PILC BOF -- Orlando IETF

[Mark Allman <mallman@lerc.nasa.gov>]

[I appologize if you receive more than one copy of this.]

A quick note to announce a BOF that will be held at the IETF meeting
in Orlando.  This BOF will be to gauage the interest in (and scope
of) possibly creating an IETF working group to document performance
implications of various less-than-ideal link characteristics.
Below is the BOF description.  At the bottom are instructions for
getting on the PILC mailing list.  Please direct all discussions of
the upcoming BOF to the PILC list.  Thanks!

allman




BOF Description:

Name: Performance Implications of Link Characteristics (PILC)
Chairs: Aaron Falk and Mark Allman 
        adalk@mail.hac.com, mallman@lerc.nasa.gov


Description:

The Internet network-layer and transport-layer protocols are
designed to accommodate a very wide range of networking technologies
and characteristics.  Nevertheless, experience has shown that the
particular properties of different network links can have a
significant impact on the performance of Internet protocols
operating over those links, and on the performance of connections
along paths that include such links.  Some examples of possibly
problematic characteristics:

	- Long delay links.  Affects timer estimation, congestion
	  adaptation. 

	- Links with high bandwidth-delay product.  Difficult to
	  fully utilize and still respond to congestion.

	- Links with variable bandwidth-delay products (such as
	  multi-channel ISDN, or LEOs).  Difficult to track how much
	  throughput a connection should try to attain.

	- Links with varying delay. Delays due to link level
	  signalling may affect the accuracy of RTO estimation.

	- Links with link layer flow control.  May have adverse
	  interactions with congestion control.

	- Links with asymmetric bandwidth.  Can lead to throughput
	  limitations such as ACK starvation.

	- Links with unusually high error rates.  Can lead to loss
	  of performance due to inappropriate perception of
	  congestion.  Can impair protocols that do not protect
	  their data with strong checksums.

	- Links with inconsistent error rates.  Can defeat attempts
	  to distinguish between congestive loss and corruption
	  loss. 

        - Links with significant monetary, resource-scarcity, or
	  delay cost for establishing connections and/or keeping
	  connections open.  These encourage not only
	  bandwidth-efficient protocols, but also protocols that
	  cluster their packet exchanges together rather than
	  spacing them out in time, and can increase the cost of
	  using standard soft-state mechanisms such as periodic
	  state refresh messages.

	- Significantly low bandwidth links.  Creates pressure for
	  mixing together otherwise separate elements (such as
	  transport and network layers, or multiple transport
	  connections) in an effort to reduce bandwidth requirements
	  via compression.

	- Unidirectional.  Can make link-layer request/response, such as ARP,
	  impossible.  (UDLR already addresses the routing elements
	  of this.)

	- Non-transitive reachability (A can reach B, and B can reach C,
	  but A can't reach C).  Breaks assumptions about subnet
	  properties.

	- Shared-channel broadcast to huge numbers of receivers.  Such
	  links stress the notion of using them for unicast traffic
	  and the viability of protocols that require a backchannel
	  such as IGMP.  Such links also incur a danger of major
	  broadcast reply implosions.

	- Links that reorder packets.  Can erroneously trigger TCP fast
	  retransmission.  Impairs protocols that adapt based on
	  timing analysis.  Can stress reassembly code if not well
	  tuned.

	- Multipathing.  Can complicate protocol adaptation since there
	  is no longer a single path property to estimate, but more
	  than one.  Diminshes efficiency of header compression
	  algorithms.

	- Links with intermittent outages.  Timers may fail to adapt
	  to outage and either falsely signal lost connectivity, or
	  back off so far that response once the outage resolves is
	  highly delayed.

	- Small MTU links.  Can lead to "black holes" if PMTU
          discovery mechanism fails.

This BOF will explore the size of this problem space: what sort of
link characteristics prove problematic, what sort of Internet
protocols are adversely affected?

One goal is to assess the utility of forming a working group to
produce informational document(s) detailing how link characteristics
interact with different IETF protocols - what well-established and
less-well-established steps can be taken to ameliorate these
problems.

Another goal is to assess the utility forming a working group(s) to
develop modifications or extensions to IETF protocols to address the
most pressing performance issues.

Note that the BOF focusses on link *characteristics* and not on link
layer *technologies* such as Frame Relay, ATM or Ethernet.
Discussing particular characteristics present in some of these
(e.g., capture effect) is in scope; discussing aspects of the
technology as a whole (e.g., CIR) is out of scope.

A mailing list for discussing the issues related to this BOF has
been setup.  Notes to the list should be sent to
pilc@lerc.nasa.gov.  To subscribe to the mailing list, send a note
to majordomo@lerc.nasa.gov with the words "subscribe pilc" in the
body of the note.  The mailing list archive is available at
http://pilc.lerc.nasa.gov/pilc/.