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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

– Extracted object’s time is subtracted from current time

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();}

}

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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: 10RTT is => 2105 msRTT is => 43 msRTT is => 31 msRTT is => 56 msRTT is => 34 msRTT is => 57 msRTT is => 32 msRTT is => 56 msRTT is => 33 msRTT is => 69 ms

Terminating link delay test...

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Lesson Outline

• intro• IP addresses• subnetting• routing/algorithms/architecture• ARP

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Fundamental, IPv4

• fundamental TCP/IP protocol

• RFC 791, other related RFCs– 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 multi-homed– 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 DecimalNotation

• 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– 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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Subnettingsubnetting 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

elsedifferent 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

• 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 -abanshee.cs.pdx.edu (131.252.20.128) at 0:0:a7:0:2d:a0pdx-gwy.cs.pdx.edu (131.252.20.1) at 0:0:c:0:f9:17longshot.cs.pdx.edu (131.252.20.129) at 8:0:11:1:44:68walt-suncs.cs.pdx.edu (131.252.21.2) at 8:0:20:e:21:25walt-cs.cs.pdx.edu (131.252.20.2) at 8:0:20:e:21:25connor.cs.pdx.edu (131.252.21.179) at 0:0:c0:c5:57:10dazzler.cs.pdx.edu (131.252.21.132) at 8:0:11:1:12:82sprite.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