North American Network Operators Group

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Re: IP-Internets

  • From: Alan Hannan
  • Date: Thu Oct 14 15:26:01 1999


  I thought Lucent and Bell Labs already knew everything! :-)

> 1- are there any all-IP backbones?  what is the layer 2 in IP 
> backbones (PPP, ATM, Frame relay...)?

  All networks which run IP are "all-IP" backbones.  Perhaps
  this definition means "A Layer 2 forwarding infrastructure
  with only IP traffic".  In this case, there are many.

  An RBOC FR network could be considered a backbone w/ partial
  IP, partial SNA, partial IPX, etc.

  Most all facilities-based ISPs run their IP networks on 
  infrastructures with dedicated L1 TDM or WDM bandwidth.

  In Marketing-ese many companies, perhaps mine, say they have a
  "pure IP network" -- where they mean that there is no ATM or
  FR in the middle, no SONET APS below that, such that all of
  the brains and intelligence is embedded in the IP stuffs.

  Most of us that use to think that was a really sexy idea now
  think MPLS is sexy, so we're calling MPLS the 'forwarding
  protocol for IP' and putting some intelligence into MPLS
  and some into IP; so as to get the proper blend of intelligence
  for constructing robust, scalable, resilient IP transport
  networks.  Except those wacky people at Qwest who just want
  to be different and are overtly influenced by brilliant
  yet obstinate swedes.

  All IP networks need a layer 2 framing protocol.  Most use
  ATM, Frame_Relay, HDLC or PPP.

------------------------ = ------------------------
wr1.sfo1#show int pos0/0
POS0/0 is up, line protocol is up 
  Hardware is Packet over SONET
  Description: WR2.SFO1-POS0/0-OC48
  Internet address is
  Encapsulation HDLC, crc 32, loopback not set
------------------------ = ------------------------

  Some stick MPLS in the middle ala IP/MPLS/HDLC or IP/MPLS/SONET.

  Additionally, IP/SONET may actually be IP/DWDM wherein the 
  Layer 2 framing protocol uses SONET frames but w/out BLSR APS.

> 2- how commonly is IP over ATM used? Do network operators really care
> about the  cell-tax?

  Fairly commonly, especially among 'mature established large backbones'.  
  Approximately 5 years ago, ATM switches had OC-12 interfaces while
  IP routers only had DS3 or partial-rate OC-3 interfaces.

  I'd estimate that 40-60% of Internet traffic traverses IP networks built on
  top of dedicated ATM networks.  This is shrinking significantly, with at
  least one large atm based network transitioning to IP/MPLS.

  Therefore the backbone trunks were done w/ ATM to provide interface
  capacity.  Since then perhaps people/systems have invested so much
  in them that it's hard to move away.
  Another benefit to using an ATM or FR infrastructure is the Traffic
  Engineering (TE) ability.

  Network operators care about the 'cell tax' but often one can 
  increase the overall efficacy of the network with TE to result
  in higher gains than are costed by the ATM 'cell tax'.

> 3- what is the approximate  number of connections to a
> router (i.e., fan-in, fan-out of a router (i) at the edge, (ii) at the
> core, and (iii) at the backbone?

  This is a function of design.

  In an ATM network, there will likely be 2 physical connections
  to the backbone ATM fabric, w/ N PVCs, in a full mesh, or truncated
  start topology.

  edge - usually 2 to the BB - 1 to 50,000 for customer aggregation
	 varies by design

  core - usually 2-64 backbone links.  most routers have 8-16
         line interface slots.  Each slot will support 1 - 4 
	 interfaces, kinda usually...
	 varies by design

  BB - see core, generally kinda the same.

> 4- what is the maximum distance between any two points in a autonm.
> system?

  I suppose this is infinite.  In practice, IP packets provide
  8 bits for the TTL, so a maximum 'diameter' is 2^8== 256 IP

  Note that IP hops are just that, Layer 3 opportunities for 
  Layer 3 forwarding decisions along w/ requisite TTL decrementation.

  These are often hidden, in networks with IP/ATM or IP/FR, and also
  sometimes with IP/MPLS.  This is psuedo analagous to not counting
  SONET switches, amplifiers, repeaters, monitors, etc...

> 5- what is the approximate ratio of copper/fiber
>    at the edge, core, backbone links?

  edge : 90% copper, 10 optical
  core:  usually inter-office cabling is mostly optical, be it
	 OC-n or gig-e.
  backbone: if BB ge OC-3; optical, if under OC-3, copper.

> 6- how tight is the physical space in the router rooms
> (i.e., is it almost a must to take out an old box in order to add one)?

  space is one of the limited resources that an ISP has.  It is
  generally a very significant issue.

  In many cases, 'silicon economics' allows folks to take out 2 boxes
  to put in one box that does 4 times as much.

  'forklift upgrades' are not always required.

  utopian designs provide 'buffer floor space' such that unused rack
  capacity is always available for new product insertion to avoid the
  need for 1:1 swap-outs during real time modifications.

> 7- in case of a node/link failure what is the average/approx period of
> time  for (i) detection and (ii) recovery

  depends.  SONET/APS is alleged to be 50ms.

  ATM and FR PVCs tend to re-route on the order of 1-5 seconds.

  IP convergence tends to re-route on the order of 30 seconds.

  MPLS tends to reroute today on the order of 30 seconds, but real
  soon now on the order of 1 second.