1 Addressing

2 Outline  Network addressing  Network packet format and forwarding  NAT, DHCP, IPv6

3 IP Addressing: introduction  IP address: 32-bit identifier for host, router interface  interface: connection between host/router and physical link  router’s typically have multiple interfaces  host may have multiple interfaces  IP addresses associated with each interface =

4 IP Addressing  IP address:  network part (high order bits)  host part (low order bits)  What’s a network ? ( from IP address perspective)  device interfaces with same network part of IP address  can physically reach each other without intervening router network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) LAN

5 IP Addressing How to find the networks?  Detach each interface from router, host  create “islands of isolated networks Interconnected system consisting of six networks How many isolated networks?

6 IP address classes Network IDHost ID 816 Class A 32 0 Class B 10 Class C 110 Multicast Addresses Class D 1110 Reserved for experiments Class E Network ID Host ID to to to to

7 Special IP Addresses  127.*.*.*: local host (a.k.a. the loopback address)  Private addresses   Class A: (10/8 prefix)  Class B: (172.16/12 prefix)  Class C: ( /16 prefix)   IP broadcast to local hardware that must not be forwarded   Same as network broadcast if no subnetting  IP of network broadcast=NetworkID+(all 1’s for HostID)   IP address of unassigned host (BOOTP, ARP, DHCP)  Default route advertisement

8 IP Addressing Problem #1 (1984)  Subnet addressing   Address problem of inefficient use of address space  For class B & C networks  Class A (rarely given out, not many of them given out by IANA)  Class B = 64k hosts oVery few LANs have close to 64K hosts oElectrical/LAN limitations, performance or administrative reasons oe.g., class B net allocated enough addresses for 64K hosts, even if only 2K hosts in that network  Class C = 256 hosts  Need simple way to get multiple “networks”  Use bridging, multiple IP networks or split up single network address ranges (subnet)  Reduce the total number of addresses that are assigned, but not used

9 Subnetting  Variable length subnet masks  Subnet a class B address space into several chunks Network Host Network HostSubnet Mask

10 Subnetting Example  Assume an organization was assigned address  Assume < 100 hosts per subnet  How many host bits do we need?  Seven  What is the network mask?  

11 IP Address Problem #2 (1991)  Address space depletion  In danger of running out of classes A and B  Class A  Very few in number  IANA frugal in giving them out  Class B  Subnetting only applied to new allocations  sparsely populated  people refuse to give it back  Class C too small for most domains  APNIC has only class C addresses

12 Some Solution  Solution  Assign multiple consecutive class C address blocks  Supernetting   Later known as Classless Inter-Domain Routing (CIDR)  

13 IP addressing: CIDR  Classful addressing:  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  CIDR: Classless InterDomain Routing  network portion of address of arbitrary length  address format: a.b.c.d/x, where x is # bits in network portion of address network part host part /23

14 IP addresses: how to get one? Q: How does host get IP address?  hard-coded by system admin in a file  Windows: control-panel->network->configuration->tcp/ip- >properties  UNIX: /etc/rc.config  Linux (Redhat): /etc/sysconfig/network-script/ifcfg-eth  DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server  “plug-and-play” (more shortly)

15 IP addresses: how to get one? Q: How does network get network part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block /20 Organization /23 Organization /23 Organization /23... ….. …. …. Organization /23

16 Hierarchical addressing: route aggregation “Send me anything with addresses beginning /20” / / /23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning /16” /23 Organization Hierarchical addressing allows efficient advertisement of routing information:

17 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 “Send me anything with addresses beginning /20” / / /23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning /16 or /23” /23 Organization

18 Where to obtain IP address 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

19 Outline  Overview of Network Layer  Network addressing  Network packet format and forwarding  NAT, DHCP, IPv6

20 Getting a datagram from source to dest. IP datagram: A B E misc fields source IP addr dest IP addr data  datagram remains unchanged, as it travels source to destination  addr fields of interest here Dest. Net. next router Nhops forwarding table in A

21 Getting a datagram from source to dest. 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 Dest. Net. next router Nhops misc fields data A B E forwarding table in A

22 Getting a datagram from source to dest. Dest. Net. next router Nhops 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  link layer sends datagram to router inside link-layer frame  datagram arrives at  continued….. misc fields data A B E forwarding table in A

23 Getting a datagram from source to dest. Arriving at , destined for  look up network address of E in router’s forwarding table  E on same network as router’s interface  router, E directly attached  link layer sends datagram to inside link-layer frame via interface  datagram arrives at !!! (hooray!) misc fields data Dest. Net router Nhops interface A B E forwarding table in router

24 IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier header checksum time to live 32 bit source IP address IP protocol version number header length (bytes) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, specify list of routers to visit. how much overhead with TCP?  20 bytes of TCP  20 bytes of IP  = 40 bytes + app layer overhead

25 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

26 IP Fragmentation and Reassembly ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =1480 fragflag =1 length =1500 ID =x offset =2960 fragflag =0 length =1040 One large datagram becomes several smaller datagrams Example  4000 byte datagram  MTU = 1500 bytes

27 Fragmentation is Harmful  Uses resources poorly  Forwarding costs per packet  Best if we can send large chunks of data  Worst case: packet just bigger than MTU  Poor end-to-end performance  Loss of a fragment  Reassembly is hard  Buffering constraints

28 Avoid fragmentation  Path MTU Discovery in IP   Hosts dynamically discover minimum MTU of path  Algorithm:  Initialize MTU to MTU for first hop  Send datagrams with Don’t Fragment bit set  If ICMP “pkt too big” msg, decrease MTU  What happens if path changes?  Periodically (>5mins, or >1min after previous increase), increase MTU  Some routers will return proper MTU  MTU values cached in routing table

29 ICMP: Internet Control Message Protocol  used by hosts, routers, gateways to communication 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

30 Traceroute  Based on ICMP : provides delay measurement from source to router along end-end Internet path towards destination.  sends three packets that will reach router i on path towards destination  Set the TTL incrementally from 1 up to 30  router i will return ICMP warning (type 11) back.  sender times interval between transmission and reply. TTL=1 TTL=2 TTL=3

31 Outline  Network addressing  Network packet format and forwarding  NAT, DHCP, IPv6

32 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

33 NAT: Network Address Translation  Motivation: local network uses just one IP address as far as outside word 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).

34 NAT: Network Address Translation Implementation A 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

35 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

36 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 an “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

37 DHCP client-server scenario A B E DHCP server arriving DHCP client needs address in this network

38 DHCP client-server scenario DHCP server: arriving client time DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 DHCP offer src: , 67 dest: , 68 yiaddrr: transaction ID: 654 Lifetime: 3600 secs DHCP request src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 Lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 Lifetime: 3600 secs

39 IPv6  Initial motivation: 32-bit address space completely allocated by  Additional motivation:  header format helps speed processing/forwarding  header changes to facilitate QoS  new “anycast” address: route to “best” of several replicated servers  IPv6 datagram format:  fixed-length 40 byte header  no fragmentation allowed

40 IPv6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). Next header: identify upper layer protocol for data

41 ver total length data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum time to live 32 bit source IP address head. len type of service flgs fragment offset protocol 32 bit destination IP address Options (if any) IPv4 vs. IPv6

42 Differences Between Ipv4 and IPv6  Address length: 32 vs. 128  Fragmentation: IPv6 has no fragmentation  Type of service (TOS): IPv6 has no TOS  Checksum: removed entirely to reduce processing time at each hop  Options: allowed, but outside of header, indicated by “Next Header” field; options specify how to deal with the packet if a header is unknown  ICMPv6: new version of ICMP  additional message types, e.g. “Packet Too Big”  multicast group management functions

43 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 approaches:  Dual Stack: some routers with dual stack (v6, v4) can “translate” between formats  Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

44 Dual Stack Approach A B E F IPv6 C D IPv4 Flow: X Src: A Dest: F data Flow: ?? Src: A Dest: F data Src:A Dest: F data A-to-B: IPv6 Src:A Dest: F data B-to-C: IPv4 B-to-C: IPv4 B-to-C: IPv6

45 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 Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Src:B Dest: E Flow: X Src: A Dest: F data Src:B Dest: E A-to-B: IPv6 E-to-F: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4

46 Backup Slides

47 IP quality of service  IP originally had “type-of-service” (TOS) field to eventually support quality  Not used, ignored by most routers  Then came int-serv (integrated services) and RSVP signalling  Per-flow quality of service through end-to-end support  Setup and match flows on connection ID  Per-flow signaling  Per-flow network resource allocation (*FQ, *RR scheduling algorithms)

48 IP quality of service  RSVP   Provides end-to-end signaling to network elements  General purpose protocol for signaling information  Not used now on a per-flow basis to support int-serv, but being reused for diff-serv.  int-serv  Defines service model (guaranteed, controlled-load)  Dozens of scheduling algorithms to support these services  WFQ, W 2 FQ, STFQ, Virtual Clock, DRR, etc.  If this class was being given 5 years ago….

49 IP quality of service  Why did RSVP, int-serv fail?  Complexity  Scheduling  Routing  Per-flow signaling overhead  Lack of scalability  Per-flow state  Route pinning  Economics  Providers with no incentive to deploy  SLA, end-to-end billing issues  QoS a weak-link property  Requires every device on an end-to-end basis to support flow

50 IP quality of service  Now it’s diff-serv…  Use the “type-of-service” bits as a priority marking  Core network relatively stateless  AF  Assured forwarding (drop precedence)  EF  Expedited forwarding (strict priority handling)

51 IPv6 Addressing (standardized in 1995)  Length: 128 bits (RFC 1752) 010RegistryProvider IDSubscriber IDSubnet ID Interface ID Provider-based unicast nm o pq Interface ID Link-local use unicast n 118-n Site-local use unicast n Subnet IDInterface ID m118-n-m 0000………………………..… Embedded IPv4 unicast 0000 IPv4 address Flags Multicast 4 Group ID ………………………..… FFFF IPv4 address Scope Anycast