1、COMP 5116Semester 2, 2012,Lecture 4 IP Addressing Extensions ICMP and Applications,1,Comer Readings,Chapter 9 Subnetting: 9.1 9.13 CIDR: 9.16 9.19 (rest for interest) Chapter 8 ICMP,2,Outline,IP addressing extensions Proxy ARP Subnet addressing and forwarding CIDR basics ICMP introduction encapsulat

2、ion/stack position basic ideas ICMP message types applications ping traceroute,3,Classful IP address problems (recap),Problems: class assignment is wasteful, especially class A ip host addresses not necessarily utilized well a single organisation needs multiple ip network addresses (one per physical

3、 network) too many networks in core routers running out of ip addresses ? How can we be more flexible?,4,Proxy ARP (the ARP hack),Works with ARP (only) Single net prefix for two (or more) networks connected to same router Router runs proxy ARP responds to ARP requests to/from other network with own

4、address spoofs hosts to send packets for machines on other network to itself routes packets from outside to correct machine Solution does not scale,5,IP Subnetting,Use single IP network address for multiple physical nets on the same site (organisation) Subnet notion converts (net, host) into slightl

5、y more hierarchical (net, subnet, host) subnet part taken from host bits, can be any number of bits (chosen by administrator) Associate subnet mask with IP address router can identify / extract the subnet part,6,Subnetting,Example: Class B, one byte of subnet: ip = subnet mask=255.255.255

6、.0 Subnet 1 in network 128.10.x.x Host 1 in Subnet 1 or, /24 slash notation, mask length = 24,7,Subnetting,Subnetting functions: you can subnet one network address and split it up on separate physical networks across routers within your own site (conserves address space) you hide your rout

7、ing structure from remote routers, thus reducing routes in their routing tables they only consider the network part of the address,8,Subnetting,Subnet part only meaningful within the subnetted network (subnet forwarding): if dest ip addr & subnet mask = my ip addr and subnet mask dest is on same sub

8、net else on different subnet (send pkt to router),9,Subnetting,Class B network Subnet mask extracts physical subnet address 8-bit subnet address supports up to 256 subnets (all 0s and all 1s subnet addresses sometimes not permitted),Comer Fig 9.2,10,Subnet Mask,Dotted decima

9、l vs CIDR notation / vs /24 Subnet forwarding uses masks in routing table Provides a unified forwarding algorithm: host, 1/32 (to specific host) subnet, /24 (to specific subnet) network, /16 (to specific net) Default, /0 = the all zeroes

10、address host route to default route most specific to least specific (forwarding algorithm),11,Unified IP Forwarding Algorithm,Table must be sorted by mask length: Host Subnet Network Default,Comer Fig 9.7,12,Subnetting Limitations,Only visible within an organisation subnet forwarding only used withi

11、n does not propagate outside (not used in the core) tight guidelines to avoid ambiguity eg fixed length only nearly all Class B networks use /24 Still does not use IP address space efficiently, especially Class A networks,13,Classless Inter-Domain Routing (CIDR) Basics,Introduced in 1993/4 in Intern

12、et core commercial explosion of the net and WWW ISPs assigned blocks of addresses associated with Autonomous Systems (later) can then assign to subscribers Network prefix occurs on arbitrary bit boundary, specified by address mask Eg block 55 2048 contiguous addresses addr

13、ess mask is /21,14,CIDR Basics,No notions of network class or fixed subnets Blocks of addresses can be partitioned on any bit boundaries to ISPs and within ISPs separately routed, outside and within ISP However, requires more complex routing tables and searching (mask length not known a priori) long

14、est prefix matching (LPM) Now used extensively in the Internet core Subnetting still used within sites (Details are beyond the scope of this Unit),15,Protocols,16,ICMP Encapsulation,ICMP transmitted within IP datagram so that it is routeable (unlike arp),17,ICMP Ideas,Considered part of IP (mandator

15、y implementation) Functionality includes: error messages (ttl exceeded, destination unreachable, router is congested, parameter problem) network management (echo request/reply) end host configuration (router advert, netmask) Error messages go from router/end host to original ip src not understood by

16、 intermediate hops,18,ICMP Ideas,Error messages typically sent at IP layer, received by source IP/UDP/TCP, which may forward to an application ICMP error messages never generated due to: ICMP error message (loop) broadcasts/multicasts Why? prevent broadcast storms Error messages contain offending IP

17、 header + 1st 8 bytes of IP data (eg tcp/udp ports),19,ICMP Header,checksum covers icmp header/data, not ip header,20,ICMP messages (not all),21,continued,22,Time Exceeded (11,0/1),If TTL value 0, discard packet and issue ICMP time exceeded, code 0, to IP source If fragments not received within a ce

18、rtain time limit at destination, discard fragments and issue ICMP time exceeded, code 1 Prevents infinite packet loops,23,Destination unreachable (3,x),Host or router cannot deliver a datagram Return IP header, first 8 bytes of datagram data Codes 0 Network unreachable 1 Host unreachable 2 Protocol

19、unreachable 3 Port unreachable 4 DF set but must fragment on next hop Detects forwarding errors (but not all),24,Source Quench (4),No flow control in IP (data rate) Source quench alerts sender: A packet was discarded Slow down transmission rate Returned is IP header plus 8 bytes of data But rarely i

20、mplemented (other congestion control mechanisms are used),25,Parameter problem (12,0/1),If the IP header format wrong Discard datagram Issue ICMP parameter problem Code 0 faulty header field, pointer field in ICMP addresses start byte of problem in IP header Code 1 required part of option is missing

21、,26,Echo request/reply (8/0),Host or router sends echo request to destination IP Destination returns echo reply Used by ping (below),27,Router solicitation (9/10),Host wants to learn about network topology issues ICMP RS message Routers reply with a router advertisement Little used (DHCP is more com

22、mon now),28,ICMP redirect (5),Limited dynamic routing table update technique Only done on same link/network Situation: 1. assume dumb host with 1 default routing table entry 2. two routers on same link, one is default, one is route to net X 3. dumb host sends pkt to net X via default router 4. defau

23、lt router sends ICMP redirect with correct router address to dumb host,29,Picture,Default router also forwards original packet correctly. Dumb host changes its routing table to reflect newly learned route to other net. Means initial configuration can be minimal, and hosts then learn. Now rarely used

24、 (DHCP).,30,Address mask (17/18),If host does not know its netmask, issue Address mask request Router on network replies with mask Can be unicasted or broadcasted Can be used at bootstrapping but now little used,31,ping - ICMP echo request/reply,ping program useful diagnostic tool, uses ICMP echo re

25、quest/reply packets ping adds identifier/sequence number fields to echo/reply packets sequence # allows you to see if packets lost ping will also do roundtrip timing,32,ping example,$ ping PING (): 56 data bytes 64 bytes from icmp_seq=0 time=8ms 64 byte

26、s from icmp_seq=1 time=8ms 64 bytes from icmp_seq=2 time=20ms - PING statistics - 3 packets transmitted, 3 packets received, 0% loss round-trip (ms) min/avg/max = 8/12/20 (MS cmd = ping),33,More ping,So what do you learn? you can route to destination end systems ip

27、 stack is working at least round trip time information are packets being lost (but doesnt tell you why) Echo reply sent by end systems ICMP, you dont know if upper layers are working.,34,traceroute,% traceroute traceroute (a command) allows you to determine the routers from one end to another Uses ICMP ttl exceeded and (UDP port unreachable or ICMP echo reply) messages (2 forms of implementation) (MS = tracert),35,traceroute example,% traceroute (from sirius.cs.pdx.ed