VLANs and QoS

• 2/25/2009

Domains

• collision domain
  • bounded by L2 device, like a switch
• broadcast domain
  • bounded by L3 device, like a router

Areas

• WAN
  • "other" side of the router
• LAN
  • broadcast domain
• VLAN
  • LAN without geographical constraints (assuming this is technically possible)

802.1Q

• IEEE VLAN standard is 802.1Q
• Q-BRIDGE-MIB from RFC 4363 (obsoletes 2674)
• our position on VLANs

Layer 2 header, Ethernet

• maximum 1518 bytes total
• destination MAC address
• source MAC address
• frame type
• payload

Layer 2 header, 802.1Q

• 4 bytes larger, maximum 1522 bytes total
• 2 octets for Tagged Protocol Identifier, 0x8100
• 3 bits for priority, 802.1p
• 1 bit for Canonical Format Indicator
• 12 bits for VLAN ID, 802.1Q

Layer 2 header, 802.1Q

• 12 bits: VLAN IDs range from 1 to 4094
• 1 is DEFAULT
• 0 and 4095 are not allowed

802.1p priority

1. priority
2. immediate
3. flash
4. flash override
5. critical
6. internetwork control
7. network control

More About 802.1p

For more on 802.1p, I like Bringing Prioritization Services to Ethernet, then maybe Directing Your Network Traffic and Implementing Prioritization for IP Networks if you want to read more. It's really a pretty painless introduction to 802.1p.

VLAN Facts of Life

1. 802.1Q adds 4 bytes to Ethernet packets between the Layer 2 (Ethernet) header and the Layer 3 leader, increasing the maximum packet size from 1518 to 1522 bytes; 3 bits are for the 802.1p Ethernet class of service and 12 bits are for the 802.1Q VLAN ID.
2. A VLAN, by definition, provides broadcast containment. Broadcasts (Windows browser elections, Bonjour) are contained within the VLAN.

VLAN Facts of Life

3. Your IP address determines your IP subnet (of course). Your IP subnet uniquely determines your 802.1Q VLAN. Each VLAN can have several subnets, but each subnet has one and only one VLAN. If the VLAN on your switch port doesn't match your IP address subnet, you can't communicate on the network (except at Layer 2, but the user reports no network connectivity).
4. For traffic to travel between VLANs, it must go to the router port defined for that VLAN (your gateway). In the simplest case, it then goes through the routing engine and out the router port for the destination VLAN. (It could also go from the routing engine on one router to the routing engine on another router, and then out the router port for the destination VLAN.) Each router knows what subnets match the VLANs it manages, and it knows what router to send packets for the VLANs it doesn't manage.

QoS

• a short walk from VLANs to QoS
• quality
• Quality of Service for packet networks is the assurance that certain special packets will encounter fewer problems on the network than all other packets.
  • QoS processing may delay all packets, up to 10x latency
  • at most 10% of packets can be special
  • QoS must be end-to-end
• flip side: policing, or dropping packets
• CoS

QoS Categories

• number of priorities
• OSI layer
• hard (absolute) or soft (relative)
• static (management) or dynamic (signaling)
• bandwidth broker or policy server
• bandwidth control, prioritization, and/or traffic control
• network engineering or traffic engineering
• active (oversubscription) or proactive (overprovisioning)
• let's fill in this chart as we go!

Simple QoS

• more bandwidth
  • very common, but requires money
  • brute force, or enlightened overprovisioning?
• switch queuing
  • very tedious

QoS: IP ToS

• ToS octet
  • 3 bits for IP precedence
  • 4 bits for DTR˘ (least Delay, most Throughput, most Reliability, least ˘o$t: pick one)
  • 1 bit zero/unused

QoS: 802.1p

• 8 levels, like ToS
• what's different?

QoS: 802.1p priorities

1. priority
2. immediate
3. flash
4. flash override
5. critical
6. internetwork control
7. network control

QoS: DiffServ

• reuse ToS octet as DiffServ Code Point
• Class Selector, also like ToS
• Per-Hop Behavior
• 3 PHBs and 5 services out of 64 values!

QoS: DiffServ

• Assured Forwarding
  1. low drop probability
  2. medium drop probability
  3. high drop probability
• Expedited (explicit) Forwarding
• don't forget, Best Effort is a type of QoS (PHB)

Best Practices for DSCP

• CS, class selector: re-use the xxx000 values from IP precedence
  • don't rewrite existing "standards"
• AF, assured forwarding: classes 1 through 4, drop probabilities 1 through 3
• EF, explicit forwarding

Best Practices for DSCP

• DSCP table

QoS: IntServ

• uses a central router for admission control
• explicit reservation with RSVP, out of band signaling
• not used

QoS: SBS

• redundant network ...
• See Andrew Odlyzko: Paris Metro Pricing for the Internet and the economics of the Internet: Utility, utilization, pricing, and Quality of Service.

WAN

• what happens on another network?
• what happens with another network administrator?

MPLS

• DTR
• EOS
• MPLS' TTL
• other terms
  • Label-Switched Paths
  • Label Distribution Protocol

QoS Comparison

• chart

When to use QoS

• scope
  • external: identify issue, work with ISP
  • internal: identify, fix
• time
  • latency: find, upgrade
  • jitter: find, then upgrade or shape traffic
• congestion
  • always: add bandwidth
  • critical times: prioritize traffic
  • sometimes: plan for upgrade

Where to use QoS

• tagging: edge

• policing: as close to edge as possible

• expediting: core

Which QoS is Best?

• least intrusive
  • best-effort bandwidth
• most effective
  • 802.1p (expedited, not policed)
• most managable
  • 802.1p with 802.1Q VLANs

QoS Selector

QoS flowchart

QoS and Network Management

• Where does QoS fit in with Network Management?
  • Are they different subjects?

VLAN Reminders

1. Your IP address determines your IP subnet (of course). Your IP subnet uniquely determines your 802.1Q VLAN. Each VLAN can have several subnets, but each subnet has one and only one VLAN. If the VLAN on your switch port doesn't match your IP address subnet, you can't communicate on the network (except at Layer 2, but the user reports no network connectivity).
2. 802.1Q adds 4 bytes to Ethernet packets between the Layer 2 (Ethernet) header and the Layer 3 leader, increasing the maximum packet size from 1518 to 1522 bytes; 3 bits are for the 802.1p Ethernet class of service and 12 bits are for the 802.1Q VLAN ID.

VLAN Reminders

3. A VLAN, by definition, provides broadcast containment. Broadcasts (Windows browser elections, Bonjour) are contained within the VLAN.
4. For traffic to travel between VLANs, it must go to the router port defined for that VLAN (your gateway). In the simplest case, it then goes through the routing engine and out the router port for the destination VLAN. (It could also go from the routing engine on one router to the routing engine on another router, and then out the router port for the destination VLAN.) Each router knows what subnets match the VLANs it manages, and it knows what router to send packets for the VLANs it doesn't manage.

Example: DHCP

• remember that a user port has one and only one VLAN
• remember that your IP address (and netmask) → your IP subnet → your VLAN
• The end-user implications for this are that if you are a mobile computer user and take your laptop to/from one location to another on campus, you must be registered for and configured to use campus DHCP service. You should also be aware of the fact that, when you do go from one building to another, you must either do a complete shutdown or an "ipconfig /release", followed by an "ipconfig /renew" in order to get a new DHCP lease that's appropriate for the routed domain to which you've moved.

Light Reading

• "Jittery about jitter?" and "Looking forward by looking backward" (Scott Bradner)
• "Measure for measure: Making metrics matter" and Light Reading message board post (David Newman)
• "The real meaning of zero loss" (Kevin Tolly)
• Nagle's Algorithm and Delayed ACKs and It's the Latency, Stupid (Stuart Cheshire)

Reading

• Stuart's Law of Networkdynamics: For every Network Service there's an equal and opposite Network Disservice by Stuart Cheshire is highly recommended! It's the longer version of the homework reading.
• Content is not king by Andrew Odlyzko is excellent.

Homework

• All of this reading material is background for the next class!
• Read It's the Latency, Stupid because I will discuss it in the next class, and even assume you understood it.
• quick questionnaire