Transcript IP Addressing, Subnetting and ARP

Network Layer
NETS 3303/3603
Week 4
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Problem: Link Delay Test
• Develop a UDP-based client/server system
to test the round-trip delay (RTD)
• PDA is chosen to be the server, which
passively open a well-known port
• Upon receiving an array of bytes, it just
echo’s the bytes
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Link Client
• Gets the host to connect and
number of link probes to send from
command line
• Create a serialised object with
current time and send to server
using ObjectOutputStream
• Waits for echoed object from server
• To find link RTD
public class Timestamp implements Serializable
{
private long time;
public Timestamp()
{
time = System.currentTimeMillis();
}
public long getTime()
{
return time;
}
public String toString()
{
return new Long(time).toString();
}
– Extracted object’s time is subtracted
from current time
}
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while (probes > 0) {
dSocket = new DatagramSocket();
time = new Timestamp();
// object to send!
bos = new ByteArrayOutputStream();
oos = new ObjectOutputStream(bos);
oos.writeObject(time);
mBuff = bos.toByteArray();
outPkt = new DatagramPacket(mBuff, mBuff.length, host, PORT);
dSocket.send(outPkt);
inPkt = new DatagramPacket(mBuff, mBuff.length);
dSocket.receive(inPkt);
ois = new ObjectInputStream(new ByteArrayInputStream(inPkt.getData()));
try
{
time = (Timestamp)ois.readObject();
}
catch (ClassNotFoundException e) {}
System.out.println("RTT is => "+ (System.currentTimeMillis()- time.getTime())");
probes--;
}
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Test Output
$ java LinkRttClient
Enter host name:
pda-wifi
Enter required probes:
10
RTT is => 2105 ms
RTT is => 43 ms
RTT is => 31 ms
RTT is => 56 ms
RTT is => 34 ms
RTT is => 57 ms
RTT is => 32 ms
RTT is => 56 ms
RTT is => 33 ms
RTT is => 69 ms
Terminating link delay test...
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Lesson Outline
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•
•
•
•
intro
IP addresses
subnetting
routing/algorithms/architecture
ARP
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Fundamental, IPv4
• fundamental TCP/IP protocol
• RFC 791, other related RFCs
–
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Inet checksum, rfc 1071, 1141, 1624
path mtu, rfc 1191
ip datagram reassembly, rfc 815
rfc 1122, communications
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Fundamental idea
• ip implements an ip logical network on top
of different kinds of network technologies
where ip address is endpoint
• hw is hidden by network layer (except for a
few things like MTU)
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what does IP do (and not do?)
• sends and recvs packets to/from ip addresses - ip
datagrams
• no retries, doesn’t promise reliable delivery
– packets due to various reasons may be lost, duplicated,
delayed, delivered out of order, or corrupted
• best effort - don’t lose them on purpose but only
when nets busy => resources unavailable
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IP functions
• route packets
– routing: process of determining path for data
– ip routes packets when they come from
• transport layer (down stack)
• link layer (up stack) - we are router and forward pkts
• fragmentation accrd. to link-layer MTU
• handle ip options
• send/recv ICMP error and control messages
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IP address
• 32 bits, “dotted-decimal” notation
– 1.2.3.4, big-endian byte order, 0..255
is range
– associated with interface, not
machine
• if machine > 1 i/f, then multihomed
– if multi-homed, not necessarily router
• ip address in UNIX assigned to i/f with
#ifconfig ed0 inet 131.253.1.2 netmask 255.255.255.0
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Example Of Dotted Decimal
Notation
• A 32-bit number in binary
– 10000000 00001010 00000010 00000011
• The same 32-bit number expressed in dotted
decimal notation
– 128 . 10 . 2 . 3
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IP address structure
• each address has structure in it: (network, host)
• Host may be divided further into (subnet, host)
• subnet mask used to determine subnet part
– operation: ipaddress & subnet mask
– (more later)
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IP Address Conventions
• When used to refer to a network
– Host field contains all 0 bits
• Broadcast on the local wire
– Network and host fields both contain all 1 bits
• Directed broadcast: broadcast on specific
(possibly remote) network
– Host field contains all 1 bits
– a packet is sent to all computers on a network
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Limited Broadcast
• All 1’s (255.255.255.255)
• Broadcast limited to local network only (no
forwarding)
• Useful for bootstrapping
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IP address problems
• assigning class by first bits means class A
takes 1/2 of range, class B 1/4, class C
1/8, etc.
• problems with this setup
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class assignment is wasteful
ip host addresses not necessarily utilized well
too many networks in core routers
running out of ip addresses ??
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Question
• How can we minimize the number of
assigned network prefixes (especially class
B) without abandoning the 32-bit
addressing scheme?
• Subnet addressing
• Proxy ARP (later)
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Subnetting
• subnet - use single IP network address to
hide multiple physical nets
• subnet notion converts (net, host) into
slightly more hierarchical
(net, subnet, host)
• associate subnet mask with i/f ip address
• Example, class B, one byte of subnet: ip
= 148.1.1.0 subnet=255.255.255.0
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Choice Of Subnet Size
• How should host portion of address be divided?
– Depends on topology at site and number of hosts per
network
• Each physical network is assigned 32-bit address
mask
• One bits in mask cover network prefix plus zero or
more bits of suffix portion
• Logical and between mask and destination IP
address extracts the prefix and subnet portions
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Subnetting
subnetting functions:
• 1. you can subnet an ip address and split it up on
separate networks across routers (conserve
address space)
• 2. you hide your routing structure from remote
routers, thus reducing routes in their routing tables
if (dest ip addr & subnet mask) == (my ip addr & subnet mask)
dest is on same subnet
else
different subnet (send pkt to router)
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Example Network
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Fixed-length Subnet Masks
• Organization uses same mask on all networks
• Advantages
– Uniformity
– Ease of debugging / maintenance
• Disadvantages
– Number of nets fixed for entire organization
– Size of physical nets fixed for entire organization
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IP encapsulation
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IP Header
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Routing
• routing - the process of choosing a path over
which to send datagrams
• hosts and routers route
• input: ip destination address
• output: next hop ip address and internally an
interface to send it out
• routing does not change ip dest address
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How configure routing table
• static routes - by hand, on unix with
% route to_dest via_next_hop
• dynamically via routing protocol daemon,
routed or gated on UNIX,
protocols=RIP/OSPF/BGP
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View routing table
• unix host
– % netstat -rn
• n is for NO dns, else you may cause DNS queries
• Linux
– % route -n
• cisco router
– (router) show ip route
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Routing table
• entries logically (destination, mask, via gateway,
metric/s)
• destination - network or host address
• mask - subnet mask for dst address
• via gateway - next hop (maybe router)
• metric/s - depends on routing table algorithm and
dynamic routing protocols
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SOME possible kinds of routes
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host, 210.1.3.21/32 (to specific host)
subnet, 131.253.1.0/24 (to specific subnet)
network, 131.253.0.0/16 (to specific net)
default route - normally the router on a net, send it
here when nothing else matches
– expressed internally as 0.0.0.0
• note: host route to default route – most specific to
least specific
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Manual route entries
• on FreeBSD unix host:
% route add default 204.1.2.3
(default route)
% route add 1.1.1.1 2.2.2.2
• 2.2.2.2 is the next-hop router for 1.1.1.1
• we must have direct connection to 2.2.2.2 (i/f must
be on same subnet and must exist)
% ifconfig ed0 2.2.2.1 (our i/f must exist)
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ARP, The problem
• problem: how does ip address get mapped to
ethernet address?
• 2 machines on same enet can only communicate if
they know MAC/hw addr
• Applications only use Internet addresses
• solutions:
– configure addresses by hand (ouch!)
– encode in IP address (48 bits in 32?)
– dynamic mapping
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Consequence
• Protocol software needs a mechanism that
maps an IP address to equivalent hardware
address
• Known as address resolution problem
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Dynamic Binding
• Needed when hardware addresses are large (e.g.,
Ethernet)
• Allows computer A to find computer B’s hardware
address
– A starts with B’s IP address
– A knows B is on the local network
• Technique: broadcast query and obtain response
• Note: dynamic binding only used across one
network at a time
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ARP
• rfc 826
• host A, wants to resolve IP addr
B,
– send BROADCAST arp request
– get UNICAST arp reply from B
• ethernet (or MAC) specific,
although protocol designed to be
extensible
• implemented in driver, not IP
• intended for LAN
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Refinements
• Cannot afford to send ARP request for each
packet
• Solution
– Maintain a table of binding
• OS will cache arp replies in arp cache (ip ,
MAC, 20 minute timeout)
– don’t need to do arp on every packet
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% arp -a (SunOs)
# arp -a
banshee.cs.pdx.edu (131.252.20.128) at 0:0:a7:0:2d:a0
pdx-gwy.cs.pdx.edu (131.252.20.1) at 0:0:c:0:f9:17
longshot.cs.pdx.edu (131.252.20.129) at 8:0:11:1:44:68
walt-suncs.cs.pdx.edu (131.252.21.2) at 8:0:20:e:21:25
walt-cs.cs.pdx.edu (131.252.20.2) at 8:0:20:e:21:25
connor.cs.pdx.edu (131.252.21.179) at 0:0:c0:c5:57:10
dazzler.cs.pdx.edu (131.252.21.132) at 8:0:11:1:12:82
sprite.cs.pdx.edu (131.252.21.133) at 8:0:11:1:12:e7
(DNS name,ip address,Ethernet address)
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Arp command, functions
• ping someone and learn MAC address
• for debugging
• delete out of date ARP entry (you changed
the IP address, and you don’t want to wait,
OR somebody mucked up)
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ARP header
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Header details
• header format is not fixed, somewhat dynamic
(not used though)
• hw type, ethernet == 1
• protocol type, ip = 0x800
• hwlen, 6 (MAC), plen 4 (ip)
• operation: (used by rarp too)
– 1: arp request, 2: arp reply
– 3: rarp request, 4: rarp reply
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More Details
• sender hw addr, 6 bytes
– the answer, if reply
• sender ip: 4 bytes
• target hw address: 6 bytes
– 0 in request
• target ip: 4 bytes
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Proxy ARP
• Allow two physical networks to share a
single IP prefix
• Arrange special system to answer ARP
requests and forward datagrams between
networks
• Hosts think they are on same network
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Proxy ARP pros, cons
• pros
– same network numbers
– transparent to hosts
– no change in IP routing tables
• cons
– does not generalize to complex topology
– can drive you nuts -- debugging
– not simple and not secure
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Summary
• IP is a best-effort network
• Main IP functions
– Routing, fragmentation, some error-handling
• Subnetting provide hierarchy => CIDR!
• ARP maps IP to hardware address
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