Network Layer. Abusayeed Saifullah. CS 5600 Computer Networks. These slides are adapted from Kurose and Ross

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1 Network Layer Abusayeed Saifullah CS 5600 Computer Networks These slides are adapted from Kurose and Ross

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

3 IP addresses: how to get one? Q: How does a host get IP address? v hard-coded by system admin in a file Windows: control-panel->network->configuration->tcp/ ip->properties UNIX: /etc/rc.config v DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play

4 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/ on ) support for mobile users who want to join network (more shortly) DHCP overview: host broadcasts DHCP discover msg [optional] DHCP server responds with DHCP offer msg [optional] host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg

5 DHCP client-server scenario / DHCP server arriving DHCP client needs address in this network / /24

6 DHCP client-server scenario DHCP server: DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 Broadcast: is there a DHCP server out there? arriving client DHCP request DHCP offer src: , 68 dest:: , 67 yiaddrr: that transaction IP address! ID: 655 lifetime: 3600 secs Broadcast: OK. I ll take src: , 67 dest: , 68 server! yiaddrr: Here s an IP transaction ID: 654 lifetime: 3600 secs Broadcast: I m a DHCP address you can use DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 lifetime: 3600 secs Broadcast: OK. You ve got that IP address!

7 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

8 NAT: network address translation rest of Internet local network (e.g., home network) / all datagrams leaving local network have same single source NAT IP address: ,different source port numbers datagrams with source or destination in this network have /24 address for source, destination (as usual)

9 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)

10 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

11 NAT: network address translation 2: NAT router changes datagram source addr from , 3345 to , 5001, updates table 2 NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 3345 D: , : host sends datagram to , S: , 80 D: , : reply arrives dest. address: , 5001 S: , 80 D: , : NAT router changes datagram dest addr from , 5001 to , 3345

12 NAT: network address translation v 16-bit port-number field: 60,000 simultaneous connections with a single LAN-side address! v 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

13 NAT traversal problem v 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 NATed address: v solution1: statically configure NAT to forward incoming connection requests at given port to server e.g., ( , port 2500) always forwarded to port client? NAT router

14 NAT traversal problem v solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to: v learn public IP address ( ) v add/remove port mappings (with lease times) NAT router IGD i.e., automate static NAT port map configuration

15 NAT traversal problem v solution 3: relaying (used in Skype) NATed client establishes connection to relay external client connects to relay relay bridges packets between to connections client 2. connection to relay initiated by client 3. relaying established 1. connection to relay initiated by NATed host NAT router

16 ICMP: internet control message protocol v used by hosts & routers to communicate networklevel information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) v network-layer above IP: ICMP msgs carried in IP datagrams 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

17 Traceroute and ICMP v source sends series of UDP segments to dest first set has TTL =1 second set has TTL=2, etc. v 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 v when ICMP messages arrives, source records RTTs stopping criteria: v UDP segment eventually arrives at destination host v destination returns ICMP port unreachable message (type 3, code 3) v source stops 3 probes 3 probes 3 probes

18 IPv6: motivation v initial motivation: 32-bit address space soon to be completely allocated. v additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed

19 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

20 Other changes from IPv4 v checksum: removed entirely to reduce processing time at each hop v options: allowed, but outside of header, indicated by Next Header field v ICMPv6: new version of ICMP additional message types, e.g. Packet Too Big multicast group management functions

21 Transition from IPv4 to IPv6 v not all routers can be upgraded simultaneously no flag days how will network operate with mixed IPv4 and IPv6 routers? v 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

22 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

23 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

24 IPv6: adoption v US National Institutes of Standards estimate [2013]: ~3% of industry IP routers ~11% of US gov t routers v Long (long!) time for deployment, use 20 years and counting! think of application-level changes in last 20 years: WWW, Facebook, Why?

25 Roadmap 1. introduction 2. virtual circuit and datagram networks 3. routing algorithms naive protocol: flooding, random link state routing distance vector routing hierarchical routing broadcast routing multicast routing routing in ad hoc networks 4. IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 5. routing in the Internet RIP OSPF BGP

26 Intra-AS Routing v also known as interior gateway protocols (IGP) v most common intra-as routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

27 RIP ( Routing Information Protocol) v included in BSD-UNIX distribution in 1982 v distance vector algorithm distance metric: # hops (max = 15 hops), each link has cost 1 DVs exchanged with neighbors every 30 sec in response message (aka advertisement) each advertisement: list of up to 25 destination subnets (in IP addressing sense) from router A to destination subnets: z u A C B D v y w x subnet hops u 1 v 2 w 2 x 3 y 3 z 2

28 RIP: example w x y A D B z C routing table in router D destination subnet next router # hops to dest w A 2 y B 2 z B 7 x

29 RIP: example A-to-D advertisement next hops dest w - 1 x - 1 z C w x y A D B z C routing table in router D destination subnet next router # hops to dest w A 2 y B 2 A z B 7 5 x

30 RIP: link failure, recovery if no advertisement heard after 180 sec --> neighbor/ link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if tables changed) link failure info quickly (?) propagates to entire net poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

31 OSPF (Open Shortest Path First) v open : publicly available v uses link state algorithm LS packet dissemination topology map at each node route computation using Dijkstra s algorithm v OSPF advertisement carries one entry per neighbor v advertisements flooded to entire AS

32 Hierarchical OSPF in large domains boundary router backbone router area border routers backbone area 3 area 1 area 2 internal routers

33 Hierarchical OSPF v two-level hierarchy: local area, backbone. link-state advertisements only in area each node has detailed area topology; only know direction (shortest path) to nets in other areas. v area border routers: summarize distances to nets in own area, advertise to other Area Border routers. v backbone routers: run OSPF routing limited to backbone. v boundary routers: connect to other AS s.

34 Internet inter-as routing: BGP v BGP (Border Gateway Protocol): the de facto inter-domain routing protocol glue that holds the Internet together v BGP provides each AS a means to: ebgp: obtain subnet reachability information from neighboring ASs. ibgp: propagate reachability information to all ASinternal routers. determine good routes to other networks based on reachability information and policy. v allows subnet to advertise its existence to rest of Internet: I am here

35 BGP basics v BGP session: two BGP routers ( peers ) exchange BGP messages: advertising paths to different destination network prefixes ( path vector protocol) v when AS3 advertises a prefix to AS1: AS3 promises it will forward datagrams towards that prefix AS1 can aggregate prefixes in its advertisement other networks 3b 3c AS3 3a 1a AS1 BGP message 1c 1d 1b 2a AS2 2c 2b other networks

36 BGP basics: distributing path information v using ebgp session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use ibgp do distribute new prefix info to all routers in AS1 1b can then re-advertise new reachability info to AS2 over 1bto-2a ebgp session v when router learns of new prefix, it creates entry for prefix in its forwarding table. other networks 3b 3a AS3 1a AS1 1c 1d ebgp session ibgp session 1b 2a AS2 2c 2b other networks

37 BGP route selection v router may learn about more than 1 route to destination AS, selects route based on: 1. shortest AS-PATH 2. closest NEXT-HOP router: hot potato routing 3. policy decision 4. additional criteria

38 Select best BGP route to prefix v Router selects route based on shortest AS-PATH v Example: select v AS2 AS17 to /22 v AS3 AS131 AS201 to /22 v What if there is a tie?

39 Hot Potato Routing v Suppose there two or more best inter-routes. v Then choose route with closest NEXT-HOP Use OSPF to determine which gateway is closest Q: From 1c, chose AS3 AS201 or AS2 AS17? A: route AS3 AS201 since it is closer 3c other networks 3b 3a AS3 1a AS1 1c 1d 1b 2a AS2 2c 2b other networks

40 BGP messages OPEN: opens TCP connection to peer and authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also used to close connection

41 BGP routing policy W A B C X legend: provider network customer network: Y v A,B,C are provider networks v X,W,Y are customer (of provider networks) v X is dual-homed: attached to two networks X does not want to route from B via X to C.. so X will not advertise to B a route to C

42 BGP routing policy (2) W A B C X legend: provider network customer network: Y v A advertises path AW to B v B advertises path BAW to X v Should B advertise path BAW to C? No way! B gets no revenue for routing CBAW since neither W nor C are B s customers B wants to force C to route to w via A B wants to route only to/from its customers!

43 Why different Intra-, Inter-AS routing? policy: v inter-as: admin wants control over how its traffic routed, who routes through its net. v intra-as: single admin, so no policy decisions needed scale: v hierarchical routing saves table size, reduced update traffic performance: v intra-as: can focus on performance v inter-as: policy may dominate over performance

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