Routing Going from one network to the next

Where is routing handled? IP serves the functions of –routing –universal naming Identification is easiest if naming has uniformity Need a function/service to map IP addresses to MAC (e.g. ethernet) addresses Transport layer hands data to IP and lets IP worry about how to carry to the right machine

Thinking about the problem Consider the routing problem by identifying three possibilities for each message –(1) message is for itself –(2) message is for a directly connected machine –(3) message is for a machine on another segment Router AB C D Consider messages from A (1) (2) (3)

How does it know if it is for self? –Knows own IP How does it know if on own segment? –Compare own subnet with the subnet of the destination address How does it deal with other routes? –Often only one (DEFAULT) place to route info, (in previous case (R)) –If more than one router available, more tricky based on what’s behind the router –Router addresses frequently hard coded (1) (2) (3)

Where is routing in OSI? Consider TCP/IP instead Application Presentation Session Transport(TCP) Network (IP) Data Link Physical Data Link not used in TCP/IP Cooperate to handle routing although done at Network (IP) Consider only the Transport/Network/Physical layers

Typical path Application Transport Network (IP) Physical Application Transport Network (IP) Physical Network (IP) Physical Router AB C D

When for itself? A couple of cases exist: consider the simple case Application Transport Network (IP) Physical A To: From: Already There! Info

Someone on it’s segment AB (2) Application Transport Network (IP) Physical To: From: Info To: From: Info To:????? From:80:3:12:2:1:0 IP 80:3:12:2:1:0 What is the MAC level address? All packets at the physical level must have the ethernet address. How can A find out B’s address? Transport IP MAC(ethernet) Legend A

ARP to the rescue an IP level protocol Network (IP) Physical To: From: To: ff:ff:ff:ff:ff:ff From:80:3:12:2:1:0 ARP What’s your MAC address? 80:3:12:2:1:0 Network (IP) Physical To: From: To: ff:ff:ff:ff:ff:ff From:80:3:12:2:1:0 ARP What’s your MAC address? 4:7:10:2:7f:2 Network (IP) Physical To: From: To: ff:ff:ff:ff:ff:ff From:80:3:12:2:1:0 ARP What’s your MAC address? c0:3:2:21:1:0 A B Router NOT ME! To: From: What’s your MAC address? YES ME! What’s your MAC address? To: From: A sends the same request to all machines ARP

ARP to the rescue Network (IP) Physical To: From: To: 80:3:12:2:1:0 From: 4:7:10:2:7f:2 ARP 4:7:10:2:7f:2 80:3:12:2:1:0 Network (IP) Physical To: From: To: 80:3:12:2:1:0 From: 4:7:10:2:7f:2 ARP 4:7:10:2:7f:2 Network (IP) Physical ARP c0:3:2:21:1:0 YES ME! 4:7:10:2:7f:2 To: From: B responds to A A B Router ARP To: From: :7:10:2:7f:2

Someone on it’s segment CONTINUED AB (2) Application Transport Network (IP) Physical To: From: Info To: From: Info To: 4:7:10:2:7f:2 From:80:3:12:2:1:0 IP 80:3:12:2:1:0 Now A know B’s Mac address! Thanks to ARP. Continue with the message. A Application Transport Network (IP) Physical To: From: Info To: From: Info To: 4:7:10:2:7f:2 From:80:3:12:2:1:0 IP 4:7:10:2:7f:2 B Yes ME!

Router Someone on another segment C A Application Transport Network (IP) Physical To: From: Info To: From: Info To: c0:3:2:21:1:0 From:80:3:12:2:1:0 IP 80:3:12:2:1:0 Network (IP) Physical To: From: Info To: From: Info To: 4:7:10:2:7f:2 From:80:3:12:2:1:0 IP c0:3:2:21:1:0 Application Transport Network (IP) Physical To: From: Info To: From: Info To: ba:5:2:7:6:4 From: 40:3:7:21:1:0 IP ba:5:2:7:6:4 To: From: Info To: ba:5:2:7:6:4 From: 40:3:7:21:1:0 IP 40:3:7:21:1:0 Not for ME! For ME! Router A C

Things to note (previous slide) The IP addresses were consistent across the transmission (END-TO-END) The ARP process between Router C has not been shown APR caches responses to save time later Router only functions at IP and below if not intended for Router. If for Router instead of C, IP would be Router’s Router has 2 interfaces->2 protocol stacks Protocol stack is actually a tree structure Ethernet indicates if IP or ARP or … going up.