l4 IP, Forwarding, and Switch Fabrics

l4 IP, Forwarding, and Switch Fabrics - Lecture 4 IP...

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Unformatted text preview: Lecture 4: IP, Forwarding, and Switch Fabrics Overview • Internet Protocol (v4)- What it provides and its header- Fragmentation and assembly • IP Addresses- Format and assignment: class-A, class-B, CIDR- Mapping, translation, and DHCP • Packet forwarding, circuits, source routing • Switch fabrics • Bisection bandwidth Internet Protocol Goal • Glue lower-level networks together R2 R1 H4 H5 H3 H2 H1 Network 2 (Ethernet) Network 1 (Ethernet) H6 Network 4 (point-to-point) H7 R3 H8 Network 3 (FDDI) The Hourglass, Revisited … FTP TCP UDP IP NET 1 NET 2 NET n HTTP NV TFTP Internet Protocol • Connectionless (datagram-based) • Best-effort delivery (unreliable service)- packets are lost- packets are delivered out of order- duplicate copies of a packet are delivered- packets can be delayed for a long time IPv4 packet format 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1 2 3 hdr len vers TOS Total Length Identification Fragment offset DM TTL Protocol hdr checksum Source IP address Destination IP address Options Padding F F Data IP header details • Routing is based on destination address • TTL (time to live) decremented at each hop (avoids loops)- TTL mostly saves from routing loops- But other cool uses . . . • Fragmentation possible for large packets- Fragmented in network if crosses link w. small frame size- MF bit means more fragments for this IP packet- DF bit says “don’t fragment” (returns error to sender) • Following IP header is “payload” data- Typically beginning with TCP or UDP header Example Encapsulation Application data Transport header IP header Link layer header Sending Receiving IPv4 packet format 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1 2 3 hdr len vers TOS Total Length Identification Fragment offset DM TTL Protocol hdr checksum Source IP address Destination IP address Options Padding F F TCP or UDP header TCP or UDP payload Other IP Fields • Version: 4 (IPv4) for most packets, there’s also IPv6 (lecture 12) • Header length (in case of options) • Type of Service (diffserv, we won’t go into this) • Protocol identifier (UDP: 17, TCP: 6, ICMP:1, why is TCP earlier?) • Checksum over the header • Let’s look at a packet with wireshark Fragmentation & Reassembly • Each network has some maximum transmission unit (MTU) • Strategy- Fragment when necessary (MTU < size of Datagram)- Source host tries to avoid fragmentation When fragment is lost, whole packet must be retransmitted!- Re-fragmentation is possible- Fragments are self-contained datagrams- Delay reassembly until destination host- Do not recover from lost fragments Fragmentation example H1 R1 R2 R3 H8 ETH FDDI PPP IP (376) PPP IP (512) PPP IP (512) (512) ETH IP ETH IP (512) ETH IP (376) IP (1400) IP (1400) R1 R2 R3 • Ethernet MTU is 1,500 bytes • PPP MTU is 576 bytes- R2 Must fragment IP packets to forward them Fragmentation example (continued) • IP addresses plus ident field identify fragments belonging to...
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l4 IP, Forwarding, and Switch Fabrics - Lecture 4 IP...

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