Pseudowire Hashing

1
Pseudowire Hashing

• Vach Kompella
• Joe Regan
• Shane Amante
• IETF 71

2
Problem Statement

• A PW can carry many different flows
• How can the multiple PSN paths in a network be
  exploited to balance the flows?

3
PW Hashing Fields

• At ingress L2 fields are available to choose
  among the first hop multi-paths
• At LSR Not determinable what is available
• Heuristic for IP look for first nibble 4/6 for
  IP
• No heuristic for L2 traffic because no protocol
  type available
• Incoming packet has label stack with tunnel encap
  and PW label
• Not enough to provide second hop diversity

4
Example

• Ethernet PW
• Typical hash fields are MAC SA/DA, ethertype,
  VLAN
• PSN supports ECMP
• Ingress only hashing gives the following paths
• Four available paths

D
C
A
B
E
F
5
Example

• Ethernet PW
• Typical hash fields are MAC SA/DA, ethertype,
  VLAN
• LAG between A-C, C-D, and D-B supports ECMP
• Ingress only hashing gives the following paths
• Four available paths

D
C
A
B
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Solution (Ingress)

• A and B exchange a range of labels called Hash
  Labels, H0 Hn
• For traffic from A to B
• A hashes L2 fields to figure out nexthop C or E,
  say C
• A hashes L2 fields to n buckets, gets index k
• A determines PW label, say PWL for this
  Attachment Circuit
• A determines PSN encap to C, say tunnel label TL1
• A pushes PWL, Hk, TL1 and sends to C

TL1
Hk
PWL
Payload
7
Solution (LSR)

• C receives label stack (TL1, Hk, PWL).
• C hashes on label stack to determine next hop
• With Hk varying over the flows of the PW, this
  will not be a single next hop
• C swaps tunnel label TL1 to TL2, leaving rest of
  label stack in place
• Alternatively, if C has a LAG in its PSN, then C
  can hash on the label stack to pick a link in the
  LAG group

TL2
Hk
PWL
Payload
8
Solution (Egress)

• B receives label stack (TL2, Hk, PWL).
• B pops TL2
• B pops Hk (action for all Hash Labels is pop
• B identifies PW using PWL and processes packet

TL2
Hk
PWL
Payload
9
Benefits of the Approach

• Ingress holds only one set of labels per peer
• Egress distributes only one set of labels
• Extensible to PSN level also
• Provides hashable information that is
• stable per flow at intermediate nodes
• uses MPLS methods
• requires no changes to CE
• minimizes the labels allocated and programmed per
  node
• Allows label stacking of Hash labels for more
  diversity
• I know that some P routers dont hash this way
  (order of labels) and I would like to understand
  if we should switch the order of labels

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Is this Extensible to PSN?

• The goal is to adapt this method for the PSN
• Avoid looking below label stack at LSRs
• General thoughts
• A, C, C, D, D, B exchange Hash labels
• A, E, E, F, F, B exchange Hash labels
• C, F, E, D exchange Hash labels

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Ingress PSN Behavior

• For PSN traffic from A to B
• A hashes incoming fields to figure out nexthop C
  or E, say C
• A hashes incoming fields to n buckets, gets index
  k
• A determines PSN encap to C, say tunnel label TL1
• A pushes TL1, Hk and sends to C

Hk
TL1
Payload
12
PSN LSR Behavior

• C receives label stack (Hk, TL1).
• C hashes on label stack to determine next hop D
  or F
• With Hk varying over the flows of the PSN, this
  will not be a single next hop, say D
• C hashes incoming label stack to n buckets, gets
  index m
• C pops Hk
• C swaps TL1 to TL2
• C pushes Hm, TL2 and sends to D

Hm
TL2
Payload
13
Questions

• Placement of Hash Label
• Before the PW label
• Leave PW label processing to the end
• Avoid switching on PW label at S-PE
• Before the Tunnel label
• Must process hash label before swapping label
• Avoid switching on tunnel label at LSR
• Existing implementations?
• Is Hash Label exchange a FEC or just a
  Capabilities exchange?
• Can we do the same for diversity for BGP label
  exchanges?
• Should this be done in mpls wg?
• Could this be used to explore PW paths like
  lsp-ping multi-path?