services
• routing (path selection)
• dealing with scale
• how a router works
• advanced topics IPv6, mobility
• instantiation and implementation in the Internet

2
Topics

• Datagram vs Virtual Circuit
• Router
• IP Internet Protocol
• Datagram format, IPv4 addressing
• ICMP
• IPv6
• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP, OSPF, BGP

3
Network layer

• transport segment from sending to receiving host
• on sending side encapsulates segments into
  datagrams
• on rcving side, delivers segments to transport
  layer
• network layer protocols in every host, router
• Router examines header fields in all IP datagrams
  passing through it

4
Key Network-Layer Functions

• analogy
• routing process of planning trip from source to
  dest
• forwarding process of getting through single
  interchange

• forwarding move packets from routers input to
  appropriate router output
• routing determine route taken by packets from
  source to dest.
• Routing algorithms

5
Interplay between routing and forwarding
6
Connection setup

• 3rd important function in some network arch.
• Virtual circuits network provides network-layer
  conn service
• used in ATM, frame-relay, X.25
• Signaling protocols used to setup, maintain
  teardown VC

6. Receive data
5. Data flow begins
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
7
VC implementation

• A VC consists of
• Path from source to destination
• VC numbers, one number for each link along path
• Entries in forwarding tables in routers along
  path
• Packet belonging to VC carries a VC number.
• VC number must be changed on each link.
• New VC number comes from forwarding table

8
Forwarding table in VC
Forwarding table in northwest router
Routers maintain connection state
information! Forwarding table is modified
whenever theres conn setup or teardown (happen
at a microsecond timescale in a tier-1 router)
9
Network service model
Q What service model for channel transporting
datagrams from sender to rcvr? a service model
defines the characteristics of end-to-end
transport of packets between

• Example services for individual datagrams
• guaranteed delivery
• Guaranteed delivery with less than certain delay
  (e.g. 40 msec)?

• Example services for a flow of datagrams
• In-order datagram delivery
• Guaranteed minimum bandwidth to flow
• Restrictions on changes in inter-packet spacing

10
Case study ATM ABR congestion control

• two-byte ER (Explicit Rate) field in RM cell
• congested switch may lower ER value in cell
• sender send rate thus minimum supportable rate
  on path across all switches
• EFCI (Explicit Forward Congestion Indication) bit
  in data cells set to 1 in congested switch to
  indicate congestion to destination host.
• when RM arrives at destination, if most recently
  received data cell has EFCI1, sender sets CI bit
  in returned RM cell

11
Network layer service models
Guarantees ?
Network Architecture Internet ATM ATM ATM ATM
Service Model best effort CBR VBR ABR UBR
Congestion feedback no (inferred via
loss) no congestion no congestion yes no
Bandwidth none constant rate guaranteed rate gua
ranteed minimum none
Loss no yes yes no no
Order no yes yes yes yes
Timing no yes yes no no
CBR constant bit rate VBR variable bit
rate ABR available bit rate UBR unspecified bit
rate
12
Datagram or VC network why?

• Internet
• data exchange among computers
• elastic service, no strict timing req.
• smart end systems
• can adapt, perform control, error recovery
• simple inside network, complexity at edge
• Additional func built in higher levels
• many link types
• different characteristics
• uniform service difficult

• VC network (e.g. ATM)
• evolved from telephony
• human conversation
• strict timing, reliability requirements
• need for guaranteed service
• dumb end systems
• telephones
• complexity inside network (e.g. network-assisted
  congestion control)

13
Topics

• Router
• IP Internet Protocol
• Datagram format, IPv4 addressing
• ICMP
• IPv6
• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP, OSPF, BGP

14
Router Architecture Overview

• Two key router functions
• run routing algorithms/protocol (RIP, OSPF, BGP)
• forwarding datagrams from incoming to outgoing
  link
• E.g. Cisco 12K, Juniper M16, Foundry SuperX

15
Input Port Functions
Physical layer bit-level reception

• Decentralized switching
• given datagram dest., lookup output port using
  forwarding table in input port memory
• goal complete input port processing at line
  speed
• queuing if datagrams arrive faster than
  forwarding rate into switch fabric

Data link layer e.g., Ethernet see chapter 5
16
Three types of switching fabrics
17
Switching Via Memory

• First generation routers
• traditional computers with switching under
  direct control of CPU
• packet copied to systems memory
• speed limited by memory bandwidth (2 bus
  crossings per datagram)

Recent development Processors in input line
cards perform lookup and storing packets into
memory ? shared mem multiprocessors E.g.
Ciscos Catalyst 8500
18
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
• 1 Gbps bus, Cisco 1900 sufficient speed for
  access and enterprise routers (not regional or
  backbone)
• E.g. 1Gbps bw supports up to 10 T3 (45- Mbps)
  links

19
Switching Via An Interconnection Network

• overcome bus bandwidth limitations
• A crossbar switch is an interconnection network
  consisting of 2n buses that connect n input to n
  output ports.
• Advanced design fragmenting datagram into fixed
  length cells at the input port, switch cells
  through the fabric and assemble at output ports.
• Cisco 12000 switches 60 Gbps through the
  interconnection network

Omega
20
Output Ports

• Buffering required when datagrams arrive from
  fabric faster than the transmission rate
• Queueing and Buffer management
• Scheduling discipline chooses among queued
  datagrams for transmission

21
Output port queueing

• buffering when arrival rate via switch exceeds
  output line speed (switching fabric speed rate
  of moving pkt from in-ports to out-ports)
• queueing (delay) and loss due to output port
  buffer overflow!
• Buffer size RTT times Link Capacity
• A packet scheduler at output port must choose
  among queued to transmit using FIFO or more
  sophisticated such as weighted fair queuing (WFQ)
  that shares the outgoing link fairly among
  different end-to-end connections.

22
Input Port Queuing

• If fabric slower than input ports combined then
  queueing may occur at input queues. It can be
  eliminated if the switching fabric speed is at
  least n times as fast as the input line speed,
  where n is the number of input ports
• Head-of-the-Line (HOL) blocking queued datagram
  at front of queue prevents others in queue from
  moving forward. Only occurs at input ports. As
  soon as the packet arrival rate on the input
  lines reaches 58 of their capacity, the input
  queue will grow to unbounded length, due to HOL
  blocking
• queueing delay and loss due to input buffer
  overflow!

23
Active Queue Management

• Drop-Tail policy
• Drop arrival packets due to overflow
• Random Early Detection (RED)
• Maintain a weighted average for the length of the
  output queue
• If queue length lt Threshold_min, admit it
• If queue length gt Threshold_max, drop it
• Otherwise, drop it with a probability (a function
  of the average queue length)
• RED drops packets before the buffer is full in
  order to provide congestion signals to senders

24
Router Processor

• Execute routing protocols
• Maintain the routing information and forwarding
  tables
• Perform network management functions

CISCO 12000 Gigabit Router Processor (GRP)
25
Forwarding table
packets forwarded using destination host
address The tables are modified by routing alg
anytime (every 15 minutes) packets between same
source-dest pair may take diff paths
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
26
Longest prefix matching
Forwarding table with 4 entries and using longest
prefix match
Prefix Match
Link Interface
11001000 00010111 00010
0 11001000 00010111 00011000
1 11001000 00010111 00011
2
otherwise
3
Examples
Which interface?
DA 11001000 00010111 00010110 10100001
DA 11001000 00010111 00011000 10101010
27
Lookup in an IP Router

• Unicast destination address based lookup

Need to be as fast as line speed!! e.g OC48
link runs at 2.5Gbps, packet256bytes ? 1
million lookups/s Low storage 100K
entries Fast updates few thousands per second,
but ideally at lookup speed
28
Route Lookup Using CAM

• Content-Address Memory Fully associative mem
  Cisco 8500
• Exact match (fixed-length) search op in a single
  clock cycle

To find the longest prefix cheaply, need to keep
entries sorted in order of decreasing prefix
lengths K. pagiamtzis, Intro to CAM
pagiamtzis.com/cam/camintro.html
29
Topics

• Router
• IP Internet Protocol
• Datagram format, IPv4 addressing
• ICMP
• IPv6
• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP, OSPF, BGP

30
The Internet Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
Link layer
physical layer
31
IP datagram format

• how much overhead with TCP?
• 20 bytes of TCP
• 20 bytes of IP
• 40 bytes app layer overhead

32
IP Fragmentation Reassembly

• network links have MTU (max.transfer size) -
  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
• IP header bits used to identify, order related
  fragments

fragmentation in one large datagram out 3
smaller datagrams
reassembly
33
IP Fragmentation and Reassembly

• Example
• 4000 byte IP datagram
• MTU 1500 bytes
• (4000-20 bytes header)3980 bytes of data to be
  fragmented
• 3 fragments (1480148010203980)
• amount of data in all but last fragment must be
  multiples of 8

offset 1480/8
1480 bytes in data field
34
IP Addressing introduction
223.1.1.1

• IP address 32-bit identifier for host, router
  interface, in dotted-decimal notation
• interface connection between host/router and
  physical link
• routers typically have multiple interfaces
• host typically has one interface
• IP addresses associated with each interface

223.1.2.9
223.1.1.4
223.1.1.3
223.1.1.1 11011111 00000001 00000001 00000001
223
1
1
1
35
Subnets (aka IP networks)
223.1.1.1

• IP address
• subnet part (high order bits)
• host part (low order bits)
• Whats a subnet ?
• device interfaces with same subnet part of IP
  address
• can physically reach each other without
  intervening router

223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
223.1.1.3
223.1.3.27
subnet
223.1.3.2
223.1.3.1
network consisting of 3 subnets
To determine the subnets, detach each interface
from its host or router, creating islands of
isolated networks. Each isolated network is
called a subnet.
36
Subnets
223.1.1.2

• How many?

223.1.1.1
223.1.1.4
223.1.1.3
223.1.7.0
223.1.9.2
223.1.9.1
223.1.7.1
223.1.8.0
223.1.8.1
223.1.2.6
223.1.3.27
223.1.2.1
223.1.2.2
223.1.3.2
223.1.3.1
37
IP addressing CIDR

• CIDR 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.
• Notation /x is subnet mask. The high order x bits
  are the network prefix.
• Before CIDR, classful addressing A (/8), B(/16),
  C(/24). Replaced by CIDRized address.

38
IP addresses how to get one?

• Q How does host get IP address?
• hard-coded by system admin in a file
• Wintel control-panel-gtnetwork-gtconfiguration-gttcp
  /ip-gtproperties
• UNIX /etc/rc.config
• DHCP Dynamic Host Configuration Protocol
  dynamically get address from as server
• plug-and-play

39
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP
router) records the subnet from which the message
was received in the DHCP message header for use
by the DHCP server.
40
IP addresses how to get one?

• Q How does network get subnet part of IP addr?
• A gets allocated portion of its provider ISPs
  address space

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
41
Hierarchical addressing route aggregation
Hierarchical addressing allows efficient
advertisement of routing information.
Organization 0
Organization 1
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16
ISPs-R-Us
Two example businesses
42
Hierarchical addressing more specific routes
Assume ISPs-R-Us has been acquired by FBN-ISP and
Org1 be transferred to ISPs-R-Us
Organization 0
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16 or 200.23.18.0/23
ISPs-R-Us
Organization 1
43
IP addressing the last word...

• Q How does an ISP get a block of addresses?
• A ICANN Internet Corporation for Assigned
• Names and Numbers www.icann.org
• allocates address space
• Top-level domain name system management
• manages DNS root servers
• Protocol identifier assignment
• assigns domain names, resolves disputes

44
NAT Network Address Translation
rest of Internet
local network (e.g., home network) 10.0.0/24
10.0.0.1
10.0.0.4
10.0.0.2
138.76.29.7
10.0.0.3
Datagrams with source or destination in this
network have 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, differe
nt source port numbers
45
NAT Network Address Translation

• Motivation local network uses just one IP
  address as far as outside world 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).

46
NAT Network Address Translation

• 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

47
NAT Network Address Translation
NAT translation table WAN side addr LAN
side addr
138.76.29.7, 5001 10.0.0.1, 3345

10.0.0.1
10.0.0.4
10.0.0.2
138.76.29.7
10.0.0.3
4 NAT router changes datagram dest addr
from 138.76.29.7, 5001 to 10.0.0.1, 3345
3 Reply arrives dest. address 138.76.29.7,
5001
48
NAT Network Address Translation

• 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 but NAT
  router need to change the transport port.
• 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

49
Skype through NAT

• NAT prevents a connection from being initiated
  from outside.
• How can Alice call Bob, both residing behind NAT
  (NAT traversal) ??
• Alice sign-in with its super-peer (Sa)
• Bob sign-in with its super-peer (Sb)
• Alice calls Bob Alice ? Sa?Sb?Bob
• If Bob takes the call, Sa and Sb select a non-NAT
  super-peer for voice relay
• See chapter 2 (4th ed) for details

50
Recap Internet Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
Link layer
physical layer
51
ICMP Internet Control Message Protocol

• used by hosts routers 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, different fields
  depending on the type/code. If its a reply type
  then it would have IP Header and first 8 bytes of
  IP datagram data.

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
52
Recap Real Internet delays and routes

• What do real Internet delay loss look like?
• Traceroute program (in Unix) or Tracert (MS-DOS)
  provides delay measurement from source to router
  along end-end Internet path towards destination.
  For all i
• sends three packets that will reach router i on
  path towards destination
• router i will return packets to sender
• sender times interval between transmission and
  reply.

3 probes
3 probes
3 probes
53
Traceroute and ICMP

• Source sends series of UDP segments to dest
• First has TTL 1
• Second has TTL2, etc.
• Unlikely port number (depends on implementation)
• When nth datagram arrives to nth router
• Router discards datagram
• And sends to source an ICMP message (type 11,
  code 0 which means TTL expired)
• Message includes name of router IP address

• When ICMP message arrives, source calculates RTT
• Traceroute does this 3 times
• Stopping criterion
• UDP segment eventually arrives at destination
  host
• Destination returns ICMP host unreachable
  packet (type 3, code 3) if port is sent. When
  source gets this ICMP, stops.
• Other Tracert implementation stops when ping
  reply is received from destination.

54
IPv6

• Initial motivation 32-bit address space soon to
  be completely allocated.
• Expanded addressing capabilities 128 bit
• Additional motivation
• header format helps speed processing/forwarding
• fixed-length 40 byte header
• no fragmentation allowed
• header changes to facilitate QoS
• Flow label and priority

These are also three most important changes
55
IPv6 Header (Cont)
Priority (8-bits) identify priority among
datagrams in flow Flow Label (20-bits) identify
datagrams in same flow. (concept offlow not
well defined). Next header (8-bits) identify
upper layer protocol for data (similar to
Upper-layer protocol in IPv4)
Traffic Class is similar to Type Of Service in
IPv4
Similar to TTL in IPv4 (8-bits)
56
Other Changes from IPv4

• 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

57
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 solutions
• Dual-stack approach IPv6 to IPv4 and vice versa
  translation of datagrams at routers that can
  understand IPv6 and IPv4. Some fields data will
  be lost.
• Tunneling IPv6 carried as payload in IPv4
  datagram among IPv4 routers

58
Tunneling
tunnel
Logical view
IPv6
IPv6
IPv6
IPv6
Physical view
IPv6
IPv6
IPv6
IPv6
IPv4
IPv4
A-to-B IPv6
E-to-F IPv6
B-to-C IPv6 inside IPv4
D-to-E IPv6 inside IPv4
59
Summary

• Network Layer Services
• forwarding, routing and connection setup in some
  networks
• Best effort network Service Models
• Router Architecture Overview
• Input/Output ports and queuing
• Switching via Memory/Bus/Interconnected network
• Network Layer Functions
• forwarding via routing protocols, routing via IP
  error reporting via ICMP
• IP Datagram Format
• IP Fragmentation and Reassembly
• IP Addressing subnets, CIDR, assignments,
  Hierarchical addressing
• Network Address Translation (NAT)
• Internet Control Message Protocol (ICMP) usage
• IPv6 motivation, datagram format and transition
  to IPv4 through Tunneling