Types of Service (TOS) The IPv4 Datagram. Fragmentation. Fragmentation Example

Transcription

1 The IPv4 Datagram Types of Service (TOS) 4 bits vers 4 bits HLen (x32b) 32 bits 8 bits 3 bits 13 bits TOS Datagram Length (bytes) Offset within original packet An early attempt to support Quality if Service (QoS). Bits 0-2: precedence (0..7, 0 is lowest) Hop count ID Flags FRAG Offset Bit 3: Delay. 1=low delay requested. TTL Protocol checksum Bit 4: Throughput. source IP Address destination IP Address <=64 KBytes Bit 5: Reliability. Bits 6-7: unused. (OPTIONS) (PAD) But most routers ignore the TOS field! More on QoS later in the course. Lecture 9 2 Lecture 9 3 Fragmentation Fragmentation Example A Problem: A router may receive a packet larger than the maximum transmission unit (MTU) of the outgoing link. Ethernet MTU=1500 bytes MTU=1500 bytes Source R1 MTU<1500 bytes R2 Destination Solution: R1 fragments the IP datagram into multiple, self-contained datagrams. Offset>0 More Frag=0 Data HDR (ID=x) Data HDR (ID=x) Data HDR (ID=x) Data HDR (ID=x) B Offset=0 More Frag=1 length =4000 ID =x fragflag =0 offset =0 One large datagram becomes several smaller datagrams length =1500 length =1500 length =1040 ID =x ID =x ID =x fragflag =1 fragflag =1 fragflag =0 offset =0 offset =1480 offset =2960 Lecture 9 4 Lecture 9 5

2 Fragmentation Time to Live (TTL) Fragments re-assembled only by the destination host Set timer when first fragment arrives, discard packet if not all fragments arrive before timeout. Fragmentation can be avoided by MTU discovery that finds the smallest MTU along the path Path MTU discovery: test for fragmentation by sending various size datagrams. But routing may change Most links today: MTU 1500 bytes. Idea: Prevent packets from remaining in the network forever. One method: use timeouts. Requires synchronized clocks! Internet method: hop count. Hop = router-router path. Sender initializes TTL (at most 255) Every time a packet reaches a router, TTL counter decremented by 1 When TTL reaches 0, packet dropped and error message sent back to sender Lecture 9 6 Lecture 9 7 Protocols ICMP: Internet Control Message Protocol Tells what type is the data. E.g., TCP=6 UDP=17 IP=4 (why need IP?) ICMP=1 See Formally: a transport layer ICMP msgs carried in IP datagrams But not general: Kind of IP control messages. Used by hosts, routers, gateways to communicate networklevel information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) ICMP message: type; code; 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 Lecture 9 8 Lecture 9 9

3 Addresses and Interfaces IP Addresses Interface (NIC): connection between host or router and the physical network link hosts may have multiple interfaces (routers always do) Interfaces have IP addresses: Hosts (or routers) don t! interface to network 10 Lecture 9 11 IP Addressing IP Addressing IP address (v4): 32 bits high order bits: network low order bits: host IP s definition of network: Set of devices that can communicate directly (in the datalink layer), without any router in the middle LAN network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) Lecture 9 12 How to find the networks? Detach each interface from router, host create islands of isolated networks Interconnected system consisting of six networks Lecture 9 13

4 Structure of IP Addresses IP Addresses: Example Originally there were 5 classes: CLASS A Net ID Host-ID CLASS B 10 Net ID Host-ID CLASS C 110 Net ID Host-ID 4 28 CLASS D 1110 Multicast Group ID 5 27 CLASS E Reserved 128 networks 16M hosts 16K networks 64K hosts 2M networks 256 hosts 256M IDs Host IDs convention: 0=self, 0xff =all, 127.* = loopback Lecture 9 14 Class A address: (0<18<128 => Class A) Class B address: ( < 192) => Class B) Some IP lingo: Dotted Decimal Notation Octets Lecture 9 15 Problem: Address classes too rigid Usually, Class C too small and Class B too big Even small organizations have > 255 hosts. But there are only 16K Class B network IDs. Wastage and shortage of addresses! Organizations with internal routers need to have a separate network ID for each link. Every router must know about every network ID in every organization large address tables. IP Addressing Hence, two solutions: Subnetting: subdivide a network ID hierarchically (used within an organization). A hack on top of the class system Classless Interdomain Routing (CIDR, supernetting ): Forget classes. Network ID can be any prefix of the IP address. Lecture 9 16 Lecture 9 17

5 Subnetting Subnetting e.g. Site e.g. Dept CLASS B e.g. Company Net ID 0000 Host-ID Subnet ID (20) Net ID Host-ID Subnet Host ID (12) Net ID Host-ID Subnet ID (22) Subnet Host ID (10) Net ID 1111 Host-ID Subnet ID (20) Subnet Host ID (12) Net ID Host-ID Subnet ID (26) Subnet Host ID (6) Subnetting is a form of hierarchical routing. Representation: an address + a bitmask. Mask 0xffff0000 (or ): the first 16 bits are the subnet ID, and the last 16 bits are the host ID. Advantage: allows for the flexible partition of large networks (typically, type B). Shortcoming: must be configured in each host with its IP address subnet is still part of its parent network: switching ISP IP addresses will change. Lecture 9 18 Lecture 9 19 CIDR Addressing Classless InterDomain Routing IP address space broken into intervals of length 2 k for an integer k 0, aligned. Representation: the common prefix. Denoted x/y, meaning y first bits of x. Example: 128.9/16 represents the addresses in the interval [ ]. 65/ / / / CIDR Addressing / / / /20 Intervals may overlap! Rule: prefer the longest matching prefix Lecture 9 20 Lecture 9 21

6 CIDR Addressing Prefix aggregation: If a service provider serves two organizations with prefixes, it can aggregate them to form a larger prefix (when?). Reduces size of routing tables. E.g. ISP serves /24 and /24, it can tell other routers to send it all packets belonging to the prefix /23. ISP Choice: In principle, an organization can keep its prefix if it changes service providers. IPv6 addresses Motivation: too few 32-bit addresses, more functionality required. new addresses: 128 bits (!) X:X:X:X:X:X:X:X where X is 4 hexadecimals structured hierarchy: 13, 24, 16, 64 bits (last is interface ID). 3 bits identify type, 8 reserved new anycast address: route to best of several replicated servers Can embed IPv4 addresses Penetration is slow... but used in China! Lecture 9 22 Lecture 9 23 Inside a Router Forwarding in 1. Forwarding Table 2. Interconnect 3. Output Scheduling Routers Forwarding Decision Forwarding Table Forwarding Decision Forwarding Table Forwarding Decision Lecture 8 24 Lecture 8 25

7 Forwarding in an IP Router Routing Tables at a router Lookup packet DA in forwarding table. If known, forward to correct port. If unknown (in particular: no default router), drop packet. Decrement TTL, update header Checksum. Forward packet to outgoing interface. Transmit packet onto link. Question: How is the address looked up in a real router? Lecture 8 26 R R2 R3 R4 e.g => Port 2 Prefix 65/ / / / / / /19 Routing table must know next hop for every network on the Internet! Next-hop Forwarding/routing table Port Lecture 8 27 Core Router Table Size Required Lookup Performance Line Line Rate Pktsize=40B Pktsize=240B T1 1.5Mbps 4.68 Kpps 0.78 Kpps OC3 155Mbps 480 Kpps 80 Kpps OC12 622Mbps 1.94 Mpps 323 Kpps OC48 2.5Gbps 7.81 Mpps 1.3 Mpps OC Gbps Mpps 5.21 Mpps source: Lecture 8 28 Lecture 8 29

8 Finding next hop in Class-based addressing Routing table IP Address Space Class A Class B Class C D Structure Class A Class B Exact Class C Match Routing Table: Port 4 Exact Match: There are many well-known ways to find an exact match in a table. Lecture 8 30 Lecture 8 31 Direct Lookup Associative Lookups Contents addressable memory (CAM) IP Address Memory Next-hop, Port Search Data 32 Associative Memory or CAM Network Address Port Number Port Number Hit? Advantages: Simple Disadvantages High Power Small Expensive Problem: With 2 32 addresses, the memory would require 4 billion entries. Lecture 8 32 Lecture 8 33

9 Hashed Lookups Lookups Using Hashing An example Memory Search Data 32 Hash Function 16 Address Memory Data Associated Data Hit? {Address log 2 N Search Data 32 Hashing Function 16 #1 #2 #3 #4 #1 #2 Associated Data Hit? Linked list of entries with same hash key. #1 #2 #3 Lecture 8 34 Lecture 8 35 Lookups Using Hashing Classless Addressing: CIDR Advantages: Simple Expected lookup time can be small Disadvantage Non-deterministic lookup time 128.9/ / / / /20 Most specific route = longest matching prefix Problem: How can we look up addresses if they are not an exact match? Lecture 8 36 Lecture 8 37

10 Ternary CAMs: CAMs with * s Trees and Tries Value Associative Memory Mask Port Binary Search Tree: Binary Search Trie: ( retrieval ) Port < > < > < > log 2 N Priority Encoder Note: Most specific routes appear closest to top of table N entries Requires 32 memory references, regardless of number of addresses. Lecture 8 38 Lecture 8 39 Search Tries Multiway tries reduce the number of memory references 16-ary Search Trie 0000, ptr 1111, ptr 0000, , ptr , , ptr Each node has 16 children, but only children that lead to an existing leaf are non-null. Why not keep increasing the degree of the trie? Lecture 8 40 a bc Longest prefix matches using Binary Tries Example Prefixes: d e 0 1 f g h j i a) b) c) d) 001 e) 0101 f) 011 g) 100 h) 1010 i) 1100 j) Lecture 8 41

11 Patricia Tries d 0 1 f g h e i Skip 5 j Example Prefixes: a) b) c) d) 001 e) 0101 f) 011 g) 100 h) 1010 i) 1100 j) IP helpers a bc Lecture 8 42 DHCP Dynamic Host Configuration Protocol Goal: dynamically obtain an IP address from network server Can renew its lease on address in use Allows reuse of addresses Support for mobile users 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 NAT Network Address Translation rest of Internet All datagrams leaving local network have same single source NAT IP address: and different source port numbers local network (e.g., home network) / / / /16 NAT reserved Datagrams with source or destination in this network have /24 address for source, destination (as usual) Lecture 9 44 Lecture 9 45

12 NAT idea Use one of three reserved blocks for all internal IP addresses (10/8, /12, /16) Within internal network all as before Connections going outside are translated by the NAT server that keeps the translation table Use transport level IDs (port numbers). NAT properties Allow a complete IP network to have one external IP address Can change internal addresses without notifying outside world Can change external address (e.g., ISP) without notifying internal devices devices inside local net not directly visible or reachable by outside world (good security, bad flexibility). Lecture 9 46 Lecture Addressing Schemes DNS & ARP Domain names: application level IP addresses: network level MAC: Data link (LAN) level Lecture 9 48 E6-E BB-4B Lecture 9 49

13 Address Translation DNS: Domain Name System DNS ARP Hostname (bakara.eng.tau.ac.il) IP address ( ) MAC address (80:20:9A:3A:99) Distributed database: a hierarchy of many name servers. Supports both queries (domain name IP address), and updates. Hierarchical authority, hierarchical queries Application-layer protocol: host, routers, name servers communicate to resolve names core Internet function implemented as application-layer protocol! This is a function for users (not only humans, though) Note: routers don t maintain any DNS-related info Lecture 9 50 Lecture 9 51 DNS: Root name servers Simplified DNS example contacted by local name server when can t resolve name root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server 13 root entities, ~300 servers source: root-servers.org host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. Contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, dns.umass.edu, if necessary local name server dns.eurecom.fr root name server authorititive name server dns.umass.edu Lecture 9 52 requesting host surf.eurecom.fr gaia.cs.umass.edu Lecture 9 53

14 recursive query: puts burden of name resolution on contacted name server heavy load? iterated query: contacted server replies with name of server to contact I don t know this name, but ask this server DNS: iterated queries iterated query local name server dns.eurecom.fr requesting host surf.eurecom.fr root name server intermediate name server dns.umass.edu 5 6 authoritative name server dns.cs.umass.edu gaia.cs.umass.edu ARP Address Resolution Protocol MAC address IP address Each IP node (Host, Router) on the LAN has ARP module and Table ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> <.. > TTL (Time To Live): timer, typically 10 s of minutes Lecture 9 55 ARP (more) Host A wants to send packet to destination IP addr XYZ on same LAN By subnet mask, knows that on same LAN go to local ARP ARP looks in its cache for IP addr XYZ; if found done. Otherwise, ARP module broadcasts ARP pkt who is XYZ? ALL nodes on the LAN accept and inspect the ARP pkt Node XYZ responds with unicast ARP pkt to A: < XYZ, MAC (XYZ) > All nodes store MAC address in their local ARP cache Entries expire after a few minutes IP Forwarding Decision 1. Given a packet: determine the network prefix of the destination in the packet (CIDR!) 2. Is the destination is on the same network? Decide by own IP address, destination IP address and subnet mask 3. If yes, immediate destination = final destination 4. Else, use routing table to find immediate destination ( which is a router) 5. Use ARP to find datalink (MAC) address 6. Send packet over to datalink immediate destination Lecture 9 56 Lecture 9 57