CMPT 471 Networking II Address Resolution IPv4 ARP RARP 1© Janice Regan, 2012

CMPT 471Networking II

Address Resolution

IPv4 ARP RARP

1© Janice Regan, 2012

Transport Layer: Socket view

© Janice Regan, 2012 2

An application on one host wishes to send some data to a particular application on another host

The applications use the transport layer interface (sockets) to transfer the data. Each application has a communication

endpoint, specified by a (port descriptor, IP address) pair

The source and destination port descriptors (addresses) are included in the transport layer header that encapsulates the application’s data in one or more segments

Transport Layer: data path Conceptually we are sending the data from one

application to the other in the transport layer In reality the data is actually passed down the

protocol stack of the source host, through the network and up the protocol stack of the destination host

Let’s review how this actually happens


Transport Layer: encapsulation

The data from the transport layer is encapsulated A transport layer header is constructed The header is prepended before the application data The transport layer header includes port addresses on

source and destination hosts. The port address indicates the process the data

belongs to on the source host and the destination host. The transport layer header and the data form a

transport layer segment. The transport layer segment is passed down to the

network layer to become the data in the IP datagram© Janice Regan, 2012 4

Network Layer: encapsulation

The transport layer segment is encapsulated by adding a network layer header The segment and the header form a IP datagram The IP header includes source and destination IP

addresses and other information necessary for the hop by hop transport of the datagram through the network

Next, the IP stack will process the datagram using the forwarding algorithm and the routing table to find the IP address of the next hop destination.

the IP datagram and the next hop address to which the datagram should be sent must be passed to the data link layer

© Janice Regan, 20125

Addressing: Data Link Layer We will need the Ethernet address of the present

router (source) and the next hop destination (destination) to use in the MAC header

Both the present router and the next hop destination will be attached to the same physical network segment

We know the Ethernet address of the present router (where the packet is now)

ARP is the protocol that allows us to determine the physical address (for example Ethernet address) of the next hop destination host given the internet address of that host


Addressing: Data Link Layer


How a packet is delivered

In the physical layer the MAC layer frame is encoded and transmitted

The internet interface (network card) on each machine has a MAC address For Ethernet the address is associated with a

unique Ethernet address (48 bit address) at the time of manufacture

Other types of LAN (using different MAC protocols) have their own types of addresses that may have different lengths from the Ethernet address


How a packet is delivered

Each network interface watches the network and can intercept packets addressed to its own address or the broadcast address of the net (also multicast packets)

The sender must know the MAC address of the receiver before it can send anything. How does it find the receivers MAC address? The problem of finding the MAC address is called Address Resolution


Using ARP ARP (address resolution protocol)

facilitates mapping of IP addresses to physical addresses for all hosts on the LAN (exceptions discussed later)

Since packets traveling in the network will be delivered using MAC addresses, the sending host needs to determine the MAC address corresponding to know IP before it can send the packet


Using ARP The host will keep a table called the ARP

cache that will contain mappings from MAC to IP addresses

When the host wishes to send a packet it will look in the ARP cache for an entry corresponding to the desired destination host’s IP address. If there is an entry it will send to the indicated

MAC address If no entry exists an new entry must be added to

the ARP cache© Janice Regan, 2012 11

Sending a packet to an IP address

© Janice Regan, 2012 12From the TCP/IP Guide

Entries in the ARP cache Entries in the ARP cache are static or dynamic.

Static entries are entered by the administrator and stay in the cache until removed

Dynamic entries are added to the cache when the local host tries to send a packet to a destination host on the LAN

When a dynamic entry is added to the cache a timer is set to indicate the lifetime of that entry When the two hosts communicate, each host updates

(reinitializes) the timer corresponding to the entry for the other host in its own ARP cache

When the timer expires the entry is removed


Why use Dynamic Entries MAC addresses of hosts may change over time

Replacement of a network card (interface) will result in a changed MAC address

Interface may no longer be reachable Host may be turned off or disconnected Interface may be disabled Interface may fail

New hosts/interfaces may be added to the network

Cache should not grow forever as new hosts are added


Using Dynamic Entries Must be able to add new entries and

remove old ones Must have a mechanism for deciding

which entries to keep and which to discard

Must keep the size of the cache reasonable


Using Dynamic Entries The detailed handling of when timers are

set and when entries expire and are removed is implementation dependent For Linux there are two significant timers one

indicates when an entry becomes stale, one indicates how often stale entries are removed

If the host for the entry is in use by other parts of the OS its entry will not be removed until it is no longer in use


Adding new entries -ARP cache If no entry in the ARP cache corresponds to the IP

address of the destination of the packet to be sent the packet will be queued or dropped and an ARP request will be sent

The ARP request is broadcast to the LAN All hosts on the LAN receive the request The host with IP address corresponding to the

destination IP address of the packet Will respond with a ARP reply unicast to the source Will add/update the entry for the source to its ARP

cache


Operation: ARP protocol

© Janice Regan, 2012 18Comer 2000: fig 5.1

ARP/RARP Message Format


ARP/RARP Message Format For an ARP request Target Hardware address

(HA) is left blank When the request is received the receiver will

updates/add to its ARP cache Construct and send an ARP reply package.

The sender HA and IP from the request become the target HA and IP in the reply. The target IP from the request becomes the sender IP for the reply and the MAC address of the receiver becomes the sender HA


Sample OPERATION field values



Sample HARDWARE field values

ARP Message Information ARP can be used by many MAC layer

protocols not just Ethernet Different protocols have different address

lengths (48 bits for Ethernet) The protocol type field indicates the MAC

protocol being used. The HLEN field indicates the length of the

hardware address for that protocol© Janice Regan, 2012 23

Gratuitous ARP Sometimes done as a host boots An ARP request containing the IP of

the booting host will be sent If a reply is received then someone

else is also using the IP of the booting machine and the booting machine should choose another IP


What is Proxy ARP Proxy ARP allows multiple

networks/network segments to share a single network address

ARP relies on broadcast requests. The broadcast address for a network (LAN) will have a limited scope.

In particular if a LAN includes segments connected by routers each segment will generally have its own broadcast address


What is Proxy ARP Since broadcast packets are not

forwarded by routers ARP requests will not travel between LAN segments connected by routers

If the segments are parts of a larger network (subnets of a larger network) we may want ARP requests to travel through the router

This is when Proxy ARP is used© Janice Regan, 2012 26

Operation of Proxy ARP Consider two segments connected by a router. A MAC frame will travel along one segment to

the router, the host sending the frame does not know the destination host is on a different segment

An ARP request from a host on segment 1 will reach the router (on segment 1 and 2) but not any host on segment 2 because broadcast packets are not forwarded


Operation of Proxy ARP If the server supports proxy ARP it will

send an ARP reply indicating its own hardware address for any IP address that is on segment 2

Later packets address to the host on segment 2 will be sent to the hardware address of the router. The router will forward them along segment 2 to the destination.


Proxy Arp


From the TCP/IP Guide

RARP Reverse address resolution protocol is designed to

allow a host to determine its own IP address This is usually an issue at boot time, when a diskless

host may not know its own IP address The host sends a RARP request to a RARP server

asking for its IP. The IP is supplied in the RARP reply RARP has been largely superseded by other protocols

Other protocols (BOOTP, DHCP) return more information than just the hardware address, increasing efficiency

Cannot be used which use dynamic assignment of hardware addresses


Operation of RARP


Operation of RARP


Documents

CMPT 471 Networking II Address Resolution IPv4 ARP RARP 1© Janice Regan, 2012