CS519: Computer NetworksLecture 1 (part 2): Jan 28, 2004Intro to Computer NetworkingCS519Remember this picture?| How did the switch know to forward some packets to B and some to D?CS519From the address in the packet header…| A packet has a header and a bodyz and, sometimes, a trailer| The header says:z Where the packet is going (address)z How big the packet is (length)z Some other stuffCS519PacketsCS519Like an envelope?| The address field is somewhat analogous to the address on an envelopez And the contents of the envelope would then be like the packet body| But this analogy doesn’t work for the length field!CS519Forwarding Table| Routers (or switches) have a forwarding tablez Router is a forwarding box that operates on IP packets| This table is indexed by the address in the header, and tells which next hop to send the packet to| Addresses can be hierarchical (like phone numbers)CS519Forwarding TableCS519Forwarding tables and routing algorithms| How did the forwarding table get there?| Typically a routing algorithm is run among the routers, and this algorithm establishes the contents of the forwarding table| In this class, we’ll look in detail at address structures and routing algorithmsCS519Two kinds of links| Routers and hosts in the Internet are typically connected by two types of links| We’ve been looking at pictures of point-to-point links| The other common kind is the broadcast linkz Usually EthernetCS519Point-to-point and broadcast linksCS519Point-to-point and broadcast linksCS519Point-to-point and broadcast linksCS519Broadcast link (Ethernet)| Well, N packets are “seen”, not really received| The Ethernet hardware filters out packets that are not for “self”z By examining the Ethernet address| The operating system (OS) never sees the packet (no packet interrupt)| Though Ethernet does have multicast and broadcast addressCS519Ethernet addresses and IP addresses???| As you all know, the Internet is a network of networksz That’s why its called the Internet| This introduces the concepts of:z Interfacez EncapsulationCS519Next hop and interface (and logical interface!)| Next hop is the next router on the path to the destination hostz Or may be the destination host itself| Interface is the input/output port over which the next hop can be reachedz May be physical (an actual wire)z Or logical (multiple interfaces on an actual wire)CS519Next hop and interface (and logical interface!)CS519So the router has another table (neighbor table)CS519Router packet forwarding procedure:| Look up dest IP address in received packetz Obtain Next Hop router (its IP address)| Look up Next Hop router in the Neighbor Table z (with a pointer from the forwarding table entry)z Obtain iface (interface) and “link” address of Next Hop router| Encapsulate IP packet in link packet and send over ifaceCS519Router packet forwarding procedure:CS519Protocol LayersCS519Packets (revisited)CS519But, what is “where”?| To an Ethernet, “where” is an Ethernet portz Ethernet address| To the Internet, “where” is a host computer on a networkz IP address| To a host computer, “where” is a processz TCP or UDP port| To a process, “where” may be a filez HTTP URLCS519A stack of headers| To deal with all these “wheres”, a packet in fact contains a stack of headers:CS519A stacked header requires one more field: “next header”Sometimes combinedCS519Header stack as protocol services| Except for the physical layer protocol, protocol peers communicate with each other by talking to a lower layerz HTTP peers use TCP, TCP peers use IP, etc.| We say that each protocol provides a service to the layer above itz Often there is a service interface that defines the serviceCS519Protocol servicesCS519Services as a protocol graphCS519Services as a protocol graphCS519EthernetExample Microsoft VPN stackApplicationTCPPPPL2TPUDPIPsecIPIPPPPPPPoEThe link layerA logical link layerA security layerA network abstraction that Microsoft finds convenientThe actual end-to-end network and transport layersA tunnelCS519EthernetExample Microsoft VPN stackApplicationTCPPPPL2TPUDPIPsecIPIPPPPPPPoETCP: Transport Control ProtocolIP: Internet ProtocolPPP: Point-to-Point ProtocolL2TP: Layer 2 Tunneling ProtocolUDP: User Datagram ProtocolIPsec: Secure IPPPPoE: PPP over EthernetCS519Protocol layers revisitedCS519Summary of lecture 1| Packet networks are more flexible than circuit networksz But have “QoS” issues of delay (latency), dropping, and jitter (versus blocking for circuit networks)| Fancy queuing can help, but ultimately traffic sources have to slow down to avoid congestionCS519Summary of lecture 1| Delay has three components, queuing, propagation, and transmit| Large Delay x Bandwidth Productpipes are becoming more common| Packets have headers that tell wherethe packet is going, and how long it is (among other things)CS519Summary of lecture 1| Routers have forwarding tables that select the next hop in a path to an addressz And neighbor tables that tell which interface and link address to use to get to the next hop| Encapsulation is used to get the IP packet from one router to another over a linkCS519Summary of lecture 1| Protocols are layered, with each layer providing a communications service to the layer above| The layering is complex, with tunnelsthat allow protocols to be layered over themselves| IP is a special layer at the waist of the Internet hourglass.CS519Next Lecture: IP| Because of IP’s special position in the Internet, it seems reasonable to start with IP, then work down and