On the Impact of Security Protocols on the Performance of SNMP J. Schonwalder and V. Marinov IEEE Transactions on Network and Service Management, 2011, pp slides Group 6 KHAN, Taimur KONG, Jing SHIH, Min SUN, Mengti YU, Chenglin

Summary 2 Question: How can SSH, TLS, DTLS be used to address limitations in SNMPv3/USM? Analyzing impact and performance between the various options to secure SNMP Answer this question by

Outline Background SNMP Architecture Message-based Security Session-based Security Performance Evaluation Key Findings Guidelines for Choosing Solutions 3

Background SSH, TLS, DTLS are introduced which can take advantage of already deployed key management infrastructures 4 SNMPv1 has no cryptographic security && the engineered solution on SNMPv2 is too complex USM (User- based Security Model) on SNMPv3 SNMPv3 and USM have to deploy another user and key management infrastructure  high cost

SNMP Architecture 5 Ref. RFC3414(USM), 3584(CSM), 5591(TSM), 5592(SSHTM), 6353(TLSTM) ASIs are the interfaces between subsystem

SNMP Architecture SNMP engine – Dispatcher organizes data flows – Subsystems contain multiple models – Abstract Service Interfaces (ASIs) are used between the communicating subsystems 6 Ref. RFC3411, 5590

SNMP Engine Subsystems Message Processing Security – User-based Security Model (USM) – Community-based Security Model (CSM) – Transport Security Model (TSM) Interacts with session-based transports through cache Access Control – View-based Access Control Model (VACM) Transport – SSH Transport Model (SSHTM) – (D)TLS Transport Model (TLSTM) 7

Message-based Security CSM (SNMPv1, SNMPv2c) – Plain-text community string which provides very little security. USM (SNMPv3) – noAuthNoPriv (nn) no authentication, no encryption – authNoPriv (an) message authentication, message integrity, timeliness checking no encryption – authPriv (ap) message authentication, message integrity, timeliness checking encryption of the payload of SNMP messages 6

TSD for Message-based Security 7

Session-based Security Transport Security Model – Negligible costs on passing existing information SSH Transport Model – Many methods for authentication TLS Transport Model for TLS – Session resumption mechanism – X.509 certificates for authentication TLS Transport Model for DTLS (over datagram) 8

TSD for Session-based Security 9

Performance Evaluation Session Establishment Latency without Packet Loss Bandwidth Usage Latency with Packet Loss Impact of Bulk Retrieval 10

11 Session Establishment PERFORMANCE OF A SINGLE SNMPGET REQUESTS (SYSDESCR.0)

Session Establishment TCP > UDP, otherwise SNMPv1 ≈ SNMPv2c USM > CSM, however USM/ap ≈ USM/nn Session establishment = significant delays especially on slow machine SSH > TLS/DTLS TLS with session resumption is close to USM in terms of bandwidth and packets exchanged 14

12 USM/ap ≈ USM/an ≈ USM/nn little benefit in removing auth/priv Slow machine Fast machine

13 USM/ap ≈ TSM/SSH ≈ TSM/(D)TLS difference of protocols do not have much impact on delays if using similar hash functions and encryption transformations Latency without Packet Loss （ II ）

14 For v3/USM/ap, v3/USM/nn, v2/CSM over UDP ≈ over TCP UDP ≈ TCP in a fast reliable network

15 USM/ap > USM/an > USM/nn > SNMPv2/TCP >SNMPv2/UDP

16 TSM/TLS > TSM/SSH > USM > TSM/DTLS

Bandwidth Usage USM/ap > USM/an > USM/nn > SNMPv2/TCP > SNMPv2/UDP – carrying security parameters – TCP header > UDP header Interestingly USM/ap-nn ≈ SNMPv2/tcp-udp TSM/TLS > TSM/SSH > USM > TSM/DTLS – TCP header > UDP header – TLS is less efficient than SSH in message encodings Interestingly TSM/SSH/TCP/ap ≈ USM/UDP/ap 20

Latency with Packet Loss TCP's retransmission algorithm clearly outperforms simple non-adaptive SNMP retransmission algorithm 17 PACKET SIZES FOR ALL SECURITY MODEL / TRANSPORT / SECURITY LEVEL COMBINATIONS

18 Impact of Bulk Retrieval ↑ max-repetitions parameter (r) ↓ number of interaction needed ↓ overall latency In the best case, r reduces the number of interactions to 1/r. But not the same factor for latency

Impact of Bulk Retrieval ↑ max-repetitions parameter (r) ↓ number of interaction needed ↓ overall latency In the best case, r reduces the number of interactions to 1/r But not the same factor for latency – larger response messages – startup costs not affected 23

Key Findings TLS session resumption feature significantly reduces the session reestablishment costs. – The resumption mechanism for SSH has yet to be standardised The difference between USM, SSH, TLS and DTLS in terms of latency is small after the session is established. The non-adaptive SNMP retransmission algorithm is outperformed by TCP algorithms in packet loss networks. The usage of GetBulk dramatically reduces the number of requests sent over the network, resulting in much better overall performance. 19

Guidelines for Choosing Solutions (SNMPv3/TSM/SSH) Good AAA Integration Easy derivation of securityName Efficient message encoding × Large session establishment overhead × Lack of session resumption mechanisms TCP Supports large messages better than UDP TCP retransmission algorithms work well 20

Guidelines for Choosing Solutions (SNMPv3/TSM/TLS) Same pros and cons of using TCP as SSHTM TLS session resumption Less session startup overhead than SSH × Less efficient in message encodings than SSH × Complicated derivation of securityName × Lack of good AAA integration × X.509 infrastructure must be in place 21

Guidelines for Choosing Solutions (SNMPv3/TSM/DTLS) Shares most of the pros and cons of TLSTM Smaller framing overhead over UDP Applications have control over retransmissions Although application retransmission timers are to be coordinated with DTLS retransmission timers 22

Guidelines for Choosing Solutions (SNMPv3/USM/UDP) Separate key management infrastructure – Ensures the availability of security regardless of the presence of other infrastructures – × Expensive to deploy Works efficiently if interactions are sporadic Applications have full control over retransmissions. SNMP/UDP may suffer from IP fragmentation 23

Related work Session resumption for the secure shell protocol Thank you! Question? 24