For end to end physcial links:
1. Delay
2. Jitter
3. Loss
For TCP traffic:
1. TCP RTT (Round Trip Time)
2. TCP SRT (Server Response Time)
3. TCP Re-transmission behavior
Tools
1. Cisco QoS + IP SLA
http://www.cisco.com/en/US/technologies/tk648/tk362/tk920/technologies_white_paper09186a00802d5efe.html
http://www.cisco.com/en/US/docs/ios/12_4/ip_sla/configuration/guide/hsjitter.html#wp1054785
Riverbed, Bluecoat, Ipanema