Transcript IP datagrams and datagram forwarding

IP datagrams
Service paradigm, IP datagrams,
routing, encapsulation,
fragmentation and reassembly
Internet service paradigm
• TCP/IP supports both connectionless and
connection-oriented services
– fundamental delivery service is connectionless
at the Internet layer
– optional reliable connection-oriented service is
layered on top of this at the transport layer
IP datagrams
• Packets of data are sent across multiple
physical networks via routers
• Internet protocols define a universal virtual
packet - the IP datagram
• The amount of data carried in a datagram is
not fixed and is determined by an application
Routers and routing tables
• Each router forwards a virtual packet by using
a local routing table
• Each entry is:
– destination address
– mask
– next hop
• IP address of a router or
• Deliver direct
• Then does address resolution
Example routing table
Best-effort delivery
• IP attempts best effort delivery and does not
guarantee to deal with:
– datagram duplication
– delayed or out of order delivery
– corruption of data
– datagram loss
• These issues are dealt with other protocol
layers
IP datagram header format
Encapsulation
• When an IP datagram is sent across a
physical network it is placed in the data
area of a frame and the frame type is set to
IP
MTU and datagram size
• Maximum transmission unit - max of data
that a frame can carry on a given network
• A packet may have to cope with different
MTU sizes as is passes over an internet
Fragmentation
• A datagram that is larger than MTU is
fragmented into smaller datagrams
Reassembly
• Is done at the final host
– routers require less state information
– fragments can take different routes
• Header fields indicate when the data is a
fragment and also where it belongs
• Whole datagram is lost if any fragment is lost
The Future of IP (IPv6)
Motivation for IPv6, Addressing,
Datagram Format, Paths
Motivation
• IP has been extremely successful at coping
with the expansion of The Internet and
changes in network hardware over 20 years!
• However:
– limited address space will soon run out
– new application requirements
• real-time audio and video require guaranteed service
• collaboration technologies require ways of sending
packets to groups of hosts
What is in a name?
• The current IP is IPv4
• The new version was originally called IP The Next Generation (IPng), but this
became associated with several proposals
• The final proposal is called IPv6
Key features of IPv6
• Connectionless like IPv4
• 128 bit address size
• Different addressing modes: unicast, multicast
and cluster
• Extension headers
• Support for audio and video
Three types of address
• unicast - address corresponding to a single
computer. Datagram sent along shortest path
• multicast –
–
–
–
–
address corresponding to a set of computers,
members can change at any time.
one copy of a datagram is delivered to each
only one copy passes over intervening networks
used for collaborative applications
• Cluster
– address corresponds to a set of computers that
share a common prefix
– a datagram is delivered to one of these
– used for replicating a service
Writing down IPv6 addresses
• Replaces dotted decimal notation with more
compact colon hexadecimal
105.220.136.100.255.255.255.255.0.0.18.128.140.10.
255.255 =>
69DC:8864:FFFF:FFFF:0:1280:8C0A:FFFF
• Zero compression further reduces space
FF0C:0:0:0:0:0:0:B1 => FF0C::B1
• Especially useful because an IPv6 address that
begins with 96 zeros contains an IPv4 address in
the last 32 bits
Datagram format
• Datagram format includes a base header
and optional extension headers
– saves space - a typical application will only use
a few IPv6 facilities
– the protocol can be extended to support new
features without being redesigned
Base Header
Paths
• Applications can be used to set up network
paths in advance
• These can be associated with different
traffic classes that provide different Quality
of Service (QoS)
• Necessary for real-time audio and video
Examples of Collaborative Applications
• 􀁺 Collaborative Virtual Environments (CVEs)
– Shared 3D virtual world
– Each user controls own viewpoint
– Interaction with objects
– Users represented by avatars
– Communication through embedded audio,
video, text and graphical gestures
CVE and network traffic
• In a CVE each user may be an active sender as
well as a receiver of various kinds of
information
• Many users may send data at the same time
• There may be hundreds of users
• As a result, CVEs can generate large volumes
of network traffic