- when multiple networks are connected together called internetwork or internet
How Networks Differ
- networks differ in many ways
- differences can be somewhat papered over
- gateway connecting networks can generate separate packets for each destination in lieu of better network support for multicasting
- QoS is difficult to ensure between multiple networks
- security difficult as well but encryption can be layered on top of one another
How Networks can be Connected
- build devices that translate packets, or add a layer of indirection and build a common layer on top of different networks
- TCP/IP protocol is foundation of modern internet
- Example
- 802.11, MPLS, and Ethernet networks want to be connected
- between MPLS and 802.11
- virtual circuit must be set up because 802.11 is connectionless, MPLS is connection oriented
- between vc and ethernet
- packet may be too large to be carried, so packet must be divided into fragments
- routers that can handle these different protocols is called a multiprotocl router
- translates protocols or leave the connection for a higher protocol layer
- higher level requires all networks implement that application example TCP
Tunneling
- encapsulating an IPv6 packet through IPv4
- used to connect isolated hosts and networks
- overlay on the network
- limitation of tunnels is an advantage with VPNs (Virtual Private Networks)
- overlay used to provide a measure of security
Internetwork Routing
- two level routing algorithm
- within each network an intradomain, interior gateway protocol is used for routing
- across networks use interdomain or exterior gateway protocol
- often referred to as BGP(Border Gateway Protocol)
- AS(Autonomous System)
- network is operated independently
- i.e. ISP network
- routing policy
- routing based on political agreements
Packet Fragmentation
- Limitations on packet size
- Hardware(size of a frame)
- Operating System(all buffers are 512 bytes)
- Protocols (the number of bits in packet length field)
- Compliance(to a standard or political agreement)
- Desire to reduce error induced retransmissions
- desire to prevent packets from occupying channel for a long time
- Max payload is 1500 bytes for ethernet
- 2272 bytes for 802.11
- Path MTU(Path Maximum transmission unit)
- know protocols from source to destination send packets small size
- MTU discovery, send packet length, if not possible return with try smaller size that is acceptable packet, until it reaches destination
- if MTU not possible break up into fragments
- first fragmentation strategy
- oversize packet arrives, router breaks it up, addressed to exit router where it is recombined
- end of packet bit provided because reassembly may not be in order
- second fragmentation strategy
- split up and recombine only at destination not at each transmitted router
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