Summary Chapter 4. Smith College, CSC 249 March 2, q IP Addressing. q DHCP dynamic addressing

Transcription

1 Smith College, CSC 49 March, 08 Summary Chapter 4 q IP Addressing Network prefixes and Subnets IP datagram format q DHCP dynamic addressing Obtain: own IP address Subnet mask, DNS serer & first-hop router IP address q NAT network address translation at end of class today

2 Oeriew of the Network Layer Network layer functions & protocols: transport layer: TCP, UDP network layer routing protocols path selection RIP, OSPF, BGP forwarding table IP protocol addressing conentions datagram format packet handling conentions ICMP protocol error reporting router signaling link layer physical layer Smith College IP Addressing Possible QUESTIONS: ) Gien a mask of What is the / notation for this? ) are the machines with IP addresses and.9..4 on the same subnet? How many hosts are supported in the range.9..00/? 4

3 IP addresses: how to get one? Q: How does network get subnet part of IP address? A: Is allocated a portion of its proider ISP s address space, which gets that from ICANN (Internet Corp. for Assigned Names and Numbers) Q: How does a host get an IP address? q hard-coded by system administrator in a file, or q DHCP: Dynamic Host Configuration Protocol: dynamically get address from as serer plug-and-play 5 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network serer when it joins a network Can renew its lease on the IP address it is using Allows reuse of addresses once one host leaes Support for mobile users to join networks DHCP oeriew: ) host broadcasts DHCP discoer msg ) DHCP serer responds with DHCP offer msg ) host requests IP address: DHCP request msg 4) DHCP serer sends address: DHCP ack msg 6

4 DHCP client-serer scenario DHCP serer:...5 DHCP discoer src : , 68 dest.: ,67 yiaddr: transaction ID: 654 arriing client time DHCP request DHCP offer src: , 68 dest:: , 67 yiaddr:...4 transaction ID: 655 Lifetime: 600 secs src:...5, 67 dest: , 68 yiaddr:...4 transaction ID: 654 Lifetime: 600 secs DHCP ACK src:...5, 67 dest: , 68 yiaddr:...4 transaction ID: 655 Lifetime: 600 secs yiaddr = your internet address broadcast address, à sent to eery host in the subnet 7 NAT: Network Address Translation q Motiation: local (home) network uses just one IP address as far as outside world iew: range of addresses not needed from ISP: just one IP address for all deices can change addresses of deices in local network without notifying outside world can change ISP without changing addresses of deices in local network deices inside local net not explicitly addressable, isible by outside world (a security plus) q Range of addresses within: /4 8 4

5 Standard Resered IP Address Blocks for Priate Network Use q /8 ( ) q / ( ) q /6 ( ) 9 NAT: Network Address Translation rest of Internet local network (e.g., home network) / All datagrams leaing local network hae same single source NAT router IP address: , but they hae different source port numbers Datagrams with source or destination in this network hae /4 address for source, destination (as usual) 0 5

6 NAT Router Tasks Implementation: NAT router must: for outgoing datagrams: replace (source IP address, port #) of eery outgoing datagram with (NAT IP address, new port #) remote clients/serers will respond using (NAT IP address, new port #) as destination address remember (in NAT translation table) eery (source IP address, port #) to (NAT IP address, new port #) translation pair for incoming datagrams: replace (NAT IP address, new port #) in destination fields of eery incoming datagram with corresponding (source IP address, port #) stored in NAT table 4- NAT: network address translation : NAT router changes datagram source addr from , 45 to , 500, updates table NAT translation table WAN side addr LAN side addr , , 45 S: , 500 D: , S: , 80 D: , 500 : reply arries dest. address: , S: , 45 D: , 80 S: , 80 D: , 45 4 : host sends datagram to , : NAT router changes datagram dest addr from , 500 to , 45 6

7 NAT Question on Handout IP6 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 er pri flow label payload len next hdr hop limit source address (8 bits) destination address (8 bits) data bits 4 7

8 NAT Controersies? q Port numbers are used by NAT to identify hosts (and the process) within the local network but ports are for addressing processes only not hosts q Routers should only process packets up to layer (ports associated with app socket) q Violates end-to-end argument NAT possibility must be taken into account by application designers, e.g., PP applications Interfering nodes should not modify IP addresses and port numbers q Address shortage should instead be soled by IP6 5 IP fragmentation & reassembly q network links hae MTU (max. transfer size) - largest possible link-leel frame different link technologies hae different MTUs q large IP datagram may be diided ( fragmented ) within a network when the link technology changes one datagram becomes seeral datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: smaller datagrams 8

9 example: IP fragmentation & reassembly A 4000 byte datagram Encounters an older link technology That can only accommodate MTU = 500 bytes 480 bytes in data field length =4000 ID =x length =500 length =500 fragflag =0 So one large datagram becomes seeral smaller datagrams ID =x ID =x offset =0 fragflag = fragflag = offset =0 offset =85 length =040 ID =x fragflag =0 offset =70 offset = 480/8 Recap: Routing. Forwarding routing algorithm local forwarding table header alue output link alue in arriing packet s header 0 8 9

10 Generalized Forwarding and SDN Each router contains a flow table that is computed and distributed by a logically centralized routing controller logically-centralized routing controller control plane data plane local flow table headers counters actions alues in arriing packet s header OpenFlow data plane abstraction q generalized forwarding: simple packet-handling rules Pattern: match alues in packet header fields Actions: for matched packet: drop, forward, modify, matched packet or send matched packet to controller Priority: disambiguate oerlapping patterns Counters: #bytes and #packets Flow table in a router (computed and distributed by controller) define router s match+action rules 0

11 OpenFlow data plane abstraction q generalized forwarding: simple packet-handling rules Pattern: match alues in packet header fields Actions: for matched packet: drop, forward, modify, matched packet or send matched packet to controller Priority: disambiguate oerlapping patterns Counters: #bytes and #packets * : wildcard. src=..*.*, dest=.4.5.* à drop. src = *.*.*.*, dest=.4.*.* à forward(). src=0..., dest=*.*.*.* à send to controller OpenFlow: Flow Table Entries Match Action Stats. Forward packet to port(s). Encapsulate and forward to controller. Drop packet 4. Send to normal processing pipeline 5. Modify Fields Switch Port VLAN ID MAC src MAC dst Eth type IP Src IP Dst IP Prot TCP sport TCP dport Link layer Network layer Transport layer

12 Examples Destination-based forwarding: Switch Port * Switch Port * MAC src Firewall: MAC dst Eth type VLAN ID IP Src IP Dst IP Proto TCP sport TCP dport Action * * * * * * * * port6 MAC src MAC dst Eth type IP datagrams destined to IP address should be forwarded to router output port 6 VLAN ID IP Src IP Dst IP Proto TCP sport TCP dport Forward * * * * * * * * drop do not forward (block) all datagrams destined to TCP port Switch Port * MAC MAC Eth VLAN IP IP IP TCP TCP src dst type ID Src Dst Prot sport dport Forward * * * * * * * * drop do not forward (block) all datagrams sent by host Oeriew of Routing q The control plane q What is the objectie of routing? q Does routing occur between hosts or routers? q What are differences between centralized (global) and decentralized algorithms? What are examples of each? Amount of information initially How information is shared/spread Synchronous or asynchronous? (see pathologies as well)

13 Routing Notation 5 Graph: G = (N,E) u x w y 5 z N = set of nodes, here nodes = routers = { u,, w, x, y, z } E = set of edges or links = { (u,), (u,x), (,x), (,w), (x,w), (x,y), (w,y), (w,z), (y,z) } 5 A Link-State Routing Algorithm Dijkstra s algorithm q Computes the shortest paths in a graph by using weights on edges as a measure of distance. Starts with complete information A path with the least number of edges may not be the path with the least weight / least cost. q Each node has global information on network topology and edge weights q A Greedy algorithm Makes the locally optimum choice, with objectie of finding the global optimum 6

14 Dijkstra Notation q c(x,y): link cost from node x to y = if not direct neighbors q D(): current alue of cost of path from source to dest. q p(): predecessor node along path from source to q N': set of nodes whose least cost path definitiely known 7 A Link-State Routing Algorithm 8 4

15 A Link-State Routing Algorithm Dijkstra s algorithm q computes least cost paths from one node ( source ) to all other nodes Determines the forwarding table for that node q The network topology and link costs are known to all nodes accomplished ia link state broadcast all nodes hae the same information q The algorithm is iteratie: after k iterations, the least cost paths to k destinations are known Dijsktra s Algorithm for node u Initialization: N' = {u} for all nodes 4 if is neighbor to u 5 then D() = c(u,) (D(): current alue of cost of path from source to dest. ) 6 else D() = 7 8 Loop 9 find some w not yet in N' such that D(w) is a minimum 0 add w to N' update D() for all adjacent to w and not in N' : D() = min( D(), D(w) + c(w,) ) /* new cost to is either old cost to or known 4 shortest path cost to w plus cost from w to */ 5 until all nodes are in set N' 0 5

16 Dijkstra s algorithm: example Step N' D() p() D(w) p(w) D(x) p(x) D(y) p(y) D(z) p(z) notes: u 7,u,u 5,u uw 6,w 5,u,w uwx 6,w,w 4,x uwx 0, 4,x uwxy,y uwxyz Construct shortest path tree by tracing predecessor nodes Construct the forwarding table by recording the next hop to the destination node What is the forwarding table?? u 5 x w y 9 z Dijkstra s algorithm: example Step start N A AD ADE ADEB ADEBC ADEBCF D(B),p(B),A,A,A D(C),p(C) 5,A 4,D,E,E D(D),p(D),A D(E),p(E) infinity,d D(F),p(F) infinity infinity 4,E 4,E 4,E 5 A B D C E 5 F 6

17 Dijkstra s algorithm: example Resulting shortest-path tree from A: A B D C E F Resulting forwarding table in A: destination B D E C F link (A, B) (A, D) (A, D) (A, D) (A, D) Routing Actiity q Each pair, or table, be a different router q Fill in table on handout using Dijkstra s algorithm, for your router letter (IP address) q Create the forwarding table (back side of handout) q Send datagrams to a distant destination, forwarding the datagrams to the appropriate next-hop using your forwarding table. 4 7

18 Link State Example Use Dijkstra s algorithm to compute the least-cost-path table for node x, and the forwarding table for x s router 5 6 8

19 Final Step: The Forwarding Table Destination S T U V W Y Z Link 7 Algorithm : Distance Vector Rather than using global information, a distance ector algorithm is: q distributed: each node communicates only with directlyattached neighbors q iteratie: continues until no nodes exchange info. self-terminating: no signal to stop q asynchronous: nodes need not exchange information or iterate in lock step! 8 9

20 Distance Vector Algorithm Bellman-Ford Equation, an important relationship among costs of least-cost paths Define d x (y) := cost of least-cost path from x to y Then d x (y) = min {c(x,) + d (y) } where min is taken oer all neighbors of x 9 Summary Forwarding: q Leads to questions of addressing Assignment of IP addresses NAT, IP6 Routing: q Routing objecties q Routing notation q Routing classification q Link state. Distance Vector q Hierarchical structure 40 0