IPv6 Datagram Format and Transition

IPv6 Datagram Format

• 27 fixed-length 40-byte header with no fragmentation allowed
• Priority: identifies priority among datagrams in a flow
• Next header: identifies upper layer protocol for data
• Ver: identifies the version of the IP protocol
• Pri: indicates the priority of the datagram
• Flow label: identifies a specific flow of datagrams
• Hop limit: specifies the number of hops the datagram can travel
• Payload length: specifies the length of the payload
• Next header: specifies the next header in the datagram
• Source address: 128-bit address of the source
• Destination address: 128-bit address of the destination
• Data: the actual payload of the datagram

Transition from IPv4 to IPv6

• Not all routers can be upgraded simultaneously
• No "flag days" for transition
• Tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers
• IPv4 header fields: IPv4 source and destination addresses
• IPv6 header fields: IPv6 source and destination addresses
• UDP/TCP payload: IPv6 datagram carried as payload in IPv4 datagram

Network Layer

• Interplay between routing and forwarding
• Routing algorithm determines end-end-path through network
• Routing algorithm determines local forwarding table
• Forwarding table determines local forwarding at this router
• Graph abstraction: represents network as a graph of nodes and edges
• Graph abstraction: costs associated with each link

ICMP

• Internet control message protocol
• Used by hosts and routers to communicate network-level information
• Network-layer "above" IP
• Error reporting: unreachable host, network, port, protocol
• Echo request/reply (used by ping)
• ICMP messages carried in IP datagrams
• ICMP message format: type, code, plus first 8 bytes of IP datagram causing error

Traceroute and ICMP

• Source sends series of UDP segments to destination
• First set has TTL = 1, second set has TTL = 2, etc.
• When nth set of datagrams arrives at nth router, router discards datagrams and sends source ICMP messages (type 11, code 0)
• ICMP messages include name of router and IP address

DHCP

• More than just IP addresses
• Can return address of first-hop router for client, name and IP address of DNS server, and network mask
• Address allocation: gets allocated portion of its provider ISP's address space
• Hierarchical addressing: allows efficient advertisement of routing information