PPPoE vs. DHCP for Internet Access

Requirements

The requirements for Internet connectivity management are broadly speaking as follows.

• CPE authentication and authorisation.
• Allocation of system parameters (DNS, local gateway addresses; STB/SIP configuration).
• IP address allocation and lease management.
• Session management (VC set-up, integrity, tear-down; QoS and service management).
• Recording of time-connected and bytes-transmitted for billing.

Point-to-Point Protocol over Ethernet (PPPoE)

Coming from an established protocol family, PPPoE makes certain things easy.

• Username/password authentication using PAP/CHAP/EAP.
• Allocation of IP address, e.g. from RADIUS.
• The establishment of a virtual circuit with session-id, providing fine-grained link management.
• Easy to apply lawful intercept.

The disadvantages are:

• Maintenance of substantial per-session state at the BRAS/Broadband Network Gateway (BNG).
• Difficulties of rapid failover (given this state).
• Needs additional and more sophisticated protocols for service management and configuration.

DHCP

DHCP is more lightweight and is a more natural fit to Ethernet access networks. It also provides more sophisticated mechanisms for IP address assignment and device configuration.

Circuit authentication can be implemented via DHCP option 82 where the CPE attached-circuit identifier (at the DSLAM or Ethernet Aggregation Switch - EAS) is sent to the DHCP server. This is better than trusting the MAC. An alternative is to use the combination of CPE VLAN tag + EAS stacked VLAN tag if these also uniquely identify the CPE.

An issue with DHCP is the lack of a convenient session ‘keep-alive’ mechanism. PPP has echo-request/echo-reply protocol units exchanged when data is not being sent. This can swiftly indicate if a circuit has failed, or if the user has disconnected – important information both for service management and billing.

Although not part of the PPP protocol, a BRAS/BNG will typically report to the RADIUS server both time-connected and bytes-transferred at session disconnect. This cannot be achieved purely by DHCP (the IP address lease mechanism is far too coarse-grained). Alternative mechanisms such as the persistent web-page/captive-portal concept of public WiFi networks have to be employed, at least until a suitable mechanism for DHCP is standardised.

Conclusion

PPP excels at session authentication and management, at the expense of costly and cumbersome state management in the BRAS/BNG.

DHCP is considerably lighter-weight and more powerful from a service configuration and management point of view, but requires the integration of additional protocols for authentication and fine-grained session management.

Penally Abbey

Just back after a few days at the Penally Abbey hotel, just a couple of miles west of Tenby. The outside temperature hovered around 1-2 degrees Centigrade, and there was a bitter east wind, particularly the second day. We were wrapped up like astronauts.

The beach at Tenby after Sunday Mass

The headland east of Manorbier

Mysterious words on the sand (click on picture to enlarge)

Clare at the castle at Carew

Nigel at Carew - dressed up like an astronaut.

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A Christmas Cold

It's Christmas day and I am the proud possessor of a cold.

It began yesterday with a certain mental lethargy as I was reviewing the pros and cons of PPPoE vs. DHCP (alternative methods of providing Internet access - if this means nothing to you, ignore it, but be warned that there'll be a write-up here sometime next week).

As the afternoon progressed a sore throat kicked in and my self-diagnosis became definitive. I went public and thereby permitted myself the general hang-dog persona of the somewhat-ill. A certain degree of sympathy was consequently elicited.

I am a keen self-observer of the cold, noting that the sore throat phase is the most unpleasant, but the running nose phase more protracted and irritating.

Today I was hoping for the in-between transition where symptoms subside, and one merely feels slightly euphoric and odd, but no, the sore throat has persisted.

And so we are under the regime of Lemsip, but unlike Andrew Motion in the Wikipedia reference, I haven't noticed any acceleration of creativity...

Update on Shadow

Time for a seasonal update on our new cat Shadow.

For the last couple of weeks it's been confined to the house as we struggled to figure out how to install a catflap. Rejecting expensive solutions through the double-glazing on the back door or tunnels through the kitchen brickwork, we eventually settled on a DIY catflap through the front door (pictured). Never mind that this is generally considered to have lowered the tone of the entire little group of houses to which we are privileged to belong.

The new catflap in the front door

Notice how much light it adds to the hall.

Shadow himself currently reacts with paranoid fear to any attempt to coax him to the scary outside. Opinions differ amongst we humans in the house as to how much persuasion and how much coercion to apply.

Shadow resting

A persuasive approach is to put the litter just outside the back door (pictured below) and then close the kitchen door. He is then meant to do what he has to do, and find his way back into the front of the house through the catflap.

Litter training

On our first trial of this strategy, it took him two hours to figure out how to do this, and the litter remained undisturbed. In fact the real objective is to 'wean' him off litter altogether, relocating him to the new sand area which Clare dug for him at lunchtime today (pictured below) at the back of the garden.

The target objective

At present rate of progress this is surely a project for 2009.

I should say that when Shadow is not being 'persuaded' to use the new catflap, or to encounter the outside world, he remains a contented and happy creature, albeit one addicted - day and night - to duvets.

Carols at Salisbury Cathedral

It should have been Winchester this year, but the tickets were snapped up far too quickly, so for the second year in a row, Alex, Clare and myself were down to Salisbury yesterday evening for the Carol Service.

The Spire of Salisbury Cathedral from the Cloisters

We passed the stall selling mulled wine (£2.75) and mince pies (50p each) and waited for almost an hour in the queue (pictured below) whch stretched all around the cloisters.

The queue waiting to get in

Inside there were seats aplenty, and eventually the Cathedral was full.

The audience faces the altar just before the service begins

The singing was as beautiful as always.

A Walk on Kimpton Down

Kimpton Down is just north of Thruxton.

Clare and Alex walking up Warren Hill

Alex taking in the view

Two deer seen in the distance

A 'wild west' image of horses feeding on straw

A Very Short Guide to VLANs

More correctly, this note is about the use of VLANs in carrier Open Access Metropolitan Ethernet Networks. An Open Access network is an access network which is independent of any particular national carrier, and which provides last mile Ethernet services in its area on a non-discriminatory basis.

VLANs - virtual LANs - were originally a method of partitioning enterprise LANs into distinct forwarding segments. A VLAN switch forwards on both the normal Ethernet destination address (DA) and the VLAN tag. A switch associates a VLAN with a specific port so that a VLAN-tagged frame can only be forwarded along a port which has enabled that tag. There is space for 4,094 different usable tags in the 12 tag-bits available (see diagram).

VLANs have local significance within a routed domain, so that if a network is partitioned into a number of layer-2 segments each connecting to its own port on a PE router (which interconnects them all at layer-3) then each of the segments can reuse the 4,094-sized tag space.

VLAN (Q-in-Q) Ethernet header

In an equal access metro network, there is a requirement to separate traffic on a per-service (VoIP, Internet, IPTV), per-user (= the final switch port facing the customer premises) or in the most fine-grained mode, a separate VLAN per-service per-user. In this case we have almost recreated a point-to-point virtual circuit.

The reason for VLAN segregation is predominantly stability and security. Broadcast traffic, e.g. ARP messages, are constrained within a VLAN thereby avoiding uncontrollable broadcast 'storms'. A positive security advantage is that broadcast traffic from one carrier's customers are not accessible by customers of rival carriers. Also, the possibilities of malicious attack are reduced.

If customers are already using VLAN tags themselves in their own enterprise networks, then carrier VLAN separation may be implemented by a second, overlaid tag. This is called "Q-in-Q" after the 802.1Q standard defining VLANs.

VLAN Q-in-Q can also be used in the metro network as a way of managing a proliferation of user-specific VLAN tags - a form of tag aggregation similar to the VC-VP usage in ATM. However, since all forwarding is done on the combination of destination address (DA) and outermost VLAN tag, scalability resulting from Q-in-Q is limited.

Getting Ethernet to the levels of manageability and scalability expected of a carrier transport layer remains a work in progress.

A Very Short Guide to Pseudowires

Pseudowires are used by carriers and Service Providers to transport layer 2 services (such as Ethernet and SONET/SDH traffic) edge-to-edge across their packet-switched (IP or MPLS) networks. So a pseudowire (PW) provides an emulated service.

The pseudowire construct - more properly Pseudo Wire Emulation Edge-to-Edge (PWE3) - is an architecture more than a protocol suite. The implementation of PWE3 doesn't, for example, necessarily need MPLS, although that's the usual implementation.

RFC3985 tells us that "The required functions of PWs include encapsulating service-specific bit streams, cells, or PDUs arriving at an ingress port and carrying them across an IP path or MPLS tunnel. In some cases it is necessary to perform other operations such as managing their timing and order, to emulate the behavior and characteristics of the service to the required degree of faithfulness."

The pseudowire definition includes higher-layer service adaptation and convergence functions which are service specific. When MPLS is the underlying packet transport network, a four byte control word may be used to carry this information (see diagram below - 'Flags, Frag, etc').

The pseudowire protocol also includes a 'pseudowire demultiplexer' (also below - 'PW Label') which specifically identifies the individual PW. A number of different PWs may be carried over the same label-switched path (LSP), which then operates as a kind of pseudowire 'trunk'.

PWE3 over MPLS

In most state-of-the-art networks, PWE3 will be carried between MPLS edge devices and will be carrying Ethernet traffic (or possibly TDM).

Pseudowires can be set up manually (through OAM&P), or via signalling using LDP or BGP.

The Day the Earth Stood Still

We were down to the Andover flicks this evening to see "The Day the Earth Stood Still" featuring Keanu Reeves as the humanity-exterminating-but-somehow-chic alien Klaatu.

Keanu has the extreme Green position that humans are causing a global biocide, and for the sake of galactically-precious-biosphere-preservation, must be removed from the scene forthwith .

The idiotic American military do their militaristic best to buttress this possibly-shaky premise, but the heroic female scientist and her adopted trans-racial child convince Keanu - sorry, Klaatu but where's the difference? - that there is hope yet for human redemption.

Redemption appears in the end to be the complete and utter destruction of technology, which would reduce the Earth's carrying capacity for humankind to maybe a few million at most. Let the die-off commence!

The CGI special effects were beautifully wonky.

We loved it!
____
Note: No angular momentum was unconserved in this film.

Note: No cliche was left unturned in this film. For example, it is well known that 'biologist Lewis Thomas was asked what message he thought should be sent to outer space aliens. He answered, "the complete works of J.S. Bach". And then he added as an afterthought, "But that would be boasting"' (here).

Inevitably then, as Keanu is in the home of the distinguished research scientist who is trying to impress him that we're worth saving, what is playing loudly in the background but the Goldberg Variations. The female scientist says helpfully "It's Bach".

Wonderful!

Viking Longship Painting

Clare has now finished her painting, previously posted here.

A Viking Longship

Offers in excess of £1 million please email to me!