slides11 Flashcards
what happens If the destination is not on the local network
send the packet to a gateway host and let it deal with where to send it next
A gateway is just a machine on more than one network
information needed to allow routing to happen
- its own address
* the address of a gateway machine
how is hopping to another machine which is not the destination machine handled header-wise?
In the non-local case, the packet is going to the gateway, so we would need to ARP for the hardware address of the gateway
The packet, with IP address of the final destination, is put into a frame with Ethernet address of the gateway
Since the packet needs to go to the gateway
This is another reason why we need both hardware and software addresses
The IP address is for the ultimate destination; the hardware address is for the next hop
EX Is ARP needed on a PPP connection?
PPP def: In computer networking, Point-to-Point Protocol (PPP) is a data link layer (layer 2) communications protocol used to establish a direct connection between two nodes. It connects two routers directly without any host or any other networking device in between. It can provide connection authentication, transmission encryption,[1] and compression.
answer: ARP should come before PPP as PPP only links two physical things and doesn’t care about IPs
how does ARP connect 2 machines
It broadcasts an ARP Request packet (protocol number 0806) in an Ethernet frame with destination hardware address ff:ff:ff:ff:ff:ff and source its own Ethernet address
(GATEWAY CASE: The packet, with IP address of the final destination, is put into a frame with Ethernet address of the gateway since the packet needs to go to the gateway)
All hosts on the local network read the frame
The target host recognises the request for its IP address
The target sends an ARP Reply packet containing its own Ethernet address (the other hosts need do nothing)
It knows the source’s Ethernet address as read from the request packet
The source gets the reply and reads out the target’s Ethernet address. It can now use that Ethernet address to send IP packets
which part of the ARP header might be incomplete
- In a request the destination hardware field is not filled in as this is what we are trying to find!
- In a reply the sender Ethernet address is the address we seek
what is gratuitous ARP
when someone joins, useful if he sends arp so all know and all update their cache so they all fast as fuq boi
ARP Bridging
If host h1 wishes to send to host h2 it must determine its hardware address (as it is on the “same” local network)
So h1 does an ARP broadcast for h2
The bridge sees this request and responds on behalf of h2 (a proxy ARP), but it supplies its own hardware address b1
Now h1 sends data to what it thinks is h2, but is actually the bridge
The bridge reads the packet, sees it is destined for h2 (by its IP address) and forwards it to the other network where h2 can read it
Furthermore, it rewrites the forwarded frame’s header to have h2 as destination and b2 as source
If h2 replies, it can either use h2 which it got from the original packet or do an ARP request, which the bridge proxies in a symmetrical way
In either case the packet goes to the bridge, which forwards it to h1, again rewriting the frame addresses appropriately
This is all transparent to h1 and h2 who believe they are on the same network
If h1 is communicating with both h2 and h3 its cache will show them to have the same hardware address b1: this is not a problem
VIRTUAL
And a common variety is 802.1q virtual bridging More commonly called Virtual LANs (VLANs)
This is a kind of reverse of the ARP bridge: it allows more than one network to run on a single physical network
EX bridge, gateway, switch, router
a bridge is a product that connects a local area network (LAN) to another local area network that uses the same protocol
A gateway joins two dissimilar networks. There can be a lot of protocol conversion work to do.
A switch, when compared to a bridge, has multiple ports. Switches can perform error checking before forwarding data
Routers support different WAN technologies but switches do not.
EX structure of a VLAN tag
Tag protocol identifier (TPID) Tag control information (TCI) Priority code point (PCP) Drop eligible indicator (DEI) VLAN identifier (VID)
EX when we add a tag we increase the size of the ethernet packet and it might go out of bounds. WAT WE DO
With the added 802.1Q tag the maximum frame size is 1522 bytes and as such would be seen as corrupt by all default configured switches and network adapters. To avoid tagged frames from being silently dropped at some point the network administrator must carefully make sure that switch ports where tagged frames might arrive is prepared for the larger frames and does allow processing of the 802.1Q vlan tag.
RARP
Reverse ARP has been replaced by BOOTP and later DHCP but Inverse ARP is solely used for device configuration. Inverse ARP is enabled by default in ATM(Asynchronous Transfer Mode) networks. InARP is used to find Layer-3 address from Layer-2 address