Chapter 4, slide: 1 Chapter 4: Network Layer r Introduction r IP: Internet Protocol  IPv4 addressing  NAT  IPv6 r Routing algorithms  Link state  Distance Vector r Routing in the Internet  RIP  OSPF  BGP

Sharing an IP address r Home networks, other small LANs r Expensive to have unique IP address for each host r Want to share internet access through just one IP address r Want to maintain security/privacy r Install router … but how does it work? Chapter 4, slide: 2

Network Address Translation r NAT is an extension of the original IP addressing scheme r Motivated by exhaustion of IP address space r Allows multiple computers at one site to share a single global IP address r Requires a device to perform packet translation r In-line configuration  All traffic entering or leaving the network must go through the NAT device  Should be transparent to all users Virtual private connection Chapter 4, slide: 3

NAT: Network Address Translation r local network uses just one IP address as far as outside world is concerned (external address) r range of addresses not needed from ISP: just one IP address for all devices r can change addresses of devices in local network without notifying outside world r can change ISP / external address without changing addresses of devices in local network r devices inside local net not explicitly addressable by outside world (a security plus). Chapter 4, slide: 4

NAT: Network Address Translation local network (e.g., home network) /24 rest of Internet Datagrams with source or destination in this network have /24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: , different source port numbers Chapter 4, slide: 5

Implementation r To send datagram out to the internet from a computer in the private network:  Computer constructs datagram with source address and destination address, sends to NAT box  NAT box translates the source address in the datagram to the site's IP address  NAT keeps source and destination addresses in its translation table  Note: checksum must be recalculated and datagram must be reconstructed Chapter 4, slide: 6

Implementation r To forward an incoming datagram from the internet to a computer in the private network:  Datagrams arrive addressed to the site's IP address  NAT finds source and destination addresses in its translation table  NAT changes the destination address in the datagram to the internal address for the target computer  NAT reconstructs the datagram (with new checksum, etc.) and forwards it to the computer in the private network Chapter 4, slide: 7

Implementation r Software solutions  Standard PC with NAT software, e.g.: –Linux masquerade –Windows RRAS (Routing and Remote Access Server) extra NIC required  OK for slower speed networks (e.g., 10 Mbps)  NAT box must translate addresses in time for the usual network functions to work detecting congestion, etc. r Hardware solutions  Special-purpose hardware for high-speed networks (e.g., gigabit Ethernet) r Hybrid solutions  Routers can incorporate software for NAT  Used in medium-speed networks (e.g., 100 Mbps) Chapter 4, slide: 8

Virtual connection r The effect of NAT is to form a virtual private connection between a computer in a private network and a remote host (internet site). r Of course, the connection may be to a computer in a separate private network (through another NAT box) r Internal communications do not use the NAT box Chapter 4, slide: 9

Problems with basic NAT r If two computers inside the private network both want to communicate with the same external site, the basic translation table is not sufficient r If one computer inside the private network is running applications with two remote hosts, the basic translation table is not sufficient r If a remote site wants to make the first contact with a computer inside the private network, there will be no translation table entry. Chapter 4, slide: 10

NAPT r Network Address and Port Translation  Most popular implementation of NAT  Usually just called NAT  Keeps track of local addresses and IP addresses  Also can keep track of (and change) TCP and UDP protocol port numbers  Allows multiple computers in the private network to communicate with a single destination multiple applications on a single computer in the private network to communicate with multiple destinations Chapter 4, slide: 11

Example NAPT table r Entry in table records protocol port number as well as IP address r Port numbers are re-assigned to avoid conflicts r Note: this requires the NAT box (router) to have some transport-layer functionality DirectionInitial valueTranslatedUnchanged out IP SRC:TCP SRC :30000 IP SRC:TCP SRC :40001 IP DST:TCP DST :80 out IP SRC:TCP SRC :30000 IP SRC:TCP SRC :40002 IP DST:TCP DST :80 in IP DST:TCP DST :40001 IP DST:TCP DST :30000 IP SRC:TCP SRC :80 in IP DST:TCP DST :40002 IP DST:TCP DST :30000 IP SRC:TCP SRC :80 Chapter 4, slide: 12

NAT table r For an out-going datagram:  Source address is changed to the site address.  Source port number is re-assigned and recorded  Checksum is recalculated  Datagram is reconstructed  Destination address / port number are not changed  Translation table records Internal source address / original port number Destination address / re-assigned source port number Chapter 4, slide: 13

NAT table r For an in-coming datagram:  Destination address is changed to the internal address recorded in the translation table.  Destination port number is changed to the port number recorded in the translation table.  Checksum is recalculated  Datagram is reconstructed  Source address / port number are not changed Chapter 4, slide: 14

NAT: Network Address Translation S: , 3345 D: , : host sends datagram to , 80 NAT translation table WAN side addr LAN side addr , , 3345 …… S: , 80 D: , S: , 5001 D: , : NAT router changes datagram source addr from , 3345 to , 5001, updates table S: , 80 D: , : Reply arrives dest. address: , : NAT router changes datagram dest addr from , 5001 to , 3345 Chapter 4, slide: 15

First contact r When initial contact is attempted from outside the site, there is no translation table entry  E.G., a private network might be running multiple servers through a NAT system Chapter 4, slide: 16

NAT traversal problem r client wants to connect to server with address  server address local to LAN (client can’t use it as destination addr)  only one externally visible NAT’ed address: NAT router Client ? Chapter 4, slide: 17

NAT traversal problem Solution 1: statically configure NAT to forward incoming connection requests at given port to server  e.g., ( , port 2500) always forwarded to port NAT router Client ? Chapter 4, slide: 18

NAT traversal problem Solution 2: Universal PnP Internet Gateway Device (IGD) Protocol. Allows NAT’ed host to: r map (private IP, private port #) with (public IP, public port #) r advertise (public IP, public port #)  So DNS can work r add/remove port mappings NAT router IGD Chapter 4, slide: 19

Summary: Network Address Translation r 16-bit port-number field:  ~65,000 simultaneous connections with a single LAN-side address! r NAT is controversial.  Objections include: routers should only process up to layer 3 address shortage should instead be solved by IPv6 Chapter 4, slide: 20

Chapter 4, slide: 21 Chapter 4: Network Layer r Introduction r Virtual circuit and datagram networks r IP: Internet Protocol  IPv4 addressing  NAT  IPv6 r Routing algorithms  Link state  Distance Vector r Routing in the Internet  RIP  OSPF  BGP

Chapter 4, slide: 22 IPv6 r Initial motivation:  32-bit address space soon to be completely allocated. r Additional motivation:  header changes to facilitate QoS r Major changes from IPv4:  Fragmentation: no longer allowed; drop packet if too big  Checksum: removed to reduce processing time; already done at transport and link layers  Options: allowed, but outside of header, indicated by “Next Header” field

New features of IPv6 r Support for audio and video  “flow labels” and “quality of service” allow audio and video applications to establish appropriate connections r Extensible  new features can be added more easily Chapter 4, slide: 23

IPv6 datagram format Chapter 4, slide: 24

IPv6 base header format Chapter 4, slide: 25

IPv6 base header r Contains less information than IPv4 header  VERSION (4 bits)  TRAFFIC CLASS (8 bits) specifies the traffic class (used to choose a route)  FLOW LABEL (20 bits) used to associate datagrams belonging to a flow or communication between two applications  PAYLOAD LENGTH (16 bits) indicates the length of data (i.e. payload) excluding header  NEXT HEADER (8 bits) points to first extension header  HOP LIMIT (8 bits)(old TTL) specifies the maximum number of hops a packet can travel through before being discarded  SOURCE ADDRESS (128 bits)  DESTINATION ADDRESS (128 bits) Chapter 4, slide: 26

NEXT header Chapter 4, slide: 27

Parsing IPv6 headers r Base header is fixed size - 40 octets r NEXT HEADER field in base header defines type of next header  Next header appears at end of fixed-size base header r Some extensions headers are variable sized  NEXT HEADER field in extension header defines type  HEADER LEN field gives size of extension header Chapter 4, slide: 28

Multiple headers r Efficiency  header only as large as necessary r Flexibility  can add new headers for new features r Incremental development  can add processing for new features Chapter 4, slide: 29

Fragmentation and Path MTU r Fragmentation information is in fragmentation extension header r IPv6 source (not intermediate routers) is responsible for fragmentation  Source must find path MTU r Routers simply drop datagrams larger than path MTU  No more fragmenting by routers  ICMP message sent to source r Must be dynamic - path may change during transmission of datagrams r Source determines path MTU  Uses path MTU discovery Source sends probe message of various sizes Gets ICMP messages until destination reached  Constructs datagrams to fit within that MTU Chapter 4, slide: 30

IPv6 addressing r 128-bit addresses r Includes network prefix and host suffix r No address classes  prefix/suffix boundary can fall anywhere  Longest matching prefix Chapter 4, slide: 31

Address notation in IPv6 r 128-bit addresses  unwieldy in dotted decimal  requires 16 numbers  example: r IPv6 uses groups of 16-bit numbers in hex separated by colons  colon hexadecimal (colon hex)  example: 69DC:8864:FFFF:FFFF:0:1280:8C0A:FFFF r Add /bits to specify netmask  example: 69DC:8864:FFFF:FFFF:0:1280:8C0A:FFFF/64 Chapter 4, slide: 32

Address shorthand in IPv6 r Zero-compression  series of zeroes indicated by two colons  example: FF0C:0:0:0:0:0:0:B1 becomes FF0C::B1 r An IPv6 address with 96 leading zeros is interpreted to hold an IPv4 address Chapter 4, slide: 33

Chapter 4, slide: 34 Transition From IPv4 To IPv6 r Can all routers be upgraded simultaneously ??  Answer: it can’t; no “flag days”  Analogy: (IP for Internet) ~ (foundation for House)  To change the foundation, you need to tear down the house!! r Solution gradually incorporate IPv6 (may take few years)  How will the network operate with mixed IPv4 and IPv6 routers?  Tunneling??

Chapter 4, slide: 35 Tunneling A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 IPv4 What is the problem here? D C Why can’t B just send an IPv4 packet to C ? Flow: X Src: A Dest: F data A-to-B: IPv6 Problem: D won’t be able to send an IPv6 packet to E? Why? Be aware that: IPv6 nodes have both IPv4 & IPv6 addresses Nodes know which nodes are IPv4 and which one are IPv6 (use for e.g. DNS)

Chapter 4, slide: 36 Tunneling A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 C D IPv4 Flow: X Src: A Dest: F data A-to-B: IPv6 Flow: X Src: A Dest: F data E-to-F: IPv6 Flow: X Src: A Dest: F data Src:B Dest: E B-to-C: IPv6 inside IPv4 Flow: X Src: A Dest: F data Src:B Dest: E B-to-C: IPv6 inside IPv4 Be aware that: IPv6 nodes have both IPv4 & IPv6 addresses Nodes know which nodes are IPv4 and which one are IPv6 (use for e.g. DNS)