Forwarding and Routing

1
Switching and ForwardingNetwork Layer Part I

• Switching and Forwarding
• Generic Router Architecture
• Forwarding Tables
• Bridges/Layer 2 Switches VLAN
• Routers and Layer 3 Switches
• Forwarding in Layer 3 (Network Layer)
• Network Layer Functions
• Network Service Models VC vs. Datagram
• ATM and IP Datagram Forwarding
• IP Addressing
• Network vs. host address blocks, longest prefix
  matching
• Address allocation and DHCP
• IP Datagram Forwarding Model and ARP Protocol
• IP and ICMP Protocols, IP Fragmentation and
  Re-assembly
• Readings Textbook Chapter 4 Section 4.1  

2
Routing ForwardingLogical View of a Router
3
IP Addressing Basics

• Globally unique (for public IP addresses)
• IP address 32-bit identifier for host, router
  interface
• Interface connection between host/router and
  physical link
• routers typically have multiple interfaces
• host may have multiple interfaces
• IP addresses associated with each interface
• Dot notation (for ease of human reading)

4
IP Addressing Network vs. Host
multi-access LAN

• Two-level hierarchy
• network part (high order bits)
• host part (low order bits)
• Whats a network ?
• (from IP address perspective)
• device interfaces with same network part of IP
  address
• can physically reach each other without
  intervening router

point-to-point link
5
Classful IP Addressing
32 bits

• Disadvantage inefficient use of address space
  address space exhaustion
• e.g., class B net allocated enough addresses for
  65K hosts, even if only 2K hosts in that network

6
Classless Addressing CIDR

• CIDR Classless InterDomain Routing
• Network portion of address is of arbitrary length
• Addresses allocated in contiguous blocks
• Number of addresses assigned always power of 2
• Address format a.b.c.d/x
• x is number of bits in network portion of address

7
Representation of Address Blocks

• Human Readable address format a.b.c.d/x
• x is number of bits in network portion of
  address, the network portion is also called the
  network prefix
• machine representation of a network (addr
  block)
• using a combination of
• first IP of address blocks of the network
• network mask ( x 1s followed by 32-x 0s

network w/ address block 200.23.16.0/23
first IP address of address block
11001000 00010111 00010000 00000000
network mask
11111111 11111111 11111110 00000000
8
More Examples
Three Address Blocks
First IP
address 11001000 00010111 00010000
00000000 Network mask

11111111 11111111 11111000 00000000
First IP address 11001000
00010111 00011000 00000000 Last IP
address
11001000 00010111 00011000
11111111 what is the network prefix?
11001000 00010111 00011000 First IP
address 11001000 00010111 00011001
00000000 Last IP address

11001000 00010111 00011111 11111111
what is the network prefix?
11001000 00010111 00011


Use longest prefix matching!
9
Another Example

• Consider a datagram network using 32-bit host
  addresses, suppose a router has four links,
  numbered 0 through 3, and packets are to be
  forwarded to the link interfaces as
  followsDestination Addr Range Link
  Interface11100000 00000000 00000000 00000000
  through
  011100000
  11111111 11111111 1111111111100001 00000000
  00000000 00000000 through
  
  111100001 00000000 11111111 1111111111100001
  00000001 00000000 00000000
  through
  211100001 11111111 11111111
  11111111O.W.
  3Provide
  the forwarding table a table containing the
  network prefix and the outgoing interface.

10
IP Addresses How to Get One?

• Q How does host get IP address?
• static assigned i.e., hard-coded in a file
• Wintel control-panel-gtnetwork-gtconfiguration-gttcp
  /ip-gtproperties
• UNIX /etc/rc.config
• Dynamically assigned using DHCP (Dynamic Host
  Configuration Protocol)
• dynamically get address from a server
• plug-and-play

11
DHCP Dynamic Host Configuration Protocol

• Goal allow host to dynamically obtain its IP
  address from network DHCP server when it joins
  network
• Can renew its lease on address in use
• Allows reuse of addresses (only hold address
  while connected as 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

12
DHCP Client-Server Scenario

13
DHCP Client-Server Scenario
14
IP Addresses How to Get One?

• Q How does a network get network part of IP
  addr?
• A gets an 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
15
IP Addressing the Last Word...

• Q How does an ISP get block of addresses?
• A ICANN Internet Corporation for Assigned Names
  and Numbers
• allocates addresses
• manages DNS
• assigns domain names, resolves disputes

16
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
10.0.0.0/8 has been reserved for private networks!
17
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).

18
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

19
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
20
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
• 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

21
IP Forwarding IP/ICMP Protocol
Network layer
22
IP Service Model and Datagram Forwarding

• Connectionless (datagram-based)
• Each datagram carries source and destination
• Best-effort delivery (unreliable service)
• packets may be lost
• packets can be delivered out of order
• duplicate copies of a packet may be delivered
• packets can be delayed for a long time
• Forwarding and IP address
• forwarding based on network id
• Delivers packet to the appropriate network
• Once on destination network, direct delivery
  using host id
• IP destination-based next-hop forwarding paradigm
• Each host/router has IP forwarding table
• Entries like ltnetwork prefix, next-hop, output
  interfacegt

23
IP Datagram Format
24
IP Datagram Forwarding Model

• IP datagram

• datagram remains unchanged, as it travels source
  to destination
• addr fields of interest here

25
IP Forwarding Table
4 billion possible entries! (in reality, far
less, but can still have millions of routes)
forwarding table entry format
destination
network
next-hop (IP address) link interface
(1st IP address , network mask )
11001000 00010111
00010000 00000000, 200.23.16.1
0 11111111 11111111
11111000 00000000 11001000 00010111 00011000
00000000, - (direct)
1 11111111 11111111 11111111
00000000 11001000 00010111 00011001
00000000, 200.23.25.6
2
11111111 11111111 11111000 00000000
otherwise
128.30.0.1
3


26
Forwarding Table Lookupusing Longest Prefix
Matching
Prefix Match
Next Hop Link Interface
11001000 00010111 00010
200.23.16.1 0 11001000
00010111 00011000 -
1 11001000 00010111
00011 200.23.25.6
2 otherwise
128.30.0.1
3
Examples
Which interface?
DA 11001000 00010111 00010110 10100001
Which interface?
DA 11001000 00010111 00011000 10101010
27
IP Forwarding Destination in Same Net
misc fields
data
223.1.1.1
223.1.1.3

• Starting at A, send IP datagram addressed to B
• look up net. address of B in forwarding table
• find B is on same net. as A
• link layer will send datagram directly to B
  inside link-layer frame
• B and A are directly connected

28
IP Datagram Forwarding on Same LANInteraction
of IP and data link layers

• Starting at A, given IP datagram addressed to B
• look up net. address of B, find B on same net. as
  A
• link layer send datagram to B inside link-layer
  frame

29
MAC (Physical) Addresses -- Revisited

• used to get frames from one interface to another
  physically-connected interface (same physical
  network, i.e., p2p or LAN)
• 48 bit MAC address (for most LANs)
• fixed for each adaptor, burned in the adapter
  ROM
• MAC address allocation administered by IEEE
• 1st bit 0 unicast, 1 multicast.
• all 1s broadcast
• MAC flat address -gt portability
• can move LAN card from one LAN to another
• MAC addressing operations on a LAN
• each adaptor on the LAN sees all frames
• accept a frame if dest. MAC address matches its
  own MAC address
• accept all broadcast (MAC all1s) frames
• accept all frames if set in promiscuous mode
• can configure to accept certain multicast
  addresses (first bit 1)

30
MAC vs. IP Addresses

• 32-bit IP address
• network-layer address, logical
• i.e., not bound to any physical device, can be
  re-assigned
• IP hierarchical address NOT portable
• depends on IP network to which an interface is
  attached
• when move to another IP network, IP address
  re-assigned
• used to get IP packets to destination IP network
• Recall how IP datagram forwarding is performed
• IP network is virtual, actually packet delivery
  done by the underlying physical networks
• from source host to destination host, hop-by-hop
  via IP routers
• over each link, different link layer protocol
  used, with its own frame headers, and source and
  destination MAC addresses
• Underlying physical networks do not understand IP
  protocol and datagram format!

31
ARP Address Resolution Protocol

• Each IP node (host, router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address timergt
• timer time after which address mapping will be
  forgotten (typically 15 min)

32
ARP Protocol

• B receives ARP packet, replies to A with its
  (B's) MAC address
• frame sent to As MAC address (unicast)
• A caches (saves) IP-to-MAC address pair in its
  ARP table until information becomes old (times
  out)
• soft state information that times out (goes
  away) unless refreshed
• ARP is plug-and-play
• nodes create their ARP tables without
  intervention from net administrator

• A wants to send datagram to B, and A knows Bs IP
  address.
• A looks up Bs MAC address in its ARP table
• Suppose Bs MAC address is not in As ARP table.
• A broadcasts (why?) ARP query packet, containing
  B's IP address
• all machines on LAN receive ARP query

33
ARP Messages
Hardware Address Type e.g., Ethernet Protocol
address Type e.g., IP Operation ARP request or
ARP response
34
ARP Request Response Processing

• The requester broadcasts ARP request
• The target node unicasts (why?) ARP reply to
  requester
• With its physical address
• Adds the requester into its ARP table (why?)
• On receiving the response, requester
• updates its table, sets timer
• Other nodes upon receiving the ARP request
• Refresh the requester entry if already there
• No action otherwise (why?)
• Some questions to think about
• Shall requester buffer IP datagram while
  performing ARP?
• What shall requester do if never receive any ARP
  response?

35
ARP Operation Illustration
36
IP Forwarding Destination in Diff. Net

• Starting at A, dest. E
• look up network address of E in forwarding table
• E on different network
• A, E not directly attached
• routing table next hop router to E is 223.1.1.4
• link layer sends datagram to router 223.1.1.4
  inside link-layer frame
• datagram arrives at 223.1.1.4
• continued..

37
IP Forwarding Destination in Diff. Net

• Arriving at 223.1.4, destined for 223.1.2.2
• look up network address of E in routers
  forwarding table
• E on same network as routers interface 223.1.2.9
  
• router, E directly attached
• link layer sends datagram to 223.1.2.2 inside
  link-layer frame via interface 223.1.2.9
• datagram arrives at 223.1.2.2!!! (hooray!)

38
Forwarding to Another LANInteraction of IP and
Data Link Layer

• walkthrough send datagram from A to B via R
• assume A knows B IP address
• Two ARP tables in router R, one for each IP
  network (LAN)
• In routing table at source host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
39
B
A
R

• A creates datagram with source A, destination B
• A uses ARP to get Rs MAC address for
  111.111.111.110
• A creates link-layer frame with R's MAC address
  as dest, frame contains A-to-B IP datagram
• As data link layer sends frame
• Rs data link layer receives frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs physical layer address
• R creates frame containing A-to-B IP datagram
  sends to B

40
IP Datagram Format Again
41
Fields in IP Datagram

• IP protocol version current version is 4, IPv4,
  new IPv6
• Header length number of 32-bit words in the
  header
• Type of Service
• 3-bit priority,e.g, delay, throughput,
  reliability bits,
• Total length including header (maximum 65535
  bytes)
• Identification all fragments of a packet have
  same identification
• Flags dont fragment, more fragments
• Fragment offset where in the original packet
  (count in 8 byte units)
• Time to live maximum life time of a packet
• Protocol Type e.g., ICMP, TCP, UDP etc
• IP Option non-default processing, e.g., IP
  source routing option, etc.

42
IP Fragmentation Reassembly Why

• network links have MTU (maximum transmission
  unit) - largest possible data gram.
• 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
43
IP Fragmentation Reassembly How

• An IP datagram is chopped by a router into
  smaller pieces if
• datagram size is greater than network MTU
• Dont fragment option is not set
• Each datagram has unique datagram identification
• Generated by source hosts
• All fragments of a packet carry original datagram
  id
• All fragments except the last have more flag set
• Fragment offset and Length fields are modified
  appropriately
• Fragments of IP packet can be further fragmented
  by other routers along the way to destination !
• Reassembly only done at destination host (why?)
• Use IP datagram id, fragment offset, fragment
  flags. Length

44
IP Fragmentation and Reassembly Exp

• Example
• 4000 byte datagram
• MTU 1500 bytes

45
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
46
ICMP Message Transport Usage

• ICMP messages carried in IP datagrams
• Treated like any other datagrams
• But no error message sent if ICMP message causes
  error
• Message sent to the source
• 8 bytes of the original header included
• ICMP Usage (non-error, informational) Examples
• Testing reachability ICMP echo request/reply
• ping
• Tracing route to a destination Time-to-live
  field
• traceroute
• Path MTU discovery
• Dont fragment bit
• IP direct (for hosts only) inform hosts of
  better routes