CSC/ECE 573Internet Protocols

ARP, RARP

Address Resolution Problem

Higher-level applications should work only with IP addresses illusion of a single, virtual network Communication is carried out by physical networks

IP datagrams are encapsulated in MAC frames MAC (hardware) addresses are needed

Address resolution problem: mapping high-level (IP) addresses to physical (MAC) addresses “Last net” problem

Solutions

1. Direct mapping: function f maps IP addresses to hardware addresses computation of f must be efficient works well when physical addresses are small and

configurable

2. Configuration file provides the mapping not scalable not easy to update

3. Dynamic binding: ARP (RFC 826) for broadcast networks such as Ethernet, Token Ring, etc. (why not just broadcast all packets?)

ARP vs. RARP

ARP Operation

ARP Packet Format

(Ethernet frame type= 0x0806)

(Reminder: Ethernet Frame Format)

Using ARP: Case 1

Using ARP: Case 2

Using ARP: Case 3

Using ARP: Case 4

ARP Cache

Sending n packets to destination x: n ARP Requests? Inefficient; one ARP Request per data packet! Better: cache ARP Replies Also, upon receipt of an ARP Request, all machines update

their cache entry for the sender of the request

Problems with caching cache space may be limited hosts move or change IP addresses solution: drop (invalidate) cache entries after “a while” (20

minutes)

Gratuitous ARP

Every machine (should) broadcast its mapping when it boots use ARP Request with own IP address as the

target IP address what should happen if an ARP Reply is received?

The arp Command

Used to query or update the ARP cacheForms arp hostname arp -a arp -d hostname arp -f filename arp -s hostname ether_address [ temp ] [ pub ]

[ trail ] P Flags P = Publish; ARP will respond to ARP requests for this address S = Static; not learned from the ARP protocol. U= Unresolved; waiting for ARP response. (M = Mapping; only used for multicast entries)

ARP Cache Contents Example

Net to Media Table

Device IP Address Mask Flags Phys Addr

------ -------------------- --------------- ----- --------------

le0 eos03ro.eos.ncsu.edu 255.255.255.255 00:d0:bc:ed:23:64

le0 perch.cacc.ncsu.edu 255.255.255.255 08:00:20:73:95:ab

le0 shrimp.cacc.ncsu.edu 255.255.255.255 08:00:20:90:a3:7f

le0 bass.cacc.ncsu.edu 255.255.255.255 08:00:20:73:98:c4

le0 multimedia 255.255.255.255 08:00:5a:8a:0c:79

le0 aphrodite 255.255.255.255 08:00:20:87:be:54

le0 artemis 255.255.255.255 08:00:20:87:bc:d4

le0 athena 255.255.255.255 08:00:20:21:87:4c

le0 BASE-ADDRESS.MCAST.NET 240.0.0.0 SM 01:00:5e:00:00:00

ARP Protocol Design

(Simplified, bare-bones ARP only; no error-checking, error-handling, packet validation, etc.)

Three modules: Output module Input Module Cache-control module

State of cache table entries: FREE, PENDING, RESOLVED

ARP Output Module (resolving an IP transmit request)

sleep until IP packet with destination IP address D is received

check cache table for entry corresponding to IP address D

if (not found)

create a queue, enqueue packet

create cache entry with state set to PENDING

send ARP Request, ATTEMPTS 1; return

if (found and state=PENDING)

enqueue packet; return

if (found and state=RESOLVED)

extract value of hardware address from entry

send packet; return

ARP Input Module(responding to ARP messages)

sleep until ARP packet (Request or Reply) arrives

check cache table for entry corresponding to this ARP packet

if (found and state=RESOLVED)

update the entry (why?)

if (found and state=PENDING)

update the entry

while (there are matching packets in the queue)

dequeue and send a matching packet

if (not found)

create an entry for this ARP packet

if (packet is an ARP request and I am the target)

send an ARP reply

return

ARP Cache-Control Module

sleep until periodic timer expires

for each entry in the cache table

if (state=FREE) continue

if (state=PENDING)

ATTEMPTS += 1

if (ATTEMPTS MAX_ATTEMPTS)

send ARP Request

else

change state to FREE; destroy queue

if (state=RESOLVED)

decrement time-out value by value of elapsed time

if (time-out 0)

change state to FREE; destroy queue

return

Proxy ARP

Proxy ARP

Maps single IP net prefix into two or more physical addresses

Router answers ARP requests for H3 or H4 from H1 or H2 with its own hardware address

Properties Assumes trusting relationship Simple: only requires changes to a single router Only works for broadcast networks

More details: see http://www.linuxdoc.org/HOWTO/mini/Proxy-ARP-Subnet/

Diskless workstations (Web PCs):

Small startup program in ROM startup program cannot contain machine's IP

address (why not?)

Startup program is used to obtain: machine's IP address IP address of file server and of nearest router initial boot image to execute

Reverse ARP

RARPOne or more RARP servers store IP addresses for hosts on their network

A diskless client uses physical network addressing to obtain its IP address from the server(s) RARP Request is broadcast RARP uses the same message format as ARP, except

Ethernet frame type= 0x8035 Op field = 3 for RARP request, 4 for RARP reply

Client repeats request if no reply is received how many times? how much delay (time-out) between retransmissions?

RARP (cont'd)

Primary RARP server provides mapping for many hosts (all diskless

machines in the network) changes message type to RARP Reply, sends

directly to client

Secondary RARP server(s) do not respond to first RARP Request respond to second copy of RARP Request

received within a short time randomly delay the reply to avoid collisions with

other non-primary servers

RARP Design and Implementation

ARP server simple, part of TCP/IP implementation, in the kernel

RARP server more complex, implemented as a user process implementation of RARP server is system-dependent

RARP Requests are transmitted as Ethernet frames with a specific type field (0x8035) the server must have a way of sending/receiving Ethernet

frames of this specific type

Drawbacks of RARP

Low-level protocol, difficult for application programmers to build a server

Supplies only one small piece of information

Cannot be used for dynamic IP address assignment

Newer protocols: BOOTP, DHCP

Also implemented as user (application) processes

Operate according to the client-server paradigm

Use UDP for message transfer (i.e., above UDP)

BOOTstraP Protocol (RFC 951, RFC1542)

Single message exchange but provides many items needed at startup, in addition to IP

address

Application uses the limited broadcast IP address (255.255.255.255) to broadcast the BOOTP request on the local net Multi-homed hosts issue a BOOTP request for each interface Source IP address = 0.0.0.0 (valid, used only during bootstrapping)

BOOTP server must also use limited broadcast for sending the reply (why?)

BOOTP “Relay Agents” permit a machine to contact a server on a non-local net

BOOTP (cont’d)

Server uses well-known UDP port 67, client uses well-known port 68

“Don’t Fragment” bit in IP header set To accommodate clients with limited memory which cannot

reassemble datagrams BOOTP reply is at least 300 bytes long

Client is responsible for ensuring reliable communication Timeout and retransmission of request

random timeout and doubling after each retransmission (why?) random delay before retransmission (why?)

BOOTP Message Format

Some BOOTP Details

Why is server name needed?

Encoding of options “TLV” = tag (1 byte), length (1 byte), value (n bytes)

Client should set Broadcast flag to 1 if it does not have a valid IP address already Otherwise, it should set client_address to it’s IP address

and be prepared to received datagrams at that address

BOOTP Relays

Clients ignore the “relay IP address” field

If a server decides to relay the request, it puts its incoming IP interface address in the “relay IP address” field and forwards to the BOOTP server

All BOOTREQUEST messages from a given client MUST be relayed to the same destination

BOOTP servers send BOOTREPLY messages directly to the relay agent identified in the ‘relay IP address' field.

All BOOTREPLY messages received by a relay agent are intended for BOOTP clients on its directly-connected networks

When a relay agent receives a BOOTREPLY message, it should examine the BOOTP ‘relay IP address', ‘your IP address', and ‘client hardware address' to deliver the BOOTREPLY message to the client

Dynamic Configuration

RARP, BOOTP designed for a relatively static environment manual configuration appropriate cannot dynamically assign values to individual machines

One problem: mobile clients

Another problem: need to reuse pool of IP addresses IP address pool may be small, pool of potential clients may

be large (e.g., dialup) automated mechanism needed

DHCP (RFC 2131)

Extends BOOTP to handle dynamic address assignment “leases” an address for a limited time (1 sec to 100 years) Backwards compatible with BOOTP clients, message format is the

same

The DHCP server Is given a set of addresses to manage

management algorithm is not standardized leases addresses to clients (for how long?) informs client of lease period, during which it will not lease same

address to another client at end of the lease period, the client must either renew or stop

using the address

DHCP Client State Diagram

INPUT

OUTPUT

+ configuration parameters