Institute of Telecommunications Warsaw University of Technology 2017 internet technologies and standards Piotr Gajowniczek Andrzej Bąk Michał Jarociński
Network Layer The majority of slides presented in this part of the course come as exact or adapted versions of slides available as the companion to the following book by Jim Kurose and Keith Ross: Computer Networking: A Top Down Approach 6 th edition, Addison-Wesley, March 2012 Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012
network layer service 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 application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical
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 dest. routing algorithms analogy: routing: process of planning trip from source to dest forwarding: process of getting through single interchange
connection, connection-less service connection oriented network signalling protocol creates entries in the forwarding table connection setup, teardown for each call before data can flow each packet carries VC identifier (not destination host address) every router on source-dest path maintains state for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) datagram network routing protocol creates entries in the forwarding table data flow can start at arbitrary time each packet carries destination host adress routers do not maintaine per flow state
connection-oriented forwarding table 12 22 32 forwarding (switching) table: VC number interface number 1 2 3 Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 Nodes maintain connection state information!
datagram forwarding table 40.40.40.40 1 2 3 Routing table: interface number Destination address host address range Outgoing interface range 4 billion of addresses IP addresses, (aggregates very large table amount entries) of potential routing table entries 10.10.10.10 10.10.10.0-255 3 20.20.20.20 20.20.20.0-255 1 30.30.30.30 30.30.30.0-255 2 40.40.40.40 40.40.40.0-255 3 Destination Address Range: Prefix:00101000 00101000 00101000 00000000 Mask: 11111111 11111111 11111111 00000000
longest prefix matching longest prefix matching when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address. Destination Address Range 11001000 00010111 00010*** ********* 11001000 00010111 00011000 ********* 11001000 00010111 00011*** ********* otherwise examples: DA: 11001000 00010111 00010110 10100001 DA: 11001000 00010111 00011000 10101010 Link interface 0 1 2 3 which interface? which interface?
router architecture overview two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link forwarding tables computed, pushed to input ports routing processor routing, management control plane (software) forwarding data plane (hardware) high-seed switching fabric router input ports router output ports
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 solution: virtual output queues switch fabric switch fabric output port contention: only one red datagram can be transferred, lower red packet is blocked, green packet could be transferred but is blocked by red (HOL)
output port queueing switch fabric switch fabric at t, packets move from inputs to outputs one packet time later one packet from each imput port can be moved to output port in one packet transmission time buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow!
the Internet network layer host, router network layer functions: transport layer: TCP, UDP network layer routing protocols path selection RIP, OSPF, BGP forwarding table IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling link layer physical layer
IP datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to ver head len. 16-bit identifier time to live type of service upper layer 32 bits flgs length fragment offset header checksum 32 bit source IP address 32 bit destination IP address options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly header error detection source and destination addresses e.g. timestamp, record route taken, specify list of routers to visit.
IP fragmentation, reassembly network links have MTU - 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 reassembly fragmentation: in: one large datagram out: 3 smaller datagrams
IP fragmentation, reassembly example: 4000 byte datagram MTU = 1500 bytes length =4000 ID =x fragflag =0 offset =0 one large datagram becomes several smaller datagrams 1480 bytes in data field length =1500 ID =x fragflag =1 offset =0 offset = 1480/8 length =1500 ID =x fragflag =1 offset =185 length =1040 ID =x fragflag =0 offset =370
IP addressing IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host typically has one or two interfaces (e.g., wired Ethernet, wireless 802.11) IP addresses associated with each interface 223.1.1.2 223.1.1.1 223.1.1.3 223.1.1.4 223.1.2.9 223.1.3.27 223.1.3.1 223.1.2.1 223.1.2.2 223.1.3.2 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1
networks vs. subnets Initial IP spec introduced address classes class A (0-126) 1 byte for network/3 bytes for hosts class B (128-192) 2 bytes for network/2 bytes for hosts class C (192-223) 3 bytes for network/1 byte for hosts IP address structure: network part - high order bits host part - low order bits what s a network? set of devices (interfaces) that can physically reach each other without intervening router Network A Network B 223.1.1.1 223.1.1.2 223.1.2.1 223.1.1.4 223.1.2.9 223.1.2.2 223.1.1.3 223.1.3.27 Network C 223.1.3.1 223.1.3.2
networks vs. subnets How many IP networks? need one IP network per layer 2 network (including point-to-point links) Inefficient use of IP address space class A, B, C networks usually bigger than physical network size Solution: subneting divide the IP networks into smaller picies (subnets) more efficient use of IP address space 223.1.9.1 223.1.1.1 223.1.2.6 223.1.2.1 223.1.9.2 223.1.9.0/24 223.1.2.2 223.1.1.2 223.1.1.3 223.1.7.0 223.1.8.0/24 223.1.8.1 223.1.8.0 223.1.3.1 223.1.1.4 223.1.1.0/24 223.1.7.0/24 223.1.7.1 223.1.3.27 223.1.3.2 223.1.2.0/24 223.1.3.0/24
IP addressing: CIDR CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address mask is associated with subnet address (and e.g. propagated by the routing protocols) allows for address aggregation (superneting) address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part host part 11001000 00010111 00010000 00000000 200.23.16.0/23
hierarchical addressing: route aggregation hierarchical addressing allows efficient advertisement of routing information: Organization 0 200.23.16.0/23 Organization 1 200.23.18.0/23 Organization 2 200.23.20.0/23 Organization 7.... ISP1 Send me anything with addresses beginning 200.23.16.0/20 Internet 200.23.30.0/23 ISP 2 Send me anything with addresses beginning 199.31.0.0/16
hierarchical addressing: more specific routes ISP 2 has a more specific route to Organization 1 Organization 0 200.23.16.0/23 Organization 2 200.23.20.0/23 Organization 7 200.23.30.0/23 Organization 1 200.23.18.0/23.... ISP 1 ISP 2 Send me anything with addresses beginning 200.23.16.0/20 Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23 Internet
IP addresses: how to get one? Q: How does a host get IP address? hard-coded by system admin in a file Windows: control-panel->network->configuration->tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play
DHCP: Dynamic Host Configuration Protocol goal: allow host to dynamically obtain its IP address from network server when it joins network simplifies network adnistration tasks allows reuse of addresses (only hold address while connected/ 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
DHCP client-server scenario 223.1.1.0/24 223.1.1.1 DHCP server 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.2.2 223.1.1.3 223.1.3.27 arriving DHCP client needs address in this network 223.1.2.0/24 223.1.3.1 223.1.3.2 223.1.3.0/24
DHCP client-server scenario DHCP server: 223.1.2.5 DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 Broadcast: is there a DHCP server out there? arriving client DHCP request DHCP offer src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 that transaction IP address! ID: 655 lifetime: 3600 secs Broadcast: OK. I ll take src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 lifetime: 3600 secs Broadcast: I m a DHCP server! Here s an IP address you can use DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 lifetime: 3600 secs Broadcast: OK. You ve got that IP address!
DHCP: more than IP addresses DHCP can return more than just allocated IP address on subnet: address of first-hop router for client (gateway) name and IP address of DNS sever network mask (indicating network versus host portion of address) lease time.. more
NAT: network address translation rest of Internet 138.76.29.7 10.0.0.4 local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.2 10.0.0.3 all datagrams leaving local network have same single source NAT IP address: 138.76.29.7 different source port numbers datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual)
NAT: network address translation motivation: local network uses just one 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 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)
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
NAT: network address translation 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table 2 NAT translation table WAN side addr LAN side addr 138.76.29.7, 5001 10.0.0.1, 3345 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 10.0.0.4 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 1 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 10.0.0.1 10.0.0.2 138.76.29.7 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3 3: reply arrives dest. address: 138.76.29.7, 5001 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4 4: NAT router 10.0.0.3 changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345
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, e.g., P2P applications address shortage should instead be solved by IPv6
NA(P)T full cone Client IP: 10.0.0.1 Port: 21 IP: 202.123.211.25 Port: 12345 NAT Binding table: 10.0.0.1:21 -- 12345 System A IP: 222.111.99.1 Port: 20202 System B IP: 222.111.88.2 Port: 10101
NAT - restricted cone Client IP: 10.0.0.1 Port: 21 IP: 202.123.211.25 Port: 12345 NAT Tablica Binding odwzorowań: table : 10.0.0.1:21 12345 (A) 10.0.0.1:21 12345 (B) System A IP: 222.111.99.1 Port: 20202 System B IP: 222.111.88.2 Port: 10101
NAT - port restricted cone IP: 202.123.211.25 Port: 12345 Client IP: 10.0.0.1 Port: 21 NAT System A IP: 222.111.99.1 Port: 20202 30303 Tablica odwzorowań: 10.0.0.1:21 12345 (A: 20202) Not accepted 30303
symmetric NAT Client IP: 10.0.0.1 Port: 21 IP: 202.123.211.25 Port: 12345 NAT IP: 202.123.211.25 Port: 45678 System A IP: 222.111.99.1 Port: 20202 System B IP: 222.111.88.2 Port: 10101 Binding table : 10.0.0.1:21 12345 (A: 20202) 10.0.0.1:21 45678 (B: 10101)
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 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
traceroute and ICMP source sends series of UDP segments to dest first set has TTL =1 second set has TTL=2, etc. unlikely port number when nth set of datagrams arrives to nth router: router discards datagrams and sends source ICMP messages (type 11, code 0) ICMP messages includes name of router & IP address when ICMP messages arrives, source records RTTs stopping criteria: UDP segment eventually arrives at destination host destination returns ICMP port unreachable message (type 3, code 3) source stops 3 probes 3 probes 3 probes
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 IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed
IPv6 datagram format 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 ver pri flow label payload len next hdr hop limit source address (128 bits) destination address (128 bits) data 32 bits
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
transition from IPv4 to IPv6 not all routers can be upgraded simultaneously no flag days how will network operate with mixed IPv4 and IPv6 routers? tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers IPv4 header fields IPv4 source, dest addr IPv6 header fields IPv6 source dest addr UDP/TCP payload IPv4 payload IPv6 datagram IPv4 datagram
tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 physical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6
tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 physical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 flow: X src: A dest: F data src:b dest: E Flow: X Src: A Dest: F src:b dest: E Flow: X Src: A Dest: F flow: X src: A dest: F data data data A-to-B: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 E-to-F: IPv6