Internetworking Outline Best Effort Service Model Global Addressing Scheme

IP Internet Concatenation of Networks Protocol Stack

Service Model 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

IP Packet Format

The IPv4 header consists of 20 byte fixed part and a variable length option part. Version indicates whether IPv4 or v6. IHL indicates the total header length in the unit of 32-bit words, so the maximum header length is 60 bytes. TOS used for QoS. Length is in Bytes, so maximum IP packet size is 65,535 bytes. Identification field is the packet number. All fragments of a packet have the same identification value.

DF bit indicates to routers that they should not fragment packets DF bit indicates to routers that they should not fragment packets. All machines are required to accept packets of 575 bytes at least. MF bit indicates whether it is a continuing or end of packet fragment. Fragment offset field indicates where in the datagram this fragment belongs in the unit of 8 bytes. TTL counts the number of hops a packet has gone through. When it hits zero, the packet is discarded. Protocol field indicates the upper layer protocol the packet belongs to, such as TCP or UDP. Header checksum field verifies the header only. It has to be recomputed at each hop since at least one header field, TTL, changes at each hop

Fragmentation and Reassembly Each network has some MTU Design decisions fragment when necessary (MTU < Datagram) try to avoid fragmentation at source host re-fragmentation is possible fragments are self-contained datagrams use CS-PDU (not cells) for ATM delay reassembly until destination host do not recover from lost fragments

Example Ident = x Start of header Rest of header 1400 data bytes Offset = 0 (b) 512 data bytes 1 Offset = 64 376 data bytes Offset = 128

IP Address Properties Dot Notation globally unique hierarchical: network + host Dot Notation 10.3.2.4 128.96.33.81 192.12.69.77 IP address is managed by Internet Corporation for Assigned Names and Numbers (ICANN), which delegate parts of the address space to regional authorities, which in turn dole out IP addresses to ISPs and other organizations.

Class-based address

Special IP addresses

Subnets An organization is often assigned a block of address with the same network number, e.g., a class B address. The problem is how to divide the block into multiple subnetworks within the organization. The solution is to divide the host portion of IP address into subnet part and host part.

A class B network subnetted into 64 subnets. The netmask is: 255.255.252.0

Routing Table A routing table include the following entries: Remote network address with all 0s in the host part, which point to output lines going to different networks. One network per entry. Local network with complete host part, which point to output lines going to different hosts on the local subnet. One host per entry. Unknown address, which point to an output line going to the default router. One entry only. Using subnet mask, given an IP address, it is a simple matter of bit-wise AND to figure out the network and host parts. Using subnets, all hosts on a subnet is represented by one entry in the routing table.

Datagram Forwarding Strategy Example (R2) Network Number Next Hop every datagram contains destination’s address if connected to destination network, then forward to host if not directly connected, then forward to some router forwarding table maps network number into next hop each host has a default router each router maintains a forwarding table Example (R2) Network Number Next Hop 1 R3 2 R1 3 interface 1 4 interface 0

Address Translation Map IP addresses into physical addresses destination host next hop router Techniques encode physical address in host part of IP address table-based ARP table of IP to physical address bindings broadcast request if IP address not in table target machine responds with its physical address table entries are discarded if not refreshed

ARP Details Request Format Notes HardwareType: type of physical network (e.g., Ethernet) ProtocolType: type of higher layer protocol (e.g., IP) HLEN & PLEN: length of physical and protocol addresses Operation: request or response Source/Target-Physical/Protocol addresses Notes table entries timeout in about 10 minutes update table with source when you are the target update table if already have an entry do not refresh table entries upon reference

ARP Packet Format TargetHardwareAddr (bytes 2 5) TargetProtocolAddr (bytes 0 3) SourceProtocolAddr (bytes 2 Hardware type = 1 ProtocolType = 0x0800 SourceHardwareAddr (bytes 4 TargetHardwareAddr (bytes 0 1) SourceProtocolAddr (bytes 0 HLen = 48 PLen = 32 Operation SourceHardwareAddr (bytes 0 ― 8 16 31

Internet Control Message Protocol (ICMP) Echo (ping) Redirect (from router to source host) Destination unreachable (protocol, port, or host) TTL exceeded (so datagrams don’t cycle forever) Checksum failed Reassembly failed Cannot fragment