1. tunneling works through a network - what about between networks

   1. also, what about tunneling overhead

   2. also, what about lame systems without enough hardware or software - think hand-helds and non-computer computers

   3. also, what about security (what about it?)

2. inter-network box terminology

   1. boxes between (or among) homogeneous networks - connectors

   2. boxes between (or among) heterogeneous networks - gateways

   3. layer-based naming

      1. physical, data-link, network connectors - repeaters, bridges, routers

      2. transport, session, presentation, application gateways

3. inter-network function

   1. translation - between dissimilar networks

   2. selection - among more than two networks

   3. translation may include selection - when to translate

   4. selection may include translation - three dissimilar networks; trivial selection

   5. selection is either hard an unscalable (centralized) or very hard and expensive (distributed) - avoid

   6. assume translation only with trivial or no selection

4. gateways translate between dissimilar clouds

5. application-level gateways do so at user space

   1. move mail between smtp and x.400 systems

   2. move ip packets between x.25 and appletalk clouds

   3. interoperability among dissimilar network endpoints

6. application gateway design and implementation

   1. gateways work by re-encapsulation or translation

   2. re-encapsulation pulls the payload out of one network pdu and stores it in another network pdu - the payload is unchanged

   3. translation mutates one application pdu into another

   4. re-encapsulation is simpler to design than translation, which can be hard

   5. gateways can be easier and to implement than can re-encapsulation

   6. translation can generate better traffic than can re-encapsulation

   7. homogeneous endpoints can use re-encapsulation; heterogeneous endpoints need to use translation somewhere

   8. gateway concurrency is important - buffering between different networks

7. gateways vs tunneling

   1. gateway translation can replace tunneling - ip packets morph

   2. user-space development

   3. newness encapsulated in the gateway

   4. the n-by-m problem; hardware intensive

   5. tunneling can provide homogeneity in the network, diversity at the endpoints

8. gateways and intermittent connectivity - the network is evanescent

9. gateways and security - gateways in the dmz; usually uses some tunneling to get into the local network

10. gateways may double the hop-count - when does this configuration make sense

11. examples

    1. cgi

    2. email to ftp, http - this is like tunneling

       1. the local client sends a request via e-mail

       2. the remote server gateway receives the e-mail, runs the request, and e-mails back the reply

       3. the local client receives the reply and perhaps un-encapsulates it

    3. slirp - a gateway between two ip networks

       1. why do this - network address translation

This page last modified on 12 April 2001.